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Soil reinforcement
Soil reinforcement 7.1
7
Introduction
With the inclusion of reinforcing elements (in the form of natural or synthetic fibres, metal strips, soil nails and anchors, and micropiles) into the soil mass, its various engineering properties are improved. The basic principle behind ground improvement using geosynthetics is that the reinforcing elements absorb the tensile loads or shear stress within the structure and thus preventing its failure due to shear or excessive deformation. The improvement in the engineering properties happens due to the friction developed at the soil–reinforcement interface and the passive resistance that occurs through the development of bearing-type stresses on transverse reinforcement surfaces in a direction normal to relative movement of soil reinforcement, as illustrated in Fig. 7.1. The contribution of reinforcing element towards ground improvement will depend upon the roughness of the surface (skin friction); normal effective stress, grid-opening dimensions; thickness of the transverse members; elongation characteristics of the reinforcement; and the level of soil–reinforcement interaction based on soil characteristics including grain morphology, grain size distribution, density, water content, cohesion, and stiffness. The reinforcing element can be either inserted into the in situ soil, or placed in the soil mass as it is constructed. The other functions of geosynthetic materials include separation, filtration, drainage, and sealing. The concept of soil nailing in the form of metal bars, tubes, or rods is to place reinforcing elements in situ at suitable distances to increase the shear strength of the soil and to restrain its displacements during and after excavation. Similarly, in case of soil anchoring, prestressed soil anchors are installed in the ground to reinforce the soil and support vertical or inclined excavations. In micropiling, small-diameter (about 10–30 cm) piles are installed vertically, or in a reticulated manner to support excavations, slopes, and foundations, or for the purpose of underpinning or retrofitting of structures. In all the reinforcement techniques mentioned above, the stress transfer between the soil and the reinforcing elements, failure surface of the reinforcing elements, strain compatibility between the soil and the reinforcement, placement method of the reinforcing element, and the durability and long-term behaviour of the reinforcing elements are the important factors that decide the effectiveness of these methods for various structures. The following sections present a brief explanation of these various reinforcing methods.
7.2
Geosynthetic materials
As per the definition of American Society for Testing and Materials (ASTM), a geosynthetic material is a planar product manufactured from a polymeric material that is Geotechnical Investigations and Improvement of Ground Conditions. https://doi.org/10.1016/B978-0-12-817048-9.00007-X © 2019 Elsevier Inc. All rights reserved.
62
Geotechnical Investigations and Improvement of Ground Conditions Normal pressure
Pull out force
(A)
Frictional force
Normal pressure
Frictional resistance Frictional resistance Passive resistance
Pullout force Passive resistance
Normal pressure Pullout force
(B) Fig. 7.1 Stress transfer mechanism for reinforcement: (A) frictional stress transfer between soil and reinforcement surfaces and (B) soil passive (bearing) resistance on reinforcement surfaces. (From Elias, V., Christopher, B.R., 2001. Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design & Construction Guidelines. Publication No. FHWA-NHI-00-043, Washington, DC.)
used with soil, rock, earth, or other geotechnical related material as an integral part of a civil engineering project, structure, or system. A geosynthetic material can be categorized as geotextile, geogrid, geomembrane, or geocomposite (e.g. geotextile-geonets, geotextile-geogrids, geotextile-geomembranes, geomembrane-geonets, geotextile-polymeric cores, or a three-dimensional polymeric cell structures). Geotextiles are permeable synthetic materials made of textile materials such as polypropylene, polyethylene, or polyester. Further, based upon their preparation, a geotextile may be woven, nonwoven, or knitted, as shown in Fig. 7.2. Aside from these traditional forms of woven and nonwoven geotextiles, there are many forms of geotextiles that have come onto market over the last few years, as depicted in Fig. 7.3. Geotextiles are widely used in road construction and railway works (for separation, filtration, drainage, and soil reinforcement); for erosion control in river canals and coastal works; as a filtering media for drainage in earthen dam, behind retaining
Soil reinforcement
Woven geotextile
63
Nonwoven geotextile
Knitted geotextile
Fig. 7.2 Woven, nonwoven, and knitted geotextiles.
walls, and in deep drainage trenches; in sport field construction like Caselon playing fields and Astro turf; and for mud control in agriculture. Geogrids are primarily used for reinforcement and they are heat-welded from strips of material (such as polyester, polyethylene, or polypropylene) or produced by punching a regular pattern of holes in sheets of material that are then stretched into a grid. As fill is placed on the geogrids, the mesh design locks soil in place providing differing levels of stability depending on the type of geogrid used. The geogrid may be uniaxial or biaxial, as shown in Fig. 7.4, depending upon their tensile strength in different directions. A uniaxial geogrid has high tensile strength in one direction and is useful for the reinforcement of retaining walls, steep slopes, and road embankments, and for repairing landslides. On the other hand, a biaxial geogrid has equal tensile strength in both directions and is useful for stabilizing roadways. It distributes loads over a larger area reducing pumping and shear failures while maximizing the load-bearing capacity of subgrades. Geomembranes are very low-permeable geosynthetic materials made from relatively thin continuous polymeric sheets (chlorosulphonated polythene (CSPE), high-density polythene (HDPE), very-low-density polythene (VLDPE), polyvinyl chloride (PVC), etc.), but they can also be made from the impregnation of geotextiles with asphalt, elastomer, or polymer. The geomembranes create an impermeable barrier that keeps contaminants and other dangerous chemicals contained so they cannot escape and damage the surrounding environment. Geomembranes are generally used as liner for sewage sludge, for the safe shutdown of nuclear facilities, for water and various waste conveyance canals, as secondary containment of underground storage tanks, as covers for solid waste landfills, as waterproof facing in various structures, to prevent infiltration of water and contaminants into sensitive areas, etc. Geocomposites are made by combining different geosynthetics materials or by combining geosynthetic materials with nonsynthetic materials, such as bentonite clay, to address specific applications in the field in the optimal manner with minimum cost. Geocomposite materials include geotextile-geonets, geotextile-geogrids, geotextile-geomembranes, geomembrane-geonets, geosynthetics clay liner (GCL), geotextile-polymeric cores, or three-dimensional polymeric cell structures. Various forms of geocomposite materials are presented in Fig. 7.5.
