Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 12.2.2 Rapid Load Test The Rapid Load Test (i.e., also known as the Force Pulse test) uses a controlled combustion of fuel in a confined pressure chamber to propel a piston against a pile or drilled shaft (Bermingham and Janes 1989). A heavy reaction mass is placed on the piston and this is forced to move as a result of the combustion. An equal and opposite reaction occurs against the pile or drilled shaft. The rapid load is applied momentarily (in around 120 milliseconds. The force on the foundation is measured with a builtin load cell and laser sensors measure the time-history of load and the displacement of the foundation. Venting of the combustion gases from the pressure chamber relieves the load on the foundation. Test loads in excess of 1000 tons can be applied with this test. The size of the load is controlled by the amount of fuel used and the size of the reaction mass deployed. 185 Depending on the orientation of the piston, a rapid load test can be performed vertically (axial compression, or laterally. In the latter case, a steel sled is used on which the reaction mass is placed. If performed over water, the sled slides on the deck of a barge. The benefits of this type of test over conventional static load tests are in the speed of the test, and there is no requirement for reaction piles. The reaction mass is considerably smaller than the equivalent load required fora static test. A Rapid Lateral Load testis a form of dynamic load, and in that sense, it is perhaps more applicable to investigation of the lateral load response of deep foundations to dynamic loading such as vessel impact, ice, wind and wave loads than static tests. Fora similar load magnitude, a Rapid Lateral Load Test maybe to 50 percent lower in cost than an equivalent static load test. Brown (2007) presented a method of analysis of the rapid lateral load test that considers inertia and rate of loading effects. He noted that it is preferable to conduct four successive and increasing load pulses as the best means to capture nonlinear load deformation response. To reload, reassemble and reposition the equipment against the test pile or shaft takes about an hour between pulses. Double integration of the acceleration time history, measured via accelerometers, provides a deformation time history. A string of eight down-hole accelerometers was noted as adequate for defining the deformation time history at each instrument location. These instruments are positioned in an inclinometer casing, and then recovered after the test. Resistance type strain gauges at intervals along the pile are also used, from which to compute bending moments. Resistance type gauges, rather than vibrating wire strain gauges, are selected because of the frequency demands of the rapid test. A high-speed data acquisition system is required to capture the data at a sampling rate in the order of 1000 samples per second. Download 6.03 Mb. Share with your friends:
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