[American Society of Civil Engineers Geo-Frontiers Congress 2011 - Dallas, Texas, United States...

Tensile Response of Pile Groups under Compression Part 1: Experimental Investigations

Akhilesh Chandra Joshi1, AMIE; Nihar Ranjan Patra2

1Senior Engineer (Civil), NTPC Limited, Tapovan Vishnugad Hydro Power Project (520MW), Joshimath, District-Chamoli, Uttarakhand, India, PIN-246443, Phone: +91 9412083460, Fax: +91 1389-222435; [email protected], [email protected] 2 Associate Professor, Department of Civil Engineering, Indian Institute of Technology, Kanpur, India; [email protected]

ABSTRACT

The contemporary state-of-the-art analysis and design of piles and pile groups

for tension neglects the compressive load imposed by superstructure on the pile foundation. The results of an experimental investigation on model single piles and pile groups embedded in a uniformly graded medium dense Indian Standard Sand, grade-2 (Ennore sand) poured to a desired density by pluviation were reported. The piles were tested under pure compressive and pure tension, as well under predefined combinations of the two. The experimental results were analyzed for various parameters such as net tensile capacity, efficiency of pile group, unit skin friction and their variations. Net tensile capacity decreased with increase in compressive load. The highest efficiency of pile groups under combined loading (Tension and Compression) was observed at 25% compressive load and mostly at 6d pile spacing.

INTRODUCTION

Tensile capacity of single piles has drawn the attention of many researchers in the past. Anand and Ayoub (1976) performed model tests on vertical and inclined piles to determine ultimate tensile capacity. Das and Seeley (1976) carried out model study for capacity of buried single piles and pile groups in a sandbox and determined efficiency versus spacing relationship for group of piles. Ismael (1989) specified the average values of skin friction for driven piles in calcareous sand. Kraft (1991) considered various parameters that influence axial capacity of pipe piles driven in sand. Alawneh et al. (1999) studied the significant variables affecting the ultimate tensile shaft resistance of a pile in dry sand. Dash and Pise (2002) performed 36 model tests on piles embedded in sand and tried to assess the effect of compressive load on tensile capacity of single pile. These researchers have set the broad guidelines for behavior of pile groups in sand in various loading types.

232Geo-Frontiers 2011 © ASCE 2011

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The specific objectives of the study were to estimate and quantify the variations of net tensile capacity, unit shaft resistance and efficiency in tension for different pile group configuration (square or line), pile spacing and stage of compressive load.

EXPERIMENTAL SETUP AND TESTING PROGRAM

To facilitate the simultaneous application of tensile and compressive loads,

experimental setup was fabricated in the laboratory. Model tank of size 990mm× 975mm× 970mm was made up of mild steel plate of 5mm thickness. The tank was large enough for specified minimum clearance even for the biggest pile group. MS Piles of square cross section of size 20mm× 20mm and length of 40 cm were threaded at top to fasten them to the pile cap by nuts and bolts. Pile caps were made of 12 mm thick MS plate for single pile and 2× 1 and 2× 2 pile group at pile spacing of 3, 4 and 6 times pile size. Hoppers composed of aluminum plate with slots opening of 4 mm were utilized to pour sand by pluviation technique from a depth of 30 cm to achieve the required uniform density of 1.58 gm/cm3( RD≈55%). Pulleys of cast iron of capacity not less than 200 kg were used to apply the tensile load with the help of wire rope of 2mm diameter. The penetrometer was fabricated and calibrated to check the accuracy and uniformity of density of deposited sand during experiment. Two dial gauges of 0.01mm sensitivity in conjunction with magnetic base plate was used to record the settlement and tension of pile groups during application of load. The figure 1 depicts the details of experimental setup for present testing program.

EXPERIMENTAL PROCEDURE: Ultimate Capacity in Compression

To determine ultimate capacity in compression, the concerned pile group was

embedded in the sand and all the required setup was prepared. Piles and pile caps of given configuration and spacing were placed in empty tank in such a way that tank boundaries don’t affect the pile behavior. Then sand was poured from 30cm height of fall using pluviation to achieve the uniform density. Dial gauges with magnetic base plate were arranged to note the displacement of the pile cap. Now the experiments under compressive load and tension load were performed separately. For compressive load case, it was loaded with incremental compressive load until it failed and displacement of the pile cap was recorded with two dial gauge and averaged out to arrive at final displacement. From the load-displacement curve, the ultimate capacity in compression was determined using double tangent technique. Ultimate Tensile Capacity

