Theme- 1. Drilled Piling SystemsSub-theme- C.   Model Testing

Experimental Study on Piled-Raft Foundation in Sand Subjected to Combined Loading

M. Samanta, Scientist, CSIR-CBRI, Roorkee, manojitbee@gmail.com

R. Bhowmik, QH Scientist Trainee, CSIR-CBRI, Roorkee, riyabhowmik89@gmail.com

Extended Abstract: Piled-rafts are used as an effective foundation system, where raft alone fails to satisfy the serviceability and/or bearing capacity criteria. Tall buildings, supported on piled-rafts are always subjected to lateral load due to wind action along with vertical dead loads. The current design procedure of piled-rafts does not consider the mutual interactions of vertical and lateral load on the vertical and lateral response of piled-raft foundation.

Present paper investigates the effect of vertical load on lateral response of piled-rafts in sand through small scale instrumented laboratory tests. Hollow cylindrical aluminium pipes of 25 mm O.D, 1 mm thickness and 1000 mm in length are used as model piles whereas aluminium plate of 200 mm x 200 mm x10 mm is used as model raft. 2 x 2 model floating piles arranged in a square pattern @ spacing of 3D c/c and fixed rigidly through nut connection to model raft is used as model piled-raft foundation. The test tank of 2000 mm x 2000 mm x1500 mm is filled with sand through rainfall technique maintaining the required relative density of 40%. The model piles are installed in test tank to simulate the field condition of cast-in-situ bored pile. Vertical load in the model piled-raft is applied through hydraulic jack and measured through load cell. Horizontal load is applied through a string-pulley-dead load arrangement attached at the side of test tank. Strain gauges along the length of the pile are used to measure the strain under applied load. The horizontal and vertical displacements of the foundation are measured through LVDTs attached at the top and side of raft.

Lateral response of piled-raft is measured and compared under pure lateral and simultaneous action of vertical and lateral load. Bending moment in the piles are calculated using beam bending theory from measured strain values along the length of pile. Shear force induced in piles was obtained by fitting a fifth order polynomial in bending moment data, and differentiating it. A substantial decrease in the bending moment and shear force values for both the rows of pile is observed for combined loading. Maximum positive moment reduces by 63% at lateral load level of 1160 N, and by 55% at 1000 N, in presence of vertical load. At the load level of 1160 N and 1000 N, reduction of 51 % and 46 % occur in maximum shear force in presence of vertical load. The horizontal load displacement curve becomes stiffer under the influence of vertical load. At the load of 1100 N, there is a reduction of 38 % in deflection.

Experimental Study on Piled-Raft Foundation in Sand Subjected to Combined Loading

Manojit Samanta1, Riya Bhowmik 2

1Scientist, CSIR-CBRI, Roorkee, manojitbee@gmail.com

2QH Scientist Trainee, CSIR-CBRI, Roorkee, riyabhowmik89@gmail.com

ABSTRACT

The current design procedure of piled-rafts does not consider the mutual interactions of vertical and lateral load on the vertical and lateral response of piled-raft foundation. Present paper investigates the effect of vertical load on lateral response of piled-rafts in sand through small scale instrumented laboratory tests. Hollow cylindrical aluminium pipes of 25 mm O.D, 1 mm thickness and 1000 mm in length were used as model piles whereas aluminium plate of 200 mm x 200 mm x10 mm was used as model raft. 2 x 2 model floating piles arranged in a square pattern @ spacing of 3D c/c and fixed rigidly through nut connection to model raft was used as model piled-raft foundation. The test tank of 2000 mm x 2000 mm x1500 mm was filled with sand at relative density of 40%. Vertical load was applied through hydraulic jack, and horizontal load through a string-pulley-dead load arrangement. Lateral response of piled-raft was measured and compared under pure lateral and simultaneous action of vertical and lateral load. A substantial decrease in the bending moment and shear force values for both the rows of pile is observed for combined loading. Maximum positive moment reduces by 63% at lateral load level of 1160 N, and by 55% at 1000 N, in presence of vertical load. At the load level of 1160 N and 1000 N, reduction of 51 % and 46 % occur in maximum shear force in presence of vertical load. The horizontal load displacement curve becomes stiffer under the influence of vertical load.

