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1Lecture 1 (cont.)
Soil and rock parameters for
design
1.7 Methods to determine soil strength and stiffness
Soil parameters for pile design Total stress analysis (undrained condition) for
clay: Apparent cohesion cu Friction angle u (= 0 for saturated clay) Youngs modulus Eu Poissons ratio u (= 0.5 for clay)
Effective stress analysis (long term) for sandand clay: c (= 0 for normally consolidated clay); ; E and
Insitu testsCone penetration test (CPT)Standard penetration test (SPT)Vane shear testPressuremeter test (stiff soil only)
Refer to Section 1.8
Laboratory tests on undisturbed soil sampleTriaxial compression testConsolidation (oedometer) testRefer to standard geotechnical engineering text 20 ton CPT rig (truck type)
(a) Cone Penetration Test (CPT)
Piezocone
ConePenetrometerpluspiezometer
Measurements:(i) Static cone resistance(ii) sleeve resistance(iii) pore pressure
Marine conepenetrationTestingsystem
2Friction angle,
Fig. 1.1 Correlation between static cone resistance qcand friction angle of sand
Fig. 1.2
Chart to aid interpretation of CPT
(Ratio of sleeve friction over cone resistance)
Cone resistance
(b) StandardPenetrationTest (SPT)
Result: standard penetration resistance (N value) or blow count
SPTFor clay,Undrained
apparent cohesion (kPa)
cu = 5N to 6 N
Friction angle,
Fig. 1.3 Relationship between N value and
(for sand)
Sand
Vane shear test setup
(c) Vane Shear Test
Fig. 1.4 Commonly used dimensions and formula for vane shear test
Measurement:Torque M to fail soil
3Fig. 1.5 Variation of against PI
Correction factor : Cu (corrected) = . Cu (vane shear) 1.8 Rock properties Weathering grade and description Rock Quality Designation RQD Standard penetration test (N value) Unconfined compression test (qu) Point load test Plate load test Pressuremeter test
(a)
(b) Rock Quality Designation RQD
RQD is the percentage of rock recovered as sound length which are 100 mm or more in length.
Sometimes this is the only rock data available! Fracture state RQD value
Very poor 0 % to 25%Poor 25% to 50%Fair 50% to 75%Good 75% to 90%Excellent 90% to 100%
(c) Standard penetration resistance N
Only suitable for very weak rockExtrapolated N value e.g. 100 blows for a penetration of 100 mm implies that the extrapolated N value is 300
Unable to perform test in medium hard rock and above due to penetration refusal during test
4(d) Unconfined compression test
Need core length 2 times core diameterFor a standard NX size core of 53 mm diameter, the required core length is 106 mm
Not suitable for weak rocks which are often highly fractured and do not have long enough samples
Diamond saw to flattenends of rock core
Unconfinedcompressiontest(Unconfinedcompressivestrength qu isalso termedas uniaxialcompressivestrength)
qu is the failure load divided by X-sectional area
Same apparatus as testing concrete
Unconfined compressive strength qu
Strength of rock qu value (MPa)Very weak < 1.25Weak 1.25 to 5Moderately weak 5 to 12.5Moderately strong 12.5 to 50Strong 50 to 100Very strong 100 to 200Extremely strong > 200
(e) Point load test Rock core length required is 1 time
core diameter Can be tested on rock samples of
different shapes and sizes Test results are correlated to point
load strength of 50 mm diameter sample termed as Is(50)
5Point load test (test can also be done on site)
Point loadStrength is the failure load divided by X-sectional area
Before test
After test
Point loadtest
Sample lengthat least 1 timecore diameter
Shape otherthancylindricalshape acceptablewithcorrectionfactor
Point load test can be performed on rock of various shapes & sizes
(f) Plate load test
Plateloadtest
Stiffness(andprobablystrength)of rockat testelevationcan bedetermined
6(g) Pressuremeter test
A very good test for stiff soil and rock Determination of pressuremeter
modulus Em and hence Youngs modulus E
Determination of soil strength cu and rock strength quNote potential errors in determining Em and strength
Pressuremeterequipment
Typicalpressuremetertest record
TypicalData plot of pressureversus radius
7Geology of Singapore
1.9 GeologyGeology of Singapore
Bukit Timah granite formation [North and centre of Singapore island] Residual soil of granite overlying fresh granite
Jurong formation (sedimentary) [West and south] Residual soil of sedimentary origin overlying great
depth of weak rocks Kallang formation [South and along rivers]
Soft marine deposit (transported soils) Old alluvium [East]
Old deposits with layered clay and cemented sands
GeologyOf WestMalaysia
Geology of Hong Kong
GeologyofSoutheasternChina
1.10 Granite (a) Bukit Timah formation
Bedrock can be shallow and very difficult to drill throughInsufficient socket length and hence base resistance is relied upon.Standard penetration resistance (N value) can be difficult to obtain.Rock can be much stronger than concrete.
