EFFECT OF PLUG REMOVAL ON LOAD TRANSFER IN PLUGGED OPEN ENDED PILE BEHAVIOR

Transcription

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 5, September-October 2016, pp , Article ID: IJCIET_07_05_015 Available online at ISSN Print: and ISSN Online: IAEME Publication EFFECT OF PLUG REMOVAL ON LOAD TRANSFER IN PLUGGED OPEN ENDED PILE BEHAVIOR Mohammed Y. Fattah Professor, Building and Construction Engineering Department, University of Technology, Baghdad, Iraq. Nahla M. Salim Assistant Professor, Building and Construction Engineering Department, University of Technology, Baghdad, Iraq. Asaad M.B. Al-Gharrawi Assistant Lecturer, Civil Engineering Department, University of Kufa, Iraq. ABSTRACT The experimental tests contain 36 tests carried out on single piles. All tubular piles tested by using the poorly graded sand from the southern city of Karbala in Iraq. The sand was prepared at three different densities using a raining technique. Different parameters are considered such as method of installation, relative density, removal of soil plug with respect to length of plug and pile length to diameter ratio. To simulate the pile load test in the field, the driving and pressing system for pile installation were manufactured. Strain gauges are used to separate skin friction with two components from and resistance bearing. The soil plug is removed by using a new device which is manufactured to remove the soil column inside open pipe piles group during driven and pressed device. The principle of soil plug removal depends on suction of sand inside pile. It was concluded that the removal of soil plug from piles will cause a reduction in pile load carrying capacity due to elimination of two components, internal friction and the confining in end bearing zone. The maximum reduction occurred in the dense state of soil with as a percentage of about 61.5%. The sand column length increases with the progressing of the installation pressure when pressing method is used, but in driven piles, there is no systematic trend of the sand column length generated in the piles. Long of piles have a higher probability of plugging and plugging penchant is also higher in dense and dilatant soils. Keywords: Pipe pile, open-ended, fully plugged, plug removal, friction, end bearing. Cite this Article: Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi, Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior. International Journal of Civil Engineering and Technology, 7(5), 2016, pp editor@iaeme.com

