This paper presents an experimental study that investigates the effects of stress history and mineralogy on the use of electrical resistivity to predict soil strength. Employing electrical resistivity as a geophysical parameter has shown advantages when surveying soil properties of seafloor soft soils because of the potential for rapid coverage rates and non-contact ability. Recent experimental studies show that the electrical resistivity, ρ, is promising in predicting the undrained shear strength, Su, for a given soil type under the same conditions (e.g., pore fluid and stress history). When employing ρ in surveying seabed sediments, the effects of natural variability in the soil, such as different stress histories and mineral compositions, need to be addressed. This study utilizes a special-made triaxial setup that can measure the electrical resistance, R, at any stage of a consolidated undrained test on reconstituted clay samples, which is used to calculate the electrical resistivity of the clay samples. The reconstituted clay samples are mixed with two different types of minerals, kaolin and bentonite, at desired soil ratios, resulting in a range of plasticity index (PI) values. Multiple triaxial tests are performed on each batch of specimens with varying stress histories (represented by the overconsolidation ratio, OCR). The results provide a correlation of Su relating to ρ, PI, and OCR, which is useful in geotechnical engineering applications where the determination of soil strength using geophysical methods is an option.

This paper seeks to contribute to the available industry literature on guidance for planning geophysical investigations to minimize ground risk and cost while maximizing value for the project. This paper reviews the successes and challenges of five case histories where geophysical ground investigations have been employed on buildings and infrastructure projects. Based on the lessons learnt a preliminary framework for planning geophysical investigations to minimize ground risk and cost while maximizing value is proposed.

Geophysical techniques are commonly employed as part of modern day ground investigation campaigns. While geophysics is not a substitute for boreholes or other intrusive testing, it is a powerful complimentary tool. An effectively planned geophysical investigation has the potential to optimize time, cost, environmental impact and ground risk on projects. However, the value of a geophysical investigation can be compromised due to poor timing of the investigation and/or poor interaction between the key parties including the geotechnical engineer or engineering geologist, client and specialist geophysical sub-contractor.

Based on real project case studies from around the world including Canada, Australia and Hong Kong which have utilized a variety of geophysical investigation techniques this paper presents successes, challenges, limitations and lessons learnt. Informed by the project case histories a preliminary framework for planning geophysical investigations is proposed. The framework covers the lifespan of project from concept to construction. It suggests the ideal timing of geophysical investigations and identifies key moments for co-ordination between stakeholders.

The authors hope that this paper will encourage practitioners to take a big picture approach when including geophysics in a ground investigation campaign and will result in investigations being targeted, cost-saving and value-adding for projects.