The reason is that MSE walls units which feature mechanical connections are generally designed to have sufficient connection capacity such that the geogrid design strength governs and not the connection capacity. Secondary reinforcement has generally not been used for MSE walls that rely upon a mechanical connection between geogrid reinforcement and MSE facing units. In these cases, the secondary layers do not connect to the facing. Secondary geogrid has also been used where it is placed behind larger MSE wall facing, such as with PCM units, gabion wire baskets, and rockery boulders. Secondary reinforcement has been used in frictional connection MSE facing that rely upon a connection based on friction of a geogrid layer inserted between the MSE wall facing. There are multiple types of MSE wall facing, such as segmental retaining wall (SRW) units, precast concrete modular (PCM) units, welded wire baskets, and rockery boulders. This redistribution reduces the connection demand of the primary geogrid layers. (See Figures 1 and 2.) The secondary geogrid reinforcement is used to redistribute high facing connection loads that occur from seismic loading and high overburden loads over a larger number of geogrid layers. Secondary geogrid layers are located at the face of the wall and do not extend for the entire length of the primary geogrid layers. In MSE walls, secondary geogrid reinforcement layers can be used between primary geogrid reinforcement layers. In allowable stress design of MSE walls, the LTDS is reduced by a factor of safety of 1.5. Reduction factors account for creep resistance, chemical durability and installation damage. The LTDS is determined by applying reduction factors to the geogrid ultimate tensile strength.

The long-term design strength (LTDS) of geogrid reinforcement is used in MSE design. The result is closely spaced geogrid layers at the face of the slope.

In slopes, the secondary geogrid layers are used to stabilize the slope face between the primary geogrid layers. – The concept of secondary geogrid layers located at the face of a mechanically stabilized earth wall is similar to the use of secondary geogrid reinforcement in MSE reinforced slopes.

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