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Connections for geogrid systems
Geotextiles and Geomembranes 9 (1990) 537-546
Connections for Geogrid Systems
Michael R. Simac Mirafi Inc., Charlotte, North Carolina 28224, USA
ABSTRACT A general discussion on connection types for geogrid reinforcement systems is presented. The various mechanical and frictional methods for connecting geogrid to geogrid, and geogrid to a structural element are defined in general terms. Basic design guidelines are recommended to assure connection integrity for geogrid reinforcement systems.
INTRODUCTION The use of geogrid reinforcement in civil engineering structures has gained general acceptance over the last few years. With hundreds of successfully completed structures performing as expected, the overall design methodology for geogrid-reinforced earth structures has proven to be appropriate. Furthermore, detailed research I into the subject has verified and refined current design methodology. To date, relatively little attention has been focused on the integrity of connections in geogrid reinforcement systems. The main reason for absence of this information is that only a few applications require connections. Most installations are plane strain problems requiring reinforcement in only one direction, eliminating any need for stress transfer between adjacent pieces of geogrid reinforcement. However, some plane strain applications such as reinforced soil-retaining walls, require connection of the geogrid to the structural element used for facing the wall. Additionally, most stabilization applications, particularly over 537 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~ 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
538
Michael R. Sirnac
large areas, require reinforcement strength in two directions to support surface loadings, placing emphasis on geogrid to geogrid connections. The requirements of an acceptable geogrid connection are two-fold. First, to develop the required strength at the intersection of load-bearing elements. Secondly, to develop the required strength within the permissible strain range of the load-bearing elements for the earth structure. This paper introduces several procedures to evaluate geogrid connections for various geogrid applications.
Anchorage capacity It is well known 2-4 that the apertures of a geogrid permit soil material to easily penetrate it, forming an efficient interlock that anchors the geogrid to the soil. The mechanism of geogrid anchorage is shown in Fig. 1 and depends upon the following variables: (a) (b) (c) (d) (e) (f)
geogrid aperture size; percentage open area of geogrid; stress transfer across geogrid junctions; frictional properties of geogrid; available soil shear strength; and available anchorage length.
The anchorage capacity is limited to the ultimate strength of a geogrid. A generally accepted method to calculate the anchorage capacity of a geogrid is as follows: A C = 2 (Ci) L a (c q- UW~(d)tanob)
(1)
where A C = anchorage capacity (kN/m)(lb/ft)
Ci = coefficient of shear stress interaction La = anchorage length (m) (ft) c = cohesion of soil (kPa) (psf) UWs = unit weight of soil (kN/m3) (pcf) d = depth of overburden (m) (ft) ~b= angle of internal shearing resistance (degrees). The coefficient of shear stress interaction, Ci, indirectly accounts for the first four factors (a, b, c, d) that influencd the anchorage capacity of a geogrid. The other two factors, soil strength and anchorage length, exert a direct influence on the anchorage capacity of a geogrid. Lastly, the available soil shear stress is mobilized both above and below the geogrid,
Connections for geogrid systems
539 Tensile Force Applied
Tendency for Grid Movement
S
"T pv
/, \,
Cross-Direction Member Passive Soil Res~stence Mobilized
by Grid Movement Soil Sheor Resistonce Mobilized by Grid Movement
Fig. 1. Anchorage of a geogrid. TABLE 1 Coefficients of Shear Stress Interaction Coefficient of shear stress interaction Soil type Silty clay, sandy clay, clayey silt (ML, CL) Silty sands, fine to medium sands (SM, SP, SW) Dense well-graded sand, sand and gravel (SW, GP, GW)
(Ci) 0-7-0.75 0.75--0.85 0.85--0.9
which for most geogrid applications is the same soil type. However, applications do occur where vastly different soil types are in contact with the top and bottom of the geogrid. The coefficient of shear stress interaction accounts for the in-situ behavior of the geogrid. There are two minimum requirements of a geogrid to obtain mechanical interlock with the soil: • there must be a minimum 50% open area in the geogrid structure, and • a geogrid's least aperture dimension must be greater than D50 of the soil and its greatest aperture dimension should be greater than D85 of the soil. The coefficient of shear stress interaction, Ci, is developed through extensive pull-out testing on a particular type geogrid. Table 1 shows a typical range of Ci derived from pull-out testing. 5
540
Michael R. Simac
Types of connections There are two basic connections for geogrid reinforcement: frictional and/or mechanical. Frictional connections will be those based solely on the available shear strength due to weight. Mechanical connections are those which depend only on additional structural elements to improve connection integrity. Anchorage capacity of a geogrid reinforcement is the cornerstone of its performance in any friction connection. The anchorage capacity of any frictional connection can be determined using eqn (1). Likewise, eqn (1) may be rearranged to calculate the minimum anchorage length, L~, for a predetermined force in the geogrid, i.e. anchorage capacity. Mechanical connections introduce an additional structural element to improve the efficiency of load (stress) transfer. All mechanical connections are very dependent on both the geogrid and the structural element used to facilitate the connection and must be tested in a laboratory to define its behavior and ultimate strength.
