PERFORMANCE OF PILED RAFT WITH VARYING PILE LENGTH ABSTRACT

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1 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 185), Pune, India PERFORMANCE OF PILED RAFT WITH VARYING PILE LENGTH Angelin Savio 1, Sreekumar.N.R,, V.Balakumar 3 ABSTRACT A piled raft foundation is fairly a new concept in which the total load coming from the superstructure is partly shared by the raft through contact with soil and the remaining load is shared by piles through skin friction. Due to the three dimensional nature of the load transfer, piled-raft foundations are regarded as very complex systems involving many interaction factors such as pile-to-pile, pile-to-raft, raft-to-soil and pile-to-soil. The economy of the foundation system for heavily loaded and settlement sensitive structures like tall slender buildings and storage tanks depends upon the method adopted to reduce the settlements to the permissible level rather than eliminating it completely. As a matter of fact when the serviceability requirements are satisfied from the point of view of permissible settlement there is no need to eliminate the settlement completely. The combined piled raft system has proved to be an ideal foundation system to satisfy the above requirement under certain favorable circumstances, namely when the bearing capacity is not a problem but settlement would be beyond the permissible requirements. Although the combined piledraft system was developed with over consolidated clay in mind, its applicability in sand also becomes important as the permissible settlement for the foundation is less than that of foundation resting on clay. Therefore the applicability of piled raft to support moderately loaded buildings and storage tanks on sand and predominantly sandy soils gains importance, further more understanding of load sharing between piles and raft is very much important for the piled raft in sand particularly when the piles are driven because the driving of piles improves the state of compaction of the sand. As the piled raft foundation system transfers the load through a complicated interaction process the effect of the parameters associated with the constituent elements, namely the piles, raft and the soil on the settlement reduction and load sharing behavior becomes very important and needs a detailed study. Although a number of published literatures are available on the effect of various parameters, they all have piles of equal lengths. In case of plaza like structures wherein the raft thickness as well as the pile length can be varied depending upon the capacity requirements, it becomes necessary to understand the effect of variation in pile length on settlement reduction and load sharing behavior of piled raft. The present work is based on the results of small scale 1g model tests conducted on piled raft models with varying configurations of varying pile length. A square raft of 150mm with 3 3 configuration and the pile 1 Angelin Savio, M.Tech Student, IES College of Engineering, Thrissur, India, angelinsavio@gmail.com Sreekumar.N.R, Asst Professor, IES College of Engineering, Thrisur, India, sreekumar.nedumpurath@gmail.com 3 V.Balakumar, Senior Consultant, Simplex Infrastructures Ltd, Chennai, India, vb_kumar00@yahoo.com

2 Angelin Savio, Sreekumar.N.R, and V.Balakumar lengths of 0.8,1 and 1.5 times the raft width are used for the study. Tests were conducted by changing the pile configuration by varying the locations of the short piles and long piles in a strategic manner. Considering the high wind or seismic loads that may act on the high rise buildings, the effect of small and large eccentric load acting on the piled raft is also studied. The load configurations are provided in such a manner to stimulate axial load and bending. From the results it was found that connecting piles of even short length has greater improvement in the raft behavior. Not much change in the raft behavior was observed with small eccentricity when compared with piled raft subjected to large eccentricity. The load settlement response was also studied and this paper discusses the result. In a specific combination of piled raft with varying pile length, it was found that the settlement profile was almost the same for the load applied vertically and for load applied with small eccentricity. Thus using dissimilar piles below a raft can optimize the design of a piled raft and is economically an innovative concept and has a further scope of study. Keywords: Differential settlement, Eccentric load, Load sharing ratio, Piled raft, Raft, Settlement, Settlement reduction ratio, Sand, Varying pile length.

