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Thrace-LINQ TECH NOTE #10
DESIGNING GEOTEXTILE RETAINING WALLS
IntroductionThe use of a Thrace-LINQ geotextile (good in tension and poor in compression) with soil (good in compression and poor in tension) allows a designer to build structures that, in the past, could only be handled by conventional gravity and cantilever wall systems.
Unlike rigid conventional systems that resist lateral earth pressure by virtue of their mass, geotextile retaining walls are built such that the embedded geotextile sections extend back into the soil. The embedded geotextile sections are restrained through frictional stresses mobilized between the geotextile and the soil. The backfill soil creates the lateral pressure on the wall face and interacts with the geotextile to resist the lateral earth pressure. Flexible earth retaining structures constructed of Thrace-LINQ geotextiles have proven themselves as cost effective alternatives to conventional methods. (See Figure 1)
Design MethodsThe most important consideration for geotextile retaining walls is to design for internal stability. Internal stability analysis determines lift thicknesses, fabric length and overlap. (See Figure 2)
To determine lift thicknesses, earth pressures are assumed to be linearly distributed using Ka (the coefficient of active earth pressure) for the soil backfill. Boussinesq elastic theory for live loads on the soil backfill is used. The following earth pressures result:
sh = shs + shq + shlwhere
sh =the total horizontal (lateral pressure) shs = the pressure due to soil shq = the pressure due to surcharge load shl = the pressure due to live loadTo determine shs,
shs =Ka gzWhere
Ka = tan2 (45 - f/2), in which f = the angle of shearing resistance of backfill soil g =the unit weight of backfill soil z = the depth from ground surface to layer in questionTo determine shq,
shq = KaqWhere
q = gD = the surcharge load on ground surface where D = depth of surcharge soilTo determine shl,
shl =P x 2z / R5Where
P = the concentrated load x = the horizontal distance load is away from wall R =the radial distance from load point on wall where pressure is being calculatedWhile the calculations for shs and shq are clear-cut, shl is more difficult to calculate, particularly for multi-wheeled truckloads where superposition of each wheel must be considered.
By taking a free body at any depth in the total lateral pressure diagram and then summing the forces in the horizontal direction, one obtains the equation for the lift thickness:
Sv = Tallow / shFSWhere
Sv =the vertical spacing (lift thickness) Tallow = the allowable stress in the geotextile sh = the total lateral earth pressure at depth considered FS = the factor of safety (usually 1.3 to 1.5)To determine embedment length, one must determine the total geotextile lengths by adding the length required for anchorage and the nonacting lengths of geotextile behind the failure plane. The following is provided:
L = Le + LRWhere
L = total fabric length Le = required embedment length (minimum of 3 feet) LR = length of geotextile behind the failure planeTo determine Le,
Le = Sv sh FS / 2 (c + gztand)Where
c = soil cohesion (zero if granular soil is used) d = the angle of friction between soil and fabricTo determine LR,
LR = (H - z) tan (45 - f/2)A conservative rule of thumb is to use an embedment length, L, equal to .7 the height, H, of the structure. Finally, the overlap distance, Lo,, can be determined:
Lo = Sv sh FS / 4 (c + gztand)Certain special considerations in determining Lo should be used. First, to determine the distance Z, measure to the middle of the desired layer. Second sh is not as large as previously illustrated, so 1/2 sh is used in the Lo calculation. Stress in reinforcement elements is greatest near the failure plane and falls off sharply to either side of the failure plane. A suggested minimum overlap, Lo, is 3 feet.
Next, you must consider external stability. This includes sliding and foundation failures. Overturning analysis is not included because internally reinforced geotextile retaining walls are not subject to the same forces as conventional, externally supported systems. For foundation failures, the designer must expect that his geotextile retaining wall is placed on a stable, firm foundation. Because of external stability force considerations, all geotextile layers should be of uniform length.
Example DesignDesign a 9 foot high geotextile wall that is to carry a storage area of equivalent dead load of 150 lbs/ft2. The wall is to be backfilled with a granular soil (SP) having properties of:
g = 105 lb/ft; f = 33o c = 0GTF 300, a woven slit film, polypropylene geotextile with an ultimate wide width tensile strength of 175/210 lb/in is intended for use. A)Determine the horizontal pressure as a function of the depth, z:
Ka = tan5 (45 - f/2) = = tan5 (45 - 33/2) Ka = 0.295 sh =shs + snh = Kagz + Kaq = (.295) (105) z + (.295) (150) sh = 31z + 44.3also, determine Tallowable of GTF 300:
Tall = Tult (1 / FScr FSid FScd)Where
Tall = long term design allowable load Tult =ultimate wide strip tensile strength FScr = partial factor of safety for creep potential FSid = partial factor of safety for installation damage FScd = partial factor of safety for degradation potential Tall = 210 (1 / 4 x 1.2 x 1.1)Note: FScr = 4 is used for this general condition. Default value of 5 is recommended by TF 27 for PP and FScr = 5 is recommended by Thrace-LINQ for critical structures.
