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Lecture 4

Design of

Rock-socketed Piles

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Rock-socketed piles

PILE

Rock

Socket

Soil

Rock

Load

In most cases, the lower part of the pile is socketed into rock.

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Performance of rock socket

Socket friction

Socket friction increases with socket

roughness

Smooth socket friction fully mobilisedat small pile settlement

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(after Horvath & Kenny, 1979)

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Performance of rock socket

Socket friction is related to socket

roughness

Horvath et al. (1983) established that

fs = 0.8 (RF)0.45 qu

where RF = roughness factor = (r Lt)/(D

L)

r = average height of asperities, Lt = total

distance along the socket wall profile, D =

socket diameter, L = socket length & qu =

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Effect of socket diameter Seidel & Harberfield (1995) found that

as the socket diameter increases, the

effect of dilation is reduced for a given

degree of socket roughness. The variations in socket friction can

be changed by a factor of up to 3, fordiameters varying between 0.5 m and

2 m.

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Rock socket frictionUnit shaft friction fs

Early days (1960s to early 1970s)

fs = 250 kPa for fragmented shale

fs = 120 to 180 kPa for weak

mudstone

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Rock socket frictionfs is related to qu

Rosenberg & Journeaux (1976)

fs = 0.375 (qu)0.515

Meigh & Wolshi (1979)

fs = 0.22 (qu)0.6

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Rock socket friction Williams & Pells (1981)

fs = qu

where and are reduction factors

reflecting rock strength and fracturestate, respectively. See Figures.

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Williams & Pells (see next figure for value of )

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(after Williams and Pells)

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Rock socket friction Horvath et al. (1983)

fs = 0.2 to 0.3 (qu)0.5

Rowe & Armitage (1987)

fs = 0.45 (qu)0.5 for a smooth socket

fs = 0.60 (qu)0.5 for a rough socket

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Rock socket friction From a large set of field data

(including those of Singapore),

Zhang & Einstein (1998) found that

fs = 0.4 (qu)0.5 for a smooth socket

fs = 0.8 (qu)0.5 for a rough socket

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Rock socket frictionRecommendations for local design

Preliminary design using Horvath

fs = 0.2 to 0.3 (qu)0.5

Detail design using Williams & Pells

fs = qu(All values need to be confirmed by load tests)

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Performance of socket base

For the same rock strength, base

resistance increases with overburdenpressure

Shallow embedment: brittle failure Deep embedment: ductile failure (i.e.

capacity is not a problem, only

settlement is a concern, see figure)

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Socket base resistanceUnit base resistance qb

Early days (60s & 70s)

Weak shale 3 MPa

Moderately strong shale 3.9-4.9

MPa

Weak mudstone 6.8 MPaModerately strong mudstone 28-58

MPa

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Correlation

between qband RQD

for a jointed

rock mass(after

Peck et al.,

1974)

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Socket base resistance Teng (1962)

qb = 5 to 8 qu

Coates (1967)

qb = 3 qu

Tomlinson

qb = 1 to 6 qu

(depending on rock fracture state)

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Socket base resistanceRecommended approach for design

Rowe and Armitage (1987)

qb = 1 qu under working stress to

ensure no yielding of rock

qb = 2.5 qu under ultimate condition

[Values subject to capacity andsettlement verification from load tests]

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Socket base resistance Based on a large number of field data

(including that of Singapore)

Zhang & Einstein (1998) found that

qb = 3.0 to 6.6 (qu)0.5

(with a mean coefficient of 4.8)

[Note qb is related to square root ofqu]

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Zhang & Einstein (1998)

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Conservative ratio of qb upon qu

(after HK GEO)

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(after HK GEO)

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Unconfined compressive strength

REMINDER

Use quc (unconfined compressive

strength of concrete) if the rock is

stronger than concrete.Poser: In view of the above, is it

necessary to socket pile into hard rock?{Answer can be yes or no!}

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Settlement of rock-socketed piles

For socketed piles with long

embedment length, capacity isnormally governed by concrete

strength. Settlement of pile needs to be

evaluated especially in suspected soft

toe cases.

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Rock socket Under working load, the load-

settlement response of a socketedpiles is fairly linear in most cases.

