Engineering properties, such as Unconfined Compressive Strength (UCS), qu and Young's Modulus, E, of mass rock have a significant effect on project design and construction operation of the rock-structure interface. A better understanding the rock engineering properties can provide a base for a more rational approach to design of civil engineering works. The estimation of rock engineering properties has historically relied on subjective assessments of rock mass quality and measurements of intact values. A recent project in Melbourne, Australia, has provided an opportunity to test rock core-samples from nearly 200 boreholes along an approximately 12 km's length of a sewerage tunnel alignment in both Siltstone and Basalt. This paper presents some previously unpublished laboratory testing data on the engineering properties of Siltstone and Basalt. Also included is an examination of the important influence of moisture content and the degree of weathering on the mechanical properties - particularly the qu and E parameters. The present findings show a good agreement with the results of previous studies on the mechanical properties of both Siltstone and Basalt.


The assessment of overall modulus (or stiffness) and strength of a fractured weak to moderately strong rock mass is a key parameter in the design of civil engineering works in these materials. It also provides the highest level of uncertainty due to scale effects, anisotropy and the influence of joint frequency and joint stiffness. Rock mass properties are difficult to measure directly and their estimation has historically relied on subjective assessments of rock mass quality and measurements of intact values. The Northern Sewerage Project (NSP) is being developed by Melbourne Water and Yarra Valley Water to address future sewerage requirements for the Upper Merri Creek and North Darebin Creek Catchments in the northern suburbs of Melbourne, Australia. The NSP involves the construction of approximately 12 km's of sewer tunnel consisting of several alignment sections, with eight main shafts and tunnel connections. The main features of the Project include a gravity sewer tunnel and several permanent access shafts. The location of the Project is shown on Figure 1. As shown in Figure 1, the NSP is in an urban area, with commercial and residential development. There were approximately 200 geotechnical investigation boreholes carried out along the sewerage alignment with depth in a range of 30m to 80m. Both insitu and laboratory testing performed on borehole and soil/rock core specimens, such as Borehole Image Processing (BIP), Gamma logging, Packer & Slug testing and unconfined compressive strength (UCS), Point Load Index (PLI), Static Middle-third Ring Young's Modulus, Sonic Velocity (P and S waves) and Dynamic Young's Modulus etc. This paper is only concentrated on the laboratory test of rock core specimens, in particular, the effect of weathering (moisture content) on strength and compressibility parameters, such as UCS and Modulus of Basalt and Siltstone.

Fig. 1 - Project location(available in full paper)


The geology of the NSP area is characterised by a mixed sequence of sedimentary and volcanic materials.

This content is only available via PDF.
You can access this article if you purchase or spend a download.