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Missouri University of Science and Technology Scholars' Mine

International Conference on Case Histories in (2004) - Fifth International Conference on Case Geotechnical Histories in

14 Apr 2004, 4:30 pm - 6:30 pm

Skin Friction and Pile Design

Akira Wada Asia Georesearch Agency Corporation Pte Ltd. (AGA), Singapore

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Recommended Citation Wada, Akira, "Skin Friction and Pile Design" (2004). International Conference on Case Histories in Geotechnical Engineering. 14. https://scholarsmine.mst.edu/icchge/5icchge/session01/14

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SKIN FRICTION AND PILE DESIGN

Akira Wada Asia Georesearch Agency Corporation Pte Ltd (AGA) Singapore

ABSTRACT

The skin friction of pile is found as a parameter of pile shaft . It will not be a simple/constant values for each type of /weathered rock. Pile load test data shows skin friction grows to maximum strength at certain displacement and then reduces to residual strength. Due to this property, the main active skin friction zone is shifted downwards with the increase of load. From the shared ratio of total skin friction in pile capacity, the share ratio of skin friction is found related with pile length. This means that for 30m long pile, the skin friction share is approx.95% of . The shared ratio shows almost constant value for each pile under 100% ~ 300% of design load if there is no failure in the pile. From this point of view, the failure of pile bearing mechanism will be due to the change of skin friction share ratio.

INTRODUCTION ULTIMATE LOAD TEST ON PILE

Today, the pile design is proceed by taking certain safety The ultimate pile load test is for to verify the ultimate bearing factor into the interpretation on the evaluated ultimate skin capacity that is evaluated by the pile design work. So the friction and ultimate bearing capacity. This procedure is applied load on the test pile is 250 ~ 300% of design load, constructed on the assumption that the skin friction and toe which is considered based on the safety factor in the design bearing capacity generates together and achieves the ultimate work. value at the same . The test piles which are studied in this paper are bored piles Based on the analysis of pile load test on bored pile, it is found with 800 ~ 1500mm in diameter and the length are 14m ~ 40m. that the skin friction and toe bearing capacity are the The object test piles have instruments such as strain gauge and parameters of pile shaft displacement. Both values did not extensometer (see fig. 2). The ground materials of these test achieve the ultimate value at the same time. This paper piles location consist of 3 types of bedding formation, such as presents the study result of the bearing mechanism of pile for granite, sedimentary rock (Jurong Formation) and Pleistocene to review the pile design. cemented layer (Old Alluvium). All these materials are heavily weathered and zone is classified as residual soil and completely weathered zone. The toe of test pile is socket into the following hard/strong materials:

Granite area Pile toe is penetrated into highly weathered zone or moderately 7 19 weathered zone. The penetration length 1 12,13 & 14 8,9,10 & 11 in highly weathered zone is 3 ~ 8m,

16 where N>100 and 1 ~ 3m in moderately 15 weathered zone, when RQD>35%. 18 LEGEND 5&6 4 Alluvium Gombak Norite 17 Old Alluvium Sajahat Formation Sedimentary rock Pile toe is penetrated into highly 2 & 3 Jurong Formation Reclaimed Land

Singapore's Shoreline weathered zone more than 3m where Bukit Timah Granite (Before Reclamation) N>100. Fig. 1. Singapore geological map and location of test piles

Paper No.1.20 1 Depth of Instrument Boring B-10 RL 105.04m Extensometer A (18.0m) Pleistocene B (20.5m) Cemented Layer Pile toe is penetrated into cemented ) n

) SPT Soil / C (22.5m) m o m h i a m ( r l pt at zone more than 5m where N>100. e

v N-Value Rock RL 105.734m ag v e De GL ( Di El Le 20 40 60 80 100 Name RL 105.00m 0 1a 1b (0.5m) From the interpretation result of the monitoring data, it shows that distribution, friction and /displacement 2 Fill of pile shaft can be obtained at each depth of pile. The flow chart in fig. 3 shows the process of data interpretation. 4 Light The phenomenon/behaviour of pile under surcharge load is