64
High modulus woven
Multilayer woven
Triaxial woven
Tubular braid
Tubular braid laid In warp
Flat braid
Flat braid laid in warp
Weft knit
Weft knit laid in weft
Weft knit laid in warp
Weft knit laid in weft laid in warp
Square braid
Square braid laid in warp
3- D Braid
3-D Braid laid in warp
Warp knit
Warp knit laid in warp
Weft inserted warp knit
Weft inserted warp knit warp in warp
Fiber mat
Stitchbonded laid in warp
Biaxial bonded
Xyz laid in system
Fig. 7.3 Different forms of geotextile materials (Frank, 2004).
Geotechnical Investigations and Improvement of Ground Conditions
Biaxial woven
Soil reinforcement
65
Fig. 7.4 Uniaxial and biaxial geogrids.
Fig. 7.5 Various forms of geocomposite materials.
So from this abovementioned information it can be concluded that the application of geosynthetic materials covers almost all types of civil engineering structures and its main functions are separation, filtration, drainage, reinforcement, protection, and waterproofing. In road construction, the application of geosynthetic materials is illustrated in Fig. 7.6.
66
Geotechnical Investigations and Improvement of Ground Conditions
Geosynthetics material
Cracks Road surface Crushed aggregate layer Native material Without geotextile
With geotextile
Geotextile used for separation
Crack sealing in presence of geotextile Geotextile
ACC Base cource
Filtration
Geotextile Transmissivity
Aggregate drainage layer Subgrade
Frictional restraint
Membrane support
Subgrade restraint
Subbase restraint Local reinforcement
Geotextile for filtration
Geotextile to provide support and strength against wheel load
Fig. 7.6 Application of geotextiles in pavements (Koerner, 1998).
When reinforcement is required to be inserted into the soil mass, such as for retaining walls and embankments, it is very important to add the backfill material (for construction of manmade structures) or insert the reinforcing element into the soil mass (for stability and/or protection of natural backfill) in a systematic way by following the proper steps. Moreover, it is necessary to include a facing wall (either soft, such as growing vegetation, or hard, such as wraparound type, gabion, precast concrete panel, or modular concrete block) for the stability and longevity of the structure. Hence, the backfill materials and facing wall along with the reinforcing elements now become an integral part of the reinforced wall, as shown in Fig. 7.7, where a precast concrete panel has been used as the facing wall. The use of geosynthetic materials in various civil-engineering structures and for ground treatment has tremendously increased in the last few decades. The advantages of using these materials are cost efficiency, convenience for transport and installation, lower repair and maintenance costs, predictability of the design, quick installation, applicability to a wide range of soils, space savings, improved performance and extended life, good-quality control due to homogeneity in nature, less environmentally sensitive, increased safety factor, and compatibility with field conditions. However, the geosynthetic materials need to meet certain requirements and to be checked and tested before using them in the field. The required properties are summarized in Table. 7.1. The new generation of geosynthetic materials includes the nanofibre membrane with superhydrophobicity and superoleophilicity for selective separation of water and low viscous oil, nanocomposite electrospun nanofiber membranes (Nanocomposite ENM) for environmental remediation and water filtration, etc. The basic principles
Soil reinforcement
67
Fig. 7.7 A typical picture of reinforced retaining wall (Holtz, 2001). Table 7.1 Requirement of geosynthetics material Property
Parameters
General properties
Material type and construction, polymer(s), mass, thickness, roll dimensions, specific gravity, absorption Strip tensile strength, grab strength, creep resistance, flexural strength, cutting – trapezoidal tear strength, shear modulus, Poisson’s ratio, burst strength, puncture resistance, penetration, flexibility (flexural strength) Abrasion resistance, UV stability, biological resistance, chemical resistance, wet/dry stability, temperature stability, long-term durability Stress–strain, creep, friction/adhesion, dynamic and cycling loading, soil retention, filtration Apparent opening size, percent open area, porosity, permeability/permittivity, soil retention ability, clogging resistance, in-plane flow capacity
Index properties
Endurance properties
Performance – soil/ fabric properties Hydraulic properties
behind the concept of nanofibre is that by reducing fibre diameter down to the nanoscale, the specific surface area increases enormously to the order of about 1000 m2/g and this reduction in dimension and increase in surface area greatly affects the chemical/biological reactivity and electroactivity of polymeric fibres.
68
7.3
Geotechnical Investigations and Improvement of Ground Conditions
Soil nailing
A soil nail can be defined as a structural member that prevents the collapse of ground material and retains it by virtue of its self-weight, bending strength, and stiffness. The internal stability of a soil-nailed system is usually assessed using a two-zone model: the active and passive zone, separated by a potential failure surface, as shown in Fig. 7.8. The soil-nailed system ties the active zone with the passive zone with its tensile, shear, and bending force. Hence, it is very useful to use soil nails as a remedial measure to treat unstable natural soil slopes or as a construction technique that allows the safe oversteepening of new or existing soil slopes. A soil-nailed system is formed by inserting relatively slender reinforcing bars into the slope. Depending upon the project cost, site accessibility, availability of working space, and the soil and groundwater conditions, soil nails can be inserted into the ground by the following methods: (1) drill-and-grout, (2) self-drilling, or (3) driving. In the drill-and-grout method soil nails are inserted into a predrilled hole that is then cement grouted under gravity or low pressure. Using this method long soil nails can be inserted, and it is possible to check the ground obstructions (if any) and also to check the alignment of the drilling holes. However, this method may require the hole to be cased in the event of collapsible ground. In the method of self-drilling, which is quite rapid because of the use of simultaneous drilling and grouting, the soil nails are directly drilled into the ground using a sacrificial drill bit. For drilling, cement grout instead of air or water is used for flushing, which maintains the hole stability and thus eliminates the requirement for any casing.
Soil-nail head
Bearing stress Passive zone Facing
Active zone Soil nail
Bending and shear Tension Shear stress
Tension
Potential failure surface
Fig. 7.8 Two-zone model of a soil-nail system. (From Geotechnical Engineering Office, 2008. Civil Engineering and Development Department, The Government of the Hong Kong Special Administrative Region, Geoguide 7: Guide to Soil Nail Design and Construction, Hong Kong.)
