Presentation for ISZA, Békéscsaba, Hungary 5-6. April 2013 2D and 2.5D multi-electrode resistivity survey on a potential tunnel construction site in Norway Presentation for ISZA, Békéscsaba, Hungary 5-6. April 2013 Balázs Rigler, Geophysical-Engineer Dorottya Bartucz, Geologist

Project objectives Potential tunnel construction site was investigated by using surface resistivity imaging technique near Drammensfjord in southern Norway. The planned track of the tunnel comes through a tilted, flat area covered by quick-clay and crystalline rock hills slashed by faults and dykes. An area (L1-7) focused mainly on mapping of soil (potential quick clay) and rock surface was surveyed with 2.5D resistivity array coverage (6 parallel ~300m long survey lines 300m long each and one survey line, which was perpendicular to the six parallel survey lines) Two long (400-500m each) profiles (P2 and P3) along the track of tunnel focused primarily on identification of zones of weakness and sediment distribution across the mountains. Weakness zones can often be identified using multi-electrode resistivity survey.

Oslo

Quick Clay Overview (1) Properties [5] [6] Quick-clay material originates from highly porous clay deposited in a marine environment during and/or following the last glacial period. The former marine clays lie currently above sea-level and have been exposed to fresh-water environment due to isostatic rebound as a result of lowering the relative sea level. Salt, which originally contributed to the bonding between the clay particles, may therefore have been leached from these materials by groundwater and percolating surface water. If sufficient leaching of salt occurred a highly sensitive or “quick” material may result. A good knowledge of the groundwater flow system is a prerequisite for mapping of potentially quick clay areas. - The presence of permeable layers, such as sand and silt layers, which are connected to surface water or other water conducting layers, greatly enhances the possibility of leaching. The topography of the bedrock can also affect leaching: if there is a local high in the surface of the bedrock underlying the soil, the outflow of groundwater may be concentrated to this point.

Quick Clay Overview (2) Geohazard Quick clay landslides are common hazard in formerly-glaciated margins like Scandinavia and Canada Specifically at construction works such as tunnel or bridge constructions quick clay must be taken seriously as geohazard Only small perturbations in stress conditions, such as human activity, erosion or excess rainwater saturation, can trigger a failure [6]. [6]

Quick Clay Overview (3) The Prospecting Area On upper side (east side) of the area are a number of source origins - It is also reasonable to assume that a significant amount of water supplied to the underlying sand layer between rock and clay by water from cracks in the granite. - This means artesian conditions in many parts of project area, which contributes to the leaching of electrolyte content of the supernatant clay formations. [1] Water-bearing fractures from granite of eastern freshwater supplies through several layers of sand, thus the whole camp volume exposed to trapping of multiple and significant contact surfaces and leaching of the electrolytes.

Classification of soil resistivity [2]

[2]

Description of acquisition system and principle - 10 channel, 72 electrode Syscal Pro (IRIS Instruments) data acquisition system - Stainless steel electrodes were used Multi-electrode 2D and 2.5D resistivity surveying was carried out with continuous profiling using 5m spacing with 42, 47, 49 and 60 electrodes on the different profiles. - Wenner-Schlumberger configuration was used for each profile with 50m penetration depth. Electrodes were planted at 5m cable takeout locations. Electrodes were watered thoroughly to avoid contact resistivity problems. Dust spray was used before connecting cables.   - Resistivity check was run by Syscal Pro before data acquisition started. Contact errors were repaired. Current injection time for WS array was 500 ms. - Topography was surveyed by RTK GPS Data processing was performed in Prosys (IRIS Instruments) software. Geophysical inversion (2D only) was performed by using the commercially available RES2DINV software (M.H. Loke, 2000) [8] Inversion results are presented as inversion models following pseudosections using a uniform color scale for quick clay site and for hard rock site to make comparison of the results easier.

Measurement Configuration

Summary Results on area L1-7 Marine clay generally have very low resistivity (1-10 Ωm) NOT OBSERVED Sensitive (quick) clay have medium low resistivity (20-100 Ωm) OBSERVED AND DELIMITED Rocks and sand with fresh pore water has very high resistivity (>100 Ωm) OBSERVED AND DELIMITED Sandy layers were observed on top of underlying bedrock and also on the surface with higher elevation The dipping surface of bedrock could be easily identified and appears with very high >400 Ωm resistivity values In the prospecting area the formation consists of sensitive clay, with the possible exception of residues of dry crust on the upper eastern portion (higher elevation). Clay layers thus seems to be air sensitive, but to varying degrees. This is probably due to different degrees of leaching. Results on Profile 2 and 3 Zones of weakness could be effectively identified by combining resistivity survey results and preliminary geological information

References [1] Final Report, Geoelektriske og Strukturgeologiske undersøkelser RV23 dagslet – Linnes. R&P Geo Services AS, 2012. [2] Solberg, Inger-L., Dalsegg, E. (2012): Resistivitetsmålinger for løsmassekartlegging i Kaldvelladalen og ved Fallan i Melhus kommune, Sør-Trøndelag. Data og tolkninger. NGU Rapport, 2012.013 [3] Solberg, Inger-L. et al. (2010): Veileder for bruk av resistivitetsmålinger i potensielle kvikkleireområder. NGU Rapport, 2010.048 [4] J.A. Mendoza, T. Dahlin (2008): Resistivity imaging in steep and weathered terrains. Near Surface Geophysics, 2008, p. 105-112 [5] G.Sauvin, I.Lecomte et al. (2012): Geophysical Investigations of Quick-clay Slide Prone Areas. Near Surface Geoscience – 18th European Meeting of Environmental and Engineering Geophysics, Paris, France, 3-5 September 2012 [6] G.Sauvin, I.Lecomte et al. (2013):Towards geophysical and geotechnical integration for quick-clay mapping in Norway. Near Surface Geophysics, 2013, 11. [7] A.Malehmir, M.Bastani et al. (2013): Geophysical assessment and geotechnical investigation of quick-clay landslides – a Swedish case study. Near Surface Geophysics, 2013, 11. [8] M.H.Loke (1998, 1999, 2000): Electrical imaging surveys for environmental and engineering studies. A practical guide to 2-D and 3-D surveys.

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