Study on Influences of Foundation Pit Excavation and Precipitation on Stresses in Support Structure and Land Deformations
|School||Taiyuan University of Technology|
|Keywords||Deep foundation pit works seepage water head boundary Typeof embedding mode waterproof curtain support structure and ground deflection|
Local variations of hydrogeological conditions due to precipitation and excavation, support structures and ambient environment constraint and interact each other during design and construction of deep foundation pit works at sites with high water level.First of all, this article sorted the low-terrace strata in Fen River, Taiyuan city, and classified the foundation pits into three embedding modes according to stratigraphic variation rule, particle size composition, permeability property and different water permeability when the curtain bottom might be embedded into layers. For each mode, it presented its characteristics of seepage variation and the respective influences on support structure and ambient environment.This article still systematically sorted and analyzed monitoring information for the deep foundation pit works of Zhonghuan No.1residential project in Taiyuan, China; and makes in-depth study on interactions among precipitation-reinjection combinations, variation in seepage condition, support structure and ambient environment through numerical simulation using Flac3D Cysoil model based on Biot’s consolidation theory; studies laws of influences of variations in water table, soil properties, permeability coefficient and reinjection condition on land subsidence around the foundation pit. Finally, in-depth studies have been made for different methods to test permeability coefficient and soil mechanical properties before and after precipitation..The main conclusions from above studies are as follows:1. The Cysoil model for material constitutive relationship is a derived Mohr-Coulomb model, the laws adopted are incremental elasticity rule, failure criterion and flow criterion. Studies demonstrate that, this model integrates precipitation-reinjection-induced seepage variation and soil mechanics constructively, coupled with Biot’s consolidation theory, it is able to reflect interaction between groundwater seepage characteristics and excavation stress change very well. By applying this model, simulation of ground subsidence can match up to90%, the horizontal displacement of the underground continuous wall can match up to60%to80%, and the support of horizontal support structures can match up to above80%, all of the above studies prove that this model is suitable for foundation pit excavation and precipitation works studied in this paper.2. Through simulation of the change of each hydrological parameter, we can see that:1) The laterally fixed hydraulic head applies to the first and second types of embedding modes, while the top-fixed hydraulic head applies to the third type of embedding mode.2) The mode of multi-step precipitation plus reinjection is most favorable for control of ground deformation. Reinjection is an effective means to control ground subsidence. It is optimum to place a reinjection well within depths0.5-1.0times the excavation depth of foundation pit.3) In order to evaluate attribution of depth of recharge well, recharge pressure, number of recharge well and curtain depth to the ground subsidence, the parameters and different combination of parameters have been provided according to orthogonal level principle, and therefore the optimum recharge combination can then be selected.4) Owing to precipitation, the curve of subsidence outside the foundation pit is "concave" shaped:The segment from foundation pit edge to the maximum subsidence point is logarithmic curve, while the segment from the maximum subsidence point to the outermost boundary of precipitation scope is linear relationship. A calculation formula for permeability coefficient versus ground deformation outside foundation pit in the second type of embedding mode was given.5) Compared with the case of not taking seepage effect into account, the case of taking seepage stress field into account shows ground subsidence decreasing by50%, pit bottom upheaval rising by35%, horizontal displacement of support structure increasing by30%, and axial force of foundation pit support structure increasing by15%. It is therefore necessary to take seepage into account during design.6) The wall horizontal displacement is limited by horizontal lateral braces, and a plump buckling shape smaller at two ends and larger at the center appears at one section. The maximum horizontal displacement of wall occurs at a distance of17m below the wall top, the horizontal displacement increment gets larger at excavation depth of8-17m, the interval with excavation depth being0.4-0.85times depth of foundation pit is the weakest portion of support structure and the key position of monitoring and supporting.7) The maximum land subsidence curve and elasticity modulus curve roughly exhibit fold-line distribution. With increase in soil modulus, the slip surface of soil deformation gets steeper and steeper with increasing slope, and the influencing scope of soil deformation is reduced. However, the increase in soil modulus has not much impact on deformation of support structure.3. From the tests and studies of hydrological parameters, we can see that:1) This article takes into account such factors as disturbance of soil sample during sampling, depth of borrow pit, stress release of soil sample, dissipation of pore water pressure, and weight of permeability coefficient on soil layer, and put forward a correction formula for permeability coefficient measured in laboratory test.2) The precipitation process is equivalent to a preloading effect of soil, which change the effective stress in soil, and after precipitation, compression modulus Es1-2increases by25%, cohesive force rises by33%, and internal friction angle increases by8%.3)"Hysteresis loops" appear for all soil samples in multi-stage high pressure consolidation and rebound test, in which slopes of unloading curves are not consistent, but rebound paths are almost a group of parallel lines.The research achievements in this article are obtained from a study on the support system of Zhonghuan No.1residential project in Taiyuan City, China; they have certain reference and guidance values for design and construction of foundation pit excavation works on the first-level terrace of any high-water-level river, and are of certain significance to the theoretical research of interactions among the seepage field in a large deep foundation pit, support structure and ambient environment.