Mobilised mass properties of embedded improved soil raft in an excavation

Abstract

An embedded improved soil raft is a layer of short overlapping soil-cement columns that are formed by jet grout piling or deep cement mixing which is often used to stabilise an excavation in soft soils. It is often installed below excavation formation level prior to excavation. As excavation proceeds, an embedded improved soil raft would be subjected to lateral compression from the inwards moving retaining walls. Thus, the mobilised mass properties in the lateral direction, rather than the material properties from elemental cores, are of direct importance in controlling the wall deflections and the associated ground movements.

In this research, numerical simulations are employed to examine in a systematic way various influencing factors, such as layering, overlapping, combined loading of lateral compression and basal uplifting, non-perfect treatment and holding piles, which affect the mobilised mass properties of an embedded improved soil raft. The analysis starts from two soil-cement columns that are assigned with linearly elastic material model, arranged just in contact with each other and being compressed laterally. Subsequently, various assumptions in the initial model are gradually relaxed so that the model can take into account other influencing factors. They are geometry arrangement, layering, lateral compression and basal uplifting, thickness of soil raft, non-perfect treatment and holding piles. Throughout this study, calibration and verification are carried out to check the numerical results against analytical solutions or field back-analysed data when possible.

The analysis shows that when the soil-cement columns are arranged in point contact, the mobilised mass stiffness is very low. This observation is echoed by reported field back-analysis of deep excavation in soft soil that is stabilised using embedded improved soil raft. The mobilised mass stiffness can be raised by introducing some degree of overlapping among neighbouring soil-cement columns. The soil-cement columns formed in the field often have layered properties and it is shown in this study that the outer layers are more important than the inner ones in determining the mobilised mass properties.

The analysis in this study shows that the uplifting pressures cause little changes in the magnitude of the mobilised mass stiffness but reduce the threshold mass strain where the mobilised mass stiffness starts to drop. The threshold mass strain is also affected by the thickness to length ratio $T/L$ of the soil raft as well as holding piles, if present. To extend the threshold mass strain, it is necessary to increase the $T/L$ ratio or to provide holding piles. But the beneficial effects of holding piles on extending the threshold mass strain depend on qualities of the interface zones between holding piles and soil raft. It is also shown that the impact of non-perfectly treated zones are dependent on the number of such zones and more importantly how these zones are distributed over the soil raft.