G E O G R I D TO G E O G R I D C O N N E C T I O N Geogrid to geogrid connections are utilized mostly for large area stabilization projects. For these applications, the geogrid reinforcement is required to maintain tension in both the major and minor principal stress directions. This generally requires a connection to maintain tension across the roll width. Geogrid to geogrid connections in the main principal stress direction are to be avoided for plane strain applications such as embankment reinforcement and reinforced soil-retaining walls. The strain associated with those connections is too large to provide effective reinforcement for the earth structure.
Overlaps for geogrid connection The most common geogrid to geogrid connection for area stabilization is overlapping adjacent geogrid sections, as shown in Fig. 2. This overlap connection is purely frictional and can be calculated by rearranging eqn (1) to equate the anchorage capacity, AC, to the force in the geogrid. L. = Fg/(2 (Ci) (c + UW~ (d) tan ~b))
(2)
Connections for geogrid systems
541
where La = minimum geogrid anchorage length to resist pullout of the geogrid (m) (ft), Fg = maximum force in grid layer (kN/m) (lb/ft). For routine stabilization applications, the maximum force in the grid layer is assumed to be the ultimate strength of the geogrid. This provides a minimum factor of safety on anchorage length, La, of greater than 1.5, since the design loads for most geogrids are less than 65% of ultimate strength, as defined by ASTM D-4595.
m -
OVERLAP ~
_- _-_
-
Fg :
-
Lo2 La = Anchoroge Length Fig. 2. Overlapconnection. The minimum overlap for a geogrid installation is equivalent to the minimum anchorage length, La, as shown in Fig. 2. The overlap connection is effected by these conditions: • The geogrids to be overlapped must each possess a percentage open area greater than 70% to maintain the minimum 50% open area criteria for the coefficient of shear stress interaction. • The same soil must be present above and below the geogrid for eqn (2) to apply. Different soils above and below the geogrid require separating the calculation, by removing the 2 in eqn (2) and summing the individual anchorage capacities from the different soil types. • A geotextile placed directly beneath a geogrid will eliminate interaction with the underlying soil. The contribution of the geotextile to underlying anchorage capacity can be calculated as follows: A C = (La) (POA) ~ (UWs(d) tan 0-677~b))
(3)
where POA = percentage open area of geogrid (decimal).
Therefore, the underlying capacity from the geotextile to the geogrid is limited to the soil in contact with the geotextile from above. The
542
Michael R. Simac
interaction between the soil and geotextile is limited to two-thirds of the strength (19) of the soil. • Any overlap connection length, L,, should be a minimum of twice the maximum anticipated differential settlement, to prevent separation of the connection. The maximum differential settlement often occurs during soil placement, due to mud-waving and/or rutting of soft soils. As a construction expedient to overlap connections, electrical cable cincture ties, tie wire, or hog-rings are placed diagonally across geogrid nodal connections to establish and maintain overlap for soil placement. These ties are placed every 300-900 mm (1-3 ft) depending upon application, to position the geogrid, and contribute little to the overall connection strength. Mechanical connections for geogrids When overlap connections are insufficient or cost prohibitive, a mechanical connection can be implemented. Mechanical connections that have previously been successful in geogrid systems consist of either ties between geogrids or interlacing of the geogrids to be joined. The tie systems utilize either electrical cable cincture ties, tie wire, or hog-rings. These ties are placed around each or alternating nodal connections of the top and bottom geogrid to be joined. The strength of the connection varies with the tie strength and nodal connection strength of the geogrids being joined.
f,..t-m
_
m
_
_
Lo U
_~-
.
