CHAPTER 10
STRUCTURAL DETAILS
10.1 STRUCTURAL DETAILS
To ensure the stability of GRS-RW and all structural components attached to it, appropriate considerations should be given to the structural details during design.
1. Fundamentals of Geosynthetic Layout
a) It is the basic principle that geosynthetic of the same quality to be used in a GRS-RW.
b) The minimum length of geosynthetic should be 1.5 m or 35% the wall height, whichever is the greater (Figure 10.1).
c) In accordance with the thickness requirement of each compacted soil layer, the geosynthetic vertical spacing is also selected as 30 cm. If buried structures are anticipated in the wall (Figure 10.2), the geosynthetic layout will be affected. The stability of the wall and facing should thus be analyzed with the buried structures.
d) It is the basic principle that the length of geosynthetic layers should be equal throughout the wall height.
If extra structures are expected on the crest (Figure 10.3) or if the wall will be subject to extra-ordinarily large lateral load, this rule does not apply. The global stability and facing stress analysis are required.
e) When the RRR is used at both sides of the structure, a few full-length geosynthetic layers are to be installed at a vertical spacing of 1.5 m (Figure 10.4).
f) In the case of embankment widening project, the geosynthetic layers are extended to the cutting edge of the slope at 1.5 m vertical interval.
If the original embankment is far from newly constructed wall, the layers are extended to a distance so that their edges form a line inclined at an angle equal to the soil internal friction angle (Figure 10.5).
At the interface of the pavement and soil, a full-length geosynthetic layer is recommended to increase its stability as well as to reinforce the pavement.
g) The geosynthetic joint is not made in the direction perpendicular to the facing (cross machine direction). In the case where joint results, a 50 cm overlapping is needed. The two sheets should be tied using strings or wires.
Along the longitudinal direction, the overlapping should be 10 cm (Figure 10.6).
2. Facing Foundation
The foundation supports the dead load of the facing. Figure 10.7 gives a schematic sketch of the foundation that satisfies external stability. For foundation resting above the rock, concrete is used to level the base.
3. Embedment Depth of Facing Foundation
The embedment depth has to be at least 40 cm.
If an excavation is anticipated in front of the wall after construction, such as for drainage channel, additional embedment depth has to be secured (Figure 10.8).
If future ground surface will be lower than existing surface, the ground on the backfill side should be excavated to the anticipated future level at where the geosynthetic layer will be placed. The embedment depth has to be extended 40 cm below this ground level.
4. Thickness and Slope of Facing
The thickness of facing is at least 30 cm and 20 cm for unreinforced and reinforced concrete, respectively (Figure 10.9).
The slope of the facing is recommended to be 1:0.05.
5. Joints
The vertical joint is constructed at less than 20 m interval with an appropriate material to account for possible expansion.
The joint to account for contraction is designed at 5 m interval by introducing a V-groove.
If the foundation is expected to give a large settlement, or the variation of temperature may be significant, the interval of joints has to be decreased. Under such circumstances, the steel reinforcement may be needed in the facing even though the requirement is not indicated in the stress analysis (Figure 10.10).
6. Facing Placement
To ensure adequate anchorage between the back of the facing and the geosynthetics, the forms are not used at the back of the facing (Figure 10.11).
In case where the presence of sound wall or electric pole gives large lateral load, or when the geosynthetic may jeopardize penetration of concrete, the anchorage needs to be reinforced.
7. Construction Joint
When unreinforced concrete is used, the reinforcing bars should be placed at the construction joint. The construction joint is normally perpendicular to the wall. The inserted bar should be at least of diameter 13 mm and length 90 cm. It is to be installed at a 30 cm interval (Figure 10.12).
8. Temporary Face Support
The wall has to be supported temporarily until the facing is constructed. This is done using gabions. The gabions render stability to the structure during construction and facilitate drainage after construction. They also act as buffer to the rigid facing and backfill soil, thus minimizing possible effects of relative settlement. Figure 10.13 shows the case where the gabions act as temporary face support.
If other suitable methods are available as temporary support, the gabions may not be required.
9. Drainage
In addition to the crush stones used in the gabions behind the facing, drainage pipes of diameter at least 6 cm are installed for every 2 to 4 m2 area (Figure 10.14).
When the gabions are used, the crushed stones automatically act as the drainage layer because of their high permeability.
If the cohesive soils are used as backfill, the geosynthetics having drainage capability, in addition to tensile reinforcement, are required. The woven-nonwoven composite geotextile is preferred. With further studies on the usage of cohesive soils, this application is expected to expand.
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