Mass behaviour of embedded improved soil raft in an excavation

Abstract

In deep excavation in soft soils, a layer of soil below formation level is often improved to stabilise the excavation. The improvement, usually by deep cement mixing or jet-grout piling, will result in the construction of a raft of overlapping short columns (termed an 'embedded improved soil raft'). Soil investigation carried out will usually provide information on vertical cores from such columns. But during excavation, the columns are loaded laterally by the inward-moving retaining wall: thus the mobilised mass properties in the lateral direction are of importance in the design. This paper examines the mechanisms of how mass properties are mobilised and their relation to the elemental properties, the variation of properties within a column, and how the columns are arranged. The analysis and the simulation of a reported field case history show that for soil-cement columns arranged just in contact with each other, the mobilised mass stiffness is less than 28% of the elemental material stiffness. The analysis also shows that the properties of the outer layer have a greater impact on the mass behaviour of the embedded improved soil raft than those of the inner layer, and this ought to be considered in any site investigations.