Although it has been established that in-situ soil mixing has improved the bearing capacity of soils, additional research is needed to better understand the effect of soil mixing on lateral resistance of pile caps. To do this, in-situ soil mixing was used to strengthen weak clay adjacent to a pile cap of a driven pile foundation. The mass stabilization method or mass mixing was used to treat an 11 ft wide, 4 ft thick, and 10 ft deep zone consisting of an average 475 psf clay that was adjacent to a 9-pile group in 3x3 pile configuration capped with a 9 ft x 9 ft x 2.5 ft, 5000 psi concrete cap. The mass mixing involved 220 cubic ft of in-situ soil and was mixed with an additional 220 cubic ft of jet grout spoils producing a mixing ratio of 1 to 1. All of the mass mixing took place after construction of the pile caps. Laboratory testing of the mass mix slurry showed an unconfined compressive strength of 20,160 psf or 140 psi. Lateral load testing of the pile foundation was then undertaken. The results of this testing were compared with similar testing performed on the same foundation with native soil conditions. The lateral resistance of the native soil was 282 kips at a pile cap displacement of 1.5 inches, and the total lateral resistance of the pile foundation treated with mass mixing was increased by 62% or 170 kips. Of the 170 kips, 90% to 100% can be attributed to the increased passive force on the face of the mass mixed zone and shear on the sides and bottom denoting that the mass mixed zone behaved as a rigid block.



College and Department

Ira A. Fulton College of Engineering and Technology; Civil and Environmental Engineering



Date Submitted


Document Type





mass mixing, mass stabilization, soilcrete, lateral soil resistance, earthquake stabilization, soil mixing