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Oriented polymer grid reinforcement
Consrrwtion md B&ding Materi&, VaI. 9, No. 6, pp. 389401, 1995 Copyright 8 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0950-0618/95/$10.00+0.00
095@-0618(95)00068-2
Oriented polymer grid reinforcement G. R. Carter and J. H. Dixon Net/on
Limited,
Received
New Wellington
21 November
Street, Blackburn,
1994; accepted
30 January
Lancashire,
BB2 4PJ, UK
1995
Tensar high strength oriented polymer gridswere developed in the UK in the late 1970s. Extensive research has been conducted in Europe and North America to investigate the various civil engineering applications for these materials. The availability of these high strength, durable reinforcing grids has led to many innovative and economical developments in geotechnical and highway engineering. This paper describes the major application areas: reinforced soil walls and slopes, reinstatement of slope failures; embankment foundations over soft soil; reinforcement of road bases for paved roads; and asphalt reinforcement. Keywords:
geogrid; reinforced soil
As long ago as 1965 potential soil reinforcement applications for polymer grids and meshes were realized in the construction of steep railway embankments’. These polymer meshes were extruded non-oriented, integral structures manufactured by the Netlon* process. In its simplest form this process consists of extruding a polymer melt through two counter rotating die lips, each one having a series of slots. The speed of rotation, the nature of the movement (including oscillation) and the profile of the slot are all factors affecting the characteristics of the mesh. By the late 1970s it had become clear that there was a need for polymer grids of greater tensile stiffness and strength than those available at the time. The most efficient means of achieving these properties from a polymer is to orient the polymer molecules. Put simply, the randomly arranged long chain molecules can be made to align in the direction of required strength. This can be achieved by drawing the polymer into strands or fibres which can be bonded into a mesh. Alternatively, a grid may be drawn from a continuous pre-punched sheet. This patented method known as the Tensar* process produces an oriented polymer grid with rigid integral joints (Figure 1). This type of junction is a critical feature in the majority of grid reinforcement applications. Reseach which started in 1978 showed clearly that uniplanar polymer sheet could, when punched with an extremely precise pattern of apertures, be stretched in either one or two directions. The strength of the interconnecting ribs and of the junctions could thus be balanced. By careful selection of the grades of high density polyethylene (HDPE) copolymer or polypropylene (PP) * Registered
trademark
of Netlon Limited in the UK and other contries.
Construction
the main polymers used - and by determining the correct shape or pattern of the often complex apertures, the short-term tensile strength of the main elements of the structures formed by orientation could, it was found, be more than 500 MPa - the equivalent of good-quality mild steel. During stretching, polymer is drawn from the junctions into the ribs as the orientation effect passes through the junction zones. There is thus no discontinuity in molecular orientation in the resultant structure. Uniaxial grids, stretched in one direction, or biaxial grids stretched in perpendicular directions, may be produced.
Reinforced soil Historicul background
The inclusion of a tensile resistant material within soil to improve the stability is not new*. Examples are known to have existed 6000 years ago, the earliest remaining example being the Agar-Quf Ziggurrat which incorporates woven reeds as reinforcement3. In the 1960s significant progress took place with the development of vertical reinforced earth walls incorporating a non-structural ‘concrete or steel facing attached to thin, galvanized steel strip reinforcement placed within good quality ‘cohesionless’ fil14. This reinforced earth technique, however, did not lend itself to the construction of reinforced soil slopes or embankments where a solid facing was not appropriate. There have also been durability problems due to corrosion of steel strips buried in ‘granular’ fil?. The use of steel reinforcement with cheaper, more chemically aggressive fine grained soils, such as industrial waste or indigenous cohesive soils, was not considered practical. An inherently inert material was and Building
Materials
1995 Volume
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Oriented polymer grid reinforcement:
G. R. Carter and J. H. Dixon
Punchedsheet
The Tensar process
Figure 1
The Tensar manufacturing process
therefore sought which could provide long-term reinforcement to a wide range of soil types. In addition, attention was focused on the most desirable characteristics of the reinforcement, including its geometrical structure. Plane surfaced strip or sheet reinforcements rely on friction to achieve stress transfer between the reinforcement and the soil. By contrast, grid reinforcement with integral high strength junctions between longitudinal ribs and transverse bars provides stress transfer predominantly through the interlocking of soil particles and their bearing on the transverse bars. The transverse bars which run parallel to the slope face must be stiff relative to their length and, for optimum efficiency, the junctions between the longitudinal ribs and transverse bars must be strong. The interaction between grids, also referred to as geogrids, and soils is extremely efficient. This has been demonstrated by laboratory and field studies”‘. In addition Jewel19 showed in the laboratory that grid reinforcement is effective in increasing both the short-term and long-term stability of cohesive soils. Tensar geogrids were developed to satisfy these requirements. The HDPE and PP polymers from which they are manufactured were selected for their outstanding resistance to chemical and biological attack. They are considered to ‘be inert to all conditions naturally found in soil”. Such materials are now widely used in the storage and processing of hazardous chemicals, in gas pipelines and in petrol containers. These polymers are not attacked by micro-organisms as confirmed by the National Water Authority’s approval for their use in conveying drinking water where it is critical that they should not support biological growth. Nor are these grades of polymer susceptible to environmental stress cracking. The polymers contain carbon black to resist ultra-violet light attack as in some applications the grids could be permanently exposed to direct sunlight.
rapid tensile test on the product may be adequate for design purposes. For many reinforced soil applications, however, the tensile load in the geogrid must be sustained over a very long time (Table 1). For these structures a rigorously researched, long-term design strength for the reinforcement is a critical parameter within the geotechnical design. The simple approach of arbitrarily applying a percentage reduction to the results of some form of rapid load test cannot be used to determine an accurate and reliable design strength, since analysis of all data confirms that there is no correlation between the long-term strength of polymer reinforcement and its short-term strength. Extensive long-term controlled environment laboratory sustained load (creep) testing, is required, to provide design data. Test procedures are described in BS6906, Part 5: (Determination of creep). Appropriate load levels are chosen and sustained for at least 10 000 hours in temperature controlled laboratories at up to 40°C. Designs based on an in-soil temperature of 10°C are recommended for temperate climates such as the UK” and for warmer climates the designer may choose a higher in-soil temperature. Data from these tests has been analysed, using methods suggested by Murray and McGown’* combined with the time-temperature superposition technique of Andrawes et al.13, to provide the safe values of long-term characteristic strength of Tensar geogrids for design lives of up to 120 years. Reference samples have continued under test for many years to verify the long-term performance of the products. Extensive field trials have been carried out to evaluate the effects of construction activities on the performance of these grids in soil. In one trial limestone fill, with various gradings in different test bays, was placed and spread over the grids by tracked plant and compacted with a heavyweight vibratory roller using compactive efforts of up to twice the level specified in the UK by the DTp14. On completion, the fill was removed and the grids were recovered and re-tested to evaluate any change in the properties due to construction activities. Results” show that there is often a small decrease in ultimate strength but that the load strain properties in
Table 1
Practical examples of loading periods
Loading period 0.01 s 0.1 s IS
1 min Ih 1 day
1 month 2 months 1 year
Fast traffic Braking/accelerating traffic Construction traffic Parked vehicle
Construction of a retaining wall Construction of a bridge abutment Consolidation of weak foundations Life of a temporary structure
Design strength
In some applications tensile loading is transmitted into the geogrid for very short periods. For such cases a 390
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and Building
Materials
10 years 60 years 120 years
1995 Volume 9 Number 6
Rapid tensile testing required
Life of a marine structure (BS 6349) Life of a retaining wall, highway structure and bridge abutment
Long-term sustained load testing required
Oriented polymer
the working range are unaltered by construction activities. It is, however, suggested that a partial factor of safety appropriate to the fill type is applied to the characteristic strength to take account of the effects of slight damage due to factors such as site handling, fill type and size, methods of compaction, undulation of fill and other construction activities. Such rigorous and comprehensive testing and evaluation of Tensar soil reinforcing geogrids has resulted in independent certification by the Institut fiir Bautechnik, Federal Republic of Germany, the British Board of Agrement and the Hong Kong Geotechnical Engineering office in certifying these geogrids for use in reinforced soil retaining walls and bridge abutments with 120 year design lives. Reinforced
soil walls
Design of reinjivced soil walls. In the United Kingdom the design of reinforced soil walls incorporating geogrids on public highway schemes is currently carried out in accordance with Department of Transport Technical Memorandum BE3/78 ‘Reinforced and Anchored Earth-Retaining Walls and Bridge Abutments for Embankments”‘. This is based on the ‘Tie Back Wedge Analysis’. Firstly the external stability of the reinforced soil block is checked against the possible failure mechanisms of sliding, overturning, tilting/bearing and slip (Figure 2). The internal stability is then checked against a series of potential failure planes which are orientated at the Rankine active wedge angle of 45” - (p/2 where cp is the friction angle of the wall fill. It is necessary to ensure that at any point within the reinforced soil block there is sufficient tensile strength in the geogrid reinforcement
(a) Tension Failure Figure 3
I
(b) Overturning Failure
(a) Sliding Failure
(c) Tilting/Bearing
Oriented polymer geogrids are extensively used in the construction of reinforced soil retaining walls. The wall can range in type from hard faced structures retaining major roads, which, in the UK, would require the geogrids to have a British Board of Agrement Roads and Bridges Certificate for a 120 year design life, to soft faced and temporary structures.
G. R. Carter and J. H. Dixon
grid reinforcement:
Figure 2
Failure
(d) Slip Failure
External stability of reinforced soil walls
across a potential failure plane and sufficient pull-out resistance behind the potential failure plane (Figure 3). A requirement of the design method is that post construction strains are limited to 1% for walls and 0.5% for abutments. Construction of reinforced soil walls. Polymer geogrids allow the use of a wide range of facings to reinforced soil walls, the choice of which depends upon aesthetic and functional considerations. Hard faced vertical reinforced soil walls have been constructed from concrete face panels, steel stanchion and concrete planks faced with an architectural finish, discrete dry bedded concrete blocks and concrete bagwork. Examples of some of these construction methods are given below.
(u) Concrete j&e panels. Concrete facings to reinforced soil structures have taken the form of either full height or incremental precast concrete face panels. In both cases, starter lengths of geogrid are cast into the rear of the concrete face pane1 at the precast factory. A full strength connection between the starter length and
(b) Pull-Out Failure
Internal failure mechanisms (schematic)
Construction
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1995 Volume
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Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Typical detail to provide a full strength SR bodkin joint
Tensar Bodkin
Figure4
Bodkin joint Figure 6
the remaining reinforcement is provided by a bodkin joint as shown in Figure 4. A typical section through an early example of a full height panel wall is shown in Figure 5. This river wall was constructed in tidal conditions on the River Derwent in Gateshead in 1984. The reinforced soil design was chosen as an alternative to a traditional anchored sheet pile structure on the grounds of a cost saving of approximately 40%. Figure 6 shows the wall during construction. Tensioning is being carried out to eliminate slack in the geogrids and the bodkin joint prior to placing and compacting fill. Temporary support, in this case in the form of chain guys and timber wedges, is removed when the fill reaches between one half and two thirds of full height. The panels were monitored for outward rotation during construction. The movement amounted to approximately l/100 of the height and was completed prior to the casting of an itz situ capping. Since the completion of this structure further uses of the full height face panel method of construction include highway retaining walls on the M74 at Lockerbie and bridge abutments on the Stirling Inner Distributor road.
