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Civil Engineering - Texas Tech University
CE 3121: Geotechnical Engineering Laboratory
Class 7
Consolidation Test on Cohesive Soil
Sources:
Soil Mechanics – Laboratory Manual, B.M. DAS (Chapter 17)
Civil Engineering - Texas Tech University
Consolidation
Definitions & Introduction
Significance
Consolidation vs Compaction
Type of Consolidations
One-Dimensional Consolidation Test
Definition
Procedure
Graphs and results
Class Outlines
Civil Engineering - Texas Tech University
Consolidation - Definition
Consolidation refers to the compression or
settlement that soils undergo as a response of
placing loads onto the ground
These loads produce corresponding increases
in the vertical effective stress, sv’
Consolidation is a time-dependent process, in
some soils it may take long time (100 years ?)
to achieve complete settlement
Civil Engineering - Texas Tech University
Significance
The amount of soil volume change that will occur is
often one of the governing design criteria of a
project
If the settlement is not kept to tolerable limit, the
desire use of the structure may be impaired and
the design life of the structure may be reduced
It is therefore important to have a mean of
predicting the amount of soil compression or
consolidation
It is also important to know the rate of consolidation
as well as the total consolidation to be expected
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Consolidation - Case Study
Palacio de las Bellas, Artes, Mexico City
Total settlement = 9ft The Leaning Tower of Pisa
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Consolidation - Introduction
The compression is caused by:
Deformation of soil particles
Relocations of soil particles
Expulsion of water or air from void spaces
Most of the settlement of a structure on clay is
mainly due to volumetric changes and rarely
due to shear strain.
Civil Engineering - Texas Tech University
Consolidation vs. Compaction
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Consolidation (cont.)
During consolidation, pore water or the water in the
voids of saturated clay gets squeezed out – reducing
the volume of the clay – hence causing settlement
called as consolidation settlement
The spring
analogy to
consolidation.
Civil Engineering - Texas Tech University
Voids Voids
Solids
H
Vv = eVs
Vs
c
e
Vv = (e - e)Vs
Vs
Solids
sz′
sz′
sz0′
sz0′
sz0′
sz0′
}sz f′
}sz f′
Before After
Consolidation (cont.)
Civil Engineering - Texas Tech University
Types of Consolidation
There are three types of consolidation:
Immediate consolidation; caused by elastic deformation of dry soil or moist and saturated soil without change in moisture content
Primary consolidation; caused as a result of volume change in saturated cohesive soils due to exclusion of water occupied the void spaces
Secondary consolidation; occurs in saturated cohesive soils as a result of the plastic adjustment of soil fabrics
Civil Engineering - Texas Tech University
Types of Consolidation (cont.)
Clayey soils undergo consolidation settlement not only under the action of “external” loads (surcharge loads) but also under its own weight or weight of soils that exist above the clay (geostatic loads).
Clayey soils also undergo settlement when dewatered (e.g., ground water pumping) – because the effective stress on the clay increases
Coarse-grained soils DO NOT undergo consolidation settlement due to relatively high hydraulic conductivity compared to clayey soils. Instead, coarse-grained soils undergo IMMEDIATE settlement.
Civil Engineering - Texas Tech University
1- D Consolidation Test
The main purpose of consolidation tests is to obtain soil data which is used in predicting the rate and amount of settlement of structures founded on clay.
The four most important soil properties determined by a consolidation test are:
The pre-consolidation stress, sp’, This is the maximum stress that the soil has “felt” in the past.
The compression index, Cc , which indicates the compressibility of a normally-consolidated soil.
The recompression index, Cr , which indicates the compressibility of an over-consolidated soil.
The coefficient of consolidation, cv , which indicates the rate of compression under a load increment.
