Geotechnical characterisation of compacted ground by passive ambient vibration techniques

Ground improvement works are commonly required to overcome poor underlying soils in conjunction with infrastructure and housing development. An extensively employed and popular improvement technique is to impart mechanical compaction to the ground in an effort to achieve adequate strength and favourable load-deformation behaviour (stiffness) for the construction of civil infrastructure, including buildings and roads. In order to assess whether a sufficient level of compaction is achieved to meet future design requirements, the means to confirm the quality of compaction and to acquire an innate knowledge of the relationships among void ratio, matric suction, moisture content and unsaturated shear strength will be imperative. There are a number of methods already available for measuring soil compaction, namely and generally, the invasive and non-invasive methods. All methods have their inherent strengths and limitations, and there will always be a trade-off in choosing one method over another. It has been particularly difficult, however, to find a cost-effective and time efficient method that can be applied to a deep and extensive compacted site. In this thesis, a non-classical characterisation method, the horizontal to vertical spectral ratio (HVSR) technique is proposed for the purpose of verifying the standard of compaction at a site. This involves a cable-less low cost, efficient and non-invasive passive ambient vibration (or microtremor) based method, which will be calibrated against in situ test data. The simplicity of the HVSR technique is manifested in having a short setup time (relative to other methods), and precluding the need for any active excitation sources. In the study a lightweight battery operated sensor was utilised to capture the prevailing ambient vibrations reflecting the structure in the ground. This thesis is centred on the following phases of research: (i) refinement and extension of current ambient vibration HVSR techniques, (ii) ambient vibration measurements of the compacted fills at Penrith Lakes quarry in Western Sydney, (iii) interpretation, validation, calibration and application of the measured data to characterise the compacted layers and the underlying soil profile, and (iv) advancing shear wave velocity (Vs) as an alternative/complimentary measure of evaluating ground compaction. The work in this thesis first involved conducting a pilot study of a general geotechnical site investigation to test the proposed techniques and theories. This pilot study was located at the Penrith (Kingswood) campus of the University of Western Sydney (UWS). Once the proposed techniques proved successful, they were applied to compacted fills, in particular by both rolling compaction and dynamic compaction methods, at the Penrith Lakes quarry in Western Sydney. The geotechnical site characterisation and the compaction assessment involved, in general: (1) interpreting the measured HVSR curves for a preliminary assessment of the soil layering, and (2) fitting the HVSR curves to a theoretical model to estimate the Vs profile of the ground. The results from the HVSR technique were verified against data from classical invasive methods (e.g. borehole data, CPT, DMT and SPT). Further verification was also made against the results from other surface wave techniques (e.g. MASW and MSOR (multichannel simulation using one receiver)). The study suggests that HVSR-based techniques could be used for characterising a site in combination with a reduced number of mechanical in situ tests, and especially to fill in the gaps in the soil stratigraphy at the locations not covered by the mechanical tests. The work in this thesis will facilitate the development of an ambient vibration technique for assessing soil compaction based on both raw data and Vs that is currently not available for practicing engineers