Lo = A n c h o r a g e Lengt ~' Bodkin Joint
Bodkin da m
m
da
= A p e r t u r e Distance
Detail
Fig. 3. Bodkin joint.
Connections for geogrid systems
543
The most efficient mechanical geogrid to geogrid connection is the bodkin joint shown in Fig. 3. The bodkin joint entails interlacing the top and bottom geogrid with a plastic (PP, PVC, HDPE) dowel that may range from one-tenth (0.1) to three-eighths (0-375) the aperture distance, da, of the geogrids to be joined. The bodkin joint is formed by pulling the main reinforcing strands of the lower geogrid, up through the upper geogrid and slipping a dowel through the loop created. The strength of a bodkin joint may vary with geogrid type, and dowel diameter or composition. Any geogrid to geogrid mechanical connection should be thoroughly tested in a laboratory, prior to field installation. Most of these connections also incorporate some overlap to provide redundancy in the connection.
G E O G R I D TO STRUCTURAL ELEMENT CONNECTION A variety of structural elements can be utilized with geogrid reinforcement to create permanent reinforced soil-retaining structures. These structural elements provide the aesthetically pleasing appearance and low maintenance durability necessary for permanent application. Each structural element requires a unique method of geogrid attachment that is specific to that facing connection. However unique each system is, the facing connection will function as either a mechanical or frictional connection.
Frictional facing connections Frictional facing connections are very similar to overlap geogrid connections, with the following exceptions. There is a defined anchorage length, usually the facia depth, and frictional resistance may be developed by the structural element and/or soil within the structural element. The connection strength, CS, for frictional connections may be calculated as follows: CS = 2 (VWtc) FD (d) tx'
(4)
where CS = connection strength (kN/m) (lb/ft) UWfc = unit weight of facia course (kN/m3) (pcf) FD = facia depth (m) (ft) d = depth of geogrid layer (m) (ft) /x' = Ci tan $ or 0.66 (~) whichever is less.
The/z' term accounts for the coefficient of shear stress interaction, Ci, with the soil friction, tO, used to fill the wall facing or interaction between
544
Michael R. Simac
the geogrid and solid facia possessing a coefficient of sliding friction,/z. For facing connections where both soil in-fill and solid facing components are supplying frictional resistance, the lowest mathematical expression for /x' should be used in the calculation. The manufacturer of the structural element for wall facing should provide the coefficient of sliding friction, /z, and unit weight of facia course, UWfc. Prior to construction, the connection strength should be verified by laboratory testing. A typical geogrid to structural element connection which is based completely on the soil properties used to fill the Geoweb T M structural element is shown in Fig. 4. Railroad tie, a typical solid wall facing connection, is shown in Fig. 5. More complex structural elements, like the KeyStone T M retaining wall system shown in Fig. 6, can also be analyzed using eqn (4). The connection strength, CS, available at each geogrid elevation must provide sufficient resistance to counteract tension in the geogrid. The maximum tension at each geogrid layer varies and may be determined using conventional design methodologies. 5 However, the facing connection rarely experiences the maximum geogrid tension except at the lowest layers of geogrid reinforcement in the structure. This is due to the location of the Rankine failure plane that moves further away from the facing near the top of the structure. Therefore, this approach represents a conservative treatment for connection design.
Mechanical facing connections Specific geometric shapes and material composition of some structural elements dictate that frictional facing connections are ineffective or inappropriate. Geogrid reinforcement may be attached to these structural elements by means of a mechanical system that increases the effectiveness of the wall facing connection. The efficiency of the connection is increased through either mechanical bonding or a mechanical advantage system. Most bonding systems are comprised of cementitious materials. The geogrid may be embedded in precast or cast-in-place concrete. Usually, a minimum of 150--300mm (6-12in) of geogrid is tied to the existing reinforcing steel cage, sometimes using a bodkin interlacing procedure. The geogrid may also be incorporated between courses of common brick or masonry block, in the regular mortar bed. Mechanical advantage connections wrap the geogrid around a structural element and return the geogrid into the reinforced soil for additional resistance as shown in Fig. 7. This mechanical advantage system has been formed with fiat steel or wood batten strips and plastic or steel pipes, depending upon the requirements of structural facing element.
