An Emperical Model To Correlate Rock Mass Classification And Hydraulic Conductivity, Geotechnical Engineering Data Acquisition And Reduction Perspective

This paper presents an empirical model for estimating rock mass hydraulic conductivity of fractured Hawkesbury Sandstone in the Sydney basin. The hydraulic conductivity of a fractured crystalline rock mass can be a critical factor in tunnelling projects and dewatering designs. The Lugeon test (commonly known as the “packer test”) is a common test used to estimate the in-situ permeability of a rock mass.

The Packer test is carried out over a specific length (typically 3-6 m) within a borehole to reduce the range of variation of the affecting parameters. To construct a detailed profile of hydraulic conductivity through the rock mass a relatively large number of Packer tests are often required which can be costly and time consuming.

The new rock mass classification system called the “HC system” was proposed by Hsu et al. (2011). The HC-system assists with the hydraulic modelling of a rock mass and is based on 4 parameters which can be readily assessed from borehole logs and borehole Televiewer data, namely Rock Quality Designation (RQD), Depth Index (DI), Gouge Content Designation (GCD) and Lithology Permeability Index (LPI). A modified new rock mass classification system called the “HC-system” or HC model has been specifically developed for Hawkesbury Sandstone in the Sydney basin.

Regression analysis was conducted to assess the correlation between the calculated HC value (using the HC model) and the corresponding hydraulic conductivity from the in-situ packer tests.

To confirm the feasibility of the proposed empirical HC model, the model was subsequently used to estimate the hydraulic conductivity of similar Hawkesbury Sandstone boreholes from a range of projects that also have corresponding Packer test data for comparison.

This empirical HC model may assist with two important hydrogeological applications. The first application is to estimate hydraulic conductivity of fractured sandstone of similar geological set up based on HC-values. By using this approach, hydraulic conductivity data in a given site can be estimated from borehole data, which increases the reliability and confidence of the packer testing. Secondly, for in-situ aquifer tests the HC-system is a valuable new rock mass classification system for estimating the degree of permeability of a borehole. The results obtained confirm the validity and flexibility of the empirical approach to handle cases of onshore and offshore data sets, in relation to data acquisition and data reduction (optimisation).