Xiaotian Liu

Laboratory model tests on water inrush in foundation pit bottom

Numerous deep foundation pits were constructed in China in the course of urbanization. Water inrush is one of the most important causes of foundation pit accidents. At present, few images and detail courses of the water inrush have been recorded and reported. The evidences for the hypothesis of water inrush calculation are not enough. In this study, model tests were performed to verify the water inrush course in foundation pit bottom. The water inrush modes of foundation pits were analyzed for the aquitards that included clay and silty clay in Shanghai, China. The deformation and failure characteristics of the layers under different water pressures were obtained. The deformation course without water inrush was divided into three stages: continuous deformation, progressive deformation, and equilibrium stages. The deformation course with water inrush was divided into four stages: continuous deformation, progressive deformation, shear failure, and water inrush and sinking stages. The course included creep deformation and micro-cracks development before failure. It was not an instantaneous phenomenon but a time-depending one. The pore water pressure was the response of aquitard to boundary water pressure, which indicated the seepage in low-permeable aquitard and the development of micro-cracks. The water inrush formula was verified and tested using the model test results. The limit equilibrium method had larger safe reserves, and the prestressed homogeneous continuous-beam method was relatively accurately.

Tectonics Dominated Hypogene Karst Geo-Hazard Mechanism Models in China’s Tunnel Construction

Hypogene karst was believed to develop only in small scale in China. However, in construction of China’s deep tunnels, large-scale hypogene karst and corresponding geo-hazards were encountered frequently. The hypogene karst uncovered by deep tunnel construction probably induce water inrushes, mud bursts, karst collapses and even geo-hazards chains. Understanding the potential geo-hazard mechanism induced by hypogene karst is significant for both karst tunnel construction and hypogene karst evolution research. Typical cases of hypogene karst geo-hazards encountered during construction of China’s deep tunnels such as Zhongliangshan tunnel group, Yuanliangshan tunnel, Yesanguan tunnel and Dayaoshan tunnel, were presented. Large-scale fold controlling model, large-scale water-rich fault model and complex tectonic deformation model based on tectonic deformation patterns and intensity were proposed and analyzed for the mechanisms of geo-hazards of hypogene karst in the tunnel construction. It can act as a model reference for hypogene karst geo-hazards prevention and control in karst tunnel construction.

Liquefaction behavior of dredged silty-fine sands under cyclic loading for land reclamation: laboratory experiment and numerical simulation

Medium-coarse sands (CS) were dredged and exhausted in land reclamation. However, the remaining silty-fine sands (FS) were wasted. The liquefaction behavior of dredged silty-FS and the possibility of utilizing the remaining silty-FS as dredger fill source for land reclamation should be investigated. Cyclic consolidation-undrained triaxial tests were performed to investigate the liquefaction resistance of dredged silty-FS under different influencing factors. The cyclic stress ratio (CSR) of dredged silty-FS increased with the increase in initial relative density and consolidation stress ratio and decreased with the increase in silt content and consolidation stress. The CSR first decreased with the increase in clay content up to a threshold value and increased with the increase in clay content. A regression model was created to estimate the relationship between CSR and silt content, clay content, initial relative density, consolidation stress, consolidation stress ratio, and cyclic resistance ratio. Response surface methodology (RSM) was employed to investigate the mutual influence among the five independent variables. On the basis of cyclic triaxial tests, particle flow code models were introduced to investigate the microscopic internal fabric changes of dredged silty-FS and the influence of extended factors on liquefaction. The average microscopic contact force and coordination number between particles controlled the macroscopic mechanical behavior of sands. Sand liquefaction was due to the cumulative loss of coordination number under cyclic loading. The average contact force between particles was linearly decreased to 0 and the coordination number sharply decreased when the sample reached initial liquefaction. On the basis of numerical tests, CSR increased with the increase in D50 and vibration frequency. The influence of vibration frequency was relatively small. In addition, the CS–FS and CS–FS–CS combination layers showed greater liquefaction resistance than the FS layer. In the filling process, the interbed of FS and CS improved the liquefaction resistance of dredged silty-FS to a certain extent.

Estimation model of sandy soil liquefaction based on RES model

In order to evaluate sandy soil liquefaction, Rock Engineering Systems (RES) was utilized to establish the estimation model of sandy soil liquefaction. Aiming at unascertained factors in the analysis of sandy soil liquefaction evaluation, earthquake magnitude, maximum ground acceleration, the value of standard penetration test, specific penetration resistance, relative density, mean particle size, and water table were selected as influencing factors of sandy soil liquefaction. The interaction matrix was utilized to describe the interaction among the factors. Expert semi-quantitative (ESQ) method was employed to code the interaction matrix. The interaction strength and dominance were analyzed by cause and effect diagram. Finally, the estimation model-based RES was proposed, and the results were in line with actual situation well. In addition, the improved estimation model-based RES was also presented and discussed, which was more convenient with the same prediction accuracy rate.

Critical distance between copper mining roadway and a vertical water-blocked body

It is vital to determine the critical distance between a mining roadway and a natural vertical water-blocked body (VWBB) where a substantial water level difference and seepage gradient have developed between the two sides of the VWBB. A VWBB critical distance was defined and then determined using a 3D numerical simulation method based on the strength reduction method. The primary value of the VWBB critical distance was determined in consideration of the potential equivalent plastic strain zone induced by the roadways. In consideration of the other influencing factors, the VWBB critical distance was suggested as 105.00 m.

Microscopic Mechanism Analysis on Liquefaction of Dredged Silty Sand in Land Reclamation Based on Discrete Element Method

Cyclic consolidation-undrained triaxial tests were performed to investigate the liquefaction behavior of dredged silty sand in land reclamation. The liquefaction behavior of dredged silty sand in the triaxial tests was simulated using discrete element method. Constant volumetric method and initial liquefaction standard were introduced in the simulations. The micro-parameters of dredged silty sand were inversed. The liquefaction micro-mechanism of dredged silty sand was analyzed based on the numerical simulations. The pore water pressure of samples reached effective consolidation pressure and average coordination number decreased quickly when the initial liquefaction was reached.