Heavy rainfall across Sydney over the first weekend of May 2015 was followed by breaking news about the appearance of two sinkholes emanating from an excavated development site at Harris Park. The sinkholes were reportedly threatening the structural integrity of adjacent buildings. One sinkhole was seen to be reaching within centimetres of the above-ground perimeter of one building. Occupants of all three neighbouring strata properties were evacuated amid mounting concerns.
CRC was called to a medium-density residential area on the fringe of the Parramatta CBD to assess the situation and direct emergency remediation works.
Sinkholes uncommon in Sydney
It’s relatively uncommon for a major sinkhole event to occur in the western suburbs of Sydney. Sinkholes are more commonly observed in regions where limestone or other soluble rock formations are partly dissolved by subterranean water, or where underground cavities such as caves and old mineshafts are prone to collapse. These types of sinkholes can occur without warning. Sometimes we see reports from other countries where the surface layer has unexpectedly given way, and the earth has swallowed part of a roadway or structure. In America, for example, there are hundreds of sinkhole occurrences every year. Most happen in remote or sparsely-populated areas, but occasionally there are reports of lost property and even human fatality.
The sinkholes at Harris Park were somewhat different in the sense that an open void had existed in the form of an excavated development site which was said to have been abandoned for a number of years. This mention alone should not cause concern to people who happen to be living or working near any building development excavation site, however. Sydney has seen many high-rise constructions stalled during the site excavation process, sometimes for many years, without causing any compromise to the integrity of surrounding buildings or infrastructure.
Investigations into the causes and effects of the Harris Park sinkholes are ongoing, but obviously rain would have combined with a number of other factors to bring about a very unfortunate event. Sympathy had to be felt for the sixty-or-so people of all ages and individual circumstances who were compelled to leave their homes at very short notice and would not be able to return properly until the entire area had been stabilised and their strata building certified for reoccupation.
Teams worked throughout the night
Between Monday 4 and Tuesday 5 May, CRC worked throughout the night at the epicentre of the Harris Park sinkhole zone, along with a team of geotechnical specialists, Council representatives, police and emergency services personnel, and construction crews. The pouring of concrete required for the primary stage of remediation was not completed until 4:00am on the second day.
The sinkholes were initially quantified at five metres in depth and filled with almost one metre of water, which is why the tremie method of concreting was used in the first instance. In simple terms, the challenge was similar to when caissons are constructed to build bridges over spans of water. Concrete is pumped to the bottom of the water through a tremie pipe. As the pour continues, the water above is displaced by the rising concrete, without diluting the concrete.
Round-the-clock team efforts made in the first few days of the sinkhole occurrences were just the beginning of a long sequence of remedial civil and construction-related works, geological and structural examinations, and expert monitoring and evaluation of results before all residents could safely re-occupy their homes. According to follow-up media reports many weeks later, dozens of people were still waiting to resume living in the two buildings most directly affected.
Sinkholes just ‘tip of the iceberg’
The visible profile of a sinkhole may represent just the tip of the iceberg, so to speak, where subterranean cavities and subsidence caused by erosion may extend far beyond visible indications, and not necessarily contiguously from where an opening has appeared on the surface. Sinkholes may manifest suddenly but the underlying erosion has usually taken a significant period of time to develop. Once a sinkhole event has occurred anywhere within close proximity of a built environment, the remedy may not be as simple as filling the void with solid materials. The conditions surrounding the Harris Park sinkholes were as complex as any that might be encountered in remedial engineering, and this degree of complexity is commensurate with the broad scope of works and protracted length of time needed to reach safe and satisfactory resolution for all concerned.
The importance of obtaining periodic engineering reports on commercial and residential strata properties – particularly multi-storey developments – cannot be over-emphasised. It is a relatively small investment in peace of mind for property owners and building occupants, and would inherently add value for owners wishing to sell their strata property at any time. When you consider all of the risks, inconveniences, and costs associated with remediation works on the scale of what has eventuated in relation to the Harris Park sinkhole occurrences, the argument in favour of proactive diagnostic reporting for all strata properties becomes very clear.
