Effects Of Groundwater Level Changes On Excavation Stability And Deformation

In excavation engineering, groundwater level changes may cause slopefailure and appreciable subsidence, which endangers buildings, structuresand underground pipelines in the vicinity. The effects of groundwater levelchanges on excavation slope stability and land deformation arerespectively discussed in this dissertation.A three-dimensional slope failure model is formulated using the upperbound limit analysis method. Under-toe failure and fold line slopes can beconsidered in this model; hydrological information is included andsituations with pore water pressure can be solved; besides, loading andreinforcement as well as anisotropy and heterogeneity with respect tocohesion of soil can be taken into account. A numerical program isdeveloped to deal with this model and obtain critical stability numbers ofslopes. The program is employed in a series of parameter studies andseveral conclusions are draw as follows: stepped excavation is beneficial tostability and gentle slopes are recommended; the larger the cohesion of soil,the more stable the slopes are; loading threatens slope stability while asmall load have no significant influences; in order to improve stability, it’sbetter to adopt long and frictional reinforcement which is set tilted.Four types of drawdown are taken to consider their effects onexcavation stability. The results show that drawdown outside slopesdecreases stability while that inside just the opposite. If both the drawdownhappens, the former effect is dominant. This means rapid drawdown insideexcavations is forbidden and sudden rise of groundwater table should beavoided. The stability of slope with groundwater table changes can be quantitatively reflected in the model and program, which can be applied inpractice.Based on Biot’s poroelastic consolidation theory, a consolidationmodel for saturated-unsaturated soil is formulated in axisymmetriccylindrical coordinates to predict land deformation of circular excavations.The heterogeneity and transverse isotropic properties with respect to bothhydraulic and mechanical properties are considered. Besides, the changesin saturation and relative hydraulic conductivity caused by undergroundwater flow and the variations in porosity and saturated hydraulicconductivity due to solid skeleton deformation are taken into account inthis model, so are underground structures. This model is applied innumerical software to simulate several groups of pumping-recovery testsperformed on circular excavations. It is concluded that:Diaphragm walls delay the water supply and restrict the soildeformation, pumping had better be taken after diaphragm walls have beenbuilt. Pumping rate should be controlled to prevent excessive subsidence.Soil deformations get smaller with larger Young’s modulus, whilegroundwater level changes get easier. As initial saturated hydraulicconductivity increases, soil deformations become smaller while itsinfluences on drawdown depend on the balance of groundwater pumpingand supply.