Using Resistivity and Ground Penetrating Radar to delineate bedrock profile and changes in construction within a trunk sewer rehabilitation project
GBG Australia were commissioned to investigate the construction of a 150m length of a main trunk sewer approximately one hundred years old. The sewer runs approximately 5m below a main road through a sandstone / sand dune environment where the construction varied from cut and fill culvert in the sand dunes and concrete lined tunnel construction in the solid sandstone. Installation of new access shafts and relining of the sewer necessitated confirmation of the location of construction changes, and the bedrock profile.
Selection of Techniques
The investigation was conducted along a Sydney suburban roadway. The depth and construction of the sewer main required that as well as Ground Penetrating Radar (GPR) data, there may be a need for augmentation from a complimentary technique. Resistivity was chosen as vehicular and pedestrian traffic precluded the use of seismic methods. It was believed that the change from sand to sand stone and the reinforcement within the tunnel lining would provide enough variation in resistances to allow plotting of the bedrock profile and relate this to the depth location of the sewer.
Methodology
Profiles of GPR and resistivity data were collected along the long axis of the sewer from the road way above.Transverse GPR profiles were acquired at selected areas running perpendicular to the direction of the sewer. . GPR data was acquired utilizing a 120MHz antenna, the digital record was saved for off-site analysis. Resistivity data was collected with automatic switching and recording equipment and a dipole – dipole array with an electrode spacing of 5m. Where the resistivity survey was along the road pavement, the asphalt was drilled to allow insertion of the electrodes. The electrodes were all watered in due to the high resistance encountered in the sand subsurface environment.
Results
Both the GPR and resistivity data revealed the changes in the sub-surface structure. The GPR data was of higher definition than the resistivity profiles and as such was used to finalise the position of construction changes and bedrock profile(refer to profiles attached figure 1 and 2) The complementary data sets cross confirmed the regions of construction changes, allowing more accurate planning and pricing of the proposed new work.
Images
Figure 1. 120MHz Profile Collected Along Street Kerb
Figure 2. Modeled Resistivity Profile
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