Static and Dynamic Lateral Load Behavior of Pile Groups Based on Full-Scale Testing. Kyle M. Rollins & Steven R. Johnson. Civil & Environmental.Engineering ...
Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference Honolulu, Hawaii, USA, May 25 –30, 2003 Copyright © 2003 by The International Society of Offshore and Polar Engineers ISBN 1 –880653 -60 –5 (Set); ISSN 1098 –6189 (Set)
Static and Dynamic Lateral Load Behavior of Pile Groups Based on Full-Scale Testing Kyle M. Rollins & Steven R. Johnson Civil & Environmental.Engineering, Brigham Young University Provo, Utah, USA
Kris T. Petersen Region 2, Utah Dept. of Transportation Salt Lake City, Utah, USA
Thomas J. Weaver Civil Engineering Dept., University of Idaho Moscow, Idaho, USA
ABSTRACT The lateral load resistance of pile groups is thought to decrease as spacing decreases due to group interaction effects. Two lateral load tests were performed on full-scale pile groups spaced at 2.8 and 5.65 pile diameters to quantify the decrease in resistance and determine appropriate p-multipliers to account for the decrease. The backcalculated p-multipliers increased from 0.4 at a spacing of 2.8D to about 0.92 as spacing increased to 5.65D for trailing row piles. Design curves show the variation in p-multipliers with spacing. Using the simple p-multiplier approach, computer models match measured response. Dynamic lateral resistance was found to be larger than static capacity even for reloaded piles.
Leading Row
Overlapping ("Shadow") Stress Zones
Trailing Row Pile
KEYWORDS: Piles; Foundations; Lateral Load; Pile Groups; Dynamic Load; Load Test.
Trailing Row
INTRODUCTION
Soil Gapping
The lateral load capacity of pile foundations is critically important in the design of structures that may be subjected to earthquakes, high winds, wave action, and ship impacts. Although reasonable methods have been developed for predicting the lateral capacity of single piles under static loads, there is very little information to guide engineers in the design of closely spaced pile groups. Because of the high cost and logistical difficulty of conducting lateral load tests on pile groups, relatively few full-scale load test results are available that show the distribution of load within a pile group. Nevertheless, the data from these limited field tests and some model studies indicate that piles in groups will undergo significantly more displacement for a given load per pile than will a single isolated pile (Brown, et al, 1987; Brown, et al, 1988; Meimon, et al, 1986; Rollins et al, 1998).
Direction of Loading
Fig.1 Illustration of reduction in lateral pile resistance due to interference with trailing row shear zones. variation in group interaction with spacing. Most full-scale pile group tests have been performed on pile groups at a spacing of 3D. Due to the lack of full-scale data, design guidelines and available computer programs to account for pile group interaction have not been thoroughly validated. As a result, engineers tend to design pile groups in a conservative manner to deal with the uncertainty. Although numerical and centrifuge models can provide some guidance regarding these issues, a reasonable number of full-scale load tests are necessary to verify these models and provide ground truth information. To improve our understanding of pile group behavior, a series of static and dynamic lateral load tests were performed on two full-scale pile groups at the Salt Lake International Airport. Results from the first pile group test were reported previously by Rollins et al (1998). This paper compares and contrasts the results from the previous testing with results from new tests and presents design guidelines based on all test data.
As closely spaced pile groups move laterally, the failure zones for trailing row piles overlap with leading row piles and decrease lateral resistance as shown in Fig. 1. The tendency for a pile in a trailing row to exhibit less lateral resistance due to location behind another pile is commonly referred to as “pile-soil-pile interaction” or the “group interaction effect”. Group interaction effects would be expected to become less significant as the spacing between piles increased; however, only model and centrifuge tests are available to define the 506
depth of about 10 m but the OCR decreases rapidly with depth. The shear wave velocities in the cohesive layer were between 120 and 150 m/s.
PILE GROUP PROPERTIES The piles in the first group were driven in a 3 x 3 pattern with a nominal spacing of 2.8 pile diameters center to center. An isolated single pile was also driven approximately 1.8 m from the pile group. The test piles were 324 mm O.D. closed-end steel pipes with a 9.5mm wall thickness and were driven to a depth of approximately 9.1 m. The elastic modulus (E) of the steel was 200 GPa and the minimum yield strength (Fy) was 331 MPa. The piles were filled with concrete prior to testing and an inclinometer casing was installed at the center of each pile. Tests conducted on concrete cylinders determined that the compressive strength of the concrete at the time of testing was 20.7 MPa (3000 psi) and the elastic modulus was 17.5 GPa (2,500,000 psi).
