EXPERIMENTAL STUDY ON PULL-OUT CAPACITY OF HELICAL PILE IN CLAYEY SOIL

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 4, April 217, pp Article ID: IJCIET_8_4_17 Available online at aeme.com/ijciet/issues.asp?jtype=ijciet&vtyp pe=8&itype=4 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EXPERIMENTAL STUDY ON PULL-OUT CAPACITY OF HELICAL PILE IN CLAYEY SOIL M.Ashni PG Student, Department of Civil Engineering, SRM University, Kattankulathur V.Janani Asst. Professor, Department of Civil Engineering, Faculty of Engineering and Technology, SRM University, Kattankulathur ABSTRACT Helical piles are used as anchors to resist uplift, compressive forces and overturning moments in various civil engineering structures. Laboratory model tests were done on helical piles by varying the number of helixes (1, 2, and 3) and relative helical spacing ratio (.67, 1.33) in medium stiff consistency clay. The vertical and oblique pulls were applied at an angle of and 15 degree and the uplift capacity of each helical pile was determined. The pull-out capacity of the helical piles increases with increase in the number of helixes, spacing between the helixes, embedment depth as well as the oblique while compared to that of oblique pull. Key words: Clay, Helical Piles, Pull-Out Test, Embedment Depth, Spacing. Cite this Article: M.Ashni and V.Janani, Experimental Study on Pull-out Capacity of Helical Pile in Clayey, International Journal of Civil Engineering and Technology, 8(4), 217, pp Type=4 1. INTRODUCTION There are many structures which are subjected to uplift, tensile and compressive force due to wind, wave or undergroundd water. The structures subjected to these forces include transmission towers, pipeline anchors, radar towers, excavation bracings, suspension bridges, offshore structures, etc. Different types of anchors have been used to resist these forces like belled, under-reamed, helical, plate etc. In this investigation helical pile is used as anchor. The major benefits of using helical piles are its rapid installation, minimal disturbance to the surrounding, eco-friendly, alll weather installation, immediate load carrying capacity, no curing period, lower cost and easily removed when not in use. The helical piles were made up of steel or concrete shaft with the helixes welded to it with the required pitch distance. It can

2 Experimental Study on Pull-out Capacity of Helical Pile in Clayey be single helical pile or multi helical pile depending on the number of helixes attached to the shaft. The multi-helical piles were of two designs, one in which the diameter of helix increases moving up the shaft and the other with constant diameter placed at uniform spacing. The helical pile behaviour depends on the embedment depth and the spacing between the helixes from which they are classified shallow and deep anchors and cylindrical and individual shear failure respectively. Many researches have been carried out on piles subjected to pull-out in cohesion-less soil of varying densities. Illamparuthi et al., (28) investigated the pull-out capacity of plate anchor in unreinforced and reinforced sand and found that the capacity of pile increases as the embedment ratio of plate anchor and density of soil increases. The results were found higher for reinforced sand with geo-grid. Ravichandran (28) conducted uplift loading test on strip anchor in sand and reinforced sand bed. The peak pull-out resistance linearly increases with width ratio of reinforcement irrespective of depth of embedment and density of soil. Verma (21) found the uplift capacity of enlarged base pile in cohesion-less soil and examined that as the diameter of base increases the resistance to uplift increases. Researches have been carried out on helical piles by varying the spacing ratios and embedment depth in cohesion-less soil under axial pull and piles subjected to oblique pull. Mittal and Muherjee (213) examined the uplift capacity of group of helical screw anchors in sand and found that the ultimate pull-out capacity increases drastically as the no. of anchors, blades and embedment depth increases. Das and Chin (1993), Krishna and Patra (26), Rao and Nasr (21) conducted model study on oblique pull-out capacity of rigid pile and found that the uplift capacity of pile increases as the angle of pull to vertical axis increases. Bhardwaj and Singh (213) and Ismael (1989) conducted oblique pull-out test on micro-pile and field test bored piles and found that the pull-out capacity of the piles decreases continuously as the angle with the vertical axis increases. Limited research had been made of helical pile with multi-helix in clayey soil NarasimhaRao et al., (1993) conducted the experiments on screw piles by varying the embedment depth and found the transition from shallow to deep anchor for H/D between 2 and 4. Lutenegger (29) conducted field test on multi-helix screw anchor and found that the individual plate bearing occurs when the spacing ratiois more than 3.Due to the forces acting on the structure, the foundation is subjected to uplift on vertical with oblique. Hence in this study an attempt was made to determine the uplift capacity of anchor piles on clayey soil by varying the number of helixes, spacing between the helixes, and embedment depth in vertical and oblique pull of 15º. 2. MATERIALS AND METHODOLOGY 2.1. Properties of Soil The soil sample was collected from Kattankulathur located in Kancheepuram district. Laboratory tests were done on the soil sample collected as per IS:272 and the soil was classified as Highly compressible Clay (CH) based on the results obtained. The properties of soil used for the test are given in Table1. Table 1 Properties of soil used in study Property Value Liquid Limit 52.4 (%) Plastic Limit 23.5 (%) Plasticity Index 28.9 (%) Bulk density (kn/m 3 ) editor@iaeme.com

