Differential Settlement. - Its Causes and How to Prevent it

Total settlement of a foundation consists of:

1. Immediate settlement

2. Consolidation settlement

3. Secondary consolidation settlement

The immediate settlement is also known as elastic settlement. It can be found by elastic theory as,

Se = qn B

where qn = Net foundation pressure B = Width of foundation

m = Poisson’s ratio Es = Modulus of elasticity

qn = Influence factor which depends upon ratio of the foundation and the thickness of

compressible layer

Consolidation settlement may be found by

Sc =

where H = Thickness of compressible layer

Cc = Compression index

eo = Initial void ratio

Po = Initial overburden pressure

Dp = Increase in the pressure due to load

Secondary consolidation settlement is small and takes place over a long period.

Plate Load Test

By loading a rigid square plate in a pit with reaction frame this test is conducted to measure the

settlement. The following points may be noted:

1. The minimum size of plate 300 mm. Commonly used plate sizes are 300 mm, 450 mm and 600 mm.

2. It is preferable to use 600 mm square plate for fissured clays and relatively loose sands and silts.

3. Width of the test pit should be at least 5 times the plate side.

4. The plate should be at the level of actual foundation proposed.

5. IS code recommends a preliminary seating load of 70 gm/cm2 before the main test.

6. Apply the load in increments and measure the settlement with dial gauge. The maximum load

corresponds to times the estimated ultimate load.

7. In cohesionless soil each incremented load is maintained for at least one hour before the settlement.

On clayey soil the load is maintained until about 75% of probable settlement has reached or for 24

hours.

8. Load settlement curve is plotted and ultimate bearing capacity is found as the load corresponding

to the settlement after which settlement rate is fast. If load-settlement curve does not indicate a

marked breaking point, a settlement equal to of the width of the test plate is considered the

failure point.

* According to IS code the permissible values of settlements in clays and sands are 40 mm and 25

mm respectively.