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NUS geotechnical engineering - CE5101
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CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
1
CE 5101 Lecture 5 SETTLEMENTS and Stress
Distribution
Sep 2011Prof Harry Tan
1
Outline Foundation Requirements
Elastic Stress DistributionElastic Stress Distribution
Concept of Effective Stress
Settlements of Soils - Immediate, Delayed, and Creep Compression
Hand Calculations
SPREADSHEET Calculations (UNISETTLE)
Finite Element Analysis (PLAXIS)2
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
2
Requirements for Foundation Design
Adequate Safety (degree of utilisation of soil strength)
Acceptable Deformations Acceptable Deformations(Movements and Settlements limits)
3
What is Adequate Safety(Lambe and Whitman Pg 196)
-2.000 -1.000 0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000 9.000 10.000 11.000 12.000
9 000 Failure Load = 866 kPaAA
4.000
5.000
6.000
7.000
8.000
9.000
E
Stress
Strain
2c
Failure Load 866 kPa
Yielded
0.000
1.000
2.000
3.000
Plastic Points
Plastic Mohr-Coulomb point Tension cut-off point
E = 34.48 MN/m2
c=167.6 kN/m2
= 0.3
Yielded Zone
4
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
3
0.00
Footing CL Settlement [m]
Chart 1
Load vs Settlement Behaviour of Flexible Footing
Settlement (m)
-0.20
-0.15
-0.10
-0.05
Point A
First Yield Local Shear
Failure
0 200 400 600 800 1.00E+03-0.35
-0.30
-0.25
Footing Load (kPa)
General Shear Failure
Load (kPa)5
4 5
Factor of Strength Reduction
Chart 2
Factor of Safety as Measure of Degree of Utilisation of Soil Strength
2.5
3.0
3.5
4.0
4.5
Point AFS = 4.338; q = 200 kPa; qult = 868 kPa
FS = 2 164; q = 400 kPa; qult = 866 kPa
0 0.05 0.10 0.15 0.20 0.25 0.30 0.351.0
1.5
2.0
Settlement (m)
FS = 2.164; q = 400 kPa; qult = 866 kPa
FS = 1.291; q = 670 kPa; qult = 865 kPa
6
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
4
What is Allowable Settlements
Effects on:
Appearance of Structure
Utility of Structure
Damage to Structure
7
Types of Settlements
Uniform settlement
Non-uniform settlement
Angular distortion = /L
L
L
Angular distortion = /(L/2)
8
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
5
Allowable Settlements
Controlled by type of settlements (Sowers 1962)y yp ( )
Total settlements
Tilting
Differential settlements
9
Limiting Angular Distortions (Bjerrum, 1963)1/150 - Considerable cracking in panel and brick walls, safe limit for flexible brick wall h/l
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
6
Limiting Angular Distortions (Bjerrum ,1963)
Max Distortion (/L) vs Max Differential Settlement :
1/500 vs 25 mm For Stiff Footing,1/500 vs 25 mm
1/300 vs 45 mm
1/200 vs 70 mm
1/100 vs 150 mm
g,
Max Diff Sett = 1/4 Max Sett
For Flexible Footing,
Max Diff Sett = 1/2 Max Sett
Typical Foundation Design,
Max Diff Sett < 25 mm
11
Limiting Angular Distortions (Bjerrum ,1963)
350 mm
200 mm
Stiff Footing max diff sett =1/4 max sett Flexible Footing max diff sett=1/2 max sett
12
100 mm
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
7
ER2010 Review Paper by Ed Cording et al Assessment of excavation-induced building damage Damage is due to combined effects of:effects of: Angular distortion; Lateral strain Bending strain
13
Basic Soil Phase RelationshipsMASSVOLUME Densities (kg/m3)
Air
Water
Solids
0
Mw Mt
Ms
Mw
Va
Vw
Vs
Vv
Vt
Total, t = Mt/VtDry, d = Ms/VtSolids, s = Ms/VsWater, t = Mw/VwSaturated, sat Ratios= (Ms+Mw+ w Va)/VtWater content, w = Mw/Ms
Void ratio, e = Vv/Vs
Porosity, n = Vv/Vt
Degree of saturation = Vw/Vv 14
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
8
Basic Soil Phase RelationshipsDefine Vs=1, Vv=e
MASSVOLUME Densities (kg/m3)
eS w = w s
Air
Water
Solids
0 t
s
Mw
(1-e)S
eS
1
e
1+e
Total, t = s (1+w)/(1+e)= d (1+w)
Dry, d = s /(1+e)Saturated, sat = (Ms+Mw+ w Va)/VtRatios
Water content, w = t/ d - 1 Void ratio, e = s/d - 1Porosity, n = e/(1+e)
Degree of saturation = S = ws / ew 15
Common soil mineral densitiesMineral Type Solid Density, kg/m3
Calcite 2800
Quartz 2670
Mica 2800
Pyrite 5000
Kaolinite 2650
Montmorillonite 2750
Illite 270016
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
9
Typical range of saturated densitiesSoil Type Saturated Density, kg/m3
Sands;gravels 1900-2300
Silts 1500-1900
Soft Clays 1300-1800
Firm Clays 1600-2100
Peat 1000-1200
Organic Silt 1200-1900
Granular Fill 1800-220017
The Textbooks on Foundations They come no better
This is one of the few showingmore than one soil layer
18
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
10
The Reality With a bit of needed W add-on
19
The Reality Getting closer, at least
20
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
11
So, with this food for thought, , g ,on to the Fundamental
Principles
21
Determining the effective stress is the key to geotechnical analysis
The not-so-good method:method:
h '' )'(' hz
= buoyant unit weight
)1(' iwt 22
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
12
It is much better to determine, separately, the total stress and the pore pressure. The effective stress is then the total stress minus the pore pressureminus the pore pressure.