Civil Engineering - Texas Tech University
Test Results
B
sp
Cr
Cc
Recompression
Index
Compression
Index
Pre-Consolidation
Stress
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Consolidation Test
Two types of consolidometers (oedometers)
commonly used:
Floating-ring
Fixed ring
This lab uses the fixed-ring consolidometer
ASTM D 2435
Civil Engineering - Texas Tech University
Laboratory Consolidation Test
Civil Engineering - Texas Tech University
Consolidation Test
Civil Engineering - Texas Tech University
Laboratory Consolidation Test
sv
sv
Solids
Voids
Solids
Voids
Vs
Vv Vv
Vs
1 Place sample in ring
2 Apply load
3 Measure height change
4 Repeat for new load
V
Confining stress
Before After
Civil Engineering - Texas Tech University
Procedure
Measure the inner diameter and height of the consolidation cutter/ring and record its mass
Prepare a soil specimen for the test by trimming and placing the soil in the ring
Determine the mass of ring + soil
Collect some excess soil for moisture content
Assume Gs = 2.7
Saturate the lower (larger) porous stone on the base of the consolidometer
Place the specimen and ring and place the upper stone/disk Follow the rest in your lab manual
Place 1.5 kg (1st day), 3kg (2nd day), 6kg (3rd day), 12kg (4th day)
Civil Engineering - Texas Tech University http://www.uic.edu/classes/cemm/cemmlab/Experiment%2011-Consolidation.pdf#search='consolidation%20test'
Civil Engineering - Texas Tech University
Calculations and Graphs - v vs w(time)
Plot of Vertical Displacement vs. Time
(P = 1000 psf)
0.42
0.4205
0.421
0.4215
0.422
0.4225
0.423
0.4235
0.00 5.00 10.00 15.00 20.00 25.00
Time (min 0.5)
Dis
plac
emen
t (in
)
Civil Engineering - Texas Tech University
v vs wtime Graph – Find t90
Plot of Vertical Displacement vs. Time
(P = 1000 psf)
0.42
0.4205
0.421
0.4215
0.422
0.4225
0.423
0.4235
0.00 5.00 10.00 15.00 20.00 25.00
Time (min 0.5
)
Dis
plac
emen
t (in
)
t90 = 2.5 min0.5
1
2 3
4
5
t90
d0
B DC
CD = 1.15 BC
A
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Calculation and Graph – v vs log(time)
Logarithm of time curve fitting
0.42
0.4205
0.421
0.4215
0.422
0.4225
0.423
0.4235
0.1 1 10 100 1000 10000
Time (min) - log Scale
Verti
cal D
ispl
acem
ent (
in)
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v vs log(time) Graph – Find t50 Logarithm of time curve fitting
0.42
0.4205
0.421
0.4215
0.422
0.4225
0.423
0.4235
0.1 1 10 100 1000 10000
Time (min) - log Scale
Vert
ical
Dis
plac
emen
t (in
)
d100 = 0.42065
t 1
t 2= 4t 1
1
2
A
3
5
4 X 6
X7
d0 = 0.42305
8
d50=0.5(d0+d100)=0.42185
d50 9
d0
d100
t50 = 10.2 min
10
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Calculation
Determine the height of
solids (Hs) of the
specimen in the mold
Determine the change
in height (H)
Determine the final
specimen height, Ht(f)
Determine the height of
voids (Hv)
Determine the final void
ratio
ws
ss
GD
WH
2
4
sftv HHH )(
s
v
H
He
Civil Engineering - Texas Tech University
Calculation (cont.)
Calculate the coefficient
of consolidation (cv)
from t90
Calculate the coefficient
of consolidation (cv)
from t50
Plot e-log p curve and
find:
sc, Cc, Cr
Plot cv – log p curves
2
90
H
tcT v
v
2
50
H
tcT v
v
Civil Engineering - Texas Tech University
Calculation Sample (Ex. pp.121)
Eq 17.2
1(in) - Hs
Hv = Hi - Hs Hi e = Hv / Hs
(1.0 + 0.9917) / 2 (0.848 x 0.99592)/(4 x 302)
t90
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Plot e vs log p
sc
R min
Cc
Cr
Civil Engineering - Texas Tech University
In Your Report
Plot all curves find t90 and t50 (10 plots)
Show your calculations in a table and find
e, cv (t90), cv (t50)
Plot e vs. log (p) and determine:
Pc
Cc
Cr
Plot cv vs. log (p) (2 plots)