Full height face panel wall during construction
(b) Stanchion and plunk j&e. In some situations an architectural facing of stone or brick is preferred to concrete. A reinforced soil system incorporating geogrids connected to a cost effective stanchion and plank face to which an architectural facing can be attached has been developed specifically for this purpose (Figure 7). The first use of this system was developed at Dewsbury by West Yorkshire County Council. A reinforced soil structure was proposed in order to provide an inherently stable structure over weak and variable foundations soils which would otherwise require piled foundations or some other form of foundation improvement. The facing system was chosen to allow the use of local Millstone Grit to harmonize with the surrounding buildings (Figure 8). A similar system has been used on a number of projects including reinforced soil bridge abutments faced with brick on the Banbury Inner Relief Road in Oxfordshire. Reinforced
steep soil slopes
The need to minimize land take and to utilize indigenous, often low quality soil fills has created a demand Guard
Capping
rail
beam--+.
___----Sealant
----
-----
-_-_-
+_-------
+---Drainage Plan detail of facing panel and joints
medium _------
MLWST P Steel / sheet pile Figure 5
392
Cross-section of full height face panel wall
Construction
and Building Materials
1995 Volume 9 Number 6
ss2 geogrids
Oriented
Figure 7 Reinforced soil wall incorporating with architectural finish
stanchion
polymer
and plank face
grid reinforcement:
First the material properties of the different components must be established. The material parameters of the soils forming the reinforced soil block and also the adjacent soils outside the reinforced block must be established using conventional sampling and testing practices. The two key parameters to be defined for the geogrid are the long-term design strength and the soil-grid interactions or bond coefficients which are usually derived from large sample shear box tests. Limit equilibrium methods are most commonly used for steep reinforced soil slopes. Modified Bishops procedure, Simplified Bishops procedure, two-part wedge and three-part wedge methods have all been utilized. The stabilizing effect of reinforcement may be incorporated into the conventional limit equilibrium analysis. Design charts’6s17are available for simple slope analysis problems. The use of the charts is limited to reinforced slopes on stable, level foundations. The charts include varying soil properties, slope angles, pore water pressures, and geometric considerations. The charts are derived from limit equilibrium analysis of logarithmic spiral and two-part wedge failure surfaces. Jewell’s revised design method” was an extension of his earlier method to enable use of the charts for wider range of reinforcement materials. This requires greater account to be taken of the bond coefficient. Reinforcement anchorage is calculated by considering a bond coefficient (Fb) between the reinforcement and the soil. This can vary between 0 and 1. For geogrids with integral rigid joints this bond coefficient or coefficient of interaction varies between 0.85 and 0.95 for sands and 0.6 and 0.7 for clays. Construction Slopes steeper
Figure 8
Dewsbury
wall during
construction
and after completion
for slopes to be constructed with face angles steeper than that at which the unreinforced soil would be stable. Design of reinjbrced steep slopes. Normal geotechnical design processes are followed in producing a reinforced soil solution. Construction
G. R. Carter and J. H. Dixon
of steep reinforced thun 1.1. Reinforced
embankments.
(a)
slopes steeper than 1:1 require a positive face restraint connected to the layers of reinforcement. Numerous types of facing systems have been employed. These include pre-cast solid concrete blocks, manhandleable hollow concrete blocks, timber (including rail sleepers), gabion baskets, grout bags and sand bags (Figure 9). Many embankments, including military blast protection traverses, have incorporated a ‘wraparound face’. With this technique the layer of reinforcement is extended outside the slope, up temporary formwork and returned horizontally back into the slope where the fill reaches the level of the next layer of reinforcement. Formwork should always be used for this operation in order not only to maintain the face profile during construction but also to achieve good face compaction. Normally this formwork takes the form of a scaffold frame with upright scaffold boards placed at regular intervals (Figure IO). As each completed layer of reinforced soil is self supporting the frame may be relatively light. In order to leave a neat finished profile, the vertical spacing of reinforcement local to the face is normally limited to 400 mm. This is often achieved by introducing intermediate layers of secondary reinforcement at the face consisting of short lengths of relatively lightweight biaxial grids. and Building
Materials
1995 Volume 9 Number 6
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Oriented polymer grid reinforcement:
G. R. Carter and J. H. Dixon
SS geogrids
at 500mm -.=-: vertical
Finished
Figure 9 Sandbags ing structure
used to face geogrid
reinforced
temporary
G.L.
retainFigure 11
Figure IO External shuttering reinforced soil structure
used to form the wraparound
face of a
The wraparound face detail can be selected to suit the environmental requirements of each project. In some instances, such as temporary work, no surface treatment is necessary (in which case the appropriate size fill or a filter is required to prevent leaching out of fines at the face). Often a grassed face is desirable, as employed on steep reinforced environmental bunds built adjacent to motorways. Standard turf or, preferably, rolls of mesh reinforced grass (typically 1 m wide x 15 m long) are often placed inside the wraparound face during construction. A thin topsoil support layer is placed behind the turf so that the grass can then grow through the geogrid apertures to create a natural appearance. Stepped faces incorporating creepers on the horizontal ledges have also been successfully used to create a vegetated face. A hard finish is sometimes required for a wraparound face such as in urban areas or arid regions where vegetation is not feasible or where ultraviolet radiation is extremely high. In these cases, sprayed concrete may be applied over the whole surface.
Geogrid
394
Construction
and Building Materials
for slopes of I:1 or less
Reinforced soil slip rep&t-J
Shallow slope failures in embankments and cuttings with conventional side slopes formed from over-consolidated clay pose serious and expensive problems2”~2’. Such failures are caused by an increase in positive pore water pressure at shallow depth local to the slope surface22,23.Laboratory tests on over-consolidated clay at low effective stress levels, corresponding to those in slopes at failure, have shown the strength of the clay to Table 2
Cost comparisons method
thun 1:l. In general, for reinforced
slopes flatter than l:l, efficient geogrid to soil interlock, combined with a sufficiently close vertical spacing of reinforcement layers, provides face stability without the need for a positive restraint. In many cases lighter, secondary geogrids are used in short horizontal layers
arrangement
placed between the more widely spaced main reinforcement (Figure 11). The wraparound facing is not used and it is only necessary to ensure that the slope surface is protected against erosion, usually by installing a flexible geosynthetic mat after the earthwork operation is complete. This construction technique is particularly economical and rapid since no formwork is required. The proximity of the grid to the face provides the stability to enable heavy compaction plant to run along the shoulder of the slope. The face can also be overfilled and trimmed with an excavator bucket in a conventional manner. This technique was used successfully by the Ontario, Canada, Ministry of Transportation and Communication for the construction of a 1 km length of highway at Brampton, Ontario. The east face of the 7 m high embankment was constructed with a 45” slope using glacial silty clay for the main fill. Geogrid reinforcement enabled a saving of over $C 720 000 (1983 prices) compared with the nearest alternative form of construction. A table of costs featuring the various alternatives that were considered was presented by Devata” (Table 2).
Construction
(b) SlopesJlatter
spacing
(millions
Construction costs
Reinforced concrete wall Bin wall Reinforced earth wall I l/4:1 slope (rockfill) 2: I slope (earthfili) I : I slope (relnforced with geogrids)
1995 Volume 9 Number
6
of dollars)
I.52 1.42 1.38 0.93 0.3 0.48
after Devata” Property costs Nil Nil Nil 0.3 0.9 Nil
Total costs I .52 I .42 1.38
1.23 1.2 0.48
Oriented
polymer
be significantly less than that inferred from conventional triaxial tests carried out at higher stressesz4. Traditional repairs have involved excavation, off-site disposal of the slipped soil and replacement with imported granular fill. These repair costs are particularly sensitive to haul distances. Recent use of soil reinforcement techniques involving the re-use of slipped clay and geogrids has enabled repairs to be carried out at between 25-70% of the traditional costZS,26.The method is also much less disruptive to local traffic. Some major slips which had remained unrepaired due to expense have been reinstated using this low cost technique such as the A406 London, UK, North Circular Road at Waterworks Corner”. A number of design solutions for geogrid reinforced soil slip repairs have been proposed, again including design charts derived from analysing potential failure surfaces2*. More recently attention has been focused on analysing potential failure planes which more closely mirror the characteristic shallow translational slips found at many sites2’. When analysing these shallow seated slips it is apparent that relatively small reinforcing forces are required to stabilize the slip surfaces and more importantly that these forces must act close to the face. Layers of reinforcement terminated at the face mobilizing their strength over a short distance, whilst resisting direct shearing of the soil over the reinforcement, are therefore required. Efficient interlock between geogrids with high junction strength and cohesive soil is particularly Important for such applications. The general construction method for these relatively flat slope reinforced soil repairs is described as follows. A series of bays of soil are excavated to accommodate the required reinforcement length. The excavation is benched and if seepage is encountered a granular drainage layer is often placed on the excavated profile. The width of strip excavated at any one time is usually limited to 30 m, enabling construction plant to operate without causing short-term slope instability. If the failed clay is too wet and weak to allow plant to operate, approximately 2% lime (sometimes included in a contract as a provisional item) may be mixed with the clay prior to compaction. The lime is not generally attributed with increasing the design strength of the soil, due to lack of data on the long-term low effective stress behaviour of saturated lime stabilized clay and the difficulty of mixing uniformly. Horizontal geogrid layers are placed on top of the layers of recompacted fill with adjacent reinforcement strips simply butted together (Figure 12). The reinforcement must then be covered with at least 150 mm of fill usually placed by a ‘4 in 1’ dozer bucket (Figure I.?), prior to trafficking with plant. All other operations follow conventional earthworks practice. Embankment
grid reinforcement:
G. R. Carter and J. H. Dixon
Figure 12 Repair of a slip failure ment of the failed soil
incorporating
geogrid
reinforce-
excavation of weak foundation soils and substitution with a granular fill. The geogrids can be employed in a number of ways. Basal reinforcement clay. The Chelmsford
of embankments
on slickensided
Bypass in Essex, UK, includes an 8.5 m high embankment transversing a London clay formation containing relatively weak pre-sheared surfaces within the upper 2 m3”. Traditional methods of dealing with this problem are either disposal of the clay and replacement by granular material or removal and replacement of the clay in compacted layers to remove the weak shear planes. Both of these methods can cause problems, particularly if working below the water level. The use of polymer geogrids allowed construction of the embankment at Chelmsford without the need for excavation. A series of three part wedge slip surfaces were analysed and the reinforcement provided to achieve a factor of safety in excess of 1.2 using residual parameters for the London clay. Four layers of Tensar SR2 geogrid at 250 mm vertical spacing were used beneath the crest with only the bottom layer extending to the toe of the embankment (Figure 14). Geogrid reinforced basal grunulur mattress. Williams3’ describes the use of biaxial geogrids encapsulating a free
base reinjhcement
Polymer geogrids can solve problems associated with the construction of embankments over soft ground, in many cases avoiding the need for piled viaducts, or Construction
Figure 13 Repair of a slip failure ment of the failed soil
and Building Materials
incorporating
1995 Volume
geogrid
reinforce-
9 Number 6
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grid reinforcement:
G. R. Carter and J. H. Dixon
Howe Green embankment
Al2Chelmsford
Byp~
Four layers of Tensar SR2 grid 8.5~
1 O.Om 13.0m
’
___________-,-,,_________--_____I _______-_______
.