INTRODUCTION

Piled raft is a well established foundation concept to support high rises (Katzenbach et. al., 2005). Its behaviour due to various interactions between its bearing elements under vertical loads has been thoroughly studied (Burland, 1995; Horikoshi & Randolph, 1998; Poulos, 2001 & Chung et. al., 2013). All of these studies were confined to the response of piled-raft under vertical load only. Studies concerning its response under lateral load are scarce. Static 1-g and centrifuge model tests were performed to investigate the response under lateral load (Horikoshi et. al., 2003; Matsumoto et. al., 2004 & Kitiyodom et. al., 2005). The results showed that the stiffness and resistance of a pile in piled- raft foundation are different from an isolated pile due to the difference in the confining stress around the piles. The proportion of vertical load carried by the piles in a piled-raft remains almost unchanged during horizontal loading, while the proportion of horizontal load carried by the piles increases as the horizontal displacement increases. This study did not investigate the effect of vertical loading on lateral response of the foundation. In case of single pile or piles in group, considerable influence of vertical load on lateral resistance of piles has been found (Anagnostopoulos & Georgiadis, 1993; Kartigeyan & Ramakrishna, 2006 & 2007). Katzenbach and Turek (2005), through 1-g model tests in cohesionless soil, observed that the presence of vertical load increases horizontal resistance of piled-raft. The raft in piled-raft system contributes most in offering resistance for lateral displacements. They also observed that similar to its singular behaviour under vertical load, a piled-raft behaves quite differently than a pile group under lateral load. Tall buildings, supported on piled-rafts are always subjected to lateral load due to wind action along with vertical dead loads. The current design procedure of piled-rafts does not consider the mutual interactions of vertical and lateral loads on the vertical and lateral response of piled-raft foundation. Present study

investigates the effect of vertical load on lateral response of piled-rafts in sand through 1-D small scale instrumented laboratory tests.

TEST DESCRIPTION

Model test tank

Geotechnical engineering division of CSIR-CBRI has a model test tank which is uniquely designed to simulate the condition of combined loading. This has facilitated the problem of studying the response of piled raft foundation under vertical, lateral and combined loading through model tests. The model tank used is a specially designed steel tank of size 2.0 m × 2.0 m × 1.5 m. The tank is stiffened with steel channels at different levels to eliminate the chances of volume change during filling. The unique feature of the model tank is the specially designed arrangement of movable vertical load assembly in both the horizontal/vertical directions. Fig. 1 shows the experimental model test set up for performing model tests. Dry Solani river sand was used for the model ground throughout the tests. The physical properties of the Solani river sand are summarised in Table 1.

Fig. 1 Experimental model test set-up

Model Pile and Raft

The dimensions of the raft and piles were so chosen that the rigid boundaries of tank have negligible effect on stresses and displacements of model foundation. From literature and numerical studies, it was found that the lateral boundary should be at least at a distance of 4.5B from the edge of the raft, and vertical boundary at a distance of L/2; where B is width of raft and L is length of pile. Accordingly, the model raft size was calculated as 200 mm × 200 mm in plan and 10 mm in thickness. It was made of aluminium alloy with modulus of elasticity 77 GPa. The model piles used were made of the same grade of aluminium alloy as that of the model raft. It was close-ended pipe with outer diameter 25 mm and wall thickness 1 mm. Length of the pile was calculated as 1000 mm.

