Bearing Capacity from SPT and PLT

By:Alka Shah

Assistant professorCivil DepartmentNirma University

Some important concepts

Bearing capacity factor can also be read from the table and chart given by terzaghi

Some important concepts

Factor of SafetyIt depends on many factors such as,

1. Type of soil2. Method of exploration3. Level of Uncertainty in Soil Strength4. Importance of structure and consequences of failure5. Likelihood of design load occurrence, etc.

Assume a factor of safety F = 3, unless otherwise specified for bearing capacity problems

Some important conceptsDensity of soil :

1. Always use dry density as it does not change with season and it is always smaller than bulk or saturated density

2. 2. If only one density is specified in the problem, assume it as dry density and use

3. 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

4. If water table is some where in between, use equivalent density

Factors influencing Bearing Capacity

1. Type of soil2. Unit weight of soil3. Surcharge load4. Depth of foundation5. Mode of failure6. Size of footing7. Shape of footing8. Depth of water table9. Eccentricity in footing load10. Inclination of footing load11. Inclination of ground12. Inclination of base of foundation

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.

Relation ship of N and Φ

Bearing Capacity From spt

Method 2

Estimating allowable bearing capacity from chart by Terzaghi and Peck

Bearing Capacity From spt

Method 2

Later chart converted in to mathematical equation by Tang

IS 6403 accepted Tang’s equation

Bearing Capacity From PLT

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)

whereB = 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.

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