50 th 2D MODEL STUDY ON SLIP SURFACE AROUND BELLED ANCHOR PILE UNDER UPLIFT IN COHESIONLESS SOIL T. Deb 1, S. K. Pal 2 ABSTRACT The belled anchor pile has proficient application in the field of geotechnical engineering as because its enlarged base can bear more compressive loads and large volume of breakout soil within slip surface move upward under tension. The bell anchor pile can be used in case of on-land and off-shore structures where there is a threat to overturning of structures due to external tension loads. The previous researchers reported qualitative and quantitative estimation of uplift resistance of belled pile for both small scale laboratory model and full-scale field models. But 2D model studies only can interpret the variation in response of 3D models. Semi-cylindrical 2D model studies are conducted in present investigation to visualize the pattern of slip surface. Herein the 2D model studies are conducted for the ratio between thickness of shaft to bell thickness of (T s /T b ) = 0.28, 0.32, 0.38 and 0.46 and all the models are tested for embedment depth ratio of 3, 4 and 5 having constant bell angle 45. Total 12 numbers of tests are conducted in dry sand bed of density 15.6 kn/m 3. The foundation bed is prepared with dyed sand maintaining 3 mm thickness of dyed sand and 18 mm dyed sand, and placed alternatively. A proving ring and dial gauges are attached with the models and a close contact of the models are kept with the side of perpex sheeted tank. Images are captured for soil deformation in case of all model tests. A void is formed underneath the base of pile and the slip surface is detected when the soil around the base collapsed to fill the voids. Here it is also observed that failure is indicated by the proving ring prior to the collapse of soil. The objective of the present paper is to investigate the nature of slip surface inside the soil and to evaluate the results on the basis of parametric study conducted in the experiments. The slip surface is indicated marking those points where nonlinearity or turning points are found in dyed layer as they are generating during uplifting. For all the bell pile it is seen that the failure initiates from the edge of belled portion of pile and move upward following a non linear path upto the ground level with a formation of heave irrespective of pile size and shape The failure points are plotted and the slope of nonlinearity is found out. It is observed that upto certain portion from the point of origin, the failure surface is close to the pile (i.e., slope of failure surface is less) and thereafter failure surface moves further away from the pile (i.e., slope of failure surface is more). The geometric configuration of belled anchor pile along with embedment 1 Deb_Tanaya1, Civil Engg. Deptt., Research scholar, NIT Agartala, Agartala, India, debtanaya88@gmail.com 2 Pal_Sujit Kumar2, Civil Engg. Deptt, Associate Professor, NITAgartala, Agartala, India, skpal1963@gmail.com
Tanaya Deb & Sujit Kumar Pal depth ratio and soil deformation around the pile should be taken care to design uplift resistant belled anchor pile. Keywords: Belled anchor pile, 2D model, embedment depth ratio, slip surface, failure surface, uplift resistance.