Table 1: Properties of sand

Fig. 2 Schematic Diagram of model raft and piled raft

The length of the pile was carefully selected so that they can behave as flexible pile. This was done because piles in deep foundation are usually flexible. The measure of flexibility, T, called as characteristic length of soil-pile system, depends on flexural rigidity of pile, EpIp and modulus of subgrade reaction, ƞh for cohesionless soil. For a pile length of 1000 mm, modulus of elasticity, Ep = 77 GPa and ƞh = 0.00026 kg/mm3 (IS: 2911 (Part I/Sec 1)-1979), T was calculated as 17.43206 (Eqn No. 1) and L/T as 5.74. As L/T > 5, the pile was considered to be long pile or flexible pile.

T= 5√ EpIpŋh … (1)

The connection between the piles and the raft was designed as rigid connection. This was done by designing a nut and threaded pile-head arrangement. The schematic view of this arrangement is shown in Fig. 2. To obtain axial stress distribution and bending moment values along the length of the pile, strain gauges were fixed at various positions along the pile length.

TEST PROCEDURE

Before installation of piled-raft, the test tank was filled with sand through rainfall technique up to the level of pile tips. The desired density in the test tank was achieved by sand raining technique. To achieve a density of 14.87 kN/m3 corresponding to RD of 40% in the test tank, a height of fall 0.15 m was determined from the height and fall method. The positions of the pile were located and were imprinted on the sand with the help of plumb bob. Piles were placed in the position and kept vertical in the tank with help of a holding device. The holding device was specially fabricated so that it maintains the verticality of the pile. The tank was filled up to the final level after placing the piles in position. The raft was then connected to the piles with nut bolt connection. Horizontal level of the raft was checked with the help of spirit level. The installation process employed in the process was maintained to create minimum disturbance to sand surrounding the piles. This procedure was used to simulate the construction of a bored pile where soil disturbance is usually small. The test program consists of 4 loading tests of model piled-raft foundation. The loading tests were preceded by a simple bending test of a cantilever pile to determine its material properties.

Lateral load on piled-raft

To measure the lateral displacements of piled-raft, two nos. of displacement transducers (LVDT) of 50 mm stroke and 0.01 % accuracy were set up horizontally. Initially, a seating load was applied so that the foundation settled down and a perfect contact between the raft and soil was maintained. When this was

Property Value

Soil type SP

G 2.67

γdmax 16.6 kN/m3

γdmin 13.9 kN/m3

c 0

φ 29.55

achieved, seating load was released and lateral load was applied in increments of 200 N up to a maximum level of 1600 N. Lateral displacements and bending strains were recorded during the test using data acquisition system (DAS) and the data were analyzed. Application of lateral load and the foundation at final stage are shown in Fig. 3 and Fig. 4, respectively.

Fig. 3 Lateral load test in process Fig. 4 Application of lateral load

Combined load on piled-raft

Two displacement transducers of 25 mm stroke were set up horizontally in the same way as in lateral load test set-up. Another two displacement transducers were placed at two corners of the raft to measure vertical settlements. One dial gauge of 50 mm probe length was also fixed on another corner of the raft. Fig. 5 shows the set-up for combined loading test. Vertical load of 2500 N was applied. It was followed by application of horizontal load with a maximum value of 1600 N, in increments of 200 N, while the vertical load remained constant. This sequence of load simulates the sequence of load application in tall buildings. During the process of application of lateral load, the alignment of the vertical load assembly was getting inclined. To prevent imposition of inclined load in place of vertical load, the alignment was adjusted each time the lateral load was applied, by moving the reaction beam forward with the help of rotating arrangement present in the model test tank set-up and adjusting the position of the loading cylinder. Fig. 6 shows the foundation at the final stage of combined loading.

Fig. 5 Set-up for combined loading on piled-raft Fig. 6 Foundation at final stage of loading

TEST RESULTS

Continuous recording of data from LVDTs and strain gauges were taken through data acquisition system (DAS) during each test. These data were processed and analyzed to get the various responses of different

foundation elements during the test. This section presents the analyzed data in form of load-displacement curve and bending moment distribution along length of pile in piled-raft.

