Design and testing of bored pile foundation to the 2 nd Penang Bridge, Malaysia

Transcription

1 Design and testing of bored pile foundation to the 2 nd Penang Bridge, Malaysia By Sing-lok CHIU, AECOM Zheng-ru Fang, CHEC Construction (M) Sdn Bhd (CHEC) Kang HUANG, China Highway Planning and Design Institute (HPDI) Presented by Dr SL Chiu Technical Director, Geotechnical Hong Kong AECOM November Page 1

2 Contents Overview of bored pile design approaches Design of bored piles for the 2 nd Penang Bridge Site characteristics Design and instrumentation of the trial bored pile Static load test on the trial bored pile Test results Conclusion Page 2

3 2 nd Penang Bridge under construction Cable-stayed section of the 2 nd Penang Bridge over the main navigation channel Page 3

4 Pier P m Pier P26 150m x 30m batu kawa 21 bored piles of 2.0 ~ 2.3 m in diameter and socketted in to sound granite bedrock at about 110m deep below sea bed Page 4

5 Overview of bored pile design approaches The load resistance capacity of a bored pile is mainly derived from the pile shaft and base resistances (Whitaker, 1976): Q ultimate = Q shaft + Q base Page 5

6 Given that: There are different resistance and settlement relationships of the shaft and base It is advisable that different load factors be applied to the calculated ultimate resistance of the shaft and the base (BS8004, 1985). Page 6

7 Partial factors or global factor of safety are applied to give allowable capacity of the bored pile Q allowable = Q shaft /F S + Q base /F b Or Q allowable = (Q shaft + Q base )/F Page 7

8 Partial factors and global factor for bored pile design Skempton (1966) suggested F s = 1.5 and F b = 3 BS 8004 (1985) suggested that the global factor of safety for a single pile is often required to be between 2 and 3 Page 8

9 Burland and Cook (1974) suggest for bored pile in stiff clay -an overall load factor of 2, and - a minimum factor of safety 3 on the base resistance be adequate. Q allowable = Q shaft + Q base /3 Or Q allowable = (Q shaft + Q base )/2, whichever is less Page 9

10 Estimate of shaft and base resistance For piles in soils (Poulos, H. G. 1989) Empirical approaches based on: In-situ tests results e.g., SPT N-value (no. of blows) f s = α + βn kn/m 2 (e.g., α = 0, and β= 1 to 5 for BP in cohesionless soil) Laboratory strength test results, e.g., undrained strength, Cu (α- method) and friction angle, φ (βmethod ): f s = αc u and f s =βσ v Page 10

11 Clay f s = αc u α=0.45 (London clay) α=0.7 times value for driven displacement pile f s = K tan δσ v K is lesser of K 0 or 0.5(1 + K 0 ) K/K 0 = 2/3 to 1; K 0 is function of OCR; δ depends on interface materials Silica sand f s =βσ v β=0.1 for φ = for φ = for φ =37 β= F tan (φ -5 ) where F = 0.7 (compression) & 0.5 (tension) Loose to medium sand Skempton(1959 ) Fleming et al.(1985) Fleming et al.(1985) Stas and Kulhawy (1984) Meyerhof (1976) Kraft & Lyons (1974) f s =βσ v β=0.2 to 0.6 *Hong Kong (Geo 2006) Page 11

12 Estimate of shaft and base resistance For piles in soils (cont d) Chinese Standard (JGJ ) and German Code (DIN 1054:2005) suggest use of presumed values based on site specific factors including soil types, physical and mechanical properties of the soils and rocks as well as pile length Page 12

13 DIN 1054:2005 CPT, qc in MPa Cohesionless soil, fs in kpa Undrained strength, Cu in kpa Cohesive soil, fs in kpa Note: Intermediate values are obtained by linear interpolation (after Vrettos, 2007) Page 13

14 DIN 1054:2005 Settlement to base diameter ratio, S/Dbase 0.02 (40mm if D= 2.0m) Pile base resistance, fb, in MPa for bored piles in cohesionless soils At an average tip cone resistance, qc of the CPT in MPa (or 700 kpa) * Note: * limiting settlement Intermediate values are obtained by linear interpolation (after Vrettos, 2007) Page 14

15 DIN 1054:2005 Settlement to base diameter ratio, S/Dbase Pile base resistance, f b, in MPa for bored piles in cohesive soils At an average shear strength, Cu of the undrained soil in MPa * Note: * limiting settlement; Intermediate values are obtained by linear interpolation; for bored piles with widened base, values shall be reduced to 75% (after Vrettos, 2007) Page 15

