Three‐dimensional finite element study of arching behavior in slope/drilled shafts system

Two‐dimensional slope stability analysis for a slope with a row of drilled shafts needs a mechanism to take into account the three‐dimensional effect of the soil arching due to the spaced drilled shafts on slope. To gain a better understanding of the arching mechanisms in a slope with evenly spaced drilled shafts socketed into a stable stratum (or a rock layer), the three‐dimensional finite element modelling technique was used for a comprehensive parametric study, where the nonlinear and plastic nature of the soil and the elastic behavior of the drilled shafts as well as the interface frictions were modelled. Various factors were varied in the parametric study to include (1) the rigidity of the drilled shafts as influenced by its diameter, modulus of elasticity, and total length; (2) shafts spacing and location on the slope; (3) the material properties of rock and the socket length of shaft; and (4) the soil movement and strength parameters. Evidences of soil arching and reduction in the stresses and displacements through the load transfer mechanisms due to the presence of the drilled shafts were elucidated through the finite element method (FEM) simulation results. Design charts based on regression analysis of FEM simulation results were created to obtain a numerical value of the load transfer factor for the arching mechanism provided by the drilled shafts on the slope. Observations of the arching behavior learned from the FEM simulations provide an insight into the behavior of drilled shafts stabilized slope. Copyright © 2009 John Wiley & Sons, Ltd.     

Limit equilibrium based design approach for slope stabilization using multiple rows of drilled shafts

In this paper, a limiting equilibrium based methodology, incorporating the method of slices and arching effects of the drilled shafts, is developed for optimizing the use of multiple rows of drilled shafts. This proposed method is focusing on the number of rows, the location of each row, the dimension and spacing of the drilled shafts. Three design criteria are used for optimization: target global factor of safety, the constructability and service limit. A PC-based program called M-UASLOPE has been coded to allow for handling of complex slope geometry, soil profile, and ground water conditions. A design example is presented to illustrate the application of the M-UASLOPE program in the optimized design of multiple rows of drilled shafts for stabilizing the example slope.     

Explicit limit equilibrium solution for slope stability

Conventional methods of slices used for slope stability analysis satisfying all equilibrium conditions involves generally solving two highly non‐linear equations with respect to two unknowns, i.e. the factor of safety and the associated scaling parameter. To solve these two equations, complicated numerical iterations are required with non‐convergence occasionally occurring. This paper presents an alternative procedure to derive the three equilibrium equations (horizontal and vertical forces equations and moment equation) based on an assumption regarding the normal stress distribution along the slip surface. Combination of these equations results in a single cubic equation in terms of the factor of safety, which is explicitly solved. Theoretical testing demonstrates that the proposed method yields a factor of safety in reasonable agreement with a closed‐form solution based on the theory of plasticity. Example studies show that the difference in values of factor of safety between the proposed method, the Spencer method and the Morgenstern–Price method is within 5%. Application of the proposed method to practical slope engineering problems is rather straightforward, but its solution is of the same precision as those given by the conventional rigorous methods of slices since it is still within the rigorous context. Copyright © 2002 John Wiley & Sons, Ltd.     

降雨入渗非饱和黏土边坡稳定性的极限平衡条分法研究

考虑非饱和黏土边坡的基质吸力和渗流力,根据水位线位于滑面上、下的临界平衡状态,分别建立了滑体条块极限平衡状态下力和力矩平衡式,获得了降雨条件下非饱和黏土边坡稳定性的极限平衡条分法计算式。通过试验、参量变换以及作用力的位置关系可以确定相关参量,并采用数值计算求解临界平衡状态下滑体条块的相互作用力系数和非饱和边坡安全系数。案例结果表明：考虑渗流力时的非饱和黏土边坡安全系数比不考虑渗流力的安全系数降低约13.8%,且考虑渗流力作用的条间力作用系数变化率明显大于不考虑渗流力的结果;当降雨强度超过一定值时,坡面径流很快形成,边坡出现不稳定的时间基本相同。     