Tall = 39.8 lb/in, or 477 lb/ftcalculate geotextile layer spacing:
Sv = Tall / sh (FS) = 477 / [31 (9) + 44.3] 1.3 = 477 / 420.3 Sv = 1.14so, 12 inch spacing for first lifts, 18 inch spacing starting at 6 feet B) Determine the length of the fabric layers and use d = 25o
Le = Sv sh (FS) / 2 (c + gztand) = Sv (31z + 44.3) 1.3 / 2 (0 + 105 ztan 25o = Sv (31z + 44.3) / 75.3z LR = (H - z) tan (45 - 33/2) = (9 - z) (.543)
|Layer #||Depth z (ft)||Spacing Sv (ft)||Le (ft)||Le min (min)||LR (ft)||L (ft)|
Note the Le values for geotextiles are almost always smaller than the minimum required value of 3.0 ft.
C) Check the overlap length Lo:
Lo = Sv sh (FS) / 4 (c + gz tan d) = Sv [31(z) + 44.3] 1.3 / 4 (0 + 105z tan 25o)
The required overlap will be greatest at the upper layer, 1.5 feet:
Lo = 1.5 [31 (1.5) + 44.3] 1.3 / 4 [ (105) (1.5 tan 25o)] Lo = 0.603, so use 3 foot minimum valueD)Determine external stability:
To resist sliding forces and bearing capacity failures at the foundation, Steward et.al. (1977) recommend that the embedment length remain constant.
E) Final Comments
GTF 300 is available in 12.5', 15', and 17.5' widths. Utilizing the 12.5' width, you can increase your L to 8 feet. For the 12 inch lifts, increase your overlap with the excess 6 inches to 3.5 feet.
Final lifts could actually be increased beyond 18 inches, but it is Thrace-LINQ's experience that geotextile wrapped faces are difficult to construct past 18 inches. If timbers or a modular block system is included, lift thicknesses should be revisited.
Finally, the face of geotextile wrapped retaining wall should be protected from UV degradation. This can be accomplished by shot-creting or spraying a bitumen over the face. Thrace-LINQ recommends you use a bitumen application because it is flexible like your structure.
Looking to the FutureRecent reinforcement design methodology with regard to geotextile retaining walls is very conservative. For example, current accepted methods believe creep is always an important consideration, and geotextile retaining walls develop full Rankine-type pressure distributions on facing elements. Work performed for the Colorado Department of Transportation by the University of Colorado suggests that facing elements such as timbers or modular blocks only receive bin pressures. Bin pressures are generated by small triangle forces between the reinforcement layers. Also, materials used for reinforcement in granular soils reach an equilibrium state where the soil will not allow the geotextile to creep. In granular soils, this suggests that allowing for creep in the geotextile is not an important consideration. Advantages of geotextile retaining walls over conventional gravity walls are numerous:
- flexible wall system
- minimum to no excavation required
- no corrosion problems
- backfill can contain fines
- unskilled labor can be used
- no heavy equipment is required
- cost per square foot of face is very low
With Thrace-LINQ's full line of geotextiles, a designer is assured of a sound retaining wall utilizing the highest quality geotextiles available in today's marketplace.
RETAINING WALL CONSTRUCTION SEQUENCE
The U.S. Forest Service has developed the following construction sequence: A wooden form of height slightly greater than the lift height is placed on the ground surface (or on the previously placed lift after the first layer is completed). This form is nothing more than a series of metal L brackets with a continuous wooden brace board running along the face of the wall.
- The fabric is then unrolled and positioned so that 3 ft. (1 m) extends over the top of the form and hangs loose. If sufficiently wide, the fabric can be unrolled parallel to the wall. This will depend on the required design length and fabric strength, which will be discussed later. If a single roll is not large enough, two of them can be sewn together. Alternatively, the fabric can be deployed perpendicular to the wall and adjacent strips can be overlapped or sewn. In this way the fabric's machine direction is oriented in the maximum stress direction.
- Backfill, preferably free-draining granular soil but not necessarily so, is now placed on the fabric for approximately three-fourths of its lift height and compacted. This is typically 9 to 18 in. (20 to 40 cm) and is done with conventional light earth-moving equipment.
- The loose end of the fabric (i.e., its "tail") is then folded back over the wooden form over the windrow.
- The remaining lift thickness is then completed to the planned lift thickness and suitably compacted.
- The wooden form is then removed from in front of, and the metal brackets from beneath, the lift and is reset on top of it in preparation of the next higher lift. Note that it is usually necessary to have scaffolding in front of the wall when the wall is higher than 5 or 6 ft. (1.5 or 1.8 m).
Christopher, Barry R., Holtz, Richard D., Geotextile Engineering Manual, Federal Highway Administration, Washington, D.C., 1985.
Koerner, Robert M., Designing With Geosynthetics, Prentice Hall, Englewood Cliffs, NJ, 1990.
Lee, K.L., Adams, B.D. and Vagneron, J.M.J., "Reinforced Earth Retaining Walls," J. Soil Mech. Fdtn. Eng. Div., ASCE, No. SMID, October 1983, pp. 745-764.
Whitcomb, W. and Bell, J.R., "Analysis Techniques for Low Reinforced Soil Retaining Walls," Proceedings 17th Eng. Geol. Soils Eng. Symp., Moscow, ID, April 1979, pp. 35-62.
Steward, J.E., Williamson, R., and Mohney, J., "Earth Reinforcement," Chapter 5 in Guidelines for Use of Fabrics in Construction and Maintenance of Low Volume Roads, U.S. Forest Service, Portland, OR, June 1977.