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