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Case Studies of

Rock-socketed piles

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Sedimentary rocks Case studies (Leung, 1996) involving

a good number of load tests carried

out in instrumented piles socketed in

weak sedimentary rock mainly inAlexandra and Keppel areas

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0 4 8 12 16 20Load (MN)

0

4

8

12

16

Settlement(mm)

Load-settlement response is reasonably linear up to working load of 10 MN

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0 5 10 15 20

Load (MN)

0

4

8

12

16

Depthbelow

groundlevel(m

)

0 200 400 600

Unit shaft friction (kPa)

Firmsilty clay(N = 12)

V. denseclayey silt(N = 130)

Weaksiltstone(qu=6.5 MPa)

Fill

Marine clay

Weaksiltstone(q

u

=3.5 MPa)

(a) (b) (c)

1 2 3 4

1 2 3 4

Applied load

1: 5 MN2: 10 MN

3: 15 MN4: 20 MN

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Results show that a large percentage of shaft friction can

be mobilised even for piles not tested to ultimate failure.

Alexandra Distripark (after Radhakrishnan & Leung, 1989)

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Rock socket adhesion factor

Pile load test results reveal that

majority of socket shaft friction can be

mobilised even for piles not tested to

ultimate failure. Rock socket adhesion factor

= fs/qu

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Sedimentary rocks For qu < 5 MPa (i.e. very weak rock)

rock socket adhesion factor is

reasonably close to theoretical values.

For qu > 5 MPa (i.e. weak rock & above), value is considerably lower than the

theoretical values. This is caused by

heavy chiselling that had significantlyweakened the rock.

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1 10 100

Unconfined compressive strength qu(MPa)

0.0

0.2

0.4

0.6

Rocksocketadhesionfa

ctor

Williams and Pells

Rowe and Armitage

Rosenberg and Journeaux

Horvath and Kenny

Field data insedimentary rocks

All piles installed by chiselling.

Chiselling affects sockets with qu > 5 MPa

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Granite Load tests are mainly carried out in the

Woodlands area (Leung, 1996). Load-settlement curve

Load transfer

Unit shaft friction

t-z curves

Rock socket adhesion factor

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0 2 4 6 8 10Load (MN)

0

5

10

15

20

25

Settleme

nt(mm)

Load-settlement response is linear up to pile working load

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0 2 4 6 8 10

Load (MN)

0

10

20

30

Depthbelow

groundlevel(m)

0 200 400 600 800

Unit shaft friction (kPa)

Clayeysilt(N =14)

Clayey siltwith sand(N = 25)

Dense siltysand(N = 120)

Granitequ=12.5 MPa

(a) (b) (c)

1 2 3 4

1 2 3 4

Applied load

1: 3.6 MN2: 6.3 MN

3: 8.1 MN4: 9.0 MN

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0 4 8 12 16 20

Shaft movement (mm)

0

200

400

600

800

Unitshaftfriction(kPa)

Depth belowground level

6.5 m

9.5 m

12.5 m

15.5 m

18.5 m

21.5 m

24.5 m

27.5 m

29.5 m

t-z curves (much of the load transfer in the granite socket)

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Granite For all qu values, is considerably

lower than the theoretical values.

Heavy chiselling, that had significantly

weakened the rock, need to done onall classes of granite as they are

generally less fractured.

[Most of the load tests were

conducted in the Woodlands area.]

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1 10 100

Unconfined compressive strength qu(MPa)

0.0

0.2

0.4

0.6

Rocksocketadhesionfa

ctor Williams and Pells

Rowe and Armitage

Rosenberg and Journeaux

Horvath and Kenny

Field datain granite

Chiselling disturbs granite more than sedimentary rocks

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Construction concern As the studies revealed heavy

chiselling process can badly damagethe rock, it is believed that better

construction tool (e.g. down the holehammer) will result in less rock

damage and hence higher shaft

resistance. [Further studies are

required to confirm this.]

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Settlement of socketed piles

The load-settlement response under

working load is found to bereasonably linear in most cases

reported earlier. The load-settlement responses of all

the previous cases are back-analysed

using a FEM program and a Elastic

theory model (Leung & Chow, 1998).

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FEM

mesh

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Settlement of socketed piles

Granite

Es = 2N (unit in MPa)

Es = Em (need more data to confirm)

Sedimentary rocks Es = Em (needs more data to confirm)

Es < 2N (exact values hard todetermine as degree of rock fracture

highly variable)

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0 2 4 6 8Measured pile settlement (mm)

0

2

4

6

8

Predictedpilesettle

ment(mm) N value

Pressuremeter modulus

Figure 4 Comparison of predicted and measured pile settlements (Singapore granite)

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0 4 8 12

Measured pile settlement (mm)

0

4

8

12

Predict

edpilesettle

ment(mm)

Figure 5 Comparison of predicted and measured pile settlements (Singapore sedimentary rocks)

N value

Pressuremeter modulus

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Pile group, cap & creep Case studies:

PSA Building

Alexandra District Park

The presence of rigid pile cap, pile groupeffect and possibly rock creep will cause

the less load being transferred to the pile

shaft in the long term. I.e. more load is

transferred to the pile base.

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PSA

Building

(Case

study by

Leung etal., 1998)

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Foundation layout and instrument plan

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Alexandra Distripark (after Radhakrishnan & Leung, 1989)

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Load transfer under short term test load is different

form that under long term service loads condition.

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Summary of findings (1) Majority of shaft friction is almost

fully mobilised for all piles [See Table1 earlier]

(2) Observed fs is close to thosepredicted by Williams & Pells and

Horvath et al. But lower than those of

Rowe & Armitage and Rosenberg &

Journeaux [see Table 2]

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Summary of findings (3) More load transferred to the pile

base under long term service loadingcondition than short term load test

condition due to interaction between piles in a group

inetraction between piles and pile cap

pile and soil/rock creep

[See previous figure and Table 3]

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