soil mechanics (土力学).ppt

第八章 地基承载力 Chapter 8 Bearing capacity ;8.1 Introduction (前言) ;the settlement of the foundation should be tolerable and, in particular, differential settlement should not cause any unacceptable damage of the structure (基础的沉降和沉降差必须在该建筑物所允许的范围之内) 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. From first requirement, qa is defined as: (地基的容许承载力 qa 定义为当上述两个条件满足时的基底最大压力.当条件一满足时qa 定义如下);8.2 Types of foundation failures (基础破坏的类型) ;(ii) Local shear failure (局部剪切破坏);(iii) Punching shear failure (冲剪破坏);8.3 Allowable bearing capacity determined from plastic zone;The following figure shows a strip foundation of width b and infinite length carries a uniform pressure q on the surface of a homogeneous and isotropic soil. In addition, an additional surcharge pressure qo acts on the soil surface (下图代表一条形基础:基础宽度为b,无限长度,均布荷载q,地基为均质土和各性同向,基础两侧均布荷载为qo) ;From elastic theory, the increases in principal stresses at a given point M are shown as follows (地基中任意点M的附加大,小主应力可从弹性理论找出);Assume the coefficient of earth pressure at-rest (Ko) is 1, the increments in principal stresses can be superimposed to the principal stresses. The final principal stresses at depth z are (假设静止土压力糸数Ko=1,这样自重应力与附加应力可以在任意方向叠加,地基中任意点M的大小主应力可从以下公式找出 );When the stresses in point M reach the limiting equilibrium, ?1 and ?3 at failure are related by the following expression from Mohr-Coulomb failure criteria (当M点达到极限平衡状态时,大小主应力必需满足以下公式);Differentiate z with respect to ?, the maximum value of z can be determined by equating dz/d? to 0 (z最大值从设导数dz/d?为零找出 );The bearing capacity equation is better to express into the following form:;If zmax is chosen as b/3;8.4. Prandtl’s Theory (普朗特公式);Plastic equilibrium exists above the assumed failure surface EDCGF such that: (在滑动面EDCGF以上土体达到朔性平衡状态) ABC is active Rankine zone (ABC为朗肯主动区) ACD and BCG are radial shear zones (ACD与BCG为径向剪切区 ) ADE and BGF are passive Rankine zones ADE and B