Case studies of geophysical imaging for road foundation design on soft soils and embankment risk assessment
Robert J. Whiteley 1 2 Richard B. Kelly 1 Simon B. Stewart 11 Coffey Geotechnics – Geophysics, 799 Pacific Highway, Chatswood Level 19, Tower B, Citadel Tower, Sydney, NSW 2067, Australia.
2 Corresponding author. Email: bob.whiteley@coffey.com
Exploration Geophysics 46(4) 394-402 https://doi.org/10.1071/EG14010
Submitted: 22 January 2014 Accepted: 22 October 2014 Published: 15 December 2014
Abstract
Population growth along the coast of eastern Australia has increased demand for new and upgraded transport infrastructure within intervening coastal floodplains and steeper hinterland areas. This has created additional challenges for road foundation design.
The floodplain areas in this region are underlain by considerable thicknesses of recently deposited alluvial and clayey marine sediments. If characterisation of these deposits is inadequate they can increase road construction costs and affect long-term road stability and serviceability. Case studies from a major coastal highway upgrade demonstrate how combining surface wave seismic and electrical geophysical imaging with conventional geotechnical testing enhances characterisation of these very soft and soft soils. The geophysical results also provide initial foundation design parameters such as void ratio and pre-consolidation pressure.
A further significant risk issue for roads is potential embankment instability. This can occur during new road construction or when upgrades of existing embankments are required. Assessing the causes of instability of existing steeper embankments with drilling and probing is often difficult and costly due to access and safety problems. In these situations combinations of electrical, ground penetrating radar and P-wave seismic imaging technologies can rapidly provide information on the likely conditions below both the roadway and embankment. Case studies show the application of these technologies on two unstable road embankments.
It is concluded that the application of both geophysical imaging and geotechnical testing is a cost-effective enhancement for site characterisation of soft soils and for risk assessment of potentially unstable embankments. This approach overcomes many of the current limitations of conventional methods of site investigation that provide point location data only. The incorporation of geophysics into a well crafted site investigation allows concentration on fewer but higher quality soil probings and geotechnical boreholes.
Key words: embankments, geotechnical, instability, radar, resistivity, roads, seismic, soils.
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