Two Rocks, WA
Background
CMW Geosciences was engaged to undertake a geotechnical site investigation across numerous residential lots of the Breakwater Estate subdivision to assess the karst risk under the proposed building envelopes. Previous preliminary investigations had identified surficial karst features (cavities/caves/pinnacles) and associated loose sands.
As the site is situated within a karst risk landform area, CMW tailored the investigation to satisfy the karst landform management plan (KLMP) as well as providing detailed remedial measures and associated engineering design parameters.
Scope
Scope of works involved the geological and geomorphological mapping of each building envelope and surrounding areas coupled with advancing 17 air core boreholes to depths of up to 20m with associated Standard Penetration Testing (SPT) and Perth Sands Penetrometer (PSP). CMW decided to use an alternative air coring method which offers excellent core recovery providing more accurate structural information of the underlying strata as well as allowing for improved drilling feedback. This method worked very well when trying to identify karsts/voids/cavities as we were able to monitor drill rates and sample return to 0.1m accuracy. Following the field investigation an interpretive geotechnical report was compiled outlining the various risks anticipated and measures to mitigate.
Findings/Challenges
The CMW scope of works involved the geological and geomorphological mapping of each building envelope and surrounding areas coupled with advancing 17 air core boreholes to depths of up to 20m with associated Standard Penetration Testing (SPT) and Perth Sands Penetrometer (PSP) tests. CMW decided to use an alternative air core drilling method which offers excellent core recovery and there by providing more accurate structural information of the underlying strata as well as allowing for improved drilling feedback. This method worked very well when trying to identify karsts/voids/cavities as we were able to monitor drill rates and sample returns to 0.1m accuracy. Following the field investigation an interpretive geotechnical report was compiled outlining the various risks anticipated and measures to mitigate.
Findings/Challenges
During the investigation SPT testing was undertaken to observe the relative density of the natural sands. Within one of the boreholes, SPTs were advanced to depths of approximately 7m in order to define the base of loose sands. It was found that these sands were decreasing in density with depth, with “N” values as low as 2, indicating the possibility that the sands were falling through to underlying cavities/voids within the limestone.
Below these loose sands, variable leached calcarenite was encountered, immediately followed by rod drop with no sample return. The rod drop, together with the overlying very loose sands indicated the presence of karst features. After this, CMW reviewed previous investigation data, including Ground Penetration Radar (GPR) and Cone Penetrometer Testing (CPT). We then proposed the relocation of the building envelope within the lot boundaries. A second borehole was then advanced within the new envelope to confirm the absence karst features in the recommended location.