Non-working piles are executed at the beginning of the project to study the soil reaction and the maximum settlement for the non-working piles. The pile will be loaded to twice of working loads. According to the test result, the pile length will be changed. If the settlement exceeds the maximum settlement then the pile length will be increased.
Test of non-working piles
Static compression load test gives an indication about the pile's reaction to the load. The settlement of non-working piles will be measured for different load increment. From this information, the design length of the pile will be decided. Maximum load for pile will be working load multiplied by 2 (working load*2) or as per local standards. The following steps explain the procedure of the test.
A. The steel cage will be prepared as per the approved shop drawing before, but the spiral will be for the full length of the steel cage. The reason behind extending the spiral is to prevent the bursting of concrete at the pile top due to the applied load.
B. Strain gauges shall be fixed on the steel cage. Each cage should be instrumented with four levels of strain gauges first gauges near the pile head and another near the toe of the pile. This strain gauge shall be very accurate with a tolerance of 1 µ strain, and each level shall be instrumented with four gauges.
C. Pile shall be cast, and the pile head must be leveled to the required level, so in a later stage, the jack can be installed in a level surface.
D. Loading system: the compression load shall be applied directly to the pile using a proper reaction system. The load will be applied using Hydraulic jacks.
E. The load applied shall be measured by pressure gauges. The pressure gauges must be calibrated before the testing, and the calibration certificate shall be available with the gauges.
F. Loading sequence: The compression load will be applied in a sequence starting from 0% up to 200%. Before loading the pile, the concrete shall reach 95% of the required strength.
There is two test arrangement.
1.Kentledge: In this arrangement, concrete blocks are used to balance the applied load. Kentledge requires a large number of blocks. This arrangement occupying considerable space.
2.Self-reaction system: This system used when the available space is limited. In this system, strand inserted and grouted inside the ground. Then strands are fastened to crown beam as shown in the figure below applied load will be transferred to these strands.
Static compressive test instrument arrangement
Pile steel cage preparation
Spiral shall be extended to the top level of the pile so it the concrete will not crack or bust due to huge stress induced by applied test load. Four levels of wire vibrating strain gauges shall be attached to reinforcement cage. Each tier shall have four wire vibrating strain gauges spaced equally. The purpose of wire vibrating strain is to measure pile strain at different levels. The first level shall be fixed near the cut of level. Similarly, the last level shall be set near the bottom of the pile. The applied load at a different level of the pile can be calculated using strain measured.
Load=strain*(Area of steel*Es+Area of concrete*Ac)
Once the concrete attained the desired strength. The area shall be excavated and leveled. Then reference beam shall be placed around the pile and fixed properly to avoid any movement. This beam shall be used as a reference for measuring pile movement. Therefore, the beam shall not move or deform during the test.
Displacement transducer is an electrical device used to measure the movement of the pile shaft in one direction. Pile shall instrument with three displacement transducers spaced equally to measure the amount of pile displacement. Displacement transducer shall be calibrated periodically.
Dial gauges also will be used to measured the settlements of piles. Dial gauges shall be calibrated periodically as recommended by the manufacturer.
Loads will be applied to pile head using hydraulic jacks. In the event of high loads, several jacks may use to apply the load. A thick steel plate is placed between the pile head and jacks to prevent pile head damage. Jacks should be calibrated and in good condition.
The load cell is a transducer used to measure the load. It is used to verify the applied load on piles. Periodic calibration should be available.
The figures below depicting the arrangement of kentledge static compression load test.
Figure 4 showing hydraulic jacks and load cell. Three jacks used to produce the required load for this test pile. In figure 5, the dial gauges and displacement transducer is shown.
Pile fails if:
A. Settlement > than 1% of least dimension.
B. Residual settlement after rebound more than 6 mm.
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