16 Chinese foundation code, JGJ Soil Type Soil properties Presumed values of f s in kpa Clay I L >1 0.75<I L <1 05<I L <0.75 Silty fine sand Coarse sand 10<N 15 15<N 30 N>30 15<N 30 N>30 21~38 38~53 53~68 22~46 46~64 64~86 74~95 95~116 Note: Intermediate values are obtained by linear interpolation an abridged version of the original Table in JGJ Page 16

17 Chinese foundation code, JGJ Soil Type Soil properties Presumed values of f b in kpa for different pile length in m 5 L<10 10 L<15 15 L <30 30 L Clay 0.75<I L <1 05<I L < ~ ~ ~ ~ ~ ~ ~ ~800 Silty fine sand 10<N 15 N> ~ ~ ~ ~ ~ ~ ~ ~1200 Coarse sand N> ~ ~ ~ ~2800 Note: an abridged version of the original Table in JGJ Page 17

18 Estimate of shaft and base resistance For bored pile founded on or socketted in sound rock Because of the great difference in stiffness of soil and the sound rock, the load carrying capacity is mainly derived from the end bearing capacity of pile on/in rock. Page 18

19 For bored pile founded on or socketted in sound rock (cont d) The estimation of end bearing is mainly based on empirical methods. Presumed values for safe working stress are recommended, being a function of the uniaxial compression strength, q c, of the rock: Page 19

20 DIN 1054:2005 Uniaxial compression strengthen of rock in Mpa Pile base resistance, f b, in MPa Pile shaft resistance, f s, in MPa Note: Intermediate values are obtained by linear interpolation. Page 20

21 For bored piles socketted in bed rock, JGJ h/d Soft rock ζ r = Hard rock Where h/d- socket depth (h) to pile diameter (d) Soft rock- UCS, f rk < 15MPa Hard rock- UCS, f rk >30MPa Page 21

22 Q rk = ζ r f rk A p Where, Q rk, is the combined shaft and base resistance of the rock socket A p, pile base area; f rk the uniaxial compression strength (UCS) of the bedrock, and ζ r a factor taking into account of the combined effect of base and shaft resistance of pile in the socket, depending on the ratio of h/d Page 22

23 Design of bored pile foundation for the 2 nd Penang Bridge Design Brief For compressive loads: Q= (Q s /2) + (Q b /3); or Q= (Q s + Q b )/2.5, whichever yields the lowest working capacity; For tensile loads (uplifts): Q=Q s /3 Where Q is the allowable pile capacity (kn), Q s is the ultimate shaft friction (kn), Q b is the ultimate end bearing (kn). Page 23

24 To evaluate the shaft resistance (Q s ) and end bearing (Q b ), the following relationships with SPT-N value as suggested by Meyerhof (1976) are used: Q s = K s * N*A s, or = f s *A s (Ks= 2.0 for cohesionless soils) Q b = K b *N b *A b, or = f b * A b (K b = 250 for silty soil and 400 for sandy soil) SPT-N value is limited to 75 Page 24

25 For end-bearing bored piles on sound bed rock Q b = q uc * (RQD) 2 *A b Where Q b = the ultimate load bearing capacity at pile base, q uc = unconfined compressive strength RQD= Rock Quality Designation For bored piles socketted in sound bed rock, Q s = f s,*a s, where Q s = the ultimate load bearing capacity of the socket be limited to: f s = 75 kpa for RQD between 0 to 25% = 150 kpa for RQD between 25 to 50% =350 kpa for RQD> 50% Page 25

26 The determination of design parameters for the bored pile foundation An instrumented trial bored pile of diameter of 2.0~2.3 m and about 125m in length was installed and tested with an Osterberg Cell (O-cell) planted in the test bored pile during construction. Page 26

27 Depth in m Site Characteristics A water depth of about 12m- sea level -9.95m (reduced level) Soft to very soft marine mud, 18 m in thickness with SPT-N value< Pier 25 (ABH1) Average SPT-N value profile Soft mud loose to medium dense medium to coarse sand N ave = 10 medium dense medium to coarse sand N ave = 22 Φ 2.3m medium dense to very dense, Alluvial fine to coarse sand with SPT-N values increasing with depth to about 100 m medium dense to dense medium to coarse sand N ave = 41 Completely weathered granite -110 Slightly weathered granite bedrock, Grade II CDG slightly weathered granite N-Value Φ 2.0m Depth zero = seabed level (reduced level m) Page 27

