Kristoffer Kåsin

Helicopter Electromagnetic Scanning as a First Step in Regional Quick Clay Mapping

Identification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects in Norway. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution airborne electromagnetics (AEM) collected from helicopter. However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.

CPTU Classification Diagrams for Identification of Sensitive Clays

When dealing with slope stability considerations in deposits where sensitive and quick clays might be encountered it is vital to map the extent of these clays. For the geotechnical engineer, the cone penetration test with pore pressure measurement (CPTU) is a powerful tool in this respect. With its combined measurement of tip resistance, pore pressure and sleeve friction, the CPTU holds a great potential for identification of quick and sensitive clays. Such interpretations can be done based on measured data directly or by combining parameters in dimensionless numbers. Amongst the more popular dimensionless numbers are the pore pressure ratio (B q ), the cone resistance number (N m ) and the friction ratio (R f ). Diagrams exist which allow classification of soils based on the combination of such numbers. Robertson (Can Geotech J 27:151–158, 1990) is one widely used example. However, In Norway, it is found that existing diagrams to a large extent fail to identify sensitive and quick clays. Based on a database of 10 Norwegian sites a new set of classification diagrams are presented with focus on identifying quick and sensitive clays. The diagrams are based on a pore pressure ratio where the tip pore pressure is used (u 1 ) rather than the u 2 -position as this is found to better capture the actual collapsible response of sensitive clays. The cone resistance number is modified to also include an effect of overconsolidation (OCR) instead of only accounting for vertical effective overburden. Also, the friction ratio is normalized with pore pressure (u 1 ) rather than the cone resistance. Electrical resistivity values from R-CPTU-soundings are also included in the considerations. The outcome is a set of revised classification diagrams that provides more accurate identification of Norwegian sensitive and quick clays compared to existing classification diagrams.

Future Strategy for Soil Investigations in Quick Clay Areas

The landslides at Rissa in 1978, and more recently at the Skjeggestad bridge in Norway, are devastating reminders of the potential threats related to quick clays. For a geotechnical engineering project it is hence important to determine if there is sensitive clay present and to clarify the extent of the quick clay deposit. Integration of geophysical and geotechnical methods has become more common in ground investigations nowadays, particularly in larger projects. In such integrated measurements, geotechnical engineers and geophysicists can cooperate, and by joint knowledge decide where geotechnical soundings, in situ tests and sampling should be located with optimal cost-efficiency. This paper describes how various investigation methods may be combined to achieve a successful strategy for detecting deposits of quick and sensitive clays. The methods presented herein include conventional soundings, CPTU and field vane test (FVT), supplemented by geophysical methods such as CPTU with resistivity measurements (R-CPTU), Electrical Resistivity Tomography (ERT) and Airborne Electromagnetic Measurements (AEM).

Airborne mapping of sensitive clay—stretching the limits of AEM resolution and accuracy

Due to postglacial uplift, lowlands in Canada, Norway, Sweden and Russia are prone to formation of highly unstable, sensitive, and leached marine clay (quick clay). Quick-clay failures are dramatic due to its high water content, resulting in liquefaction. It thus poses a major hazard for society and construction projects in particular, and knowledge of its extent is of vital importance. Quick-clay assessment is usually undertaken by geotechnical boreholes having the disadvantage of giving only information at the borehole location. To overcome this limitation, geophysical ground-based methods like electrical resistivity tomography have been used successfully. However, when a larger area has to be investigated, electrical resistivity tomography surveys become costly and time consuming. We show results from an airborne electromagnetic survey aiming at detection of different clay units for a road project in southeastern Norway. Airborne electromagnetic data clearly show structures within the sediment layer that correspond well with results from geotechnical boreholes. While a clear distinction between clay and quick clay cannot be derived from airborne electromagnetic alone, our study shows that this method has high-enough resolution and accuracy to map differences in clay units, which can subsequently be probed at specified locations. Thus, by using airborne electromagnetics to target borehole locations, the costs for the geotechnical drilling program can be reduced significantly.