Site characterisation and permeation curtain grouting for deep basement excavation

This paper describes the geological site characterisation for the design and construction of a 26m deep basement excavation in Fitzroy, Melbourne. Key geotechnical features include an extensive, 10-40m thick felsic dyke within Melbourne Formation rock which generated geotechnical implications for excavation stability and high groundwater inflows. As a mitigation effort a hybrid retention system consisting of a solider pile wall and an infill grout curtain was subsequently developed.

Four rounds of geotechnical and hydrogeological investigations were completed to delineate the extent of the dyke and identify groundwater flow characteristics. Site trials were completed to provide permeation grouting parameters within the dyke. Pumping tests are proposed to validate the effectiveness of the grout curtain in mitigating groundwater inflow to the excavation.

A Discrete Fracture Network (DFN) of rock defects was parametrically scripted in three dimensions to assist with the design of grout volumes and wedge failure modelling. The DFN was developed from seed boreholes using investigation results from acoustic and optical televiewer borehole imagery.

The grout curtain design consisted of a primary and secondary grouting programme based on the DFN model and results of the grout trial.