Abstract:
Md Nur Basit Zaman, Ph.D., E.I.T. 1, Billy Fisher 2
Installation of deep foundation elements can be extremely challenging in unstable overburden soils due to the tendency for soils to cave during the drilling operation. Caving soils result in the need for temporary steel casings or utilization of slurry. This could significantly increase the cost and construction duration for the project. In such a situation, auger cast piles (ACP) can be a more suitable alternate to conventional drilled piers due to the speed of construction and relatively low cost of production.
The capacity of the auger cast pile is highly dependent upon the experience and quality of installation procedure. Therefore, it is often deemed necessary to incorporate a full-scale pile load test to verify the performance-based design.
Current case study involves implementation of a bi-directional (Osterberg cell) full scale load test to evaluate the capacity of a H-pile reinforced auger cast pile for a Regional Rail System project. In addition to being the reinforcement for the ACP, the H-pile also will act as the structural element to transfer the load from the railroad bridge to the pile. Therefore, like any other structure, it is crucial for the engineer to design an ACP capable of handling the required loads.
There were three major purposes for the study:i) to demonstrate the proper installation of ACP; ii) to demonstrate the ability to install the H-pile completely vertical with minimum margin of error; and iii) to perform a load test to determine the ultimate capacity of the ACP.
For this study, two 24-inch (610mm) test piles were installed to the desired depth, socketed five feet (1.52m) into the bedrock. Each of the test ACPs were instrumented with a hydraulic jack capable of applying a bi-directional load of up to 450 kips (2, 002kN) – which is more than twice the design capacity for the piles. Three strain gauges were attached to the H-pile; two within the socket in order to determine the capacity within the rock socket, and one at the lower zone of the overburden soil. A displacement transducer was used to measure pile displacement. The load test data showed that both piles performed similarly. The upper section of the piles within overburden soil failed at 395 kips (1, 757kN) for test pile 1 and 389 kips (1, 730kN) for test pile 2. However, the lower portion socketed into the rock only reached 0.36 inch (9.14mm) and 0.47 inch (11.94mm) of settlement respectively. The bi-directional test can help determined the ultimate unit side resistance for the overburden soils which allowed for a more confident and efficient design. The bi-directional load test proved the ACP had adequate capacity to satisfy the design criteria.