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OCR for page 23
23 Horizontal deflection (in) A steel reinforcing cage was installed at the top of each test -1.0 0.0 1.0 2.0 3.0 4.0 pile to connect the test piles to the pile cap. The reinforcing -5 cage consisted of six #8 reinforcing bars that were confined within a #4 bar spiral with a diameter of 8 in. and a pitch of 6 in. The test piles typically extended about 2 ft above the base of the pile cap and the reinforcing cage extended 2.25 ft 0 above the base of the cap and 8.75 ft below the base. The steel Depth from Ground Surface (ft) pipe pile was filled with concrete that had an average uncon- fined compressive strength of 5000 psi. 5 A pile cap was constructed by excavating 2.5 ft below the 0.5 in surface of the surface clay layer. The concrete was poured 0.75 in directly against vertical soil faces on the front and back sides of 1.0 in 1.5 in each pile cap. This procedure made it possible to evaluate pas- 10 2.0 in sive force against the front and back faces of the pile caps. In 2.5 in contrast, plywood forms were used along the sides of each cap String Pot Above Load Pt. String Pot At Load Pt. and were braced laterally against the adjacent soil faces. This 15 construction procedure created a gap between the cap side- wall and the soil so that side friction would be eliminated. Steel reinforcing mats were placed in the top and bottom of each cap. 20 Figure 3-12. Deflection vs depth curves at several deflection increments for single pile 3.5 Pile Group Testing Procedure lateral load test. Lateral load was applied using MTS actuators with the load centered at a height of 11 in. above the top of the pile cap. Each of deflection increments. The shape array provides horizon- actuator could produce 600 kips in compression and 450 kips tal deflection values at 1 ft intervals from the top of the pile, in tension. Another pile group or groups provided a reaction for which was approximately 40 in. above the load point. the applied load. In all cases, the reaction pile group or groups Without any corrections, the computed deflection curves were located 32 ft away from the test pile group to minimize obtained from the shape arrays are consistent with the max- interference between the two pile groups during lateral load- imum pile head deflections measured by the string poten- ing. Each actuator was fitted with two 8.67-ft extension pieces tiometers at the load point. The deflected shape curves also to span the 32.1-ft gap between the pile groups. The actuator are consistent with the free-head (zero-moment) boundary was attached to a concrete corbel atop each pile cap using steel condition. tie-rods that extended through PVC sleeves in the corbel and were bolted to the back face of the corbel. This allowed load to be applied without affecting the soil around the pile group. The 3.4 Pile Group Properties tie-rods were prestressed to minimize displacement of the steel A total of 16 lateral load tests were performed on the 4 pile during the load tests. A three-dimensional swivel head was groups. Schematic drawings of the pile group layout and the located at each end of the actuator to provide a zero moment soil improvement geometries are provided in Appendix A. All or "pinned" connection. Each swivel could accommodate ± 5° pile groups consisted of nine test piles, which were driven in of pile cap rotation about a horizontal line and ± 15° of pile cap a 3 × 3 arrangement with a nominal center to center spacing rotation about a vertical line. of 3 ft. The tests piles were 12.75-in. outside diameter pipe The lateral load tests were carried out with a displacement piles with a 0.375-in. wall thickness and they were driven control approach with target pile cap displacement increments closed-ended with a hydraulic hammer to a depth of approx- of 0.25, 0.5, 0.75, 1.0, and 1.5 in. During this process, the actu- imately 44 ft below the excavated ground surface. The steel ator extended or contracted at a rate of about 40 mm/min. conformed to ASTM A252 Grade 3 specifications and had a Additionally, at each increment, 10 cycles with a peak pile cap yield strength of 58.6 ksi based on the 0.2% offset criteria. The amplitude of ±0.1 in. were applied with a frequency of approx- moment of inertia of the pile itself was 279 in.4; however, angle imately 1 Hz to evaluate dynamic response of the pile cap. After irons were welded on opposite sides of two to three test piles this cyclic loading at each increment, the pile group was pulled within each group, which increased the moment of inertia back to the initial starting point prior to loading to the next to 324 in.4. higher displacement increment.
OCR for page 24
24 Load Test Instrumentation the length of each pile cap to evaluate pile cap rotation. On both caps, string potentiometers were located 2 in. from the Applied load was measured directly by the load cell on the north and south edges of the corbel, with a distance of 44.72 in. actuator, which was calibrated in the laboratory prior to test- between the potentiometers on Cap 1 and a distance of 108 in. ing in the field. Lateral pile cap displacement was measured (9 ft) for Cap 2 as shown in Figure 3-13. Each potentiometer using two string potentiometers attached to the pile cap at the was attached to an independent reference beam supported at a elevation of the loading point (0.92 ft above the top of the cap) distance of about 6 ft from the side of the pile cap. The pile on the east and west sides of the actuator attachment point as rotation, , was determined using the following equation: shown in Figure 3-13. Lateral pile cap displacement also was measured on the back side of each corbel with two string - 2 = tan -1 1 H (4) potentiometers attached 1.75 ft (21 in.) and 0.375 ft (4.5 in.) above the top of the pile cap directly in line with the load direc- tion. Therefore, the vertical distance between these two string where 1 and 2 are the vertical pile cap deflection at two pots was 1.375 ft (16.5 in.) as shown in Figure 3-13. Finally, ver- points on the pile cap and H is the distance between the tical pile cap displacement was measured at two points along measurements. Figure 3-13. Typical instrumentation layout for piles caps with a partial-length corbel (Caps 1 and 4) and a full-length corbel (Caps 2 and 3).