The in-situ material behind and below the existing retaining wall is a silty clay. This material retains a substantial amount of moisture and due to the high plasticity, loses bearing strength. The increased lateral pressure behind the wall in conjunction with the decreasing bearing pressure below the wall has caused the wall to begin to move. The solution has been to Install 21 micropiles through the existing footer and 24 tiebacks through the existing wall. The micropiles and tiebacks are tied together with a new retaining wall poured in front of the existing retaining wall.
Typically micropiles and tiebacks are filled with grout via a tremie pipe that uses gravity head to fill the borings. In this material, it was not possible to achieve enough bond strength of have an adequate factor of safety on this design. The micropiles had to be pressure grouted, which involves injecting grout into the boring at 100 to 150-psi. The tiebacks were pressure grouted and then post-grouted, which involves fracking the bond zone through a perforated tube at 600 to 1000-psi.