Chapter8BearingCapacityofFoundations..docVIP

Chapter 8 Bearing Capacity Part 1 [地基承载力] 8.1 Introduction A foundation is that part of a structure which transmits loads directly to the underlying soil. The ultimate bearing capacity (qu) is defined as the pressure which would cause shear failure of the supporting soil immediately below and adjacent to a foundation. [当基底压力增大到极限承载力时,地基出现剪切破坏] A foundation must satisfy two fundamental requirements: (1) the factor of safety against shear failure of the supporting soil must be adequate, a value between 2 and 3 normally being specified; (2) the settlement of the foundation should be tolerable and, in particular, differential settlement should not cause any unacceptable damage of the structure. The allowable bearing capacity (qa) is defined as the maximum pressure which may be applied to the soil such that the above two requirements are satisfied. [设计基础要满足两个要求: (1)地基达到剪切破坏的安全糸数要适当,一般在2至3之间,(2)基础的沉降和沉降差必须在该建筑物所允许的范围之内,地基的容许承载力定义为当上述两个要求满足时的基底最大压力] 8.2 Types of shear failure [剪切破坏的形式] Three distinct modes of failure have been identified and these are illustrated in Fig. 8.1. In general shear failure, continuous failure surfaces develop between the edges of the footing and the ground surface. The state of plastic equilibrium is fully developed throughout the soil above the failure surfaces. Heaving of the ground surface occurs (see Fig.8.1a). This mode of failure is typical for soils of low compressibility (i.e. dense or stiff soils). The ultimate bearing capacity is well defined. [整体剪切破坏:当基底压力达到极限荷载时,基础两侧地面向上隆起, 地基形成连续滑动面而破坏] In local shear failure, there is significant compression of the soil under the footing and only partial development of the state of plastic equilibrium. The failure surfaces, therefore, do not reach the ground surface and only slight heaving occurs (see Fig.8.1b). This type of failure is associated with soils of high compressibility and is characterized by the occurrence of relatively large settlements. The ultimate bearing capacity is not