Doctor of Philosophy
School of Civil, Mining and Environmental Engineering
Ribeiro Heitor, Ana Paula, Assessment of post-compaction characteristics of an unsaturated silty sand, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2013. http://ro.uow.edu.au/theses/3732
Conventional field compaction control methods including nuclear gauge, sand cone and rubber balloon, are effective at the time of placement. However, their measurements are discrete and have a limited depth of investigation, which may not be suitable for postconstruction compaction quality assessments of deeper fills or larger surface areas. In this situation, classical destructive geotechnical surveys (i.e. boreholes, cone penetration tests) are sought to evaluate the current fill conditions. Nevertheless, these methods often do not provide the level of required information because only certain locations are tested and furthermore they have tremendous implications in terms of cost. The use of available nondestructive cost and time effective methodologies, such as shear wave velocity surveys (i.e. SASW, spectral analysis of surface waves or HVSR, horizontal-to-vertical spectral ratio), offers a valuable alternative to efficiently control compaction over large areas during postconstruction stages, and locate areas within the existing formations where the soil was not sufficiently compacted. In fact, shear wave velocity (Vs) has been used in the past to evaluate the quality of compaction or the in-situ void ratio, but in most of these studies the effect of partial saturation has been neglected. While this may not be a major concern for natural ground profiles where the ground water level (GWL) is close to the surface, for reclaimed fill areas it is a significant problem where the GWL is usually located deeper. This is particularly noteworthy, because, high in situ Vs may not truly represent a higher degree of densification. Furthermore, compacted soil is under unsaturated condition, which means that in-situ suction has an important role in controlling the shear strength.
This doctoral thesis addresses the effects of partial saturation in the implementation of a field methodology based on the propagation of shear wave velocity and suction for evaluating the compaction quality. It encompasses the use of both small and large strain range in relation to laboratory and field approaches to characterise the behaviour of materials under different compaction conditions, as well macrostructure characterization using X-ray CT-scan techniques. The small strain behaviour was characterized using Bender elements for both as compacted and post-compaction conditions. Specimens representative of different compaction conditions, that is water content and energy levels, were tested straight after compaction and subjected to isotropic confining pressures while maintaining constant water content conditions to evaluate the effect of in-situ overburden stress. Moreover, cycles of wetting and drying were also imposed to the compacted specimens in an effort to understand the effect of climatic variations on the measured Vs and its relative importance for the field site in Penrith. During these tests the suction was controlled and measured using an array of different techniques including axis translation technique, filter paper method and a small tip tensiometer.
The mechanical behaviour of the silty sand soil was investigated through constant water content direct shear (CWDS) tests, given that in-service strain is likely to exceed the small strain domain in which Vs is measured. While suction during the test was not measured, a simple formulation based on the pore air pressure associated with the volumetric changes of the specimen undergoing undrained shearing was proposed. Specimens were tested for different compaction conditions (i.e water content and energy levels) in both as compacted and post compacted states.
A new empirical formulation for evaluating the current void ratio or degree of compaction based on shear wave propagation and suction or water content is proposed in this thesis. The performance of the methodology developed was first calibrated for site-specific silty sand soil in laboratory and then assessed for field site located in Penrith, in which the evaluation of the current compaction degree is of paramount importance for the future redevelopment of the site. The thesis also addresses numerous practical guidelines and recommendations for the future.