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Sub-Soil ‘Foundation Study’

 

The Bridge Foundation Study  focused on mapping vertical variations of electrical resistivity to enable detection various layers with different lithology in the subsurface at the abutment locations. The objectives of the survey are as the following: To establish 2D Electrical Resistivity sections of the ground profile showing different layers of overburden and bedrock .To find out weak zones and water bearing zones in the specified survey areas.

GD-10 Multi-function DC Geo-electrical Res/IP system deployed for foundation engring strata study using 2D ERI Method in Kaligandaki river Nepal.

Interpretation for the ERT Line I : This profile runs parallel to Kaligandaki River in its right bank. The 2D ERT result up to 45 m depth and respective lithology are shown in ERT I.  The position of proposed abutment is also shown in the same Figure. The ERT result is discussed below. There are low to medium resistivity contours in the top part of the profile near chainage 80m, abutment location and near chainage 250m in the upstream part (marked by “a”). It indicates presence of overburden composed of alluviums with more gravels and boulders. Second layer is marked by low resistivity contours (marked by “b”) near chainages 80m. 150m (near abutment) and 250m. The low resistivity suggests presence of alluvial deposit with more saturated sands in these locations. These alluvial layers together extend to about 30m depth near chainage 80m, 12m near abutment site and 8m near chainage 250m. Near chainage 125m (20m downstream of present abutment site) the alluvial layer has mainly gravel boulder and extend only to about 9m depth. The underlying layer is marked by medium to high resistivity contours. This is may be layer with more gravels and boulders or weathered bedrock.

Interpretation for the ERT Line II : This profile runs parallel to Kaligandki river in its left bank. The centre of profile is at the proposed abutment site. The ERT profile layout is shown in  ERT II. There are medium resistivity contours in the top layer from chainage 80m to 180m, including proposed abutment site. The medium resistivity suggests presence of gravel boulder layer. It extends to about 7m depth from ground surface. The low resistivity contours are present near chainages 50m, 80m to 180m and 240m. These are the zones with dominantly saturated sands. This zone extends to about 24m depth near the proposed abutment site and 9m depth near chainage 250m. The underlying layer is represented by higher resistivity contours. This may be hard bedrock layer, which may extend from 20m depth to well over 45m depth. However, at locations near chainages70m (previously drilled location) and 200m the bedrock may extend from a depth of 6m to over 45m depth.