Pile foundations are principally used to transfer the loads from buildings, through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth, increasing the effective size of a foundation and resisting horizontal loads. They are typically used for large structures and in situations where soil is not suitable to prevent excessive settlement.
Pile load tests are generally performed to either prove that the piles are capable of sustaining the ultimate design load (“proof test”) or to gain more detailed information that will enable a more efficient design (“load-deformation test”). Static and dynamic pile load tests can be performed on drilled or driven piles to evaluate either axial or lateral capacities. Axial capacity refers to the vertical (up/down) force that the pile will have to withstand while the lateral capacity refers to the piles ability to withstand loads from the side’s perpendicular to it. Static tests consist of loading piles and measuring deflection. Dynamic tests attempt to obtain static pile capacities generally using stress wave analyses of pile deflection caused by dynamic loads. Axial capacity refers to the vertical (up/down) force that the pile will have to withstand while the lateral capacity refers to the piles ability to withstand loads from the side’s perpendicular to it. This short article briefs on one of the static load tests used by AEL, the O-Cell Test.
O-Cell Test Setup on-site (P2)
The O-Cell Test
The award winning Osterberg Cell®, or "O-cell", gets its name from the inventor, Dr. Jorj Osterberg. The O-cell is a hydraulically driven, high capacity, sacrificial loading device installed within the foundation unit. As the load is applied to the O-cell, it begins working in two directions (bi-directional); upward and downward.
By virtue of its installation within the foundation member, the O-cell load test is not restricted by the limits of overhead structural beams and tie-down piles. Instead, the O-cell derives all reaction from the soil and / or rock system and the foundation element itself.
Each O-cell is specially instrumented to allow for direct measurement of the O-cell’s expansion. By measuring the top of shaft movement and compression, the upward and downward O-cell movement is determined. Strain gauges are often used to separate stratigraphic zones (layers of soil). The test provides a detailed data report containing tables, graphs, calibrations, a detailed description of the tested element and subsurface.
O-cells range in capacities from 90.7 ton-force to 2721.55 ton-force (903.73 kN to 27117.56 kN). By using multiple O-cells on a single horizontal plane, the available test capacity can be designed to obtain virtually any load. By utilizing multiple O-cells on different planes, distinct elements within a shaft or pile can be isolated for testing. Using the O-cell, the application of deep foundation load testing has transformed from expensive, time-consuming, small-scale field tests to state-of-the-art, short-duration, full-scale load testing of production shafts and piles.
Access Engineering piling division employed this technology in the Havelock City project to test 1500mm and 1800mm piles recently. A new generation of O-cells called “Super Cells”, which consisted of multiple inflatable jacks (Super-jacks), that can produce large loads were used.
In comparison with traditional load test methods such as; kentledge tests; which use blocks of concrete or iron, or anchor reaction tests; which utilize secondary piles to load the main piles, these bidirectional super cells offer multiple advantages. This hi-tech design combined with the light weighted low profile enables it to be handled more easily on site. As it can be even operated in a very limited working area, saving time and money while improving the safety and increasing the test loading capability.