Chidanand Jadar 10CV64
Asst. Prof
UNIT-6
FOUNDATION SETTLEMENT
Definitions
Settlement is the vertically downward movement of structure due to the compression of
underlying soil because of increased load.
Uniform Settlement Differential Settlement
Smax
Smax
Fig. 1 : Concepts of uniform and differential settlement
Maximum Settlement : It is the absolute maximum downward movement of any part of
building element. Maximum Settlement = Smax
Differential Settlement : It is the maximum difference between two points in a building
element.
Differential Settlement = Smax - Smin
Angular Distortion : It is another method of expressing differential settlement.
Angular Distortion = Differential Settlement/Length of element
= (Smax - Smin)/L
Fig. 2 represents soil movement under different circumstances at the ground level. The
fluctuation in the elevation of ground level depends on seasonal changes in expansive (Indian
Black Cotton) soils and
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Fig. 2 : Soil movement in different situations
Table 1 presents the different types of movements experienced by various soils. It can be
noticed that only few soils such as clay can experience swell. Further, consolidation settlement
and creep settlement are more pronounced in clay. Immediate or elastic settlement is observed
in each and every soil. First three, namely Immediate, Consolidation and Creep settlement
cause downward movement of ground while swell causes upward movement of ground.
1: Types of Movement in different soils
Principal Type of Movement
Soil
Type Immediate Consolidation Creep Swell
Rock Yes No No Some
Gravel Yes No No No
Sand Yes No No No
Silt Yes Minor No Yes
Clay Yes Yes Yes Yes
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Organic Yes Minor Yes Yes
Extract from IS 1904 -1986 : General Requirements for Design &
Construction of Foundation
IS 1904-1986 presents Table 1 which gives details about the permissible settlement in steel
structures, reinforced concrete structures, multi-storeyed buildings and water towers and silos
in two different types of soils, namely (1) Sand and hard clay and (2) Plastic clay. The
settlements considered are maximum settlement, differential settlement and angular distortion
or tilt. The details in this table can be followed in the absence of more precise settlement
suggested by the user. In case of multi storeyed buildings both RC frames and load bearing
wall structures are considered. Load bearing structures with L/H 2 and 7 are dealt with. Two
types of foundations considered are isolated footing and raft foundation. Table 8.2 gives the
extract of IS code and Table 8.3 presents the same table in different form for steel and RC
structures. A maximum settlement of 75 mm, differential settlement of 0.0015L and angular
distortion of 1 in 666 is permitted for isolated footings.
Table 2 : Permissible uniform and differential settlement and tilt for shallow foundations
Table 3 : Permissible uniform and differential settlement and tilt for shallow foundations
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Sand & Hard Clay Plastic Clay
Max. Diff. Angular Max. Diff. Angular
Settlement Settlement distortion Settlement Settlement distortion
Isolated
foundation 50mm 0.0033L 1/300 50mm 0.0033L 1/300
i) Steel ii) 50mm 0.0015L 1/666 75mm 0.0015L 1/666
RCC
Raft foundation
i) Steel ii) 75mm 0.0033L 1/300 100mm 0.0033L 1/300
RCC 75mm 0.002L 1/500 100mm 0.002L 1/500
Table 4 : Limiting Values of movement for Geotechnical Structures
Design Application Parameter Typical Movement
Shallow Allowable Bearing Pressure 25 mm for buildings
Foundation
Deep Foundation Skin Friction 10 mm for skin friction to
mobilize
Retaining Wall Active & Passive earth pressure 0.1% H for Ka & 1% H for Kp to
mobilize in dense sand
Reinforced Earth wall Friction & Dilatncy to load transfer 25 to 50 mm for geogrid
in soil & reinforcement 50 to 100 mm for geotextile
Pavement Rut depth based on strain due to no. 20 mm rut depth in major roads &
of repetitions 100 mm rut depth in minor roads
Embankment Self weight settlement 0.1% height of embankment
Drainage Total settlement 100 to 500 mm
Total Settlement
Total foundation settlement can be divided into three different components, namely Immediate
or elastic settlement, consolidation settlement and secondary or creep settlement as given
below.