28 1.88m 0.44m m m Construction of the test bored pile pile head 2.3m -8.50m -9.95msea bed m Reinforcement details: 2.0m Main 150mm c/c Binder: T to 300 mm c/c Concrete cover: 75mm Concrete Grade: G40/20 Upper plate of load cell Load cell lower plate of load cell m m level of pile toe Page 28

29 Instrumentation of the test bored pile data acquisition system displacement transducers reference beam hydraulic pump with pressure gauge oil pipe shaft side shear steel telltale rods telltale casings load cell shaft end bearing Page 29

30 Instrumentation of the test bored pile 5 hydraulic jacks of a maximum stroke of 200mm Access of tell-tale rods to bottom plate Type Outer diameter (mm) Diameter of Cylinder (mm) Upper plate thickness (mm) Lower plate thickness (mm) Height (mm) max. stroke (mm) YG Page 30

31 Instrumentation of the test bored pile TGCL-1 Vibrating wire type strain gauge Operational range: 2500 με Resolution: 0.4 ~ 1 με Waterproof 150m under water Temperature: -20 to 80 WDL-50TZ Linear Variable Differential Transformer (LVDT) displacement transducers Page 31

32 Layout of testing platform refrence pile 1 refrence beam P25 test pile Reference pile 2 Unit:mm; Page 32

33 Instrumentation of the test bored pile 6 LVDT displacement transducers were installed, namely 2 for upward movements of the top plate of load cell 2 for downward movements of the bottom plate of load cell 2 for upward movements of the pile head. Page 33

34 Vibrating wire type strain gauges Tell tale access O-Cell Page 34

35 Static load test of the trial bored pile NO. Design strength of concrete P25 G40/20 Location in Chainage CH Socket depth (m) Pile Diameter (m) Anticipat ed Level of Pile Toe(m) Level of pile top (m) Bottom Level of Load Cell Box(m) Working Load (kn) ~ Note: The diameter is 2.3 meters from level to ; and the diameter is 2 meters from level to reduced level referred to NGVD NO. Type of drilling rig Verticality Concrete filling rate Density of slurry Level of pile top (m) Level of pile toe (m) Socket depth (m) Filter cake thickness (mm) P25 ZJD-300 1/ ~ Page 35

36 Static load test of the trial bored pile (cont d) Load increment No. Percentage of Working Load (%) Test load, in kn, Q Applied load at load cell in kn, Q up Minimum Maintained Time (hour) 1 to to to to to Note: design working load= kn; maximum design testing load = kn Page 36

37 Test Results When loaded from kn to kn, the pile moved upward for more than 46mm (i.e., from mm to mm while the lower part moved downward for 0.6mm (i.e., from 4.21mm to 4.81mm). As the test load was released to zero, the residual settlements measured at the top and bottom plates of the O- cell were 24.33mm and 0.06mm respectively. The residual movement remained at the pile head was 19.84mm Page 37

38 Test results (cont d) kn mm Equivalent load settlement curve for the test pile subjected to equivalent head down loading Page 38

39 depth(m) Depth in m Test results (cont d) Shaft friction of P25 0 Axial force(kn) estimated measured Max shaft resistance, fs in kpa Page 39

40 Conclusion 1. The Design Brief for bored pile foundation to the 2 nd Penang Bridge was based on Malaysian practice which is an empirical approach on the basis of the British Standard BS8004 (1985). 2. The geotechnical parameters for bored pile foundation design were verified by in-situ loading test on an instrumented trial pile. 3. The test was carried out by O-cell method on the trial pile, 2~ 2.3 m in diameter, 115 m in length including a socketted depth of 4.3m in sound granite (Grade III/II) bed rock. Page 40

41 4. The measured shaft friction was less than the estimated probably because of the influence of direction of loading (uplift) which worked against the overburden thus leading to a reduction of shaft friction; and the long construction time that might have led to softening of the soil around the pile shaft The slurry cake might not be completely removed by the concreting 5. The ultimate rock socket friction is 798 kpa under uplift conditions whereas the maximum rock socket friction in compression is 941 kpa. Page 41

42 6. It is noted from the test result that the ending bearing capacity was only slightly mobilised. The load carrying capacity of the test bored pile can be significantly increased if the end bearing capacity of the pile is considered. Page 42

43 Thank You Page 43