A CPT with resistivity measurement has been developed as a tool to map quick clay depsits. Quick clay may be a serious geohazard in Scandinavia and North America.

Standard CPT (left), with the resistivity measurement (right).

Leached marine clay and consequently quick clay in coastal, post-submarine areas in Scandinavia and North America is a serious geohazard, and large efforts are being taken to map its occurrence and extent.

Quick clay hazard zones have traditionally been mapped by virtue of geotechnical field investigations such as rotary pressure soundings and soil sampling. Within a three year research effort NGI, in cooperation with other groups in Norway, Ireland and Sweden, has started to investigate the applicability of resistivity logging CPT (RCPT) in concert with surface resistivity methods such as ERT, CSRMT, IP, etc. to identify and map quick clay.

Results from RCPT sounding (click for enlarged picture in a new window)

As the electrical resistivity of sediments is generally governed by the clay content and pore fluid chemistry, resistivity appears to be a good indicator for quick clay. Within a potential quick clay layer the bulk resistivity thus indicates quick clay or unleached clay. First results from RCPT field data and core measurements in the lab confirm our assumptions.

When it comes to extending RCPT soundings laterally and spatially, surface resistivity methods come to mind and have in fact been successfully applied in Norway before. The challenges in combining RCPT and surface resistivity are the different survey parameters such as geometry and frequency, as we are dealing with a presumably anisotropic and polarizable material. Tackling these challenges is substance of our ongoing research.
 

Description - CPTU and quick clay

CPTU - "Cone Penetration Test with pore pressure measurement" is a steel cylinder with cone tip which can measure tipresistance, pore pressure and skin friction as the cylinder is penetrated into the ground, either from the surface or from the bottom of a borehole. Measured results are used to characterize soil profiles, such as layering, and to calculate strength parametres, such as shear strength in clay and friction angle in sand. CPT is a common tool for soil investigations.

Quick clay
can be described as a highly sensitive, marine clay. It has been deposited in a coastal marine environment during the last glacial period. Due to post-glacial heaving these areas are now up to 200 - 300 m above sea level. The former marine clay has consequently been exposed to fresh ground water and percolating surface water which has led to a reduction in pore water salinity.

Marine clay has large pore volumes between the clay minerals and the structure is kept stable by the salt in the pore water. When the salt leaches, the bindings between the clay minerals become weaker, and the clay becomes sensitive. Some of the most densely inhabited areas of Norway, e.g. the areas around the Oslo- and Trondheimsfjord, are located in potential quick clay areas and hence detailed risk assessment is needed.