)( hz uz '
23
Determining pore pressure
u = w hu w hThe height of the column of water (h; the head representing the water pressure)is usually not the distance to the ground surface nor, even, the distance to thegroundwater table. For this reason, the height is usually referred to as thephreatic height or the piezometric height to separate it from the depth belowthe groundwater table or depth below the ground surface.
The pore pressure distribution is determined by applying the facts that
(1) in stationary conditions, the pore pressure distribution can be assumed to be linear in each individual soil layer
SAND Hydrostatic distribution
GW
PRESSURE
(2) in pervious soil layers that are sandwiched between less pervious layers, the pore pressure is hydrostatic (that is, the vertical gradient is unity)
CLAY Non-hydrostatic distribution, but linear
SAND Hydrostatic distribution Artesian phreatic head
DEPTH
24
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
13
Distribution of stress below a a small load area
The 2:1 method
)()(0 zLzBLBqqz
The 2:1-method can only be used for distributions directly under the centerof the footprint of the loaded area. It cannot be used to combine (add)stresses from adjacent load areas unless they all have the same center. it isthen only applicable under the area with the smallest footprint. 25
Boussinesq Method for stress from a point load
2/522
3
)(23
zrzQqz
26
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
14
Newmarks method for stress from a loaded area
Newmark (1935) integrated the Boussinesq equation over a finite area and obtained a relation for the stress under the corner of aarea and obtained a relation for the stress under the corner of a uniformly loaded rectangular area, for example, a footing
40CBAIqqz
2222
22
112nmnm
nmmnA
(1)
12
22
22
nmnmB
2222
22
112arctannmnm
nmmnC
m = x/zn = y/zx = length of the loaded areay = width of the loaded areaz = depth to the point under the corner
where the stress is calculated
27
Eq. 1 does not result in correct stress values near the ground surface. To represent the stress near the ground surface, Newmarks integration applies an additional equation:
40CBAIqqz
F h 2 + 2 + 1 2 2
(2)
For where: m2 + n2 + 1 m2 n2
28
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
15
Stress distribution below the center of a square 3 m wide footing
0 0.25Eq. (2) Eq. (2)
-4
-2
DE
PTH
(m)
0.10
0.15
0.20
INFL
UE
NC
E F
AC
TOR
, IEq. (1)
Eq. (1)
0 20 40 60 80 100-6
STRESS (KPa)0.01 0.10 1.00 10.00
0.00
0.05
m and n (m = n)
29
00 25 50 75 100
STRESS (%)
Boussinesq
Westergaard
00 25 50 75 100
SETTLEMENT (%)
Boussinesq
Westergaard
1
2
3
EPTH
(di
amet
ers)
2:1
1
2
3
EPTH
(di
amet
ers)
q
2:1
Comparison between Boussinesq, Westergaard, and 2:1 distributions
4
5
DE
4
5
DE
30
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
16
00 25 50 75 100
STRESS (%)
Westergaard
00 25 50 75 100
SETTLEMENT (%)
Westergaard
1
2
3
PTH
(di
amet
ers)
Boussinesq
1
2
3
PTH
(di
amet
ers)
Boussinesq
4
5
DEP 2:1
4
5
DEP
2:1
31
0
1
0 25 50 75 100
STRESS (%)
Westergaard
0
1
0 25 50 75 100
SETTLEMENT (%)
Westergaard
1
2
3
EPTH
(di
amet
ers)
2:1
Boussinesq
Characteristic Point; 0.37b from center
1
2
3
EPTH
(di
amet
ers)
Boussinesq
2:1 Characteristic Point; 0.37b from center
4
5
D
4
5
D
Below the characteristic point, stresses for flexible and stiff footings are equal 32
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
17
Now, if the settlement distributions are so similar, why do we persist in using
Boussinesq stress distribution instead of the much simpler 2:1 distribution?
Because a footing is not alone in this world; near by, there are other footings, and fills,near by, there are other footings, and fills,
and excavation, etc., for example:
33
The settlement imposed
00 25 50 75 100
SETTLEMENT (%)
BoussinesqOutside Point Boussinesq
Center PointThe settlement imposed outside the loaded
foundation is often critical
1
2
3
PTH
(di
amet
ers)
Center Point
4
5
DE
Loaded area
34
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
18
Calculations using Boussinesq distribution can be used to determine how stressapplied to the soil from one building may affect an adjacent existing building.
