Post on 21-Jul-2020
Consolidation Tests of Soils
Hsin-yu ShanDept. of Civil Engineering
National Chiao Tung University
Some Definitions
Settlement: change in elevationCompression: change in thickness
Consolidation Tests
An effort to measure stress-strain-time relationship for soils under partial or complete drainage
Loading
Porous Stone
Dial gauge 1
3456
789
0
Soil Specimen
Porous Stone
2
Restrictions of Consolidation Tests
Suitable for cohesive soilsSand compresses instantly, difficult to measure strain-time relationshipSudden shock makes the measured deformation inaccurate
Simplified stress surface (profile of u vs. H)Coupled with theory
History of Consolidation Test
1809 Thomas Telform was the first to use the term “consolidation”1901 Consolidation test was invented1910 D. E. Morgan invented the floating ring1923 Karl Terzaghi worked on consolidation theory
We do not run consolidation test on sandVolume change takes place instantly
We can still get the relationship between volume change and stress
σlog
rebound
reloadσ
ε
εSemilog plot:Improves linearityEasier to use in low stress range
Nowadays, we don’t have to use semilog plot anymoreInput all experimental data, compute compression through linear interpolation
Some Corrections of the Measured Change in Height
Machine deflection
Settlement-Time Relationship
Square root time method - TaylorLog time method – Casagrande
Primary consolidation – the period of consolidation where the volume change of soil is due to the drainage of water driven by excess pore water pressure
Square Root Time Method
tS = U Su
2HtcT v=s
vcHTt
2
=
2
4UT π
=Fox’s equation, for U ≤ 60%
t = FS2 At least for U ≤ 60%, t1/2 ~ S is a straight line if T’s theory is valid
t
s
Initial portion may be higher or lower than S0
Straight line portion
d50
9/5 d50
s90
Peat
Extension from the straight line portion
t
s
For some soils, such as peat, k changes very much when subjects to change in effective stress; thus, Terzaghi’s theory does not work at all.In addition, Terzaghi’s theory does not account for secondary effect.
How Do We Get the Factor 1.15?
197.059
59
848.0
50
90
Fd
Fd
⋅=
=
15.11526.1197.05
9848.0
59
50
90 ≈=⋅
=F
Fd
d
It makes no difference if we use d40 and T40 to construct d90 and T90, we get the same results
90100 910 SS ∆=∆
The square root time method works well based on the assumption of NO secondary consolidationUsed strictly for vertical drainage
Log Time Method
Log t
t1
t2
∆s1Pick t1, t2, so that t2=4t1 to get s0
∆s1
s
s100
Most of the time, it is difficult to get this straight line, since the secondary effect does not necessarily produce a straight line
Square Root t vs. Log t
There was a war going on between MIT and HarvardTo decide which one is correct compare with k valueGenerally, square root t method is betters100 from log t method is ambiguous
Square root t method only gives one cv, log tmethod gives different cv by different people
Square root t method usually gives higher cv
kc computed from cv almost always smaller than km actually measuredSecondary effects lead to a delay in settlement in addition to that caused by the real kIf all causes of delayed compression are lumped into kc kc < km
Smaller k slower compression
Special Aspects of Consolidation Testing
Rapid loadingIncrease loading as soon as s100 is reachedContinuous loading
Boundary impedenceRing frictionEffect of temperatureRates of dissipation of pore water pressuresEffects of non-linear stress-strain curve
Rapid Loading
Standard
cv
Rapid
σlogSmaller cv means smaller kRapid loading reduces the “delay” effect of secondary consolidation
Boundary Impedence
t
Old porous stone PLUS filter paper
Old porous stone, NO filter paper(likely to be clogged by fine particles)
Impedence factor I increases
HkkHI
d
d=s
New porous stone, NO filter paper
Hd is the thickness of the porous discThe smaller the better
kd is the hydraulic conductivity of the porous disc
The larger the betterFree drainage I = 0I should be kept less than 0.01I affects the time rate of consolidation, but it does not affect the total settlement
How to Reduce I
Keep the porous stone cleanUse filter paper to keep the fine particles from entering itBoil the porous stone before use to get the trapped air out
Ring Friction
Effects of ring frictionDuring loading reduce stress acted on the specimen specimen compress lessDuring rebound reduce the swelling tendency specimen swell less
Flatten the swelling curve at low stress level
Taylor (1942)
λpq =
)]4exp(1[4 H
DK
KH
Do
o
µµ
λ −−= Make λ 1 to reduce friction
Q
Q = P – τ z π DP
The stress in the ring is not uniform
zτ