Soil reinforcement
69
However, the disadvantages of this method are that (1) it cannot penetrate through rocks or ground containing corestones and hence is not suitable under such ground conditions, (2) it is difficult to check and maintain the alignment of the soil nails, and (3) there are durability issues due to a lack of specified minimum grout cover and coating. In the third method of installation, i.e. ‘driven’, soil nails are directly driven into the ground using a compressed air launcher using the percussive method and hammering equipment or using the vibratory method and vibrating equipment. Compared to the other two methods, this method is more rapid and causes minimal ground disruption. However, there are basic limitations and disadvantages to this method, which are that (1) only relatively short-length nails can be installed, (2) there is more chance of damage to soil nails due to excessive buckling stress induced during the installation process, especially in case of stiff soil, and (3) the nails are highly susceptible to corrosion if noncorrodible reinforcement is not used, since the nails are in direct contact with the surrounding ground. The basic components of a soil-nailed system are shown in Fig. 7.9. It consists of the following components: (i) Reinforcing elements, typically a solid high yield deformed steel bar or, in some cases, a fibre-reinforced polymer can also be used. (ii) Reinforcement connector or coupler, which is used to connect sections of soil nailreinforcing bars. (iii) Grouting, which is done in a predrilled hole after the insertion of the soil nails, transfers stresses between the ground and the soil nails, and also acts as part of the corrosion protection measures. (iv) Corrosion protection measures, using epoxy-coated bars, provides a protection cover in the form of hot-dip galvanizing and corrugated plastic sheathing, heat-shrinkable sleeves made of polyethylene, or anticorrosion mastic sealant material.
Stud welded to plate Geocomposite drainage strip
Threadbar (Epoxy coated)PVC Centralizer
Grout Reinforcing steel
Drill length
Temporary shotcretewall Permanent shortcrete or cip wall Bar length
Soil nail length
Fig. 7.9 Schematic diagram of a soil-nailed cut slope (Tuozzolo, 2003).
70
Geotechnical Investigations and Improvement of Ground Conditions
(v) Centralizers, which are placed around the soil nail to maintain an even thickness of grout around the bar. (vi) Soil nail head, which comprises a reinforced concrete pad, steel bearing plate, and nuts, and its primary function is to provide a reaction for individual soil nails to mobilize tensile force and to make the ground stable near the slope surface. (vii) Geocomposite drainage strip, for protection against the adverse effects of surface runoff or perched water in the soil-nails interface, which may cause problems during construction or for the shotcrete, which may have a problem adhering to the face if surface water is present. (viii) Wall facing in the form of a permanent shotcrete wall or isolated soil nails head plates. Alternatively, rabbit-proof wire mesh and environmental erosion control fabrics may also be used in conjunction with flexible mesh facing, as demanded by environmental conditions.
A soil-nail system is basically designed for its stability, durability, and serviceability, keeping in mind the economic and environmental considerations. The soil-nail system may become unstable for one or more of the following reasons: 1. 2. 3. 4. 5. 6. 7. 8.
Failure of ground around soil nails Soil-nail head bearing failure Local failure between soil nails Tensile failure of soil nails Pullout failure at grout–ground and/or grout–reinforcement interface Bending or shear failure of soil nails Structural and connection failure of soil-nail head Structural and connection failure of facing.
The allowable tensile capacity (TT), pullout resistance provided by the soil-grout bond length in the passive zone (TSG) and the allowable pullout resistance provided by the grout–reinforcement bond length in the passive zone (TGR) are critical parameters and are to be determined properly during the design stage as follows: TT ¼ fy A0 =FT
(7.1)
TSG ¼ ðc0 Pc L + 2 D σ v 0 μ LÞ=FSG
(7.2)
TGR ¼ β √fcu Pr L =FSG
(7.3)
where fy is the characteristic yield strength, A0 is the effective cross-sectional area, and FT is the factor of safety against tensile failure of soil nail reinforcement. c0 is the effective cohesion of the soil, Pc is the outer perimeter of the cement grout sleeve, L is the bond length of the soil nail reinforcement in the passive zone, D is the outer diameter of the grout sleeve, σ v0 is the vertical effective stress in the soil calculated at mid-depth of the soil-nail reinforcement in the passive zone with a minimum value of 300 kPa, μ is the coefficient of apparent friction of soil (equal to tan φ0 where φ0 is the angle of shearing resistance of soils under effective stress), FSG is the factor of safety against pullout failure at the soil–grout interface, β is the coefficient of friction at the
Soil reinforcement
71
grout interface, fcu is the characteristic strength of the cement grout, Pr is the effective perimeter of the soil-nail reinforcement. In addition to stability, it is also necessary to consider the serviceability of the soilnail systems. At any period of time during its service life, it should not affect nearby structures or facilities. It should not cause any ground deformation beyond the acceptable limit, there should not be any damage to the slope facing and drainage system, and the soil-nail system should be in good condition appearance-wise. Moreover, the spacing between soil-nails (generally 1.5–2.0 m) and the inclination (generally 5° to 20°) should be determined properly. Wide-spacing may cause local instability whereas close spacing may not be economical to install. Having discussed the installation method and understood the working principles, now it is easy to identify the advantages and disadvantages of the soil-nail systems, which are summarized in Table 7.2.
7.4
Soil anchoring
Soil or ground anchors also known as ‘tiebacks’ are prestressed tendons or cables installed inside a borehole that apply a restraining force to rock faces, cuttings and slopes, or tunnels via a plate in order to stabilize it. The end of the anchor is either grouted or locked mechanically deep in sound strata. The basic difference between Table 7.2 Advantages and disadvantages/limitations of soil-nail system Advantages Allows in situ strengthening with minimum backfilling and excavation and so it is a timesaving, cost-effective and environmentally friendly method Requires light machinery and equipments and so can be used where accessibility or space is a major problem It can cope with site constraints and variations in ground conditions encountered during construction by adjusting the location, length, and inclination of the soil nails The failure mode of a soil-nailed system is likely to be ductile and thus it provides a warning sign before failure l
l
l
l
Disadvantages/limitations May not be suitable under certain ground conditions such as with cobbles, boulders, highly fractured rocks, open joints, voids, or obstructions to drilling or driving. Moreover, problems may arise from the collapse of a borehole and loss of grout through fractured rock mass, open joints, and cavities. In addition, for coarse-grained soil and soft clays that have less self-support time and where the soil is prone to creeping this method is not applicable l
l
l
Loss in serviceability and instability of slope and facing due to various factors related to soil, reinforcement, grout, soil-nail, grout-nail interaction The presence of utilities, underground structures, or other buried obstructions poses restrictions to the length and layout of soil nails. Moreover, future development may be restricted in that area sterilized with the presence of soil nails
72
Geotechnical Investigations and Improvement of Ground Conditions
a soil nail and a soil anchor is that a soil nail is a passive bearing element, which relies on soil movement to mobilize the shear strength along the nail, whereas an anchor is prestressed to mobilize shear strength. The basic components of a ground anchor are presented in Fig.7.10. They mainly consist of l
l
l
l
l
the anchor head, which transmits the anchor force to the structure via the bearing plate the free stressing unbonded length from the head to the near end of the anchorage the grouted anchor bond length, by which the tensile force is transmitted to the surrounding ground through the anchor grout. This anchor bond length should be located behind the critical failure surface the sheath, which is a portion of the tieback and encases the tendon in the stressing unbonded length only. The sheath, also known as the ‘bond-breaker’, enables the tendon bar in the unbonded length to elongate without obstruction during testing and stressing and leaves the tendon unbonded after locked-off to the specified load the trumpet, which transmits the prestressing force from the tendon to the supported structure and is sealed to the bearing plate and overlaps the unbonded length corrosion protection by at least 10 cm. The trumpet is generally long enough to accommodate movements of the structure and the tendon during testing and stressing without damaging the encapsulation.
