Soil cushion versus deep soil mixing for settlement control of a multi-storey building in a seismic region

Multi-storey housing in the seismically active regions of Uzbekistan is increasingly built on weak, highly compressible soils, where excessive total and differential settlements generate an unfavourable pre-earthquake stress state in the structure and reduce its seismic reliability. This paper compares, under identical conditions, the efficiency of two ground-improvement strategies for a typical 12-storey residential building: a 6-m compacted soil cushion and deep soil mixing (DSM) with 231 soil-cement columns of 0.6 m diameter and 12 m length. Three finite-element models — unimproved base, cushion and DSM — were analysed in Plaxis 3D on a 200x200x40 m domain discretised into 8621 tetrahedral elements. On the unimproved base the maximum settlement reaches 6.46 cm with a pronounced non-uniform distribution under the raft. The cushion reduces the settlement to 5.6 cm (−13 %) by spreading the load within the upper layers, but leaves the deep compressible thickness unaffected. The DSM scheme stiffens the deep layers, transfers the load below the soft stratum and limits the settlement to 4.33 cm (−33 %). The two methods are shown to be complementary in depth, and the choice between them for seismic regions should follow the depth distribution of soil compressibility.

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