3. SUBGRADE SOIL STRENGTH
The factors on which the strength characteristics
of soil depend are :
1. Type of soil
2. Moisture content
3. Dry density
4. Internal structure of the soil
5. The type and mode of stress application
4. it is very difficult to predict the stress-strain
relationship of soil, because of the diversity in the soil
types and non–homogenous nature of the soil. Generally ,
the highway engineer is interested in the stability or the
deformation of the soil under the stress applications.
In a soil mass, the deformation is largely due to
slippage between soil particles. Hence the shearing
resistance in soil represents the strength. The sliding
mechanism in soils is complicated as the shear
deformation cause reorientation of the particles resulting
in change in volume. Valence bond between particles,
thickness and other properties of adsorbed layer of water.
5. Shearing resistance in soil mass is commonly
attributed to internal friction and cohesion parameters of
the soil. Coulomb (1776), in his investigations observed
that one component of the shearing strength called
apparent cohesion is constant for given soil and is
independent of the applied stress. The other component
namely, the frictional resistance, varies directly as the
magnitude of the normal stress on the plane of rupture.
The shear strength (S) of a soil at a point on a
particular plane was expressed by coulomb as liner
function of the normal stress on that plane as
S = C + σ tan Ф ........ Coulomb’s equation
6. Where,
S = shear strength of the soil
C = apparent cohesion
σ = normal stress on the failure plane
Ф = angle of internal friction
S = C + σ tan Ф
C and Ф are also referred to as the shear strength
parameters of the soil. The value Ф depends on,
• dry density of the soil
• grain size distribution
• Shape and texture of soil
7. Cohesion C is the resistance of soil grains to
displacement by bond developed at the surface of
contact by very fine grained soils as a result of
intermolecular and electrochemical forces of attraction.
The value of C depends on,
• The type of clay mineral
• Size of clay mineral
• The surface charges
• The proportion of the clay
• Water content etc.
8. Evaluation of soil strength
The tests used to evaluate the strength properties of
soils may be broadly classified into three groups.
1. Bearing tests
2. Shear tests
3. Penetration tests
1. Bearing tests :
Bearing tests are loading tests carried out on
subgrade soils in-situ with a load bearing area. Plate
load test is very popular bearing test to determine the
bearing capacity of soils.
9. 2. Shear tests :
Shear tests are usually carried out on relatively
small soil sample in the laboratory. The commonly
known shear tests are,
• Direct shear test
• Triaxial compression test
• Unconfined compression test
• Vane shear test
10. 3. Penetration tests :
Penetration test may be considered as small scale
bearing tests in which the size of the loaded area is
relatively small. These tests are carried out in the field
or in the laboratory.
The commonly known penetration tests are,
• The California bearing ratio test
• Cone penetration tests.
