Centrifuge model study on spudcan footprint interaction 4

Chapter – Experimental Setup and Procedures . Chapter Experimental Setup and Procedures 3.1 Introduction This chapter discusses the centrifuge modeling concept and experimental setup for full and half-spudcan tests and experimental procedures in the present study. Most centrifuge model tests were performed on the NUS geotechnical beam centrifuge. Ten (10) model tests were performed on the drum centrifuge facility in University of Western Australia (UWA). The experimental setups for both centrifuge facilities will be discussed separately. The experimental setup includes sample preparations, transducers, instrumentations, in-flight soil strength characterization devices, calibration of instruments etc. Lastly, the strategies and experiment procedures are presented. 3.2 Centrifuge modelling In a keynote lecture by Schofield (1998), he pointed out that geotechnical centrifuge model testing is as valuable as the observational method, however geotechnical centrifuge model tests help solve problems where “the observational method” cannot be used. One of the conditions that cannot be replicated for full scale test purposes is foundation behaviour of a mobile independent leg jack-up rig deployed offshore (Schofield, 1998). Some studies in relation to the behaviour of spudcan footings of jack-ups have been undertaken using centrifuge modeling technique in the past few decades 54 Chapter – Experimental Setup and Procedures . including Murff et al. (1991), Dean et al. (1993 and 1995), Tsukamoto (1994) and more recent works by Stewart and Finnie (2001), Cassidy et al. (2004), Hossain et al. (2004), Purwana (2007), Teh et al. (2008) and etc. The offshore industry already has confidence in geotechnical centrifuge modeling (Murff, 1996). In addition, owing to complexity of soil behaviour and the limitations in numerical modeling, physical modeling provides necessary data for the validation of numerical approaches and the refinement of the empirical solution for foundation response (Randolph and House, 2001). The main focus of the present study is to investigate the footprint characteristics and the interaction between spudcan and footprint. Both require creation of spudcan footprints by performing spudcan penetration and extraction. The footprint characteristics required evaluation of soil shear strength profiles beneath and around a footprint at various times. Soil consolidation condition is deemed to be important in defining the soil condition beneath a footprint. The time factor, Tv, for soil consolidation in centrifuge model and prototype can be expressed as follows: Tv  cvM .t M hM  cvp .t p hp (3.1) where t is time for consolidation, cv is the coefficient of consolidation and h is the thickness of the soil. The subscript M refers to model and p refers to prototype. To achieve the stress similarity, the scaling relation for linear dimension between model and prototype is deduced as hM  hp N (3.2) 55 Chapter – Experimental Setup and Procedures . where N is the ratio of centrifugal acceleration to the earth’s gravitational acceleration. cv is assumed to be the same for the same soil used in model and prototype. Substitute Equation 3.2 into Equation (3.1) gives tM  N2 (3.3) To achieve the same degree of consolidation, the time required in the model test is shortened by N2 times than that required in prototype. This is not only fastened the consolidation process during the sample preparation, but also the soil consolidation after a footprint is formed. Unlike surface footing, the spudcan test involves considerable penetration into the model ground of particularly in soft clay. The soil failure mechanism was found to change from shallow failure to deep failure over a range of penetration (Hossain et al., 2004 and Purwana, 2007). A correctly modeled over-burdened stress is required to trigger the soil back flow. In the situations where considerable cavity formed above the infilled spudcan, the effect of the weight of the infilling (or the over-burdened stresses) on the net bearing resistance becomes significant. Also, for spudcan extraction, the infilled soil provides a seal against transient suction developed at the base of the spudcan (Hossain et al., 2004). It is readily seen that the correctly modelled soil overburdened stress is essential to ensure that a more realistic soil failure mechanism is simulated. Centrifuge model testing therefore allows the use of small model structures to simulate a full size prototype stress field. 