EXISTING NEW 0 20 40 60 80 100STRESS (%)
ADJACENT BUILDING
BUILDING WITH LARGE LOAD OVER FOOTPRINT
AREA
2 m2 m 4 m
0
5
10
15
DEP
TH (
m) STRESSES
UNDER THE FOOTPRINT OT THE LOADED BUILDING
STRESSES UNDER AREA
BETWEEN THE TWO BUILDINGS
20
25
30
STRESSES ADDED TO THOSE UNDER THE FOOTPRINT OF THE ADJACENT BUILDING
35
Calculation of Stress DistributionDepth 0 u0 0 1 u1 1(m) (KPa) (KPa) (KPa (KPa) (KPa) (KPa)
Layer 1 Sandy silt = 2,000 kg/m30.00 0.0 0.0 0.0 30.0 0.0 30.0STRESS (KPa)0.00 0.0 0.0 0.0 30.0 0.0 30.0
GWT 1.00 20.0 0.0 20.0 48.4 0.0 48.42.00 40.0 10.0 30.0 66.9 10.0 56.93.00 60.0 20.0 40.0 85.6 20.0 65.64.00 80.0 30.0 50.0 104.3 30.0 74.3
Layer 2 Soft Clay = 1,700 kg/m34.00 80.0 30.0 50.0 104.3 30.0 74.35.00 97.0 40.0 57.0 120.1 43.5 76.66.00 114.0 50.0 64.0 136.0 57.1 79.07.00 131.0 60.0 71.0 152.0 70.6 81.48.00 148.0 70.0 78.0 168.1 84.1 84.09.00 165.0 80.0 85.0 184.2 97.6 86.6
10.00 182.0 90.0 92.0 200.4 111.2 89.211.00 199.0 100.0 99.0 216.6 124.7 91.912.00 216.0 110.0 106.0 232.9 138.2 94.613.00 233.0 120.0 113.0 249.2 151.8 97.414.00 250.0 130.0 120.0 265.6 165.3 100.315.00 267.0 140.0 127.0 281.9 178.8 103.116.00 284.0 150.0 134.0 298.4 192.4 106.0
0
5
10
15
0 100 200 300 400 500
( )
DEP
TH (m
)
SAND
CLAY
17.00 301.0 160.0 141.0 314.8 205.9 109.018.00 318.0 170.0 148.0 331.3 219.4 111.919.00 335.0 180.0 155.0 347.9 232.9 114.920.00 352.0 190.0 162.0 364.4 246.5 117.921.00 369.0 200.0 169.0 381.0 260.0 121.0
Layer 3 Silty Sand = 2,100 kg/m321.00 369.0 200.0 169.0 381.0 260.0 121.022.00 390.0 210.0 180.0 401.6 270.0 131.623.00 411.0 220.0 191.0 422.2 280.0 142.224.00 432.0 230.0 202.0 442.8 290.0 152.825.00 453.0 240.0 213.0 463.4 300.0 163.4
20
25
SAND
HYDROSTATIC PORE PRESSURE DISTRIBUTION
36
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
19
Calculation of Stress DistributionSTRESS DISTRIBUTION (2:1 METHOD) BELOW CENTER OF EARTH FILL (Calculations by means of UniSettle)
ORIGINAL CONDITION (no earth fill) FINAL CONDITION (with earth fill and artesian pore pressure in sand)
Depth 0 u0 0 1 u1 1(m) (KPa) (KPa) (KPa (KPa) (KPa) (KPa)
Layer 1 Sandy silt = 2,000 kg/m30.00 0.0 0.0 0.0 30.0 0.0 30.0
GWT 1 00 20 0 0 0 20 0 48 4 0 0 48 4GWT 1.00 20.0 0.0 20.0 48.4 0.0 48.42.00 40.0 10.0 30.0 66.9 10.0 56.93.00 60.0 20.0 40.0 85.6 20.0 65.64.00 80.0 30.0 50.0 104.3 30.0 74.3
Layer 2 Soft Clay = 1,700 kg/m34.00 80.0 30.0 50.0 104.3 30.0 74.35.00 97.0 40.0 57.0 120.1 43.5 76.66.00 114.0 50.0 64.0 136.0 57.1 79.07.00 131.0 60.0 71.0 152.0 70.6 81.48.00 148.0 70.0 78.0 168.1 84.1 84.09.00 165.0 80.0 85.0 184.2 97.6 86.6
10.00 182.0 90.0 92.0 200.4 111.2 89.211.00 199.0 100.0 99.0 216.6 124.7 91.912.00 216.0 110.0 106.0 232.9 138.2 94.613.00 233.0 120.0 113.0 249.2 151.8 97.414.00 250.0 130.0 120.0 265.6 165.3 100.315 00 267 0 140 0 127 0 281 9 178 8 103 115.00 267.0 140.0 127.0 281.9 178.8 103.116.00 284.0 150.0 134.0 298.4 192.4 106.017.00 301.0 160.0 141.0 314.8 205.9 109.018.00 318.0 170.0 148.0 331.3 219.4 111.919.00 335.0 180.0 155.0 347.9 232.9 114.920.00 352.0 190.0 162.0 364.4 246.5 117.921.00 369.0 200.0 169.0 381.0 260.0 121.0
Layer 3 Silty Sand = 2,100 kg/m321.00 369.0 200.0 169.0 381.0 260.0 121.022.00 390.0 210.0 180.0 401.6 270.0 131.623.00 411.0 220.0 191.0 422.2 280.0 142.224.00 432.0 230.0 202.0 442.8 290.0 152.825.00 453.0 240.0 213.0 463.4 300.0 163.4
Aquifer with artesian head
37
Stress Distribution
00 100 200 300 400 500
STRESS (KPa)
00 100 200 300 400 500
STRESS (KPa)Stress from Fill
5
10
15DEP
TH (m
)
5
10
15DEP
TH (m
)
SAND
CLAY
20
25
20
25
SAND
Artesian Pore Pressure HeadThe distribution for the hydrostatic case
38
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
20
Effective Stress Concept
Effective stress = Total stress - Pore pressure
= - uUnit weight = g (kN/m3)g assumed to be 10 m2/s
T t l ti l t b dTotal vertical stress or overburden stress, z = t.z
39
Pore Water Pressure (PWP)
Pore water pressure u = uw = uss + uexcuss = Steady state condition, hydrostatic or steady seepage