Elastic theory can be employed toevaluate the pile settlement

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0 4 8 12 16 20

Load (MN)

0

4

8

12

16

Settlem

ent(mm)

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

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Rock socket modulus Er Tomlinson

Er= j Mr quc (see tables)

Rowe and Armitage

Er= 215 (qu)0.5

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Mass factor j (after Tomlinson and BS8004)

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Modulus ratio Mr (after BS8006)

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Settlement of rock socket RECOMMENDED APPROACH

Rowe & Armitage (1987)

Assumptions:

Soil above rock socket has nosignificant contribution in resistance

Elastic solutions are developedassuming a fully bonded socket using a

FEM.

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(after

Rowe &Armitage,

1987)

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Rowe and Armitage Assumptions (continued)

The dimensionless settlement I defined

by I = Ed D/Qt

where is the socket settlement, Ed isthe design socket shaft modulus, D is

socket diameter and Qt is the working

load. Design charts are developed for various

pile geometry and socket/concrete

modulus ratios

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Rowe and ArmitageDESIGN CONCEPT

Satisfying a user-specified design

settlement

Ensuring an adequate factor of safetyagainst failure

Limit state design concept: partial safety

factors applied to deformation andstrength parameters

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Rowe and ArmitageDESIGN PROCEDURES

Step 1: Given

design settlement d

socket diameter D applied working load Qt

Concrete modulus Ep [Factored] Unconfined compressive strength of

rock qu

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Rowe and Armitage Step 2 (a): Determine

fs = 0.45 (qu)0.5

Er= 215 (qu)0.5

Determine Eb

if qu

at pile base is different

Step 2(b):Apply partial factor of safety

Design unit shaft resistance d = f fs

Design rock mass modulus Ed = fE Er

Partial factors f and fE are taken as 0.7 or 0.5(for probability of 30% and 11% exceeding d )

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Rowe and Armitage Step 3: Determine Ep/Ed and Eb/Er

Step 4(a): Determine maximum

socket length assuming no base

resistance(L/D)max = Qt/( D

2 d)

Step 4(b): Determine thedimensionless settlement I

I = Ed

D/Qt

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Rowe and Armitage Step 5(a): Determine (L/D)d and

(Qb/Qt)d from the intersection of thefactored design line for (L/D)max with

the contour for Id using design chartsallowing for slip

Step 5(b): If there is an intersection,

calculation is okay & proceed to step

6.

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Qb =100%,

I.e.

Full base

resistance

Qb = 0%, I.e. Full shaft resistance

Intersection

refers to

estimated

Qb(after

Rowe &

Armitage,1987)

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Rowe and Armitage Step 5(C): Determine the value of (L/D)d

for given Id If there is a value of (L/D)d can be found,

determine (Qb/Qt)d for the (L/D)d and proceed

to step 6 If there is no value, redesign is necessary, go

back to step 1.

Refer to paper for details for design involvinga recessed pile

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Determine the

value of (L/D)dfor given Id

Elasticsocket

(no slip)

(after Rowe &

Armitage, 1987)

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If there is a value of (L/D)d can be found,

determine (Qb/Qt)d for the (L/D)d

(after Rowe & Armitage, 1987)

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Rowe and Armitage Step 6: Check for base condition

qt = Qt/( D2/4)

Under working load

qb = (Qb/Qt)d qt and need to be < qu Under worst condition

qbu

= qt

- 4(L/D)d

(0.3 fs

) and < 2.5 qu

assuming only 0.3 fs can be mobilised

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Summary

of designcurves

(1) Full slipEb/Er = 0.5

(after

Rowe &

Armitage,

1987)

Fig. 6.1

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Summary

of designcurves


(after

Rowe &

Armitage,

1987)

Fig. 6.2

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Summary

of designcurves


(after

Rowe &

Armitage,

1987)

Fig. 6.3

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Summary

of designcurves

(4) Elasticsocket

(No slip)

(after

Rowe &

Armitage,

1987)

Fig. 6.4

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Summaryof design

curves

(5) Elastic

socket:

baseresistance

(after

Rowe &Armitage,

1987)

Fig. 6.5

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Axially loaded rock-socketed piles

Examples 4.1 [from Rowe & Armitage Page 138 Case

(i)]

Given:

Design settlement d = 8.5 mmPile diameter D = 0.71 m

Design working load Qt = 4.45 MN

Modulus of pile material Ep = 37,000 MPa

Unconfined compressive strength of rock qu = 6.75 MPa

[Rock at pile base is more fractured and Eb = 0.75 Er]

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CalculationStep 2(a): Unit shaft friction fs = 0.45 (qu)