Incorporating set-up into LRFD method for drilled shafts

The increase of pile resistance with time is referred to as ‘set-up’. This behaviour of driven piles has been widely discussed in many studies by researchers. Meanwhile, there has been little, if any, information regarding this aspect for drilled shafts. Performing a bearing capacity test for a shaft over time, however, requires higher costs and more complicated rigs compared to a driven pile. A database including results from five Osterberg cell-tested drilled shafts conducted at two different stages is considered, from which the set-up effect is statistically analysed. The reliability-based analysis technique using Monte Carlo simulation (MCS) is used to develop separate resistance factors to account for different degrees of uncertainties associated with the predicted reference resistance and the predicted set-up resistance in the framework of the load and resistance factor design (LRFD) method. By incorporating set-up into design, shaft length or number of shafts can be reduced and economical design of drilled shafts can be achieved.     

Evaluating accuracy for two empirical methods in predicting settlement of drilled shafts

Several theoretical, empirical and semi-empirical methods are available in the literature to predict settlement of drilled shafts in sandy soils. In the Arabian Gulf countries, specifically in the United Arab Emirates, equations and procedure from the rest of the world are being used in analysis and design of drilled shafts without proper validation. It is the aim of this study to assess the applicability and evaluate the accuracy of two well known, and commonly used methods for pile prediction in the United Arab Emirates (UAE), namely Vesic (1977) and Poulos (1979), via comparison with data from field pile load tests conducted on shafts drilled in the region. Some of these tests were conducted for the purpose of this study, while others were made available through the courtesy of International Piling Contractors who are active in the region (e.g. Bauer International and Swiss Borings). Pile load test data were analyzed to back-calculate the model parameters related to settlement under different loading stages. Geological data and soil properties were obtained from studies conducted at the relevant sites. An effort is made to correlate soil properties with the prediction models. Statistical analysis is conducted to assess the accuracy of the results obtained from the two methods at different stages of loading via those obtained from pile load tests. Moreover, a detailed parametric study is conducted to assess the effect of the related parameters on the predicted pile settlement and the estimated settlement at different stages of loading. The study concluded with a recommendation of the most appropriate models and procedures to be followed for predicting the settlement of drilled shafts in the UAE, together with useful charts and correlation relations. Results showed that settlement values predicted by Vesic (1977) and Poulos (1979) overestimates the true values. This revised version was published online in July 2006 with corrections to the Cover Date.     

基于整体刚体平衡法的膨胀土边坡稳定分析

膨胀土边坡危险滑面受裂隙面影响,常位于强弱风化层界面,且近似为直线,滑坡后缘一般为垂直张拉裂缝,下部主滑床呈近水平状。基于上述特性,提出滑坡体的三折线刚体简化模型,滑体内土体强度采用多次干湿循环后稳定值,并根据整体刚体的静力平衡推导建立边坡安全系数求解公式。以秦淮东河膨胀土边坡为实例,开展膨胀土干湿循环室内试验,并利用推导公式求解边坡长期稳定系数,计算结果与FLAC强度折减法求解结果接近。设计程序分析了强风化层厚度和膨胀土层厚度对边坡安全系数求解值的影响,发现后者随前两者的变化先急剧减小后趋于平缓,在一定范围内影响较大。考虑浅层土体强度和强风化层深度在干湿循环不同阶段的变化,求解了各阶段强弱风化层交界面上滑面的安全系数,发现其呈现指数衰减的趋势,边坡主要在6次干湿循环后发生失稳,滑面深度在1.83 m左右。     

计算边坡安全系数的坡向离心法

边坡稳定分析方法发展最引人注目,它是经典土力学最早试图解决而至今仍未圆满解决的课题。在常用边坡分析方法的基础上,从边坡失稳机制出发,提出一种更方便的安全系数计算方法--坡向离心法。该法通过不断增大水平加速度,直至边坡失稳为止,依据水平加速度与重力加速度失稳影响机制求得安全系数。通过算例与传统极限平衡法和有限元强度折减法相比较,并对各物理参数进行敏感性分析。结果表明,坡向离心法在边坡工程稳定分析中的应用是切实可行的,其弹性模量和泊松比对该法所求安全系数影响不大。     