__I_ j - -----____ -----_--_-_-_-----
//..~/xV// _I tl.5m London clay head material to 3m depth with polished slicken sides (@‘= Figure 14
Arrangement
of geogrids
to reinforce
embankment
over pre-sheared
draining granular mattress to form the base of an embankment constructed on the Great Yarmouth Bypass in 1983 (Figure 1.5). The embankments, which were a maximum of 8 m high, were constructed on up to 22 m of soft alluvial deposits of soft and very soft organic clay and silt with some layers of peat. The geogrids were provided for three reasons: l
l
l
To provide sufficient factor of safety against slip circle failure, calculated on an effective stress basis. To minimize lateral spread of the embankment on the soft, compressible soils. To control differential settlement.
Geocell matress. The Geocell Mattress is a 1 m deep open cellular structure constructed from a biaxial grid base layer with uniaxial grids forming vertical cells (Figure 16) which are then filled with graded granular fill. The filled Geocell creates a rigid, high strength foundation for the embankment, a construction platform for earthworks plant, and a drainage layer at the base of the embankment. The Geocell provides a cost-effective alternative to removal and replacement of soft foundation soils which are underlain by firmer material. Potential failure planes are intersected and the rigidity of the Geocell forces them deeper into firm strata. The critical failure mechanism becomes that of plastic failure of the soft layer. The rough interface at the base of the Geocell ensures mobilization of the maximum shear capacity of the foundation soil and significantly increases stability. Differential settlement and lateral spread are also minimized.
Reinforced base layer provided by a ‘Tensar’ encapsulated granular mattress as used on the A47 Great Yarmouth By-pass.
Figure 15
396
Geogrids
Construction
encapsulating
and
a basal granular
Building
mattress
Materials
1995
Volume
London
17.0m 2
30.0m /
14”) at 1.5 to 2.0m depth
clay
Geocells using Tensar geogrids have, to date, been used in more than 25 locations around the world. Robertson and Gilchrist3’ describe the design and construction of a Geocell to support a highway embankment over very soft ground on the A807 road at Auchenhowie near Glasgow. Here a 4.5 m high embankment was constructed on a 4.0 m thick layer of soft silty clay with an average undrained cohesion of 15 kN/m2 underlain by stiffer material. The use of a Geocell represented a saving of 3 1% over the excavation and replacement option. Construction of the Geocell was carried out in very poor conditions during the winter of 1985/86 (Figure 17). The Geocell enabled the overlying embankment to be constructed rapidly. Performance since construction has been good. Geogrid reinjhced load transjkr platform for piled embankments. Piled foundations are used to support
embankments on very soft ground in situations where no significant post-construction settlement can be tolerated. To transfer embankment loads onto the piles individual pilecaps are used. However, this can lead to settlement of the subgrade between the pilecaps which can reflect upwards to the road surface. This problem can be solved by incorporating a continuous concrete load transfer slab across the piles. The development of geogrid reinforced load transfer platforms has provided a more cost-effective and more rapidly constructed solution to this problem. The design concept of the load transfer platform is one of enhanced arching. The natural arching of the granular layer is enhanced by incorporating layers of stiff geogrid reinforcement, which is analogous to increasing the angle of friction of the granular material. Guido et af.33*34have shown that multi-layer Tensar geogrid reinforcement can increase the angle of load spread through a soil layer. Using an optimum of three layers of biaxial geogrid, the angle of load spread in a geogrid reinforced granular layer can conservatively be taken as 45“. The thickness of the granular layer, pile cap size and pile spacing is selected so that uniform support is provided at the top of the platform (Figure 18). The first application of this approach was at the Royal Albert Dock Spine Road in London in 198835, where piled road embankments incorporating Tensar biaxial geogrids were constructed over fill and alluvial 9 Number
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Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Figure 16 The Geocell mattress
Figure 17
Partially filled mattress
soils. In this case no significant settlement between the embankments and the adjacent piled reinforced concrete trough carrying the Docklands Light Railway could be accepted. More recently, a similar system has been used at the second Severn Crossing where the toll plaza area is being founded on a granular platform incorporating Columns Tensar geogrids over Vibro Concrete (vcc’s)36. Polymer geogrids in road pavements Geogrid reinforced grunulur layers
Since the development of polymer geogrids in the 1970s a major market has developed in the use of geogrids to reinforce the granular layers of road pavements, i.e. the capping layer and sub-base over soft subgrades. In
addition there have been successful uses of geogrids in the reinforcement of railway ballast. The type of geogrid used in this application is always a biaxial geogrid. The geogrid reinforces the granular material by the mechanism of interlock. When a granular material is compacted onto the geogrid, some of the granular material penetrates the geogrid apertures to form a positive interlock (Figure 19). The granular particles are restrained by the ribs of the grid and therefore do not oscillate under the application of transient loadings from traffic passing over the surface of the granular material. This prevention of particle movement has the dual effects of separation and reinforcement. The separation is achieved by preventing the pumping and subsequent mixing of the granular particles with the soft subgrade. This has two benefits. Firstly; expensive granular material is not lost in the underlying subgrade, and secondly, the stone layer does not become contaminated with soft material and it therefore retains its good load-carrying and drainage properties. The more important result of the interlock, however, is that of reinforcement. By restraining the bottom of the granular layer and preventing movement of the individual particles under heavy wheel loads, deformations within the granular layer are reduced. By careful design, taking into account the CBR of the formation and the volume of traffic travelling over the unsurfaced granular layer during construction, it is possibie to limit rut depths at the surface to 40 mm, and eliminate deformations at the formation level altogether. The elimination of these ruts at the bottom of the granular layers is more important as they cannot be detected and corrected during construction, and, if left, water will collect in them leading to softening and premature failure of the road. Enhanced /
soil arching angle
Geogrid
Geogrid
reinforced mattress 1
_______-
I
1 cd
4 Figure 18
Geogrid reinforced load transfer platform
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and Building Materials
1995 Volume 9 Number 6
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Oriented polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Figure 19 The mechanism of interlock
In order for the interlock mechanism to work effectively it is necessary to use a geogrid which has three basic characteristics: Good stiffness at low strain. The strain at which the geogrid is working in road pavements is very low i.e. generally less than 2%, and it is necessary that the grid is able to give a good strength response at this strain level. High junction strength. The interlock mechanism depends on the ribs of the grid restraining the stone and preventing movement. Therefore the ribs must not only be strong in themselves, but should also have good integrity at the junctions. A good rib profile. The ribs of the grid should be rectangular and therefore present a positive profile to the stone3’. An aerofoil shaped rib with a feather edge is more likely to allow stone movement. Since a correctly designed geogrid can reduce deformations at the surface of a granular layer then, for a particular level of acceptable deformation, usually 40 mm, the granular layer thickness can be reduced38. Stone thickness savings of up to one third can be achieved. This will normally provide an economic solution on formation CBR'S of 5% or less. Even more saving can be achieved for temporary haul roads over soft ground where larger rut depths will normally be acceptable providing the surface remains serviceable. There have been numerous projects over the past 15 years where Tensar biaxial geogrids have been used to reinforce granular layers over soft ground in both temporary and permanent roads. A selection of these are described in Reference 39. In many cases a single layer of geogrid placed on the formation at the base of the granular layer is sufficient. However, if long-term differential settlement is a potential problem, for instance on ground subject to excessive consolidation, compaction or decomposition, then a 398
Construction
and Building
Materials
multi-layer system of geogrids may be required. The geogrids will restrain a layer of granular fill and spread the load, in the manner described above to produce a flexible raft which will minimize the effects of differential settlement. Although a movement free surface cannot be guaranteed, the multi-layer system of geogrids is considerably cheaper than excavation and replacement or other forms of ground treatment. For roads or parking areas where some long-term maintenance is acceptable, this system is very cost-effective. One of the first examples of this type of construction was at Bugsby’s Way in the London Borough of Greenwich in 1981 (Figure 20). Three layers of Tensar geogrid placed within 1 m of granular fill were used as an alternative to piling on waterlogged ground consisting of waste overlying soft alluvium. Twelve years after construction the performance of the road is good, as proved by surveys carried out by the London Borough of Greenwich, with negligible deterioration and no requirements for maintenance at this time. Asphult reinjbrcement Introduction. Stiff polypropylene geogrids for the reinforcement of asphalt were first introduced in 1982, the first project being at Canvey Island. Between 1981 and 1983 an extensive programme of research was carried out by Professor S. F. Brown at Nottingham University. Laboratory test work investigated the benefits of the grid with respect to the control of deformation, the control of reflective cracking and the improvement of the fatigue life of the pavement. The research identified a number of significant benefits due to the use of a stiff polypropylene geogrid within the asphalt layers. The work was carried out over approximately four years and consisted of simulative testing leaving installation techniques to be developed in the field. Details of the laboratory work have been published by Brown et a1.40,41.
1995 Volume 9 Number 6
Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
significant increase in performance was confirmed at full scale. Futigue. Similar beams to those used in the reflective
Figure
20
Installation
of
Tensar
geogrids
at
Bugsby’s
Way,
crack test were used to investigate the efforts of the geogrid with regard to fatigue resistance of the pavement. A slightly less stiff base was used but no gap was incorporated. A saw cut was made in the base of the slab to fix the position of the onset of fatigue cracking. Only vertical cyclic loading was used and strains were measured at various points over the depth of the slab. Results showed that in general the fatigue life of the pavement was increased by a factor of 10 when the geogrid was installed at the base of the layer (Figure 23).
