Pile foundations are principally used to transfer the loads from superstructures, 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 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. 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.This short article briefs on one of the static load tests used by AEL, the O-Cell Test.
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. The test provides a detailed data report containing tables, graphs, calibrations, a detailed description of the tested element and subsurface.
Graphs include load and movement data, t-z and q-z plots, strain gage load distribution and an equivalent top load plot.
O-cells range in capacities from 200 kips to 6,000 kips (90.7 ton-force [metric] to 2721.55 ton-force [metric]). 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 “Supercells” consist of multiple inflatable jacks (Super-jacks) which can produce large loads were used. The low height and lightweight design enable it to be more easily handled on site. Also, it can be readily secured inside the rebar cage without the use of bearing plates. With the unique cone-shape flow guiding mechanism, the surface of the Super Cell can be seamlessly covered by pre-casted concrete in high strength; and residues from the pile bottom may be diverted up through the load cell smoothly.
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