2.0 LECTURE 2 SOIL MECHANICS- BEARING CAPACITY IF SOIL.pdf

Soil assessment for road construction

1. MISUNGWI COMMUNITY DEVELOPMENT
TECHNICAL TRAINING INSTITUTE

2. BEARING CAPACITY OF SOIL
Foundationis the lowest part f the substructure, that transmit
the load of the structure to the soil beneath it.
The foundation should be designed such that
a)The soil below does not fail in shear
b)Settlement is within the safe limits.

3. Terms used in Bearing
Capacity of soil
1.Ultimate Bearing Capacity (q
�??????�): The maximum pressure
that the soil can bear before failure occurs.
2.Net Ultimate Bearing Capacity (q
�??????�,�??????�): The ultimate
bearing capacity minus the weight of the soil above the
foundation base.
3.Allowable Bearing Capacity (q
??????????????????�??????): The maximum
pressure that can be applied to the soil considering a factor of
safety.

4. Factors affecting bearing
capacity of the soil
1.Soil Type and Properties: Different soils (clay, silt, sand, gravel) have different
bearing capacities.
2.Depth of Foundation: Deeper foundations generally have higher bearing
capacities.
3.Width of Foundation: Wider foundations distribute loads over a larger area,
increasing bearing capacity.
4.Water Table: High water tables can reduce the effective stress in the soil,
decreasing bearing capacity.
5.Load Eccentricity: Eccentric loads can reduce the bearing capacity of the soil.
6.Type of Load: Static loads vs. dynamic loads
7.Inclination of the ground
8.Inclination of the load.

5. MODES OF BEARING CAPACITY
FAILURES
Shear failure of soil under a foundation Can be
classified into three categories depending on
the type of soil & location of foundation.
a)General shear failure
b)Local shear failure
c)Punching Shear failure

6. .
General shear
General shear failure is a type of soil failure that occurs when a
well-defined shear surface develops from the edges of the
foundation to the ground surface, resulting in a sudden and
catastrophic collapse of the soil structure. (Common in dense
sands, gravels, or stiff clays)

7. .
Local shear failure:
Local shear failure occurs when the soil fails
progressively without the formation of a well-defined
failure surface. It is characterized by partial development
of shear surfaces and moderate soil
deformations.(common in loose sand)

8. Cont….
Punching shear failure:
Is the shear failure of the structural member due to high
concentrated load or highly localized force. (Occurs in very
loose or soft clay soils with low shear strength)

9. Terzaghi's Bearing Capacity
Equation (Applies for General
shear failure)

10. Bearing Capacity Factors

11. Table for bearing capacity factors

12. Graph for bearing capacity factors

14. Modified Bearing Capacity
Factors for Local Shear
Failure
For local shear failure Ø, the Terzaghi bearing capacity factors
are modified as follows
•Ø
′
= ??????????????????
−
(0.67tan Ø)
•??????
′
= 0.67C

16. Example Problem
•

17. CONT….

18. Example Problem
•

19. Cont…
From Table of factors, for 0 = 25°,
Nc = 25.1, N = 12.7,
Ny = 9.7
Now from the bearing capacity equation

20. Effect of water table on
Bearing Capacity
The equation for ultimate bearing capacity by
Terzaghi has been developed based on
assumption that water table is located at a great
depth .
If the water table is located close to foundation,
the equation needs modification

21. Case 01. when the water table
is located above the base of
footing

22. Case 2. when the water table
is located at the depth below
the footing base
Surcharge term is not affected.
Thus: qu= c’Nc+ γ Df Nq + 0.5B γ Nγ
When y = B ; W.T. at B below base of footing.
qu= c’Nc+ γ Df Nq + 0.5 B γ Nγ
Hence when ground water table is at b ≥ B, the equation is not
affected.

23. Field and Laboratory
Testing
1.Standard Penetration Test (SPT): Provides N-values
correlated to bearing capacity.
2.Cone Penetration Test (CPT): Provides continuous profile
of soil properties.
3.Plate Load Test: Measures settlement under a load to
determine bearing capacity.

24. Method of increasing
bearing capacity
a)Draining the soil
b)Compacting the soil
c)Confining the soil
d)Replacing the poor soil
e)Using grouting material
f)Stabilizing the soil with chemicals

25. ASSIGMENT
QUESTION 01:
A strip footing of width 2.5 m is to be founded at a depth of 1.5
m in a well drained sand stratum having the following
properties: ∅' =38
0
, c = 0, y = 18 ൗ
????????????
??????
3Determine the ultimate
bearing capacity using Terzaghi's bearing capacity equation

26. .
QUESTION 02:
A footing of 2.4 m2 is to be founded at a depth of 1.6 m in a
soil having the following shear strength parameters; c' = 0 and .
: ∅' =39
0
. Determine the ultimate bearing capacity when the
ground water level is
a)well below the foundation,
b)at the base of the foundation, and
c)at the ground surface.
The unit weight of the sand above the water table is y = 18
ൗ
????????????
??????
3the saturated unit weight is 20 ൗ
????????????
??????
3

27. REFFER TO BS 8004 S1-T1

28. QUESTION 03:
From the PSD and consistency test
you conducted in your groups, classify
the soil and give us the assumed
bearing capacity of your soil referring
to BS 8004 Section 1 Table 1

29. NEXT SESSION IN SOIL MECHANICS
NEXT SESSION
Chapter 3

30. FOR MORE CONSULTATION
PREPARED BY ENG. ABDULBAST IDRISA
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