τ = σv Ko µ
Use consolidation rings made of cadmium, nickel, hard chrome will be fineDon’t use Teflon, although it has the lowest friction coefficient
Teflon is too soft and will be scrapped by soils with hard materials and get stuck
λ of greased steel > ungreasedplastic>cadmium coated steelλ increases as consolidation pressure goes up less error
Use larger D/H ratio, the larger D/H the smaller the effect of friction
Wide and thin specimenFor D/H about 3, greased steel, total error is about 10%
Effect of Temperature
Stress-strain curveThe higher the temperature, the lower the e-log p curve
This may due to the change of volume of the consolidation ring at higher temperature
The ring expands such that the friction decreases
σlog
Reduction in eAs temperature goes up
e
Effect on cv and kAs temperature goes up, cv increases since kdecreases as the viscosity of water gets smallerThe water flows faster
Effects of Non-Linear Stress-Strain Curve
Effect on settlement-time relationshipEffect on
Effect on cv and av
i
b
uu
Effect on Settlement-Time Relationship
σ
(2)NC clay, concave upwardsResultant cv decreases
(1)
e
(3)OC clay, concave downwardsResultant cv increases
t
(2) NC clay
(3) OC clayU = 90%
U
Influence of Secondary Compression on Subsequent Behavior
Stress-strain behaviorSubtract the secondary compression to get the stress-strain curve (e – log p)?
Settlement-time relationshipLoading rate in the lab is very highSlow loading in the fieldDrainage distance in the field is far greater than in the lab
May be difficult to see secondary effect
log t
H = 5.5”
H = 0.37”
s
Almost totally secondary compression
Secondary compression may not affect the primary consolidation of next load incrementalMaterials such as peat also has primary stage, but its properties changes a lot during this period
Difficult to obtain meaningful parameters
Quick Loading Test
Monitor the s ~ t relationshipApply next incremental load once the primary consolidation completesEliminate the secondary effect?
Continuous Loading Tests
Constant rate of strain (CRS) testControlled deformation rate
Constant gradient testConstant rate of loading test
Constant Gradient Test
Constant isochroneMaintain the same stress surface
Use stepping motor ~ 200 to 2000 step/revolution to control the stepping rate
iu
bu
Transducer
Constant Rate of Strain Test
In CRS test, the strain rate is uniformly high throughout the testIn conventional incremental loading test, the strain rate is very small at the end of any load
standardCRS
vσ
cv
cv,CRS ~ cv,inc
Advantages of CRS Test
Reduced testing timeAutomated data collection and reductionContinuous definition of properties
Not at specific loadsReduced extrusion of solidsReduced space requirements
Disadvantages of CRS Test
Increased capital costIncreased maintenance costRequire better-trained techniciansPeriodic “down-time” when something went wrongRequire backpressuringNo data on secondary effects
Secondary effects are hidden in primary stages
Higher strain rates than in the field
Consolidation Test with kMeasurement
Performed after primary consolidationNo free drainage in the bottom of the consolidation cellWater flow through the specimen from bottom to top
Can perform constant head test or falling head test
Constant head test is better, at least the state of stress of the specimen remains the same throughout the test
Can also use additional air pressure to raise the hydraulic gradient
Do not use high gradient because Darcy’s law is valid only for constant volume condition
σlog
Elevate pore pressure at the bottom
e
Average stress during k test
Radial Flow Consolidation Test
Drainage column in the center of the specimen
A hole is drilled in the center of the specimen and filled with sand or other drainage material
Use radial flow theory for data reduction
Effect of Partial Saturation
Soils with inter-connected air voidsk increases with the degree of saturation
Soils with occluded air voidsMajor reason for instantaneous initial compressionAs the soil becomes saturated almost no instantaneous initial compression
Backpressure saturationCan be done with some types of consolidation cells
tGas = 5%
6%
9%
kwU
12%
Effect of Sampling Disturbance
Stress-strain curveRound offSmaller e for givenThe difference between the slope of e –log p curve of undisturbed and remolded soil is not very large
cv decreasedReduced secondary effects, cα decreasedObtained soil properties
maxσ
σ
σlog3” tube
2” tube
remolded Sampling disturbanceStress relief
e
cv
There are fissures in the field, but we tend to use “uniform” and “good” sample in the lab reduced k and cv