Brown explained as follows: (see fig. 4). 6 2a 2b (6.0m) Medium - The stress develops in the pile shaft by surcharge load on top

Coarse of the pile and transfer the stress downwards with . 8 Grain Each depth of pile is deformed by the stress developed at each

Sand depth. Then the stress at the pile toe is transferred into the 10 ground material. The ground material under the pile toe is 93.24 11.80 compressed due to the transfer of stress. The compression of 12 3a 3b (12.0m) Very Soft ground material and the deformation of pile shaft will accumulate from the pile toe to the top. Then it is monitored Greenish 14 as the displacement of pile shaft and settlement of pile top. Grey 4a 4b (15.0m) The skin friction is generated by the displacement of pile shaft Marine 16 and its magnitude is found as the parameter of displacement.

87.06 18.00 18 5a 5b (18.0m) Residual Soil SKIN FRICTION 85.04 20.00 20 6a 6b (20.0m) Highly W. Mudstone Properties of Skin Friction 82.44 22.60 7a 7b (21.5m) 22 8a 8b (22.5m) The relationship between the skin friction and pile shaft 24 Moderately Weath. displacement under the increase surcharge load is shown in fig. Sandstone 79.24 25.80 5. The skin friction is generated with the increment of pile 26 shaft displacement and the maximum skin friction is observed H.W.Sandstone/ Mudstone at 5 ~ 15mm displacement. Then it reduces to residual 77.04 28.00 28 strength when the displacement continues. The maximum Fig. 2. Sample of instrumentation in test pile skin friction is confirmed as the parameter of of soil and as shown in table 1 and figs. 6 & 7. In general, the magnitude of displacement which generates the maximum skin friction has good relationship with the shear strength and elastic modulus of soil, such as: ε at Each Gauge Area of Pile Section Load / Skin Friction Strain Gauge Settlement ε at Each Depth P ε ε Monitoring Depth Sn-1 Concrete Volume Pile Dia at Each Zone Length (Area) of Each of Pile ∆l 1 ∆l 2 = P τ τn P ∆l l l i Monitoring Depth AE Uncon. Comp. Test E of Concrete of Test Cube S P ε ε Monitoring Depth n δl Load at Each Depth P = ε.E.A ∆S : Monitoring Shortening of Pile

Depth of Strain Gauge Length of Each Area ∆S = S (top) - S (toe) ( l ) Distribution of P Deformation of Pile at Each Area ∆l : Deformation of Pile At Each Area Along the Pile σ at Each Area ∆l 2 = P.l / A.E Sb : Settlement at Pile Toe

Adjustment Factor Comparision between Extensometer Skin Friction (fn) α = ∆l / Σl2 data and evaluated data Along the Pile

Deformation at Each Area Surface Area of Pile at δl = α ∆l 2 Each Area

Accumulate Displacement at Each Area

si = ∆l + Sb Unit Skin Friction τi at Each Area Shown in Graph Relationship of τi and si

Show in Table and Figure Unit Skin Friction For Design at Each Area

Fig. 3 Flow chart of analysis work of instrumentation Paper No.1.20 2 2 Unit Skin Friction (τ) t/m Increase Stress in Pile, (t/m2) Displacement of Pile (mm) 0Pile Pile Pile 0

) ) 2 m

h ( h (m pt pt e De D

Fig. 4. Increase of stress and magnitude of pile displacement along the pile

24 32 4.0-8.0m 4.20-6.20m 12.0-16.0m 30 6.20-8.20m 22 16.0-20.0m 8.20-10.20m 28 20.0-24.0m 10.20-16.20m 20 24.0-28.0m 16.20-18.20m 28.0-32.0m 26 18.20-20.20m 32.0-36.0m 18 36.0-40.0m 24 20.20-22.20m 40.0-42.0m 22.20-23.00m 22 16 ) ) 2 2 20 /m 14 on/m (ton