Un
bo
Trumpet
nd
ed
len
gth
Anchor head Sheath
Bearing plate
An
ch
Te
nd
on
Wall
or
bo
nd
bo
nd
len
gth
len
gth
Unbonded tendon Anchor grout Bonded tendon
An dia cho me r ter
Fig. 7.10 Basic components of a ground anchor. (From Sabatini, P.J., Pass, D.G., Bachus, R.C., 1999. Geotechnical Engineering Circular No. 4 Ground Anchors and Anchored Systems, Report No. FHWA-IF-99-015.)
Soil reinforcement
73
Fig. 7.11 (A) Bar tendons and (B) strand tendons used for ground anchoring.
The drill hole diameter is generally less than 15 cm, except for hollow stem augured anchors, in which case it is about 30 cm in diameter. The total anchor length generally varies from 9 m to 18 m, with a minimum unbonded length of 3 m for bar tendons and 4.5 m for strand tendons (shown in Fig. 7.11). The inclination of anchor varies from 10° to 45°.
7.5
Micropiling
Micropiles are smaller in diameter (about 90–300 mm) compared with piles and are installed closer together than the conventional pile foundations. The installation method for micropiles is depicted in Fig. 7.12. The drilling of boreholes with casings is done using a suitable drilling technique for the soil/site conditions. Afterwards the drilling rod and drilling bits are removed and reinforcing bars (usually corrosion protected steel bars) are inserted into the boreholes. Grouting is then done in sequence under pressure and the casing is removed gradually. Micropiles are generally executed by use small-sized equipment and hence the method can be carried out in small areas and with limited access. The field for the use of micropiles application is very wide; it can be executed in areas with limited
Geotechnical Investigations and Improvement of Ground Conditions
Steps of Installation of a micropile
74
(1)
(2)
(3)
(4)
(5)
(1) Beginning of drilling and installation of casing (2) Completion of depth of drilling (3) Removal of drill bit and rod (4) Introduction of the reinforcement (5) Grouting (6) Removal of casing and further grouting under pressure
(6)
Fig. 7.12 Installation method of micropiles.
vertical clearance like basements and under bridge structures. Some other examples of critical areas where the micropiles can be installed are inside industrial buildings, small tunnels, mountain trails, rice fields, mountainous forested areas, steep slopes, etc. In addition, micropiles can be installed through preexisting foundations and for the underpinning of structures and repair of defective foundations. According to the design applications micropiles could be classified into two groups. The first group includes micropiles being loaded either axially or laterally. This group of micropiles transfers the structural loads to the competent strata below the foundation (e.g. the underpinning of structures) or they may be used to resist the movement of failure planes (i.e. stabilization of slopes). The second group includes micropiles being used to strengthen the soil mass by forming a reinforced soil composite. On the other hand, micropiles also could be classified into four groups (i.e. Groups A, B, C, and D) based upon the methods of grouting, as shown in Fig. 7.13. For the group A micropiles grouting is placed under gravity. In group B grout is injected into the hole under pressure, but the pressure is limited to avoid hydrofracturing of the surrounding grounds. The installation process for group C micropiles involves two steps: first a primary grout is applied under pressure to cause hydrofracturing of the surrounding ground, and then a secondary grout is injected via a manchette tube before the primary grout hardens. Group ‘D’ micropiles are similar to the group ‘C’, except that the secondary grout is injected after hardening of the primary grout.
Soil reinforcement
75
Fig. 7.13 Micropiles classification based on grouting method.
Type A
Type B
Type C
Type D
References Frank, K.K., 2004. From Textile to Geotextiles, Seminar in honour of professor Robert Koerner, September 13, 2004. Department of Materials Science and Engineering, Drexel University, Philadelphia University, Philadelphia, PA, pp. 20. Holtz, R.D., 2001. Geosynthetics for soil reinforcement. In: The Ninth Spencer J. Buchanan Lecture, November 9, 2001. Koerner, R.M., 1998. Designing with Geosynthetics, fourth ed. Prentice-Hall, Secaucus, NJ, pp. 761. Tuozzolo, T.J., 2003. Soil nailing: where, when and why-a practical guide. In: Presented at the 20th Central Pennsylvania Geotechnical Conference, Hershey, PA.
Further reading Bonaparte, R., Holtz, R.D., Giroud, J.P., 1987. Soil reinforcement design using geotextiles and geogrids. In: Fluet, J.E. (Ed.), Geotextile Testing and the Design Engineer. American Society for Testing and Materials, pp. 69–116 Philadelphia: ASTM, Special Technical Publication. Koerner, R.M., Hsuan, Y.G., 2001. In: Rowe, R.K. (Ed.), Geosynthetics: Characteristics and Testing. Chapter 7 in Geotechnical and Geoenvironmental Handbook. Kluwer Academic Publishers, New York, pp. 173–196. Liew, S.S., Fong, C.C., 2003. Design and construction of micropiles. In: Geotechnical Course for Pile Foundation Design & Construction. ipoh (29–30 September 2003). Meccai, K.A., Hasan, E.A., 2004. Geotextiles in transportation applications. In: 2nd Golf Conference on Roads, Abu Dhabi, March. Sabatini, P.J., Pass, D.G., Bachus, R.C., 1999. Geotechnical Engineering Circular No. 4 Ground Anchors and Anchored Systems. Report No. FHWA-IF-99-015.
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Geotechnical Investigations and Improvement of Ground Conditions
Shong Ir, L.S., Chung, F.C., 2003. Design and construction of micropiles. In: Geotechnical Course for Pile Foundation Design and Construction.pp. 29–30. Ipoh. September. Tuozzolo, T.J., 2003. Soil nailing: where, when and why – a practical guide. In: Presented at the 20th Central Pennsylvania Geotechnical Conference, Hershey, PA.
