Design and installation of steel open end piles in weathered basalt. Luc Maertens*

Transcription

1 Design and installation of steel open end piles in weathered basalt Luc Maertens* * Manager Engineering Department Besix, Belgium, lmaertens@besix.com Associate Professor Catholic University Leuven, luc.maertens@bwk.kuleuven.ac.be Abstract The Dabhol Power Company (DPC) is constructing India s first Liquefied Natural Gas (LNG) Terminal on a remote strip of the western coast along the Arabian Sea about 160-km south of Mumbai. Designed to handle the largest LNG carriers, the Terminal s marine facilities includes a Jetty that extends 1750-m into open sea to reach adequate depth, a jetty head supporting the unloading arms and the control tower, four berthing dolphins, four mooring dolphins, walkway support dolphins, three navigation dolphins and one tug berth. 473 steel open-end piles (dia.762-mm) support all these structures and are driven into weathered basalt to support working compression loads up to 4000 kn. Tensile loads up to 2000 kn are supported by 610-mm diameter sockets that are bored beneath the pile tip in the underlying basalt. The subsoil consist of three subsequent layers: 1. Soft clay layer with a thickness varying between 0 and 6-m. 2. Weathered Basalt with a thickness of 1 to 5-m s, and a RQD value varying between 0 to 90%. 3. Sound Basalt with unconfined compression strength varying between 29 and 115 MPa. A significant problem consists of defining a installation procedure for the piles, which reconcile the requirement to guarantee an adequate bearing capacity with a risk of damaging the pile tip, and the requirement of limiting the deformation of the pile tip in such way that the installation of the socket through the steel open end pile remains possible without damaging the bore hammer. An onshore test program was performed to solve this problem. To confirm the findings of the test program, Dynamic Pile Tests were carried out on offshore piles. In this way the bearing capacity of the installed piles was controlled. This paper discusses the test program which was carried out to define the installation conditions (Hammer Energy and Penetration per blow) in order to guarantee the bearing capacity in compression and tensile together. For compression piles, a penetration of 1 mm per blow is required and for tension piles 5 mm per blow for 16 m piles and 2,5 m per blow for 19 m piles is not to exceed 1

2 Description of the project. Dabhol LNG Project. The Marine Works of the Dabhol LNG project were awarded by Enron Engineering and Construction Company to the Belgian Contractor BESIX as a design and built contract in December The jetty consists of a concrete deck that is supported by prestressed beams. The beams are supported by bents 30-m apart. Vertical and raked steel open-end piles support the bents. Every 120-m is a construction joint and in the centre of each section of 120-m is a strong point bent. Berthing, mooring, navigation and walkway support dolphins as well as the jetty head are slab-structures supported by the same piles. Two types of piles are used: piles dia.762, th.16-mm and piles dia.762, th.19-mm. All piles are provided with toe reinforcement consisting of a 12-mm thick steel plate welded inside the pile tip. The height of this reinforcement is 380-mm. The length of the piles varies from 20 to 35 meters and the steel grade is X60 (413 N/mm²). The marine structures are protected from the sea attack by a breakwater of 2300-m parallel to the shoreline. However, during the first monsoon, the progress of the breakwater will not be sufficient to give an adequate protection. For this reason all structures are designed to resist to wave loads with a significant height of 9-m. Other loads to consider are: a current of 1 m/sec, live loads on the decks and last but not least an earthquake load with a ground acceleration of 0.16-g. Mooring and berthing dolphins have to resist to horizontal forces of 6000 kn. 2

3 Pile hammer and boring equipment. 1. Pile Hammer: The driving hammer used for the installation of the piles is an IHC Hydrohammer S90. The hammer is hydraulically operated and the weight of the hammer is 92 MN, the weight of the ram is 45 MN. The hammer has an operating impact energy operation rate of 2 to 90 kj. 2. Boring equipment: The drilling equipment used in Dabhol is specially designed and build for the site by Geotec International (Belgium) and consists of a Numa Reversh Circulation Hammer (Massachusetts, USA) combined by a RCD rotary head (NCB, Italy). It allows for boring a 610- m socket in the weathered and sound basalt trough the 762-m piles. RCDS-3 Drilling Equipment. The RCDS-3 drilling equipment consists in (see figure): 1. Casing clamp 2. Working platform 3. Raking cylinder 4. Mast inclination cylinder 5. Rotary head 6. Mast 7. Pull-down hydraulic gear motor 8. Suction pipe 9. Drill rod 10. Casing 11. Stabiliser 12. Down-the-hole hammer 3