Greenwich
Permanent deformation testing. Testing was carried out on slabs made from various asphalt mixes and loaded on a very stiff base to eliminate the effects of the lower pavement. Repeated wheel loading was applied at a constant speed and at an elevated temperature. Comparison between unreinforced and reinforced slabs showed that the reinforcement provided significant resistance to lateral flow and hence the build up of permanent deformation. The reduction in rutting was found to be as high as 70% (Figure 21). Reflective trucking. 100 mm deep asphalt beams were supported on a stiff base and subjected to vertical cyclic loading and repeated wheel loading. A 10 mm wide joint in the base simulated a joint or crack in a rigid pavement. In the reinforced sample with the grid installed directly above the joint it was shown that crack propagation could be considerably retarded, in fact on some samples cracking was eliminated entirely (Figure 22). These small-scale tests used to examine pavement deformation and reflective cracking were repeated in the pavement test facility at Nottingham University and the
20 Typical rolled asphalt
IS-
Permanent deformation (mm)
O! IO’
Figure 21
Effect of geogrid
I
IO’ Number of load applications
reinforcement
on rutting
I
IO5
performance
Construction
Full safe triuls. Many full scale trials have been carried out in different geographical locations throughout the world to confirm the results of the laboratory testing and to develop efficient methods of installing the geogrid. From the results of the laboratory and field trials it was possible to derive an analytical design method incorporating stiff polypropylene geogrids based on the strain capacity improvement of the pavement related to the number of axle passes. Developments. Although the methods developed for installing the geogrid gained rapid acceptance, methods to improve the technique were constantly being investigated. Work began in 1990 to examine the benefits of bonding a geotextile to the geogrid in order to provide a material which could be installed without the need to fix, tension and dress prior to paving by machine. Geotextile fabrics are commonly used on their own to help improve the bond between layers and also to waterproof any subsequent cracking that occurs during use. They have, however, no intrinsic reinforcing properties and cannot prevent reflective cracking. To produce the Tensar AR-G composite a stiff polypropylene geogrid is thermally bonded to a thin, non-woven, needle punched polypropylene/polyester geotextile. The installation requires the regulated road surface to be sprayed with either a rich bitumen emulsion or straight 200 pen bitumen at a net rate of about 1.0 l/m’. The composite material is then rolled out either by hand or by machine under light tension and brushed to ensure intimate contact with the pavement. When the bitumen has fully cured and the fabric is fully stuck down then mechanical paving can commence. Several trial installations have been carried out in the UK, Europe and USA using bitumen emulsions, polymer modified emulsions and straight bitumen. In these full scale trial pavement installations, cores have been taken and subjected to shear testing which has confirmed that there is no significant loss in bond when compared with the geogrid alone. The geogrid/geotextile composite is resistant to normal asphalt paving temperatures. and Building
Materials
1995 Volume
9 Number 6
399
Oriented polymer grid reinforcement:
Figure 22
Effect of geogrid
reinforcement
G. R. Carter and J. H. Dixon
on reflective
cracking
400 Dense bitumen Macodclm
~~~~
loo-
1.0
Figure 23
Effect of geogrid
reinforcement
on fatigue
Oriented polymer geogrids were first developed in the late 1970s to fulfil a perceived need within the civil engineering industry. Since then their current usage within civil engineering has grown to between 15 and 20 million square metres, world-wide, each year. Major application areas are reinforced soil walls and slopes, the base of embankments over soft ground, the reinforcement of road sub-bases over soft ground and the reinforcement of bituminous overlays in highways maintenance. Tensar geogrids have been accepted by the Department of Transport in the UK, as being available for use in vertical retaining structures with a 120 year design life. Similar certification has been received in both Germany and Hong Kong. Pressure on land in countries such as the UK and Japan means that development often takes place on congested sites with poor ground conditions. Oriented polymer geogrids can solve many of the problems raised by Construction
life (mso)
performance
Conclusion
400
100
10 Design
and Building Materials
such sites. For this reason, and because the use of geogrids is spreading to more countries, their use in civil engineering applications is likely to continue to increase.
References Iwasaki and Watanabe. Reinforcement of railway embankments in Japan. Symposium on Earth Reinforcement, ASCE, 1978 Jones, C. J. F. P. Eurth Reinforcement and Soil Structures. Butterworths, London, 1985 Bagir, T. Iraq J. 1994, pp 5-6 Schlosser, F. Experience in reinforced earth in France. Supplementary Report 457, Transport and Road Research Laboratory, Paper 9, 1977 Blight, G. E. and Dane M. S Deterioration of a wall complex constructed of reinforced soil. Geotechnique 39, (I), Thomas Telford, 1989, pp. 47-53 Chang, J. C., Hannon, J. D. and Forsyth, R. A. Pull-out resistance and interaction of earthwork reinforcement and soil. Transportation Research Record 640, Washington DC, 1977 Forsyth, R. A. Alternative earth reinforcement. Symposium on Earth Reinforcement ASCE, 1978 Peterson, L. M. Pull-out resistance of welded wire mesh embedded in soil. MSc, Utah State University, 1980 Jewel], R. A. Some effects of reinforcement on the mechanical behaviour of soils. PhD Thesis, University of Cambridge, 1980
1995 Volume 9 Number
6
Oriented IO
11
12
13
14 15
polymer
Wrigley, N. E. Durability and long-term performance of Tensar polymer grids for soil reinforcement. Muter. Sci. Technol. March 1987, 3,. 161-72 Reinforced and anchored earth retaining walls and bridge abutments for embankments. Department of Transport, UK. Technical Memorandum (Bridges) BE 3/78 (revised 1987) Murray, R. T. and McGown, A. Assessment of the time dependent behaviour of geotextiles for reinforced soil applications. Durability of Geotextiles, RILEM, Chapman and Hall, 1988, pp. 52--73 Andrawes, K. Z., McGown, A. and Murray, R. T. The loadstrain-time temperature behaviour of geotextiles and geogrids. Proc. Third Int. Conf on Geotextiles, Vol III, Session 7A, Vienna, 1986, pp. 707-712 Specification for Highways Works, Department of Transport, 1986 Bush, D. I. Evaluation of the effects of construction on the physical properties of polymeric soil reinforcing elements. In Proc.
27
28
29
30
31
In!. -Gebtechnical -Symp. on Theory and Praciice of Earth Reinforcement. Fukuoka. October. Balkema. Rotterdam, 1988
16 Jewel], R. A.,‘Paine, N.‘and Woods, R. I.‘Design methods for steep reinforced embankments. In Proc. Symp. on Polymer Grid Reinforcement, Thomas Telford, London, 1984, pp. 70-81 17 Guidelines for the Design and Construction of Embankments over Stable Foundations using Tensar Geogrids, Netlon Limited, 1988 18 Jewel], R. A. Revised design charts for steep reinforced slopes.
32
Proc. Symp. Reinforced Embankments: Theory and Practice in the British Isles, Cambridge, September Thomas Telford, London,
34
I9
20 21
22
23
24
25 26
1989 Devata, M. S. Geogrid reinforce earth embankments with steep side slopes. Proc. Symp. Polymer Grid Reinforcement, Thomas Telford, London, 1984, pp. 82-87 Studies of Slope Stability in Highway Engineering. TRRL leaflet LF934, Transport and Road Research Laboratory, 1983 Parsons, A. W. and Perry, N. Slope stability problems in ageing highway earthworks. In Proc. Symp. on Failures in Earthworks, Thomes Telford, London, pp. 63-78 Chandler, R. J. and Skempton, A. W. The design of permanent cutting slopes in stiff fissured clays, Geotechnique, 1974, 24, (4), pp. 457466 Anderson, M. G. and Kneale, P. E. Pore water pressure changes in a road embankment. J. Inst. Highw. Eng. 1980, May, pp. I l-17 Atkinson, J. H. and Farrar, D. M. Stress path tests to measure soil strength parameters for shallow landslips. I lth ICSMFE, San Francisco, 1985, pp. 983-986 Maintenance and repair of highway embankments: Studies of seven methods of treatment. Research Report 30, Transport and Road Research Laboratory, 1985 Oliver, T. L. H. Reinforced soil techniques for the reinstatement
Construction
33
35
36
37
38 39 40
41
grid reinforcement:
G. R. Carter and J. H. Dixon
of failed soils using geogrids. In Proc. Symp. Failures in Earthworks, Tech Note 7, Thomas Telford, pp. 417419 Dixon, J. H. Geogrid reinforced soild repair of a slope failure in clay - North Circular Road, London, UK. Geosynthetic Case Histories, 35 years of Experience (eds G. P. Raymond and J. P. Giroud) ISSMFE, Chapter 10, 1993 Murray, R. T. Reinforcement techniques in repairing slope failures. In Proc. Symp. on Polymer Grid Reinforcement, Thomas Telford, London, 1984, pp. 47-53 Greenwood, J. R., Holt, D. A. and Herrick, G. W. Shallow slips in highways-embankments constructed of overconsolidated clay. In Proc. Symp. on Failure in Earthworks, Thomas Telford, London, 1985, pp. 79-92 Dixon, J. H. Geogrid basal reinforcement for embankment on slickensides clay - Chelmsford Bypass, United Kingdom. Geosynthethics Case Histories, 35 years of Experience. (eds G. P. Raymond and J. P. Giroud) ISSMFE, Chapter IO, 1993 Williams, D. Reinforced embankments at the Great Yarmouth Bypass. In Symposium on Polymer Grid Reinforcement in Civil Engineering, ICE, London, 1984, pp. 88-94 Robertson, J. and Gilchrist, A. J. T. Design and construction of a reinforced embankment across soft lakebed deposits. Ing Conf on Foundations and Tunnels, 1987 Guido, V. A., Knueppel, J. D. and Sweeny, M. A. Plate loading tests on geogrid reinforced earth slabs. Geosynthetic 87 Conference, New Orleans, 1987, pp. 216225 Guido, V. A. Bearing capacity of shallow foundations reinforced with Tensar geogrids. Tensar Technical Note TN:BRN9, Tensar Corp, 1986 Card, G. B. and Carter, G. R. A case history of a piled embankment in London’s Docklands. The Engineering Geology of Construction, 1992 Bell, A. L., Jenner, C., Maddison, J. D. and Vigroles, J. Embankment support using geogrids with vibro concrete columns. Fifth International Conference on Geotextiles. Geomembranes and Related Products, Singapore, 1994 Webster, S. L. Geogrid reinforced base courses for flexible pavements for light aircraft - test section construction, behaviour under traffic laboratory tests and design criteria. Final Report DOT/FAA/RD - 92125, 1992 Ground stabilisation - design curves showing reduced sub-base thickness using Tensar geogrids. Netlon Limited, 1993 Tensar geogrid reinforced sub-bases - Case Studies. Netlon Limited, 1993 Brown, et al. Polymer grid reinforcement of asphalt. Annual meeting of the Association of Asphalt Paving Technologists, San Antonio, Texas, 1985 Brown, et al. The use of polymer grids for improved asphalt performance. Eurobitumen Conference, The Hague, Netherlands, 1985
and Building
Materials
1995 Volume 9 Number 6
401
095@-0618(95)00068-2
Oriented polymer grid reinforcement G. R. Carter and J. H. Dixon Net/on
Limited,
Received
New Wellington
21 November
Street, Blackburn,
1994; accepted
30 January
Lancashire,
BB2 4PJ, UK
1995
Tensar high strength oriented polymer gridswere developed in the UK in the late 1970s. Extensive research has been conducted in Europe and North America to investigate the various civil engineering applications for these materials. The availability of these high strength, durable reinforcing grids has led to many innovative and economical developments in geotechnical and highway engineering. This paper describes the major application areas: reinforced soil walls and slopes, reinstatement of slope failures; embankment foundations over soft soil; reinforcement of road bases for paved roads; and asphalt reinforcement. Keywords:
geogrid; reinforced soil
As long ago as 1965 potential soil reinforcement applications for polymer grids and meshes were realized in the construction of steep railway embankments’. These polymer meshes were extruded non-oriented, integral structures manufactured by the Netlon* process. In its simplest form this process consists of extruding a polymer melt through two counter rotating die lips, each one having a series of slots. The speed of rotation, the nature of the movement (including oscillation) and the profile of the slot are all factors affecting the characteristics of the mesh. By the late 1970s it had become clear that there was a need for polymer grids of greater tensile stiffness and strength than those available at the time. The most efficient means of achieving these properties from a polymer is to orient the polymer molecules. Put simply, the randomly arranged long chain molecules can be made to align in the direction of required strength. This can be achieved by drawing the polymer into strands or fibres which can be bonded into a mesh. Alternatively, a grid may be drawn from a continuous pre-punched sheet. This patented method known as the Tensar* process produces an oriented polymer grid with rigid integral joints (Figure 1). This type of junction is a critical feature in the majority of grid reinforcement applications. Reseach which started in 1978 showed clearly that uniplanar polymer sheet could, when punched with an extremely precise pattern of apertures, be stretched in either one or two directions. The strength of the interconnecting ribs and of the junctions could thus be balanced. By careful selection of the grades of high density polyethylene (HDPE) copolymer or polypropylene (PP) * Registered
trademark
of Netlon Limited in the UK and other contries.
