CHAPTER THREE. SOIL STRENGTH AND SOIL FORCES. 3.1 INTRODUCTION. In terms of Soil Mechanics, there are two groups of soil properties: 3.1.1 Internal properties: i) Friction in the soil is a factor and depends on the normal load.
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SOIL STRENGTH AND SOIL FORCES
3.2.1 External Properties: In the soil surface, put a slider. Apply a normal force, N and apply a shear force, F.
If the soil moisture content is increased, another set of points are obtained as shown in the second line. There is now adhesion between the metal plate and the soil. In this general case,
where: is N/A = normal stress
Shear Strength is defined as the maximum resistance of the soil to
shearing stress under any given conditions
Cross-sectional area of sample =
Additional vertical pressure =
Cell pressure(kN/m2 ) Additional vertical pressure (kN/m2 ) Total vertical
50 84 134
150 134 284
250 186 436
Consider a soil element, with bulk density, . The shear stress
on the soil element exerted by a mass of soil on top of the
element ( i.e. vertical stress), , where Z is the distance
from the soil surface to the element.
Note: At the point of soil failure, the Mohr circle will
just touch the Coulomb line at a tangent point.
> ), the circle will go leftwards as
= . being larger than means that the major force causing failure on the soil element is the vertical stress and then the soil above the element is referred to as being ACTIVE (See figure above) because it was doing the work. If is larger like the bulldozer blade, then the soil above the soil element acts as if it is dormant waiting for a horizontal stress to shear it. The soil is then said to be PASSIVE.
If = 1 > , then = 3 and the soil is said to be ACTIVE
If = 1 > , then = 3 and the soil is said to be PASSIVE
NOTE: Soil normally fails at an angle to the plane on which the major principal stress acts.
Consider a simple case of a retaining wall with a vertical back supporting a cohesionless soil with a horizontal surface (see figure below).
Let the angle of shearing resistance of the soil be and the unit weight, be of a constant value.
The vertical stress acting at a point Z below the top of the wall is equal to .
If the wall is allowed to yield i.e. move forward slightly, the soil is able to expand and there will be an immediate reduction in the value of lateral pressure at depth Z, but if the wall is pushed slightly into the soil then the soil will tend to be compressed and there will be an increase in the value of the lateral pressure.
The above indicates that there are two possible modes of failure that can occur within the soil mass. If we assume that the value of the vertical pressure at depth Z remains unchanged at Z during these operations, then the minimum and maximum values of lateral earth pressure that will be achieved can be obtained from the Mohr circle diagram below.
The lateral pressure can reduce to a minimum value at which the stress circle is tangential to the strength envelope of the soil; this minimum value is known as the active earth pressure.
The lateral pressure can rise to a maximum value (with the stress circle again tangential to the strength envelope) known as the passive earth pressure. It can be seen from the Mohr circle diagram that the vertical pressure due to the soil weight ( Z) is a major principal stress when considering active pressure and that when considering passive pressure, the vertical pressure due to the soil weight ( Z) is a minor principal stress.
For weighty and cohesive soil, take moments, Mo thus: Pr . 2/3 Z + Pc. Z/2 = (Pr + Pc)L . From this equation, L which is the vertical distance, the force acts, can be obtained.
If friction exists on the wall, then the Rankine equations break down. Wall friction produces shear stress i.e. horizontal and vertical planes are no longer major and minor principal planes.
In the active case, the friction at wall prevents the free sliding of the soil down the wall and in the case of the passive one, the friction at wall prevents free sliding of soil up the wall.
In the presence of wall friction, for the active soil pressure, the analysis can be done using the Coulomb Trial Wedge Analysis. For the Passive Earth Pressures with wall friction, especially for tillage and traction, the Log. Spiral or the General Soil Mechanics Equation can be used.