Abstract: In the excavation engineering, the soil settlement outside the excavation caused by pressure reduction in the excavation is closely related to the soil properties and stress state of the aquifer. However, the soil state and the settlement regularity of confined aquifers in Tianjin still lack of systematic research. Firstly, the stress states of the soils in the first and second aquifer groups of Tianjin are analyzed through the historical groundwater level changes. On this basis, combined with two groups of field pumping tests, the settlement and deformation laws of confined aquifers in the process of dewatering and recovery are analyzed. According to the analysis of historical data, the upper part of the first aquifer group in Tianjin is in the normal consolidation and light over-consolidated state affected by recharge and natural conditions. However due to the over-exploitation from 1960s to 1980s, the drawdown cone centered on the urban area was formed in deep aquifers of Tianjin, which caused serious surface subsidence. Now the groundwater level has been partially recovered due to the limitation of exploitation from 1980s. Therefore, the bottom of the first aquifer group and the second aquifer group are seriously over-consolidated. Based on the analysis of the pumping test results, the results show that the deformation of the first confined aquifer in the normal consolidation state is mainly plastic deformation during the pumping-recovery process, while the second confined aquifer in the over-consolidated state is mainly elastic deformation; and the plastic deformation will still occur in the cycle pumping-recovery process. When the groundwater level was recovered, the settlement of the soil cannot be fully recovered. Meanwhile, affected by “soil arching”, the location of the maximum settlement caused by the aquifer pumping was not on the ground surface, but near the aquitard over the pumping aquifer.

Key words: historical groundwater level, foundation pit dewatering, over-consolidation, plastic deformation