3 M.Ashni and V.Janani 2.2. Test Tank and Model Helical Piles The test tank used for the study was made of iron with dimension of 75mm 75mm 1mm. The tank was designed considering the failure zone and skin friction mobilization. At the pile tip the failure zone extends to a radial distance of about 2.5 times the diameter of the helical plate and total width of failure plane is about three to four times the diameter of the pile (Bowels, 1982).The schematic diagram for experimental set-up for pull-out tests are shown in Figure 1. A steel wire rope was used which was connected on one side to the top of the pile and its other side was connected to the load hanger passing through two pulleys. The displacement of pile was measured using two deflectometers of.1mmm sensitivity. Four different helical piles were used for this study with the same geometrical properties of plates. The piles were made of solid mild steel rod and the helixes were welded to it at the required position. The other end of the pile was threaded so that it facilitates the ease of holding the steel wire rope. Provision had been made for placing the dial gauge. The parameters like the diameter of the shaft (15mm), the diameter of the helix (75mm), the thickness of the helix (2mm), the pitch of each helixes (15mm), distance from the pile bottom to the bottom helix (5mm) and the total length of pile (7mm) were kept constant throughout for all the piles. The numbers of helixes weree varied as one, two and three and the spacing between the helixes in triple helical pile were varied as 5mm and 1mm. The helical pile details are given in Table 2. Figure 1. Experimental set-up for pull-out test Table 2 Detailed specifications of helical piles Pile type No. of helixes Spacing b/w helixes (mm) Distance b/w top and bottom helix (mm) Distance between pile top and top helix (mm) Distance b/w pile bottom and bottom helix (mm) Total length of pile (mm) P1 1 - P2 2 1 P3 3 1 P editor@iaeme.com

4 Experimental Study on Pull-out Capacity of Helical Pile in Clayey 3. RESULTS AND DISCUSSION The helical pile was centrally placed in the tank and the clay was filled after mixing it up to the required consistency (I C =.67). Samples were checked for full saturation and homogeneity. The displacements were measured using two dial gauges of.1mm sensitivity. The loads were applied in equal increment in the load hanger. The next increment of load was applied when the rate of displacements becomes.2mm per hour. Loads were applied until the pile comes out the soil bed and it was considered as the ultimate pull-out capacity of the piles. Total of 8 tests were done and the effect of spacing, embedment depth and number of helixes were studied. The consistency of the soil was maintained for all the tests. The angle of pull was varied from axial to oblique pull of 15º and the capacity of piles were found and compared Effect of number of helixes The number of helical plates attached to the shaft is varied between 1, 2 and 3 and the ultimate capacity was determined. For the piles P1, P2 and P3 the load-displacement curves are drawn as shown in Figure 2 for the embedment depth of The piles were compared for varying no. of helixes with same spacing and same embedment depth. The pull-out capacity of pile was found to increase as the number of helixes increases. This increase in capacity was due to the resistance offered by the helixes for the pull. The ultimate capacities obtained in experimental study are summarized in Table 3. Table 3 Variation of pull-out capacities by varying number of helixes under axial pull Pile type No. of helixes Consistency Index, I c Ultimate pull-out Capacity, (kg) P P P P1 P2 P Figure 2 Comparison on pull-out capacity of piles with varying number. of helixes editor@iaeme.com

5 M.Ashni and V.Janani P3 P Figure 3 Comparison on pull-out capacity of piles by varying the spacing 3.2. Effect of spacing between the helixes Spacing is an important parameter for the helical piles. The spacing ratio is the ratio of the distance between the helixes to the diameter of helix. The spacing ratio between the helixes was varied as.67 and Here the pile P3 had spacing ratio of 1.33 and the pile P4 has spacing ratio of.67 with 3 helixes. Table 4 summarizes the experimental results of piles P3 and P4 under axial pull. As the spacing between the plates increased the pile capacity also got increased. For piles with more spacing more clay occupies the spacing between the helixes and due to the cohesive nature of clay resistance towards pull was increased. The load displacement curves for piles P3 and P4 of varying spacing is shown in Figure 3 Table 4 Variation of pull-out capacities by varying the spacing under axial pull Pile type No. of helixes Spacing Ratio Consistency index, I c Ultimate pull-out Capacity (kg) P P Effect of embedment depth Embedment depth is the distance from the ground surface to the top helix. The embedment depth ratio (H/D) is the ratio of embedment depth to the diameter of the helix. For the pile P1 the embedment depth is varied as 2.67 and 4 and the effect was found. Table 5 shows the experimental results obtained for pile P1. The pull-out capacity of pile was found to increase as the embedment depth increases. This is due to the additional surcharge which acts over the pile for higher embedment depths. Figure 4 shows the load-displacement curves of single helical pile of two different embedment ratios. Table 5 Variation of pull-out capacities by varying embedment depth ratio under axial pull Pile type H/D Consistency Index, I C Ultimate Pull-out Capacity, (kg) P P editor@iaeme.com