3 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 185), Pune, India PERFORMANCE OF PILED RAFT WITH VARYING PILE LENGTH Angelin Savio, M.Tech Student, IES College of Engineering, Thrissur, Sreekumar.N.R, Asst Professor, IES College of Engineering, V.Balakumar, Senior Consultant, Simplex Infrastructures Ltd, Chennai. ABSTRACT: The combined piled raft system has proved to be an ideal foundation system to satisfy the above requirement under certain favourable circumstances, namely when the bearing capacity is not a problem but settlement would be beyond the permissible requirements. Although a number of published literatures are available on the behavior of piled raft foundation system, they all have piles of equal lengths. In most of the conventional designs, piles in the foundation have uniform lengths. The present work is based on the results of small scale 1g model tests conducted on piled raft models in sand with configurations of varying pile length at varied load eccentricity. Studying the settlement response and load sharing behaviour it was found that using dissimilar piles below a raft can optimize the design of a piled raft and is economically an innovative concept and has a further scope of study. INTRODUCTION In designing raft foundations, engineers frequently encounter situations in which the bearing capacity of the raft is quite adequate, but the settlements are estimated to be excessive. In such cases, the combined use of a raft, along with a limited number of piles, could be an economical counter measure in which the piles are used to reduce the settlements to an acceptable level. The present state of knowledge on the behaviour of piled raft is mostly centred on the piled raft seated on deep deposits of over consolidated clay, However it is quite possible that this system can be adopted to support structures sensitive for settlement and are to be supported on loose to medium dense sand. Although majority of the piled raft and pile supported raft have piles of uniform length it has observed that peripheral piles take higher load based on the tributary area of the raft settlement profile remains more or less uniform depending on the nature of the raft. The behavior of the piledraft with sand as bed material has been studied by researchers like Balakumar and Ilamparuthi (009), Sahu and Bajad (009) and so on. Extensive parametric studies have also been carried out by many researchers using 1g model studies (Balakumar 008) and centrifuge models (Horikoshi,1995). The small scale studies included 1g model tests and centrifuge tests. In order to solve this complex problem of piled raft, several methods such as numerical and analytical model studies were done. With an advancement of the computer, more rigorous methods such as Finite Element Method (FEM) are also used in some of recent researches (Chow et. al, (005), Liu et. al., (009), Joy et. al, (01)). Some researchers conducted both small scale model studies and analytical model studies for more accurate results (Baziar et. al, (009), Jeyapriya and Venkateshwaran (013), Balakumar et. al., (013)). From the literature, it was found that a 1g model test on piled raft foundation in sand deposit is very limited. But in the case of sand the permissible settlement is lesser than clay. Further more understanding of load sharing between piles and raft is very much important for the piled raft in medium or loose sand particularly when the piles are driven because the driving of piles can improve the state of compaction of the sand. In the traditional design of group piles or piled rafts, the piles are often arranged uniformly (i.e., using the same pile diameter length, and spacing). However, research showed that peripheral piles either carry a greater proportion of loads than the central piles under a rigid cap, or suffer larger differential

4 Angelin Savio, Sreekumar.N.R, and V.Balakumar settlements under a flexible cap, as a result of pilesoil-raft interaction. Therefore, the traditional design often does not result in the best performance in overall stiffness or differential settlement. To overcome these aforementioned problems, the use of piles with different lengths or positions below a raft can optimize the design of a piled raft. Using a piled raft in areas subjected to high wind or seismic loading, may increase the stability of the building and may reduce the tilt of the raft foundations that are subjected to eccentric loading. This paper focus on the behaviour of piled raft with varying pile length located in a strategic manner subjected to small and large eccentricity. A series of 1g tests were conducted at various eccentricities, on unpiled