3.2.1 Centrifuge scaling laws and model error The scaling relationships between the model and its prototype can be derived either by dimensional analysis or by considering the governing equations and 56 Chapter – Experimental Setup and Procedures . system mechanics. A standard basic scaling law is needed to ensure the consistency response between the model and the prototype. The centrifuge scaling relations is shown in Table 3.1. Unlike the earth’s gravity which is relatively uniform throughout the depth, the acceleration field simulated in centrifuge varies in a model both magnitude and direction. This is because the inertial acceleration field in centrifuge is given by R2 where  is the angular rotational speed and R is the radius from the centre of rotation to any element in the soil model. Hence, the artificial gravitational acceleration is expressed as Ng = R2 (3.4) where N is the ratio of centrifugal acceleration to gravitational acceleration and g is the earth gravitational acceleration with a value of 9.81 ms -2. A strategically selected nominal radii, Re can be adopted to minimize the shortcoming of g-level variation throughout the entire soil model. As illustrated by Taylor (1995), the stress variation can be minimized to lower than 5% if the Re is set to be the distance from the centre of rotary to one-third of model height. 3.3 Experimental setup 3.3.1 NUS geotechnical centrifuge – Full spudcan test The centrifuge model tests presented in this report were conducted on the beam centrifuge at National University of Singapore (NUS), see Figs. 3.1(a) and (b). The working area of the platform is 750 mm × 700 mm and the headroom available is about 1200 mm. This m radius centrifuge is designed for a payload capacity of 40g-tonnes. A stack of 100-tracks silver-graphite slip 57 Chapter – Experimental Setup and Procedures . rings is mounted on top of rotor shaft for power and signal transmission between the centrifuge machine and the control room. More detail information about NUS geotechnical centrifuge can be found in Lee et al. (1991) and Lee (1992). All tests were conducted at 100g. The photograph and schematic diagram of the centrifuge model set-up for the present study are shown in Figs. 3.2(a) & (b). As the tests were to simulate foundation behaviour in offshore environment, all tests were conducted underwater. The model setup for full spudcan tests comprises two main components, namely the model container and the loading platform mounted on top of the container. Details of each component are described in the following sections. 3.3.1.1 Model container The model containers are cylindrical tubs made by stainless steel. Each container has a diameter of 550 mm diameter and height of 400 mm. Water valve is attached to the base of the container to facilitate water circulation. The water valve is opened during soil sample consolidation and closed during the test. A complete test involves consolidation of soil sample and followed by performing spudcan penetration without intervention of the centrifugal motion. Prior to spudcan penetration, the valve was closed mechanically in-flight using downward movement of the hydraulic piston. 3.3.1.2 Loading platform and actuators The loading frame is made by stainless steel with a plan area of 320 mm x 650 mm and height of 330 mm (see Figs. 3.3a and b). A 60 mm × 550 mm opening was made at the middle of the loading frame. This is to provide a continuous 58 Chapter – Experimental Setup and Procedures . passage for the vertical loading actuators to move at one axis. Two vertical loading actuators were engaged, the bigger one (denotes VA1) was used to perform spudcan penetration and extraction, while the smaller one (VA2) was employed to perform T-bar tests. The loading actuator VA1 is a hydraulic cylinder with a piston of 75 mm diameter and a rod of 37.5 mm diameter. At maximum hydraulic pressure of about 60 bars, the big loading actuator is able to provide a maximum compressive (downward) force of 2.65 tonnes and tensile (uplifting) force of tonnes. The difference between these two forces is due to smaller effective area of the piston at the tensile side (to exclude the rod area). The loading actuator VA2 is a hydraulic cylinder with a piston of 30 mm diameter and rod of 16 mm in diameter, and with capacity of 0.43 tonnes in compression and 0.3 tonnes in tension under the maximum supplied hydraulic pressure. The maximum stroke is 300 mm for VA1 whereas 250 mm for VA2. A flow divider was needed