4 Soil conditions at onshore test location. DEPTH (m) Boring at test location TCR RQD (%) TCR 5 RQD To perform the onshore trial pile test, a series of onshore borings were carried out in order to find a location with a geological profile which was as similar as possible as the available offshore borings. The aim was to find a location with a sufficient thick layer of weathered basalt. A typical boring at the test location is given here beside. Nine unconfined rock core tests were performed, giving an UCS of respectively: 72,5 43,1 61,6 50,8 113,3 52,6 65, and 42,8 MPa Typical boring at test location. Definition of the Static Resistance from Hammer Records. IHC carried out wave equation runs made with the TNOWAVE software of TNO Delft (Netherlands), taking into account the following parameters: 1. The soil conditions as given in chapter The geometry of the piles, including the 12-mm toe reinforcement. 3. Hammer characteristics (IHC S-90) The calculations include runs for 100, 75 and 50% hammer energy and for different depths, resulting in a matrix of Energy, Penetration per blow, Stress during driving, and SRD (Static Resistance to Driving). These calculations were performed for the 19-mm piles as well as for the 16-mm piles. 4

5 A mathematical treatment of all this results allowed Besix engineers to define specifically for the site a general equation for the SRD in function of the number of blows for 100-mm of penetration (N) and the applied hammer energy (E). This gives the following dimensional formulas: SRD = (21.70*E )*ln(N) *E for the 762x16-mm piles [1] SRD = (28.39*E )*ln(N) *E for the 762x19-mm piles [2] SRD in kn, E in kj, N = number of blows for a penetration of 100-mm. In the figure here below, one can see the mathematical approach (lines) against the calculated by IHC applying TNOWAVE (dots). TNOWAVE calculations against Dimensional Formula for 19 mm piles SRD = (28.39 E ) ln(n) E kj ,5 kj SRD (kn) kj TNOWAVE Blows per 100 mm penetration 8000 TNOWAVE calculations against Dimensional Formula for 16 mm piles SRD = (21.70 E ) ln(n) E kj 67,5 kj SRD (kn) TNOWAVE 45 kj Blows per 100 mm penetration 5

6 Static Compression Test. Static Compression Test A pile test up to 8000 kn was performed on pile C2. The pile characteristics are as follows: The pile with an external diameter of 762-m and a wall thickness of 19-mm was embedded over a length of 3,28-m. The total length of the pile was 4,40-m. The ultimate bearing capacity can be calculated from the formula: Q ub = α * q ub * A b with: α = installation coefficient = 0,5 for driven open end piles [1] q ub = ultimate bearing pressure A b = section of the pile basis = 0,456 m² q ub = c * N c with: c = cohesion = α * q uc q uc = unconfined compression test of the rock ( 30 MPa) α = 0,1 for RQD = 0 70% [2] N c = bearing factor = 15 for φ = 35 (= internal angle of friction for basalt) [3] Q ub = 0,5 * 0,1 * 30 * 15 * 0,456 = 10,26 MN The results of the compression test are given in the figures below. Ultimate bearing capacity was not reached for a load of 8000 kn. By extrapolating the settlement curves it was found that a bearing capacity of kn was found for a penetration of 7,65-m which is only 1% of the pile diameter and still far below the 10% for the conventional ultimate bearing capacity. 6

7 Pile test C2 (23-24/09/1999) : Average settlement Load (ton) Load Settlement Average settlement (mm) Time (min) 0 Pile load test C2 (23-24/09/1999) - Loading and unloading curves Load (kn) Average Settlement (mm) loading WORKING LOAD unloading 1 unloading 2 loading 2 7 The bearing capacity was also measured by a local subcontractor, using a dynamic test procedure together with the well-known CAP-WAP program but unfortunately, the results were not trustable. Below gives the driving records for pile C2 together with the interpretation for the bearing capacity using formula [2] and the not trustable dynamic test results are given. 7