(t 18

i i τ τ

,

12 n 16 ion, t

tio c i r F Fric 14

n i 10 in Sk t it Sk

i 12 n n U

U 8 10

6 8

6 4 4

2 2

0 0 024681012 0 4 8 12 16 20 24 28 32 Displacement of Pile Shaft, Si (mm) Displacement of Pile Shaft, Si (mm)

Fig. 5. Unit skin frictio n with displacement

Table 1. Skin friction of pile in various soil and weathered rock

Ground Material Skin Friction Soil Parameters Max. Skin Friction Residual Skin Friction Ratio Soil / Rock Formation Displacement Strength Displacement Strength N Cu Weathered τres (mm) 2 (mm) 2 2 τmax (KN/m ) τres (KN/m ) τmax (KN/m ) Sandy Clay / Sandy 10.0 44 19.7 31 0.68 7 20 Top Soil / Fill Silty / Reclaimed Sand 8.8 110 14.0 25 0.21 14 24 Marine Clay 7.5 60 16.0 52 0.85 14 24 Silty Clay 4.0 64 - - - 10 29 Alluvium Organic Clay/ Organic sand / 7.8 40 16.0 31 0.77 2 11

Sandy Clay / Clayey Sand / Silty Sand 13.8 90 24.4 32 0.45 17 30 Residual Soil of Limestone (VI) 9.3 117* - - - 12 99 Compl. Weathered Mudstone (V) 7.5 195 - - - 28 210 Jurong Formation Highly Weathered Mudstone (IV) 8.0 400* - - - >100 2100 Moderately Weathered Limestone (III) 5.7 1520* - - - >100 -

Residual Soil of Granite (VI) 7.1 77 16.6 59 0.69 19 50 Completely Weath. Granite (V) 7.2 135 8.3 108 0.77 46 203 Bukit Timah Completely Weath. Granite (V) 8.1 287 12.8 253 0.94 72 - Granite Highly Weathered Granite (IV) 5.0 690* 10.0 430 0.91 >100 2643 Moderately Weathered Granite (III) 5.0 2600* - - - >100 -

Residual Soil of Boulder Clay 5.1 60 9.5 52 0.79 19 45 Residual Soil of Boulder Sandstone 5.9 86 - - - 25 - Boulder Clay Weathered Zone of Boulder Clay 8.0 144 12.0 58 0.52 37 98 Cemented Zone of Boulder Clay 7.1 398 11.0 253 0.87 >100 175 Weathered Zone of Old Alluvium 14.0 98 16.3 62 0.63 32 60 Old Alluvium Cemented Zone (I) of Old Alluvium 14.5 258* - - - >100 - Cemented Zone (II) of Old Alluvium 9.8 820 14.5 790 0.96 >100 - Note : (*) - Under Progress Pape r No.1.20 3 300 500 Type of Weathered Rock Type of Weathered Rock 270 450 B.T. Granite Limestone / Mudstone

) 240 = 4 N 2 400 max Boulder Clay ) τ

2

m Bukit Timah Granite / m / N 210 Old Alluvium 350

N (k

Boulder Clay (k

x

max 180 300 τ ma Old Alluvium , τ n

max = 1.8 Cuu , o τ i n t 150 250 o c i i t ic r Fr 120 200 n i F

k in k S . 90 150 x S . x Ma 60 a 100 M 30 50

0 0

0 30 60 90 120 150 180 210 0 10203040 5060708090100 C (kN/m2) N - Value By SPT uu

Fig. 6. Relationship between τmax and Cuu Fig. 7. Relationship between τmax and N-Value

τi - Si τi - P Unit Skin Friction Unit Skin Friction 2 2 0 Pile Pile Pile 10 20 30 40 50 t/m Pile 10 20 30 40 50 t/m

τ τ

) ) ) m m m τ τ Max Skin Friction Depth ( Depth ( Depth (

τ τ

τ τ A w

τ τ At 1234567 Displacement of Magnitude of Loading Stage 1 Active Skin Friction Pile Shaft (Si) Applied Load (P) Loading Stage 2 Loading Stage 5 Loading Stage 4 Loading Stage 7