2 Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi 1. INTRODUCTION Normally, vertical piles are used to carry compression loads from superstructures such as buildings, bridges, etc. The piles are used in groups joined together by pile caps. The loads carried by the piles are transferred to the adjacent soil. If all the loads coming on the tops of piles are transferred to the tips, such piles are called end-bearing or point-bearing piles. However, if all the load is transferred to the soil along the length of the pile such piles are called friction piles. If, in the course of driving a pile into granular soils, the soil around the pile gets compacted, such piles are called compaction piles. Small-displacement piles are either solid (e.g. steel H-piles) or hollow (open-ended tubular piles) with a relatively low cross-sectional area. This type of pile is usually installed by percussion method. However, a soil plug may be formed during driving, particularly with tubular piles, and periodic drilling out may be necessary to reduce the driving resistance. A soil plug can create a greater driving resistance than a closed end, because of damping on the inner-side of the pile. The soil inside an open-ended pile may form a plug during the driving process, preventing new soil from entering at the toe, referred to as soil plugging. Soil plugging changes the driving characteristics of the open-ended pile to that of a closed-ended pile, often accompanied by increased driving resistance. It follows that if the plugging behavior of the pile is not correctly understood, the result is either that unnecessarily powerful and costly hammers are used because of high predicted driving resistance or that the pile plugs (Karlowskis, 2014). The behavior of open-ended piles is more complex, with a response generally intermediate between that of non-displacement and displacement piles. As an open-ended pile is driven into the soil, a soil column (or soil plug) forms inside the pile. The length of this plug may be equal to or less than the pile driving depth. If it is the same, the pile has been driven in a fully coring or unplugged mode throughout. If driving takes place in a partially or fully plugged mode, at least during part of the way, the length of the soil plug within the pile will be less than that of the pile. It may be possible to observe all three driving modes (fully coring, partially plugged or fully plugged) during the driving of a single pile (Paikowski et al., 1989). The main objectives of this work are to offer a better realization regarding the performance of soils and pipe pile group under vertical loading with soil plug, and to provide valuable geotechnical data and parameters necessary for the numerical simulations and foundation design. Paik and Salgado (2003) stated that during the driving of open-ended pipe piles, some amount of soil will initially enter into the hollow pipe. Depending on the soil state (dense or loose) and type (fine-grained or coarse grained), diameter and length of pile, and the driving technique, the soil inside the pile may or may not allow further entry of soil into the pipe. If soil enters the pipe throughout the driving process, driving is said to take place in a fully coring mode and the behavior is more like that of a non-displacement pile. However, if the soil forms a plug at the pile base that does not allow further entry of soil, then driving is said to be done in a fully plugged mode. Shijhait (2013) and Fattah and Al-Soudani (2016a) studiedthe effect of different parameters such as pile diameter to length ratio, type of installation in sand of different densities on the bearing capacity of closed and open-ended piles. Removal of plug was dense in three stages (50%, 75% and 100%) with respect to length of plug. The total number of 84 model pile tests were carried out to assess the effect of soil plug on the model pile capacity. The piles were embedded in sand of different densities with different lengths of piles and types of pile ends. Piles with circular cross section are tested under the effect of vertical static compression load.itwas concluded that the percentage of reduction in pile load capacity for open ended pile increases with increase of the length of removal of the soil plug and increase of sand density and The pile reached a fully plugged state (for which IFR would be equal to zero) for pressed pile in loose and medium sand and partially plugged (IFR = 10%) in dense sand. For driven pile, the IFR is about 30% in loose sand, 20% in medium sand and 30% in dense sand. Fattah and Al-Soudani (2016b) investigated that the changes in the soil plug length and incremental filling ratio IFR with the penetration depth during pile driving and showed that the open-ended piles are partially plugged from the outset of the pile driving. The pile reached a fully plugged state for pressed piles in loose editor@iaeme.com

3 Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior and medium sand and partially plugged (IFR = 10%) in dense sand. For driven piles, the IFR is about 30% in loose sand, 20% in medium sand, and 30% in dense sand. The pile load capacity increases with increases in the length of the plug length ratio PLR.The rate of increase in the value of the pile load capacity with PLR is greater in dense sand than in medium and loose sand. The objective of this study is carrying out experimental laboratory models for closed and open pipe pile, with plugged and unplugged case, for different cases including investigation and auditing of load settlement results, demonstration of internal and external friction components, and isolation of end bearing and total friction to know which of them is most influential on plug formation. 2. EXPERIMENTAL WORK AND MATERIALS The soil used in the research was brought from a site at the center of Karbala city in Iraq. The soil is classified as SP type (Poorly graded clean sand). Standard experiments were performed to determine the physical of the sand. The properties details are listed in Table 1. Laboratory tests carried out on soil used included the grain size distribution, specific gravity, direct shear test, and maximum and minimum dry unit weights. Table 1 Chemical and physical properties of sand. Physical properties Value Specification D 60,(mm) 0.95 D 30,(mm) 0.63 D 10,(mm) 0.33 Gravel (%) 0 ASTM D Sand (%) 95.8 ASTM D Silt and caly (%) 4.2 ASTM D Coefficient of uniformity (Cu) 2.88 Coefficient of curvature (Cc) 1.27 Specific gravity (Gs) 2.65 ASTM D Maximum dry unit weight (kn/m 3 ) 18.8 ASTM D Minimum dry unit weight (kn/m 3 ) 15.1 ASTM D Maximum void ratio (e max. ) 0.73 Minimum void ratio (e min. ) 0.42 Direct shear box experiment was performed according to ASTMD The direct shear box experiment has many particle sizes to boxsize Demands when preparing specimens for experiment. The minimum specimen width should not be less than 10 times the maximum particle size diameter and the minimum initial specimen thickness should not be less than six times the maximum particle diameter. The values of (φ) for loose, medium and dense sands are 31 o, 36 o and 40 o, respectively. The relative densities for loose, medium and dense sand were 20%, 45% and 75%, respectively. 3. MODEL SETUP FORMULATION To simulate the pile load experiment in the area, is useda new apparatus which manufactured by Shighait (2013) and modified in this work. It consists of the following parts: Steel container, steel base, steel loading frame, axial loading system, raining frame, impact hammer device, mechanical jack, load cell, digital weighing indicator, gear box, AC Drive (speed regulator), and pile driving or pressing system installation. The following parts are also included: Soil plugs removal and measurement system editor@iaeme.com

