土力学-10-地基承载力.ppt

1、Chapter 8 Bearing capacity Part 1 (地基 承载力 ) In this lecture, well learn Types of foundation failures (基础破坏 的 类型 ) Ultimate bearing capacity of shallow Foundations (浅基础地基极限承载力 ) Prandtls method: smooth foundation and weightless soil (普朗特公式 :基底光滑与土无重量 ) Terzaghis method: rough foundation (太沙基公式 :基底粗糙

2、) 8.1 Introduction (前言 ) 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: (设计基础要满足两个要求 ) the factor of s

3、afety Fs against shear failure of the supporting soil must be adequate, a value between 2 and 3 normally being specified (地基达到剪切破坏的安全糸数 Fs 要适当 ,一般在 2至 3之间 )8.1 Introduction (前言 ) the settlement of the foundation should be tolerable and, in particular, differential settlement should not cause any una

4、cceptable 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 q

5、a 定义如下 )8.2 Types of foundation failures (基础破坏 的 类型 ) (i) General shear failure (整体剪切破坏 ) State of plastic equilibrium is fully developed throughout the soil above the failure surfaces (地基形成连续滑动面而破坏 ) Heaving of the ground surface occurs (基础两侧地面向上隆起 ) Typical failure mode for soils of low compressib

6、ility, i.e. dense or stiff soils (密砂或硬粘土的典型破坏模式 )PressureSettlementSliding surface (滑动面 )Ground surface (地面 )8.2 Types of foundation failures (基础破坏 的 类型 ) (ii) Local shear failure (局部剪切破坏 ) Partial development of the state of plastic equilibrium (剪切破坏区限制在地基内部某一区域 ) Slight heaving occurs (基础两侧地面只是微微隆

7、起 ) It is characterised by the occurrence of relatively large settlements (这种破坏型式的特征是出现相对大的沉降 )PressureSettlementSliding surface (滑动面 )Ground surface (地面 )8.2 Types of foundation failures (基础破坏 的 类型 ) (iii) Punching shear failure (冲剪破坏 ) There is relatively high compression of the soil under the fou

8、ndation accompanied by shearing in the vertical direction around the edges of the foundation (基础边缘下地基产生垂直剪切破坏 ) No heaving of the ground surface (基础两侧地面不出现隆起 ) PressureSettlementSliding surface (滑动面 )Ground surface (地面 )8.3 Ultimate bearing capacity of shallow foundations (浅基础地基极限 承载力 ) Foundations

9、can be classified as shallow and deep foundations. In general, if the depth of a foundation (D) is smaller than or equal to its breadth (B), i.e. D/B 1, the foundation is classified as shallow foundation. (一般认为 ,当基础的埋深 D小于或等于基础宽度 B时 ,称为浅基础 )8.3 Ultimate bearing capacity of shallow foundations (浅基础地基

10、极限 承载力 ) 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)

11、Ground surfaceqbqoGround surfaceqbdo8.3.1 Prandtls method (普朗特公式 ) Assumption: the base of the foundation is smooth and the soil is weightless. (假设 基底光滑与土体无重 ) The failure mechanism for a strip foundation is shown in following figure. (下 图代表条形基础的假设滑动面 )qu qoqob8.3.1 Prandtls method (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 BGF (ADE与BGF为朗肯被动区 )qu qoqob