Shallow Foundation

SHALLOW FOUNDATION

Introduction

A foundation is a integral part of the structure which transfer the load of the superstructure to the soil. A foundation is that member which provides support for the structure and it's loads. It includes the soil and rock of earth's crust and any special part of structure that serves to transmit the load into the rock or soil. The different types of the foundations are given in fig. 4.1

Different types of footings

Fig. 4.1 Different types of footings

Methods of determining bearing capacity The various methods of computing the bearing capacity can be listed as follows:

Presumptive Analysis

Analytical Methods

Plate Bearing Test

Penetration Test

Modern Testing Methods

Centrifuge Test

Prandtl's Analysis

Prandtl (1920) has shown that if the continuous smooth footing rests on the surface of a weightless soil possessing cohesion and friction, the loaded soil fails as shown in figure by plastic flow along the composite surface. The analysis is based on the assumption that a strip footing placed on the ground surface sinks vertically downwards into the soil at failure like a punch.

Prandtl analysed the problem of the penetration of a punch into a weightless material. The punch was assumed rigid with a frictionless base. Three failure zones were considered.

Zone I is an active failure zone

Zone II is a radial shear zone

Zone III is a passive failure zone identical for O=0

Terzaghi's Bearing Capacity Theory  Assumptions in Terzaghi's Bearing Capacity Theory

Depth of foundation is less than or equal to its width. Base of the footing is rough.

Soil above bottom of foundation has no shear strength; is only a surcharge load against the overturning load

Surcharge upto the base of footing is considered.

Load applied is vertical and non-eccentric. 

The soil is homogenous and isotropic. 

L/B ratio is infinite.

Consider a footing of width B and depth D_f loaded with Q and resting on a soil of unit weight r. The failure of the zones is divided into three zones as shown below. The zone1 represents an active Rankine zone, and the zones 3

are passive zones.the boundaries of the active Rankine zone rise at an angle of 45+a/2, and those of the passive

zones at 45 - a/2 with the horizontal. The zones 2 are known as zones of radial shear, because the lines that constitute one set in the shear pattern in these zones radiate from the outer edge of the base of the footing. Since the base of the footings is rough, the soil located between it and the two surfaces of sliding remains in a state of

equilibrium and acts as if it formed part of the footing. The surfaces ad and bd rise at a to the horizontal. At the instant of failure, the pressure on each of the surfaces ad and bd is equal to the resultant of the passive earth pressure

P_P and the cohesion force C_a. since slip occurs along these faces, the resultant earth pressure acts at angle a to the normal on each face and as a consequence in a vertical direction. If the weight of the soil adb is disregarded, the equilibrium of the footing requires that