Construction
the main polymers used - and by determining the correct shape or pattern of the often complex apertures, the short-term tensile strength of the main elements of the structures formed by orientation could, it was found, be more than 500 MPa - the equivalent of good-quality mild steel. During stretching, polymer is drawn from the junctions into the ribs as the orientation effect passes through the junction zones. There is thus no discontinuity in molecular orientation in the resultant structure. Uniaxial grids, stretched in one direction, or biaxial grids stretched in perpendicular directions, may be produced.
Reinforced soil Historicul background
The inclusion of a tensile resistant material within soil to improve the stability is not new*. Examples are known to have existed 6000 years ago, the earliest remaining example being the Agar-Quf Ziggurrat which incorporates woven reeds as reinforcement3. In the 1960s significant progress took place with the development of vertical reinforced earth walls incorporating a non-structural ‘concrete or steel facing attached to thin, galvanized steel strip reinforcement placed within good quality ‘cohesionless’ fil14. This reinforced earth technique, however, did not lend itself to the construction of reinforced soil slopes or embankments where a solid facing was not appropriate. There have also been durability problems due to corrosion of steel strips buried in ‘granular’ fil?. The use of steel reinforcement with cheaper, more chemically aggressive fine grained soils, such as industrial waste or indigenous cohesive soils, was not considered practical. An inherently inert material was and Building
Materials
1995 Volume
9 Number
6
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Oriented polymer grid reinforcement:
G. R. Carter and J. H. Dixon
Punchedsheet
The Tensar process
Figure 1
The Tensar manufacturing process
therefore sought which could provide long-term reinforcement to a wide range of soil types. In addition, attention was focused on the most desirable characteristics of the reinforcement, including its geometrical structure. Plane surfaced strip or sheet reinforcements rely on friction to achieve stress transfer between the reinforcement and the soil. By contrast, grid reinforcement with integral high strength junctions between longitudinal ribs and transverse bars provides stress transfer predominantly through the interlocking of soil particles and their bearing on the transverse bars. The transverse bars which run parallel to the slope face must be stiff relative to their length and, for optimum efficiency, the junctions between the longitudinal ribs and transverse bars must be strong. The interaction between grids, also referred to as geogrids, and soils is extremely efficient. This has been demonstrated by laboratory and field studies”‘. In addition Jewel19 showed in the laboratory that grid reinforcement is effective in increasing both the short-term and long-term stability of cohesive soils. Tensar geogrids were developed to satisfy these requirements. The HDPE and PP polymers from which they are manufactured were selected for their outstanding resistance to chemical and biological attack. They are considered to ‘be inert to all conditions naturally found in soil”. Such materials are now widely used in the storage and processing of hazardous chemicals, in gas pipelines and in petrol containers. These polymers are not attacked by micro-organisms as confirmed by the National Water Authority’s approval for their use in conveying drinking water where it is critical that they should not support biological growth. Nor are these grades of polymer susceptible to environmental stress cracking. The polymers contain carbon black to resist ultra-violet light attack as in some applications the grids could be permanently exposed to direct sunlight.
rapid tensile test on the product may be adequate for design purposes. For many reinforced soil applications, however, the tensile load in the geogrid must be sustained over a very long time (Table 1). For these structures a rigorously researched, long-term design strength for the reinforcement is a critical parameter within the geotechnical design. The simple approach of arbitrarily applying a percentage reduction to the results of some form of rapid load test cannot be used to determine an accurate and reliable design strength, since analysis of all data confirms that there is no correlation between the long-term strength of polymer reinforcement and its short-term strength. Extensive long-term controlled environment laboratory sustained load (creep) testing, is required, to provide design data. Test procedures are described in BS6906, Part 5: (Determination of creep). Appropriate load levels are chosen and sustained for at least 10 000 hours in temperature controlled laboratories at up to 40°C. Designs based on an in-soil temperature of 10°C are recommended for temperate climates such as the UK” and for warmer climates the designer may choose a higher in-soil temperature. Data from these tests has been analysed, using methods suggested by Murray and McGown’* combined with the time-temperature superposition technique of Andrawes et al.13, to provide the safe values of long-term characteristic strength of Tensar geogrids for design lives of up to 120 years. Reference samples have continued under test for many years to verify the long-term performance of the products. Extensive field trials have been carried out to evaluate the effects of construction activities on the performance of these grids in soil. In one trial limestone fill, with various gradings in different test bays, was placed and spread over the grids by tracked plant and compacted with a heavyweight vibratory roller using compactive efforts of up to twice the level specified in the UK by the DTp14. On completion, the fill was removed and the grids were recovered and re-tested to evaluate any change in the properties due to construction activities. Results” show that there is often a small decrease in ultimate strength but that the load strain properties in
Table 1
Practical examples of loading periods
Loading period 0.01 s 0.1 s IS
1 min Ih 1 day
1 month 2 months 1 year
Fast traffic Braking/accelerating traffic Construction traffic Parked vehicle
Construction of a retaining wall Construction of a bridge abutment Consolidation of weak foundations Life of a temporary structure
Design strength
In some applications tensile loading is transmitted into the geogrid for very short periods. For such cases a 390
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and Building
Materials
10 years 60 years 120 years
1995 Volume 9 Number 6
Rapid tensile testing required
Life of a marine structure (BS 6349) Life of a retaining wall, highway structure and bridge abutment
Long-term sustained load testing required
Oriented polymer
the working range are unaltered by construction activities. It is, however, suggested that a partial factor of safety appropriate to the fill type is applied to the characteristic strength to take account of the effects of slight damage due to factors such as site handling, fill type and size, methods of compaction, undulation of fill and other construction activities. Such rigorous and comprehensive testing and evaluation of Tensar soil reinforcing geogrids has resulted in independent certification by the Institut fiir Bautechnik, Federal Republic of Germany, the British Board of Agrement and the Hong Kong Geotechnical Engineering office in certifying these geogrids for use in reinforced soil retaining walls and bridge abutments with 120 year design lives. Reinforced
soil walls
Design of reinjivced soil walls. In the United Kingdom the design of reinforced soil walls incorporating geogrids on public highway schemes is currently carried out in accordance with Department of Transport Technical Memorandum BE3/78 ‘Reinforced and Anchored Earth-Retaining Walls and Bridge Abutments for Embankments”‘. This is based on the ‘Tie Back Wedge Analysis’. Firstly the external stability of the reinforced soil block is checked against the possible failure mechanisms of sliding, overturning, tilting/bearing and slip (Figure 2). The internal stability is then checked against a series of potential failure planes which are orientated at the Rankine active wedge angle of 45” - (p/2 where cp is the friction angle of the wall fill. It is necessary to ensure that at any point within the reinforced soil block there is sufficient tensile strength in the geogrid reinforcement
(a) Tension Failure Figure 3
I
(b) Overturning Failure
(a) Sliding Failure
(c) Tilting/Bearing
Oriented polymer geogrids are extensively used in the construction of reinforced soil retaining walls. The wall can range in type from hard faced structures retaining major roads, which, in the UK, would require the geogrids to have a British Board of Agrement Roads and Bridges Certificate for a 120 year design life, to soft faced and temporary structures.
G. R. Carter and J. H. Dixon
grid reinforcement:
Figure 2
Failure
(d) Slip Failure
External stability of reinforced soil walls
across a potential failure plane and sufficient pull-out resistance behind the potential failure plane (Figure 3). A requirement of the design method is that post construction strains are limited to 1% for walls and 0.5% for abutments. Construction of reinforced soil walls. Polymer geogrids allow the use of a wide range of facings to reinforced soil walls, the choice of which depends upon aesthetic and functional considerations. Hard faced vertical reinforced soil walls have been constructed from concrete face panels, steel stanchion and concrete planks faced with an architectural finish, discrete dry bedded concrete blocks and concrete bagwork. Examples of some of these construction methods are given below.
(u) Concrete j&e panels. Concrete facings to reinforced soil structures have taken the form of either full height or incremental precast concrete face panels. In both cases, starter lengths of geogrid are cast into the rear of the concrete face pane1 at the precast factory. A full strength connection between the starter length and
(b) Pull-Out Failure
Internal failure mechanisms (schematic)
Construction
and Building Materials
1995 Volume
9 Number 6
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Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Typical detail to provide a full strength SR bodkin joint
Tensar Bodkin
Figure4
Bodkin joint Figure 6
the remaining reinforcement is provided by a bodkin joint as shown in Figure 4. A typical section through an early example of a full height panel wall is shown in Figure 5. This river wall was constructed in tidal conditions on the River Derwent in Gateshead in 1984. The reinforced soil design was chosen as an alternative to a traditional anchored sheet pile structure on the grounds of a cost saving of approximately 40%. Figure 6 shows the wall during construction. Tensioning is being carried out to eliminate slack in the geogrids and the bodkin joint prior to placing and compacting fill. Temporary support, in this case in the form of chain guys and timber wedges, is removed when the fill reaches between one half and two thirds of full height. The panels were monitored for outward rotation during construction. The movement amounted to approximately l/100 of the height and was completed prior to the casting of an itz situ capping. Since the completion of this structure further uses of the full height face panel method of construction include highway retaining walls on the M74 at Lockerbie and bridge abutments on the Stirling Inner Distributor road.
