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KSCE Journal of Civil Engineering 2010 14 4 453 460 DOI 10 1007 s12205 010 0453 5 453 Coastal and Harbor Engineering Verification of Laboratory Model Testing and Calibration of Suction Pile Installation in Sand Ki Chul Park Terje Preber and In Chul Kim Received July 6 2009 Revised October 5 2009 Accepted October 30 2009 Abstract As a part of the feasibility study of the Mobile Offshore Bases suction piles are currently being studied to provide the necessary mooring capability This paper presents the results of a series of suction pile installation tests performed in a model tank consisting of 59 2 cm diameter and 176 8 cm height Suction piles ranging in diameter from 15 9 cm to 29 8 cm were tested with various surcharge weights and initial pile penetration depths in loose through medium sand conditions to measure the relationship between the applied suction pressure inside the pile and the resulting pile penetration from the beginning till the complete installation The results have been used to verify the laboratory model testing and calibrate the mobilized soil friction angle included in the analytical solution of the suction pile installation i e the suction pressure vs pile penetration relationship This mobilized soil strength is described as a function of non dimensional parameters characterizing the variation and transition of the soil strength during pile installation The non dimensional parameters include all pertinent pile and soil parameters that are thought to govern the pile behavior during installation Keywords suction pressure suction pile anchor pile penetration offshore mobilized soil strength 1 Introduction During last several decades the concept of using differential water pressure as a driving force to install piles into the seafloor has been increasingly applied in offshore activities Albert et al 1989 This paper presents that the suction pile provides an adequate mooring technique for the large scale floating structure in deep sea Since the vertical and lateral loads caused by the weight of superstructures upon the pile itself and by a mountain of wave expected from the floating structure such as the Mobile Offshore Bases MOB and Tension Leg Platforms TLP are to be extremely large in magnitude any conventional underwater mooring techniques may not provide adequate resistance Bang and Cho 2003 Suction piles are different from traditional steel pipe piles in several ways Traditional offshore structures have been compos ed of steel pipe piles which are driven into the deep seafloor A substitutable way for easer pile installation process has been desired because pile driving is difficult in deep sea Wang et al 1978 The primary distinction between suction anchors and suction piles is that suction anchors are subject to a continuously applied suction and are usually much smaller than suction piles in size Due to its small size and requirement of continuous suction maintaining anchorage the use of suction anchors is limited Consequently suction anchors are typically used for temporary anchorage of small rigs Hogervorst 1980 The principle of the suction pile is using suction of the water and air contained inside the skirt compartment to create the downward driving force to be added to the submerged self weight Once the pump is in operation a differential pressure is developed at the portion of the pile top which results in a net downward force working over across sectional area of the top of the pile And a water pressure gradient due to the differential pressure inside the portion of the pile top caused by the applied suction pressure takes place in the sand and around pile inside surface submerged below the water Senpere and Auvergne 1982 Rognlien et al 1991 This water pressure gradient induces the decrease of the soil effective stress by lateral and vertical water flow from outside soil surface through the tip of the pile to inside surface of the pile This means that the strength of the sand is reduced by loosening sand density particularly inside sand because of water flow from outside to inside Bang et al 1998 The objectives of this research are to verify the laboratory model tests with additional tests of expanding diameters sur charge conditions and various initial penetrations and to calibrate Member Lecturer Dept of Civil Engineering Dongseo University Busan 617 716 Korea E mail dongseo94 yahoo co kr Emeritus Professor Dept of Civil and Environmental Engineering