6 Experimental Study on Pull-out Capacity of Helical Pile in Clayey H/D=2.67 H/D= Figure 4 Comparison on pull-out capacity of piles by varying the embedment depth ratio P1 P2 P Figure 5 Load-displacement characteristics of helical piles under oblique pull 3.4. Effect of angle of pull-out For the pull angle of 15 degree one of the pulleys was fixed in a required position and then the load is applied. Both the horizontal and the vertical displacement are noted using the dial gauges. The piles P1, P2, P4 were tested for oblique pull and the results are summarized in Table 6. Initially the horizontal displacement was found more and finally the vertical displacement increased. Das and Chin (1993) used the following formula (1)to calculate the final displacement for oblique pull-out from horizontal and vertical displacements values obtained in dial gauges Displacement, = Δh +Δv (1) In the above equation Δhis the horizontal displacement and Δv is the vertical displacement. Figure 5 shows the load displacement behaviour of piles subjected to oblique pull of 15 degree. The pile P4 even though it was with more number of helixes compared to pile P2 the capacity was less because of its lesser spacing between the helixes. Table 6 Variation of pull-out capacity of helical pile under oblique pull of 15º Pile type No. of helixes Spacing Ratio Consistency index, I C Ultimate Pull-out Capacity (kg) P P P editor@iaeme.com

7 M.Ashni and V.Janani As the angle of pull of helical piles increases the uplift capacity of piles increases accordingly. Figure 6 shows the comparison of pile P2 subjected to axial and oblique pull. From this it s found that the pull-out capacity of helical piles was more under oblique pull compared to axial pull. This shows that the helical piles can take more loads under oblique pull Figure 6 Comparison of pull-out capacity of pile P2 under axial and oblique pull 4. CONCLUSION For the helical piles the number of helixes, spacing between the helixes, embedment depth are the most important parameters to be considered. From the model study conducted on medium-stiff consistency clay, following inferences are made. 1. As the number of helixes increases, the pull-out capacity of the pile also increases. The percentage increase in pull-out capacity for one helix to 3 helixes was found as 128%. 2. The ultimate capacity of piles increases as the embedment depth and spacing between the piles increases. The percentage increase in pull-out capacity for spacing of 1.33 compared to.67 was 71% and for embedment depth ratio of 4 compared to 2.67 was 25%. 3. From the oblique pull-out test it was found that the helical piles subjecting to oblique pull the pile capacity is more compared to axial pull. REFERENCES [1] IlamparuthiK, Ravichandran P.T and Mohammed ToufeeqM (28), Study on uplift behaviour of plate anchor in geogrid reinforced sand bed, Geotechnical earthquake engineering and soil dynamics (IV), 1-1 [2] Ravichandran P.T and Illamparuthi K (28), Uplift behavior of strip anchor in sand and reinforced sand bed, Indian Geotechnical Journal, 38(2), [3] Verma A.K and Joshi Ronak K (21), Uplift load carrying capacity of piles in sand Indian Geo-technical Conference, [4] Satyendra Mittal and Sanjeev Mukherjee (213), Vertical uplift capacity of a group of helical screw anchors in sand, Indian Geotechnical Journal, 43(3): pp [5] Das B.M. and Shin E.C (1993), Uplift Capacity of Rigid Vertical Metal Pile in Clay under Inclined pull, International Journal Offshore Polar Engineering, 3, (3); pp [6] Krishna P. and Patra N.R (26), Effect of Compressive load on Oblique Pull-Out Capacity of Model Piles in Sand, Geotechnical Geology Engineering, 24, (3); pp editor@iaeme.com

8 Experimental Study on Pull-out Capacity of Helical Pile in Clayey [7] Rao S.V.K. and Nasr A.M. (21), Behaviour of Vertical Piles Embedded in Reinforced sand Under Oblique Pullout Loads, International Journal Geotechnical Engineering.,4,(2); pp [8] Bhardwaj S and Singh S.K. (213), Pullout Capacity of Model Micropiles under oblique loads, Procedings of Indian Geotechnical Conference, pp. 13, IIT Roorkee [9] Ismael, N.F. (1989), Field Test on Bored Piles Subject to Axial and Oblique Pull, Journal Geotechnical. Engineering Division ASCE, 12, (9); pp [1] Rao S, Prasad Y and Veeresh C (1993), Behaviour of embedded model screw anchors in soft clays. Geotechnique 43(4): pp [11] Lutenegger A.J (29), Cylindrical Shear or Plate Bearing?-Uplift Behavior of Multi- Helix Screw Anchors in Clay, Contemporary Issues in Deep Foundations Institute, Vol.5, No.1 pp [12] IS: 272 (Part , ), "Methods of tests for soil Liquid Limit and Plastic Limit, Bureau of Indian Standards, New Delhi. [13] IS: 272 (Part 7) 198, "Methods of tests for soil - Determination of compaction characteristics, Bureau of Indian Standards, New Delhi. [14] J.E.Bowels (1982), Foundation Analysis and Design McGraw Hill, Auckland, Third edition editor@iaeme.com