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
S = SI + SC + SS
Here, S = Total Settlement
SI = Immediate / Elastic Settlement
SC = Consolidation Settlement SS =
Secondary Settlement
8.4 Immediate Settlement
• Immediate settlement is also called elastic settlement.
• It is determined from elastic theory.
• It occurs in all types of soil due to elastic compression.
• It occurs immediately after the application of load
• It depends on the elastic properties of foundation soil, rigidity, size and shape of
foundation. •
Immediate settlement is calculated by the equation mentioned below.
Here,
SI = Immediate settlement
μ = Poisson’s Ratio of foundation soil
E = Young’s modulus of Foundation Soil
q = Contact pressure at the base of foundation B = Width of
foundation
I = Influence Factor
Table 8.5 presents the typical values of Poisson’s ratio in different soils. Table 8.6 represents
the ranges of soil modulus in clayey soil of different consistencies in undrained state. In the
absence of more accurate data, the values in tables can be used. The influence factor I depends
on the shape and flexibility of footing. Further, in flexible footing I is not constant. Table 8.7
presents the different values of I .
Table 5 : Typical Range of Poisson’s Ratio for different soils
Type of Soil Poisson’s Ratio
Saturated Clay 0.5
Sandy Clay 0.3 – 0.4
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Unsaturated Clay 0.35 – 0.4
Loess 0.44
Silt 0.3 – 0.35
Sand 0.15 – 0.3
Rock 0.1 – 0.4
Table 6 : Typical Range of Soil Modulus in undrained state
Soil Type Soil Modulus (kPa)
Very Soft Clay 400 – 3000
Soft Clay 1500 – 4000
Medium Clay 3000 – 8500
Hard Clay 7000 – 17000
Sandy Clay 28000 – 42000
Table 7 : Typical Values of Influence Factors I
Shape of Footing Flexible Rigid
Center Corner Mean
Circle 1.00 0.64 0.85 0.80
Rectangle L/B = 1 1.12 0.56 0.95 0.90
Rectangle L/B = 1.5 1.36 0.68 1.20 1.09
Rectangle L/B = 2 1.52 0.77 1.31 1.22
Rectangle L/B = 5 2.10 1.05 1.83 1.68
Rectangle L/B = 10 2.52 1.26 2.25 2.02
Rectangle L/B = 100 3.38 1.69 2.96 2.70
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Problem 1
Determine the elastic settlement of a footing 3 m X 3 m resting on sandy soil given Es = 45000
kPa and = 0.3. Footing carries a load of 2000 kN. Take I = 0.82 (Feb 2002)
q = 2000/32 =222.22 kPa B = 3
m SI = 0.011 m = 11 mm
I = 0.82 SI = 1−µ2 E qBIρ
E = 45000 k Pa
= 0.3
Problem 2
Estimate the immediate settlement of a concrete footing 1 m X 1.5 m in size, if it is founded at
a depth of 1 m in silty soil whose compression modulus is 9000 kPa. Footing is expected to
transmit unit pressure of 200 kPa. Assume
I = 1.06, = 0.3
Data
E = 9000 kPa SI = 1−µ2 E qBIρ
= 0.3 q =
200 kPa B =
1m
I = 1.06
SI = 0.214 m
Problem 3
A series of plate load tests was conducted on three plates 300 mm, 450 mm and 600 mm square
plates. The loads and corresponding settlements in the linear portions of P – curves are as
follows at a site. Find the immediate settlement of a footing 2 m X 2 m subjected to a load of
1000 kN.
Table 8.8 : Details of
Plate size (mm) Load (kN) Settlement (mm)
Load settlement for
different plate sizes
300 X 300 4.50 1.00
Civil Engg Dept450 X 450 8.71 1.50 AIT-Bengaluru
600 X 600 14.40 2.00
Chidanand Jadar 10CV64
Asst. Prof
Table 8.9 : Variation of qB with settlement for different plate sizes
B (m) P (kN) q S (m) qB
(kPa) (kN/m)
0.30 4.50 50.00 0.0010 15.00
0.45 8.71 43.01 0.0015 19.36
0.60 14.40 40.00 0.0020 24.00
1−µ2
SI = E qBIρ
µ
SI = 1− 2 Iρ
∴
qB E
1−Eµ2 I = 0.000111(kPa)−1 ρ
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
24
22
20
qB (kN/m)
18
16
14
12
10
0.001 0.0012 0.0014 0.0016 0.0018 0.002 0.0022
Settlement (m)
Fig. 8.3 : Variation of qB with settlement for different plate sizes
Data B = 2 m q = 1000/(2*2) =
250 kPa
1
SI = −µ2 Iρ qB
E
SI = 0.0555 m
Problem 4
The following are the results of plate load test on granular soil. Find the allowable bearing pressure if B
= 2 m, Bp = 0.3 m, permissible settlement in field = 12 mm.