7
Introduction
With the inclusion of reinforcing elements (in the form of natural or synthetic fibres, metal strips, soil nails and anchors, and micropiles) into the soil mass, its various engineering properties are improved. The basic principle behind ground improvement using geosynthetics is that the reinforcing elements absorb the tensile loads or shear stress within the structure and thus preventing its failure due to shear or excessive deformation. The improvement in the engineering properties happens due to the friction developed at the soil–reinforcement interface and the passive resistance that occurs through the development of bearing-type stresses on transverse reinforcement surfaces in a direction normal to relative movement of soil reinforcement, as illustrated in Fig. 7.1. The contribution of reinforcing element towards ground improvement will depend upon the roughness of the surface (skin friction); normal effective stress, grid-opening dimensions; thickness of the transverse members; elongation characteristics of the reinforcement; and the level of soil–reinforcement interaction based on soil characteristics including grain morphology, grain size distribution, density, water content, cohesion, and stiffness. The reinforcing element can be either inserted into the in situ soil, or placed in the soil mass as it is constructed. The other functions of geosynthetic materials include separation, filtration, drainage, and sealing. The concept of soil nailing in the form of metal bars, tubes, or rods is to place reinforcing elements in situ at suitable distances to increase the shear strength of the soil and to restrain its displacements during and after excavation. Similarly, in case of soil anchoring, prestressed soil anchors are installed in the ground to reinforce the soil and support vertical or inclined excavations. In micropiling, small-diameter (about 10–30 cm) piles are installed vertically, or in a reticulated manner to support excavations, slopes, and foundations, or for the purpose of underpinning or retrofitting of structures. In all the reinforcement techniques mentioned above, the stress transfer between the soil and the reinforcing elements, failure surface of the reinforcing elements, strain compatibility between the soil and the reinforcement, placement method of the reinforcing element, and the durability and long-term behaviour of the reinforcing elements are the important factors that decide the effectiveness of these methods for various structures. The following sections present a brief explanation of these various reinforcing methods.
7.2
Geosynthetic materials
As per the definition of American Society for Testing and Materials (ASTM), a geosynthetic material is a planar product manufactured from a polymeric material that is Geotechnical Investigations and Improvement of Ground Conditions. https://doi.org/10.1016/B978-0-12-817048-9.00007-X © 2019 Elsevier Inc. All rights reserved.
62
Geotechnical Investigations and Improvement of Ground Conditions Normal pressure
Pull out force
(A)
Frictional force
Normal pressure
Frictional resistance Frictional resistance Passive resistance
Pullout force Passive resistance
Normal pressure Pullout force
(B) Fig. 7.1 Stress transfer mechanism for reinforcement: (A) frictional stress transfer between soil and reinforcement surfaces and (B) soil passive (bearing) resistance on reinforcement surfaces. (From Elias, V., Christopher, B.R., 2001. Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design & Construction Guidelines. Publication No. FHWA-NHI-00-043, Washington, DC.)
used with soil, rock, earth, or other geotechnical related material as an integral part of a civil engineering project, structure, or system. A geosynthetic material can be categorized as geotextile, geogrid, geomembrane, or geocomposite (e.g. geotextile-geonets, geotextile-geogrids, geotextile-geomembranes, geomembrane-geonets, geotextile-polymeric cores, or a three-dimensional polymeric cell structures). Geotextiles are permeable synthetic materials made of textile materials such as polypropylene, polyethylene, or polyester. Further, based upon their preparation, a geotextile may be woven, nonwoven, or knitted, as shown in Fig. 7.2. Aside from these traditional forms of woven and nonwoven geotextiles, there are many forms of geotextiles that have come onto market over the last few years, as depicted in Fig. 7.3. Geotextiles are widely used in road construction and railway works (for separation, filtration, drainage, and soil reinforcement); for erosion control in river canals and coastal works; as a filtering media for drainage in earthen dam, behind retaining
Soil reinforcement
Woven geotextile
63
Nonwoven geotextile
Knitted geotextile
Fig. 7.2 Woven, nonwoven, and knitted geotextiles.
walls, and in deep drainage trenches; in sport field construction like Caselon playing fields and Astro turf; and for mud control in agriculture. Geogrids are primarily used for reinforcement and they are heat-welded from strips of material (such as polyester, polyethylene, or polypropylene) or produced by punching a regular pattern of holes in sheets of material that are then stretched into a grid. As fill is placed on the geogrids, the mesh design locks soil in place providing differing levels of stability depending on the type of geogrid used. The geogrid may be uniaxial or biaxial, as shown in Fig. 7.4, depending upon their tensile strength in different directions. A uniaxial geogrid has high tensile strength in one direction and is useful for the reinforcement of retaining walls, steep slopes, and road embankments, and for repairing landslides. On the other hand, a biaxial geogrid has equal tensile strength in both directions and is useful for stabilizing roadways. It distributes loads over a larger area reducing pumping and shear failures while maximizing the load-bearing capacity of subgrades. Geomembranes are very low-permeable geosynthetic materials made from relatively thin continuous polymeric sheets (chlorosulphonated polythene (CSPE), high-density polythene (HDPE), very-low-density polythene (VLDPE), polyvinyl chloride (PVC), etc.), but they can also be made from the impregnation of geotextiles with asphalt, elastomer, or polymer. The geomembranes create an impermeable barrier that keeps contaminants and other dangerous chemicals contained so they cannot escape and damage the surrounding environment. Geomembranes are generally used as liner for sewage sludge, for the safe shutdown of nuclear facilities, for water and various waste conveyance canals, as secondary containment of underground storage tanks, as covers for solid waste landfills, as waterproof facing in various structures, to prevent infiltration of water and contaminants into sensitive areas, etc. Geocomposites are made by combining different geosynthetics materials or by combining geosynthetic materials with nonsynthetic materials, such as bentonite clay, to address specific applications in the field in the optimal manner with minimum cost. Geocomposite materials include geotextile-geonets, geotextile-geogrids, geotextile-geomembranes, geomembrane-geonets, geosynthetics clay liner (GCL), geotextile-polymeric cores, or three-dimensional polymeric cell structures. Various forms of geocomposite materials are presented in Fig. 7.5.