4 Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi Strain gauges. Data logger. Linear variable differential transformer ( LVDT ). Digital dial gauges The steel container has dimensions of 750 mm in length, 750 mm in width, and 750 m in height. It is made of five separated parts, one for the base and the others for the four sides. Each container part was made of 4 mm thick steel plate. At the internal sides of the container, a steel bar was welded along three sides with 1 cm 2 cross sectional area and the front side was kept free. These shafts are welded each (100 mm) from the container bottom. Steel plate (740mm 740 mm) with 8 mm thickness movable plate at any specific height was designed; it is inserted inside the container and put on the welded bar and rested on it. This arrangement designed allows changing the height of soil bed that is being used. A steel base was designed for this study to support the container and the loading frame weight. The box is rested on two channels with the ability of lateral movement. A steel loading frame was manufactured to support the gear box motor, axial loading system and mechanical jack, as shown in Figure 1. The load is submitted through a mechanical jack joint by a gear box motor and AC drive (speed regulator), which in turn controls the speed of the gear box motor as shown in Figure 2. The maximum load that can be submitted is1 ton. The loading rate is kept constant at 1 mm/min as recommended by ASTM D The model has become after development possible to move in horizontal and vertical directionsto a pile group, while the original model was move only in one direction which is perpendicular to the cross section of piles, and so one can make sure that the load is in the middle of the pile system and there is no evidence of any eccentricity. Figure 1 Steel loading frame and axial loading system editor@iaeme.com

5 Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior 4. SAND RAINING FRAME The raining frame consists of two columns with changeable height. It was made to achieve any desired elevation. The frame height change is done by holes with equidistance steps (15 cm). It is joined from top and bottom to the column with two to 4joints beams together and these beams are bolted at their ends. To support the U-section beams, the two beams in the longitudinal direction have (U-section) and the other beams are used. Another beam was designed as a roller, which are rests on the longitudinal beams to move along these beams. This (rolled-beam) is jointed from the bottom with another beam, it is supported with screw and it can be moved horizontally along with the beam; this beam was designed to carry the cone that is used to pour the sand. The raining frame is illustrated in Figure 2. This raining frame configuration helps get a uniform density by controlling the height of fall. The rolled-beam and the screw which is jointed with the cone to ensure each particle drop at equal height and uniform intensity. A mesh piece (the aperture diameter is 10 mm) is put inside the cone to reduce the effect of the particles (Shighait 2013). To control the fall height, additional tubular elongations at the cone end were usedby using adapted tubes. Figure 2 Raining frame used for controlling sand density. 5. PILE DRIVING PRESSING SYSTEM The piles setup system consists of a base plate with dimensions of (850 mm 200 mm) and 4 mm in thickness. This plate includes nine holes (31 mm) in diameter and the spacing between holes is (91 mm), these holes are considered as focus place for the piles to penetrate the soil in the box. To support the two beams that designed from stainless steel the two tube columns with (28 mm) diameter are fixed vertically. The main part in the driving hammer is the aluminum rod, it contains steel helmet in the rod head and steel cylinder, which is used as a base for dropping the hammer weight. The steel helmet was designed with various holes that are suitable for all model pile sizes used in the experiments. These grooves are designed to ensure the piles are as fixed as possible to reserve the vertical direction for penetration of pile without tilting through the driving process, and these parts are shown in Figure 3. Mechanical jack is used for pressing pile into the soil at a constant rate. This jack is fixed to the pile during installation system, and these parts are shown in Figure editor@iaeme.com