50 th 2D MODEL STUDY ON SLIP SURFACE AROUND BELLED ANCHOR PILE UNDER UPLIFT IN COHESIONLESS SOIL Tanaya Deb, Research scholar, Civil Engg. Deptt., NIT Agartala, E-mail: debtanaya88@gmail.com Dr. Sujit Kumar Pal, Associate Professor, Civil Engg. Deptt., NIT Agartala, E-mail: skpal1963@gmail.com ABSTRACT: Experimental studies are conducted on semi-cylindrical 2D or panel models of bell anchor and the models possess the thickness ratio (T b /T s ) = 0.28, 0.32, 0.38 and 0.46 and all these are installed to attain embedment ratio of 3, 4 and 5. The models have constant bell angle and shaft thickness 45 and 26 mm respectively. Experiments are conducted on total 12 numbers of models in dry sand bed of density 15.6 kn/m 3. The foundation bed is prepared with dyed and non-dyed sand and placed in alternate layers to attain predetermined depth to investigate the generated slip surface inside the soil due to uplift on the basis of parametric study. The failure surfaces are fairly distinct, similar in nature but changed in shape due to embedment depth. For all the bell pile it is seen that the failure initiates from the edge of bell making angle within the range ϕ/2-1 to ϕ/2+1 with vertical and in a non-linear path raise upto the ground level with a formation of heave irrespective of pile size and shape and where the final angle makes within the range of ϕ+21 to ϕ+29. INTRODUCTION The design of structures like transmission line towers, radio tower, television tower, radar tower have incorporate wind load higher than their self weight. And high ambitious structures like silos, overhead water tank and tall chimneys are not only concerned about transmission of large compressive load in sub-soil profile but uplift loads too. More over water front structures and off-shore structures in hostile environment are subjected to wave action, wind gust and thrust from floating vessels result in tension in foundation. In such cases the belled pile may be an acceptable option for geotechnical engineers; because its enlarged base can offer more load bearing capacity and in contrast higher load carrying capacity during uplift; as large volume of breakout soil within slip surface tends to move upward under tension and offers passive resistance. Till date the available uplift capacity theories are established based on the assumed slip surface around the pile. But there is paucity of available literatures exploring influence of pile characteristics and soil characteristics on the pattern of slip surface formed during uplifting through laboratory model test. The failure pattern in belled pile is reported by [6] and [20] in cohesionless soil by pictorial evidence. Therefore the present study aims to investigation of failure pattern, shape and mechanism around 2D or panel model of belled anchor pile in cohesionless soil. [14] presented a vertical slip surface model whereas, [18] introduced soil-cone method, where the soil wedge was in the shape of truncated cone and a function of ϕ. [22] was in argument that soilcone method to be conservative for shallow anchor, but quite opposite for deep anchor. [1] Said that the slip surface followed the trend of non-linear tangential curve and approximated to circular arc. [8] And [19] presented inverted cone model and the failure angle was equal to ϕ with vertical. [4] for dynamic pullout resistance of anchors and [2] in case of helical anchors estimated uplift capacity surface to be an inverted cone model whereas the failure angle was equal to ϕ/2 with vertical. [17] Said that slip surface was pyramidal in shape and varies from ϕ/2 to ϕ/4. [3] Proposed a mathematical model for the estimation ultimate vertical resistance of horizontal anchor plates in sand for shallow and deep anchors by assuming an exponential equation for the slip surface by
Tanaya Deb & Sujit Kumar Pal satisfying the boundary conditions considered by [1]. [10] Studied on half cut flat and deep curved circular models and the failure initiated at an angle of ϕ/2 ±2. In case of shallow strip anchor having aspect ratio within the range of 1 to 8, [7] through PLAXIS finite element analysis shown the soil displacement contours intended to raise upto soil surface. Failure surface mechanism was studied by [6] and [20] by using semi-cylindrical modeling. [13] Suggested that for foundation of transmission line tower and pole under uplift, breakout soil cone as frustum of cone make maximum angle 30 with vertical and recommended it to be 20 in cohesionless soil for design purpose. [9] for screw anchor segregated the failure mechanism, into (a) shallow anchors, (b) deep anchors and (c) transit anchors based on the lateral extent of heave observed at the surface of the sand bed as the observations made through a glass plate. MECHANISM OF FORMATION OF SLIP SURFACE Uplift load occurs different stress changes in different location of soil. During the primary stage of uplift loading there is relief of total stress below the anchor and simultaneously just above the anchor there is increase of total stress. At that time there is gradual formation of cavity or gap below the anchor and upward movement of soil are observed. The cavity or gap below the anchor is continued to expand upto the plastic stage. At the plastic stage, shear strength is completely mobilized; bulging of soil occurs, tends