64
High modulus woven
Multilayer woven
Triaxial woven
Tubular braid
Tubular braid laid In warp
Flat braid
Flat braid laid in warp
Weft knit
Weft knit laid in weft
Weft knit laid in warp
Weft knit laid in weft laid in warp
Square braid
Square braid laid in warp
3- D Braid
3-D Braid laid in warp
Warp knit
Warp knit laid in warp
Weft inserted warp knit
Weft inserted warp knit warp in warp
Fiber mat
Stitchbonded laid in warp
Biaxial bonded
Xyz laid in system
Fig. 7.3 Different forms of geotextile materials (Frank, 2004).
Geotechnical Investigations and Improvement of Ground Conditions
Biaxial woven
Soil reinforcement
65
Fig. 7.4 Uniaxial and biaxial geogrids.
Fig. 7.5 Various forms of geocomposite materials.
So from this abovementioned information it can be concluded that the application of geosynthetic materials covers almost all types of civil engineering structures and its main functions are separation, filtration, drainage, reinforcement, protection, and waterproofing. In road construction, the application of geosynthetic materials is illustrated in Fig. 7.6.
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Geotechnical Investigations and Improvement of Ground Conditions
Geosynthetics material
Cracks Road surface Crushed aggregate layer Native material Without geotextile
With geotextile
Geotextile used for separation
Crack sealing in presence of geotextile Geotextile
ACC Base cource
Filtration
Geotextile Transmissivity
Aggregate drainage layer Subgrade
Frictional restraint
Membrane support
Subgrade restraint
Subbase restraint Local reinforcement
Geotextile for filtration
Geotextile to provide support and strength against wheel load
Fig. 7.6 Application of geotextiles in pavements (Koerner, 1998).
When reinforcement is required to be inserted into the soil mass, such as for retaining walls and embankments, it is very important to add the backfill material (for construction of manmade structures) or insert the reinforcing element into the soil mass (for stability and/or protection of natural backfill) in a systematic way by following the proper steps. Moreover, it is necessary to include a facing wall (either soft, such as growing vegetation, or hard, such as wraparound type, gabion, precast concrete panel, or modular concrete block) for the stability and longevity of the structure. Hence, the backfill materials and facing wall along with the reinforcing elements now become an integral part of the reinforced wall, as shown in Fig. 7.7, where a precast concrete panel has been used as the facing wall. The use of geosynthetic materials in various civil-engineering structures and for ground treatment has tremendously increased in the last few decades. The advantages of using these materials are cost efficiency, convenience for transport and installation, lower repair and maintenance costs, predictability of the design, quick installation, applicability to a wide range of soils, space savings, improved performance and extended life, good-quality control due to homogeneity in nature, less environmentally sensitive, increased safety factor, and compatibility with field conditions. However, the geosynthetic materials need to meet certain requirements and to be checked and tested before using them in the field. The required properties are summarized in Table. 7.1. The new generation of geosynthetic materials includes the nanofibre membrane with superhydrophobicity and superoleophilicity for selective separation of water and low viscous oil, nanocomposite electrospun nanofiber membranes (Nanocomposite ENM) for environmental remediation and water filtration, etc. The basic principles
Soil reinforcement
67
Fig. 7.7 A typical picture of reinforced retaining wall (Holtz, 2001). Table 7.1 Requirement of geosynthetics material Property
Parameters
General properties
Material type and construction, polymer(s), mass, thickness, roll dimensions, specific gravity, absorption Strip tensile strength, grab strength, creep resistance, flexural strength, cutting – trapezoidal tear strength, shear modulus, Poisson’s ratio, burst strength, puncture resistance, penetration, flexibility (flexural strength) Abrasion resistance, UV stability, biological resistance, chemical resistance, wet/dry stability, temperature stability, long-term durability Stress–strain, creep, friction/adhesion, dynamic and cycling loading, soil retention, filtration Apparent opening size, percent open area, porosity, permeability/permittivity, soil retention ability, clogging resistance, in-plane flow capacity
Index properties
Endurance properties
Performance – soil/ fabric properties Hydraulic properties
behind the concept of nanofibre is that by reducing fibre diameter down to the nanoscale, the specific surface area increases enormously to the order of about 1000 m2/g and this reduction in dimension and increase in surface area greatly affects the chemical/biological reactivity and electroactivity of polymeric fibres.
68
7.3
Geotechnical Investigations and Improvement of Ground Conditions
Soil nailing
A soil nail can be defined as a structural member that prevents the collapse of ground material and retains it by virtue of its self-weight, bending strength, and stiffness. The internal stability of a soil-nailed system is usually assessed using a two-zone model: the active and passive zone, separated by a potential failure surface, as shown in Fig. 7.8. The soil-nailed system ties the active zone with the passive zone with its tensile, shear, and bending force. Hence, it is very useful to use soil nails as a remedial measure to treat unstable natural soil slopes or as a construction technique that allows the safe oversteepening of new or existing soil slopes. A soil-nailed system is formed by inserting relatively slender reinforcing bars into the slope. Depending upon the project cost, site accessibility, availability of working space, and the soil and groundwater conditions, soil nails can be inserted into the ground by the following methods: (1) drill-and-grout, (2) self-drilling, or (3) driving. In the drill-and-grout method soil nails are inserted into a predrilled hole that is then cement grouted under gravity or low pressure. Using this method long soil nails can be inserted, and it is possible to check the ground obstructions (if any) and also to check the alignment of the drilling holes. However, this method may require the hole to be cased in the event of collapsible ground. In the method of self-drilling, which is quite rapid because of the use of simultaneous drilling and grouting, the soil nails are directly drilled into the ground using a sacrificial drill bit. For drilling, cement grout instead of air or water is used for flushing, which maintains the hole stability and thus eliminates the requirement for any casing.
Soil-nail head
Bearing stress Passive zone Facing
Active zone Soil nail
Bending and shear Tension Shear stress
Tension
Potential failure surface
Fig. 7.8 Two-zone model of a soil-nail system. (From Geotechnical Engineering Office, 2008. Civil Engineering and Development Department, The Government of the Hong Kong Special Administrative Region, Geoguide 7: Guide to Soil Nail Design and Construction, Hong Kong.)