Connections for geogrid systems
.
545
GEOWEB@A4 STANDARD UNIT ~ COMPACTED ¢,r'OGRin NATIVE
~
~
•
i
[
SOIL
1
~
~m~m,~m~m_~m,_=,, 8 " ] ~ l ~ f ~ n ~ r
()
~
COMPACTED AGGREGATE
L ~ ' ~ , ~ , ~ E
AND ~E~,NO
L_ ~. ~L~..~~ Fig. 4. Geoweb T M cellular confinement system. 10' GALV. ST. SPIKE 5,/8" MINIMUM 3 PER 8' TIMBER R"y F,::\\ 4.5" 8" COMPACTEDDRAINAGE ~:~-~ED~ r ;i I" -I AGGREGATE LANDSCAPEX J ~ ~
TIMBER Jr'~llll L ~
PLYWOOD~,\I~,U A ' ~ . . GEOGRID BATTEN 1 / 2 ~ 1 H : : : I I J--~l E l l k=l 11 i. ~~ J ~ - ~ f-3~-k.h i u'TTLL~T~j jJJ_,3"~J. l FACING . -" ; : : J- (~ STRIP ~j --' " 1,0,,PENNY //~]~[ [~I l ~ l l \ . . COMPACTED
~
' ~ r { l ~ I I ~ ~ , ~m
~ I ~ ) t Z ~
"A'"
\RE'N~ORCE°SO,L
Fig. 5. Railroad tie facing. FIBERGLASS DOWEL
KEYSTONE@ GEOGRID HOOKED STANDARDUNIT AROUND ..i.~--. . . . . . ~ COMPACTED FIBERGLASS ~~l.,,~J GEOGRID REINFORCED DOWEL____ ~ZAli~dlmaK~-~ SOIL
/-/'--
G
-
~
¢OMPAOTED AGGREGATE
L-F~IJ]ilIE~
"~. BETWEEN, INSIDE AND
L~ ~,.._L~2 :
Fig. 6. Keystone TM retaining wall system. 10" GALV. ST. SPIKE 3/8" MINIMUM 3 PER 8' TIMBER 6" X 8 " ~ " " " " ' ~ " RAILROAD ~ TIE ~ ~
HEAVY~
~
.
r
r
L
8"
U
COMPACTED DRAINAGE d AGGREGATE
~
r
AT SPACING Lq V/AI~~I ~ l l l l i l l l ~ - ~ - , ~ ; ,,T-,~_u.~,,,,.-=,,,.~,. ,~ DESIGNATED /t[]~ I I I l i / ~ .~=lil=llL~ll=l El BY E N G I N E E R / / I/It I | / \ / i m m m K \ COMPACTED SOIL
~ } } A \ ~ } } ~ ~NATIVE
Fig. 7. Treated landscape timbers.
546
Michael R. Simac
All the mechanical connections outlined above are geogrid, structural element, and bonding agent or mechanical advantage system dependent. Therefore, all mechanical connections to structural elements of wall facing should be tested in a laboratory to define the connection strength for use in design. Manufacturers of different geogrid reinforcement and structural elements for wall facing may provide test data from previous projects to assist a designer.
SUMMARY The principles governing anchorage of geogrid reinforcement to soil were presented as the basis to understand and analyze frictional connections of geogrids. A systematic calculation procedure is offered to analyze frictional connections of geogrids to other geogrids and to structural elements normally used for facing reinforced soil-retaining structures. Details for various frictional geogrid connections were illustrated. Mechanical connection of geogrid reinforcement to adjacent geogrid reinforcement and to structural elements used for reinforced soil-retaining wall facing was discussed in general terms. Several potential mechanical attachment methods for geogrids were described. Mechanical connections require laboratory testing to verify the efficiency and ultimate strength for the particular components of the proposed connection eliminating any potential for an analytical prediction of connection strength.
REFERENCES 1. Christopher, B. R. & Holtz, R. D. Geotextile Engineering Manual, Report No. FHWA-TS-861203, Federal Highway Administration, Washington, DC, 1986. 2. Mirafi Inc. Miragrid Booklet. Engineering test data and promotional literature, Mirafi Inc., Charlotte, NC, 1988. 3. Koerner, R. M. Designing With Geosynthetics. Prentice Hall, Englewood Cliffs, N J, 1986. 4. Jones, Colin J. F. P. Earth Reinforcement and Soil Structures. Butterworths, London, 1985. 5. Mirafi Inc. Miragrid Engineering Concepts, Design Methodology Reinforced Soil Retaining Walls. Mirafi Inc., Charlotte, NC, 1988.