6 Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi The hammer masses used in the experiments are circular in shape and formed from steel material. They have holes in the center to enable lifting and lowering along the hammer rod. The hammer mass which is lifted to a specified height (220 mm) by means of half- plastic plate with handle is fixed to small steel. Figure 3 Pile group system installation. 6. SOIL PLUGS REMOVAL AND MEASUREMENT SYSTEM This study is based on the system of cleaning pile raise plug. The mechanism is based on pulling out of sand from inside the pile after the completion of the pressure or driven piles. This system consists of motor with specifications following input operating vacuum (675mm Hg) and output-air flow (26 LPM ) and voltage v, AC 50 Hz connected to the flask to collect sand banter from inside the pile and then measuring the sand column height inside the pile, or a pile group. The instrument is relying on the withdrawal of sand and measurement system rod which consists of aluminum diameter 1.5 mm with different lengths based on the lengths of the pile that has been relegated in the sand as it was the work of signals every 10 mm to measure the length of the sand column after the clouds to find out the sand size that has been removed from inside the pile. 7. STRAIN GAUGE TECHNIQUE When a material is stretched (or compressed), corresponding inside stress generates by use the force. This stress in turn generates a proportional tensile strain (or compressive strain) that deforms the a trial by L+ L (or L- L), where L is the original material length and L is the length change. When this happens, the ratio of L to L is called strain. There are a many ways of measuring strain mechanically and electrically, but the vast majority of stress measurement is carried out using strain gauges because of their superior measurement features. 8. PIPE PILE PROPERTIES Regarding the physical properties of the adopted piles' sections, piles were experimented as per ASTM E8 with the collaboration of the Central Organization for Standardization and Quality Control. Four aluminum pipe pile samples have been taken randomly and experimented then the average value has been reported for each property as shown in Table editor@iaeme.com

7 Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior Experiment Table 2 Aluminum pipe pile properties. Sample B1 Sample B2 Results Sample B3 Sample B4 Tensile strength (N/mm 2 ) Yeild strength (N/mm 2 ) Elongation (mm) Modulus of elasticity (N/mm 2 ) Outer diameter ( mm) Thickness (mm) Two types of piles have been used in this study, these were: open ended and closed ended tube piles. All the piles in the experimental work were used with 32.8 mm diameter and 1.44 mm thickness. The embedment length of the model piles,which is considered in the experimental programs of the experiments, based onthe ratio of embedment length to pile diameter, (L/d) ratio. The pile type, outside diameter, and length of each pile size are shown in Table 3. Table 3 Model pile types and dimensions used in the experiment Pile designation Pile type Soil plug situation DOF Open - ended Full Plug DOU Open - ended Unplugged Diameter d (mm) Length ( mm ) L/d=12 L/d= SETTING UP OF MODEL PRESSED AND DRIVEN PILES Firstly, the bed of sand is prepared with controlled density as mentioned before. 10. SETTING UP OF MODEL DRIVEN PILES Strain gauge is sticking on the piles and then driving hammer is fixed to the box to penetrate the model piles to the desired length. The weight that is used to drive the model piles is determined approximately. The weight calculation takes into consideration many factors that affect the pile capacity. The model piles are vertically installed in specific hole that is made in the hammer plate and the rod of hammer is lowered to the model piles until the pile helmet is connected with the model pile. After the model pile head enters inside the helmet, the process of driving begins with dropping a specific weight from a specified height, and the blows number results are recorded each (25 mm) of model pile length until reaching the final desired length of penetration. The ram weight used to drive the model piles equals (1.9 kg). This weight was chosen to determine the best driving energy based on the weight ratio of pile to hammer (P/W) where P is the pile weight and W is the hammer weight. 11. SETTING UP OF MODEL PRESSED PILES In the installation frame the mechanical jack is used and fixed on the box to press the model pile to penetrate the desired length editor@iaeme.com

