Soil Bearing Capacity and Case study of Black Cotton Soil

Soil Bearing

Capacity

What is Soil Bearing Capacity?

The Bearing Capacity of Soils is perhaps the most important of all the

topics in soil engineering.

In geotechnical engineering, Bearing Capacity is the capacity

of soil to support the loads applied to the ground. The bearing

capacity of soil is the maximum average contact pressure between

the foundation and the soil which should not produce shear failure

in the soil.

Soils behave in a complex manner when loaded so, it is important to

know the bearing capacity of soils.

Why is Soil Bearing Capacity required?

The stability of foundation depends on:

1) The bearing capacity of the soil beneath the

foundation.

2) The settlement of the soil beneath the `

foundation.

There are, therefore, two independent stability conditions

to be fulfilled since the shearing resistance of the soil

provides the bearing capacity and the consolidation

properties determine the settlement.

Important Terminologies related to

Soil Bearing Capacity

Ultimate Bearing Capacity (qu)

The ultimate bearing capacity is the gross pressure at the base of

the foundation at which soil fails in shear.

Net ultimate Bearing Capacity (qnu):

It is the net increase in pressure at the base of foundation that

cause shear failure of the soil.

Net Safe Bearing Capacity (qns):

It is the net soil pressure which can be safely applied to the soil

considering only shear failure.

Gross Safe Bearing Capacity (qs):

It is the maximum pressure which the soil can carry safely without shear failure.

Net Safe Settlement Pressure (qnp):

It is the net pressure which the soil can carry without exceeding allowable settlement

Net Allowable Bearing Pressure (qna):

It is the net bearing pressure which can be used for design of foundation.

Calculation of Soil Bearing Capacity

FIELD TESTS LAB TESTS

Classified Based on the location where the tests are conducted.

Field Tests:for direct determination of SBC

Plate Load Test

Vane Shear test

Dynamic cone penetration

Field-Density (approximation)

Field observation

Previous Knowledge

Field Sample Collection

Plate Load Test

Field Tests:calculation based on engineering properties

Shear tests: measuring shear strength of soil

Triaxial test: measurement of shear strength in all three

dimensions

Consolidation test: expulsion of water under static

sustained load.

Settlement Analysis: analysis of load bearing based on

settlement of soil.

LAB TESTING EQUIPMENT

Case Study

Soil Bearing Capacity in Black Cotton Soil

Case Study Site Location

Solapur Road, Keshwapur, Hubli

Purpose:

To procure soil samples for assessing the physical properties, which

will help in design and mode of construction of proposed

foundation.

To determine the nature, thickness and variation of soil strata in the

region.

SITE INVESTIGATION

Methods of Soil Exploration

Open test pits or Trial pits

Sub-surface soundings

Borings (Tube, Auger, etc.)

Geophysical methods – seismic and electrical resistivity

methods

Vane test for shear strength

Open pits or Test pits

Most common and simplest method

Used for small depths (up to 3m)

(At greater depths, ground water causes problems in case of open pit

exploration)

Provides reliable information about the sub-surface conditions

3m

Results of Laboratory Tests

Calculated by conducting the above mentioned tests. [Ref-1]

Sr. No. Laboratory Test Result

1 Grain Size Distribution 74.69% fine

2 Specific Gravity (G) 2.65

3 Water Content (Natural) (w) 9.29%

4 Liquid Limit (WL) 56.50%

5 Plastic Limit (WP) 29.10%

6 Plasticity Index (IP or P.I) 27.40%

7 Free Swell Index (F.S.I) 30

8

Optimum Moisture Content

(O.M.C.) 20.02%

9 Maximum Dry Density (M.D.D.) 1.63 g/cc

10 California Bearing Ratio (C.B.R.) 2.33%

11

Unconfined Compressive Strength

(U.C.S.) at OMC 2.69 Kg/cm2

Outcomes Based on Laboratory Tests:

PROBLEMS IN BLACK COTTON SOIL

Soil grains oriented in the form of platelets or sheets.

Ingress of moisture between these sheets causes increase in

volume causing swelling.

During summer season, moisture moves out of the soil

causing

1) Shrinkage cracks (almost 10 to 15cm wide on the

ground and up to 2 m in depth)

2) Loss of support beneath the footings, resulting in high

settlements.

Some expansive and shrinkable soils stick to the

footing base and pull the footing down when they

shrink causing horizontal cracks in the walls.

Cracks are formed due to movement of the

ground on account of alternate swelling and

shrinkage. The cracks thus formed are sometimes 15

to 20 cm wide and 2.5 to 4 m deep.

METHODS OF INCREASING SOIL BEARING CAPACITY OF BLACK

COTTON SOIL

1) Increasing Depth of Foundation:

In granular soil the bearing capacity increases with the depth due to the

confining weight of overlying materials.

2) Compaction of soil:

Due to the compaction of soil the bearing capacity of the soil increases resulting in reduction in resting settlements

3) Drainage of soil:

Due to the presence of water in the soil the bearing capacity of the soil is decreased hence by draining out water the void ratio is improved resulting

in higher bearing capacity.

4) Grouting:

Bore holes in sufficient numbers are driven in the ground and cement

grout is forced through these under pressure

Due to which the bearing capacity of soil is increased.

5) Chemical Treatment:

In this treatment certain chemicals are grouted in the place of cement

grout. The chemical used is such that it can solidify and gain early

strength.

Safety Analysis for Soil Bearing Capacity Of Black Cotton Soil

The foundation should be taken at Ieast 50 cm lower than the depth of

moisture movement. This depth should also be much more than depth of

tension cracks.

Where this soil occurs only in top layer, and where the thickness of this

layer does not exceed 1 to 1.5 m, the entire layer of black cotton soil (or

other expansive soil) should be removed, and the foundation should be

laid on non-shrinkable non- expansive soil.

Where the depth of clay layer is large, the foundation or footing should

be prevented from coming in contact with the soil. This can be done by

excavating wider and deeper foundation trench and interposing layer of

sand/mooram around and beneath the footing.

Where the soil is highly expansive, it is very essential to have minimum

contact between the soil and the footing. This can be best achieved

by transmitting the loads through deep piles or piers and by supporting

wall loads on capping beams which are kept some distance (5 to 15

cm) above the ground surface, to permit free expansion of the soil.

Where the bearing capacity of soil is poor, or soil is very soft, the bed of

the foundation trench should be made firm or hard by ramming

mooram and ballast mio it.

The foundations should be constructed during dry season. Also suitable plinth protection around the external wall should be made on the

ground surface, with its slope away from the wall, so that moisture does

not penetrate the foundation during rainy season.

Conclusion:

Scope of the work was to Study the Soil Bearing Capacity and Properties of

Black Cotton Soil for Civil Engineering works.

Following conclusions are made on the basis of test results

The property of volumetric changes with the change of

atmospheric conditions makes black cotton soil dangerous to be

founded buildings.

It swells excessively when wet and shrinks excessively when dry resulting terrible cracks in soil without any warning.

Hence due to the low Bearing Capacity of Black cotton soil which

makes it dangerous for construction of high rise buildings. So all the safety and precautionary methods must be followed before

construction of the building.