sábado, 22 de agosto de 2009

THE OSTERBERG CELL


The Osterberg Cell is another innovative technique that has been evaluated by FHWA during the 1990's to determine its applicability and potential for reducing time and costs of performing foundation load testing, especially for drilled shafts. The drilled shaft designer must face both the problems of predicting subsurface soil and rock strength and compressibility characteristics, and the difficulty of estimating the impact of construction installation technique on the completed shaft. Model testing and laboratory investigations don't provide sufficient insight to assess the complex interaction between soil (especially intermediate geomaterials) and the concrete shaft. The Osterberg Cell, commonly called the "O-cell", is well suited for this problem and has provided an attractive short duration alternative for testing drilled shafts (31).

The O-cell is a hydraulically driven, high-capacity sacrificial jack-like device that is installed at the bottom of the reinforcement cage (figure 25) of a drilled shaft or at the tip of a driven pile. Unlike the conventional static and dynamic load tests that apply a compressive force at the top of the pile or shaft, the O-cell loads the unit in compression, but from its bottom end. It requires no overhead reaction frame, dead load, or other external system. As the O-cell is pressurized and expands, the shaft and the soil provide reaction to the applied loads. The end bearing support provides reaction for the side friction along the shaft, and vice versa, until reaching the capacity of the cell or either of the two support components. O-cell tests automatically separate the end bearing support component from side friction values. Thus, an O-cell test load placed at the bottom of the shaft has twice the testing effectiveness of that same load placed at the top of the shaft. Production shafts can also be tested, if the device is grouted after testing is completed.

Movements of the foundation element during an O-cell test are measured by electronic gauges connected to a computerized data acquisition system. Linear vibrating wire displacement transducers (LVWDT's) are attached to the bottom plate of the O-cell. Telltales are used to measure both the compression of the test pile and the upward movement of the top of the O-cell. The downward movement of the bottom plate is obtained by subtracting the upward movement of the top of the O-cell from the total extension of the O-cell as determined by the LVWDT's. The upward movement of the top of the test pile or shaft is measured by digital gauges mounted on a reference beam. The loading mechanism has evolved from the original bellows-type expansion cell to the current piston-type jack. However, the piston extends downward instead of upward like that of a conventional load test.

The O-cell test offers a number of obvious advantages over conventional load testing, including economy, high load capacity, safety, reduced work area, and the ability to separate the end-bearing and side-friction components. Disadvantages include the need for advance installation, sacrificial expendability, unsuitability for certain types of piles, and the balanced component limitation, i.e., the test capacity is limited to twice the capacity of the support component reaching ultimate first. It has been noted that the majority of load tests on drilled shafts are now being done with the O-cell.

Boston Engineers were the first to use the O-cell in a practical application in 1987 on a bridge site near Boston, Massachusetts and later that same year at Rochester, New York. After testing, they recovered the cell and used it on another test site. In 1988, two more tests were performed with FHWA research funds at a bridge in Port Orange, Florida. More than 200 additional tests have been performed with O-cells in the United States on piles and drilled shafts during the 1990's, including some lateral load tests.

The Minnesota DOT recently completed a major load-testing research program featuring the use of O-cell devices in both axial and lateral load-testing modes. The shafts were 58 m deep and 1.3 m in diameter. One of the test shafts was instrumented with Sister Bar strain gauges to develop a better understanding of the load transfer distribution. The first-ever embedded O-cell lateral load test was on a second test shaft on this same project.

Other interesting applications in FHWA-sponsored projects include two previous record- setting performances in Massachusetts and Kentucky on highway bridge projects. The Owensboro, Kentucky record of 53.4 MN (6000 tons) was soon broken by the Boston Central Artery project record of 55.9 MN (6,280 tons), which was easily broken by the Florida project. The site conditions on Boston's Southeast Expressway (I-93) were treacherous because the test shaft was drilled between the two fast lanes inside two jersey barriers only 2 m apart. The testing was completed without obstructing or disrupting traffic flow.

No job appears to be too big or too small for the use of an O-cell. Several world records for load testing have been set recently, including the current world record for total load of 135 MN (more than 15,000 tons) set in Florida in 1997. Whether inside a building or under a bridge with limited access or low headroom, or within a cofferdam in the middle of a river, O-cell testing can perform well. It can be used to isolate portions of a pile or shaft by installing multiple cells, and is fast becoming a favorite of engineers and contractors because of its speed of installation and cost advantages.

http://www.fhwa.dot.gov/engineering/geotech/pubs/century/02.cfm

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