Evaluating the effectiveness of rolling impact compaction at a brownfield site with high and low frequency seismic surface waves and geotechnical testing

Abstract

ABSTRACTUsing a case study from a brownfield site in Australia that contained substantial uncontrolled crushed rock fill, we demonstrate that innovative geophysics and ground improvement using rolling dynamic compaction can provide alternatives to costly excavation, removal and re‐engineering of soils. This approach can also overcome some of the limitations of conventional geotechnical testing in such conditions. The process we have developed involves selection and levelling of trial areas within larger brownfield sites. These are initially screened with electromagnetic geophysics to target environmental sampling sites, buried objects and local areas of potentially unsuitable or contaminated soils for removal and replacement. High and low frequency surface wave testing is then completed along closely spaced lines within the trial areas using specially adapted Multi‐channel Analysis of Surface Waves (MASW) methods and a modified land streamer. This provides S‐wave velocity distributions within fill and deeper materials over two depth ranges, at high vertical resolution from approximately 0 to 2 m and at lower resolution from 2 to ~10 m. These results are compared with geotechnical engineering test information. Ground improvement works are then undertaken using rolling dynamic compaction with a ‘square’ impact roller.Within this brownfield trial area, geotechnical testing showed that post‐compaction in‐situ densities increased by about 6 to 10%, while S‐wave velocities increased by up to 40%. This clearly demonstrates the sensitivity of S‐wave velocities to soil and fill modulus. The S‐wave velocity sections also showed the consistency of the landform created by impact rolling and that both high and low frequency MASW testing is needed for more accurate assessment of the densification of both shallow and deeper fill. Normalized S‐wave velocity difference sections and Cone Penetrometer Testing (CPT) provided a clear indication of the depth of influence of the impact rolling.A relationship was developed between post‐compaction S‐wave velocity and Dry Density Ratio (DDR) that formed the basis of a viable geotechnical verification strategy for the ground improvement works on the larger brownfield site. Electromagnetic and advanced surface wave geophysics, combined with innovative ground improvement methods by rolling dynamic compaction, represent an advance over current practice for brownfield site development.