The underlying cohesionless soil layer consists of poorly graded medium-grained sands and silty sands classifying as SP or SM according to the USCS. SPT blowcounts and CPT tip resistances indicate that the sand is dense to very dense with corresponding relative densities (Dr) of 65 to 85%. The measured shear wave velocity was approximately 150 m/s. LOAD TEST SET-UP AND INSTRUMENTATION
Load Test Set-up
The piles in the second group were also driven in a 3 x 3 pattern with a nominal spacing of 5.65 pile diameters center to center in the direction of loading. An isolated single pile was also driven near the pile group as a reference. The test piles were 324 mm O.D. closedend steel pipes with a 9.5-mm wall thickness and were driven to a depth of approximately 11.9 m. The elastic modulus (E) of the steel was 200 GPa and the average yield strength (Fy) was 400 Mpa (58,700 psi). The moment of inertia of the piles was 1.16 x 108 mm4 (279 in4). However, two angle irons were attached to each pile to protect the strain gages, which increased the moment of inertia to 1.43 x 108 mm4 (344 in4). Based on the soil profile and properties, pore water pressures generated during the driving operations had adequate time to dissipate prior to load testing; however, no pore pressure measurements were made.
The load was applied to the pile groups using hydraulic jacks reacting against a sheet pile wall in one case and a 1.3 m diameter drilled shaft in another case. The hydraulic jacks applied load to a steel frame consisting of I-beams and channel sections that were essentially rigid in comparison with the lateral pile stiffness. Load was transferred from the load frame to each pile using a pinned connection (zero moment) tie-rod. Each tie-rod was instrumented with strain gages so that the load carried by each pile could be independently measured. The load was applied at 0.4 m and 0.48 m above the ground surface for the first and second pile groups, respectively. The load frame was constructed with lubricated steel casters that traveled on steel beams placed on the ground surface. This provided a low-friction mechanism for load transfer. A free-head load test was also performed on adjacent isolated single piles with the load applied at the same heights for comparison purposes.
SOIL PROFILE AND PROPERTIES
Instrumentation
A geotechnical investigation was carried out to characterize the soil profile and properties at the test site. This investigation consisted of conventional sampling and laboratory testing as well as in-situ testing. Conventional sampling included 76.2 mm (3 inch) diameter thinwalled shelby tube undisturbed samples and disturbed soil samples. In-situ tests included standard penetration testing (SPT), cone penetrometer testing (CPT), dilatometer testing (DMT), pressuremeter testing (PMT), vane shear testing (VST), and downhole wave velocity measurements. Laboratory testing performed on the field samples consisted of particle size distribution, Atterberg limit, soil classification, shear strength, and consolidation testing. A detailed summary of all the geotechnical testing is provided elsewhere (Peterson and Rollins, 1996)
The total load applied to the group was measured using load cells and the load carried by each pile was measured with tie-rod load cells. Generally the measured loads were typically within about 5% of one another. Pile head deflection and rotation were measured using LVDTs attached to an independent reference frame. In the first pile group (2.8D spacing), inclinometer casing was installed at the center of the single pile and each of the nine piles in the group. Inclinometer readings were taken at 0.61 m intervals at each load increment. “Sister-bar” type strain gages were installed in the single pile and in one pile in each of the three rows in the group. Strain gages in the group were located at eight depths below the natural ground surface.
Based on the results of all the field and laboratory testing, a composite soil profile was developed as shown in Fig. 2. The gravel fill shown in Fig. 2 was excavated prior to the pile driving. Because laterally loaded piles receive most of their support from the soil in the upper 10 to 15 pile diameters, the shallow surface layers are of greatest interest for this study. The soil profile near the surface consists of layers of low-plasticity silts and clays underlain by a sand layer. The water table was located near the original natural ground surface during the testing.
In the second pile group (5.65D Spacing), strain gages were placed on opposite outside faces of the pile at 14 depth levels below ground level to determine bending moment profiles versus depth. To minimize difficulties with analysis, the steel pipes were not filled with concrete so inclinometers could not be placed in the piles.
LOAD TEST PROCEDURE For the first pile group test (2.8D spacing), load was applied incrementally in one direction with a hydraulic jack until deflection targets were reached. Once the full static load test was completed in one direction, the statnamic sled was placed against the load frame and two test firings were performed in the opposite direction to the static loading. This approach provided a comparison of static and dynamic resistance for nearly virgin loading conditions.
The cohesive surface soils consisted of low-plasticity silts and clays classifying as ML, CL-ML, or CL according to the Unified Soil Classification system (USCS). Hydrometer analyses indicate that a majority (50 to 75%) of the cohesive soil near the ground surface (1.7 to 4.5 m) consisted of silt-size particles with a clay-size content generally between 10 and 25. As shown in Fig. 2, the undrained shear strength is typically between 25 and 60 kPa, although some surface layers had strengths of over 100 kPa likely due to dessication. Consolidation testing indicates that the soils are overconsolidated to a
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Fig. 2 Soil profile and results from field and laboratory testing at the Salt Lake City, Utah airport test site.
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For the second pile group test (5.65D spacing), load was also applied incrementally in one direction with a hydraulic jack until a target deflection was reached. Then the load was released and 14 additional cycles were applied to reach the same deflection level. At several deflection levels, the statnamic sled was then used to apply a 16th load cycle in the same direction as the static loading. The statnamic force was selected so that the deflection would be approximately the same as that for the static test. This approach provided a comparison of static and dynamic resistance for repeated loading conditions.
Figs. 5 and 6 present the average pile load-deflection curves for each row of piles in comparison with the respective single pile loaddeflection curve for the pile groups at 2.8 and 5.65D spacings, respectively. For the pile group at 5.65D spacing (see Fig. 6), group effects are relatively small and the load-deflection curve for the front row piles is quite similar to that for the single pile. However, the trailing row piles do exhibit somewhat lower resistance for a given deflection. The lateral resistance decreases progressively from the front to the back row piles. Group effects appear to become more significant as the deflection increases. For the larger spacing, greater movement is likely necessary for shear zones to overlap. For the pile group at 2.8D spacing (see Fig. 5), there is a much greater decrease in the load carried by the rows in the group relative to that in the single pile. For a given displacement, front row piles carried the greatest load while middle and back row piles carried significantly lower loads. In fact, the back row pile actually carried somewhat higher load than the middle row pile for a given deflection.
STATIC TEST RESULTS Figs. 3 and 4 present plots of load vs. deflection for the static load test on the 9 pile groups at 2.8D and 5.65D spacings, respectively. Load vs. deflection curves for the respective single pile tests are also shown for comparison. To facilitate comparisons, an average load was computed by dividing the total load on the pile group by the number of piles. For the pile group at 2.8D spacing, the average load carried by each pile in the group is noticeably smaller than the single pile. In fact, for the same average pile load, the displacement of the pile group is 2 to 2.5 times higher than that of the single pile. In contrast, for the pile group at 5.65D spacing, the average load carried by the pile group is just slightly smaller than that of the single pile. This result highlights the fact that group interaction effects decrease as pile spacing increases.
No consistent trends were observed in the load distribution among piles in the same row. This finding is consistent with results from previous full-scale tests (i.e. Brown et al, 1987, Brown et al, 1988); however, they conflict with elasticity-based solutions which predict that corner piles will carry more load than average while center piles will carry less. 200
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Fig. 3 Average pile load vs. deflection curve for pile group at 2.8D spacing relative to curve for single pile.
Fig. 5 Average pile load vs. deflection curve for each row of piles in the group at 2.8D spacing relative to the single pile curve.
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Fig. 4 Average pile load vs. deflection curve for pile group at 5.65D spacing relative to curve for single pile.
Fig. 6 Average pile load vs. deflection curve for each row of piles in the group at 5.65D spacing relative to the single pile curve.
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ANALYSIS OF STATIC TESTS
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Lateral pile response is typically analyzed using finite-difference or finite element models of the pile along with non-linear springs to represent the resistance provided by the soil. The load-displacement curves for the soil are known as p-y curves, where p is the horizontal soil resistance (force per length) and y is the horizontal displacement. Generic p-y curves have been developed for soft clays, stiff clays, and sands and have been widely incorporated in computer models. For closely spaced piles, Brown et al (1987) proposed that the p-y curve for a pile in a group be obtained using p-multipliers (PM) to reduce all the p-values on a single pile p-y curve. With this approach it is possible to reduce the computed load carrying capacity of the piles in a group relative to the single pile capacity as observed in load test results.
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In this study, p-multipliers were back-calculated using computer models. Initially, computer models were used to obtain a good match between computed and measured response for the single pile test. The soil profiles used in the analysis were developed using lab and field data to model the variations in the stratigraphy. Although the same soil profile was used for both pile groups, it was necessary to vary the strength in the upper clay layer to obtain a reasonable match with the measured response. The p-y curves used in the soil profile models were calculated using the equations developed by Matlock (1970) for the clay layers and Reese et al (1974) for the sand layers. All single pile analyses were performed using the computer program LPILE (Reese and Wang, 1994).
Fig. 8 Comparison of measured load-deflection curve for single pile near pile group at 2.8D spacing with curve computed using LPILE (Reese and Wang, 1994). The first estimate of p-multipliers came from taking the ratio of the average load carried per pile in each row of the pile group to the load carried by the single pile at the same pile head displacement. These ratios were initially input into the computer program GROUP (Reese et al, 1996) as user-defined p-multipliers. These p-multipliers were then decreased until the computer analysis yielded results similar to the measured pile group response. As these adjustments were made, the ratios of the middle and back row p-multipliers to the front row pmultipliers were generally maintained equal to the respective row load ratios measured in the group load test. The p-multipliers are smaller than the row load ratios because the lateral resistance of the pile itself does not decrease. Therefore, the total decrease in lateral resistance must result from a decrease in soil resistance. The back-calculated pmultipliers for each row in each group are summarized in Table 1. The p-multiplier for the first row at 2.8D spacing is the lowest backcalculated value from any full-scale test.
Pile head load vs. deflection curves computed for the companion single pile tests near the two pile groups are presented in Figs. 7 and 8. As shown in Figs. 7 and 8, variations in the soil strength profiles made it possible to obtain an excellent match between the measured and computed load vs. deflection curves in both cases. Once the single pile match had been obtained, the soil profile and properties were held constant in the analysis of the pile group behavior. The match between measured and computed pile group behavior was then obtained by varying the p-multipliers.
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Table 1 Summary of back-calculated p-multipliers for each row in each pile group the for the Salt Lake City airport tests. Pile Group Front Row Middle Row Back Row Spacing/Pile Diameter p-multiplier p-multiplier p-multiplier (S/D) 2.8 0.6 0.38 0.43 5.65 0.98 0.95 0.88
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The computed total load vs. deflection curves obtained using the pmultipliers in Table 1 are shown in Figs. 9 and 10 for the pile groups at 5.65D and 2.8D, respectively, along with the measured curves. In both cases, the agreement is very good. In addition, with these pmultipliers the computed maximum moment is also quite close to the measured values. These results illustrate that even with a relatively simple adjustment factor like the p-multiplier, reasonable estimations of pile group response can be obtained.
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Fig. 7 Comparison of measured load-deflection curve for single pile near pile group at 5.65D spacing with curve computed using LPILE (Reese and Wang, 1994).
Based on the results of the static pile group load testing and computer analysis, preliminary design curves showing p-multipliers for leading and trailing row piles as a function of pile spacing were developed for low plasticity silts and clays as shown in Fig. 11. These curves are
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Fig. 9 Comparison of measured load-deflection curve for pile group at 5.65D spacing in comparison with curve computed using GROUP (Reese et al, 1996) with p-multipliers developed in this study.
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Fig. 11 P-multiplier values as a function of spacing for (a) leading row piles and (b) trailing row piles based on results of full-scale pile group load tests conducted in this study in comparison with recommendations by Reese et al (1996) and AASHTO (2000).
Fig. 10 Comparison of measured load-deflection curve for pile group at 2.8D spacing in comparison with curve computed using GROUP (Reese et al, 1996) with p-multipliers developed in this study. only slightly reduced from interim design curves proposed by Rollins et al (1998). Curves showing p-multipliers vs. normalized spacing recommended by Reese et al (1996) and AASHTO are also shown in Fig. 11. The recommendations by Reese et al (1996) and AASHTO (2000) are based on scale model tests and theoretical considerations. The curves based on the full-scale testing in this study suggest that recommendations by Reese et al (1996) will significantly overestimate the lateral resistance of piles in groups. This conclusion is also consistent with back-calculated p-multipliers from other fullscale tests in other soil types (Brown et al, 1987; Brown et al, 1988; Meimon et al, and Ruesta and Townsend, 1997). In contrast, the AASHTO approach appears to significantly underestimate the resistance of piles in groups particularly for leading row piles. The AASHTO p-multiplier of 0.25 at 3D spacing is lower than has been measured by any of the full-scale tests conducted to date.
stiffness will lead to a longer period of vibration. Use of a longer period will typically reduce the seismic design forces in comparison to what might actually be the case. Because it is not always possible to assess in advance what might ultimately be conservative or nonconservative, the best approach is to use the most accurate estimate of the p-multiplier.
DYNAMIC LOAD TEST RESULTS As the fuel pellets in the statnamic sled combusted, a dynamic horizontal force was produced against the pile group foundation and the 160 kN sled moved away from the pile group. The resulting load pulse duration was typically 300 milliseconds with a rise time of 130 milliseconds. The maximum accelerations for the two test firings were 0.65 and 0.87 g’s. These acceleration levels were similar to what would be produced in an earthquake. However, the maximum velocities were 200 and 350 mm/s, respectively which are lower than
Using a lower p-multiplier might initially appear to be an acceptable conservatism. However, the use of an inappropriately soft foundation
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relative ground velocities that might be produced in an earthquake. Therefore, the damping contribution, which is proportional to velocity, would likely be greater than measured in the testing.
and the peak dynamic resistance eventually exceeds the peak loaddeflection curve for the 15th cycle of static loading. In fact, the dynamic peak resistance is typically 25 to 35% higher than the static peak resistance at the same deflection. This increase is about the same as was observed for the dynamic lateral load testing of the pile group under virgin load conditions. These results suggest that damping resistance can still produce significant dynamic resistance even when gaps are initially present around the piles.
Continuous load vs. deflection curves are shown in Fig. 12 for the two statnamic lateral load tests conducted on the pile group at 2.8D spacing. It should be noted that the maximum load does not coincide with the maximum displacement. Although the statnamic load is decreasing, the momentum of the pile group produces additional horizontal movement. The maximum displacement is reached when the pile group velocity drops to zero just before the group rebounds in the opposite direction. Since static lateral load tests were performed previously on the same free-head pile group with the load oriented at 180° to the direction of the statnamic loading, static lateral load vs. deflection curves are also presented in Fig. 12 for comparison purposes. The dynamic resistance is typically 20 to 30% higher than the static resistance at a given displacement. In fact, the dynamic load-deflection curves approach the stiffness of the curve for the single isolated pile. Apparently, any reduction in resistance that might have developed due to group interaction was more than compensated for by damping and inertia forces during dynamic loading.
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Fig. 13 Comparison of load-deflection curves for dynamic (statnamic) lateral load tests on the pile group at 5.65D spacing relative to the peak resistance for the 15th cycle static load cycle.
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The plots in Fig. 14 provide a comparison of the full load-deflection curve for the 15th cycle of static loading to the full load-deflection curve for the 16th cycle which was produced by the statnamic loading device. The size of the loop for the dynamic case relative to the static curve clearly suggests that significant damping resistance is present during the dynamic loading.
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Fig. 12 Comparison of load-deflection curves for static and dynamic (statnamic) lateral load tests on the pile-group at 2.8D spacing.
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The large loops formed by the dynamic load-deflection curve for the statnamic load tests in Fig. 12 suggest that there is a significant damping component as the piles interact with the surrounding soil. However, that series of dynamic load tests was performed for virgin load conditions where the soil was initially in contact with the soil prior to the loading. For repeated loadings, as in the case of earthquakes, gaps would likely develop in the clay surrounding the piles and this could conceivably decrease the dynamic resistance. To evaluate the dynamic resistance for cases involving repeated loadings, two dynamic (statnamic) lateral load tests were performed on the pile group at 5.65D spacing after 15 cycles of static loading had already been applied and gaps had formed. Fig. 13 presents the load-deflection curve for the pile group obtained by connecting the peak load for the 15th cycle of loading at each increment. In addition, Fig. 13 shows the continuous load-deflection curve for two dynamic load tests conducted after 15 cycles of static loading to deflection levels of 25 mm and 100 mm, respectively. The initial segment of the dynamic load-deflection curves clearly show the presence of a gap and the dynamic lateral resistance is significantly reduced; however, as the deflection level increases, the pile makes contact with the soil
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Fig. 14 Continuous load-deflection curves for the 15th cycle of static loading in comparison with the 16th cycle of dynamic (statnamic) loading for two load levels on the pile group at 5.65D spacing.
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BYU, David Anderson, for data acquisition and test set-up assistance for both field load tests.
CONCLUSIONS 1.
2.
The lateral resistance of a pile in a closely-spaced group in low plasticity silt and clay is a function of row location within the group, rather than location within a row as suggested by elastic theory.
REFERENCES AASHTO (2000) Bridge Design Specifications, American Assoc. of State Highway and Transportation Officials, Washington, D.C.
The average load-deflection curves for rows in the pile group were softer than the comparable single pile curve but the difference became less significant as pile spacing increased from 2.8 to 5.65 pile diameters.
Brown, DA, Morrison, C, and Reese, LC, (1988). "Lateral load behavior of a pile group in sand, J. Geotech. Engrg., ASCE, 114(11), 1261-1276.
3.
The leading row piles in the groups carried the greatest load, while the middle and back row piles carried smaller loads for a given displacement due to pile-soil-pile interaction.
Brown, DA, Reese, LC, and O'Neill, MW, (1987). "Behavior of a large scale pile group subjected to cyclic lateral loading", J. Geotech. Engrg., ASCE, 113(11), 1326-1343.
4.
Back-calculated p-multipliers based on the test results increased as the pile spacing increased from 2.8 to 5.65 diameters. Extrapolation of the test results suggests that group reduction effects can be neglected for pile spacings greater than about 6.5 pile diameters.
Matlock, H (1970). “Correlations for design of laterally-loaded piles in soft clay,” Procs., Second Annual Offshore Technology Conf., Paper No. OTC 1204, Vol. 1, 577-594.
5.
6.
7.
Meimon, Y, Baguelin, F, and Jezequel, JF, (1986). "Pile group behaviour under long time lateral monotonic and cyclic loading," Proc., Thrd Int'l. Conf. on Numerical Methods in Offshore Piling, Inst. Francais Du Petrole, Nantes, p. 286-302.
Recommendations for p-multipliers as a function of spacing made by Reese et al (1996) significantly overestimate the lateral resistance for closely spaced pile groups, while recommendations provided by AASHTO (2000) greatly underestimate the pmultipliers based on the full-scale load tests conducted during this study.
Peterson, KT and Rollins, KM (1996). “Static and dynamic lateral load testing of a full-scale pile group in clay”, Civil Engineering Dept. Research Report CEG.96-02, Brigham Young University, Provo, Utah.
Using the simple p-multipliers obtained in this study, reasonable estimates of the load-deflection and maximum bending moment values could be obtained using lateral pile analysis programs such as LPILE (Reese and Wang, 1994) and GROUP (Reese et al, 1996).
Reese, LC, Cox, WR, and Koop, FD (1974). “Analysis of laterally loaded piles in sand,” Procs., Fifth Annual Offshore Technology Conf., Paper No. OTC 2080, Houston, Texas. Reese, LC and Wang, ST (1994) Documentation of computer program LPILE version 4.0 for windows, Ensoft, Inc., Austin, Texas, 365 p
Dynamic lateral resistance of the free-head pile groups was typically 20 to 35% higher than the static lateral resistance for both virgin load conditions and re-loading conditions at the peak deflection.
Reese, LC., Wang, ST, Arrellaga, JA, and Hendrix, J (1996) “Computer program GROUP for Windows, User’s Manual, version 4.0”, Ensoft, Inc., Austin, Texas, 370 p.
ACKNOWLEDGEMENTS
Ruesta, PF and Townsend, FC (1997). “Evaluation of laterally loaded pile group at Roosevelt Bridge, J. Geotech. and Geoenv. Engrg., ASCE, 123(12), 1153-1161.
We express appreciation to the Salt Lake Airport Authority for allowing use of the test site. Funding for the testing described in this paper was provided by the FAA, MCEER, Utah DOT and the National Science Foundation under Grant No. CMS-0100363. This support is gratefully acknowledged. The conclusions from this study do not necessarily reflect the views of the sponsors. The statnamic testing was coordinated with Applied Foundation Testing, Inc. and Berminghammer, Inc. Finally, we thank our Dept. technician at
Rollins, KM, Peterson, KT, and Weaver, TJ (1998) “Lateral Load Behavior of a Full-Scale Pile Group in Clay”, J. Geotech. and Geoenv.. Engrg. ASCE, (124)6, 468-478.
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