The self-stability height of the foundation pit sidewall is an important criterion for evaluating the safety degree and designing the supporting structure. The strength reduction elastic-plastic finite element numerical calculation method has been adopted in this paper. Based on comparative analysis of the stability characteristics for deep foundation pit in binary strata of upper soil and lower rock under multiple working conditions, the potential fracture surface of deep foundation pit and the evolution law of corresponding safety factor have been revealed under different Hs and H. A new idea that the vertical soil sidewall height (Hs) and the vertical rock sidewall height (Hr) are used as two independent evaluation indexes, respectively, for deep foundation pit stability in binary strata of upper soil and lower rock has been put forward. The distribution characteristics and variation law of Hs0 and Hr0 under different Hs and different H have been revealed, respectively. The spatial distribution map of the self-stabilizing height for deep foundation pit vertical sidewall in upper soil and lower rock binary stratum has been constructed, and the mathematical fitting equation between Hr0 and Hs has been obtained. Finally, combined with the implementation effect of the deep foundation pit project of Ningxia Road Station for Qingdao Metro Line 3, the rationality of the conclusions is verified. The research results provide theoretical basis for quickly determining the self-stability characteristics of foundation pit vertical sidewall.
The moisture content of municipal sludge is relatively high, which increases the cost of sludge transportation and treatment. To reduce the volume of the sludge, sludge dewatering is needed. This paper proposes the theory of sludge dewatering and facilitates efficient and economical technology of sludge dewatering. Sludge dewatering tests were carried out by using homemade rapid sludge dewatering devices. There were two groups of tests with single- and double-drainage conditions, and all test runs were loaded with a negative vacuum pressure at the bottom. During the experiments, the vacuum degree and the pore water pressure in the sludge were monitored in real time. After the experiments, the data were compared and analyzed. At the initial stage, the sludge dewatering extent and the sludge dewatering velocity for double-drainage conditions were much higher than those for single-drainage conditions. The vacuum occurring for single-drainage conditions lagged behind that for double-drainage conditions in the sludge. The value of vacuum degree for single-drainage conditions was lower than that for double-drainage conditions, and the vacuum attenuation for single-drainage conditions was considerable. The excess pore water pressure for double-drainage conditions dissipated faster than that for single-drainage conditions in the sludge. The pore water pressure for single-drainage conditions at the top and middle of the sludge layer first increased and then decreased in the early loading stage, resembling the Mandel effect. Overall, with a vacuum negative pressure load at the bottom, the sludge dewatering efficiency for double-drainage conditions was much higher than that for single-drainage conditions. This study provides an experimental and theoretical basis for engineering applications in the sludge treatment industry.
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