NEES-2006-0163: In Situ Determination of Soil Modulus and Damping as a Function of Level of Strain
| | Organizations | Texas A & M University TX, United States University of Texas at Austin TX, United States |
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| | Start Date | 2005-08-01T00:00:00 |
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Description:
In this study, the new field technique is developed to evaluate the linear and nonlinear shear moduli of soils. This method involves: (1) applying static vertical loads near surface of a soil, (2) applying dynamic horizontal loads to generate elastic waves within the soil mass, and (3) measuring the response of the soil mass beneath the loaded area using the embedded instrumentation. Both static and dynamic loads are applied using the mobile vibroseis, called Thumper (small-capacity vibroseis) and T-Rex (large-capacity vibroseis), of nees@UTexas. As a source of wave propagation the circular concrete footing is utilized. Before constructing the concrete footing velocity transducers (geophones), are embedded beneath the proposed footing location to monitor the propagation of waves. On the concrete footing the static loads are applied in different amplitude levels and these static loads provide different confining states of stress in the instrumented soil mass. At a certain level of static load, a set of dynamic loads is applied from the small-strain to large-strain levels. Applying horizontal dynamic loads in the small-strain range generates shear wave propagation in the small-strain range and the measurements of this shear wave propagation allow us to obtain the linear modulus of soil. In the same manner the horizontal dynamic loads in the medium- to large-strain ranges generate shear wave propagation in the medium- to large-strain ranges and the measurements of those wave propagation allow us to obtain the nonlinear shear modulus. The set of dynamic loads (from small- to large-strain ranges) is applied at each of the different static load levels. Therefore, the variations of G values with strain amplitude are able to be determined with different confining states of stress in the field.
In addition to this technique, small-strain crosshole and downhole testing can be performed with the same set-up of the field testing described above. Since the crosshole and downhole tests have been validated and accepted as standard field methods to evaluate the shear modulus in the small-strain range, these tests provide opportunity to better understand the state of stress under the testing area. The crosshole and downhole tests can be conducted with static loads applied on the concrete footing; hence, it becomes possible to evaluate the small-strain stiffness with confinement state in the field. Also, from the crosshole and downhole testing results, it is possible to investigate any changes in the soil skeleton before and after the large-strain dynamic test using the vibroseis.
Field experiments has been conducted at two different locations of the Fitzpatrick Ranch in south Austin in spring 2006 and National Geotechnical Experimental Site (NGES) on the Riverside campus of Texas A&M University (TAMU) in spring 2007. One 3-ft diameter footing was examined in the Fitzpatrick Ranch. In NGES sand site, the proposed field testing were performed on one 3-ft diameter and one 1.5-ft diameter footings. For the testing restults from the NGES site, however, data from the only 3-ft diameter footing will be included in this data set, because more investigation should be performed for the data from measurements with the 1.5-ft diameter footing.
Three main experiment folders have been created like:
(1) 3-ft diameter footing in the Fitzpatrick Ranch
(2) 3-ft diameter footing in NGES at Riverside campus of TAMU, and
(3) Resonant Column and Torional Shear Tests for the specimen from (i) Fitzpatrick, and (ii) NGES sites.
Under each experiments folder, description of the field testing at each site can be founded.
The results of this study would be: (1) to develop the field testing method to evaluate the linear and nonlinear shear moduli of soils, (2) to validate the current laboratory method to obtain the nonlinear shear modulus of soil by comparing the field measurements, (3) to provide the reliable fiel