to spread laterally taking definite shape and tries to move upward. At the plastic stage, tension cracks are formed in soil around the base and a plastic flow move downward to fill the voids. But there is no alternation of break out soil wedge once formed and hence the domain next to break out soil wedge is in rest position. OBJECTIVES The literature review reveals that clear understanding between failure mechanism studies and uplift resistance is often lacking in dry sand. So, it is expected that the present investigation on trend and behavior study of slip surface would offer excellent opportunity to reflect mechanical behavior in uplift resistance and simultaneously understanding the performance of large scale model. Fig. 1 Schematic diagram of 2D model anchor pile with generated failure surface TEST PROGRAMME A panel belled pile model may be characterized for its bell angle, bell width (T b ), shaft width (T s ), width ratio (T s /T b ) and embedment ratio (L/T b ). In the present investigation the constant and variable parameters are clearly distinguished as under: Constant Parameters Tank size;. Sand type and density; Installation method and loading arrangement; Shaft thickness (mm); Bell angle ( ); Variable Parameters width ratio : T s /T b = 0.28, 0.32, 0.38 and 0.46. embedment ratio: L/ T b = 3, 4, 5 for each width ratio The above parameters are used as variables in present study. Total 12 no of tests are conducted. MATERIALS The sand used in present study is collected from local market. The air dried soil is used in all experiments and is predominantly medium grained. The sand is used in the present study because it is easy to control the test condition and the density within the testing tank. Fig. 2 shows grain size distribution curve of sand sample. The sand used in present study maintaining density 15.6 kn/m 3 and it is achieved by raining technique and the free fall
50 th Percentage finer (%) is from the height of 70 cm. The rain-fall method is followed by [5]. The rain fall technique involves no segregation of sand particles. The physical and engineering properties are presented in Table 1. Here the sand is used in layers of two different in the same line. Keep the details of each author brief enough to be colors in order to clearly distinguish the failure surface. The thickness of dyed and nondyed layer is 3 mm and 18 mm respectively and thus total thickness of one layer is 21 mm as convenient to prepare test set up. For characterization of sand in predetermined density following experiments are conducted in accordance with ASTM standards as under: Specific gravity test Grain size distribution analysis Direct shear test Model Anchors and installation technique In practical, belled pile is possible in clayey type of soil; the bottom part of pile is enlarged by belling out with belling tools and practically they are constructed of concrete. Hence to simulate the actual field condition, the model surface is made rough by attaching the same sand on its surface. Though [21] concluded negligible effect of surface roughness in case of plate anchor and [5] in case of belled pile when pulled vertically upward. The panel anchors are made of steel plates. The panels are 57± 0.5 cm in length and hence sufficient to consider that plain strain condition would prevail. The surface is roughened by attaching same sand by glue. The models are placed parallel to the width of the tank. The sands are attached to develop the frictional contribution though [21] told friction is vital for shallow plate anchor (H/B = 1, where H = embedment depth and B = plate width) and [5] reported only about 10% of contribution of friction to enhance the uplift capacity of belled pile. A compacted sand datum of fixed thickness 10 cm is made and over it the model is placed vertically allowing no eccentricity. Sand is filled upto predetermined height. Fig. 2 represents details of 2D model geometry. Here the method of model pile installation can never be attained in field but it simulates the behavior of bored pile. Testing Tank The testing tank was 750 mm (L) 600 mm (W) in plan and 70 mm in depth. The box was made of perpex sheet in four sides to have a clear view from outside. The sheet was 12 mm in thickness to make it stiffer and to prevent any deflection in outside. The size of the tank is large enough to avoid boundary effects as the heave formed in the surface is maximum 4 times of bell thickness as concerned in the present test. 100 90 80 70 60 50 40 30 20 10 0 0.01 0.1 1 10 Particle diameter (mm) Fig 2 Grain size distribution of sand EXPERIMENTAL SET-UP AND TEST PROCEDURE A schematic diagram of loading frame, anchor inside the tank and other accessories is presented in Fig. 3. With a screw rod the anchor is suspended in the empty tank which was filled with 100 cm thick compacted sand bed. The load is measured by 5 kn capacity proving ring and corresponding displacement was measured by two dial gauges of 0.002 mm accuracy. The dial gauges are fixed with magnetic base and base is placed on a piece of steel angle run parallel to the width of the tank. The each tip of two dial gauge is placed over the two wings projected in both sides of panel. After filling the tank with sand the model is pulled upward and corresponding load and displacement is recorded.