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69
However, the disadvantages of this method are that (1) it cannot penetrate through rocks or ground containing corestones and hence is not suitable under such ground conditions, (2) it is difficult to check and maintain the alignment of the soil nails, and (3) there are durability issues due to a lack of specified minimum grout cover and coating. In the third method of installation, i.e. ‘driven’, soil nails are directly driven into the ground using a compressed air launcher using the percussive method and hammering equipment or using the vibratory method and vibrating equipment. Compared to the other two methods, this method is more rapid and causes minimal ground disruption. However, there are basic limitations and disadvantages to this method, which are that (1) only relatively short-length nails can be installed, (2) there is more chance of damage to soil nails due to excessive buckling stress induced during the installation process, especially in case of stiff soil, and (3) the nails are highly susceptible to corrosion if noncorrodible reinforcement is not used, since the nails are in direct contact with the surrounding ground. The basic components of a soil-nailed system are shown in Fig. 7.9. It consists of the following components: (i) Reinforcing elements, typically a solid high yield deformed steel bar or, in some cases, a fibre-reinforced polymer can also be used. (ii) Reinforcement connector or coupler, which is used to connect sections of soil nailreinforcing bars. (iii) Grouting, which is done in a predrilled hole after the insertion of the soil nails, transfers stresses between the ground and the soil nails, and also acts as part of the corrosion protection measures. (iv) Corrosion protection measures, using epoxy-coated bars, provides a protection cover in the form of hot-dip galvanizing and corrugated plastic sheathing, heat-shrinkable sleeves made of polyethylene, or anticorrosion mastic sealant material.
Stud welded to plate Geocomposite drainage strip
Threadbar (Epoxy coated)PVC Centralizer
Grout Reinforcing steel
Drill length
Temporary shotcretewall Permanent shortcrete or cip wall Bar length
Soil nail length
Fig. 7.9 Schematic diagram of a soil-nailed cut slope (Tuozzolo, 2003).
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Geotechnical Investigations and Improvement of Ground Conditions
(v) Centralizers, which are placed around the soil nail to maintain an even thickness of grout around the bar. (vi) Soil nail head, which comprises a reinforced concrete pad, steel bearing plate, and nuts, and its primary function is to provide a reaction for individual soil nails to mobilize tensile force and to make the ground stable near the slope surface. (vii) Geocomposite drainage strip, for protection against the adverse effects of surface runoff or perched water in the soil-nails interface, which may cause problems during construction or for the shotcrete, which may have a problem adhering to the face if surface water is present. (viii) Wall facing in the form of a permanent shotcrete wall or isolated soil nails head plates. Alternatively, rabbit-proof wire mesh and environmental erosion control fabrics may also be used in conjunction with flexible mesh facing, as demanded by environmental conditions.
A soil-nail system is basically designed for its stability, durability, and serviceability, keeping in mind the economic and environmental considerations. The soil-nail system may become unstable for one or more of the following reasons: 1. 2. 3. 4. 5. 6. 7. 8.
Failure of ground around soil nails Soil-nail head bearing failure Local failure between soil nails Tensile failure of soil nails Pullout failure at grout–ground and/or grout–reinforcement interface Bending or shear failure of soil nails Structural and connection failure of soil-nail head Structural and connection failure of facing.
The allowable tensile capacity (TT), pullout resistance provided by the soil-grout bond length in the passive zone (TSG) and the allowable pullout resistance provided by the grout–reinforcement bond length in the passive zone (TGR) are critical parameters and are to be determined properly during the design stage as follows: TT ¼ fy A0 =FT
(7.1)
TSG ¼ ðc0 Pc L + 2 D σ v 0 μ LÞ=FSG
(7.2)
TGR ¼ β √fcu Pr L =FSG
(7.3)
where fy is the characteristic yield strength, A0 is the effective cross-sectional area, and FT is the factor of safety against tensile failure of soil nail reinforcement. c0 is the effective cohesion of the soil, Pc is the outer perimeter of the cement grout sleeve, L is the bond length of the soil nail reinforcement in the passive zone, D is the outer diameter of the grout sleeve, σ v0 is the vertical effective stress in the soil calculated at mid-depth of the soil-nail reinforcement in the passive zone with a minimum value of 300 kPa, μ is the coefficient of apparent friction of soil (equal to tan φ0 where φ0 is the angle of shearing resistance of soils under effective stress), FSG is the factor of safety against pullout failure at the soil–grout interface, β is the coefficient of friction at the
Soil reinforcement
71
grout interface, fcu is the characteristic strength of the cement grout, Pr is the effective perimeter of the soil-nail reinforcement. In addition to stability, it is also necessary to consider the serviceability of the soilnail systems. At any period of time during its service life, it should not affect nearby structures or facilities. It should not cause any ground deformation beyond the acceptable limit, there should not be any damage to the slope facing and drainage system, and the soil-nail system should be in good condition appearance-wise. Moreover, the spacing between soil-nails (generally 1.5–2.0 m) and the inclination (generally 5° to 20°) should be determined properly. Wide-spacing may cause local instability whereas close spacing may not be economical to install. Having discussed the installation method and understood the working principles, now it is easy to identify the advantages and disadvantages of the soil-nail systems, which are summarized in Table 7.2.
7.4
Soil anchoring
Soil or ground anchors also known as ‘tiebacks’ are prestressed tendons or cables installed inside a borehole that apply a restraining force to rock faces, cuttings and slopes, or tunnels via a plate in order to stabilize it. The end of the anchor is either grouted or locked mechanically deep in sound strata. The basic difference between Table 7.2 Advantages and disadvantages/limitations of soil-nail system Advantages Allows in situ strengthening with minimum backfilling and excavation and so it is a timesaving, cost-effective and environmentally friendly method Requires light machinery and equipments and so can be used where accessibility or space is a major problem It can cope with site constraints and variations in ground conditions encountered during construction by adjusting the location, length, and inclination of the soil nails The failure mode of a soil-nailed system is likely to be ductile and thus it provides a warning sign before failure l
l
l
l
Disadvantages/limitations May not be suitable under certain ground conditions such as with cobbles, boulders, highly fractured rocks, open joints, voids, or obstructions to drilling or driving. Moreover, problems may arise from the collapse of a borehole and loss of grout through fractured rock mass, open joints, and cavities. In addition, for coarse-grained soil and soft clays that have less self-support time and where the soil is prone to creeping this method is not applicable l
l
l
Loss in serviceability and instability of slope and facing due to various factors related to soil, reinforcement, grout, soil-nail, grout-nail interaction The presence of utilities, underground structures, or other buried obstructions poses restrictions to the length and layout of soil nails. Moreover, future development may be restricted in that area sterilized with the presence of soil nails
72
Geotechnical Investigations and Improvement of Ground Conditions
a soil nail and a soil anchor is that a soil nail is a passive bearing element, which relies on soil movement to mobilize the shear strength along the nail, whereas an anchor is prestressed to mobilize shear strength. The basic components of a ground anchor are presented in Fig.7.10. They mainly consist of l
l
l
l
l
the anchor head, which transmits the anchor force to the structure via the bearing plate the free stressing unbonded length from the head to the near end of the anchorage the grouted anchor bond length, by which the tensile force is transmitted to the surrounding ground through the anchor grout. This anchor bond length should be located behind the critical failure surface the sheath, which is a portion of the tieback and encases the tendon in the stressing unbonded length only. The sheath, also known as the ‘bond-breaker’, enables the tendon bar in the unbonded length to elongate without obstruction during testing and stressing and leaves the tendon unbonded after locked-off to the specified load the trumpet, which transmits the prestressing force from the tendon to the supported structure and is sealed to the bearing plate and overlaps the unbonded length corrosion protection by at least 10 cm. The trumpet is generally long enough to accommodate movements of the structure and the tendon during testing and stressing without damaging the encapsulation.
