1. Session 7 – 8 SETTLEMENT OF SHALLOW FOUNDATION
Course : S0484/Foundation Engineering
Year : 2007
Version : 1/0
Session 7 – 8 SETTLEMENT OF SHALLOW FOUNDATION

2. Consolidation Settlement
SHALLOW FOUNDATION
Topic:
General
Immediate Settlement
Consolidation Settlement

3. GENERAL
The settlement of shallow foundation may be divided into three broad categories:
Immediate settlement, which is caused by the elastic deformation of dry soil and of moist and saturated soils without any change in the moisture content. Immediate settlement are generally based on equations derived from the elasticity theory
Primary consolidation settlement, which is the result of a volume change in saturated cohesive soils because of expulsion of the water that occupies the void spaces.
Secondary consolidation settlement, which is observed in saturated cohesive soils and is the result of the plastic adjustment of soil particles.
This course will focus at immediate and primary consolidation settlement only.

4. IMMEDIATE SETTLEMENT

5. General Equation (Harr, 1966)
IMMEDIATE SETTLEMENT
General Equation (Harr, 1966)
Flexibel Foundation
At the corner of foundation
At the center of foundation
Average
Rigid Foundation ; ;
H = 
Es = Modulus of elasticity of soil
B = Foundation width L = Foundation length

6. IMMEDIATE SETTLEMENT

7. IMMEDIATE SETTLEMENT
If Df = 0 and H < , the elastic settlement of foundation can be determined from the following formula:
(corner of rigid foundation)
(corner of flexible foundation)
The variations of F1 and F2 with H/B are given in the graphs of next slide

8. IMMEDIATE SETTLEMENT

9. IMMEDIATE SETTLEMENT

10. EXAMPLE
Problem:
A foundation is 1 m x 2 m in plan and carries a net load per unit area, qo = 150 kN/m2. Given, for the soil, Es = 10,000 kN/m2, s 0.3. Assuming the foundation to be flexible, estimate the elastic settlement at the center of the foundation for the following conditions:
a. Df = 0 and H = 
b. Df = 0 and H = 5 m

11. EXAMPLE Solution: Part a. Part b. For L/B = 2/1 = 2    1.53, so
For L’/B’ = 2, and H/B’ = 10  F1  and F2  0.033, so

12. General Equation (Bowles, 1982)
IMMEDIATE SETTLEMENT
General Equation (Bowles, 1982)
Es = Modulus of elasticity of soil
H = effective layer thickness, ex B below foundation
At the center of Foundation
and F1 time by 4
At the corner of Foundation
and F1 time by 1

13. IMMEDIATE SETTLEMENT
For saturated clay soil

14. IMMEDIATE SETTLEMENT For sandy soil where:
Iz = factor of strain influence
C1 = correction factor to thickness of embedment foundation = 1 – 0.5x[q/(q-q)]
C2 = correction factor due to soil creep
= 1+0,2.log(t/0,1)
t = time in years
q = stress caused by external load
q =  . Df

15. IMMEDIATE SETTLEMENT Circle Foundation or L/B =1 z = 0  Iz = 0.1
Young Modulus
Circle Foundation or L/B =1
z =  Iz = 0.1
z = z1 = 0,5 B  Iz = 0.5
z = z2 = 2B  Iz = 0.0
Foundation with L/B ≥ 10
z =  Iz = 0.2
z = z1 = B  Iz = 0.5
z = z2 = 4B  Iz = 0.0

16. EXAMPLE
A shallow foundation 3 m x 3 m (as shown in the following drawing). The subgrade is sandy soil with Young modulus varies based on N-SPT value (use the following correlation: Es = 766N)
Determine the settlement occur in 5 years (use strain influence method)

17. EXAMPLE

18. EXAMPLE Depth (m) z Es (kN/m2) Iz (average) (m3/kN) 0.0 – 1.0 1.0
8000
0.233
0.291 x 10-4
1.0 – 1.5
0.5
10000
0.433
0.217 x 10-4
1.5 – 4.0
2.5
0.361
0.903 x 10-4
4.0 – 6.0
2.0
16000
0.111
0.139 x 10-4 
1.55 x 10-4

19. CONSOLIDATION SETTLEMENT

20. CONSOLIDATION SETTLEMENT
Normal Consolidation
Over consolidation or or
po + p < pc
po < pc < po+p

21. CONSOLIDATION SETTLEMENT
where:
eo = initial void ratio
Cc = compression index
Cs = swelling index
pc = preconsolidation pressure
po = average effective pressure on the clay layer before the construction of the foundation
=  ’.z
p = average increase of pressure on the clay layer caused by the foundation construction and other external load, which can be determine using method of 2:1, Boussinesq, Westergaard or Newmark.
Alternatively, the average increase of pressure (p) may be approximated by:
pt = the pressure increase at the top of the clay layer
pm = the pressure increase at the middle of the clay layer
pb = the pressure increase at the bottom of the clay layer

22. CONSOLIDATION SETTLEMENT

23. EXAMPLE
A foundation 1m x 2m in plan is shown in the following figure. Estimate the consolidation settlement of the foundation Assume the clay is normally consolidated.

24. EXAMPLE po = (2.5)(16.5) + (0.50)(17.5-10) +(1.25)(16-10) = 52.5 kN/m2
2:1 method

25. ALLOWABLE SETTLEMENT