再论悬臂式抗滑桩合理桩间距的计算方法

为了进一步合理确定悬臂式抗滑桩桩间距,在对以往关于开挖边坡抗滑桩桩间距分析模型缺陷讨论的基础上,提出在抗滑桩桩后局部所形成的拱脚处,两侧土拱在此交汇形成的是倒梯形受压区。在计算确定悬臂式抗滑桩桩间距时,除满足桩间静力平衡条件、土拱跨中及拱脚处的强度条件等基本控制条件外,还需满足桩计算宽度条件、桩土变形协调条件等附加条件,得到了确定桩间净距的方程组,通过迭代试算可以求解。依托一工程实例,定量地说明了在其他因素不变的情况下,桩间净距随桩后土体黏聚力或内摩擦角的增大而非线性增大,且桩间净距受黏聚力的影响更为敏感,同时桩间净距随着桩后坡体推力的增大而呈非线性减小。所提出的方法除可用于黏性土外,还可用于无黏性土。     

边坡稳定分析的非线性有限元混合解法

在对有限元强度折减法和有限元迭代解法的优缺点分析的基础上,提出了边坡稳定分析的非线性有限元混合解法,该法将有限元强度折减法和有限元迭代解法联合运用于边坡稳定分析,充分利用了两种方法的优点。即由有限元强度折减法搜索边坡可能滑动面,将可能滑动面在网格中画出,由迭代解法逐步迭代求得其滑动面的安全系数。并以算例和工程实例说明了此种方法的正确性和合理性。     

膨胀土边坡长期稳定性的一种研究途径

由于降雨等原因,边坡的地下水位往往会在一定范围内反复变化,使得在稳定的地下水位以上的部分岩土体经常处于干湿循环的状态.这对膨胀土边坡的稳定尤其不利.边坡稳定的极限分析方法只适用于比例加载的情况.当其上所受到的全部或部分载荷发生反复变化时,应该采用安定分析来评价边坡的安全性.本文根据率形式的Koiter机动安定定理,提出了考虑地下水位往反变化情况下边坡安定的上限分析方法.利用抗剪强度参数折减定义的安全系数,能够定量地计算载荷变化对边坡稳定性的影响.最后,通过一个直立边坡的简单算例证明了本文方法的有效性.     

锚固力在边坡加固中的工程应用

假定滑面正应力分布为含有待定参数的插值函数,利用力和力矩平衡方程推导出锚固力作用下边坡安全系数显式解和给定安全系数要求的锚固力系数解析解。并把这种方法用在边坡加固的实际工程中,发现在锚固力作用下,滑面上正应力分布比较连续,条间力分布也比较合理,而且其方法计算过程简单,易编程实现,便于工程应用,其计算精度也在严格极限平衡法限定范围内,同时克服了传统方法导致滑面正应力反常的缺点。另外,该方法与岩土边坡动力学方法相比较,结果是一致的,说明该方法是可行的。     

基于离散元的边坡矢量和稳定分析方法研究

基于离散元方法对获取的块体单元、节理的应力场处理后得到坡体整体应力场,结合矢量和法安全系数的定义提出了一种边坡稳定性分析方法即VSM-UDEC法。首先通过滑块试验验证了多种情况下UDEC接触应力计算和单元应力的精确性,结果表明,UDEC接触应力和块体单元应力的精确程度很高（接触应力相对误差基本低于0.2%）,能够满足矢量和安全系数计算要求。采用VSM-UDEC法分别对直线型滑面和圆弧型滑面算例进行了分析,并与相应的理论解和严格极限平衡法（Morgenstern-Price法）进行了对比。结果表明,对于直线型滑面VSM-UDEC法安全系数与理论解几乎相同,而对于圆弧型滑面该方法与极限平衡法结果基本一致。最后将VSM-UDEC法运用到锦屏I级水电站左岸边坡工程实例中,结果表明,假设块体为刚体时VSM-UDEC法安全系数与极限平衡法（Sarma法、Morgenstern-Price法）结果吻合较好,此外,VSM-UDEC法能够考虑坡体内结构面对稳定性的影响,较极限平衡法更能反映边坡的变形分布情况。鉴于UDEC在模拟边坡破坏方面优点较多且矢量和法安全系数物理意义明确,VSM-UDEC法有望在边坡稳定性分析中有较好的应用前景。     