8 Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi 12. RESULTS AND DISCUSSION The effect of removing the soil plug column which is formed during driving or pressing methods of installation is studied. The models include single and groups of pipe piles with different sand densities and length of piles. The effect of soil plug removal can be studied through testing a certain open ended pile configuration with different parameters such as, sand density, length to diameter ratio and two different methods of installation (driving and pressing ). The process of removal of soil plug, which is developed from driving or pressing of the pile, was done with several stages. In each stage, the amount of the elevated sand was measured to avoid elevating amount of the sand which is higher than the plug value that has been measured before removing process. Figures 4 and 5 show the effect of soil plugged removal on the pile load capacity with different sand densities, i.e. loose, medium and dense. The relationship between loading magnitude and the settlement for full plug and 100% plug removal has been drawn. All results are presented as a relationship between the applied load and the average of two dial gauge readings. From these figures, it can be noted that, the bearing capacity of the pile after removing the plug soil decreased with different values depending on the type of the soil, L/D ratio and pile installation. As shown in Figure (4 a, b and c), for driven pile in loose, medium and dense sand, the bearing resistance is reduced by 58.3%, 51.2% and 40.8% respectively for L/D =15 and 51%, 52% and 40.5% respectively for L/D =12 as shown Figure (4 d, e and f). It can be noted that the reduction in bearing resistance in the case of dense soil is smaller than that of loose and medium states. This may be due to the large contribution of the external friction resistance and low disturbance of bearing soil due to soil plug removal. The base and friction resistance have been separated by computing the friction resistance along the external pile surface using strain gauges. Figure 5 shows the bearing and friction resistance components of single pile. It can be seen that the pile with full plug has capacity higher than that of 100% soil plug removal. This is due to the presence of internal friction in fully plugged case, while the bearing resistance at 100% plug removal has been reduced till it reaches to zero value. The losses of the base resistance is due to the losing of plug weight and disturbance of soil below the pile wall. Figure 5 shows the bearing capacity for the pile after changing its length from 450 mm to 360 mm (L/D = 12 and L/D=15). It can be observed that the behavior is rarely the same for both cases of fully plugged and soil plug removal with taking into consideration the difference in bearing capacity and friction values for piles as shown in Table 4. Table 4 shows the complete details of the bearing capacity for single pile with both components of internal and external friction, bearing values when the pile was placed in the soil and after soil plug removal progress and for two cases of execution (i.e. driven and pressing) with different values of L/D and three different values of soil density. From the table mentioned above, it can be noted that the decrease in the bearing capacity of pile as a result of soil plug removal which leads to that the value of IFR value decreases up to zero in addition to a reason related to the distortion of the zone surrounding the end bearing and an obvious loss in the value of this component. To illustrate the contribution ratio for each of external and internal components and end bearing to the total bearing capacity of the pile, Table 5 was prepared to show the ratio of participation for end bearing which ranged between 54.7% to 78.3% for fully plugged cases, while in case of soil plug removal, the ratio decreased to the range between 14.4% to 28.7%. Table 5 illustrates the contribution ratios for components of internal and external friction and the end resistance to the total bearing capacity of pipe piles tested for various cases of densities of the soil, length of piles and the method of installation for both cases aforementioned and for fully plugged and 100% soil plug removal. From the table, it can be noticed that there is apparent decrease in the components of friction and end bearing in dense soil is which up to zero in both cases of loose and medium soil editor@iaeme.com

9 Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior Figure 4 Load-settlement curve for driven single pipe pile with unplugged and fully plugged conditions under static load test. (a) loose, L/D=15, (b) medium, L/D=15,(c) dense, L/D=15, (d) loose, L/D=12, (e) medium, L/D=12, (f) dense, L/D= editor@iaeme.com

10 Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi Figure 5 Load-friction and load- bearing curves for driven single pipe pile with unplugged and fully plugged conditions under static load test. (a) loose, L/D=15, (b) medium, L/D=15, (c) dense, L/D= editor@iaeme.com

11 Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior Table 4 Pile bearing capacity according to tangent method with the values of external friction, internal friction and end bearing for single pile editor@iaeme.com

