Qian-qing Zhang

Investigation into Skin Friction of Bored Pile Including Influence of Soil Strength at Pile Base

Traditional pile design methods of separately assessing the shaft and base capacities have been doubted by some model pile tests and numerical analyses. In this study, two case histories in China are investigated to look through the influence of the material strength beneath the pile base on the shaft resistance mobilized under working load and at failure. The case studies involve a series of full-scale load tests on long bored piles instrumented with strain gauges along the shafts. Comparative parameters include the formation of end-bearing soils and the craftwork of base-grouting, all attributing to one unified factor of material strength at the pile base. The field observations reveal that an increase in the material strength at the pile base puts significant and positive impact on mobilizing the shaft resistance. A simple analysis procedure based on soil mechanics principles is likely to capture the mechanism that the slip displacement over the whole shaft and the radial stress acting on the shaft at deep depth are essentially responsible for the phenomena. This study is an attempt to initiate design approaches with optimistic evaluation on the shaft capacity given that special techniques of enhancing the pile base strength have been made.

Analysis of Compression Pile Response Using a Softening Model, a Hyperbolic Model of Skin Friction, and a Bilinear Model of End Resistance

AbstractFor a single pile subjected to compressive load, a softening model and a hyperbolic model are adopted in this work to simulate the relationship between pile-soil relative displacement and unit skin friction developed along the pile-soil interface, whereas a bilinear model is used to describe the unit end resistance–displacement response developed at the pile tip soil. Based on the proposed three models, two simple analytical approaches for nonlinear analysis of the load-displacement response of a single pile are presented. One method uses a softening model of skin friction and a bilinear model of end resistance to capture the pile response, and the other approach is based on a hyperbolic model of skin friction and a bilinear model of end resistance to describe the pile load-displacement response. As for the analysis of the response of a single pile subjected to a compression load, a highly effective iterative computer program is developed using the two proposed approaches and the load-transfer cur...

Study on Longitudinal Deformation Profile of Rock Mass in a Subsea Tunnel

During tunnel excavation, the deformation of surrounding rock due to the unloading of rock mass will vary with time. However, the measured displacement of surrounding rock is only a part of the actual longitudinal deformation profile. There is a need to analyze the longitudinal deformation profile to identify the deformation state and evaluate the stability of surrounding rock mass. In the present article, the variation of pre-deformation of surrounding rock due to excavation was analyzed, and the release coefficient was obtained from the measured results. For the Qingdao subsea tunnel, the measured crown settlement of surrounding rock was analyzed using the regression analysis method, and the longitudinal deformation profile of rock mass was simulated using the numerical calculation method. Moreover, based on the conditions of the subsea tunnel, a solid-fluid coupling model test was carried out to check the reliability of the numerical calculation results. The results of the model test were consistent with the numerical calculation results.

Analysis and Design Implications on Stability of Cofferdam Subjected to Water Wave Action

Based on Power-Law nonlinear failure criterion and the upper bound theorem of limit analysis, a numerical solution was developed to analyze the shape of collapsing block and landslide thrust of cofferdam subjected to residual pore pressure which was caused by longstanding water wave action. In the upper bound analysis, the effect of water pressure assumed to be a work rate of external force was considered. To evaluate the validity of the present method, the results were compared with previous findings. The good agreement between the present results and the previous findings indicated that the proposed method was valid to assess the shape of collapsing block and landslide thrust of cofferdam subjected to residual pore pressure. As for the influence of the factors on the shape of collapsing block and landslide thrust, a parametric study was conducted. This parametric study was carried out to optimize the design of cofferdam supporting structures and improve analysis of the stability of cofferdam subjected to water wave action.

Parameters Optimization of Curtain Grouting Reinforcement Cycle in Yonglian Tunnel and Its Application

For practical purposes, the curtain grouting method is an effective method to treat geological disasters and can be used to improve the strength and permeability resistance of surrounding rock. Selection of the optimal parameters of grouting reinforcement cycle especially reinforcement cycle thickness is one of the most interesting areas of research in curtain grouting designs. Based on the fluid-structure interaction theory and orthogonal analysis method, the influence of reinforcement cycle thickness, elastic modulus, and permeability on water inflow of tunnel after grouting and stability of surrounding rock was analyzed. As to the water inflow of tunnel after grouting used as performance evaluation index of grouting reinforcement cycle, it can be concluded that the permeability was the most important factor followed by reinforcement cycle thickness and elastic modulus. Furthermore, pore water pressure field, stress field, and plastic zone of surrounding rock were calculated by using COMSOL software under different conditions of reinforcement cycle thickness. It also can be concluded that the optimal thickness of reinforcement cycle and permeability can be adopted as 8 m and 1/100 of the surrounding rock permeability in the curtain grouting reinforcement cycle. The engineering case provides a reference for similar engineering.

Analysis of the Response of an Axially Loaded Pile Considering Softening Behavior of Pile-Soil System

The skin friction degradation was analyzed by using a softening model, and the relationship between unit end resistance and pile end displacement was simulated using a hyperbolic model. Determinations of the parameters related to the softening model of skin friction were presented. As to the bearing behavior of an axially loaded pile, an iterative computer program was developed by using the proposed softening model of skin friction and hyperbolic model of end resistance. Comparisons of the response of an axially loaded pile demonstrate that the proposed method is generally in good agreement with the field-observed behavior, and can assess the failure characteristic of skin friction. For practical purposes, different load transfer functions can be selected in the proposed method to capture the response of an axially loaded pile.

Response of a Single Pile Subjected to Tension Load by Using Softening Models

This paper proposes a single exponential model and a double exponential model to simulate the relationship between pile displacement and unit skin friction developed at the pile-soil interface. Based on the proposed two models, two simple analytical approaches were presented for nonlinear analysis of the response of a single tension pile. A highly effective iterative computer program was developed by using the load transfer method. Comparisons of the present computed results, the calculated results derived from other analytical methods, and measured results were made to demonstrate the reliability of the proposed analytical methods.

A simplified approach for settlement analysis of single pile considering pile end settlement induced by skin friction in multilayered soils

A simplified approach for the nonlinear analysis of the load–displacement response of a single pile is presented using the load-transfer approach. In the present method, a hyperbolic model is adopted to simulate the load–displacement response of both the pile base and the shaft. Using the Mindlin displacement solution of the elastic half-space, the pile end settlement induced by the skin friction of pile is calculated. Based on the present analytical method, a highly effective iterative computer program is developed for the analysis of the response of a single pile. The validity of the present method is then demonstrated with the results of the load tests carried on instrumented piles. The calculated results indicate that at all loading levels, the proposed method is generally in good agreement with the well-documented field test results and the calculated results derived from other approaches. It is noted the pile head displacement obtained from the present approach increases with increasing failure ratio of skin friction and end resistance at the same loading level. It is also observed that the influence of the failure ratio of skin friction to the pile head displacement is slightly larger than the influence of the pile end resistance.