raft (plain raft) and on piled raft. The purpose was to compare load settlement response of unpiled raft with piled raft with uniform and varying pile length. The Messeturm tower in Frankfurt is a standard example (Leung et. al., 010) for the piled raft with varying pile length. Further it may also become necessary to add certain short piles when the foundation has to be reused or due to loading changes. Hence this study has gained considerable importance. EXPERIMENTAL WORK A model scale of 1/100 was used in all the tests. It should be noted that the models selected for the present work were not intended to simulate behaviour of a specific prototype foundation at a specific location. The results from the 1g model tests are analysed to understand the role of various pile parameters in reducing settlement of the raft and in sharing the applied load. Test Setup The test facility developed in this study consists of a loading frame, a steel box, sand raining and controlled tamping device, piled raft and raft models and loading arrangement. The vertical compressive load was applied to pile raft model through a screw jack of 0kN capacity, which was supported centrally at the bottom flange of the steel girder made of channel sections. It was operated manually by rotating the lever. The load was transferred to the model through a proving ring, which was fixed on the bottom of jack and the rigid loading platen was connected to the proving ring through an extension rod. The schematic view of the experimental set up is given in Fig. 1. Fig. 1 General arrangement of experimental set up Test Box A rectangular rigid steel test box of inner dimensions 0.75m 0.75m 0.5m with 10mm wall thickness was fabricated and used for conducting the experiments on the model piled raft in the laboratory. With a wall thickness of 10mm and several stiffeners on the outer side, the box is considered to be rigid. The inside face of the tank was graduated at every 50mm depth intervals to aid preparation of sand bed in layers by sand raining technique. Raft and Pile Models A square piled raft model was chosen for the study. This represented the behaviour of foundation of a square raft of multi-storied building. Model raft Raft modelled to prototype scale of 1/100 were fabricated from a single Perspex sheet. The size of the model was chosen to represent the prototype foundation of 15m wide for square raft. The thickness of model raft was maintained uniform, though thickness of raft of real size piled raft foundations were in general varied from the edge to centre. The model of sides 150mm 150mm and with thickness 8mm was used in the study. Model Piles The model piles were of circular shaped Perspex rods. The pile diameter was kept constant to

5 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 185), Pune, India represent pile diameter of 1000mm. Perspex rod of 10mm diameter was used as model piles. The length of piles tested were 187.5mm (1.5 B), 150mm (B) and 10mm (0.8 B), representing the tip of pile below pressure bulb, at pressure bulb and above pressure bulb respectively. Here after we can call it as long (L), medium (M) and short piles (S). The length of the piles were chosen in such a way that the depth of the bed below the tip was sufficiently thick so that, bottom rigidity does not affect the pile behaviour and pile functions purely as friction pile. Threads were provided at the top end of piles to facilitate the proper connection and to generate monolithic action between the piles and the raft. Piled Raft Models Raft and pile models of required diameter and length were assembled by fitting the piles in the holes of the raft and connecting them together with stainless steel screws. A total of 9 piles are arranged in 3 3 grid pattern with spacing 5 times the diameter of the pile. Piled raft models were tested for different arrangements of varying lengths of piles. The purpose was to examine the influence of pile length configuration of piles on load sharing and settlement reduction. Various pile configurations tried in this study for the square raft are shown in Fig.. The various combinations are named as combination 1,, 3,, 5,, and 7 respectively. Test Medium Uniformly graded clean river sand was used in all the experiments as a test medium. The index properties obtained are presented in Table 1 and the sand is classified as poorly graded sand (SP). Load Eccentricity The presence of certain amount of eccentricity of loading in the footing can induce moments in the building. In case of tall structures the effect of load eccentricity may arise due to horizontal load acting on it. In the present study two cases of eccentricity has been