uexc = Excess pwp due to soil loading
Buoyant unit weight
= t - w for hydrostatic condition = t - w + i wi = hydraulic gradient (head diff/ distance)
i is negative for upward artesian flow 40
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
21
Elastic Stress Distribution with DepthQ
QqL
B
1(H):2(V)zL+z
B+z
qo
qz
BLQqo
))(( zLzBBLqq oz
Only can be used for stress at centre ofOnly can be used for stress at centre of loaded area
Cannot use for combined effects of two or more loaded areas, unless they have same centres
41
Boussinesq Distribution (1885)Assumes: isotropic linear elastic halfspace, Poisson ratio = 0.5
Q
z
For Point load Q kN
2/522
3
)(2)3(zrzQqz
Integrate for Line Load, P kN/m
32Pr
qz 222
3
)(2
rzzPqz
Integrate for Rectangular Area, get Fadums Influence Chart Fig.1; for Circular Area, get Foster and Alvin Chart Fig.2
42
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
22
Weastergaard Distribution (1938)Assumes: isotropic linear elastic halfspace, Poisson ratio = 0, rigid horizontal layers
Q
z
, g y
For Point load Q kN
2/322 ))/(21( zrzQqz
Differences with Boussinesq is
rqz
qsmall
For wide flexible loaded areas Westergaard method is preferred
43
UniSettle 2.4 21 Jan 2000EX02.STL page 1
Effective Stress Comparison, ( 4.11 , 8.11 )-----------------------------------------------------------------
BOUSSINESQ WESTERGAARD 2:1Depth Ini. Fin. Ini. Fin. Ini. Fin.
Stress Stress Stress Stress Stress Stress(m) (kPa) (kPa) (kPa) (kPa) (kPa) (kPa)
-----------------------------------------------------------------
Comparison of stresses under a 3m square footing below its
Layer 1 Any Soil 0. kg/m^30.00 100.0 0.0 100.0 0.0 100.0 0.00.10 97.9 0.0 89.3 0.0 93.7 0.00.20 94.3 0.0 79.4 0.0 87.9 0.00.30 89.4 0.0 71.4 0.0 82.6 0.00.40 83.2 0.0 64.4 0.0 77.9 0.00.50 76.8 0.0 58.4 0.0 73.5 0.00.60 70.9 0.0 53.2 0.0 69.4 0.00.70 65.6 0.0 48.9 0.0 65.7 0.00.80 61.1 0.0 45.1 0.0 62.3 0.00.90 57.2 0.0 41.9 0.0 59.2 0.01.00 53.7 0.0 39.1 0.0 56.3 0.01 10 50 8 0 0 36 7 0 0 53 5 0 0
q gcharacteristic point
Characteristic point is point where vertical stress is equal for both rigid and flexible footing, this point is located at 0.37B and 0.37L from centre of rectangular footing, or 0.37R of 1.10 50.8 0.0 36.7 0.0 53.5 0.0
1.20 48.1 0.0 34.5 0.0 51.0 0.01.30 45.8 0.0 32.5 0.0 48.7 0.01.40 43.7 0.0 30.8 0.0 46.5 0.01.50 41.7 0.0 29.2 0.0 44.4 0.01.60 40.0 0.0 27.8 0.0 42.5 0.01.70 38.3 0.0 26.4 0.0 40.7 0.01.80 36.8 0.0 25.2 0.0 39.1 0.01.90 35.4 0.0 24.1 0.0 37.5 0.02.00 34.0 0.0 23.0 0.0 36.0 0.0
g g,circular footing
Results show that under characteristic point 2:1 method is similar to Boussinesq result
44
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
23
AA
-1.000 0.000 1.000 2.000 3.000 4.000 5.000 6.000
Elastic Stress Bulb for Circular Footing
BCDEFGHIJKLMNOPQRS
2.000
3.000
4.000
5.000
[ kN/m2]
A : -0.900
B : -0.850
C : -0.800
D : -0.750
E : -0.700
F : -0.650
G : -0.600
H : -0.550
I : -0.500
J : -0.450
K : -0.400
L : -0.350
M : -0.300
N : -0.250
O : -0.200
P : -0.150
-0.000
1.000
E ffective mean stressesExtrem e effec tive mean stress -882.98*10
-3 kN/m
2
Q : -0.100
R : -0.050
S : 0.000
T : 0.050
45
Elastic Settlement for Circular Flexible LoadSurface settlements is given by Terzaghi,1943 as:
R
q
R
D =
3.),( FigseerfisI
IEqR
Edge settlement = 0.7 centre settlement
Centre settlement is :
footingFlexibleforERqz )1(2
2
footingRigidforERqz )1(2
2 46
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
24
Flexible and Rigid Footing
-0.300 -0.200 -0.100 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100
Flexible Footing-0.300 -0.200 -0.100 0.000 0. 100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100
Rigid Footing
0.08 0.16 0.24 0. 32 0.40 0.48 0.56 0.64 0.72 0.80 0.88 0.96 1.04
10.40
10.48
A
A
9.300
9.400
9.500
9.600
9.700
9.800
9.900
10.000
10.100
10.200
9.200
9.300
9.400
9.500
9.600
9.700
9.800
9.900
10.000
10.100
10.200
