We discussed in previous post that soilcrete column can be constructed either by deep soil mixing or jet grouting. We have learned that deep soil mixing may be wet mix and dry soil mix. Now we will learn about jet grouting techniques to form soilcrete column to strengthen soft or weak foundation soil to have a better founding medium to transfer structural loads.
This technique is used for stabilization of tunnel, underpinning purposes especially in confined condition, bottom seals for excavation, supporting utility premises. To strengthen existing older structure to comply with recent earthquake codes or to solve settlement problem of existing buildings, low headroom drilling rig is generally used.
Now we will learn about grouting techniques. This is a process where cement slurry, named grout, is injected into ground to replace or erode the existing soft soil with the grout (as termed before). Holes of (4-8) inch diameter are drilled in to ground to designed depth. After reaching desired depth grouting is injected through a system of nozzles attached to drill rods near bottom of it. Regarding depth limit, it can be said that 120’ depth is typical but a greater depth can be used to injecting grouts.
Through these nozzles a pressure up to 500 psi is used to pump a mixer of grout with water or air. As the pumping is progressed, the drill rods are withdrawn rotating slowly from the hole. This procedure forms a typical (3-10) ft diameter soilcrete column. These rigid soilcrete columns show average compressive strength in confined condition ranging from (250-1250) psi.
As we discussed earlier and in previous post that soilcrete is a technique to strength soil against foundation settlement and increasing bearing capacity to mitigate earthquake generated liquefaction hazards. The application of drilled shaft and piling or by replacing existing weak soil, we can also mitigate such hazard. But soilcrete is a cheap way to serve these purposes. The foundation engineers should consider this option before selecting more expansive solution as described above.