基于上限法的地基极限承载力计算模式探讨

当前地基极限承载力理论研究及实践应用的基本框架,仍采用的是Terzaghi所建议的三项叠加法,即分别考虑土体粘聚力、超载和地基土重对承载力的贡献,然后将三项进行叠加,由于土体并非线弹性材料,应用叠加原理在理论上是欠妥的。从理论上讲,采用整体考虑影响承载力的各种因素直接进行极限承载力计算更为恰当。为探讨采用三项叠加法与整体考虑各种影响因素直接计算的差别,基于多块体上限方法,分别采用两种计算方法对中心荷载作用下粗糙及光滑基础地基极限承载力问题进行了计算,探讨两种计算方法的差异,并从地基破坏面所反映出的信息揭示了两种计算存在差异的内在原因。     

Design of Raft Foundations for High-Rise Buildings on Jointed Rock

This paper presents calculations of displacements and bending moments in a 2-m-thick reinforced-concrete foundation slab using three-dimensional finite-element software. A preliminary paper was presented by Justo et al. (Rock Mech Rock Eng 43:287–304, 2010). The slab is the base of a tower of 137 m height above foundation, supported on jointed and partly weathered basalt and scoria. Installation of rod extensometers at different depths below foundation allowed comparison between measured displacements and displacements calculated using moduli obtained from rock classification systems and three material models: elastic, Mohr–Coulomb and hardening (H). Although all three material models can provide acceptable results, the H model is preferable when there are unloading processes. Acceptable values of settlement may be achieved with medium meshing and an approximate distribution of loads. The absolute values of negative bending moments (tensions below) increase as the rock mass modulus decreases or when the mesh is refined. The paper stresses the importance of adequately representing the details of the distribution of loads and the necessity for fine meshing to obtain acceptable values of bending moments.     

系数法计算柱下双向偏心矩形扩展基础承载力探讨

为解决基础工程设计中确定基础底面尺寸繁琐的问题 ,探索一种简捷计算柱下双向偏心矩形扩展基础承载力方法 ,在研究分析现行单向偏心系数增大法的基础上 ,推导了一种用偏心矩K(e) 对应特定基础底面尺寸的地基承载能力系数计算基础承载力的方法。通过工程实例计算分析比较 ,该法计算过程简捷直观、精度可靠 ,具有较好的实用价值     

粘土中圆形隧道上限解的研究

根据极限分析上限理论,基于Davis等人提出的隧道垮落模型,提出了剪切区参数α等于1时的新的隧道垮落模型。由不可压缩性条件和速度连续性条件求出了剪切区速度和间断线表达式,计算了该模型间断线上能量耗损、剪切区能量耗损和总外力所做的功。利用功能原理,求出了稳定率的表达式,并计算了相应的稳定率。从求解结果来看,所得上限解小于Davis等人的结果,因而该上限解将更接近真实解。从与实验结果比较来看,该上限解位于实验值附近,能与实验结果比较好地吻合,从而说明该模型更加接近真实隧道垮落情形。该上限解可为隧道支护提供理论依据。     

广州地区软岩承载力的讨论

根据原位和室内试验资料,对广州地区软岩天然地基承载力的确定及嵌岩桩承载力计算参数取值中存在的一些问题进行分析讨论,提出了解决思路和处理建议.     

Allowable Bearing Capacity of Shallow Foundations Based on Shear Wave Velocity

Firstly, the historical background is presented for the determination of ultimate bearing capacity of shallow foundations. The principles of plastic equilibrium used in the classical formulation of the ultimate bearing capacity are reviewed, followed by a discussion about the sources of approximations inherent in the classical theory. Secondly, based on a variety of case histories of site investigations, including extensive bore hole data, laboratory testing and geophysical prospecting, an empirical formulation is proposed for the determination of allowable bearing capacity of shallow foundations. The proposed expression corroborates consistently with the results of the classical theory and is proven to be reliable and safe, also from the view point of maximum allowable settlements. It consists of only two soil parameters, namely, the in-situ measured shear wave velocity, and the unit weight. The unit weight may be also determined with sufficient accuracy, by means of another empirical expression, using the P-wave velocity. It is indicated that once the shear and P-wave velocities are measured in-situ by an appropriate geophysical survey, the allowable bearing capacity is determined reliably through a single step operation. Such an approach, is considerably cost and time-saving, in practice.     

Estimating the Strength of Jointed Rock Masses

Determination of the strength of jointed rock masses is an important and challenging task in rock mechanics and rock engineering. In this article, the existing empirical methods for estimating the unconfined compressive strength of jointed rock masses are reviewed and evaluated, including the jointing index methods, the joint factor methods, and the methods based on rock mass classification. The review shows that different empirical methods may produce very different estimates. Since in many cases, rock quality designation (RQD) is the only information available for describing rock discontinuities, a new empirical relation is developed for estimating rock mass strength based on RQD. The newly developed empirical relation is applied to estimate the unconfined compressive strength of rock masses at six sites and the results are compared with those from the empirical methods based on rock mass classification. The estimated unconfined compressive strength values from the new empirical relation are essentially in the middle of the estimated values from the different empirical methods based on rock mass classification. Similar to the existing empirical methods, the newly developed relation is only approximate and should be used, with care, only for a first estimate of the unconfined compressive strength of rock masses. Recommendations are provided on how to apply the newly developed relation in combination with the existing empirical methods for estimating rock mass strength in practice.     