In-situ density of sand in the test tank was checked after each test. From observations, mean value of in-situ density was found to be 15.22 kN/m3 with standard deviation (SD) and co-efficient of variation (COV) of 0.37 and 2.45 %, respectively.

Lateral load on piled-raft

From the recorded data during the test, load- deflection characteristics of the piled-raft foundation subjected to lateral load was drawn. Fig. 7 shows the lateral load – deflection curve of piled- raft foundation. Load - deflection curve of piled-raft shows an initial straight portion followed by a nonlinear portion.

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Fig. 7 Lateral load vs. deflection of piled-raft

Moment along the length of pile for front and back pile was also drawn for different level of lateral load. Bending moments in the piles were calculated using beam bending theory from measured strain values along the length of pile. Fig. 8 (a) & (b) depicts bending moment along the length of the pile.

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Fig. 8 (a) Bending moment vs. depth of front pile Fig. 8(b) Bending moment vs. depth of back pile

As obvious, with increasing the lateral load, maximum bending moment in pile also increases and occurs at a depth of 8 -12 times the diameter of pile. Maximum bending moment in front pile is nearly 10 times greater than that of back pile. This is due to “shadowing effect”, which occurs due to overlapping of failure zone of piles with each other, resulting in reduction of lateral resistance for piles in trailing rows (McVay et. al.,1998, Patra & Pise, 2001). The values of maximum negative moment at head of piles are very low. This indicates that the designed nut and bolt connection was not fully successful in achieving rigid pile head connection.

Combined load on piled-raft

Fig. 9 shows the comparison of lateral load–deflection behaviour of piled-raft under combined loading with that of lateral load – deflection behaviour of piled raft. Influence of vertical load on lateral response of piled-raft has been brought out in the figure. The comparison shows that a significant decrease in lateral deflection of the piled-raft was observed in presence of vertical loading. At the load of 1100 N, there is a reduction of 38 % in deflection, i.e., the lateral response of piled raft becomes stiffer in presence of vertical load.

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Fig. 9 Comparison of lateral load vs. deflection

Fig. 10 shows the comparison of bending moment along the pile depth for back pile, with and without vertical load. Bending moment distribution shows a decrease in its magnitude, in presence of vertical load. Maximum positive moment reduces by 63% in presence of vertical load of 1160 N, and by 55% at 1000 N.

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Fig. 10 Comparison of bending moment of back pile Fig. 11 Comparison of shear force of back pile

Comparison for shear force profile for the same pile is shown in Fig. 11. Shear force profile was obtained by fitting a fifth order polynomial in bending moment data, and differentiating it. Similar to bending moment, shear force induced on pile also reduces in presence of vertical load. At the load level of 1160 N and 1000 N, reduction of 51 % and 46 % occur in maximum shear force in presence of vertical load. Even at the low load level of 200 N, maximum shear force reduces by 65 %.

CONCLUSIONS

Present analysis investigated the effect of vertical load on lateral resistance of piled raft, through 1-D small scale model tests. Following conclusions can be drawn from the study:

a. Vertical load plays an influential role on the lateral response of piled-raft. b. Lateral deflection of the pile within the piled-raft is very much dependent on the applied vertical

load on the raft. The horizontal load displacement curve becomes stiffer under the influence of vertical load. At the load of 1100 N, there is a reduction of 38 % in deflection.

c. Maximum positive and negative bending moment was also dependent on the vertical load. Both the maximum moments decreased with increase in vertical load on the raft. Maximum positive moment reduces by 63% at lateral load level of 1160 N, and by 55% at 1000 N, in presence of vertical load.

d. Maximum bending moment in pile occurred at a depth of 8 -12 times the diameter of pile. e. Bending moment of front pile was about 10 times higher than that of back pile, due to

“shadowing effect” which results in reduction of lateral resistance of back pile.f. Reduction in shear force was also obtained in presence of vertical load. At the load level of 1160

N and 1000 N, reduction of 51 % and 46 % occur in maximum shear force in presence of vertical load.

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