Full height face panel wall during construction
(b) Stanchion and plunk j&e. In some situations an architectural facing of stone or brick is preferred to concrete. A reinforced soil system incorporating geogrids connected to a cost effective stanchion and plank face to which an architectural facing can be attached has been developed specifically for this purpose (Figure 7). The first use of this system was developed at Dewsbury by West Yorkshire County Council. A reinforced soil structure was proposed in order to provide an inherently stable structure over weak and variable foundations soils which would otherwise require piled foundations or some other form of foundation improvement. The facing system was chosen to allow the use of local Millstone Grit to harmonize with the surrounding buildings (Figure 8). A similar system has been used on a number of projects including reinforced soil bridge abutments faced with brick on the Banbury Inner Relief Road in Oxfordshire. Reinforced
steep soil slopes
The need to minimize land take and to utilize indigenous, often low quality soil fills has created a demand Guard
Capping
rail
beam--+.
___----Sealant
----
-----
-_-_-
+_-------
+---Drainage Plan detail of facing panel and joints
medium _------
MLWST P Steel / sheet pile Figure 5
392
Cross-section of full height face panel wall
Construction
and Building Materials
1995 Volume 9 Number 6
ss2 geogrids
Oriented
Figure 7 Reinforced soil wall incorporating with architectural finish
stanchion
polymer
and plank face
grid reinforcement:
First the material properties of the different components must be established. The material parameters of the soils forming the reinforced soil block and also the adjacent soils outside the reinforced block must be established using conventional sampling and testing practices. The two key parameters to be defined for the geogrid are the long-term design strength and the soil-grid interactions or bond coefficients which are usually derived from large sample shear box tests. Limit equilibrium methods are most commonly used for steep reinforced soil slopes. Modified Bishops procedure, Simplified Bishops procedure, two-part wedge and three-part wedge methods have all been utilized. The stabilizing effect of reinforcement may be incorporated into the conventional limit equilibrium analysis. Design charts’6s17are available for simple slope analysis problems. The use of the charts is limited to reinforced slopes on stable, level foundations. The charts include varying soil properties, slope angles, pore water pressures, and geometric considerations. The charts are derived from limit equilibrium analysis of logarithmic spiral and two-part wedge failure surfaces. Jewell’s revised design method” was an extension of his earlier method to enable use of the charts for wider range of reinforcement materials. This requires greater account to be taken of the bond coefficient. Reinforcement anchorage is calculated by considering a bond coefficient (Fb) between the reinforcement and the soil. This can vary between 0 and 1. For geogrids with integral rigid joints this bond coefficient or coefficient of interaction varies between 0.85 and 0.95 for sands and 0.6 and 0.7 for clays. Construction Slopes steeper
Figure 8
Dewsbury
wall during
construction
and after completion
for slopes to be constructed with face angles steeper than that at which the unreinforced soil would be stable. Design of reinjbrced steep slopes. Normal geotechnical design processes are followed in producing a reinforced soil solution. Construction
G. R. Carter and J. H. Dixon
of steep reinforced thun 1.1. Reinforced
embankments.
(a)
slopes steeper than 1:1 require a positive face restraint connected to the layers of reinforcement. Numerous types of facing systems have been employed. These include pre-cast solid concrete blocks, manhandleable hollow concrete blocks, timber (including rail sleepers), gabion baskets, grout bags and sand bags (Figure 9). Many embankments, including military blast protection traverses, have incorporated a ‘wraparound face’. With this technique the layer of reinforcement is extended outside the slope, up temporary formwork and returned horizontally back into the slope where the fill reaches the level of the next layer of reinforcement. Formwork should always be used for this operation in order not only to maintain the face profile during construction but also to achieve good face compaction. Normally this formwork takes the form of a scaffold frame with upright scaffold boards placed at regular intervals (Figure IO). As each completed layer of reinforced soil is self supporting the frame may be relatively light. In order to leave a neat finished profile, the vertical spacing of reinforcement local to the face is normally limited to 400 mm. This is often achieved by introducing intermediate layers of secondary reinforcement at the face consisting of short lengths of relatively lightweight biaxial grids. and Building
Materials
1995 Volume 9 Number 6
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Oriented polymer grid reinforcement:
G. R. Carter and J. H. Dixon
SS geogrids
at 500mm -.=-: vertical
Finished
Figure 9 Sandbags ing structure
used to face geogrid
reinforced
temporary
G.L.
retainFigure 11
Figure IO External shuttering reinforced soil structure
used to form the wraparound
face of a
The wraparound face detail can be selected to suit the environmental requirements of each project. In some instances, such as temporary work, no surface treatment is necessary (in which case the appropriate size fill or a filter is required to prevent leaching out of fines at the face). Often a grassed face is desirable, as employed on steep reinforced environmental bunds built adjacent to motorways. Standard turf or, preferably, rolls of mesh reinforced grass (typically 1 m wide x 15 m long) are often placed inside the wraparound face during construction. A thin topsoil support layer is placed behind the turf so that the grass can then grow through the geogrid apertures to create a natural appearance. Stepped faces incorporating creepers on the horizontal ledges have also been successfully used to create a vegetated face. A hard finish is sometimes required for a wraparound face such as in urban areas or arid regions where vegetation is not feasible or where ultraviolet radiation is extremely high. In these cases, sprayed concrete may be applied over the whole surface.
Geogrid
394
Construction
and Building Materials
for slopes of I:1 or less
Reinforced soil slip rep&t-J
Shallow slope failures in embankments and cuttings with conventional side slopes formed from over-consolidated clay pose serious and expensive problems2”~2’. Such failures are caused by an increase in positive pore water pressure at shallow depth local to the slope surface22,23.Laboratory tests on over-consolidated clay at low effective stress levels, corresponding to those in slopes at failure, have shown the strength of the clay to Table 2
Cost comparisons method
thun 1:l. In general, for reinforced
slopes flatter than l:l, efficient geogrid to soil interlock, combined with a sufficiently close vertical spacing of reinforcement layers, provides face stability without the need for a positive restraint. In many cases lighter, secondary geogrids are used in short horizontal layers
arrangement
placed between the more widely spaced main reinforcement (Figure 11). The wraparound facing is not used and it is only necessary to ensure that the slope surface is protected against erosion, usually by installing a flexible geosynthetic mat after the earthwork operation is complete. This construction technique is particularly economical and rapid since no formwork is required. The proximity of the grid to the face provides the stability to enable heavy compaction plant to run along the shoulder of the slope. The face can also be overfilled and trimmed with an excavator bucket in a conventional manner. This technique was used successfully by the Ontario, Canada, Ministry of Transportation and Communication for the construction of a 1 km length of highway at Brampton, Ontario. The east face of the 7 m high embankment was constructed with a 45” slope using glacial silty clay for the main fill. Geogrid reinforcement enabled a saving of over $C 720 000 (1983 prices) compared with the nearest alternative form of construction. A table of costs featuring the various alternatives that were considered was presented by Devata” (Table 2).
Construction
(b) SlopesJlatter
spacing
(millions
Construction costs
Reinforced concrete wall Bin wall Reinforced earth wall I l/4:1 slope (rockfill) 2: I slope (earthfili) I : I slope (relnforced with geogrids)
1995 Volume 9 Number
6
of dollars)
I.52 1.42 1.38 0.93 0.3 0.48
after Devata” Property costs Nil Nil Nil 0.3 0.9 Nil
Total costs I .52 I .42 1.38
1.23 1.2 0.48
Oriented
polymer
be significantly less than that inferred from conventional triaxial tests carried out at higher stressesz4. Traditional repairs have involved excavation, off-site disposal of the slipped soil and replacement with imported granular fill. These repair costs are particularly sensitive to haul distances. Recent use of soil reinforcement techniques involving the re-use of slipped clay and geogrids has enabled repairs to be carried out at between 25-70% of the traditional costZS,26.The method is also much less disruptive to local traffic. Some major slips which had remained unrepaired due to expense have been reinstated using this low cost technique such as the A406 London, UK, North Circular Road at Waterworks Corner”. A number of design solutions for geogrid reinforced soil slip repairs have been proposed, again including design charts derived from analysing potential failure surfaces2*. More recently attention has been focused on analysing potential failure planes which more closely mirror the characteristic shallow translational slips found at many sites2’. When analysing these shallow seated slips it is apparent that relatively small reinforcing forces are required to stabilize the slip surfaces and more importantly that these forces must act close to the face. Layers of reinforcement terminated at the face mobilizing their strength over a short distance, whilst resisting direct shearing of the soil over the reinforcement, are therefore required. Efficient interlock between geogrids with high junction strength and cohesive soil is particularly Important for such applications. The general construction method for these relatively flat slope reinforced soil repairs is described as follows. A series of bays of soil are excavated to accommodate the required reinforcement length. The excavation is benched and if seepage is encountered a granular drainage layer is often placed on the excavated profile. The width of strip excavated at any one time is usually limited to 30 m, enabling construction plant to operate without causing short-term slope instability. If the failed clay is too wet and weak to allow plant to operate, approximately 2% lime (sometimes included in a contract as a provisional item) may be mixed with the clay prior to compaction. The lime is not generally attributed with increasing the design strength of the soil, due to lack of data on the long-term low effective stress behaviour of saturated lime stabilized clay and the difficulty of mixing uniformly. Horizontal geogrid layers are placed on top of the layers of recompacted fill with adjacent reinforcement strips simply butted together (Figure 12). The reinforcement must then be covered with at least 150 mm of fill usually placed by a ‘4 in 1’ dozer bucket (Figure I.?), prior to trafficking with plant. All other operations follow conventional earthworks practice. Embankment
grid reinforcement:
G. R. Carter and J. H. Dixon
Figure 12 Repair of a slip failure ment of the failed soil
incorporating
geogrid
reinforce-
excavation of weak foundation soils and substitution with a granular fill. The geogrids can be employed in a number of ways. Basal reinforcement clay. The Chelmsford
of embankments
on slickensided
Bypass in Essex, UK, includes an 8.5 m high embankment transversing a London clay formation containing relatively weak pre-sheared surfaces within the upper 2 m3”. Traditional methods of dealing with this problem are either disposal of the clay and replacement by granular material or removal and replacement of the clay in compacted layers to remove the weak shear planes. Both of these methods can cause problems, particularly if working below the water level. The use of polymer geogrids allowed construction of the embankment at Chelmsford without the need for excavation. A series of three part wedge slip surfaces were analysed and the reinforcement provided to achieve a factor of safety in excess of 1.2 using residual parameters for the London clay. Four layers of Tensar SR2 geogrid at 250 mm vertical spacing were used beneath the crest with only the bottom layer extending to the toe of the embankment (Figure 14). Geogrid reinforced basal grunulur mattress. Williams3’ describes the use of biaxial geogrids encapsulating a free
base reinjhcement
Polymer geogrids can solve problems associated with the construction of embankments over soft ground, in many cases avoiding the need for piled viaducts, or Construction
Figure 13 Repair of a slip failure ment of the failed soil
and Building Materials
incorporating
1995 Volume
geogrid
reinforce-
9 Number 6
395
Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Howe Green embankment
Al2Chelmsford
Byp~
Four layers of Tensar SR2 grid 8.5~
1 O.Om 13.0m
’
___________-,-,,_________--_____I _______-_______
.