considered namely small eccentricity, and large eccentricity. Eccentricity is provided on the basis of e/b ratios. B is the raft width and e corresponds to eccentric distance from the centre of raft. For small eccentricity case e/b= 0.05 is considered and for large eccentricity case e/b = 0.15 is considered. Fig. Piled raft with different combinations of pile arrangement Table 1 Index properties of sand Parameter Value Effective size (D 10 mm) 0.7 D 30 (mm) 0.3 D 0 (mm) 0.78 Uniformity coefficient (Cu).87 Coefficient of curvature (Cc) 0.8 Type of soil SP Specific gravity G.7 Maximum dry density( k N/m 3 ) Minimum dry density( k N/m 3 ) 15.7 Specimen Dry Sand Relative Density 8% Dry density (k N/m 3 ) Undrained cohesion (C) 0 Angle of internal friction (Φ) 3.5 o

6 Settlement (mm) Angelin Savio, Sreekumar.N.R, and V.Balakumar In accordance with the provision IS 911, relating to pile foundation, the possible deviation of the pile permitted is 0-75mm on a smaller diameter of a pile thus works out to 13.3%, considering 50mm as the diameter of the pile. If 75mm is permitted on a 500mm diameter pile, the eccentricity works out to 15%. However on a larger diameter pile, say 1m, the permissible eccentricity works out to 7.5%. As a parameter of study we have considered 15% for the present. No eccentricity case is also studied for comparison. Each combination of pile arrangement is subjected to all the three loading conditions. The density of sand bed was achieved by the combination of sand raining and compaction. Preweighed sand was rained in layers and controlled compaction was adopted to achieve the required density. The installation of pile was planned in such a way that represents a real time pile installation. Piles were connected to the raft with suitable arrangement to ensure monolithic action. The foundation was vertically loaded using a screw jack fitted to a loading frame and the load applied was monitored using a proving ring of 50 kn capacity. Settlement of piled raft was measured using dial gauges having travel of 5 mm and least count of 0.01 mm. RESULT ANALYSES & DISCUSSION In order to study the load sharing behaviour of the pile group with varying pile length seven combinations of pile configuration have been studied. As a basic data the load settlement response of plain raft has also been studied so that the effect of piles of varying length on the behaviour of the composite system can be quantified. Further the effect of load eccentricity has also been studied. Since the present study mainly concentrates on the load sharing response, much concentration is not shown on the settlement reduction behaviour of the composite system. Behaviour of Plain Raft It is seen that in the initial stages of loading the behaviour does not show any appreciable variation, but as the loading increases the variation in the settlement increases. In the case of large eccentricity condition the loss of raft stiffness is more rapid. This indicates that when there is an eccentricity of loading, a larger eccentricity can induce distress in the raft. Fig. 3 presents the load settlement response of the plain raft of 150mm square and 8mm thick tested in dense sand bed, under three loading conditions Load (kn) Fig. 3 Load settlement response of plain raft e/b = 0 e/b = 0.05 e/b = 0.15 Effect of Pile Groups with Equal Pile Length Pile Length Shorter than Raft Width In all the three cases of loading namely no eccentricity case, small eccentricity case and large eccentricity condition the behaviour is identical as seen from the load settlement response of plain raft. As the load increases in the case of small eccentricity and large eccentricity the loss of the foundation stiffness is very rapid, this is perhaps in the first case of no eccentricity the applied load is resisted by the shaft friction since the pile group and the soil prism in between acts as a single pier. Further the enhancement of confining pressure around the pile is uniform and the system takes a much higher load; but when the load increases further to overcome the frictional resistance the system loses its stiffness more rapidly. Pile Length Equal to Raft Width It is seen from the Fig. that the behaviour is regular in the sense that the loss of stiffness is more or less at the same settlement level namely 8mm to 10mm.This is mainly because the pile group is subjected to a uniform confining pressure for the entire length of the pile and also the enhancement of the confining pressure around the piles is also uniform. This is mainly due the reason that the pile