to divide the hydraulic flow into two separate passages into the two hydraulic control systems from a single outlet of hydraulic supply. When the loading actuator is engaged to perform the downward or upward movements, a digital command is sent to a servo amplifier through a switch box to form a closed-loop circuit with feedback signals. The actuator would then move whenever there is difference between the command and feedback signals and stay stationary when these two signals are identical. These two vertical loading actuators on top of a movable platform comprising a pair of minirails, a stepper motor, a gearbox, a ball screw, a control device etc. (see Fig. 3.3b). Each minirail consists of a frictionless linear slider and six (6) compatible bearings. The load carrying capacity per 59 Chapter – Experimental Setup and Procedures . each bearing and slider set is 350 kg. The coefficient of friction of the linear slider and bearing sets is as small as 0.2. The moving platform facilitates the measurements of soil shear strength at locations away from the centre of the model container. Potentiometers are used to monitor the elevation of the spudcan and the location of the moving platform. The overall system allows a model spudcan footprint to be created by performing a model spudcan penetration and extraction at the centre of the model container. The entire platform is capable to move in one axis at 100g. 3.3.1.3 Model spudcans The model spudcan is a cylinder made of aluminium alloy with a conical base and a pointed tip at the bottom of the conical base. The model spudcans used in the experiments have different diameters, namely 100, 80 and 60 mm for specific test purpose that will be discussed in the later chapters. All spudcan have the same base angle of 11o and pointed tip of 80o. A schematic crosssection and photograph of the model spudcans are shown in Figs. 3.4(a) & (b). 3.3.1.4 Model jack-up leg As discussed in Session 1.1, there are two basic types of jack-up leg design; columnar and open-truss legs. The open-truss legs are more common as they are stronger in both bending and axial loadings. In order to achieve the same ratio of bending and axial strengths in the proposed model, intricate model building technique to construct the truss-braces structure was involved. This seems to be impractical as different rigs have different leg designs and instrumentation on the model legs would be extremely difficult. Since the soil reaction at the spudcan level is the main interest of this study, the spudcan leg 60 Chapter – Experimental Setup and Procedures . is modeled as a thin walled circular hollow section (columnar type) with flexural rigidity similarity. In the selection of a more realistic leg flexural rigidity, a literature review on the model legs used in other studies were carried out and summarized in Tables 3.2. The actual field jack-up leg stiffness available published in the public domain are given in Table 3.3. The leg stiffness, EI generally ranges from 1.01×1012 Nm2 to 2.75×1012 Nm2 (except for Murff et al., 1991 where a much lower stiffness was used) for spudcan diameter ranging from 7.6 m to 18 m. Foo et al. (2003) made a comparison of equivalent stiffness EI/L3 for four different jack-up rigs over the leg length L (Fig. 2.7). They pointed out that the equivalent stiffness difference became less distinct when the jack-up operates in deeper water or has longer leg length. Owing to complexity of spudcan-soil interaction, uniqueness of installation location and unknown lateral load, it is impossible to determine the level of stiffening required for a jack-up leg (Foo et al., 2003). It is apparent that there is no typical leg stiffness and the equivalent leg stiffness is dependent on the leg length (effective length from spudcan to hull). In present study, a stiffness value of 1.18×1012 Nm2 (the leg stiffness of rig type 116C) was chosen for the first model leg (denotes Leg 1) fabricated. Owing to limited headrooms in the NUS centrifuge, the maximum allowable leg length is 280 mm or 28 m in prototype. This represents the spudcan installation in shallow water. To account for the spudcan installation in deeper water, the second leg (Leg 2) with much lower stiffness was fabricated to simulate the equivalent stiffness in water depth of 70 m due to the leg length limit. By adopting the equivalent stiffness similitude shown 61 Chapter – Experimental Setup and Procedures . below, Leg was designed to have EI magnitude of 15.6 times lower than that of Leg 1.  