Fig. 8. Magnitude of skin friction under load test

Pile with Strain Maximum Skin 100% Loading Stage 200% Loading Stage 300% Loading Stage Gauge Friction Skin Friction Skin Friction Skin Friction

G - 1

fa fa (100) fa (200) fa (300) G - 2 ) m

( fb (100) fb (200) fb (300)

th fb p G - 3 De fc fc (100) fc (200) fc (300) G - 4

fd fd (100) fd (200) fd (300) G - 5 fe fe (100) fe (200) fe (300) G - 6

f (max) = fa + fb + . . . . . Active Skin Friction f(100) Active Skin Friction f(200) Active Skin Friction f(300) fi = τi x φi x li fa(100)+fb(100)+fc(100 )+.. fa(200)+fb(200)+fc(200)+.. fa(300)+fb(300)+fc(300)+..

f(100) ...… for 100% loading stage Ratio of Active Skin Friction = f(max)

Fig. 9. Active skin friction

Paper No.1.20 4

Relatively small displacement - at the soil of big shear Active skin friction strength/large elastic modulus. Fig. 9 shows the illustration of active skin friction. In fig. 9, Relatively big displacement - at the soil of low shear the maximum skin friction during 300% loading is shown as strength/small elastic f(max). Then the active skin friction at 100% loading, 200% modulus. loading and 300% loading show f(100), f(200) and f(300) respectively. The ratio of active skin friction with maximum This relation means the magnitude of displacement is a skin friction is evaluated using each load test data and shown parameter of elastic modulus of soil. in fig. 10. By this evaluation, the active skin friction is found as follows: When the skin friction is monitored during the pile load test, (1) The ratio of active skin friction is not shown 100% of the active skin friction under certain loading stage does not f(max) at any loading stage. The ratio under 300% loading show good relation with shear strength of soil because the skin stage is still 95% of f(max). friction being a parameter of pile shaft displacement. (2) The ratio of active skin friction under 100% and 200% The active skin friction under increase loading stage is shown loading stage is 38% and 73% respectively, which is slightly in fig. 8. on the safety side of the actual design. o Left figure shows the relationship between the skin friction and the pile shaft displacement at several depths. Table 1 summarises the ratio between maximum skin friction o The second figure shows the skin friction at each loading and residual skin friction. The ratio at loose sand of fill stage. The load increases from number 1 to 7. The material and alluvium formation show less than 50% and at maximum skin friction at each depth generates different cohesive soil is in the range of 0.7 ~ 0.85. Also, the ratio in loading stage. weathered rock is in the range of 0.7 ~ 0.95. o The third figure shows active skin friction with depth. With The displacement at the moderately strong ~ strong rock near the increment of loading, the depth of maximum skin the pile toe is generally small, such as less than 5mm and the friction shifts downwards and total skin friction increases. correct residual value are not monitored. Due to this, the ratio o The fourth figure shows the relations between the active in weathered rock will change if it can be mobilize up to skin friction at certain loading stage and the maximum skin 10mm. friction along the pile shaft. As shown in this figure, the This range of ratio is almost the same as factor. skin friction at upper zone is at residual strength stage and lower zone is at the increment stage to maximum strength.

350 350 Pile Length L > 30m L < 25m L= 25~30m d 300 300 L < 25m oad Loa L L = 25m~30m n

gn g i i

s 250 s 250 L>30m e e D on 200 200 d e

ed on D s a b bas ) ) 150 150

% % ( ad ( 100 Load Lo 100 ed i l ed i

l p p p 50 p 50 A A

0 0 0 102030405060708090100 40 50 60 70 80 90 100

Share Ratio of Skin Friction (% ) Ratio of Activ e Skin Friction (% )

Fig. 10. Active skin friction with increment of load Fig. 11. Share Ratio of skin friction with increment of load

