Study On The Over-consolidation Effect Induced By Excavation Process

With the development of urban construction,the environmental effect of excavation is increasingly significant,therefore,it is essential to study deformation properties of soils during excavation in order to estimate deformation of excavation and take effective control measures.In soft soil areas,dewatering and excavation are frequently used in pit construction,which cause repeated loading and unloading of soils.The over-consolidation effect induced by excavation will influence the deformation and mechanical properties of soils below excavation base.In this paper,SLB-1 type triaxial shearing and permeability apparatus controlled by stress and strain was used to study the soils at different depths below excavation base during dewatering and excavation.The undrained shearing tests were done after the stress path tests.The results indicated that the influence of dewatering and excavation on soil deformation was smaller as the soils became deeper and the influence depth of the construction concerned in this paper was approximately 2.2 times of the excavation depth.For the first dewatering step the settlement of soil decreased with depth,which showed the deformation modulus increased with depth.The increasing of deformation could be related to the under-consolidation effect.And the deformation modulus also increased with depth during excavation process because the total resilience of soil decreased with depth.The deformation modulus was becoming smaller and smaller with the process of excavation and the change could be related to over-consolidation effect.The changing trend and rate of deformation modulus were closely related to the basic physical and mechanical properties of soils.The influence of over-consolidation effect could be expressed in the linear relationship between deformation modulus E ? 1/(OCR-1)for excavation path.In addition,strain softening was found in the un-drained shearing test,which also supported the effect of over-consolidation induced by excavation.Based on the influence of over-consolidation effect on deformation modulus,the numerical simulation software PLAXIS was used to study the resilience of soils,deformation of retaining structures and ground settlement during excavation.The results showed that the over-consolidation effect caused the increase of deformation and the change of deformation distribution,so it is essential to consider the over-consolidation effect induced by excavation when studying deformation in excavation.The over-consolidation effect during excavation has an effect on deformation properties of soils,therefore,the deformation results of soils and retaining structures will be totally different if over-consolidation effect is taken into consideration.When estimating the deformation of excavation and establishing control measures,there is a need to consider the dynamic changing process of over-consolidation effect of different depths during excavation and the variation of deformation with the over-consolidation ratio for different stages and positions.