Connections for Geogrid Systems
Michael R. Simac Mirafi Inc., Charlotte, North Carolina 28224, USA
ABSTRACT A general discussion on connection types for geogrid reinforcement systems is presented. The various mechanical and frictional methods for connecting geogrid to geogrid, and geogrid to a structural element are defined in general terms. Basic design guidelines are recommended to assure connection integrity for geogrid reinforcement systems.
INTRODUCTION The use of geogrid reinforcement in civil engineering structures has gained general acceptance over the last few years. With hundreds of successfully completed structures performing as expected, the overall design methodology for geogrid-reinforced earth structures has proven to be appropriate. Furthermore, detailed research I into the subject has verified and refined current design methodology. To date, relatively little attention has been focused on the integrity of connections in geogrid reinforcement systems. The main reason for absence of this information is that only a few applications require connections. Most installations are plane strain problems requiring reinforcement in only one direction, eliminating any need for stress transfer between adjacent pieces of geogrid reinforcement. However, some plane strain applications such as reinforced soil-retaining walls, require connection of the geogrid to the structural element used for facing the wall. Additionally, most stabilization applications, particularly over 537 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~ 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
538
Michael R. Sirnac
large areas, require reinforcement strength in two directions to support surface loadings, placing emphasis on geogrid to geogrid connections. The requirements of an acceptable geogrid connection are two-fold. First, to develop the required strength at the intersection of load-bearing elements. Secondly, to develop the required strength within the permissible strain range of the load-bearing elements for the earth structure. This paper introduces several procedures to evaluate geogrid connections for various geogrid applications.
Anchorage capacity It is well known 2-4 that the apertures of a geogrid permit soil material to easily penetrate it, forming an efficient interlock that anchors the geogrid to the soil. The mechanism of geogrid anchorage is shown in Fig. 1 and depends upon the following variables: (a) (b) (c) (d) (e) (f)
geogrid aperture size; percentage open area of geogrid; stress transfer across geogrid junctions; frictional properties of geogrid; available soil shear strength; and available anchorage length.
The anchorage capacity is limited to the ultimate strength of a geogrid. A generally accepted method to calculate the anchorage capacity of a geogrid is as follows: A C = 2 (Ci) L a (c q- UW~(d)tanob)
(1)
where A C = anchorage capacity (kN/m)(lb/ft)
Ci = coefficient of shear stress interaction La = anchorage length (m) (ft) c = cohesion of soil (kPa) (psf) UWs = unit weight of soil (kN/m3) (pcf) d = depth of overburden (m) (ft) ~b= angle of internal shearing resistance (degrees). The coefficient of shear stress interaction, Ci, indirectly accounts for the first four factors (a, b, c, d) that influencd the anchorage capacity of a geogrid. The other two factors, soil strength and anchorage length, exert a direct influence on the anchorage capacity of a geogrid. Lastly, the available soil shear stress is mobilized both above and below the geogrid,
Connections for geogrid systems
539 Tensile Force Applied
Tendency for Grid Movement
S
"T pv
/, \,
Cross-Direction Member Passive Soil Res~stence Mobilized
by Grid Movement Soil Sheor Resistonce Mobilized by Grid Movement
Fig. 1. Anchorage of a geogrid. TABLE 1 Coefficients of Shear Stress Interaction Coefficient of shear stress interaction Soil type Silty clay, sandy clay, clayey silt (ML, CL) Silty sands, fine to medium sands (SM, SP, SW) Dense well-graded sand, sand and gravel (SW, GP, GW)
(Ci) 0-7-0.75 0.75--0.85 0.85--0.9
which for most geogrid applications is the same soil type. However, applications do occur where vastly different soil types are in contact with the top and bottom of the geogrid. The coefficient of shear stress interaction accounts for the in-situ behavior of the geogrid. There are two minimum requirements of a geogrid to obtain mechanical interlock with the soil: • there must be a minimum 50% open area in the geogrid structure, and • a geogrid's least aperture dimension must be greater than D50 of the soil and its greatest aperture dimension should be greater than D85 of the soil. The coefficient of shear stress interaction, Ci, is developed through extensive pull-out testing on a particular type geogrid. Table 1 shows a typical range of Ci derived from pull-out testing. 5
540
Michael R. Simac
Types of connections There are two basic connections for geogrid reinforcement: frictional and/or mechanical. Frictional connections will be those based solely on the available shear strength due to weight. Mechanical connections are those which depend only on additional structural elements to improve connection integrity. Anchorage capacity of a geogrid reinforcement is the cornerstone of its performance in any friction connection. The anchorage capacity of any frictional connection can be determined using eqn (1). Likewise, eqn (1) may be rearranged to calculate the minimum anchorage length, L~, for a predetermined force in the geogrid, i.e. anchorage capacity. Mechanical connections introduce an additional structural element to improve the efficiency of load (stress) transfer. All mechanical connections are very dependent on both the geogrid and the structural element used to facilitate the connection and must be tested in a laboratory to define its behavior and ultimate strength.
