Data was acquired within 6 weeks in summer 2015 and final bedrock models delivered in autumn 2015. In 2016 parts of the data were further analysed in terms of soil properties delivering a probability for hazardous, highly sensitive marine clay (quick clay). Identification of potential weakness zones in bedrock is a further pending task.

Challenge

Major linear infrastructure development is currently ongoing in Norway replacing vintage, often windy and slow, road and railway corridors with minimum curvature e.g. high-speed, double track rail alignments. Most of these new corridors cross complicated geological settings with little to no prior geotechnical data. The "InterCity project" encompasses a total of 230 km new track stretching north and south from Norway's capital, Oslo, and aims at a completion in 2030. Given the large area and desired rapid project execution, timely ground investigations are a critical factor.  

Method

High-resolution time-domain AEM bedrock mapping has previously been used by NGI with great success for both road, railway and tunnel design projects. It has been shown that in favourable cases AEM may even distinguish saline, marine clay from leached and potentially sensitive clay. These experiences provided the basis for the large-scale InterCity project survey.

In partnership with Danish AEM specialist SkyTEM Surveys we covered about 600 km2 extending over 9 separate geotechnical project areas. The AEM data were used to create a geomodel (bedrock topography and sediment type) for the detailed railway alignment and design.

Results

To handle the uncertainty inherent in any geophysical method, the 3D resistivity model was interpreted manually incorporating geotechnical data where available, geological- and topographical maps as well as aerial photographs. Based on that integration an expected shallowest and deepest bedrock model was delivered for further alignment selection works and borehole ground investigation planning. In areas with moderate bedrock topography, drilling largely confirmed the bedrock model. Due to the method's limited lateral resolution, partial mismatches where found in areas with strongly undulating bedrock (steeper than 30⁰ dip).

Beside of the bedrock model, the 3D resistivity model was further processed with respect to sediment properties, in particular to distinguish marine clay from highly sensitive "quick clay". As the salt content of such deposits (and thus electrical conductivity) strongly correlates with sensitivity, a method was developed to quantify quick clay hazard from the sedimentary resistivity model.

 

NGI

Left: Quaternary geology map along with flight lines (red) and existing railway (thin black); Inset: Resistivity model along red line with assumed bedrock (blue and black lines showing deepest and shallowest assumption) and drilling results (green bars). Right: Bedrock topography model derived from AEM data over an aerial image