Table 8.10 : Values of Load Settlement from Plate Load Test
P (kN) 5 10 20 30 40 50 60
(mm) 0.14 0.31 0.63 0.91 1.24 2.50 8.07
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Load (kN)
0 20 40 60 80
0
-1
-2
S e ttle m e n t (m m )
-3
-4
-5
-6
Pf = 50 kN;
-7 qf = 555.6 kPa
-8
-9
Fig. 8.4 : Load – Settlement curve for Plate Load Test data
sp = Bp (Bf +0.3) 2
sf Bf (Bp +0.3)
0.3(2+0.3) 2
=
2(0.3+0.3)
∴ sp = 1mm
Based on settlement
Permissible plate settlement ~ 1 mm
ABP = 32 kN/(0.3X0.3) = 355.6 kPa
Problem 5
The following results were obtained from a plate load test conducted on dry sandy stratum using square
plate of 0.3 m width. Determine the settlement of square footing 1.5 m wide when the intensity of
loading is 120 kPa.
Table 8.11 : Values of Load Settlement from Plate Load Test
Pressure (kPa) 50 100 150 200 250
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Settlement (mm) 1.2 2.4 4.8 9.6 32.0
Data
Sandy stratum
BF = 1.5 m
BP = 0.3 m
SP = 3.2 mm
Plate Load Test Result
0 50 100 150 200 250 300
0
-5
-10
)m
m
( -15
tn
e
m
e -20
lt
te
S
-25
-30
-35
Soil Pressure (kPa)
Fig. 8.5 : Load – Settlement curve for Plate Load Test data
Consolidation Settlement
1. It occurs due to the process of consolidation.
2. Clay and Organic soil are most prone to consolidation settlement.
3. Consolidation is the process of reduction in volume due to expulsion of water under an
increased load.
4. It is a time related process occurring in saturated soil by draining water from void.
5. It is often confused with Compaction.
6. Consolidation theory is required to predict both rate and magnitude of settlement.
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
7. Since water flows out in any direction, the process is three dimensional.
8. But, soil is confined laterally. Hence, vertical one dimensional consolidation theory is
acceptable.
9. Spring analogy explains consolidation settlement.
10. Permeability of soil influences consolidation. 2
SP = BP (BF +0.3)
SF BF (BP +0.3)
Table 8.12 : Compaction Vs Consolidation 3.2 0.3(1.5+0.3) 2
COMPACS TION= 1.5(0 CONSOLIDATION
.3+0.3)
1. Man made SFF = 8.89mm 1. Natural
2. Volume reduction due to expulsion 2. Volume reduction due to
of air expulsion of water 3. Gradual
3. Sudden (Short duration) 4. Dry density increases water content
4. Dry density increases water content decreases
does not change 5. Applicable for saturated soils
5. Applicable for unsaturated soils
Compaction Consolidation
Fig. 8.6 : Comparison between Compaction and Consolidation
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Fig. 8.7 : Concept of Consolidation settlement
Oedometer Test
1. Oedometer test is also called consolidometer test.
2. It is a laboratory test to determine one dimensional consolidation properties of soil.
3. Compression index and Coefficient of consolidation are the important properties
determined from this test.
4. At a given sustained pressure, rate of compression of saturated specimen with time is
measured.