0.08 0. 16 0.24 0.32 0.40 0.48 0.56 0.64 0.72 0.80 0.88 0.96 1.04
10 40
10.48
AA*
9.76
9.84
9.92
10.00
10.08
10.16
10.24
10.32
E=1000 kPa, =0, q=10 kPa, sett= 20 mm
AA*
9.76
9.84
9.92
10.00
10.08
10.16
10.24
10.32
10.40
E=1000 kPa, =0, q=10 kPa, sett= 16 mm47
Approximate Ratio at corner, centre and edge to average settlement
Flexible Load Area RigidgFooting
FoundationDepth
Corner/Ave
Edge/Ave
Centre/Ave
Rigid/Ave
H/L= 0.6 0.9 1.2 0.9H/L=1 0.5 0.7 1.3 0.8
H/L=1/4 0.4 0.7 1.3 0.8
48
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
25
Elastic Settlement for Other Flexible LoadCorner settlements is given by Terzaghi,1943 as:
B
D =
4.)/(
)1( 2
FigseeLBfisI
IE
qB
Points other than corner for any combination of rectangles can be obtained by superposition
qL
For centre of square loaded area:
)1(12.1 2 EBqz
49
Superpostion Principle for Rectangles
= + + +z
- - +=z
50
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
26
Elastic Settlement for Other Flexible Load
For Flexible Rectangular Loaded area, Corner settlement , g , ,with finite depth of elastic layer, use Steinbrenner chart
)21()1(
)1(
22
2
FFI
IE
qB B
qL
5.
)21()1(
21
22
12
FigingivenFandFFFI
FiniteD
51
Equivalent Footings for Pile Groups SettlementsGround level
Soft Clay
Ground level
L2/3L
Equivalent FtgL
2/3L
Soft Clay
Equivalent Ftg
2
1Homogeneous Clay
2
1Firm Layer
52
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
27
The end result of a geotechnical design analysis
is
Settlement
53
Movement, Settlement, and CreepMovement occurs as a result of an increase of stress, but the term should be reservedto deformation due to increase of total stress. Movement is the result of a transfer ofstress to the soil (the movement occurs as necessary to build up the resistance to theload), and when the involved, or influenced, soil volume successively increases as thestress increases. For example, when adding load increments to a pile or to a plate in astatic loading test (where, erroneously, the term "settlement" is often used). As a term,mo ement is sed hen the in ol ed or infl enced soil ol me increases as the loadmovement is used when the involved, or influenced, soil volume increases as the loadincreases.
Settlement is volume reduction of the subsoil as a consequence of an increase ineffective stress. It consists of the sum of "elastic" compression and deformation due toconsolidation. The elastic compression is the compression of the soil grains (soilskeleton) and of any free gas present in the voids, The elastic compression is oftencalled "immediate settlement. It occurs quickly and is normally small (theelastic compression is not associated with expulsion of water). The deformation due toconsolidation on the other hand is volume change due to the compression of the soilconsolidation, on the other hand, is volume change due to the compression of the soilstructure associated with an expulsion of waterconsolidation. In the process, theimposed stress, initially carried by the pore water, is transferred to the soil structure.Consolidation occurs quickly in coarse-grained soils, but slowly in fine-grained soils. Asa term, settlement is used when the involved, or influenced, soil volume stays constantas the effective stress increases.
54
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
28
Movement, Settlement, and Creep
Creep is compression occurring without an increase of effective stress.Creep is usually small, but may in some soils add significantly to thecompression of the soil skeleton and, thus, to the total deformation of thesoil It is then acceptable to talk in terms of creep settlement
The term "settlement" is normally used for the deformation resulting from thecombined effect of load transfer, increase of effective stress, and creep duringlong-term conditions. It is incorrect to use the term settlement to meanmovement due to increase of load such as in a loading test.
soil. It is then acceptable to talk in terms of creep settlement.