__I_ j - -----____ -----_--_-_-_-----
//..~/xV// _I tl.5m London clay head material to 3m depth with polished slicken sides (@‘= Figure 14
Arrangement
of geogrids
to reinforce
embankment
over pre-sheared
draining granular mattress to form the base of an embankment constructed on the Great Yarmouth Bypass in 1983 (Figure 1.5). The embankments, which were a maximum of 8 m high, were constructed on up to 22 m of soft alluvial deposits of soft and very soft organic clay and silt with some layers of peat. The geogrids were provided for three reasons: l
l
l
To provide sufficient factor of safety against slip circle failure, calculated on an effective stress basis. To minimize lateral spread of the embankment on the soft, compressible soils. To control differential settlement.
Geocell matress. The Geocell Mattress is a 1 m deep open cellular structure constructed from a biaxial grid base layer with uniaxial grids forming vertical cells (Figure 16) which are then filled with graded granular fill. The filled Geocell creates a rigid, high strength foundation for the embankment, a construction platform for earthworks plant, and a drainage layer at the base of the embankment. The Geocell provides a cost-effective alternative to removal and replacement of soft foundation soils which are underlain by firmer material. Potential failure planes are intersected and the rigidity of the Geocell forces them deeper into firm strata. The critical failure mechanism becomes that of plastic failure of the soft layer. The rough interface at the base of the Geocell ensures mobilization of the maximum shear capacity of the foundation soil and significantly increases stability. Differential settlement and lateral spread are also minimized.
Reinforced base layer provided by a ‘Tensar’ encapsulated granular mattress as used on the A47 Great Yarmouth By-pass.
Figure 15
396
Geogrids
Construction
encapsulating
and
a basal granular
Building
mattress
Materials
1995
Volume
London
17.0m 2
30.0m /
14”) at 1.5 to 2.0m depth
clay
Geocells using Tensar geogrids have, to date, been used in more than 25 locations around the world. Robertson and Gilchrist3’ describe the design and construction of a Geocell to support a highway embankment over very soft ground on the A807 road at Auchenhowie near Glasgow. Here a 4.5 m high embankment was constructed on a 4.0 m thick layer of soft silty clay with an average undrained cohesion of 15 kN/m2 underlain by stiffer material. The use of a Geocell represented a saving of 3 1% over the excavation and replacement option. Construction of the Geocell was carried out in very poor conditions during the winter of 1985/86 (Figure 17). The Geocell enabled the overlying embankment to be constructed rapidly. Performance since construction has been good. Geogrid reinjhced load transjkr platform for piled embankments. Piled foundations are used to support
embankments on very soft ground in situations where no significant post-construction settlement can be tolerated. To transfer embankment loads onto the piles individual pilecaps are used. However, this can lead to settlement of the subgrade between the pilecaps which can reflect upwards to the road surface. This problem can be solved by incorporating a continuous concrete load transfer slab across the piles. The development of geogrid reinforced load transfer platforms has provided a more cost-effective and more rapidly constructed solution to this problem. The design concept of the load transfer platform is one of enhanced arching. The natural arching of the granular layer is enhanced by incorporating layers of stiff geogrid reinforcement, which is analogous to increasing the angle of friction of the granular material. Guido et af.33*34have shown that multi-layer Tensar geogrid reinforcement can increase the angle of load spread through a soil layer. Using an optimum of three layers of biaxial geogrid, the angle of load spread in a geogrid reinforced granular layer can conservatively be taken as 45“. The thickness of the granular layer, pile cap size and pile spacing is selected so that uniform support is provided at the top of the platform (Figure 18). The first application of this approach was at the Royal Albert Dock Spine Road in London in 198835, where piled road embankments incorporating Tensar biaxial geogrids were constructed over fill and alluvial 9 Number
6
Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Figure 16 The Geocell mattress
Figure 17
Partially filled mattress
soils. In this case no significant settlement between the embankments and the adjacent piled reinforced concrete trough carrying the Docklands Light Railway could be accepted. More recently, a similar system has been used at the second Severn Crossing where the toll plaza area is being founded on a granular platform incorporating Columns Tensar geogrids over Vibro Concrete (vcc’s)36. Polymer geogrids in road pavements Geogrid reinforced grunulur layers
Since the development of polymer geogrids in the 1970s a major market has developed in the use of geogrids to reinforce the granular layers of road pavements, i.e. the capping layer and sub-base over soft subgrades. In
addition there have been successful uses of geogrids in the reinforcement of railway ballast. The type of geogrid used in this application is always a biaxial geogrid. The geogrid reinforces the granular material by the mechanism of interlock. When a granular material is compacted onto the geogrid, some of the granular material penetrates the geogrid apertures to form a positive interlock (Figure 19). The granular particles are restrained by the ribs of the grid and therefore do not oscillate under the application of transient loadings from traffic passing over the surface of the granular material. This prevention of particle movement has the dual effects of separation and reinforcement. The separation is achieved by preventing the pumping and subsequent mixing of the granular particles with the soft subgrade. This has two benefits. Firstly; expensive granular material is not lost in the underlying subgrade, and secondly, the stone layer does not become contaminated with soft material and it therefore retains its good load-carrying and drainage properties. The more important result of the interlock, however, is that of reinforcement. By restraining the bottom of the granular layer and preventing movement of the individual particles under heavy wheel loads, deformations within the granular layer are reduced. By careful design, taking into account the CBR of the formation and the volume of traffic travelling over the unsurfaced granular layer during construction, it is possibie to limit rut depths at the surface to 40 mm, and eliminate deformations at the formation level altogether. The elimination of these ruts at the bottom of the granular layers is more important as they cannot be detected and corrected during construction, and, if left, water will collect in them leading to softening and premature failure of the road. Enhanced /
soil arching angle
Geogrid
Geogrid
reinforced mattress 1
_______-
I
1 cd
4 Figure 18
Geogrid reinforced load transfer platform
Construction
and Building Materials
1995 Volume 9 Number 6
397
Oriented polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
Figure 19 The mechanism of interlock
In order for the interlock mechanism to work effectively it is necessary to use a geogrid which has three basic characteristics: Good stiffness at low strain. The strain at which the geogrid is working in road pavements is very low i.e. generally less than 2%, and it is necessary that the grid is able to give a good strength response at this strain level. High junction strength. The interlock mechanism depends on the ribs of the grid restraining the stone and preventing movement. Therefore the ribs must not only be strong in themselves, but should also have good integrity at the junctions. A good rib profile. The ribs of the grid should be rectangular and therefore present a positive profile to the stone3’. An aerofoil shaped rib with a feather edge is more likely to allow stone movement. Since a correctly designed geogrid can reduce deformations at the surface of a granular layer then, for a particular level of acceptable deformation, usually 40 mm, the granular layer thickness can be reduced38. Stone thickness savings of up to one third can be achieved. This will normally provide an economic solution on formation CBR'S of 5% or less. Even more saving can be achieved for temporary haul roads over soft ground where larger rut depths will normally be acceptable providing the surface remains serviceable. There have been numerous projects over the past 15 years where Tensar biaxial geogrids have been used to reinforce granular layers over soft ground in both temporary and permanent roads. A selection of these are described in Reference 39. In many cases a single layer of geogrid placed on the formation at the base of the granular layer is sufficient. However, if long-term differential settlement is a potential problem, for instance on ground subject to excessive consolidation, compaction or decomposition, then a 398
Construction
and Building
Materials
multi-layer system of geogrids may be required. The geogrids will restrain a layer of granular fill and spread the load, in the manner described above to produce a flexible raft which will minimize the effects of differential settlement. Although a movement free surface cannot be guaranteed, the multi-layer system of geogrids is considerably cheaper than excavation and replacement or other forms of ground treatment. For roads or parking areas where some long-term maintenance is acceptable, this system is very cost-effective. One of the first examples of this type of construction was at Bugsby’s Way in the London Borough of Greenwich in 1981 (Figure 20). Three layers of Tensar geogrid placed within 1 m of granular fill were used as an alternative to piling on waterlogged ground consisting of waste overlying soft alluvium. Twelve years after construction the performance of the road is good, as proved by surveys carried out by the London Borough of Greenwich, with negligible deterioration and no requirements for maintenance at this time. Asphult reinjbrcement Introduction. Stiff polypropylene geogrids for the reinforcement of asphalt were first introduced in 1982, the first project being at Canvey Island. Between 1981 and 1983 an extensive programme of research was carried out by Professor S. F. Brown at Nottingham University. Laboratory test work investigated the benefits of the grid with respect to the control of deformation, the control of reflective cracking and the improvement of the fatigue life of the pavement. The research identified a number of significant benefits due to the use of a stiff polypropylene geogrid within the asphalt layers. The work was carried out over approximately four years and consisted of simulative testing leaving installation techniques to be developed in the field. Details of the laboratory work have been published by Brown et a1.40,41.
1995 Volume 9 Number 6
Oriented
polymer
grid reinforcement:
G. R. Carter and J. H. Dixon
significant increase in performance was confirmed at full scale. Futigue. Similar beams to those used in the reflective
Figure
20
Installation
of
Tensar
geogrids
at
Bugsby’s
Way,
crack test were used to investigate the efforts of the geogrid with regard to fatigue resistance of the pavement. A slightly less stiff base was used but no gap was incorporated. A saw cut was made in the base of the slab to fix the position of the onset of fatigue cracking. Only vertical cyclic loading was used and strains were measured at various points over the depth of the slab. Results showed that in general the fatigue life of the pavement was increased by a factor of 10 when the geogrid was installed at the base of the layer (Figure 23).