South Dakota School of Mines and Technology Rapid City SD 57701 USA E mail Terje Preber sdsmt edu Member Associate Professor Dept of Civil Engineering Dongseo University Busan 617 716 Korea Corresponding Author E mail cvkic dong seo ac kr Ki Chul Park Terje Preber and In Chul Kim 454 KSCE Journal of Civil Engineering the previously developed analytical solution based on laboratory model tests on suction pile installation in sand with the result of the present model tests An analytical solution has been calibrat ed through many model tests on suction piles to investigate the pertinent mobilized soil friction angle ratio resulting from the relationships between the applied suction pressure inside pile top and the pile penetration at a given depth and many parameters on the shape of suction pile as well In order to produce this analytical solution performing suction pile installation in the field without any instability the carefully selected raw data had to be obtained so that 3 to 5 of identical tests in each series as shown in Tables 2 and 3 were conducted to minimize any potential errors and the effect of water flow from outside soil through the tip of the pile to inside soil resulting from the differential pressure caused by the applied suction pressure Preber et al 2001 Park 2001 The lateral flow of the water at the tip of the pile and the upward flow inside the pile reduce the strength of sand by loosening its density To simulate this sand loosening the concept of the mobilized soil friction angle ratio has been introduced by Bang et al 1998 It is expressed as 1 where m mobilized soil friction angle necessary for the equilibrium between the external force and the pile bearing capacity fully available soil friction angle The variation of has been determined from the results of laboratory tests by matching the calculated pile penetration with the observed pile penetration at given conditions during testing In this study the laboratory model tests utilized two circular plexiglas tubes with two different diameters and thicknesses i e piles II and III Totally six different surcharge weights were mounted on the top of the pile to simulate the real steel pile or concrete pile and the superstructures in the field Also approxi mately three different pile initial penetration depths were con ducted to examine its effect on the pile installation The results of laboratory model tests on these two piles would be added up to that of the preceded one on pile I as shown in Table 1 in order to calibrate the mobilized soil friction angle ratio 2 Test Facility 2 1 Model Test Tank The test tank consisted of two sections of 61 cm diameter heavy duty PVC water pipe The lower section was made from the flange of the pipe and was attached with turnbuckles to 1 27 cm thick steel plate In order to provide a water proof seal 1 27 cm thick neoprene sheet was placed between the steel plate and the pipe flange For backwash and drainage purpose a perforated steel plate was covered with a geofabric to prevent the sand from washing into the backwash chamber 149 35 cm straight pipe section was then installed into the top portion of the flange and sealed with a standard pipe seal 5 08 cm diameter of drain pipe was located at 3 81 cm below the top of the test tank to keep a constant water level in the test tank for the duration of the test so as not to induce variable water head levels Prior to testing the test tank was filled with water and sand was poured into the test tank The sand was placed to within 15 24 cm of the top of the pipe The density of the sand was filled constant by maintaining the height of each test through a combination of backwashing and agitation 2 2 Model Piles Two different model piles in diameter were used to carry out the laboratory model tests and each one had 9 series which consisted of 3 to 4 tests to minimize the potential errors that could happen due to manual operation The large model pile consisted of 29 18 cm inside diameter and 153 0 cm long plexiglas pipe with wall thickness of 0 65 cm The small one had 15 24 cm inside diameter and 154 9 cm long plexiglas pipe with wall thickness of 0 65 cm at which additional pile II with porous disk plate was located at 26 67 cm below the top of the pile Six different surcharge weights were applied on the top of the pile to simulate the weights of the pile and the superstructure in the field In addition three different initial pile penetration depths were used to examine its effect on the