5. The pressure level is gradually increased with loading and decreased with unloading.
Consolidation Settlement in normally consolidated clayey soil is given by the expression,
Sc = Consolidation Settlement
Cc= Compression Index
eo = Initial Void Ratio
H = Thickness of clay layer
σo = Initial overburden pressure at the middle of clay layer
∆σ= Extra pressure due to the new construction
8.5.2 : Computation of time taken for consolidation settlement in field
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
At any degree of consolidation, comparison in time taken for consolidation between laboratory
& field with respect to drainage path is
Here,
w= Water content
G = Specific Gravity of Soil Solids (assume 2.7)
S = Degree of Saturation (=1 for soil experiencing consolidation)
8.5.3 : Load dispersion in particulate soil medium
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Fig. 8 : Concept of Load dispersion
Coefficient of Consolidation
It is one of the important properties of consolidation theory that helps in evaluating
consolidation settlement. It is determined from oedometer test. The below
mentioned formula is used to determine coefficient of consolidation.
CV = T dV
2
t
Here, Tv is the time factor, d is the drainage path and t is time taken for
consoldation.
Time Factor
Time factor is obtained from the formulae shown below. It depends on the degree of
consolidation.
TV =1.7813−0.9332log (100−U%)
10
Commonly time factor at 50 % and 90 % degrees of consolidation are used and are
as mentioned below.
(TV)50 = 0.197
(TV)90 = 0.848
Secondary Compression
1. This settlement starts after the primary consolidation is completely over.
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
2. During this settlement, excess pore water pressure is zero.
3. The reasons for secondary settlement are not clear.
4. This is creep settlement occurring due to the readjustment of particles to a stable
equilibrium under sustained loading over a long time.
5. This settlement is common in very sensitive clay, organic soils and loose sand
with clay binders.
C = Coefficient of secondary compression
H = Thickness of clay layer
tsec = Time taken for secondary compression (usually life span of structure)
tprim = Time taken for primary consolidation to complete
(EPWP to become zero)
Coefficient of Secondary Compression
Table: Typical values of Coefficient of Consolidation in different soils
Type of Soil C
N. C. Clay 0.005 – 0.02
Organic Clay > 0.03
O. C. Clay (OCR > 2) < 0.001
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Fig. 11 : Time Vs Settlement curve representing coefficient of secondary settlement
Factors Influencing Settlement
Many factors influence the settlement of foundation soil when a structure is built on
it. The following are a few important factors to be considered in the evaluation of
settlement.
1. Elastic properties of soil
2. Shape of footing
3. Rigidity of footing
4. Contact pressure
5. Width of footing
6. Compressibility characteristics of soil
7. Initial conditions of soil (Density, void ratio etc.)
8. Degree of saturation
9. Over Consolidation Ratio
10. Time available for settlement
11. Thickness of soil layer
12. Load dispersion angle
Modulus of Subgrade Reaction
1. Modulus of subgrade reaction is defined as the ratio of contact pressure to
the corresponding settlement
2. Its units are in kN/m3
3. It depends on shape, rigidity and size of footing, depth of embedment and
type of soil
Civil Engg Dept AIT-Bengaluru
Chidanand Jadar 10CV64
Asst. Prof
Probable Questions
1. Distinguish between
1. Consolidation Settlement and Immediate settlement
2. Consolidation settlement and Secondary settlement
3. Uniform settlement and Differential settlement
2. Explain the concept of elastic settlement.
3. What is influence factor ? What factors influence it ?
4. What is contact pressure ? What factors influence the same ?
5. How are soil modulus and poisson’s ratio determined in laboratory ? Explain.
6. Explain the concept of consolidation theory.
7. What is consolidation settlement ? How is it determined ?
8. Explain the use of laboratory consolidation settlement in finding settlement of
soil.
9. List the factors influencing settlement.
10. Explain the concept of creep / secondary consolidation settlement
11. Define the terms
• Coefficient of secondary compression
• Compression Index
• Coefficient of Consolidation
• Influence factor
• Contact Pressure
12. What are the ill effects of total and differential settlement of soil on buildings ?
13. What are the different types of settlements of footings ?
14. Mention the relations used to compute elastic and consolidation settlements.
15. Write a note on BIS specifications for different types of settlements. 16. Explain
the use of plate load test in estimating the probable settlement of footing.
17. Define modulus of subgrade reaction and indicate the factors affecting it.
Civil Engg Dept AIT-Bengaluru