8 Ultimate Bearing Capacity - Pile C2 (19 mm) Comparison of values calculated with the dimensional formula [2] and from Dynamic Test. Ultimate Bearing Capacity: * from dimensional formula and hammer records : 9200 kn * from dynamic test : 6600 kn 0,0 1,0 Penetration (m) 2, ,0 4,0 5, Ultimate Bearing Capacity (kn) 8

9 Static Tensile Tests. Tensile pile test arrangement Testpiles 762x16mm Sockets 61mm 9,59 9,60 T1 T2 8,54 8,59 5,95-0,05 4,35 1,35 Two static tensile pile tests were performed on pile T1 with a socket of 6-m and on pile T2 with a socket of 3-m. A tensile test up to 4000 kn was performed on pile T1 and a tensile test up to 2000 kn on pile T2. As the result of the tensile test on both piles T2 and T1 was totally satisfactory, it was decided to carry out a pull out test on pile T2 up to 5000 kn (limit due to the strength of the testing frame). The pile characteristics are given in Figure 12. The results of the T2-test and the pullout test are given here below. 9

10 Pile load test T2 (15-16/09/99) - Interpretation of socket load Tensile Load (kn) First Loading Slope 0,1 Load supported by friction on steel pile 0,2 Second Loading Slope Average uplift (mm) 0,3 0,4 0,5 0,6 Unloading 0,7 0,8 T2 - superposition of loading and unloading curves to 2000 kn and 5000 kn 0 Tensile Load (kn) ,5 1 Average uplift (mm) 1,5 2 2,5 3 3,5 Working load 10

11 As the tensile capacity of the pile is not only generated by the friction on the socket, but also by the friction on the steel pile, and since the uplift design is neglecting the last, it was advisable to split up both. In Figure 17 one can distinguish two slopes in the loading curve. We assumed that the change in slope corresponds to the start of mobilisation of the friction on the socket. One can see that it was found that the friction on the steel pile was 1250 kn or 123 kn/m² (= 0,123 MPa). Results of the tensile test: Pile Socket length (m) Working load (kn) Deflection at working load (mm) Test load (kn) Deflection at test load (mm) T T T Table 1 Evaluation of the Uplift Capacity. The uplift capacity is defined by: (1) bond between the concrete socket & rock (2) mass of mobilised soil (3) bond between the steel pile and the rock (4) bond between steel and concrete (inside the pile). (2): The mass of mobilised soil can easily be calculated. It is not measured since the foundations of the reaction supports of the tensile test are inside the influence core. (1) (3): Only the sum of both is measured. The split of both is done according to pile load test T2 figure. (4): The length of the socket plug inside the pile was 2,3-m. The bonded surface is thus : π * 0,73 * 2,3 = 5,3 m² According to BS 5400, the bond stress between the steel pile and the infill concrete is 0,4 MPa for ULS. During pull-out test, the bond stress was 0,71 MPa (1): The bond stress between the concrete socket and the rock was: 3750 /(π * 0,61 * 3) = 0,65 MPa According to Tomlinson [4] the bond stress is 0,36 MPa in ULS. Under working load, a bond stress equal to 0,12 MPa was assumed in the design. Bond stress (MPa) ULS Test Test/ULS Steel pile concrete 0,4 (*) 0,75 1,8 Concrete socket - rock 0,36 0,65 1,8 (*) according to BS 5400 Part 5 art

12 Installation procedure for piles. Damaged pile tip Damage of pile tip as shown above can not be accepted since excessive damage of pile tip prevents the installation of the sockets through the piles. The TNOWAVE analyses as documented in chapter 4 also gives the stress during driving. The combination of the SRD, Stress during driving, Hammer Energy and penetration per blow leads to graphs as given in below for a 16-mm compression pile. It shows that the stresses during driving, for a same SRD, decrease significantly with the hammer energy, and increase slightly with the number of blows for a penetration of 100-mm. 12