pile installation The tip of the pile was beveled at an angle of approximately 30o with the longitudinal axis of the pile The details of the test piles are summarized in Table 1 The top of the pile was capped with a plexiglas disk and a vacuum pipe leading to a vacuum pump was attached near the top of the pile as shown in Fig 1 Pressure transducer to record the level of vacuum was attached near the top of the pile and on the pipe outside the pile The vacuum pipe from the pile to the vacuum pump was equipped with two vacuum reservoir tanks and two desiccator chambers to reduce the amount of moisture entering the vacuum pump Fig 1 A vacuum valve was attached to the pipe between the desiccator chamber and the vacuum pump The piles were tan m tan Fig 1 Schematics of Pile and Vacuum System Verification of Laboratory Model Testing and Calibration of Suction Pile Installation in Sand Vol 14 No 4 July 2010 455 marked in 1 27 cm increment on the outside surface to measure the penetration of the pile manually Automatic readings were taken with a roller device attached to the outside of the pile For pile II in order to eliminate possible slippage of the roller a narrow sand paper strip was attached along the outside surface of the pile to improve the roller friction 2 3 Vacuum Reservoir Tank The vacuum reservoir tank consisted of a cylindrical steel tank 157 58 cm long and 51 81 cm in diameter It was installed between the suction pile and the desiccator chambers Only one vacuum tank was used for pile II But during the tests on pile III two vacuum reservoir tanks were necessary because the amount of water drawn through the cross section area was 4 times greater than that of the pile II 2 4 Soil Properties The soil used for the experiment consisted of cohesionless fine sand obtained commercially The material was sub rounded poorly graded sand with 100 passing the number 16 and less than 1 passing the number 100 and 200 US standard sieves The friction angle of the sand was 30o for loose sand and 36 5o for medium dense sand The total unit weights for loose and medium sands were 17 kN m3 and 18 86 kN m3 respectively 3 Experiment Details 3 1 Pile Installation Prior to installation of the pile backwashing and agitating of the sand were performed by applying water through the backwash valve attached to the lower part of the tank and the 1 9 cm diameter agitation pipe in order to raise its surface to a preset level corresponding to the desired dry unit weight The pile was then manually pushed into the sand to a preset initial penetration depth A level was used to make sure the pile be vertical Final adjustment of the sand level corresponding to a dry unit weight of 18 86 kN m3 was performed by tapping around the tank with a rubber hammer keeping the drain valve attached to lower part of tank open for drainage of excess water The vertical position of the pile was then rechecked Surcharge was loaded on the top cap of the pile to simulate the pile in the field with care not to affect the vertical placement 3 2 Test Procedure After the pile was completely seated at its initial penetration depth all instruments were set to zero reading on the digital Data Acquisition System DAS and the soil and water column heights rechecked Vacuum was then carefully applied in increments by opening and adjusting the vacuum control valve shown in Fig 1 until the pile started to move As long as the pile moved manual pressure readings were taken at every 1 27 cm penetration DAS increments were set at 10 second intervals During this process extreme care was taken in increasing the vacuum pressure not to induce a sand boiling condition as this would fill up the pile with sand quickly and prevent further penetration However the soil column inside the pile was observed to rise slightly It was considered that this was due to both the displacement of the sand caused by the pile penetration and the soil volume expansion caused by the upward water flow gradient inside the pile The soil column heights were recorded As the pile penetration progressed under constant vacuum level the time interval between movement increments increased When the time interval between increments reached 1 to 2 minutes or longer the vacuum pressure inside the pile was increased gradually until the movement commenced again and the procedure repeated until the pile was fully penetrated or until a sand boiling condition inside the pile occurred Tests were conducted with two phases in three types of piles