Seismic retro-fit of an historic earth dam using grouted stone columns

An earth dam in Victoria, constructed in 1885 as part of Melbourne’s water supply system, has been in continuous use and will continue to form an integral element of the system. A safety review between 2004 and 2006 identified remedial works requirements to address a number of issues including embankment stability. Analysis of geotechnical information for the site, in conjunction with geological modelling, concluded there were layers and zones of soils that exhibited the potential for liquefaction under earthquake loads, thus constituting a significant risk to the stability of the embankment and the water supply. Design parameters were agreed and three areas were specified for treatment. Ground improvement options were assessed and the solution adopted was to install grouted stone columns. The design was undertaken, reviewed and accepted and the work carried out between July and September 2011. Grouted stone columns were formed using the bottom feed vibro system, conventionally used for the construction of vibro stone columns. A cement-bentonite slurry mix formed the grout, which was added to the stone prior to its placement into the vibro rig’s hopper. Several trial columns were carried out, with attention being paid to the stone size and grading; the grout components and proportions; and installation procedures. This paper presents the background to the solution adopted to retro-fit the dam against future seismic events and presents the results of testing, initial trials and the implementation of the ground improvement.