55
StrainLinear Elastic Deformation (Hookes Law)
E'
= induced strain in a soil layer= imposed change of effective stress in the soil layer
E = elastic modulus of the soil layer (Youngs Modulus)
Youngs modulus is the modulus for when lateral expansion is allowed, which may be the case for soil loaded by a small footing, but not when the load is applied over a large area. In the latter case, the lateral expansion is constrained (or confined). The constrained modulus, D, is larger than the E-modulus. The constrained modulus is also called the oedometer modulus. For ideally elastic soils, the ratio between D and E is:
'
)21()1()1(
ED
= Poissons ratio56
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
29
Stress-Strain
57
Stress-strain behavior is non-linear for most soils. The non-linearity cannot be disregarded when analyzing compressible soils, such as silts and clays, that is, the elastic modulus approach is not appropriate for these soils.
Non-linear stress-strain behavior of compressible soils, is conventionally modeled as follows.
where = strain induced by increase of effective stress from 0 to 1
0
1
0
1
0 ''
lg''
lg1
CR
eCc
y 0 1Cc = compression indexe0 = void ratio0 = original (or initial) effective stress1 = final effective stressCR = Compression Ratio = (MIT)
01 eCCR c
58
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
30
In overconsolidated soils (most soils are)
)''lg
''
lg(1
1 100 p
cp
cr CCe
where p = preconsolidation stressp pCcr = re-compression index
59
The Janbu Method
The Janbu tangent modulus approach, proposed by Janbu (1963; 1965; 1967; 1998), and referenced by the Canadian Foundation Engineering Manual, CFEM (1985; 1992), applies the same basic principles of linear and non-linear stress-strain behavior. The method applies to all soils, clays as well as sand. By this method, the relation between stress and strain is a function of two non-dimensional parameters which are unique for a soil: a stress exponent, j, and a modulus number, m.
Janbus general relation is
])'()'[(1 01 jj ])'
()'
[( 01 jr
j
rmj
where r is a reference stress = 100 KPa
j > 0
60
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
31
The Janbu Method
Dense Coarse-Grained Soil j = 1
'1)''(1 01 mm
'1)''(1
KPa
ksf
Cohesive Soil j = 00
1
''ln1
m
2)(
2 01
mmksf
)''(51
01 m KPaSandy or Silty Soils j = 0.5
pm''(2 1 ksf
61
There are direct mathematical conversions
between m and the E and Cc-e0
For E given in units of KPa (and ksf), the relation between the modulus number and the E-modulus is
m = E/100 (KPa)m = E/2 (ksf)
For Cc-e0, the relation to the modulus number is
22
00 69.02lg10ln13.21
10ln
cc Ce
Cem
62
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
32
Typical and Normally Conservative Virgin Modulus Numbers
SOIL TYPE MODULUS NUMBER STRESS EXP.
Till, very dense to dense 1,000 300 (j = 1)
Gravel 400 40 (j = 0.5)
Sand dense 400 250 (j = 0.5)compact 250 150 - " -loose 150 100 - " -
Silt dense 200 80 (j = 0.5)compact 80 60 - " -loose 60 40 - " -
Silty clay hard, stiff 60 20 (j = 0)and stiff, firm 20 10 - -
Clayey silt soft 10 5 - -
Soft marine claysSoft marine claysand organic clays 20 5 (j = 0)
Peat 5 1 ( j= 0)
For clays and silts, the recompression modulus, mr, is often five to tentimes greater than the virgin modulus, m, listed in the table
63
1 00
1.20
p'c 20
25
Evaluation of compressibility from oedometer results
0.40
0.60
0.80
1.00
10 100 1 000 10 000
Voi
d R
atio
(- -
)
m = 12(CR = 0.18)
0
5
10
15
10 100 1,000 10,000
Stra
in (
%)
p 10p
Cc
Cc = 0.37
e0 = 1.01 p'c
p 2.718p
1/m
Slope = m = 12
10 100 1,000 10,000
Stress (KPa) log scale Stress (KPa) log scale
Void-Ratio vs. Stress and Strain vs. Stress Same test data
64
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
33
Comparison between the Cc/e0 approachand the Janbu method
0 30
0.35
30
35
m
0.05
0.10
0.15
0.20
0.25
0.30
CO
MPR
ESSI
ON
IND
EX, C
c Do these values indicate a
compressible soil, a medium compressible
soil, or a non-compressible soil?5
10
15
20
25
30
VIR
GIN
MO
DU
LUS
NU
MB
ER, m
Data from a 20 m thick sedimentary deposit of medium compressibility.
0.000.40 0.60 0.80 1.00 1.20
VOID RATIO, e0
00.400.600.801.001.20
VOID RATIO, e0
65
75
100
125
150
OR
MA
LIZE
D C
c (%
)
75
100
125
150
OR
MAL
IZED
m (
%)
The Cc-e0 approach implies that the the compressibility varies by 30 %.