To determine ultimate total tensile capacity, for pure tensile load (also the 0% compressive load case) concerned pile group was loaded with incremental tensile load through double pulley arrangement. Ultimate total tensile capacity was determined from load displacement curve for tensile load using single tangent technique. For tension cum compressive load compressive load was applied in stages of 25%, 50%, 75% and 100% of ultimate capacity in compression of respective pile


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group at a given spacing. Corresponding pile group was first loaded with the required stage of compressive load and allowed to settle, and then it was subjected to incremental tension till failure. Ultimate tensile load in case of tension cum compression was also determined from load-displacement curve using single tangent method as earlier for pure tension case. Net tensile capacity under tension cum compression was determined by deducting the stage of compressive load and weight of pile cap and weight of piles and other accessories of respective pile group from the total tensile capacity for a particular stage of compressive load.

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Legends 1. Wire rope 2. Aluminum strip 3. Pulley 1 4. Compressive load 5. Magnetic base plate 6. Dial gauge

7. Pile cap 8. Piles 9. Sand deposit 10. Model tank 11. Uplift load 12 .Pulley 2


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Fig. 1: Details of Experimental Setup

EXPERIMENTAL RESULTS Load Displacement Characteristics

Load displacement diagrams were plotted for all 42experiments. The general

trend of all load displacement curves was found to be similar. At the initial stage of loading the load displacement response of model piles was linear, which gradually deviated to a non-linear response with increased load, and again after some load increase it regained some linearity. Typical load displacement response is shown in figure 2 for 22 × pile group at 3d pile spacing under compressive load and tensile load with and without compressive load.

Figure 2: Load displacement responses for 2x2 pile group (3d spacing)

Net Tensile Capacity

Net tensile capacity is the central concern of the most research problems associated with tensile behavior of pile groups.

Variation of net tensile capacity: A general observation made from the entire experimental program was that for almost all cases net tensile capacity decreases with increase in stage of compressive loading. a) With pile group configuration


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Higher net tensile capacity for almost all stages of compressive load was associated to 2x2 pile group as compared to 2x1 pile group for a given pile spacing. The trend of variation (Figure 3) was same for 2x2 and 2x1-pile group at all spacing. The difference between the net tensile capacity for 2x1 and 2x2-pile groups lies in the range of 5-50% for 3d spacing, 40-60% for 4d spacing, and 75% to 80% for 6d spacing. This indicates that the pattern of variation of net tensile capacity for 2x1 and 2x2 pile groups attains higher similarity at higher spacing.

Figure 3: Net tensile capacity variation at 6d spacing

b) With pile spacing

Higher net tensile capacity was observed to be associated with higher pile spacing for a given pile group configuration (i.e. highest net tensile capacity for 6d spacing and lowest for 3d spacing). The same is described in figure 4 for 2x2 pile group. The net tensile capacity for 3d and 4d pile spacing for 2x2 pile group was practically same up to 50% compressive load.

Figure 4: Net tensile capacity variation for 2x2pile group


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Net Tensile Capacity per Pile Net tensile capacity per pile can be obtained by dividing the net tensile

capacity of a particular pile group by number of piles in that group.

Variation of net tensile capacity per pile: For all the conditions a decrease was observed in net tensile capacity per pile with increasing stage of compressive load. a) With pile group configuration

Observations revealed that2x1-pile group has the highest net tensile capacity

per pile for a given pile spacing, followed by 2x2- pile group. Net tensile capacity per pile of both the pile groups was lower than the net tensile capacity of single pile under the same conditions. (Shown in Figure 5)

Figure 5: Variation of net tensile capacity per pile @6d


For a given pile configuration higher spacing exhibits higher net tensile

capacity per pile (maximum at 6d spacing and minimum at 3d spacing). At all stages of compressive loads single pile has the highest net tensile capacity per pile. Typical variation of net tensile capacity per pile is shown in figure 6 for 2x2 pile group. The individual pile was contributing more in transferring tensile loads at higher spacing and the efficiency of overall group in tension got enhanced.


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Figure 6: Variation of net tensile capacity per pile for 2x2 pile group

Group Efficiency

Group efficiency ( gη ) is an important index to judge the effect of grouping on pile behavior.

Efficiency in compression

For 2x1-pile group efficiency increases linearly with pile spacing while in a

2x2- pile group efficiency first increases from 3d to 4d spacing then drastically decreases as the pile spacing increases to 6d (Shown in Figure 7).

Figure 7: Efficiency vs. spacing (compression)

Efficiency in compression for both the pile-groups was greater than 100% and it may go even up to 300%. This was due to localized compression of sand in near pile zone. The difference in 2x1 and 2x2 pile groups was due to dilative and contractive behavior of sand at different void ratios. Such observations were similar to Kezadi (1957), Combefort (1953), Vesic (1964), Lo (1967).


