Lecture 3.0 Compressibility of Soil
Transcript of Lecture 3.0 Compressibility of Soil
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Dr. J. BERLIN P. JUANZON CE, MBA, MSCM
CE 152
Geotechnical Engineering
2
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CONTENT
SETTLEMENT
CONSOLIDATION TIME RATE OF CONSOLIDATION
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Leaning tower of Pisa is one of the most famous buildings in the world. It
has been built before hundreds years ago.
Even today the great mass
continues to sink very slowly. It is a
question of about 1 mm. every year.
Since nobody can state with
mathematical security that thissinking will continue in the future at
the present yearly rate, without its
ceasing, remedies by means of
adequate measures, based onscientific studies and projects, are
under consideration. In the
meantime supervision with
instruments of very high precision is
continuously being carried out.
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SETTLEMENT Definition
The total vertical deformation at the surfaceresulting from :
External Load
Dewatering Settlement Components
Immediate Settlement ; Se
Primary Consolidation Settlement ; Sc Secondary Settlement (Creep) ; Ss
sce SSSS
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SETTLEMENTPurpose
Study the settlement behaviorDetermine the settlement value and time
Study the settlement influence to the
structure stability
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SETTLEMENT INFLUENCE
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IMMEDIATE SETTLEMENT
Defined as settlement which occurred directlyafter the application of a load, without a change inthe moisture content.
Caused by soil elasticity behavior
The magnitude of the contact settlement willdepend on the flexibility of the foundation and thetype of material on which it is resting.
For clay, the immediate settlement generally verysmall comparing to the consolidation settlement,therefore this immediate settlement mostlyignored.
Usually considered at sand or sandy soil.
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IMMEDIATE SETTLEMENT
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IMMEDIATE SETTLEMENT
General Equation
Se= Elastic Settlement
Cs= shape and foundation rigidity factor
q = P/B2(net vertical pressure applied)
Es= Elasticity modulus of soil
B = Foundation width
Ip= Influence factor
= Poissons ratio of soil
EsqBCS se
21 Eqn 1.0
Eqn 2.0 pe IEsqBS
21
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Problem 1.0
Estimate the immediate settlement of a column footing 1.5m in diameter that is constructed in an saturated claylayer, given that the total load carried by the column
footing is 150kn, Es = 7,000kn/m2
and = 0.25. Assume thefooting to be rigid.
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CONSOLIDATION When the stress on a saturated clay layer in the field is
increased, the pore water pressure in the clay willincrease.
Because the coefficients of permeability of clays are verylow, it will take some time for the excess pore waterpressure to dissipate and the stress increase to betransferred to the soil skeleton gradually.
Consolidation is the process of dissipation of excess porewater pressure in a row of time.
Note:Dissipation of pore water pressure occurs simultaneously with the squeezing out ofthe pore water. Therefore the consolidation time depend on:
The distance of pore water to be squeezed out
The coefficient of permeability of soft soil
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CONSOLIDATION Consolidation Type
Normal consolidation
Pre-consolidation pressure (Pc) just equals the
existing effective vertical overburden pressure (Po) Over consolidation
If the soil whose pre-consolidation pressure (Pc) isgreater than the existing overburden pressure
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CONSOLIDATION
Normal Consolidation
oc pp OR 1p
p
o
c o
oc
oc
p
ppH
e
CcS
log..
1
Where:
Sc = primary consolidation settlement
Cc = Compression Index
H = thickness of clay layer
eo= in situ void ratio
Po = average effective stress at mid-height of clay layer orgeostatic pressure
P = average increase of effective stress on clay layer
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CONSOLIDATION
oc pp OR 1o
c
p
p
po+ p < pc
o
oc
oc
p
ppH
e
CsS
log..1
po< pc< po+pc
oc
oo
cc
oc
pppH
eCc
ppH
eCsS
log..
1log..
1
Over Consolidation
Cs = swell index (ranges from 1/5 to 1/10 of Cc
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DETERMINATION OF CONSOLIDATION
PROPERTIES
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DETERMINATION OF CONSOLIDATION
PROPERTIES
Procedures :
1. Determine the point O on thee-lop p curve that has thesharpest curvature (that is,the smallest radius of
curvature)2. Draw a horizontal line OA
3. Draw a line OB that is tangentto the e-log p curve at O
4. Draw a line OC that bisectsthe angle AOB
5. Produce the straight lineportion of the e-log p curvebackward to intersect OC. Thisis point D. The pressure thatcorresponds to the point p isthe preconsolidation pressure,pc.
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DETERMINATION OF CONSOLIDATION
PROPERTIES
1
2
21
p
plog
eeCc
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DETERMINATION OF CONSOLIDATION
PROPERTIES
3
4
43
p
plog
eeCs
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CONSOLIDATION SETTLEMENT
Other equation
p.H.mS cvc
Where :
mv= Compression Index
Hc = Thickness of soft soil layer
p = The stress increment due to theexternal
load
o
vv
'1
'2
21v
e1
am
pp
eea
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SECONDARY CONSOLIDATION (CREEP)
p
pc
p
st
ttlog.H.
e1CS
Where :ep= void ratio at the end of primary consolidationtp= time at the end of primary consolidationt = time incrementt2= t
p+t
p
2
t
t
log
eC
Is the additional settlement that occurs at a constant value of effetive
stress after excess pore water pressure has been dissipated. The process
typically continues slowly for a long period of time. Secondary settlement is
usually small compared to primary consolidation settlement.
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SECONDARY CONSOLIDATION (CREEP)
PROBLEM 2 0
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PROBLEM 2.0A 2.1m layer of clay is buried beneath a 3.0m stratum of verycompact granular soil. Compact sand underlies the clay. The layer
of granular soil is composed of material having a unit weightof20.46kN/m3. The clay unit weight is 16.52 kN/m3. A laboratorycompression test indicates a compression index of 0.40 and anatural void ratio of 1.3. A planned building loading will cause a26.38kPa stress increase at the middle of the clay layer.
1. what amount of primary compression occurs in the clay forthe indicated conditions?
2. How much primary compression of the clay layer wouldresult if the groundwater table was the ground surface.
3. How much clay layer compression would occur if the claywas an over consolicated material, the past maximum pressurewas 95.94kPa and th Cs value was 0.10? Assume a deep watertable