However, the Janbu methods shows it to vary only by 10 %. The modulus number, m, ranges from 18 through 22; It would be unusual to find a clay with less variation
500.40 0.60 0.80 1.00 1.20
VOID RATIO, e0
N 500.400.600.801.001.20
VOID RATIO, e0
NO
find a clay with less variation.
What about Immediate Settlement and Consolidation? 66
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
34
CE 5101 - SETTLEMENTS AND 1D Compression TheorySettlements of Soils - Immediate, Delayed, and Creep Compressionp p
Delayed Consolidation Compressionmv methode-logP methodJanbu method
Terzaghis Theory of 1D ConsolidationTerzaghi s Theory of 1D ConsolidationEffects of Drainage and Initial Stress Distribution
SPREADSHEET Calculations (UNISETTLE)Finite Element Analysis (PLAXIS)
67
Foundation Settlement Issues
How Much settlements will occur?
Interested in Ultimate settlements in fully drained state, as well as long-term creep settlements
How Fast and how long will it take for most of settlements to occur?
Involved Consolidation and SecondaryInvolved Consolidation and Secondary Compression theories to estimate rate of settlements, and
Methods to accelerate settlements and minimise long-term settlements
68
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
35
Types of Ground Movements and Causes of Settlements
Compaction - due to vibrations, pile driving, earthquake
El ti V l t i S ttl t i OC Cl i iElastic Volumetric Settlement in OC Clay in recompression, use E and or recompression index, Cr or Immediate or Undrained Settlement - Distortion without volume change, use Eu and uMoisture changes - Expansive soils, high LL and PI, high swelling and shrinkage
S ll P t ti l (%) 0 1(PI 10) l ( / )Swell Potential (%) = 0.1(PI-10) log (s/p)
Effects of vegetation - related to moisture changes by root system
Effects of GWT lowering - shrinkage ad consolidation
69
Types of Ground Movements and Causes of Settlements
Effects of temperatures - Frost heaving, drying by furnace and boilers
Effects of seepage and scouring - Erosion byEffects of seepage and scouring - Erosion by piping, scouring and wind action, mineral cement dissolved by GW eg limestone, rock salts and chalk areas
Loss of lateral support - Footings beside unsupported excavation, movement of natural slopes and cuttings
Effects of mining subsidence - collapse of ground cavities
Filled ground - settlements of the fill soils, compaction, consolidation, and creep
70
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
36
Shrink/ swell potential is function of Clay Activity = %clay fraction/ PI
Swell Potential (%) = 0.1(PI-10) log (s/p)
Where s=suction before construction and p=final bearing pressure71
1D Settlements
Soil deformations are of two types: Distortion (change of shape 2D effects)Compression (change of volume)
Components of Settlements:
distortionsettlementimmediateswheressss
i
scit
,
ncompressiosecondarysncompressiosettlementionconsolidats
distortionsettlementimmediateswhere
s
c
i
,
,
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CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
37
Undrained Immediate Settlements
Distortion of clay layer:Calculate by elastic theory eg JanbuCalculate by elastic theory eg Janbu Chart
by JanbufactorsessdimensionlIandIarealoadedflexibleforsettlementimmediateswhere
EqBIIs
i
i
)1( 210
layerclay of ratio sPoisson' layerclay of modulus Undrained E
B dthfootong wi on loadAverage qby JanbufactorsessdimensionlIandI 10
73
Immediate Settlements in Clays by Janbu
74
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
38
Example on Janbu Chart: Foundation 4x2m with q=150kPa, located at 1m in Clay layer 5m thick with Eu=40MN/m2. Below is second clay layer of 8m thickness and Eu=75 MN/m2.What is average settlement under foundation? Assume =0.5
D:MN/m40Ewithlayer,clay upperConsider(1)
9.0 0.5, 1/2D/B 2;4/2L/B Now2
u
0
H
DB mm5.35.01
402*150*7.0*9.0s
7.0 2,4/2L/B and 24/2H/By ,ypp( )
21i
1
u
mm3.25.0175
2*150*85.0*9.0s
85.0 2,4/2L/B and 612/2H/B:MN/m75E withcombined, layers two Consider (2)
22i
1
2u
mm9.15.0175
2*150*7.0*9.0s
7.0 2,4/2L/B and 24/2H/B:MN/m75E withlayer, upper Consider (1)
23i
1
2u
mm 3.9 1.9 -2.3 5.3sssss
;inciplePrionSuperposit By
i
3i2i1ii
75
1D Primary Consolidation Settlements
Time delayed Primary Consolidation Compression:
