Seepage Characteristics And Numerical Simulation Under TRD Bottom-fall Curtain For Ultra-deep Foundation Pit

The number of the ultra-deep super-large foundation pits in Wuhan area is increasing recently.TRD(also known as construction method of diaphragm wall under equal thickness cement soil)has been introduced to separate the hydraulic connection between the pit and outside owing to the growing excavation depth,more complex surrounding environment and strict groundwater control requirements.There should be no hydraulic connection between the pit and outside theoretically.However,water is still found flowing into the pit continually during the precipitation process.It is generally believed that the leakage is caused by leakage of the sidewall and leakage at the bottom of the pit.Few researches on the leakage at the bottom of the pit are carried out presently while most of the researches focus on the leakage of the sidewall of the foundation pit.A lot of projects had shown that the amount of leakage at the bottom of the pit which must be paid attention to had been affecting the precipitation design of the foundation pit seriously.The deep foundation pit project of Wuhan Changhang Center Building was the first foundation pit project in Wuhan area using the TRD method.Based on the on-site pumping test and numerical simulation,the precipitation method of the deep foundation pit for Wuhan special geological structure was carried out.The fact showed that research ideas and routes were in line with the reality.The main research contents and conclusions were as the follows:(1)The previous researches results of the seepage theory of deep foundation pit and the principle of Two-dimensional seepage finite element theory were Summarized,and the application status of TRD falling bottom water curtain in deep foundation pit was analyzed;(2)Based on the foundation pit project of Wuhan Yangtze River Shipping Center,a systematic overall pumping experiment was carried out to study the seepage law of groundwater in deep foundation pits under the conditions of TRD falling bottom water curtain,which can also provide verification basis for analyzing the subsequent numerical simulation and precipitation design methods.The experimental results showed that the TRD ground wall had a positive effect on preventing groundwater from flowing into the pit from the side wall,but the groundwater could still penetrate into thepit through the bottom of the pit,and there was still a hydraulic connection between the foundation pit and outside;The law of the deepening of the water level inside and outside the pit was affected by other factors such as the water level of the Yangtze River and the location of the pumping well;(3)The concept of Equivalent Permeability Coefficient was proposed to characterize the permeability characteristics of the weathered rock formation comprehensively.The numerical simulation method was used to verify the calculation with the measured values of the experiment based on the SEEP/W software.The empirical formula for the design of the precipitation of the pit project was carried out finally which could provide a certain reference for the deep foundation pit precipitation of other similar projects in Wuhan area.