G E O G R I D TO G E O G R I D C O N N E C T I O N Geogrid to geogrid connections are utilized mostly for large area stabilization projects. For these applications, the geogrid reinforcement is required to maintain tension in both the major and minor principal stress directions. This generally requires a connection to maintain tension across the roll width. Geogrid to geogrid connections in the main principal stress direction are to be avoided for plane strain applications such as embankment reinforcement and reinforced soil-retaining walls. The strain associated with those connections is too large to provide effective reinforcement for the earth structure.
Overlaps for geogrid connection The most common geogrid to geogrid connection for area stabilization is overlapping adjacent geogrid sections, as shown in Fig. 2. This overlap connection is purely frictional and can be calculated by rearranging eqn (1) to equate the anchorage capacity, AC, to the force in the geogrid. L. = Fg/(2 (Ci) (c + UW~ (d) tan ~b))
(2)
Connections for geogrid systems
541
where La = minimum geogrid anchorage length to resist pullout of the geogrid (m) (ft), Fg = maximum force in grid layer (kN/m) (lb/ft). For routine stabilization applications, the maximum force in the grid layer is assumed to be the ultimate strength of the geogrid. This provides a minimum factor of safety on anchorage length, La, of greater than 1.5, since the design loads for most geogrids are less than 65% of ultimate strength, as defined by ASTM D-4595.
m -
OVERLAP ~
_- _-_
-
Fg :
-
Lo2 La = Anchoroge Length Fig. 2. Overlapconnection. The minimum overlap for a geogrid installation is equivalent to the minimum anchorage length, La, as shown in Fig. 2. The overlap connection is effected by these conditions: • The geogrids to be overlapped must each possess a percentage open area greater than 70% to maintain the minimum 50% open area criteria for the coefficient of shear stress interaction. • The same soil must be present above and below the geogrid for eqn (2) to apply. Different soils above and below the geogrid require separating the calculation, by removing the 2 in eqn (2) and summing the individual anchorage capacities from the different soil types. • A geotextile placed directly beneath a geogrid will eliminate interaction with the underlying soil. The contribution of the geotextile to underlying anchorage capacity can be calculated as follows: A C = (La) (POA) ~ (UWs(d) tan 0-677~b))
(3)
where POA = percentage open area of geogrid (decimal).
Therefore, the underlying capacity from the geotextile to the geogrid is limited to the soil in contact with the geotextile from above. The
542
Michael R. Simac
interaction between the soil and geotextile is limited to two-thirds of the strength (19) of the soil. • Any overlap connection length, L,, should be a minimum of twice the maximum anticipated differential settlement, to prevent separation of the connection. The maximum differential settlement often occurs during soil placement, due to mud-waving and/or rutting of soft soils. As a construction expedient to overlap connections, electrical cable cincture ties, tie wire, or hog-rings are placed diagonally across geogrid nodal connections to establish and maintain overlap for soil placement. These ties are placed every 300-900 mm (1-3 ft) depending upon application, to position the geogrid, and contribute little to the overall connection strength. Mechanical connections for geogrids When overlap connections are insufficient or cost prohibitive, a mechanical connection can be implemented. Mechanical connections that have previously been successful in geogrid systems consist of either ties between geogrids or interlacing of the geogrids to be joined. The tie systems utilize either electrical cable cincture ties, tie wire, or hog-rings. These ties are placed around each or alternating nodal connections of the top and bottom geogrid to be joined. The strength of the connection varies with the tie strength and nodal connection strength of the geogrids being joined.
f,..t-m
_
m
_
_
Lo U
_~-
.
