Layers of geosynthetic reinforcement are used to stabilize slopes against potential deep-seated failure using horizontal layers of primary reinforcement. The reinforced slope may be part of slope reinstatement and (or) to strengthen the sides of earth fill embankments as shown in Fig. 7.11.

The reinforcement layers allow slope faces to be constructed at steeper angles than the unreinforced slope. It may be necessary to stabilize the face of the slope (particularly during fill placement and compaction) by using relatively short and more tightly spaced secondary reinforcement and (or) by wrapping the reinforcement layers at the face. In most cases the face of the slope must be protected against erosion. This may require geosynthetic materials including thin soil-infilled geocell materials or relatively lightweight geomeshes that are often used to temporarily anchor vegetation.

Fig. 7.11 Geosynthetic reinforced soil slope over stable foundation. |

O Fig. 7.12 Circular slip analysis of reinforced soil slope over stable foundation. |

The location, number, length and strength of the primary reinforcement required to provide an adequate factor-of-safety against slope failure is determined using conventional limit-equilibrium methods of analysis modified to include the stabilizing forces available from the reinforcement. The designer may use a “method of slices” approach together with the assumption of a circular failure surface, composite failure surface, two-part wedge or a multiple wedge failure mechanism. Figure 7.12 shows circular slip analysis of reinforced soil slope over

stable foundation. The reinforcement layers are assumed to provide a restraining force at the point of intersection of each layer with the potential failure surface being analysed. A solution for the factor-of — safety using the conventional Bishop’s Method of analysis can be carried out using the following equation:

where Mr and Md are the resisting and driving moments for the unreinforced slope, respectively, a is the angle of tensile force in the reinforcement with respect to the horizontal, and Tallow is the reinforcement maximum allowable tensile strength. Since geosynthetic reinforcement is extensible the designer can assume that the reinforcement force acts tangent to the failure surface in which case RT cos a = R. The potential failure surfaces must also include those passing partially through the reinforced soil mass and into the soil beyond the reinforced zone as well as those completely contained by the reinforced soil zone. Figure 7.13 shows the use of geosynthetics as a primary reinforcement of a slope and the completed reinforced embankment.

The construction of embankments on soft soils can be a challenging task. The use of geosynthetics to improve embankment stability is one of the most effectives and well-tried forms of the soil reinforcement technique. Figure 7.14 shows four examples for the use of geosynthetics for the improvement of embankment stability over soft soils. In case of

Fig. 7.13 Use of geosynthetics as a primary reinforcement of a slope and the completed

reinforced embankment.

Fig. 7.14 Use of geosynthetics for the improvement of embankment stability over soft soils. |

limited reinforcement effect, the so-called “basal reinforced piled embankment” can be used. Prefabricated piles or improved soil piles can be employed, as shown in Fig. 7.14(d).

The stability level of a reinforced embankment on soft soil can be evaluated by the definition of safety factors (Fs), as shown in Fig. 7.15:

• For overall stability

where MD — soil driving moment

MR — soil resisting moment AMr — geosynthetic contribution against failure

• For stability against sliding failure

active thrust from the fill (from active earth pressures)

Fig. 7.16 Uses of geosynthetics as reinforcement for the improvement of

embankment stability.

The efficiency of geosynthetics as reinforcements of embankments on soft soils can be visualized by the following figures. Figure 7.16 shows applications of geosynthetics for the improvement of embankment stability over soft soils.