EI   EI     3  L  Leg  L  Leg1 (3.5) Details of the NUS model leg and spudcan are summarized in Table 3.4. It was noted that the model axial area was unrealistic high. This is because a circular hollow section was used to simulate a real jack-up leg which is practically a lattice structure. The problem of using circular hollow section is that the model cannot correctly model the ratio of bending stiffness (EI) to axial stiffness (EA) of the prototype. Since the main interest of this study is to investigate the rotational moment and horizontal force acting at the spudcan during spudcan penetration, the model leg was designed to have an equivalent bending stiffness to the prototype but slightly stiffer in axial stiffness than that of the prototype. Both model legs were instrumented with levels of full bridge axial gauges and levels of full bridge bending gauges, as shown in Figs. 3.6a & b. Full bridge strain gauges were selected as differential temperature can be compensated. To avoid the strain gauges being disturbed by soil back flow during spudcan penetration, a steel tube was used as a shaft protector to separate the soil and water from the model legs. Consistent results obtained for two identical tests indicated no undesirable influences acting at the strain gauges. However, the limitation of engaging the shaft protector is that it substantially reduces the volume of back flow soil on top of the spudcan. Comparison between the thin model leg of 16 mm diameter and the Leg with protected 62 Chapter – Experimental Setup and Procedures . shaft of 48 mm diameter were made as shown in Fig. 3.7. The difference in axial force during penetration in soft clay is insignificant up to a depth of m and increasing gradually with depth as the soil back flow takes place. However, within the test range, the difference in axial force is less than 10% and even lower for penetration in firmer clay where the soil back flow is negligible. Thus, this suggests that the influence of the protected shaft on the axial force can be accepted. 3.3.1.5 Calibration of the strain gauges of the model leg Two levels of full bridge axial gauges namely A2 and A4 were installed at positions shown in Fig. 3.6a. When the model leg experienced stress, the corresponding strain would be captured by the axial gauges through the TML strainmeter that was installed on-board. The axial force can be computed by converting the strain to force by either adopting the theoretical approach or direct load calibration. Theoretically, the axial force can be calculated based on the following equation (in elastic state): Axial force  EA a (3.6) where a is the axial strain and a1 to are the strain measured by each gauge at the same level (in microstrain), given as follow   a1   a   a   a  6   10   a   E is the Young Modulus of the material used and A is the material crosssectional area. The accuracy of the measured force is dependent on how close the measured strains to the actual strains of the material. Legs and are circular steel tubes with diameters of 35 mm and 20.1 mm respectively. The strain 63 Chapter – Experimental Setup and Procedures Table 3.6 Engineering properties of UWA kaolin clay (after Stewart, 1992) M’ 83 Chapter – Experimental Setups and Procedures a) Schematic diagram (after Lee at el. 1991) b) Photo Fig. 3.1 NUS geotechnical centrifuge 84 Chapter – Experimental Setups and Procedures (a) (b) Fig. 3.2 a) Photograph of the centrifuge model set-up; and b) schematic diagram of front view of the model set-up 85 Chapter – Experimental Setups and Procedures (a) Schematic diagram Stepper Motor Linear potentiometers VA2 VA1 (b) Photo Fig. 3.3 Elevation view of the loading frame 86 15 Chapter – Experimental Setups and Procedures 11° 8.9 80° 80° 11° 7.2 6.4 12 100 80° 11° 80° 11° 11° 80° 60 5.4 11° 4.8 80 80° Fig. 3.4 Schematic diagram and photograph of model spudcans of 100, 80 and 60 mm diameter (all dimensions shown are in mm unless otherwise stated) 87 Chapter – Experimental Setups and Procedures 18 18 12mm  Male Thread 12mm  Male Thread Full Bridge Axial Gauges Full Bridge Axial Gauges Full Bridge Bending Gauges Full Bridge Bending Gauges Stainless Steel Stainless Steel 34.9mm outer diameter 34.9mm outer diameter 22.2mm inner diameter 22.2mm inner diameter Wall thickness 6.35mm Wall thickness 6.35mm Y 300 300 280 mm Y dia. 34.9 dia. 34.9 dia. 22.2 dia. 22.2 X Full Bridge Bending Full Bridge Bending GaugesGauges X Full Bridge Axial Gauges Full Bridge Axial Gauges Full Bridge Bending Full Bridge