Save Time and Costs on Load Testing
The Osterberg cell (O-cell) bi-directional testing method enables relatively low-cost, high-capacity static load testing of bored piles that were otherwise prohibitively expensive or technically impractical. The genius behind the innovation is a specially designed hydraulic jack (O-cell assembly) cast directly into the pile at a predetermined location (Figure 4). After curing or set-up, the O-cell is hydraulically pressurized from the surface, simultaneously loading the pile section above the O-cell and the pile section below it. By loading the pile internally, the pile component above the O-cell acts as reaction for loading the pile component below the O-cell, and vice-versa. As the load is applied during testing, electronic sensors measure the displacement of both pile sections. In this way, the O-cell simultaneously tests the end bearing and skin friction and quantifies their resistances individually, thereby maximizing the information obtained.

The O-cell method improves safety and saves time and money because of the reduced effort required to prepare for testing. While the O-cell test has become the premier method for static load testing of bored piles and auger cast in place (ACIP) piles, until recently, it had never been used to test a helical pile.

O-cell Testing on Helical Piles
The idea to use an O-cell to test a helical pile originally came to HPS Chief Engineer, Tom Bradka, while attending a seminar by Dr. Bengt H. Fellenius, a distinguished foundation engineer in the piling industry. Dr. Fellenius discussed the concept of O-cell bi-directional testing on bored concrete and driven steel piles, expounding on the advantages of testing piles in this manner. Tom soon began working on a concept to apply the same method to helical piles.

After the concept was presented to LTI, engineers from both companies jointly developed a method to incorporate the O-cell into a helical pile. Using a specially designed torque-transfer mechanism, the O-cell is installed in a space near the base of a 2-piece helical pile (Figure 5).  This patent-pending mechanism allows the pile to transfer torque across the plane of the O-cell while also allowing the pile to separate under load.  The pile is then advanced into the ground using the traditional helical pile installation method. Once installed, all of the hydraulic and electronic connections are made and the test is carried out in general accordance with ASTM D1143, applying incrementally greater loads on the pile over specified time intervals.

The test site was located in Fort Saskatchewan; the centre of the industrial heartland of Alberta, Canada.  This location was selected in part because a conventional static pile load test was completed on that site at an earlier date. Two piles were prepared for the test site; both fabricated at 219-mm in shaft diameter, with a single 457-mm diameter helix embedded 4 meters below the ground surface to duplicate the dimensions used in the comparison test.  One of the piles contained two additional helices above the O-cell location to provide additional upward resistance to loading, allowing the pile tip to be loaded to a higher displacement.  A photo of the O-cell test in progress is included as Figure 6. The results from both O-cell test piles were then compared to the conventional load test results obtained from the earlier test program.

The Final Analysis
The advantages of this innovative application were clear from the beginning.  Where traditional top-down pile load tests would normally take days to set up and complete, the O-cell load test was completed in only four hours by a team of engineers assembled by HPS and LTI. After the test was complete, the helical pile was extracted by reconnecting the drive head and reversing the torque motor. The O-cell and ancillary equipment were then removed for re-use on a subsequent test pile. Figures 7 and 8 show the helical pile after extraction.

This re-use was innovative in and of itself because when bored piles or ACIP piles are tested by this method, the O-cell cannot be retrieved and is considered to be sacrificial.  In this case, two similarly equipped helical piles were installed, tested and removed all in a single day.

The O-cell test method allowed the LTI and HPS team of engineers to directly measure the resistances developed by the shaft friction (90 kN) and the helix and tip bearing (160 kN) independently. This information is critical in refining design parameters and confirming assumptions about how the load resistance is distributed in the helical pile. The data was also combined to produce an equivalent top-loaded curve to predict the head displacements that would occur in a conventionally loaded pile. The curve is presented in Figure 9.

Implications
Helical piles are quickly proving to be one of the most cost effective and versatile foundation systems available. The savings resulting from using helical piles in lieu of concrete or driven piles in some cases could be significant. Having an inexpensive and quick means of verifying capacity only adds to their allure.

Alvin Pyke, P.Eng. Chief Executive Officer for HPS, states that this test method will reduce the cost of load testing by as much as 80%. By eliminating uncertainty through load testing and optimizing the foundation design, potentially significant cost savings can be passed on to the client.  HPS has already decided to include O-cell testing as standard on all future helical pile projects. By adopting this approach HPS hopes to lead the industry toward performance-based design as a standard, allowing helical piles to make significant inroads as a preferred foundation alternative.