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Efficiency in tension Variation of efficiency in tension: For any pile group, at any spacing, efficiency in tension first increases from 0% to 25% compressive load and a continuous decrease was observed afterwards i.e. maximum efficiency was observed at 25% compressive load and minimum efficiency was observed at 100% compressive load. a) With pile group configuration

For a particular spacing, a 2x1-pile group has more efficiency as compared to

a 2x2-pile group at almost all stages of compressive load (Shown in Figure 8). The difference in efficiency for 2x1 and 2x2-pile groups may vary from 15% to 80%.

Figure 8: Efficiency in tension vs. % compressive load (3d spacing)

b) With Pile spacing

At a given spacing and stage of compressive load, efficiency increased with

increase in pile spacing. The highest efficiencies were observed at 25% compressive load at all spacing, then at 0% compressive load and least efficiencies were found at 100% compressive load (shown in Figure 9). It can be observed that the efficiency at 0% and 25% compressive load approaches unity at 6d pile spacing. So 6d-pile spacing may be designated as isolation spacing for both the pile groups.


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Figure 9: Efficiency in tension vs. Pile spacing (2x2 pile groups)

Unit Shaft Resistance (Unit Skin Friction) Unit shaft resistance or skin friction is the resistance being acted upon at

the unit surface of pile shaft. Under the action of tensile loads, the overall resistance comes from the shaft resistance only as the end bearing is nonexistent. Variation of unit shaft resistance: Normally unit shaft resistance decreases with increase in stage of compressive loading. a) With pile group configuration

For a given spacing, single pile has the highest unit shaft resistance, followed

by 2x1 pile group. The least unit shaft resistance was observed for a 2x2-pile group. This pattern was very prominent for 3d spacing. Typical comparison of unit shaft resistance is shown in Figure 10.

Figure 10: Unit shaft resistance vs. % Compressive loads (3d spacing)


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For a given stage of compressive load, a marked increase in unit shaft

resistance was observed with increasing pile spacing at all stages of compressive load. Figure 11 describes the variation of unit shaft resistance for a 2x2-pile group with pile spacing.

Figure 11: Unit shaft resistance vs. % Pile spacing (2x2 pile group)

CONCLUSIONS

The following conclusions were drawn based on the present experimental

study on tensile behavior of model single piles and pile groups with and without compressive load:

1. Net tensile capacity decreased with increase in compressive load. The maximum

and minimum values of the net tensile capacity were obtained at 0% and 100% compressive loads respectively.

2. For pile groups embedded in medium dense sand bed, the efficiency in compression was always greater than unity. The same may be even up to 300%. Thus medium dense sand strata were exceptionally good in transferring compressive loads.

3. Efficiency in tension initially increased from 0% to 25% compressive loads and continuously decreased upto 100% compressive load. The highest efficiency in tension was observed at 25% compressive load for almost all cases.

4. Efficiency in tension nearly approached 100% at 6d pile spacing at 0% and 25% compressive load. Hence 6d pile spacing may be suggested as the isolation spacing in tension for both the pile groups.

5. Unit shaft resistance under tensile loads decreased with increase in compressive load. Single pile had the highest skin friction followed by 2x1 group and least for 2x2 group.


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REFERENCES Awad, A., and Ayoub, A. (1976). ‘‘Ultimate uplift capacity of vertical and inclined

piles in cohesionless soil.’’ Proc., 5th Conf. on Soil Mechanics and Foundation Engineering, Budapest, Hungary, 221–227.

Alawneh, A.S., Malkawi, A.I.H. and AI-Deeky, H. (1999). “Tension Tests on Smooth and Rough Model Piles in Dry Sand.” Canadian Geotechnical Journal, 36: 746-753.

Das, B.M., Seeley, G.R. and Smith, J.E. (1976). " Uplift Capacity of Pile Groups in Sand." Journal of Geotechnical Engineering, ASCE, 102(GT3): 282-286.

Dash, B.K. and Pise, P.J. (2003). “Effect of compressive load on uplift capacity of model piles.” Journal of Geotechnical and Geoenvironmetal Engineering, ASCE, 129 (11): 987-992.

Ismael, N. F. (1989). “Skin friction of driven piles in calcareous sands.” Journal of Geotechnical Engineering, ASCE, 115(1): 135-139.

Kraft, L. M. (1991). ‘‘Performance of axially loaded pipe piles in sand.’’ Journal of Geotechnical Engineering, ASCE, 117(2): 272–296.


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