7 50 th IGC Settlement (mm) Settlement (mm) 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 185), Pune, India tip coincides with the tip of the pressure bulb. It is also seen that the increase in the load between the case of small eccentricity and no eccentricity is smaller than the difference between large eccentricity and small eccentricity Fig. Load settlement response of piled raft with combination Pile length larger than the raft width One very important observation is to be made here Load (kn) Load (kn) e/b = 0 e/b = 0.05 e/b = 0.15 e/b = 0 e/b = 0.05 e/b = 0.15 Fig. 5 Load settlement response of piled raft with combination 3 It is seen from the load settlement response shown in Fig. 5 that the load taken by the group with larger pile is only marginally more than the previous case namely the pile group with length equal to the raft width. This clearly indicates that the enhancement of the confining pressure due to the applied load through the raft takes place only within the pressure bulb and so increasing the pile length does not produce any distinct advantage and that increasing pile length beyond pressure bulb does not produce any distinct advantage. Combination and 5 Various combinations by varying the pile lengths have been studied by keeping the long pile at the centre. In the combination 5, where the long pile at the centre is surrounded by medium length piles, the load at failure is kn for no eccentricity case. This is marginally more than combination (9.89 kn), where the long pile at the centre is surrounded by short length piles. This marginal difference is mainly due to the increase in the pile length. And the influence of the pile soil interaction through the short piles and the soil. The load settlement curves showed similar trend as in previous cases. Combination and 7 Fig. and Fig. 7 show the load settlement response of piled raft with combination and 7 respectively. In combination 7 the smaller piles are provided at the corners and in combination medium length piles are given at the corners. In these two cases it is peculiar to see that there is no much difference in the load taken at any given settlement level in the case of no eccentricity and small eccentricity. However the load at failure is higher for no eccentricity case. This means that so long as the longer pile remains in the centre, the location of the smaller pile does not affect the behaviour of piled raft with smaller eccentricity compared to the piled raft of no eccentricity. But in the case of larger eccentricity combination showed better results than combination 7. This means that with the small piles in the corner the performance of the foundation system is not satisfactory. It is also seen that the fall of stiffness is at relatively smaller load level compared to the previous cases. Hence when the eccentricity is higher the location of smaller piles affects the performance of the piled raft.

8 Load Sharing Ratio Settlement (mm) Load Sharing Ratio Settlement (mm) Angelin Savio, Sreekumar.N.R, and V.Balakumar Fig. Load settlement response of piled raft with combination Load (kn) Load (kn) e/b = 0 e/b = 0.05 e/b = 0.15 e/b = 0 e/b = 0.05 e/b = 0.15 Fig. 7 Load settlement response of piled raft with combination 7 Considering all the combinations in various loading conditions we can say that combination is most effective. Even though the load taken was marginally lower than combination 3 with all long piles, it look more load than combination with all medium length piles for all loading conditions except in the case of large eccentricity. But this reduction in the case of large eccentricity is also marginal. Since we are using a combination of small, medium and long length piles in the piled raft system this can effectively reduce the cost of construction. Generalised Load Sharing Behaviour The load sharing behaviour of the pile group of piled raft under on eccentric load is presented in the form of a bar chart in Fig. 8. The load sharing ratio at corresponding settlement is given by: (1) Where = Load sharing ratio; = Load carried by piled raft; = Load carried by raft mm mm mm 8mm 10mm Settlement Fig. 8 Load sharing ratio for various combinations in no eccentricity case It is seen that from 10mm settlement level as the settlement increases the load sharing ratio decreases gradually indicating that the pile group essentially functions as settlement reducer. In the case of no eccentricity the combination 3 and 7 has established a better performance mm mm mm 8mm 10mm Settlement Fig. 9 Load sharing ratio for various combinations in small eccentricity case Fig. 9 showes that when there is small eccentricity the combination 3, and 7 performs better as the load increases. However we can see that as load increases the combination shows good result than combination 7. This indicates that when there is

