2-D Electrical Resistivity In A Complex Geological Environment

Construction of a new office park / housing development East of Seattle, adjacent to the<br>l-90 corridor, requires construction of a new access road, the proposed South<br>Sammamish Plateau Access Road (SSPAR). The steep slopes adjacent to the<br>interstate highway were identified as a possible deep-seated ancient landslide.<br>Initially, numerous borings were completed with somewhat ambiguous results with<br>regard to delineating the presence of a slide. Seismic Refraction lines and<br>Electromagnetic profiles were also initially completed at the site by others with similar<br>results.<br>Based upon our 2-D Electrical Resistivity data, the site interpretation was indicative of<br>very complex geology, which was further substantiated with new boring data. The<br>subsurface geology appears to range from a mixture of clayey silt deposits up to large<br>ice rafted boulders, the size of a house. The primary conclusion drawn from the 2-D<br>Electrical Resistivity data defined what appear to be subsurface channels cut roughly<br>parallel to the steep site slopes. These channels are thought to be erosional from<br>meltwater flows of alpine glaciers. They may also result from the catastrophic draining<br>of a large glacial lake just East of Seattle. The channels were subsequently infilled with<br>soils derived from nearby deposits, causing the difficulty in both visual and earlier<br>geophysical exploration.