water
solids
0e
1
e0H
H
)()(
00
0
volumeverticalc
0volumevertical
HHHse1e
HH
'vv P where P), vs(e methodm e
Pe -ility compressib of coeffnav
a1e0e
fe
fP0P P
0
v
0v e1
aP1
)e(1e -change volume of coeffnm
00
vc
0v00
c
HPe1
a s
HPm He1e sH
76
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
39
mv and Constrained Modulus D
For wide loaded area, get 1D compression d K diti l ti d l t lunder Ko condition, elastic modulus to apply
is called the Constrained Modulus D defined by:
hlftC ffi ih211
1Em1D
v
0.35)( ratio Poisson drained Soil elasticity of modulus drained Soil E
changevolume oftCoefficienmwhere v
77
Sc by (e vs logP) MethodNormally Consolidated Clays - non-linear stress strain for soil, but linear in logP Pge
0e
fe
indexncompressioCc
0
fc0fc P
PlogCPlogPlogCe
0
f
0
c0c P
Plog
e1CHHs
fP0P Plog
0
cc e1
CC CRRatio, nCompressio
78
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
40
Sc by (e vs logP) MethodOver-Consolidated Clays (Pf < Pc), Preconsolidation Pressure
e
0efe
fP0P Plog
indexncompressioCc
0
fr0fr P
PlogCPlogPlogCe
0
f
0
r0c
ssettlementsmallPP
loge1
CHHs cP
indexswellingCindexionrecompressC
s
r
f0 Plog
cr C than smaller times10 to5 is C asssettlementsmall
0
rr e1
CC RR, Ratio ncompressioRe
c
79
Sc by (e vs logP) MethodOver-Consolidated Clays (Pf > Pc), Preconsolidation Pressure
Pe0ece
fP0P Plog
cC
0
cr0cr1 P
PlogCPlogPlogCe
e1eeHHs 210c
cP
sr CorC
fe
c
fccfc2 P
PlogCPlogPlogCe
f0 Plog
ssettlementlarge mean willThisPP
logCPPlogC
e1H
e1
c
fc
0
cr
0
0
0
c
80
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
41
Sc by Janbu Tangent Modulus MethodCanadian Foundation Manual 1985
Janbu (1963, 1965,1967,1998); For Cohesionless Soils; j>0
Th i d i P U iS ttl (b UNISOFT C d )Theory is used in Program UniSettle (by UNISOFT, Canada)
P stresseffective vertical initial
stresseffective vertical inchange by induced strain vertical where
mj1 get to Integrate m'M
0,
0
v
j
,
r
,
0
j
,
r
,
fv
j1
'r
''rt
kPa100 stresseffective verticalReference
test field or lab from obtained number, modulus Janbu mexponent stress Janbuj
P stresseffective vertical final
,
r
f,
f
0
81
Cohesionless Sands and Silts; j=0.5Normally Consolidated Cohesionless Soils
,0,fv
,
r
5m1
kPa100 0.5j SOIL TYPE mSAND DENSE 400-250
MEDIUM 250-150
LOOSE 150-100
SILT DENSE 200-80
MEDIUM 80-60
LOOSE 60-40
82
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
42
Cohesionless Sands and Silts; j=0.5Over-Consolidated Cohesionless Soils
kPa100 'P'f
,
r0.5j; for CASE
,
0
,
fr
v
,
r
'p
'f
5m1
kPa,100 ; for CASE ,0.5j
m than larger times10 to5 usually is m number, modulus OC
kPa inpressure dationpreconsoli
where5m
15m
1
f
r
,
P
,
P
,
f
,
0
,
Pr
v
r
83
Cohesive Soils; j=0
Normally Consolidated Clays
, kPa1000j SOIL TYPE mSILTY HARD, 60-20
c
0
c
0
,
0
,
f
0
c,
0
,
fv
r
Ce13.2
Ce110lnm
loge1
Clnm1
kPa100 0j
SILTY
CLAYSHARD,STIFF
60 20
AND STIFF,FIRM
20-10
CLAYEYSILT
FIRM TOSOFT
10-5
SOFTMARINEC S
20-5
CLAYSORGANICCLAYS
10-3
PEAT 5-1
84
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
43
Over-consolidated Clays; j=0Over- Consolidated Clays
,
P
,
f
0
c,
0
,
P
0
rv
,
P
,
f,
0
,
P
rv
loge1
Cloge1
C
lnm1ln
m1
r
0
r
0r
c
0
c
0
Ce13.2
Ce110lnmand
Ce13.2
Ce110lnm
85
Linear Elastic Soil; j=1
j,j,
Em100E,therefore
m1001
mj1 ,
0
,
f,
r
,
0,
r
fv
100Em
86
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
44
Janbu compared to e-logP
87
Oedometer Results in linear scale by Janbu
88
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
45
Calculation of Settlement
Determine soil profile to get initial effective stresses
Determine soil compressibility parameters, e-logP, Cc, Cr and Pc OR Janbu modulus number, m and mr
Determine final effective stresses due to imposed loads, excavations, fills, GWT changes etc
Divide each soil layer into sublayers, calculate strain caused by change from initial to final effective stresses in each sublayer
Calculate the settlement for each sublayer and the accumulated settlement
89
UNISETTLE Calculation of Settlement
90
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
46
Hand Calculation
91
UniSettle Input
92
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
47
InputLoading and Excavation
93
Results
94
CE5101 Consolidation and SeepageLecture 5
Prof Harry TanSEP 2011
48
Settlements Distribution
95
Alternative Conditions
96