边坡稳定分析三维极限平衡条柱间力的讨论

根据在自重作用下潜在滑体在斜面上运动的规律,提出条柱底滑面上抗滑力方向垂直于底滑面与水平面交线的假定,并引用条间抗剪强度发挥系数的概念,运用各条柱的3个方向力的平衡和边坡整体3个方向的力矩平衡,提出了一种计算边坡稳定性的三维极限平衡方法,通过对算例的计算结果分析发现：当忽略条柱间全部剪力作用时,得到的安全系数与二维Spencer法计算结果相近;考虑部分条柱间剪力作用时,得到的安全系数与文献[5]计算结果相近;当考虑条柱间横向剪力对边坡稳定性的贡献时,即考虑条柱间全部作用力,得到的安全系数最高,这种假定更能反映边坡实际受力情况,因此,所得的结果更接近于实际     

Load transfer analysis of rock-socketed drilled shafts by coupled soil resistance

The load distribution and deformation of rock-socketed drilled shafts subjected to axial loads are evaluated by a load transfer method. The emphasis is on quantifying the effect of coupled soil resistance in rock-socketed drilled shafts using 2D elasto-plastic finite element analysis. Slippage and shear-load transfer behavior at the pile–soil interface are investigated by using a user-subroutine interface model (FRIC). It is shown that the coupled soil resistance acts as pile-toe settlement as the shaft resistance is increased to its ultimate limit state. Based on the results obtained, the coupling effect is closely related to the ratio of the pile diameter to soil modulus (D/E_s) and the ratio of total shaft resistance against total applied load (R_s/Q). Through comparison with field case studies, the 2D numerical analysis reasonably estimated load transfer of pile and coupling effect, and thus represents a significant improvement in the prediction of load deflections of drilled shafts.     

土坡等圆心角斜条分稳定性分析法

在土坡稳定性计算中,条分法作为一种简单而又实用的方法在实际工程中得到了广泛的应用。传统的条分法大多采用垂直条分,等圆心角斜条分法以斜条分代替垂直条分,使得条间力的假设更为合理。在斜条分的基础上,结合重新定义的应力水平安全系数,使问题的求解也更为简单、迅速,并且求解的结果还可以反映出土条应力状态的差异。     

基于变分法的均质土坡稳定性分析

在平面应变条件下,从考虑强度折减的滑动土体极限平衡方程出发,推导出一般情况下均质土坡稳定分析的泛函极值等周模型。依据泛函取极值时必须满足的欧拉方程,通过坐标变换对变分问题求解,得到滑裂面函数及其上分布的法向应力函数。对于给定的边界条件及横截条件,土坡稳定问题转化为求解以两个拉格朗日乘子、安全系数和滑裂面一个端点的x坐标为4个未知量的函数极值问题,可以通过数值计算得到最小安全系数及所对应的最危险滑裂面。考虑坡顶拉裂缝,通过算例分析了不同的滑裂面位置及对应的安全系数。通过对ACADS边坡考题算例分析,在不考虑坡顶开裂的条件下,采用该方法的分析结果与依据极限平衡法得到的各裁判答案非常接近,在考虑坡顶开裂时,安全系数略有减小。     

膨胀土边坡稳定分析方法研究

Bishop法等边坡稳定分析的经典方法用于分析膨胀土边坡时本身并无问题,只是由于计算条件的考虑不足,使得计算结果不能反映出膨胀土上的滑坡特点。在Bishop法的基础上,通过完善其计算条件,考虑了裂缝开展导致的土体强度降低、裂缝开展的深度、降雨时裂缝群中形成的渗流,以及可能的裂缝侧壁静水压力的作用等影响因素,完成了对膨胀土边坡稳定分析方法的改进。改进后的方法反映出裂缝对膨胀土边坡稳定性的影响,计算结果真实再现了浅层性、牵引性、长期性、平缓性和季节性等膨胀土边坡的滑坡特点。