Un
bo
Trumpet
nd
ed
len
gth
Anchor head Sheath
Bearing plate
An
ch
Te
nd
on
Wall
or
bo
nd
bo
nd
len
gth
len
gth
Unbonded tendon Anchor grout Bonded tendon
An dia cho me r ter
Fig. 7.10 Basic components of a ground anchor. (From Sabatini, P.J., Pass, D.G., Bachus, R.C., 1999. Geotechnical Engineering Circular No. 4 Ground Anchors and Anchored Systems, Report No. FHWA-IF-99-015.)
Soil reinforcement
73
Fig. 7.11 (A) Bar tendons and (B) strand tendons used for ground anchoring.
The drill hole diameter is generally less than 15 cm, except for hollow stem augured anchors, in which case it is about 30 cm in diameter. The total anchor length generally varies from 9 m to 18 m, with a minimum unbonded length of 3 m for bar tendons and 4.5 m for strand tendons (shown in Fig. 7.11). The inclination of anchor varies from 10° to 45°.
7.5
Micropiling
Micropiles are smaller in diameter (about 90–300 mm) compared with piles and are installed closer together than the conventional pile foundations. The installation method for micropiles is depicted in Fig. 7.12. The drilling of boreholes with casings is done using a suitable drilling technique for the soil/site conditions. Afterwards the drilling rod and drilling bits are removed and reinforcing bars (usually corrosion protected steel bars) are inserted into the boreholes. Grouting is then done in sequence under pressure and the casing is removed gradually. Micropiles are generally executed by use small-sized equipment and hence the method can be carried out in small areas and with limited access. The field for the use of micropiles application is very wide; it can be executed in areas with limited
Geotechnical Investigations and Improvement of Ground Conditions
Steps of Installation of a micropile
74
(1)
(2)
(3)
(4)
(5)
(1) Beginning of drilling and installation of casing (2) Completion of depth of drilling (3) Removal of drill bit and rod (4) Introduction of the reinforcement (5) Grouting (6) Removal of casing and further grouting under pressure
(6)
Fig. 7.12 Installation method of micropiles.
vertical clearance like basements and under bridge structures. Some other examples of critical areas where the micropiles can be installed are inside industrial buildings, small tunnels, mountain trails, rice fields, mountainous forested areas, steep slopes, etc. In addition, micropiles can be installed through preexisting foundations and for the underpinning of structures and repair of defective foundations. According to the design applications micropiles could be classified into two groups. The first group includes micropiles being loaded either axially or laterally. This group of micropiles transfers the structural loads to the competent strata below the foundation (e.g. the underpinning of structures) or they may be used to resist the movement of failure planes (i.e. stabilization of slopes). The second group includes micropiles being used to strengthen the soil mass by forming a reinforced soil composite. On the other hand, micropiles also could be classified into four groups (i.e. Groups A, B, C, and D) based upon the methods of grouting, as shown in Fig. 7.13. For the group A micropiles grouting is placed under gravity. In group B grout is injected into the hole under pressure, but the pressure is limited to avoid hydrofracturing of the surrounding grounds. The installation process for group C micropiles involves two steps: first a primary grout is applied under pressure to cause hydrofracturing of the surrounding ground, and then a secondary grout is injected via a manchette tube before the primary grout hardens. Group ‘D’ micropiles are similar to the group ‘C’, except that the secondary grout is injected after hardening of the primary grout.
Soil reinforcement
75
Fig. 7.13 Micropiles classification based on grouting method.
Type A
Type B
Type C
Type D
References Frank, K.K., 2004. From Textile to Geotextiles, Seminar in honour of professor Robert Koerner, September 13, 2004. Department of Materials Science and Engineering, Drexel University, Philadelphia University, Philadelphia, PA, pp. 20. Holtz, R.D., 2001. Geosynthetics for soil reinforcement. In: The Ninth Spencer J. Buchanan Lecture, November 9, 2001. Koerner, R.M., 1998. Designing with Geosynthetics, fourth ed. Prentice-Hall, Secaucus, NJ, pp. 761. Tuozzolo, T.J., 2003. Soil nailing: where, when and why-a practical guide. In: Presented at the 20th Central Pennsylvania Geotechnical Conference, Hershey, PA.
Further reading Bonaparte, R., Holtz, R.D., Giroud, J.P., 1987. Soil reinforcement design using geotextiles and geogrids. In: Fluet, J.E. (Ed.), Geotextile Testing and the Design Engineer. American Society for Testing and Materials, pp. 69–116 Philadelphia: ASTM, Special Technical Publication. Koerner, R.M., Hsuan, Y.G., 2001. In: Rowe, R.K. (Ed.), Geosynthetics: Characteristics and Testing. Chapter 7 in Geotechnical and Geoenvironmental Handbook. Kluwer Academic Publishers, New York, pp. 173–196. Liew, S.S., Fong, C.C., 2003. Design and construction of micropiles. In: Geotechnical Course for Pile Foundation Design & Construction. ipoh (29–30 September 2003). Meccai, K.A., Hasan, E.A., 2004. Geotextiles in transportation applications. In: 2nd Golf Conference on Roads, Abu Dhabi, March. Sabatini, P.J., Pass, D.G., Bachus, R.C., 1999. Geotechnical Engineering Circular No. 4 Ground Anchors and Anchored Systems. Report No. FHWA-IF-99-015.
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Shong Ir, L.S., Chung, F.C., 2003. Design and construction of micropiles. In: Geotechnical Course for Pile Foundation Design and Construction.pp. 29–30. Ipoh. September. Tuozzolo, T.J., 2003. Soil nailing: where, when and why – a practical guide. In: Presented at the 20th Central Pennsylvania Geotechnical Conference, Hershey, PA.