The tests of the first phase were performed on the pile I with a surcharge load corresponding to 100 of the weight of a steel pile with the same dimensions The tests of the second phase were carried out on the piles II and III with surcharge weights corresponding to 125 and 150 of steel piles with the same dimensions All tests of two phases were performed with initial pile penetrations of approximately 0 3 0 6 and 0 9 meters In addition three to four tests were performed at each initial pile penetration in order to check for repetitive test results For the laboratory model testing more than 1600 data points were obtained Preber et al 2001 Park 2001 4 Test Results and Discussions 4 1 Test Results Laboratory model tests on suction piles in sand were con ducted with two different piles in diameter Nine series of tests were performed on each pile size Each series consisted of 3 to 5 nearly identical tests to minimize any potential errors due to manual operation and unexpected behaviors possibly caused by non uniform sand Series 1 through 9 were conducted on pile II 15 24 cm in diameter and series 10 through 18 were performed on pile III 29 18 cm in diameter For pile II series 1 through 3 comprised of ten tests with surcharge load of 304 7 N mounted on the top of the pile Average initial penetration depths were 29 85 60 96 and 90 81 cm Series 4 through 6 comprised of nine tests with surcharge load of 380 3 N mounted on the top of the pile and initial pile penetration depths were 31 12 59 69 and 90 81 cm Series 7 through 9 comprised of nine tests with surcharge load of 459 5 N and initial pile penetration depths were 32 39 60 96 and Table 1 Details of Test Piles Pile Buoyant Weight N Length cm Inside Diameter cm Thickness mm I42 7152 411 986 7 II62 3154 915 246 5 III93 4153 029 186 5 Ki Chul Park Terje Preber and In Chul Kim 456 KSCE Journal of Civil Engineering 90 17 cm For pile III the series 10 through 12 comprised of twelve tests with surcharge load of 418 1 N mounted on the top of the pile Initial pile penetration depths were 31 12 58 42 and 88 90 cm Series 13 through 15 comprised of twelve tests with surcharge load of 525 8 N and initial pile penetration depths of 29 85 59 06 and 88 27 cm Series 16 through 18 comprised of eleven tests with surcharge load of 634 3 N and initial pile penetration depths of 34 77 62 23 and 87 0 cm The details of the model tests are summarized in Tables 2 and 3 During the model testing pile penetration depth and water rise inside the pile corresponding to the applied suction pressure were carefully recorded For piles II and III the surcharge loads were approximately equivalent to 100 125 and 150 of the weight of the steel piles of the same dimensions This means that the laboratory model tests were performed with almost the same surcharge loads in the field condition 4 2 Test Discussions Fig 2 from Test 2 series 1 A shows that the pile penetration increases as the suction pressure increases which indicates the Table 2 Details of Model Test Series 1 9 Series Pile Type Effective Surcharge Plus Pile Weight N Initial Pile Penetration cm Final Penetration cm 1 AII 304 7 90 81 129 54 1 BII 304 7 90 81 122 55 1 DII 304 7 90 81 120 65 2 AII 304 7 60 96 123 19 2 BII 304 7 62 87 113 03 2 CII 304 7 62 87 120 65 2 DII 304 7 60 33 118 11 3 AII 304 7 29 85 110 49 3 BII 304 7 29 85 111 76 3 CII 304 7 29 85 113 03 4 AII 380 3 88 27 120 02 4 CII 380 3 90 81 123 19 4 DII 380 3 93 98 119 38 5 AII 380 3 62 87 115 57 5 BII 380 3 92 87 116 84 5 CII 380 3 59 69 114 30 6 AII 380 3 31 12 116 84 6 BII 380 3 29 85 113 03 6 CII 380 3 31 75 113 03 7 AII 459 5 89 54 120 65 7 BII 459 5 90 81 121 92 7 CII 459 5 90 17 121 92 7 DII 459 5 90 81 124 46 8 AII 459 5 61 60 118 11 8 BII 459 5 60 96 119 38 8 DII 459 5 60 96 116 84 9 AII 459 5 32 39 114 30 9 BII 459 5 32 39 114 30 Table 3 Details of Model Test Series 10 18 Series Pile Type Effective Surcharge Plus Pile Weight N Initial Pile Penetration cm Final Penetration cm 10 BIII 418 1 31 12 129 54 10 CIII 418 1 29 85 125 73 10 DIII 418 1 31 12 130 81 10 EIII 418 1 29 85 132 08 11 AIII 418 1 59 06 119 38 11 BIII 418 1 59 06 134 62 11 CIII 418 1 54 61 134 62 11 DIII 418 1 58 42 127 00 12 AIII 418 1 85 73 133 35 12 BIII 418 1 88 90 133 35 12 CIII 418 1 89 54 137 16 12 DIII 418 1 87 63 133 35 13 AIII 525 8 29 85 134 62 13 BIII 525 8 28 58 127 00 13 CIII 525 8 30 48 120 65 13 DIII 525 8 29 85 127 00 14 AIII 525 8 59 06 129 54 14 BIII 525 8 59 06 125 73 14 CIII 525 8 57 15 133 35 14 DIII 525 8 57