Tanaya Deb & Sujit Kumar Pal The model is continued to be pulled little after the failure as indicated by proving ring till the surrounding soil at the base of anchor is collapsed and move downward to fill the void formed underneath the anchor and at that moment the experiment is stopped to record the deformation in sand. In the failure mechanism study of [20], the test was continues so far surrounding soil at the base of anchor is collapsed to fill the void. A straight line is already drawn through the centre of each model prior to placing which is vividly shown through the plaxiglass and with the scale the horizontal extent of failure in both side of model is recorded with respect to the corresponding depth. So, for each layer breaking point is found out from that very point, where upward transition started with respect to horizontal line. It is found that in both sides of model horizontal extent of failure is almost similar. For all the models in general it is found that in a sand layer vertical extent of sand displacement reduce gradually upto failure point. In the upper layers vertical displacement of soil is lesser than lower layers. Fig. 3 Schematic diagram of experimental set up RESULTS In all the cases it is observed that the sand is expanded, pushed upward and deforms laterally. The failure curves are plotted in the co-ordinate axes system and centre of the plate is taken as origin. A close photographic view of displaced sand is presented in Figs. 4 and 5. Here, horizontal extent of failure with respect to depth is presented in Figs. 6 to 8. Discussions on Test Results On the basis of experiment results, this section presents the effect of various physical parameters on slip surface around belled anchor pile under uplift in cohesionless soil. The effect of bell diameter, embedment ratio and pedestral height on failure pattern are discussed for the models having constant bell angle 45. Here the maximum extent of failure surface and initial and final angle of slope surface is also discussed. Table 1 Physical and engineering properties of sand used in model test Physical parameters Experime -ntal data Medium sand, 2.00 to 0.60 mm (%) 91.00 Fine sand, 0.600 to 0.075mm (%) 9.00 Silt and clay, 0.075 mm (%) Nil Effective grain size (D 10, mm) 0.74 Average grain size (D 50,mm) 0.9 Coefficient of curvature (C c ) 1.10 Coefficient of uniformity (C u ) 1.28 Classification SP Specific gravity (G s ) 2.62 Minimum void ratio 0.60 Maximum void ratio 0.85 Minimum density (γ min ), (kn/m 3 ) 14.22 Maximum density (γ max ), (kn/m 3 ) 16.35 Relative density (%) 67.90 Internal friction angle of sand ( ) 31 Interface friction angle ( ) 21 Table 2 Details of 2D model geometry Anc hor no. Shaft thicknes s (mm) Bell thicknes s (mm) Bell angle ( ) Height of bell (mm) 1. 26 92 45 33 2. 26 80 45 27 3. 26 68 45 21 4. 26 56 45 15
Embedment depth (cm) Embedment depth (cm) 50 th diameter the slope of slip surface shows more nonlinearity than slip surface formed in higher diameter model. Fig. 4 Pictorial view of model T s /T b = 0.47at L/ T b = 3 Maximum extent of slip surface due to failure The maximum extent of failure surface increases with higher embedment depth and width ratio. The lateral extent of heave were within the range of 17.7 to 22 cm, 15.5 to 20 cm and 14.8 to 18.1 cm and 12.8 to 17.2 cm for anchor no 1, 2 3 and 4 respectively.the finding is in conformity with [11]. 28 24 20 16 12 8 Fig.5 Pictorial view of model T s /T b = 0.28 at L/ T b = 5 Effect of Embedment Ratio on Failure Pattern With the increase of embedment depth, the failure surfaces are shifted gradually left side as presented in Figs. 4 and 5 that may be due to the effect of confinement. Higher embedment ratio enhanced the overburden pressure, lesser sliding and more interlocking. [21] Recorded that extensive contained plastic deformation starts before collapse and after commencement of collapse there is unrestricted plastic deformation towards surface. At lower embedment depth slip surfaces have tendency to move outward due to dialatency effect during plastic deformation as reported by [10]. Effect