Studies Of The Dewatering Effect On Deep Excavation And Surroundings In Soft Clay

Dewatering is necessary in the excavation under the ground water level in softclay, which provide a dry enviroment for the excavation and is benefit for the slopestability and reducing the harm induced by the groundwater. But dewatering canmake the seepage field more complex around the excavation,error may come intobeing if the seepage field can not be analyzed correctly.Dewatering have importanteffect on the behaviour of the foundation pit and the surrounding soils, seepageinduced by dewatering from outside to the inside of the foundation pit have effect onthe stability and the deformation of the foundation pit and surface settlement for thevertical consolidation by the underground-water drawdown out of the foundationpit. So it is very important to analyse and estimate the dewatering effect on the deepexcavation and surroundings.The primary research work of this paper is as follows:1. Based on the 46 case histories and result of the finite element methodcalculation, the deformation characteristic of the deep excavation engineering in softclay is analyzed,which include the characteristic of the maximum lateral deflectionand basal heave and surface settlement.The deformation control criterion is alsostudied.2. The effect of preconsolidation pressure by dewatering on the soil in thefoundation pit is analyzed, the harm of the seepage to the stability and the seepagefield′s characterize is analyzed also,The earth pressure due to dewatering isinvestigated.The formula of the critical thickness of the soil layer withstandingpressure water in the columniform foundation pit is also derived.3. The density of soil and stress increment induced by dewatering around thefoundation pit is investigated,the behaviors of settlement induced by dewatering wereanalyzed by FLAC program and three main effective factors such as Young'smodulus, dewatering depth and permeability were taken into account. A simplifiedmethod for calculating settlement by the dewatering is derived, the characteristicsand the rule of settlement induced by the dewatering is also analyzed. 4. Numerical analysis of considering consolidation and interaction between theexcavation support and the soil by using FLAC3D software in the super deepexcavation of the underground transformer substation for the world exposition ofshanghai 2010. The results not considering dewatering are compared with the resultconsidering dewatering, including wall deflections, basal heave, surfacesettlement, The analysis of considering leakage of the wall and leakage betweenconfined aquifer is provided also. The analysis shows that although the difference issmall in soft clay due to the low permeability of the soil, dewatering enhance thedeformation of the foundation pit and the foundation pit is inclined to be not securityif dewatering is not considered, the effect of leakage of the wall can be obvious onthe surface settlement.In the end, this paper study the dewatering effect on deep excavation andsurrounding, especially the surface settlement induced by excavation and dewateringand provide some reasonable and efficient support for the design and construction ofthe foundation pit.