The area of interest had been provisionally selected for the development of an electrical power generating plant, aimed at servicing a nearby metal alloy refinery (Figure 1). Geotechnical investigations were concerned with characterizing the geology and physical properties of the near surface as these would influence the foundation design of such a structure. Eleven geotechnical boreholes and several shallow investigation trenches had already been developed.
GAP was originally approached to conduct cross-hole seismic tomography surveys to attempt to determine the S-wave seismic velocity of the weathered zone. However, the geotechnical boreholes were found to be too far apart to conduct such surveys with any confidence. Additional boreholes would have to be drilled if the S-wave velocity was to be determined using this traditional approach.
Figure 1 : Area of interest over Google Earth Image. Red dots : location of passive seismic (HVSR) soundings (with posted weathered zone thickness estimates), Pink lines :seismics refraction traverses. Borehole locations are given in white.
GAP proposed the use of a relatively new passive seismic method called HVSR. Passive seismic (HVSR) soundings were conducted at each borehole and along transverses between boreholes to estimate the S-wave velocity and to map the thickness of the weathered zone over the site.
To compliment the passive seismic surveys, traditional seismic refraction surveys were undertaken along 100m long traverses at each borehole to confirm weathered zone thickness estimates and to provide P-wave velocity estimates of the same.
The S-wave velocity of the weathered zone at each borehole was calculated from the passive seismic soundings. Weathered zone thickness estimates obtained from borehole geology logs and seismic refraction surveys were in excellent agreement (Figure 2). Poisson’s ratio for the overburden was calculated based on estimated weathered zone P and S-wave velocities.
Figure 2 : A) Plot of weathered zone thickness from borehole logs against estimates of the same from seismic refraction surveys. B) Plot of P-wave and S-wave velocities of the weathered zone, showing a bimodal grouping.
Through the use of seismic refraction and passive seismic GAP was able to estimate the S-wave velocity and thickness of the weathered zone over the site. Calculated Poisson’s ratio values indicated that the site was characterized by stiffer lithologies in the east than in the west (Figure 3). This indicated either preferential weathering or a change in the composition of the weathered zone material. Field work was completed in two days and came in under the client’s original project budget. Survey mapping objectives were exceeded.
Figure 3 : Distribution of Poisson’s ratio estimates over the site. Higher Poisson’s ratio values – red, lower Poisson’s ratio values – white.
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