Paper No.1.20 5

BEARING MECHANISM OF PILE (2) Today, it is difficult to obtain the accurate skin friction from laboratory/in-situ shear test result on hard soil and The share ratio of skin friction in pile bearing capacity is weathered rock due to the following reasons: summarised in fig. 11. As shown in this fig. 11, more than o There are limits in the accuracy in the laboratory test 80% of bearing capacity is obtained from skin friction. The result which use undisturbed sample from stiff, hard share ratio of skin friction increase with longer pile length, soil and weathered rock, where large skin friction is such as if pile is more than 30m long, 95% of bearing capacity generated. comes from skin friction. The large share ratio of skin friction This is according to the disturbance of specimen during is considered to come from the pile deformation which sampling, handling and treatment in laboratory. generates maximum skin friction. For example, the total o The relations between the skin friction and shear deformation of pile under 300% of design load is strength of ground material is not clearly analysed. approximately as follows: Generally, this relationship is shown as certain index 15m long pile approximately 5.0 ~ 10.0mm factor and sometimes keeps in the black box. 30m long pile approximately 10.0 ~ 20.0mm o The presence of disturbance and its shear strength of ground material during piling work is not studied. If the total pile deformation is 15mm, the maximum skin The shear failure takes place at the weak zone such as friction and residual skin friction will take place at most of the the disturbed zone. So the magnitude of skin friction depth of pile shaft. If the total pile deformation is 7.5mm, the depends on the shear strength of the disturbed zone. skin friction at more than 50% of the pile shaft will be under Due to this reason, the skin friction and toe bearing are the development stage and the maximum skin friction will not designed using N value for usual pile design. generate in pile shaft. Another one remarkable point is the share ratio of skin friction (3) For pile design, the monitored skin friction from pile load will not change by magnitude of loading. It is almost constant test is kept in a database for each type of soil, and uses under 100% ~ 300% loading for non failure pile. This point this as guide value. In future, this database can be suggests that if the failure of bearing mechanism takes place in worked for analysis the bearing mechanism of pile. pile, above constant share ratio will change significantly. Most possible case is the toe bearing cannot generate due to problem of soft toe. But if there is any problem in the skin CONCLUSION friction, it also causes the failure of bearing mechanism. Through the study of pile load test, the following properties The share ratio shows in this paper is for Singapore’s case are evaluated. study. It will change according to the stratigrahic structure of (1) Skin friction value is not simple/constant and it is soil layer. Also, the share ratio will change for steel pile generated by pile shaft displacement. which can transfer large stress to pile toe. (2) Skin friction has maximum strength at certain displacement. It reduces to residual strength if displacement In the studied pile load test data, the relationship of settlement increases. and bearing capacity of pile toe material cannot be obtained (3) The depth of main active skin friction zone is shifted because settlement is usually small. So the allowable downwards by increase of load. settlement of pile toe and ultimate toe bearing capacity for (4) The active skin friction will not achieve 100% of several types of soil/rock is not found in this study. theoretical skin friction because skin friction is the parameter of pile shaft displacement. (5) The share ratio of skin friction shows almost constant DESIGN OF PILE value however load increase from 100% to 300% of design load. The constant value is related with pile length. Based on this study, the following points are recommended to take into the consideration of pile design.

(1) The share ratio of skin friction in total bearing capacity of REFERENCES pile. The share ratio of skin friction is found as a parameter of Broms B.B., Chang M.F. & Goh A.T., (1988), Bored Piles in pile length. In the pile design, it is better to study the Residual Soil and Weathered Rock in Singapore bearing capacity of pile base on the share ratio. Meyerhof G.G. (1956), Penetration Test and Bearing Capacity Also the share ratio is understood to depend on the of Cohesionless Soil, Soil Mech. Found. Vol. 82 stratigraphic structure of ground material and type of pile. M.J. Tomlinson (1994), Pile Design and Construction Practice Due to this point, the share ratio shall be further studied (4th Edition) Chapter 4. for a variety of and pile types. Poulos H.G. (1989), Pile Behaviour – Theory and Application Geotechnique Vol. 39, No. 3.

6 Paper No.1.20