Bending GaugesGauges 18 18 Section CrossCross Section 1-1 1-1 x Half Bridge Bending Gauges (see Cross-section 1-1) 25 40 Male Thread 12mm 12mm  MaleThread (a) (b) Fig. 3.5 a) Schematic diagram of Leg (all dimensions are in mm); b) Photograph of the model leg Full Bridge Axial Gauges 280 Stainless Steel x Half Bridge Bending Gauges (see Cross-section 1-1) dia. 17 X Full Bridge Axial Gauges 15 25 Bending Gauge dia. 20 25 Y x Half Bridge Bending Gauges (see Cross-section 1-1) Bending Gauge Cross Section 1-1 20 (a) (b) Fig. 3.6 a) Schematic diagram of Leg (all dimensions are in mm); b) Photograph of the model leg 88 Chapter – Experimental Setups and Procedures V (MN) -2 10 15 20 Depth 48mm 16mm 10 12 Fig. 3.7 Comparison of axial load obtained for legs with 16mm diameter (measured by loadcell) and 48mm diameter (measured by axial gauges) 2000 1800 1600 Load (N) 1400 1200 1000 800 600 Calibration -Loading 400 Calibration-unloading 200 Theoretical 0 10 20 30 Strain (x10-6) 40 50 60 Fig. 3.8 Calibration factor and theoretical value for axial gauges (A1) 89 Chapter – Experimental Setups and Procedures 45000 40000 Moment (Nmm) 35000 30000 25000 20000 15000 10000 Calibration 5000 Theoretical 0 50 100 150 Strain (x10-6) 200 250 300 Fig. 3.9 Calibration factor and theoretical value for bending gauges (B1) 90 Chapter – Experimental Setups and Procedures H (MN) 0.2 0.4 0.6 -2 H (MN) 0.8 1.2 Simple beam Beam column 0.6 0.8 1.2 Simple beam Beam column Depth (m) 10 10 12 12 a) Leg b) Leg M (MNm) M (MNm) -2 10 -2 Simple beam Beam column 10 Simple beam Beam column Depth (m) Depth (m) 0.4 Depth (m) 0.2 -2 10 10 12 12 c) Leg d) Leg Fig. 3.10 Evaluation of HM using simple beam and beam column formulations 91 Chapter – Experimental Setups and Procedures Control Markers Textured clay Fig. 3.11 Close view of the frontal face of the soil sample VA1 Loading frame Half Spudcan Clay Camera Fig. 3.12 Half-spudcan test setup 92 Chapter – Experimental Setups and Procedures Fig. 3.13 UWA drum centrifuge with central tool table in position (after Stewart et al., 1998) Fig. 3.14 Schematic diagram of cross-sectional view of UWA drum centrifuge (not to scale) (after Stewart et al., 1998) 93 Chapter – Experimental Setups and Procedures (a) (b) Fig. 3.15 a) Schematic cross-section view of spudcan used in UWA (model scale in mm); & b) the model spudcan leg Clay surface Tool table Leg Spudcan Fig. 3.16 Top view of the model spudcan and leg prior to centrifuge spinning 94 Chapter – Experimental Setups and Procedures (a) 5mm ball 5mm AL 6061 rod with 2mm hole along the centre Strain gauge wires (b) Fig. 3.17 a) Miniature ball-penetrometer, and b) T-bar used in NUS M3 thread to screw into 5mm rod 2.5 mm dia. (Al 6061) rod with 1mm dia hole 1mm dia hole to be filled with epoxy 2.5 mm dia. rod (Epoxy) i.e. a groove in the mould (5 times of upper rod diameter) Strain gauge wires Strain gauges mm dia. ball (Epoxy) Teflon mould (a) (b) Fig. 3.18 a) Photograph, and b) schematic diagram of mm ball penetrometer (courtesy of Lee (2009)) 95 Chapter – Experimental Setups and Procedures su (kPa) 10 20 30 40 Nball = 13.5 NT-bar = 10.5 NT-bar = 10.5 Depth (m) Empirical: su = 0.185v' 10 15 20 25 Fig. 3.19 Measured su profiles by T-bar and ball with the empirical su Soil sample achieves 90% consolidation activate movable platform to a position further from the intended footprint position Penetrometer test (to acquire undisturbed su) move to the intended footprint position at/near the container centre Formation of a spudcan footprint move to various positions from the footprint centre (to evaluate the soil condition of a footprint) move to the desired spudcan installation position Spudcan re-installation at desired position Ball penetrometer tests at various position from the footprint centre move to offset distance of 0.5 times spudcan diameter from from the footprint centre Fig. 3.20 Flow chart showing test procedures 96 Chapter – Experimental Setups and Procedures Fig. 3.21 Schematic layout of ball penetrometer tests and spudcan repenetration Fig. 3.22 Effect of strain rate and partial consolidation on penetration resistance (after Low et al., 2008) 97 Chapter – Experimental Setups and Procedures ` Fig. 3.23 Layout of test orientation (D is spudcan diameter, 10cm) (All dimensions are in cm) 98 [...]