Repetitive Dynamic Loading Of Soils And Highly Weathered Rocks Due To Foreshore Construction Activities

Rockwall construction work was carried out on the foreshore of Port Phillip Bay at Mount Eliza in 1995 involving the delivery of many truckloads of boulders, the mass of each boulder generally being in the range of 0.8 to 3 tonnes. After being dumped onto the beach from tip-trucks, these boulders were then individually placed either in rockwalls at the back of the beach or dropped onto the beach sand to form a seawall for erosion protection purposes. Nearby residents complained that the foreshore construction activities generated noticeable ground vibrations at their houses. The occupants of one house, located 95 metres (m) from the point where trucks were dumping their loads of boulders, reported that hanging objects in the house were set swinging by the vibrations, which also could be distinctly felt when lying on lounges and beds within the house. The ground vibrations generated by the 1995 foreshore construction works were not monitored at that time, but on 13th November 1998 the 1995 foreshore works were replicated and monitored, so as to determine the magnitude of the resulting ground vibrations and their attenuation with increasing distance.

The 1995 works continued over a period of several weeks. The repetitive nature of the vibrations, involving about one hundred significant dynamic loadings each day, raised the possibility of fatigue weakening of the soils and highly weathered rocks in the cliffs behind the foreshore. The soils and highly weathered rocks in the area have unconfined compressive strengths less than 250 kPa. The adjacent cliff developed tension cracks just behind its crest, prior to completion of the foreshore construction works in June 1995, and failed on 8th July 1995. The cliff was not instrumented at the time of failure and hence the precise cause(s) of failure remain a matter of opinion. The aim of the 1998 test work was to ascertain if the vibrations generated by these foreshore construction works were sufficiently powerful for fatigue weakening to be a feasible, contributory failure mechanism for the cliff.

Comprehensive description of vibration requires measurement of the amplitude and frequency of the ground motion, and how these parameters vary over time, ie recording of a time series of the vibration rather than just the peak particle velocity. This allows for the recordings to be decomposed using Fourier analysis to show the frequency composition and additionally allows for conversion by differentiation or integration between displacement, velocity and acceleration. The ground vibration from each of the 1998 tests described in this paper was recorded as a time series.