A qualitative geophysical assessment was undertaken for Clarence Valley Council as part of the conception stage of the Yamba ebb-tide release where NSW Public works is the project manager. Lessons learned from a similar project in Iluka were utilised to improve the methodology for the Yamba project. It should be noted that this project is currently in the design phase.
The Iluka ebb-tide release utilised simple geotechnical techniques for creating a ground model. Various historical boreholes were available and traditional desktop interpretations were conducted to create a rudimentary model of the geology (Figure 1). This model proved to be inadequate leading to variations during the construction stage of the project. Specifically, the ground model failed to identify weak spots in the indurated sands (coffee rock) leading to delays as a result of hole collapses in larger gravel pockets.
Figure 1: A 2 dimensional geological cross section generated solely by joining similar structures from nearby boreholes often creates more problems than it solves (diagram courtesy Clarence Valley Council).
The Yamba ebb-tide release improved on the methodology used for previous projects and included an Early Tenderer Involvement (ETI) process. A need for robust ground model was also identified and an early form of OEMG’s IDGM was utilised. This ground model combined results from an Aquares geophysical survey with historical borehole data (Figure 2). The IDGM was then presented to design and construct contractors as part of the ETI process prior to tenders being submitted, allowing geological risk to be established at a detailed level. Results were presented in such a way that they were also able to be interpreted by people without a background in geophysics.
Figure 2: For these works 14 line kms of data was collected comprising approximately 14,000 shot points over two days. The very large volume of data points allowed the creation of an accurate 4 dimensional ground model. Generally, loose sediments are seen above indurated sands. An Iso-surface along the top of the indurated sands was created (below) and the data below excluded. This then highlighted any weak areas extending past the target depth of the under-bore and the design group was able to project an ideal path for the pipe.
Of interest, the historical boreholes GH5 and GH7 appear to not conform with the findings of the Aquares model (Figure 3). However, after revievwing the borelogs, and inparticular, the Standard Penetration Test (SPT) results, it was possible to re-interpret the findings of the boreholes to conform to the Aquares model. This highlights the benefit of multiple and unrelated ground modelling tools, the IDGM and that boreholes are a remote sensing tool and subject interpretation.
Figure 3: It can be seen from a cross section along the centre line that boreholes GH6 and 3 conform to the predictions of the Aquares model, but GH5 and 7 do not. However, after reviewing the original borelogs and in particular the SPT values, the client was able to re-interpret the findings of the boreholes to conform to those of the Aquares model.
Despite the project being in the design phase, the improved ground model has already provided tangible benefits to the project. Probably the most important was the reduced risk levels for contractors as weak spots in the “Coffee rock” were identified up front with a high degree of confidence and the findings of the ground model could be effectively communicated through the use of the IDGM to all parties. This gave NSW Public works and Clarence Valley Council confidence to assume the risk for potential variations as a result of latent geological conditions. Results complemented the ETI process very well, giving surety to contractors for their designs.
A number of sustainability outcomes were achieved due to the geophysical collection methods and associated ground model including: