Selected Full-Scale Lateral Load Test Case Studies

Gill (1968) presented the results of lateral load tests carried out at Hamilton Air Force Base and Naval Civil Engineering Laboratory in two papers. In Gill (1968), the San Francisco Bay pile tests were performed to study the horizontal load-displacement characteristics of natural soil deposits and to associate these characteristics with the behavior of laterally loaded piles. 4.5, 8.6, 12.8 and 16 in diameter open ended pipe piles were driven in both the dry area and flooded area. In the flooded area, no tests were carried out until the shear strength of the soil stabilized. Each pile was sufficiently embedded to insure flexible rather than rigid behavior. Lateral loads were applied 30 in. above the ground surface so that the loading consisted of both a horizontal load and a bending moment. Displacement and slope at ground surface were measured versus load. The horizontal displacements determined experimentally and the theoretically for all pile sizes were in fairly close agreement.

Singh and Verma (1973) reported the results of lateral load tests on single piles and pile groups; of mild steel pipes, 2.5 in outside diameter and 16.5 ft. long. The group consisted of four piles arranged in a square pattern at three diameters center to center

spacing with a rigidly welded pile cap. The pile groups and single piles were subjected to incremental lateral load applied at ground surface. The horizontal deflection and rotation of the cap at ground level were measured. Plots showed the pile group with pile spacing of three diameters offers less resistance to deflection compared to a single pile under similar conditions of loading. The results also showed that with an increase of deflection, the resistance of both single piles and pile groups decreased, with the resistance of groups decreasing faster than that of the single piles.

Cox et al. (1974) conducted lateral load tests on two 24 in diameter steel pipe piles with a wall thickness of 0.75 in., driven into sand. One pile was subjected to cyclic loads and the other was loaded statically. The piles penetrated to a depth of 69 ft. into clean fine sand to silty fine sand below water. The friction angle of the sand was 39o (Reese et al., 1974) and the buoyant unit weight was 66 pcf. The lateral load was applied at 1 ft. above the ground surface. The calculated values of lateral loads using the Characteristic Load Method which uses dimensional analysis to characterize the nonlinear behavior of laterally loaded piles by means of relationships among dimensionless variables were compared to measured values for lateral load. The results showed that the calculated deflections were about 10% higher than the measured values. The calculated maximum bending moments agreed quite well with the measured values for maximum bending moments.

Reese et al. (1975) conducted lateral load tests on two 24 in. and one 6 in. diameter pipe piles driven into stiff clay. The piles were instrumented to measure bending moments. On both the 6 and 24 in piles, short-term and cyclic loads were applied and the water table was maintained a few inches above the ground surface. The two 24 in. piles

were placed horizontally and connected at the ends to create simple beam supports, the two were then jacked apart with hydraulic ram and a load cell in series. The 6 in. pile was connected to a 24 in. pile by tension straps, and a jack was placed between the piles to push the piles apart. The results of the tests were analyzed to obtain the families of curves showing the soil resistance p as a function of pile deflection y. In the case of the 24 in. piles, the comparison between the computed and the measured p-y curves showed excellent agreement. While there was also a reasonable agreement for maximum bending moment for the 6 in. pile, the deflection at ground-line was poor.

Reese and Nyman (1978, as referenced in Reese and VanImpe, 2001) reported the results of an instrumented drilled shaft installed in vuggy limestone in the Florida Keys. The test was performed to gain information for the design of foundations for highway bridges. The drilled shaft diameter was 4 ft. and penetrated about 43.7 ft. into the limestone. A maximum lateral load of 150 kips was applied to drilled shaft at about 11.5 ft. above the limestone elevation. The maximum deflection at the point of load application was about 0.71 in, and about 0.02 in at the top of the rock. Although the load versus deflection curve was nonlinear, there was no indication of rock failure.

The Mechanical Research Department, Ontario, Canada, in an effort to examine the foundation behavior of rigid piers, carried out a full scale tests on two instrumented 5.0ft. diameter drilled shafts. The test results, analyzed and reported by Ismael and Klym (1978), were used to determine the accuracy with which the elastic method and the p-y method could predict the pier is lateral response. Lateral loads were applied to the piers at the ground surface. Displacement readings were taken after each 10 kip load increment. At 40 kips, the load was cycled. The incremental load was increased from 20 kips to a

maximum of 160 kips. The elastic solution was unable to model the true non-linear behavior of the pier and the p-y method only provided a conservative estimate.

Brown et al. (1987) reported the results of cyclic lateral load tests on a large- scale pile group and a single pile. The piles consisted of nine 10.75 in. diameter 0.365 in. thick steel-pipe piles in a closely-spaced arrangement. The piles were installed close- ended in a 3 by 3 arrangement with spacing of 3-pile diameter centers to a depth of 43 ft. The results showed greater deflection under the load of piles in group than that of a single pile under a load equal to the average load per pile. Also, the bending moments in the piles in the group were greater than those for the single pile.

Brown et al. (1988) reported the results of a large-scale group of steel pipe piles and an isolated single pile subjected to two-way cyclic lateral loading. The tests were carried out in a submerged firm to dense sand that was placed and compacted around the piles. The pile group consisted of nine 10.75 in. diameter 0.365 in. thick steel pipe piles, arranged in a 3 by 3 group and spaced at three times the diameter. The ultimate objectives of the test were to compare the response of the piles in the group with the response of the single pile and measure the variation in soil resistance within the group. The piles were instrumented to measure the distribution of load to each pile, bending stresses along the length, and the slope at the top for comparison.

Several conclusions that were presented in the report are, the deflections of the piles in the group were significantly greater than that of the single pile under equal average load; the reduced efficiency of the pile group was due to the effect of shadowing; and the piles in the leading row had similar bending moment with the single

pile under the same load per pile. Due to the two-way cyclic loading, significant densification occurred in the sand.

Caltrans (Speer 1992 as referenced in Reese and Vanimpe, 2001) performed lateral load tests on two 7.4 ft. diameter drilled shafts. Shaft A, penetrated 41 ft. into the rock, and shaft B penetrated about 45 ft. into the rock. Both drilled shafts were tested simultaneously. Load was applied incrementally at 4.6 ft. above the ground line for shaft A and 4.1 ft. for shaft B. The load test results showed that shaft A apparently had a structural weakness, so only shaft B was used in developing the recommendations for p-y curves. Groundline deflection of 0.7 in. was measured at a 1,800 kips lateral load, but the deflection increased to about 2.0 inches at a lateral load of about 2,010 kips.

Ruesta and Townsend (1997) reported full-scale lateral load tests on a single pile and pile group consisting of 16 (4 x 4) prestressed 30 in. square concrete piles 54 ft long at the Roosevelt Bridge in Stuart, Florida. The objectives of the test were to provide a better understanding of the lateral resistance of closely spaced (3 diameters) driven piles in a group and whether it could be numerically related to the behavior of a single isolated pile through p-y multipliers, evaluate techniques for determining p-y curves based on in situ tests, verify the latest version of the program FLPIER and provide a general guideline for future load tests and lateral load design recommendations. The test program consisted of a single isolated 30 in. square pile and two 16 pile groups with three diameter spacing. From the lateral load tests, it was concluded that the average pile group response was softer than the single pile response, the p-y multipliers worked well to account for the group effect, and the maximum bending moments for the leading row were higher than the trailing rows.

Rollins et al. (2005a and b) reported the results of lateral load tests performed on a full-scale pile group and single pile in liquefied and preliquefied sand. The studies show the effect of liquefaction as the piles were loaded laterally. In the test before liquefaction, the objective was to evaluate pile-soil-pile interaction effects and improve the understanding of pile group behavior. The test pile was a 12.75 in. outside diameter steel pipe with a 0.375 in. wall thickness driven open ended to a depth of 37.7 ft. below the excavated ground surface. The pile group was arranged at 3 by 3 at 3.3 diameters spacing. The piles were driven into a soil profile of loose to medium dense sand underlain by clay and were instrumented to measure the distribution of load to the top of each pile, bending stresses along the length of each pile, and the slope at the top of each pile for comparison. Pre-liquefaction results showed a reduction in lateral resistance for the pile group relative to the single pile due to the group interaction effects. In addition, outer piles in the row carried about 20-40% greater lateral load than the middle pile in each row. This shows that lateral resistance was a function of position within the row. In contrast to pre-liquefaction tests, group interaction effects were insignificant after liquefaction. The lateral resistance of each pile in the group was similar and about the same as for the single pile.

Rollins et al. (2008) carried out lateral static and Stat NAMIC load tests on two 8.5 ft. diameter drilled shafts at the Cooper River Bridge site in Charleston, South Carolina after liquefying the soil to a depth of 42 ft. using controlled blasting. The intent was to determine the impact of soil liquefaction (similar to that from an earthquake) on the lateral response of the drilled shafts. The interpreted static load-deflection curve indicates that the liquefied sand provided significant lateral resistance and that the

reasonable estimate of response could be obtained using a p-y curve for liquefied sand (Dr ≈ 50%) developed by Rollins et al (2005) which include diameter effects.