Field Evaluation of Ground Stiffness by Static and Dynamic Tests
Research Associate, School of Civil Engineering,
Information of ground stiffness are important for geotechnical applications. This paper applies static and dynamic approaches for evaluating ground stiffness. The moduli by seismic methods are found to be around 8 times those by static plate load tests. In addition to this, a falling plate test is designed for evaluation of ground stiffness and estimate soil properties at shallow depth. The testing equipment includes a steel plate with accelerometer tightly attached. To make a measurement, the falling plate is dropped from specified height (the height is set depending on the depth of ground to evaluate). The motion of the plate during impact is recorded by the accelerometer. Load-displacement curves can be obtained by processing the acceleration record. The portion on the load-displacement curves after force registration is completed are found to be similar to those measured by static plate load tests. The peak acceleration of the falling plate is found to be highly correlated to shear wave velocity of soil at the ground surface. This makes the falling plate test fast and convenient for evaluating earthwork compaction such as for highway foundations. The test set up fully utilizes the fact that the highest sensitivity of ground displacement to excitation force appears at the impact point as seen in the falling weight defelctometer data. In addition, unlike the dynamic plate test where the energy imparted into the ground varies, the same amount of energy is dispatched into the ground by the falling plate. This helps to prevent the dependency of results on the measurement system. The frequency response function of the ground under impact load generated by falling plate is obtained from Fourier transformation of the recorded signal, which is found to resemble that of vibration of single degree of freedom with damping. However, the resonance frequency from measured frequency response function is found to be much larger than predicted by lumped system model commonly used in machine foundation analysis. Details of this will be further investigated. With improved theoretical model for the impact process, inversion analysis can be used to determine more soil properties. This will make falling plate test a fast and reliable field tool for evaluation of earthwork compactions.
KEYWORDS: Ground stiffness, falling plate test, accelerometer, frequency response function
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