... the model container The spudcan penetration and extraction was then performed A footprint was created soon after the spudcan was completely pulled out from the model ground To measure the soil condition beneath the footprint, the movable platform was shifted to various positions from the footprint centre at which the penetrometer tests would be carried out Upon completion of the penetrometer tests, spudcan. .. instrumented on the model leg (see Session 3.6) for spudcan penetration at certain offset distance from footprint centre 3.3.2 NUS centrifuge – Half spudcan test 3.3.2.1 Half -spudcan test setup Half -spudcan tests were performed to investigate the soil failure mechanisms during initial spudcan penetration and extraction and the spudcan repenetration at certain offset distance from the initial penetration site... re-penetration would be conducted at the opposite side (see Fig 3.21) For some tests, the spudcan penetration at certain offset distance from the footprint centre was performed straightaway without the procedure of evaluation of the footprint condition 3.6.1 Penetration rate The penetration rates for all tests were evaluated under undrained condition To quantify the undrained response of the penetration tests,... VHM acting at the spudcan One of the main interests of the present study is to investigate the interaction between a spudcan and a footprint As the fully restrained connection is modelled, the interaction is reflected in terms of vertical load (V), horizontal load (H) and moment (M) acting at the load reference point (L.R.P.) The V is measured by the axial gauges at A1 & A2, and either one can be used... pore pressure transducers, wireless connection is used to transfer the measurements to the data processor in the control room 3.3.3.1 Model spudcan and leg The model spudcan with largest diameter of 14. 55 m in prototype was used in this study A schematic cross-section of the model spudcan is shown in Fig 3.15(a) The model leg is a thin walled circular hollow section with prototype flexural stiffness... randomly onto the frontal surface of the clay block The front wall of the container is detachable which can be disassembled for sprinkling the flocks and bits on the white clay surface When it is assembled, the connection between the front wall to the two side walls is water-tight The half -spudcan was fabricated to have the dimension of exactly half of the model spudcan with a diameter of 100 mm (Fig 3 .4( a))... shows top view of the model spudcan and leg attached on the centre tool table prior to the centrifuge spinning 3 .4 Sample preparation 3 .4. 1 NUS beam centrifuge Malaysia kaolin clay was chosen in this study instead of marine clay, mainly due to its relatively high permeability which could reduce the time for consolidation significantly In addition, the physical properties are more consistent which is capable... based on the 100 mm diameter spudcan and cv of 40 m2/year, the corresponded V was 79 (30 < V < 300) Therefore, the undrained condition was preserved at penetration rate of 1 mm/s The field spudcan installation is closer to load controlled penetration as the penetration is triggered by loading up the ballast tanks located at hull However, Teh (2007) found the difference between displacement controlled... to the top of the drum centrifuge for changing the testing tool such as spudcan, T-bar, laser scanning device etc Data acquisition consists of two onboard computers, one serving the tool table and the other one serving the channel Communication between the centrifuge testing tools and computers in the control room takes place serially via sliprings For additional instrumentations installed inside the... increased pre-consolidation pressures at 1g before subject to selfweight consolidation at 100g The magnitude of final pre-consolidation pressure is dependent on the desired clay shear strength profile The higher the final pre-consolidation pressure, the stronger the clay is The shear strength profile of the undisturbed clay sample was evaluated using T-bar and ball penetrometers 3 .4. 2 UWA drum centrifuge . present study is to investigate the interaction between a spudcan and a footprint. As the fully restrained connection is modelled, the interaction is reflected in terms of vertical load (V), horizontal load. geotechnical centrifuge model testing is as valuable as the observational method, however geotechnical centrifuge model tests help solve problems where “the observational method” cannot be used. One of the conditions. procedures in the present study. Most centrifuge model tests were performed on the NUS geotechnical beam centrifuge. Ten (10) model tests were performed on the drum centrifuge facility in University