软土地基上静压预制桩承载力的工后增大效应

分析了软土地基上预制桩承载力的时效性机理，介绍了几种研究承载力时效性的经验回归公式，结合工程说明应充分认识承载力时效性的科学性和利用这种规律潜在的巨大经济效益。     

Directional Support Method for Tunnel Jointed Rock Mass

Geotechnical and Geological Engineering - In order to explore the directional support method of jointed rock mass and ensure the stable operation of roadway, the support effect of jointed rock mass...     

Theoretical Methods for Wave Propagation across Jointed Rock Masses

Different methods are presently available for the analysis of wave propagation across jointed rock masses with the consideration of multiple wave reflections between joints. These methods can be divided into two categories. One is based on the displacement discontinuity model for representing rock joints, where the displacements across a joint are discontinuous and the tractions are continuous, and the other is the equivalent medium method. For the first category, there are three methods, i.e., method of characteristics (MC), scattering matrix method (SMM) and virtual wave source method (VWS). MC solves the equation of motion by using the theory of characteristic curves. SMM is based on the definition of the scattering matrix in which the reflection and transmission coefficients of a set of joints are stored. VWS method replaces the joints in the rock mass with a virtual concept. For the second category, equivalent medium model treats the problem in the frame of continuum mechanics and simplifies it from an explicit wave propagation equation. The objective of this paper is to review and compare these theoretical methods. The comparison shows that the four solutions agree very well with each other. Some additional considerations about the advantages and disadvantages of these methods are also given in the paper.     

The Effect of Spatial Correlation of Cone Tip Resistance on the Bearing Capacity of Shallow Foundations

The effect of directional behaviour of correlation structure of cone tip resistance on the bearing capacity of shallow strip footing resting on cohesionless soil deposit in 2-D random field is analysed using probabilistic approach. The results obtained from the analysis show that the assumption of perfect (or infinite) correlation of cone tip resistance data leads to lower values of probability of failure. In contrast, the isotropic assumption of correlation behaviour based on vertical scale of fluctuation leads to higher values of probability of failure. The results also show that the transformation model would play a major role in the evaluation of variability of design property. In conclusion, the need for a proper evaluation methodology for calculation of correlation lengths of soil properties and their influence in foundation design is highlighted.     

节理岩体中锚索加固作用分析

简要介绍了锚索加固技术的应用,并对全长粘结式锚索在节理岩体中的锚固作用机理进行初步分析,同时对节理岩体中锚索加固的抗拉粘结强度及抗剪切强度的影响因素进行了分析。     

Pseudo-static Seismic Bearing Capacity of Shallow Foundations in Unsaturated Soils Employing Limit Equilibrium Method

There exist many structures founded on unsaturated soil deposits. Shear strength augmentation due to the evolution of the matric suction within the unsaturated porous media enhances the bearing capacity of the overlying foundation. This paper presents the evaluation of the pseudo-static seismic bearing capacity of the shallow foundations resting on unsaturated soil deposits using limit equilibrium method. Adopting the Coulomb failure mechanism and Bishop effective stress concept, the bearing capacity equations are solved. The distribution of the matric suction beneath the footing is assumed to be linear. The results of the bearing capacity evaluation are validated against some experimental data found in literature for the static condition. For the seismic loading consideration, the pseudo-static method is utilized. The dual effect of the earthquake acceleration vertical component is thoroughly discussed and a suction transition point is introduced in which the minimum bearing capacity is observed to bear the same value for both upward and downward directions. The increase in the matric suction throughout the soil deposit leads to the increase in the soil shear strength, thus posing more resisting forces as well as higher ultimate bearing capacity. The offered solution is deemed a consistent and useful tool for the accurate prediction of the seismic bearing capacity of shallow footings resting on unsaturated soil deposits.

     

节理岩体三维偶应力弹性理论

节理的存在使得岩体的力学特性十分复杂。偶应力理论考虑了介质内颗粒尺度的影响，它以具有一定尺度的微元体为研究对象，引进了偶应力及弯曲曲率的概念，从而考虑了弯曲效应对介质变形特性的影响，可以用来解决岩土工程中的地下洞室底鼓、顶板开裂及反倾向层状岩体的弯曲倾倒等弯曲变形及弯曲破坏问题。本文考虑了节理间距的影响，对节理岩体的空间间题提出了弹性偶应力理论并建立了有限元计算模式，最后通过算例说明用偶应力理论解决上述问题的有效性。     

A Simple Limit Equilibrium Approach for Calculation of Ultimate Bearing Capacity of Shallow Foundations on Two-Layered Granular Soils

The bearing capacity of shallow foundations in a non-homogeneous soil profile has been a challenging task in geotechnical engineering. In this paper, a limit equilibrium method is used for calculating bearing capacity factors of shallow foundations constructed on a two-layered granular soil profile. The main objective has been to determine the ultimate bearing capacity computed from equivalent bearing capacity factors N_q and N_γ and comparing that with numerical analysis using finite element methods. It will be shown that the data obtained form the developed method are well comparable with those obtained from FE approach, specially when the difference between shear strength parameters of layers is low which is a practical case for sedimentary soil profiles and also for artificially compacted soils. A computer program has been developed to investigate the influence of various parameters on bearing capacity factors.     

不同实验条件下软质岩石地基承载力分析

现行《岩土工程勘察规范》(GB50021-2001)及《建筑地基基础设计规范》(GB50007-2002)规定,确定岩石地基承载力特征值常用方法有取岩样作室内试验和现场原位载荷试验等。软质岩的岩石试样作室内试验时,因环境条件与实地有明显的差异,试验结果多不理想,现场静载试验在实际环境条件下进行,能更好地挖掘软质岩石地基承载力特征值的潜力,减少基础工程造价。