13 Definition of refusal for compression pile 16 mm Maximum SRD = 2 Maximum Working Load = 2 * 2620 kn = 5250 kn 90 kj 67,5 kj SRD (kn) kj Blows per 100 mm penetration Maximum Driving Stress (MPa) Yield stress = 415 MPa Allowable stress = 332 MPa Blows per 100 mm penetration On another hand, analysis of the pile toe damage during driving (using the IHC Hammer at full Energy = 90 KJ), leads to the following conclusions: Pile Thickness (mm) Max blows for 100-mm penetration (at 90 kj) Maximum Driving Stress (from output of TNOwave model) (MPa) Damage at toe (m) T T T

14 As one can see, the maximum stresses during driving were close to the yield stress (415 MPa). In fact these maximum driving stresses are computed by the IHC model with the assumption that the stresses are uniformly distributed over the entire cross section. This is of course never true in reality, and an appropriate safety factor has to be used in the definition of the refusal criteria. Considering the stress analysis in view to guarantee the SRD, together with the damage analyses leads to the final installation criteria. It was decided to allow 80% of the yield stress (=332 MPa) for compression piles and 55% (= 225 MPa) for tension piles, since tension piles need a socket. Finally the installation procedure was as follows: Compression Pile 19-mm Tension 16-mm (*) 19-mm (*) Refusal criteria for permanent works Hammer Energy Blows per 100- mm penetration (kj) (% of full energy) 16-mm (*) This criterion was checked by installing two additional raking piles on the test location onshore. After inspection, no damage at pile tip was observed as shown below. Pile tip after driving 14

15 Dynamic offshore test. Since dynamic onshore test was not trustable, dynamic offshore test was carried out by another Al-Futtain Tarmac from Dubai. Parameters used for PDA analysis are: 1. Parameters concerning the driving hammer: 2. Parameters concerning the piles: Characteristic impedance = kg.m -2. sec -1 Wave velocity = 5172 m/sec 3. Parameters concerning calculation of the bearing capacity with the CASEmodel. As damping coefficient, Jc = 0,1 have been used. 4. Overview of the results : Structure Bent Rake Loading Type Thickn. (mm) Ultimate Resistance SRD (hammer records) SRD (dynamic test) Jetty Approach Jetty Approach Jetty Approach Jetty Approach Jetty Approach 16 V Compression /3 Compression /3 Tension V Tension /3 Tension Jetty Head - 1/3 Tension Jetty Head - 1/3 Tension Jetty Head - 1/3 Tension Jetty Head - 1/3 Tension Jetty Head - V Tension An overview of these results is also given in the figure below. 15

16 Dynamic offshore tests Resistance (kn) Required Ultimate Resistance SRD (dimensional formula) Dynamic Offshore Test Conclusions Installation of open end piles in rock (weathered basalt) can be controlled efficiently under the condition that a continuous controlled hammer is used together with an adequate pile recorder. In the case of Dabhol, all hydraulic functions of the IHC-S90-Hammer are electronically regulated. This ensures optimum control of the energy blow rate and an optimum control of the penetration of the pile to the required depth without damaging the pile toe. This is essential in the case that sockets have to be drilled through the pile toes after driving. Onshore tests and preliminary calculations allowed for a prediction of the Ultimate Bearing Capacity in function of the applied Hammer Energy and the penetration per blow. Offshore dynamic tests confirmed the presumed bearing capacity. References [1] Ministère de l Equipement, du Logement et des Transports (1993). Règles techniques de conception et de calcul des fondations des ouvrages de génie civil.. Fascicule 62, Titre V. Paris. [2] Kulhawy F. and Goodman R.E. (1987). Foundations in rock, chapter 15 of Ground Engineering Reference Book, ed. F.G. Bell, Butterworth, London. [3] Wyllie D.C. (1991). Foundations on rock. E & FN Spon, London. [4] Tomlinson M.J. (1995). Pile design and construction practice. E & FN Spon, London. 16