IV- Mechanical Properties

Soil Strength

What is soil Strength ???????????????????

** Strength is considered to be the ability of a material to resist failure.

** It is the upper limit of the stress strain relationship

 

SOIL FAILURE

 

- Compression

 

- Tension

 

 

- Shear

 

 

 

** Failure occurs usually along well defined surface.

** In some cases soil failure is considered at 20% strain (Normally Consolidated Clay or loose sand).

Soil Strength = Shear Strength =

(Mohr-Coulomb yield criterion)

* Based on the drainage condition of the soil (permeability and boundary conditions), the strength of the soil is defined as:

1- Undrained Strength (),
2- Drained Strength ().

Basic Shear Parameters

Components of shear strength in soil are:

Angle of Internal Friction & Cohesion

* Shear Tests are performed to determine

* are determined for SHORT TERM ANALYSIS or TOTAL STRESS ANALYSIS.

* are determined for LONG TERM ANALYSIS or EFFECTIVE STRESS ANALYSIS.

Stress Conditions on Shear Plane

 

 

 

 

 

 

 

 

 

 

 

 

STRENGTH AND DEFORMATION

** The design of most of geotechnical structures requires a quantitative assessment of the strength and deformation characteristics of soils.

** The term strength is refers to shear strength

** The behavior of a soil mass is controlled not only by the external total stress but also by the water and air pressures developed in the pores of the soil.

** Terzaghi was first to recognize that the controlling feature was the resultant interparticle forces, and expressed this fact for a saturates soil by what he termed the EFFECTIVE STRESS LAW.

** He postulated that the strength of a soil depend on the difference between the total stress and the pore water pressure ( termed the effective stress).

** Another simple but fundamental concept is that of the "state" of drainage. Two "limiting" conditions are recognized- the UNDRAINED STATE in which the pore water is prevented from draining, which would otherwise occur as a result of the pressure induced in the pore water; and the DRAINED STATE in which the pore pressures are controlled to specific values, commonly atmospheric pressure.

Principle of Effective Stress:

An understanding of the strength characteristics of soils requires that the multi-phase structure of a soil be considered.
In particular, that part of the total stress applied to a soil which is carried by the soil skeleton must be separated from the part carried by the water and air phases.
The later cannot carry shear stress, but only normal (hydrostatic) stress.
Thus, all shear stresses applied to a soil must be carried by interparticle forces in soil skeleton, but in general only a proportion of the normal stresses will be carried by the soil skeleton.

The statement of the amount of stress carried by the soil skeleton is called the principle of effective stress; the actual stress itself being called the effective stress.


DETERMINATION OF SOIL STRENGTH

** The shear strength of a soil is usually determined experimentally by one of the following tests:

1- Direct shear test


 

2- Laterally confined compression test, known also as the triaxial compression test

 


 

3- Unconfined compression test

 

 

TYPES OF SHEAR TESTS:

** Depending on the nature of the drainage conditions of the soil, the shear strength is usually measured using one of the following methods:

a) the undrained test or the quick test (UU test)
b) the consolidated - undrained test or the consolidated - quick test (CU - test)
c) the consolidated - drained test or the consolidated
- slow test (CD - test)

** In the direct shear and the triaxial tests the three methods can be performed.

** In the unconfined compression test the first method is the only one that can be applied.

** The CD test is always used to determine the shear strength of cohesionless soils.
Why ????????????

 

TYPES OF APPLIED LOADING:

There are two types of applied loading in shear testing of soils; these are:

1- Controlled stress
2- Controlled strain

Both of these methods can be applied for shear strength measurements; however, the second one is preferable because the residual strength of the soil can be determined.