12 Mohammed Y. Fattah, Nahla M. Salim and Asaad M.B. Al-Gharrawi Table 5 Ratios of sharing for pile external friction, internal friction and end resistance to ultimate carrying single pipe pile capacity. 13. CONCLUSIONS 1. The behavior of open-ended piles with fully plugged is same as the closed-ended piles. Furthermore, the pile capacity of closed-ended piles is larger than that of open-ended piles in both loose and medium states of soil by about 10%, while in dense sand state of soil the capacity of the open ended pipe piles with fully plugged is larger than that closed-ended piles by 42% and 50% for length to diameter 12% and 15% respectively. 2. The removing of soil plug from piles will cause a reduction in pile load carrying capacity due to elimination of two components, internal friction and the confining in end bearing zone editor@iaeme.com

13 Effect of Plug Removal on Load Transfer in Plugged Open Ended Pile Behavior 3. The maximum reduction occurred in the dense state of soil with as a percentage of about 61.5%. 4. Based on the method of pile installation, it can be concluded that using of pressing method will yield a high pile capacity for loose and medium states of soil while the driving method of installation gives a higher capacity for dense state. 5. The sand column length increases with the progressing of the installation pressure when pressing method is used, but in driven piles, there is no systematic trend of the sand column length generated in the piles. 6. Long of piles have a higher probability of plugging and plugging penchant is also higher in dense and dilatant soils. REFERENCES [1] ASTM D3080, (1998), Standard Test Method for Direct Shear Test of Soils under Consolidated Drained Conditions, American Society for Testing and Materials. [2] ASTM D422, (2000), Standard Test Method for Particle-Size Analysis of Soils, American Society of Testing and Materials. [3] ASTM D854, (2005), Standard Test Method for Specific Gravity of Soil Solids by Water Pycnometer, American Society for Testing and Materials. [4] ASTM D4253, (2007), Standard Test Method for Maximum Index Density and Unit Weight of Soils Using a Vibratory Table, American Society for Testing and Materials. [5] ASTM D4254, (2007), Standard Test Method for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density, American Society for Testing and Materials. [6] ASTM D1143, D1143M, 07, (2013), Standard Test Method for Piles under Static Axial Compressive Load, American Society of Testing and Materials. [7] Fattah, M. Y., Al-Soudani, W. H. S., (2016), "Bearing Capacity of Open-Ended Pipe Piles with Restricted Soil Plug", Ships and Offshore Structures, Vol. 11, 5, pp , Taylor & Francis, DOI: / , London W1T 3JH, UK. [8] Fattah, M. Y., Al-Soudani, W. H. S., (2016), " Bearing Capacity of Closed and Open Ended Pipe Piles Installed in Loose Sand with Emphasis on Soil Plug", Indian Journal of Geo-Marine Sciences Vol. 45(5), May 2016, pp [9] Karlowskis, V., (2014), Soil Plugging of Open-Ended Piles During Impact Driving in Cohesionless Soil, Master of Science Thesis, Royal Institute of Technology (KTH), Department of Civil and Architectural Engineering, Stockholm, Sweden [10] Paikowski, S.G., Whitman, R.V. and Baligh, M.M. (1989), A New Look at the Phenomenon of Offshore Pile Plugging, Marine Geotechnical, Vol. 8, pp [11] Paik, K. and Salgado, R. (2003), "Determination of bearing capacity of open-ended piles in sand, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 129, No. 1, pp [12] Shighait, W. H., (2013), Effect of Plugging on the Load Carrying Capacity of Closed and Open Ended Pipe Piles in Sands, M.Sc. Thesis, University of Baghdad, Iraq. [13] Kavitha P. E., Dr. Beena K. S. and Dr. Narayanan K. P., Analytical Study on Soil-Pile Interaction Effect in the Variation of Natural Frequency of a Single Pile. International Journal of Civil Engineering and Technology, 5(12), 2014, pp [14] Mohammed M. Salman And Prof. Dr. Abdulaziz Abdulrassol Aziz, The Effect of Improvement Surrounding Soil on Bored Pile Friction Capacity. International Journal of Civil Engineering and Technology, 7(1), 2016, pp editor@iaeme.com