考虑浅埋破坏的拖曳锚在黏土中安装运动特性分析

拖曳锚是海洋工程中一种常见的系泊基础,因造价低廉和高承载特性而得到广泛应用.其在海床中的安装轨迹和运动特性受到锚与土体之间复杂相互作用的影响,使得精确定位仍存在挑战.目前已有的塑性屈服面方法被广泛用于计算拖曳锚的运动特性,即假设整个拖曳过程为深埋板在不同深度的破坏过程.实际上,拖曳锚的安装是从浅埋到深埋的连续贯入,因此该方法不能考虑浅埋破坏对拖曳锚运动特性的影响,从而可能导致预测的轨迹不准确.通过有限元分析研究了锚板方位角及埋深比对单向承载和复合荷载下屈服面的影响,确定了锚板浅埋破坏时的屈服面,补充了塑性屈服面法对浅埋破坏效应的考虑;进一步地,考察了锚板方位角、承载系数、浅埋区域大小对拖曳锚轨迹预测和运动特性的影响,并与传统仅假设深埋破坏情况对比分析.结果表明:浅埋破坏时锚板方位角与浅埋区域大小决定了锚板的运动特性和轨迹;合理考虑浅埋破坏后,与纯假设深埋破坏情况比,锚板在达到稳定状态之前的预测埋深和锚链力要小,但极限嵌入深度一致.

基础下浅埋洞室安全顶板厚度研究

在参考已有资料的基础上 ,分析了山区城市浅埋地下洞室安全顶板厚度的影响因素。采用有限单元数值模拟法研究了基础与洞室相互作用下的破坏机制 ,以及各影响因素对浅埋地下洞室安全顶板厚度的影响及相关变化规律。并运用逐步线性回归的方法得出了一个能综合考虑各影响因素的安全顶板厚度的预测模型。

Catastrophe stability analysis for shallow tunnels considering settlement

Limit analysis of the stability of geomechanical projects is one of the most difficult problems.This work investigates the influences of different parameters in NL failure strength on possible collapsing block shapes of single and twin shallow tunnels with considering the effects of surface settlement.Upper bound solutions derived by functional catastrophe theory are used for describing the distinct characteristics of falling blocks of different parts in twin tunnels.Furthermore the analytical solutions of minimum supporting pressures in shallow tunnels are obtained by the help of the variational principle.Lastly,the comparisons are made both in collapsed mechanism and stability factor with different methods.According to the numerical results in this work,the influences of different parameters on the size of collapsing block are presented in the tables and the limit supporting loads are illustrated in the form graphs that account for the surface settlement.

Determination of minimum cover depth for shallow tunnel subjected to water pressure

Prediction of the state of roof collapse is a big challenge in tunnel engineering, while the limit analysis theory makes it possible to derive the analytical solutions of the collapse mechanisms. In this work, an exact solution of collapsing shape in shallow underwater tunnel is obtained by using the variation principle and the upper bound theorem based on nonlinear failure criterion. Numerical results under the effect of river water and supporting pressure are derived and discussed. The maximum water depth above the river bottom surface is determined under a given buried depth of shallow cavities and the critical depth of roof collapse is obtained under a constant river depth. In comparison with the previous results, the present solution shows a good agreement with the practical results.

Safe range analysis of clear distance of twin shallow tunnels based on limit analysis and reliability theory

A method of analyzing the stability of twin shallow tunnels was presented using both limit analysis with nonlinear failure criterion and reliability theory.In the condition of nonlinear failure criterion,the critical clear distancesof twin shallow tunnels were obtained by analyzing the change of surrounding pressure.A reliability model was established based on limit state equation,and the failure probability was solved by virtue of Monte Carlo method.Safety factor and corresponding clear distance of different safety levels were obtained by introducing a target reliability index.The scope of clear distance for different safety levels is described,which can be used as a supplement and improvement to the design codes of tunnels.

Upper bound solutions for supporting pressures of shallow tunnels with nonlinear failure criterion

Linear failure criterion is widely used in calculation of earth pressure acting on shallow tunnels. However, experimental evidence shows that nonlinear failure criterion is able to represent fairly well the failure of almost all types of rocks, A nonlinear Hock-Brown failure criterion is employed to estimate the supporting pressures of shallow tunnels in limit analysis framework. Two failure mechanisms are proposed for calculating the work rate of extemal force and the internal energy dissipation. A tangential line to the nonlinear failure criterion is used to formulate the supporting pressure problem as a nonlinear programming problem. The objective function formulated in this way is minimized with respect to the failure mechanism and the location of tangency point. In order to assess the validity, the supporting pressures for the proposed failure mechanisms are calculated and compared with each other, and the present results are compared with previously published solutions when the nonlinear criterion is reduced to linear criterion. The agreement supports the validity of the proposed failure mechanisms. An experiment is conducted to investigate the influences of the nonlinear criterion on collapse shape and supporting pressures of shallow tunnels.

Energy analysis of stability of twin shallow tunnels based on nonlinear failure criterion

Based on nonlinear Mohr-Coulomb failure criterion, the analytical solutions of stability number and supporting force on twin shallow tunnels were derived using upper bound theorem of limit analysis. The optimized solutions were obtained by the technique of sequential quadratic programming. When nonlinear coefficient equals 1 and internal friction angle equals 0, the nonlinear Mohr-Coulomb failure criterion degenerates into linear failure criterion. The calculated results of stability number in this work were compared with previous results, and the agreement verifies the effectiveness of the present method. Under the condition of nonlinear Mohr-Coulomb failure criterion, the results show that the supporting force on twin shallow tunnels obviously increases when the nonlinear coefficient, burial depth, ground load or pore water pressure coefficients increase. When the clear distance is 0.5to 1.0 times the diameter of tunnel, the supporting force of twin shallow tunnels reaches its maximum value, which means that the tunnels are the easiest to collapse. While the clear distance increases to 3.5 times the diameter of tunnel, the calculation for twin shallow tunnels can be carried out by the method for independent single shallow tunnel. Therefore, 3.5 times the diameter of tunnel serves as a critical value to determine whether twin shallow tunnels influence each other. In designing twin shallow tunnels,appropriate clear distance value must be selected according to its change rules and actual topographic conditions, meanwhile, the influences of nonlinear failure criterion of soil materials and pore water must be completely considered. During the excavation process, supporting system should be intensified at the positions of larger burial depth or ground load to avoid collapses.