of width ratio on Failure Pattern In fig. 5 it is observed that with the increase in bell width the failure wedge expand more laterally. For anchor no 1, 2 and 4 at embedment ratio 5, 4 and 3 depth yields 28 cm, 27.2 cm and 27.6 cm respectively. After reaching to ground surface the heave extended laterally though upto a small extent. So, from the observation made in fig.6 this argument can be placed that in case of small 4 0 0 2 4 6 8 10 12 14 16 18 20 Horizontal extent of failure surface (cm) Fig. 6 Observed failure surface in anchor no. 1 30 25 20 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20 Horizontal extent of failure surface (cm) Fig. 7 Observed failure surface in anchor no. 3
Embedment depth (cm) Tanaya Deb & Sujit Kumar Pal 45 40 35 30 25 20 15 10 5 0 0 5 10 15 20 25 Observed failure surface with model with (cm) Fig. 8 Observed failure surface in same depth similar depth with different width ratio. Though the authors reported that in sand deposit having ϕ value 34.5 lateral extent of heave were not clearly observed for embedment ratios 2 to 4. Table 3 Horizontal extent of Failure surface in ground surface, Initial and final inclination angle of slip surface with vertical and inclination angle of slip surface with horizontal at ground surface Sl. no. T s /T b L/T b Depth (cm) X max (cm) β i ( ) β f ( ) 90- β f ( ) 1. 0.28 3 27.6 17.7 16 52 38 2. 4 36.8 19.5 14.5 53 37 3. 5 46.0 22.0 14.5 56 34 4. 0.33 3 24.0 15.5 16 57 33 5. 4 32.0 18.7 16 55 35 6. 5 40.0 20.0 15 55 35 7. 0.38 3 20.4 14.8 16 60 30 8. 4 27.2 16.7 16 52 38 9. 5 34.0 18.1 16 55 35 10. 0.46 3 16.8 12.8 23 53 37 11. 4 22.4 14.5 18.5 53.5 36.5 12. 5 28.0 17.2 16 55 35 Initial and final inclination angle of slip surface with vertical due to failure A number of literatures are already presented in the earlier sections where a range of initial inclination angle (β i ) of slip surface with vertical due to failure is well established by previous researchers. In Table no 3, the values of β i angle is presented and these values are within the range of 14.5 to 23. In general the range of the β i value is ϕ/2-1 to ϕ/2+1 except in anchor no.1 at embedment ratio =3 and 4, where β i = ϕ/2+7.5 and β i = ϕ/2+4.5. Inclination angle of slip surface with horizontal at ground surface [3] Assumed that at ground surface slip surface made angle (45 - ϕ/2) and hence β f value can be written as (45 +ϕ /2). Here, the β f value is within the range of 52 to 60, though as per the convention of [3] the β f value would be 60.5. We can write 90 β f = 45 - ϕ /2 = 29.5, but in this test condition these values are within the range of 30 to 38. The variation in the values of β i and β f represent the nonlinearity in the failure surface. CONCLUSIONS Laboratory experimental programme has been carried out to represent the soil deformation patterns around belled type pile during uplift. 2D anchors having bell angle 45 0, embedded ratio 3 to 5 and width ratio 0.28 to 0.46 are studied here. Based on the experimental results and discussion the following conclusions are made. The horizontal extent of failure shows the tendency to dilate more in lower embedment depth irrespective to the width ratio having a range taken into consideration in this present study. With increase of both width ratio and embedment ratio heave formation in ground surface is more extended. In same embedment depth having lower width ratio slip formation is less expanded in model having lowest width ratio but nonlinearity is higher in model having highest width ratio. The failure planes initiate from the edge of bell and extend outward making angle ϕ/2-1 to ϕ/2+1 in general, with vertical and ultimately reach the ground surface making the angle within the range 30 to 38 with ground surface for the embedment ratio taken into consideration. REFERENCES 1. Balla A (1961). The resistance to breaking out of mushroom foundations for pylons. Proc 5th Int Conf Soil Mech Found Engg. 1 (1), 569 576. 2. Bobbitt, D.E., and Clemence, S.P., (1987). Helical anchors: application and design criteria.
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