Greenwich
Permanent deformation testing. Testing was carried out on slabs made from various asphalt mixes and loaded on a very stiff base to eliminate the effects of the lower pavement. Repeated wheel loading was applied at a constant speed and at an elevated temperature. Comparison between unreinforced and reinforced slabs showed that the reinforcement provided significant resistance to lateral flow and hence the build up of permanent deformation. The reduction in rutting was found to be as high as 70% (Figure 21). Reflective trucking. 100 mm deep asphalt beams were supported on a stiff base and subjected to vertical cyclic loading and repeated wheel loading. A 10 mm wide joint in the base simulated a joint or crack in a rigid pavement. In the reinforced sample with the grid installed directly above the joint it was shown that crack propagation could be considerably retarded, in fact on some samples cracking was eliminated entirely (Figure 22). These small-scale tests used to examine pavement deformation and reflective cracking were repeated in the pavement test facility at Nottingham University and the
20 Typical rolled asphalt
IS-
Permanent deformation (mm)
O! IO’
Figure 21
Effect of geogrid
I
IO’ Number of load applications
reinforcement
on rutting
I
IO5
performance
Construction
Full safe triuls. Many full scale trials have been carried out in different geographical locations throughout the world to confirm the results of the laboratory testing and to develop efficient methods of installing the geogrid. From the results of the laboratory and field trials it was possible to derive an analytical design method incorporating stiff polypropylene geogrids based on the strain capacity improvement of the pavement related to the number of axle passes. Developments. Although the methods developed for installing the geogrid gained rapid acceptance, methods to improve the technique were constantly being investigated. Work began in 1990 to examine the benefits of bonding a geotextile to the geogrid in order to provide a material which could be installed without the need to fix, tension and dress prior to paving by machine. Geotextile fabrics are commonly used on their own to help improve the bond between layers and also to waterproof any subsequent cracking that occurs during use. They have, however, no intrinsic reinforcing properties and cannot prevent reflective cracking. To produce the Tensar AR-G composite a stiff polypropylene geogrid is thermally bonded to a thin, non-woven, needle punched polypropylene/polyester geotextile. The installation requires the regulated road surface to be sprayed with either a rich bitumen emulsion or straight 200 pen bitumen at a net rate of about 1.0 l/m’. The composite material is then rolled out either by hand or by machine under light tension and brushed to ensure intimate contact with the pavement. When the bitumen has fully cured and the fabric is fully stuck down then mechanical paving can commence. Several trial installations have been carried out in the UK, Europe and USA using bitumen emulsions, polymer modified emulsions and straight bitumen. In these full scale trial pavement installations, cores have been taken and subjected to shear testing which has confirmed that there is no significant loss in bond when compared with the geogrid alone. The geogrid/geotextile composite is resistant to normal asphalt paving temperatures. and Building
Materials
1995 Volume
9 Number 6
399
Oriented polymer grid reinforcement:
Figure 22
Effect of geogrid
reinforcement
G. R. Carter and J. H. Dixon
on reflective
cracking
400 Dense bitumen Macodclm
~~~~
loo-
1.0
Figure 23
Effect of geogrid
reinforcement
on fatigue
Oriented polymer geogrids were first developed in the late 1970s to fulfil a perceived need within the civil engineering industry. Since then their current usage within civil engineering has grown to between 15 and 20 million square metres, world-wide, each year. Major application areas are reinforced soil walls and slopes, the base of embankments over soft ground, the reinforcement of road sub-bases over soft ground and the reinforcement of bituminous overlays in highways maintenance. Tensar geogrids have been accepted by the Department of Transport in the UK, as being available for use in vertical retaining structures with a 120 year design life. Similar certification has been received in both Germany and Hong Kong. Pressure on land in countries such as the UK and Japan means that development often takes place on congested sites with poor ground conditions. Oriented polymer geogrids can solve many of the problems raised by Construction
life (mso)
performance
Conclusion
400
100
10 Design
and Building Materials
such sites. For this reason, and because the use of geogrids is spreading to more countries, their use in civil engineering applications is likely to continue to increase.
References Iwasaki and Watanabe. Reinforcement of railway embankments in Japan. Symposium on Earth Reinforcement, ASCE, 1978 Jones, C. J. F. P. Eurth Reinforcement and Soil Structures. Butterworths, London, 1985 Bagir, T. Iraq J. 1994, pp 5-6 Schlosser, F. Experience in reinforced earth in France. Supplementary Report 457, Transport and Road Research Laboratory, Paper 9, 1977 Blight, G. E. and Dane M. S Deterioration of a wall complex constructed of reinforced soil. Geotechnique 39, (I), Thomas Telford, 1989, pp. 47-53 Chang, J. C., Hannon, J. D. and Forsyth, R. A. Pull-out resistance and interaction of earthwork reinforcement and soil. Transportation Research Record 640, Washington DC, 1977 Forsyth, R. A. Alternative earth reinforcement. Symposium on Earth Reinforcement ASCE, 1978 Peterson, L. M. Pull-out resistance of welded wire mesh embedded in soil. MSc, Utah State University, 1980 Jewel], R. A. Some effects of reinforcement on the mechanical behaviour of soils. PhD Thesis, University of Cambridge, 1980
1995 Volume 9 Number
6
Oriented IO
11
12
13
14 15
polymer
Wrigley, N. E. Durability and long-term performance of Tensar polymer grids for soil reinforcement. Muter. Sci. Technol. March 1987, 3,. 161-72 Reinforced and anchored earth retaining walls and bridge abutments for embankments. Department of Transport, UK. Technical Memorandum (Bridges) BE 3/78 (revised 1987) Murray, R. T. and McGown, A. Assessment of the time dependent behaviour of geotextiles for reinforced soil applications. Durability of Geotextiles, RILEM, Chapman and Hall, 1988, pp. 52--73 Andrawes, K. Z., McGown, A. and Murray, R. T. The loadstrain-time temperature behaviour of geotextiles and geogrids. Proc. Third Int. Conf on Geotextiles, Vol III, Session 7A, Vienna, 1986, pp. 707-712 Specification for Highways Works, Department of Transport, 1986 Bush, D. I. Evaluation of the effects of construction on the physical properties of polymeric soil reinforcing elements. In Proc.
27
28
29
30
31
In!. -Gebtechnical -Symp. on Theory and Praciice of Earth Reinforcement. Fukuoka. October. Balkema. Rotterdam, 1988
16 Jewel], R. A.,‘Paine, N.‘and Woods, R. I.‘Design methods for steep reinforced embankments. In Proc. Symp. on Polymer Grid Reinforcement, Thomas Telford, London, 1984, pp. 70-81 17 Guidelines for the Design and Construction of Embankments over Stable Foundations using Tensar Geogrids, Netlon Limited, 1988 18 Jewel], R. A. Revised design charts for steep reinforced slopes.
32
Proc. Symp. Reinforced Embankments: Theory and Practice in the British Isles, Cambridge, September Thomas Telford, London,
34
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20 21
22
23
24
25 26
1989 Devata, M. S. Geogrid reinforce earth embankments with steep side slopes. Proc. Symp. Polymer Grid Reinforcement, Thomas Telford, London, 1984, pp. 82-87 Studies of Slope Stability in Highway Engineering. TRRL leaflet LF934, Transport and Road Research Laboratory, 1983 Parsons, A. W. and Perry, N. Slope stability problems in ageing highway earthworks. In Proc. Symp. on Failures in Earthworks, Thomes Telford, London, pp. 63-78 Chandler, R. J. and Skempton, A. W. The design of permanent cutting slopes in stiff fissured clays, Geotechnique, 1974, 24, (4), pp. 457466 Anderson, M. G. and Kneale, P. E. Pore water pressure changes in a road embankment. J. Inst. Highw. Eng. 1980, May, pp. I l-17 Atkinson, J. H. and Farrar, D. M. Stress path tests to measure soil strength parameters for shallow landslips. I lth ICSMFE, San Francisco, 1985, pp. 983-986 Maintenance and repair of highway embankments: Studies of seven methods of treatment. Research Report 30, Transport and Road Research Laboratory, 1985 Oliver, T. L. H. Reinforced soil techniques for the reinstatement
Construction
33
35
36
37
38 39 40
41
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G. R. Carter and J. H. Dixon
of failed soils using geogrids. In Proc. Symp. Failures in Earthworks, Tech Note 7, Thomas Telford, pp. 417419 Dixon, J. H. Geogrid reinforced soild repair of a slope failure in clay - North Circular Road, London, UK. Geosynthetic Case Histories, 35 years of Experience (eds G. P. Raymond and J. P. Giroud) ISSMFE, Chapter 10, 1993 Murray, R. T. Reinforcement techniques in repairing slope failures. In Proc. Symp. on Polymer Grid Reinforcement, Thomas Telford, London, 1984, pp. 47-53 Greenwood, J. R., Holt, D. A. and Herrick, G. W. Shallow slips in highways-embankments constructed of overconsolidated clay. In Proc. Symp. on Failure in Earthworks, Thomas Telford, London, 1985, pp. 79-92 Dixon, J. H. Geogrid basal reinforcement for embankment on slickensides clay - Chelmsford Bypass, United Kingdom. Geosynthethics Case Histories, 35 years of Experience. (eds G. P. Raymond and J. P. Giroud) ISSMFE, Chapter IO, 1993 Williams, D. Reinforced embankments at the Great Yarmouth Bypass. In Symposium on Polymer Grid Reinforcement in Civil Engineering, ICE, London, 1984, pp. 88-94 Robertson, J. and Gilchrist, A. J. T. Design and construction of a reinforced embankment across soft lakebed deposits. Ing Conf on Foundations and Tunnels, 1987 Guido, V. A., Knueppel, J. D. and Sweeny, M. A. Plate loading tests on geogrid reinforced earth slabs. Geosynthetic 87 Conference, New Orleans, 1987, pp. 216225 Guido, V. A. Bearing capacity of shallow foundations reinforced with Tensar geogrids. Tensar Technical Note TN:BRN9, Tensar Corp, 1986 Card, G. B. and Carter, G. R. A case history of a piled embankment in London’s Docklands. The Engineering Geology of Construction, 1992 Bell, A. L., Jenner, C., Maddison, J. D. and Vigroles, J. Embankment support using geogrids with vibro concrete columns. Fifth International Conference on Geotextiles. Geomembranes and Related Products, Singapore, 1994 Webster, S. L. Geogrid reinforced base courses for flexible pavements for light aircraft - test section construction, behaviour under traffic laboratory tests and design criteria. Final Report DOT/FAA/RD - 92125, 1992 Ground stabilisation - design curves showing reduced sub-base thickness using Tensar geogrids. Netlon Limited, 1993 Tensar geogrid reinforced sub-bases - Case Studies. Netlon Limited, 1993 Brown, et al. Polymer grid reinforcement of asphalt. Annual meeting of the Association of Asphalt Paving Technologists, San Antonio, Texas, 1985 Brown, et al. The use of polymer grids for improved asphalt performance. Eurobitumen Conference, The Hague, Netherlands, 1985
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