Experimental Studies of Dynamic Response of Foundations

An investigation was made into the behavior of rigid foundations and structures resting on the surface or embedded in a cohesionless soil and subjected to transient active or passive excitations and forced vibrations using the centrifuge modeling technique. The investigation was aimed at studying both low and high amplitude vibrations of foundations under machine type loadings, earthquake or wave induced vibrations, and other sources of dynamic loads. Rigid "prototype" foundations of mass and size comparable to foundations of a low rise building were simulated in the centrifuge at a centrifugal acceleration of 50g. Rigid model structures (aluminum towers) attached to foundations of different shapes, sizes, masses, and moments of inertia were tested. The effect of soil depth, boundary conditions, and depth of foundation embedment were investigated. Mainly rocking and horizontal modes of vibration were studied. The impulse rocking-horizontal excitation of the models was provided by actively perturbing the model structures using explosive energy or by passively exciting them by shaking the whole soil bucket using a hydraulic shaking system. The forced vibration was produced by a miniature air-driven counterrotating eccentric mass shaker mounted on the model structures. During the tests detailed measurements of the static and dynamic contact pressure distributions, displacement components of the model, and acceleration amplitudes at different elevations of the model structure were obtained. The acceleration ratios were used to determine the modes of vibration of the foundation systems. Natural frequencies and damping coefficients of the modes were calculated by fitting the amplitude-frequency response of a single degree of freedom mass-spring-dashpot oscillator to the experimental response curves derived from the test data. Experimental results provided information regarding the influence of different geometrical, inertial, and loading conditions on the vibrational characteristics of the soil-structure system. In particular the effect of foundation embedment was to increase the model resonant frequencies and to cause an appreciable change in contact pressure distribution underneath the footing. However, the resonant frequencies predicted by the lumped parameter analysis for a simple two-degree-of-freedom (rocking and translation) model were about 20 to 55 per cent higher than those measured experimentally. These results were consistent with the comparisons made in similar theoretical and experimental studies such as those performed by Morris in the Cambridge centrifuge and those performed on full-scale footings by Stokoe and Richart. Damping ratios of the rocking-sliding vibration did not change considerably when footing size or depth of embedment changed. The existence of rigid boundaries around the soil mass in the bucket, and an inefficient contact between soil and the foundation side walls and lower surface could account for these observations. Uplift and nonlinear large amplitude vibrations were consistently observed during the steady-state vibration tests. Uplift led to a softer vibrating system which behaved non-linearly. As a result the frequency of vibration decreased with the amount of lift-off. In transient vibration uplift reduced the intensity of higher frequency vibration. Soil around the foundation edge yielded and plastic deformations and subsequent softening of the contact soil increased the material damping while it decreased the resonant frequency of the system. It was concluded that elastic half-space theory does not satisfy the needs for analysis of foundation behavior under high amplitude vibrations and more sophisticated methods of analysis are required.</p

In Situ Dynamic Testing of Pore Pressure Transducers at Treasure Island, California using "Caltech Piezometer"