0.5 = 1.17 MPa

Youngs modulus of rock Ef= 215 (qu)0.5 = 560 MPa

Step 2(b): Take partial factor of safety f = fE = 0.7[Probability of 30% not exceeding d]Design unit shaft friction d = f fs =0.82 MPaDesign rock mass modulus Ed = fE Er= 390 MPa

Step 3: Ep/Ed = 95 (therefore use Ep/Ed = 100 chart)Eb/Er= 0.75 (interpolation between Eb/Er =1 [Fig. 6.2]

and = 0.5 [Fig. 6.1])

Step 4(a):Assume no base resistance, maximum socket length

(L/D)max = Qt / (D2 d ) = 3.4

Step 4(b): Dimensionless settlement I = Ed D/ Qt = 0.53

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Calculation (cont.)Step 5(a):

By drawing a line from (Qb/Qt)d = 100 % to (L/D) = 3.4in Fig. 6.2(d) (i.e. for Eb/Er= 1) show that (Qb/Qt)d =63 % and the corresponding design socketlength/diameter ratio (L/D)d = 1.3 for Id = 0.53.

Similarly for Eb/Er= 0.5 from Fig. 6.1(d),(Qb/Qt)d = 27 % and (L/D)d = 2.5

By interpolation, (Qb/Qt) = 45% and (L/D)d = 1.9

Hence design socket length = 1.9 D = 1.35 mand proceed to Step (6)

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Calculation (cont.)Step (6) Unit loading pressure

qt = Qt / (D2

/4 ) = 11.2 MPa Under working condition, allowable base pressure

qb = (Qb/Qt)d qt = 5.1 MPa < qu OKAY

Under worst condition where only 0.3 fs can bemobilised qbu = qt 4(L/D)d (0.3 fs)

= 8.5 MPa < 2.5 qu OKAY

Design completed and required socket length is

1.35 m.

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Example 4.2Given:

d = 5 mm, D = 1 m, Qt = 7.8 MN,Ep = 34,000 MPa, qu = 20 MPa, Eb = Er

Calculations:

Step 2(a) & (b): d = f fs = 0.7 x 0.45 qu = 1.4 MPa

Ed = fE Er= 0.7 x 215 qu = 673 MPa

Step 3: Ep/E

d= 34,000 / 673 = 50

Step 4: (L/D)max = Qt / (D2 d ) = 1.77

I = Ed D/ Qt = 0.43

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Calculation (cont.)Step 5(a): Fig. 6.2(c) No intersection!

Step 5(b): Fig. 6.4(a)

for I = 0.43 & Ep/Ed = 50

(L/D) required 1.2

Fig. 6.5(a)for (L/D) = 1.2 & Ep/Ed = 50

(Qb/Qt) = 30%

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Calculation (cont.)Step 6: qt = Qt / (D

2 /4 ) = 9.93 MPa

Under working conditionqb = (Qb/Qt)d qt = 0.3 x 9.93 MPa = 2.98 < qu

(20MPa) OKAY

Under worst working conditionqbu = qt 4(L/D)d (0.3 fs)

= 9.93 4(1.2) (0.3 x 0.45 qu)

= 7.03 MPa < 2.5 qu OKAY

Required socket length = 1.2 x 1 = 1.2 m

and there is no slip in the socket.

References for

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References for

Design of rock- socketed piles

Related publications by C F Leung1. Leung C F and Radhakrishnan R (1985). Observations of an

instrumented pile-raft foundation in weak rock, Proc. 11th Int.Conf. on Soil Mech. and Fdn. Engr., San Francisco, pp. 1429-1432.

2. Leung C F, Radhakrishnan R and Wong Y K (1988).

Observations of an instrumented pile-raft foundation in weakrock, Proc. Instn. Civ. Engrs., Part 1, Vol. 84, pp. 693-711.

3. Leung C F and Radhakrishnan R (1990). Geotechnicalproperties of weathered sedimentary rocks, GeotechnicalEngineering, Vol. 21, pp. 29-48.

4. Leung C F, Radhakrishnan R and Tan S A (1991).Performance of precast driven piles in marine clay, J. ofGeotech. Engr., ASCE, Vol. 117, pp. 637-657.

5. Leung C F (1996). Case studies of rock-socketed piles,Geotechnical Engineering, Vol. 27, pp. 51-67.

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Related publications by C F Leung (cont.)

