This presentation contains information about Soil Bearing Capacity (SBC) and Case on Black Cotton Soil of Hubli-Dharwad region study performed by me

1. Soil Bearing
Capacity

2. 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.

3. 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.

4. 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.

5.  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.

6. Calculation of Soil Bearing
Capacity
FIELD TESTS LAB TESTS
Classified Based on the location where the tests are conducted.

7. 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

8. Plate Load Test

9. 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.

10. LAB TESTING EQUIPMENT

11. 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.

12. SITE INVESTIGATION

14. 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

15. 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

16. 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

17. 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.

18.  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.

19. 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.

21.  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.

23. 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.

24.  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.

25. 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.