Change from completely weathered soil to fresh rock can be sudden
Inclined rock surfaces pose problems
8Granite
Granite (Hillview area, Singapore)
200 400 600 800 1000Extrapolated N value
0
10
20
30
40
Dep
th b
elow
gro
und
leve
l (m
)
0 50 100 150qu (MPa)
Singapore Granite
Unconfined compressive strength
Not possible to obtain N value forhard granite
Granite can be much stronger than concrete
-3.6 -3.2 -2.8 -2.4 -2.0 -1.6Ln (DV/V)
0
40
80
120
Pres
sure
(kg/
cm2 )
Granite
Gradient = cu = 62.5 kg/cm2 = 6.25 MPa qu = 2cu = 12.5 MPa
Pressuremeter test results on Singapore granite
Not possible to obtain strength of hard granite
Granite formation (Thomson area, Singapore)Note dense sand layer at soil/rock interface
(b) soft toe problem
Thin layer of dense sand at soil/rock interface with high coefficient of permeability causing soil to collapse into borehole (hence soft toe at pile base exists)
Borehole needs casing or bentonite to stabilise under this situation
Hong Kong igneous rock
RQD almost 100% in many places
9(c) Granite formation (Hong Kong)
Typical subsurface profilePlace Soil Rock weathering
overburden Grade 3 Grade 2HK Cyber Port 68 m 0 m > 5mTai Lam tunnel 30 m > 3 mWest Rail Central 26 m 1 m > 7mSham Shui Po 29 m > 4 mKwai Hing 7 m > 10 mWanchai 28 m > 5 m
1.11 Sedimentary rock (a) Singapore
Highly weathered and fractured in excess of 50 m deep (N values can be obtained & RQD is highly variable)
Thus unable to found pile base into fresh/hard rock (qu of rock < qu of concrete)
Rock strength very difficult to determine as rocks are highly fractured
Highly fractured and very low RQD values
Kallang formation (soft marine clay) above Jurong formation(sedimentary soils and rocks)
(Keppel area,Singapore)
Weathered sedimentary rock outcrop
10
Sedimentary rock (Alexandra area, Singapore)
200 400 600 800 1000
Extrapolated N value
0
5
10
15
20
25
30
Dep
th b
elow
gro
und
leve
l (m
)
0 20 40 60 80
RQD (%)0 10 20 30
qu (MPa)
(a) (b) (c)
Singapore Sedimentary rocks
Rock Quality Designation
-4.8 -4.4 -4.0 -3.6 -3.2 -2.8Ln (DV/V)
0
40
80
120
Pres
sure
P(k
g/cm
2 )
Gradient = cu = 44 kg/cm2 = 4.4 MPa
qu = 2cu = 8.8 MPa
Sedimentary rock
Pressuremeter test on Singapore sedimentary rock
Correlation As it is difficult to obtain properties of
Singapore sedimentary rocks, correlations of different rock test results are attempted!
Between qu and RQDNo correlation if rocks at site are not of the
same type Correlation may be possible if the rock is
fairly homogeneous(Thus on a case by case basis)
Correlation not possible for different rock typesSite mainly siltstone and correlation may be possible
11
Another site with mainly siltstone and correlation also possible
Correlation between qu & Is(50) Weak Sedimentary rock (Singapore)
qu = 6 Is(50)
Weak Granite (Singapore & Malaysia)qu = 10 Is(50)Both are conservative correlation and should be checked for large projects
Correlation between qu against Is
Correlation between qu and N
Weak sedimentary rock (Singapore)
qu = 10[-3.29 + 1.45 log10(N)]
for 100 < N < 1200
Correlation of qu against N value
Need more data to confirm the promising correlation between qu and Em (pressuremeter modulus)
12
Rock parameters for design
Recommended tests Unconfined compression test
(need long samples) Pressuremeter tests (highly
recommended) Point load test (preliminary test) RQD (if rock at site is homogeneous) Plate load test (limited to test elevation) N values (generally too crude)
Residual soils of sedimentary origin
Soil is stiff when it is dry and soft when it is wet.
Sedimentary and volcanic rocks (b) Hong Kong
Repulse Bay Formation (Volcanic rocks)
Weathering is generally shallow with an average depth of about 10 m
Depth of weathering may vary considerably over short distances
Gradual change from weathered to fresh rock in most places
Deep weathering to depth of 40 m or more along faults and shear zones
Sedimentary and volcanic rocks (cont.)
1.12 Old alluvium (Singapore)
Cemented sand (casing may be necessary)
Stiff clay
Typical oldalluviumformationSingapore
Alternatingdense sandand stiffclay layers
13
1.13 Soft soilsSoft marine clay (grey)Soft peaty clay (black)
Kallang formation
Pile installation problem and negative skin friction