9 50 th IGC Load Sharing Ratio 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 185), Pune, India eccentricity in the loading the position of smaller pile influences the performance of the foundation system. The load sharing ratio in the case of large eccentricity is given by Fig 10. It is seen that at higher loading eccentricity, the pile group has to take higher load. When the e/b ratio is 0.15 at any given settlement level the load sharing ratio is higher than the other two cases. Also where there is no load eccentricity the pile group functions more as a settlement reducer as the load sharing ratio is smaller. When there is loading eccentricity the pile group is more stressed and its efficiency as load sharing element reduces mm mm mm 8mm 10mm Settlement Fig.10 Load sharing ratio for various combinations in large eccentricity case CONCLUSIONS From the results obtained it was observed that, at any given settlement the load taken by piled raft is greater than plain raft. When the piled raft was subjected to eccentric loads, not much change in raft behaviour was observed between small eccentricity and no eccentricity case when compared with large eccentricity case. Connecting piles of even shorter length showed greater improvement in the raft behaviour. It was observed that connecting a long pile in middle can reduce settlement considerably provided no much eccentricity acts on the system. For a given area ratio and density of sand, the stiffness of piled raft is higher for longer piles. However, pile lengths more than 80% of the lateral dimension of the raft did not show any appreciable results. From the load settlement behaviour of various combinations it was found that in no eccentricity case, the piled raft with varying length can be used effectively. When eccentric load was at minimum, position of smaller pile had no appreciable effect. The combination showed better results than all other cases. This was because the position of short pile plays an important role under eccentricity loading. Therefore, in case of large eccentric loads it is preferable not to have any variation in pile length. The study establishes that for practical problems piled raft can have piles of varying length provided they are placed strategically depending on requirement. Thus we can reduce the cost of construction by limiting the length of the piles in an effective manner. As a further study numerical modelling can be done as a validation of the thesis. Further, the effect of varying pile length can be studied on raft thickness, spacing, pile diameter and size of raft. As an extension to this thesis, the interaction effect of superstructure stiffness can also be analysed. REFERENCES 1. Balakumar, V. (008), Experimental Studies of Model Piled Rafts on Sand and Field Study of Proto Type Behaviour, Ph.D. Thesis, Anna University, Chennai. Balakumar, V., and Anirudhan, I.V. (011), Piled raft behaviour model studies and field performance, Proceedings of IGC, Kochi, pp Balakumar, V., and Ilamparuthi, K. (009), Effect of pile layout on the behaviour of circular piled raft on sand, IGC, Guntur, India, pp El Sawwaf, M. (009), Experimental and numerical study of eccenrically loaded strip footing resting on reinforced sand, Jl. of Geotech. and Geoenv. Engineering, ASCE, vol 135, pp El Sawwaf. M. (010), Experimental Study of Eccentrically Loaded Raft with Connected and Unconnected Short Piles, Jl. of Geotech.

10 Angelin Savio, Sreekumar.N.R, and V.Balakumar and Geoenv. Engineering,, ASCE, vol 13, pp Garg, P., Singh, H., and Jha,J. (011), Optimisation of piled-raft foundation, UKIERI Concrete Congress - Innovations in Concrete Construction. pp Liang, F., Li, J., and Chen, L. (00), Optimization of Composite Piled Raft Foundation with Varied Rigidity of Cushion, Foundation analysis and design, ASCE proceedings, pp Sahu, R.B., and Bajad, S.P. (01), An Experimental Investigation on Interference of Piled Rafts in Soft Soil, Civil and Environmental Research, vol (), pp Singh, A.K., and Singh, A.N. (013), Study of piled raft foundation, Proceeding of Indian Geotechnical Conference, Roorkee, pp Singh, A.K., and Singh, A.N. (011), Experimental study of piled raft foundation, Proceedings of Indian Geotechnical Conference, Kochi, pp Singh, B., and Singh, N.T. (011), Influence of piles on load settlement behavior of raft foundation, International Journal of Engineering Science and Technology, vol 3, pp Venkateshwaran,N., and Jeyapriya, S.P.(013), Experimental Observations and Parametric Study of Piled Raft Foundation Using Plaxis, Proceedings of Indian Geotechnical Conference, pp