1. Bearing Capacity from SPT and PLT
By:
Alka Shah
Assistant professor
Civil Department
Nirma University

2. Some important concepts
Bearing capacity factor can also be read from the table and chart given by terzaghi

5. Some important concepts
Factor of Safety
It depends on many factors such as,
1. Type of soil
2. Method of exploration
3. Level of Uncertainty in Soil Strength
4. Importance of structure and consequences of failure
5. Likelihood of design load occurrence, etc.
Assume a factor of safety F = 3, unless otherwise specified for bearing capacity problems

6. Some important concepts
Density of soil :
Always use dry density as it does not change with season and it is always smaller than bulk or saturated density
2. If only one density is specified in the problem, assume it as dry density and use
If the water table correction is to be applied, use saturated density in stead of dry density. On portions above the water table, use dry density
If water table is some where in between, use equivalent density

7. Factors influencing Bearing Capacity
1. Type of soil
2. Unit weight of soil
3. Surcharge load
4. Depth of foundation
5. Mode of failure
6. Size of footing
7. Shape of footing
8. Depth of water table
9. Eccentricity in footing load
10. Inclination of footing load
11. Inclination of ground
12. Inclination of base of foundation

8. Bearing Capacity From spt
Method 1
The ultimate bearing capacity of cohesion less soil is determined from the standard penetration number N
The standard penetration test is conducted at a number of selected points in the vertical direction below the foundation level at intervals of 75 cm or at point where there is a change of strata.
An average value of N is obtained between the level of the base of footing and the depth equal to 1.5 to 2 times the width of the foundation.
Obtained value of Φ from the N value from chart and the bearing capacity factors are found from that value of Φ.

9. Relation ship of N and Φ

10. Bearing Capacity From spt
Method 2
Estimating allowable bearing capacity from chart by Terzaghi and Peck

11. Bearing Capacity From spt
Method 2
Later chart converted in to mathematical equation by Tang
IS 6403 accepted Tang’s equation

12. Bearing Capacity From PLT

13. Bearing Capacity From PLT
From Fig. the safe bearing pressure for medium and dense sands could be read, corresponding to a settlement ( St ), which shall be calculated as under ( St taken as permissible settlement of footing)
where
B = the size of footing
BP = size of test plate
SP, = settlement of test plate
St = settlement of footing
From this formula total settlement of footing ( St ) is calculated taking Sp as observed total settlement of plate.

14. Example
A plate load test was carried out on a ground having a uniform sand stratum up to sufficient depth. The size of plate used was 30 cm X 30 cm. Plot load settlement curve. Also determine bearing capacity and load that can be taken by a column footing of size 1.2 X 1.2 m in this soil, for an allowable settlement of 2 cm.
Load Q (kN)
Settlement (mm)
4.5
0.75 9
1.25 18 2 27
3.5 36
5.38 45
7.75 54
10.75