6. Leung C F and Tan G P (1996). Load carrying capacity of spunpiles, Proc. 12th Southeast Asian Geotech. Conf., Kuala Lumpur,Vol. 1, pp. 423-428.

7. Leung C F and Chow Y K (1998). Settlement of rock-socketedpiles, Proc. 5th Int. Conf. on Tall Buildings, Hong Kong, Vol. 2, pp.884-889.

8. Radhakrishnan R, Leung C F and Subrahmanyam R (1985).Load tests on instrumented large diameter bored piles in weak

rock, Proc. 8th Southeast Asian Geotech. Conf., Kuala Lumpur,Vol. 1, pp. 2.50 2.53.

9. Radhakrishnan R and Leung C F (1989). Load transfer behaviourof rock-socketed piles, J. of Geotech. Engr., ASCE, Vol. 115, pp.755-768.

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Other publications

1. BSI (1986). British Standard Code of Practice for Foundations(BS8004:1986), British Standards Institution, London, UK.

2. Cole K W and Stroud M A (1977). Rock socket piles at

Coventry Point, Market Way, Coventry" Piles in Weak Rock,Instn. of Civil Engrs., UK, pp. 47-62.

3. GEO (1996). Pile Design and Construction, GEO PublicationNo. 1/96, Geotechnical Engr. Office, Hong Kong.

4. Horvath R G and Kenny T C (1979). Shaft resistance of rock-

socketed drilled piers, Symp. on Deep Foundations, ASCE,pp. 182-214.

5. Horvath R G, Kenny T C and Kozicki P (1983). Methods ofimproving the performance of drilled piers in weak rock, Can.Geotech. J., Vol. 20, pp. 758-772.

6. ISRM (1985). Suggested method for determining point loadstrength, Int. J. of Rock Mech. and Mining Sci., Vol. 22, pp.51-60.

7. Irfan T Y and Powell G E (1991). Foundation Design ofCaissons on Granitic and Volcanic Rocks, GEO Report No. 8,

Geotech. Control Office, Hong Kong.

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Other publications (cont.)

8. Lam T S K, Yau J H W and Premchitt J (1991). Side resistance ofa rock-socketed caisson, Hong Kong Engineer, Vol. 19, No. 2,pp. 17-28.

9. Meigh A C and Wolski W (1979). Design parameters for weakrock, Proc. 7th European Conf. On Soil Mech. and Fdn. Engr.,Brighton, Vol. 5, pp. 59-79.

10. Osterberg J O and Gill S A (1973). Load transfer mechanism forpiers socketed in hard soils on rock, Proc. 9th Canadian Symp. on

Rock Mech., pp. 235-262.11. Peck R B, Hanson W E and Thornburn T H (1974). Foundation

Engineering, 2nd Edition, John Wiley and Sons, New York, USA.

12. Rosenberg R K and Journeaux N L (1976). Friction and endbearing tests on bedrock for high capacity socket design,

Canadian Geotech. J., Vol. 13, pp. 324-333.13. Rowe R K and Armitage H H (1987a). Theoretical solutions for

axial deformation of drilled shafts in rock, Canadian Geotech. J.,Vol. 24, pp. 114-125.

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14. Rowe R K and Armitage H H (1987b). A design method for drilledpiers in soft rock, Canadian Geotech. J., Vol. 24, pp. 126-142.

15. Seidel J P and Harberfield C M (1994). A new approach to theprediction of drilled pier performance in rock, Proc. Int. Conf. onDesign and Construction of Deep Foundations, Orlando, Vol. 2,pp. 556-570.

16. Shiu Y K and Chung W K (1994). Case studies in prediction andmodelling of the behaviour of foundation on rock, Proc. 8th Int.

Congress on Rock Mech., Tokyo, Vol. 3, pp. 1243-124.17. Teng W C (1962). Foundation Design, Prentice-Hall, USA.

18. Tomlinson M J (1995). Foundation Design and Construction, 6thEdition, Longman, UK.

19. Williams A F and Pells P J N (1981). Side resistance rock

sockets in sandstone, mudstone and shale, Can. Geotech. J.,Vol. 18, pp. 502-513.

20. Zhang L and Einstein H H (1998). End bearing capacity of drilledshafts in rock, J. of Geotech. and Geoenvir. Engr., Vol. 124, pp.574-584.

Other publications (cont.)

Documents

Piles - 4a