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Skin Friction and Pile Design

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Skin Friction and Pile Design Missouri University of Science and Technology Scholars' Mine International Conference on Case Histories in (2004) - Fifth International Conference on Case Geotechnical Engineering Histories in Geotechnical Engineering 14 Apr 2004, 4:30 pm - 6:30 pm Skin Friction and Pile Design Akira Wada Asia Georesearch Agency Corporation Pte Ltd. (AGA), Singapore Follow this and additional works at: https://scholarsmine.mst.edu/icchge Part of the Geotechnical Engineering Commons 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 This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. 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 displacement. It will not be a simple/constant values for each type of soil/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 bearing 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 bearing capacity. 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 time. 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 top 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 stress distribution, friction and deformation/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 damping. 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. Clay 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 shear strength 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 Settlement Strain Gauge 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 8 U 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.
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