Liquefaction in saturated sandy deposits is one of the most dramatic causes of damage to structures during earthquakes. The onset of liquefaction has been observed through the occurrence of sand boils, building settlen1ents, lateral spreading, and other phenomena during numerous earthquakes. The most recent reminders of liquefaction and its destructive effects were observed in Kobe, Japan during the 17 January 1995 Hanshin earthquake. A thorough understanding of liquefaction phenomena, and formulation and calibration of analytical techniques to predict liquefaction occurrence and its effects, require data from instrumented sites where in-place soil properties have been appropriately established, and where ground motions have been measured for an earthquake strong enough to have generated significant increases in the pore pressures. So far, among few potentially liquefiable sites around the world that have been permanently instrumented with pore-pressure transducers and accelerometers, only the Wildlife Site in Imperial Valley, California has liquefied during an event. Acceleration and pore-pressure data were recorded at the Wildlife Site during the magnitude 6.6 Imperial Valley earthquake of24 November 1987 which resulted in liquefaction of the site. However, some unusual aspects were observed in the data, including long rise times of the pore pressures and a time lag between the strong ground shaking and maximum pore pressure development. Conditions associated with installation of the pore-pressure transducers might have been the cause of the anomalies observed in the data. In a con1prehensive experimental study under a grant from the U.S. Geological Survey (USGS), the authors performed in-situ dynamic inspection and calibration of the USGS piezometers installed at the Wildlife Site versus a reference pore-pressure transducer carefully installed close to the existing transducers. The field and the laboratory tests performed as part of this research provided valuable information on overall response of the USGS transducers and on the techniques to in1prove the installation and subsequent in-situ inspection of piezometers at any future site. Following the Wildlife field study a prototype unit of a new pore-pressure measuring probe, designed at the California Institute of Technology (Caltech), was fabricated and used in a field testing program at Treasure Island, San Francisco, at the National Science Foundation (NSF) U.S. Geotechnical Test Site to develop standard field techniques for installation and in-situ calibration of pore-pressure transducers. A large number of tests was performed using the "Caltech Piezometer" and several types of USGS pore-pressure transducers at the Treasure Island site in February 1994. The piezometers were all installed at sin1ilar depths, and pore pressures were generated either by dropping a 1300 lb. (590 kg) weight on the ground surface or by detonating explosives at some depth below ground surface at equal horizontal distances from the piezometers. The new Caltech probe performed well in all the tests conducted, and provided a ready means of introducing a deaired saturated probe into the ground to any required depth in the field. On the other hand, the performance of the USGS piezometers deployed was quite variable. The ease of on-site preparation, calibration, and installation of the "Caltech Piezometer" suggests a number of possible uses for it such as a) long-term installation and monitoring of pore pressures during earthquakes, b) testing piezometers at existing instrumented sites, c) rapid deployment and measurement of pore pressures during aftershocks, d) monitoring dynamic compaction of hydraulic or liquefiable natural fills, and e) in-situ field measurement of liquefaction potential of granular soils

In-place calibration of USGS pore pressure transducers at Wildlife Liquefaction Site, California, USA

Acceleration and pore-pressure data were recorded at the Wildlife Site during the magnitude 6.6 Imperial Valley earthquake of 24 November 1987 which resulted in liquefaction of the site, alone among instrumented sites. Some unusual aspects were observed in the data, including long rise times of the pore pressures resulting in a time lag between the strong ground shaking and maximum pore pressure development. It was not clear from the data obtained whether the liquefaction process at the Wildlife Site was different from that observed in other saturated sand deposits, or if the pore-pressure transducers were not responding correctly. In December 1989, the authors performed an in-situ dynamic calibration of the USGS piezometers installed at the Wildlife Site with respect to reference pore-pressure transducers showing that only one of the transducers was performing nearly correctly. Several hypotheses have been examined to explain the data recorded during the earthquake, including particularly transducer malfunction due to air bubbles in the transducer chambers

In-situ calibration of USGS piezometer installations

Among the few potentially liquefiable sites around the world that have been permanently instrumented with pore-pressure transducers and accelerometers, only the Wildlife Site in the Imperial Valley, California has liquefied during an earthquake. Acceleration and pore pressure. data were recorded at the Wildlife Site during the magnitude 6. 6 Imperial Valley earthquake of 24 November 1987 which resulted in liquefaction of the site. Some unusual aspects were observed in the data, including long rise times of the pore pressures and a time lag between the strong ground shaking and maximum pore pressure development

Dynamic behavior of pile group in liquefied sand deposit

Dynamic centrifuge tests were performed to develop an understanding of soil-pile foundation interaction and to provide data for verification of an earthquake response analysis method for pile foundations in liquefiable soil during earthquakes. Centrifugal test results indicate that pile foundation response in nonlinear liquefied sand is greatly affected by soil behavior due to large ground displacement and excess pore water pressure buildup. The numerical model which consists of beam elements and nonlinear lateral Winkler springs, taking into account the changing effective stress, is discussed by comparing predicted results with measured results