Lo = A n c h o r a g e Lengt ~' Bodkin Joint
Bodkin da m
m
da
= A p e r t u r e Distance
Detail
Fig. 3. Bodkin joint.
Connections for geogrid systems
543
The most efficient mechanical geogrid to geogrid connection is the bodkin joint shown in Fig. 3. The bodkin joint entails interlacing the top and bottom geogrid with a plastic (PP, PVC, HDPE) dowel that may range from one-tenth (0.1) to three-eighths (0-375) the aperture distance, da, of the geogrids to be joined. The bodkin joint is formed by pulling the main reinforcing strands of the lower geogrid, up through the upper geogrid and slipping a dowel through the loop created. The strength of a bodkin joint may vary with geogrid type, and dowel diameter or composition. Any geogrid to geogrid mechanical connection should be thoroughly tested in a laboratory, prior to field installation. Most of these connections also incorporate some overlap to provide redundancy in the connection.
G E O G R I D TO STRUCTURAL ELEMENT CONNECTION A variety of structural elements can be utilized with geogrid reinforcement to create permanent reinforced soil-retaining structures. These structural elements provide the aesthetically pleasing appearance and low maintenance durability necessary for permanent application. Each structural element requires a unique method of geogrid attachment that is specific to that facing connection. However unique each system is, the facing connection will function as either a mechanical or frictional connection.
Frictional facing connections Frictional facing connections are very similar to overlap geogrid connections, with the following exceptions. There is a defined anchorage length, usually the facia depth, and frictional resistance may be developed by the structural element and/or soil within the structural element. The connection strength, CS, for frictional connections may be calculated as follows: CS = 2 (VWtc) FD (d) tx'
(4)
where CS = connection strength (kN/m) (lb/ft) UWfc = unit weight of facia course (kN/m3) (pcf) FD = facia depth (m) (ft) d = depth of geogrid layer (m) (ft) /x' = Ci tan $ or 0.66 (~) whichever is less.
The/z' term accounts for the coefficient of shear stress interaction, Ci, with the soil friction, tO, used to fill the wall facing or interaction between
544
Michael R. Simac
the geogrid and solid facia possessing a coefficient of sliding friction,/z. For facing connections where both soil in-fill and solid facing components are supplying frictional resistance, the lowest mathematical expression for /x' should be used in the calculation. The manufacturer of the structural element for wall facing should provide the coefficient of sliding friction, /z, and unit weight of facia course, UWfc. Prior to construction, the connection strength should be verified by laboratory testing. A typical geogrid to structural element connection which is based completely on the soil properties used to fill the Geoweb T M structural element is shown in Fig. 4. Railroad tie, a typical solid wall facing connection, is shown in Fig. 5. More complex structural elements, like the KeyStone T M retaining wall system shown in Fig. 6, can also be analyzed using eqn (4). The connection strength, CS, available at each geogrid elevation must provide sufficient resistance to counteract tension in the geogrid. The maximum tension at each geogrid layer varies and may be determined using conventional design methodologies. 5 However, the facing connection rarely experiences the maximum geogrid tension except at the lowest layers of geogrid reinforcement in the structure. This is due to the location of the Rankine failure plane that moves further away from the facing near the top of the structure. Therefore, this approach represents a conservative treatment for connection design.
Mechanical facing connections Specific geometric shapes and material composition of some structural elements dictate that frictional facing connections are ineffective or inappropriate. Geogrid reinforcement may be attached to these structural elements by means of a mechanical system that increases the effectiveness of the wall facing connection. The efficiency of the connection is increased through either mechanical bonding or a mechanical advantage system. Most bonding systems are comprised of cementitious materials. The geogrid may be embedded in precast or cast-in-place concrete. Usually, a minimum of 150--300mm (6-12in) of geogrid is tied to the existing reinforcing steel cage, sometimes using a bodkin interlacing procedure. The geogrid may also be incorporated between courses of common brick or masonry block, in the regular mortar bed. Mechanical advantage connections wrap the geogrid around a structural element and return the geogrid into the reinforced soil for additional resistance as shown in Fig. 7. This mechanical advantage system has been formed with fiat steel or wood batten strips and plastic or steel pipes, depending upon the requirements of structural facing element.
Connections for geogrid systems
.
545
GEOWEB@A4 STANDARD UNIT ~ COMPACTED ¢,r'OGRin NATIVE
~
~
•
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L ~ ' ~ , ~ , ~ E
AND ~E~,NO
L_ ~. ~L~..~~ Fig. 4. Geoweb T M cellular confinement system. 10' GALV. ST. SPIKE 5,/8" MINIMUM 3 PER 8' TIMBER R"y F,::\\ 4.5" 8" COMPACTEDDRAINAGE ~:~-~ED~ r ;i I" -I AGGREGATE LANDSCAPEX J ~ ~
TIMBER Jr'~llll L ~
PLYWOOD~,\I~,U A ' ~ . . GEOGRID BATTEN 1 / 2 ~ 1 H : : : I I J--~l E l l k=l 11 i. ~~ J ~ - ~ f-3~-k.h i u'TTLL~T~j jJJ_,3"~J. l FACING . -" ; : : J- (~ STRIP ~j --' " 1,0,,PENNY //~]~[ [~I l ~ l l \ . . COMPACTED
~
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Fig. 5. Railroad tie facing. FIBERGLASS DOWEL
KEYSTONE@ GEOGRID HOOKED STANDARDUNIT AROUND ..i.~--. . . . . . ~ COMPACTED FIBERGLASS ~~l.,,~J GEOGRID REINFORCED DOWEL____ ~ZAli~dlmaK~-~ SOIL
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Fig. 6. Keystone TM retaining wall system. 10" GALV. ST. SPIKE 3/8" MINIMUM 3 PER 8' TIMBER 6" X 8 " ~ " " " " ' ~ " RAILROAD ~ TIE ~ ~
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.
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AT SPACING Lq V/AI~~I ~ l l l l i l l l ~ - ~ - , ~ ; ,,T-,~_u.~,,,,.-=,,,.~,. ,~ DESIGNATED /t[]~ I I I l i / ~ .~=lil=llL~ll=l El BY E N G I N E E R / / I/It I | / \ / i m m m K \ COMPACTED SOIL
~ } } A \ ~ } } ~ ~NATIVE
Fig. 7. Treated landscape timbers.
546
Michael R. Simac
All the mechanical connections outlined above are geogrid, structural element, and bonding agent or mechanical advantage system dependent. Therefore, all mechanical connections to structural elements of wall facing should be tested in a laboratory to define the connection strength for use in design. Manufacturers of different geogrid reinforcement and structural elements for wall facing may provide test data from previous projects to assist a designer.
SUMMARY The principles governing anchorage of geogrid reinforcement to soil were presented as the basis to understand and analyze frictional connections of geogrids. A systematic calculation procedure is offered to analyze frictional connections of geogrids to other geogrids and to structural elements normally used for facing reinforced soil-retaining structures. Details for various frictional geogrid connections were illustrated. Mechanical connection of geogrid reinforcement to adjacent geogrid reinforcement and to structural elements used for reinforced soil-retaining wall facing was discussed in general terms. Several potential mechanical attachment methods for geogrids were described. Mechanical connections require laboratory testing to verify the efficiency and ultimate strength for the particular components of the proposed connection eliminating any potential for an analytical prediction of connection strength.
REFERENCES 1. Christopher, B. R. & Holtz, R. D. Geotextile Engineering Manual, Report No. FHWA-TS-861203, Federal Highway Administration, Washington, DC, 1986. 2. Mirafi Inc. Miragrid Booklet. Engineering test data and promotional literature, Mirafi Inc., Charlotte, NC, 1988. 3. Koerner, R. M. Designing With Geosynthetics. Prentice Hall, Englewood Cliffs, N J, 1986. 4. Jones, Colin J. F. P. Earth Reinforcement and Soil Structures. Butterworths, London, 1985. 5. Mirafi Inc. Miragrid Engineering Concepts, Design Methodology Reinforced Soil Retaining Walls. Mirafi Inc., Charlotte, NC, 1988.