Ground water pumping of Mackay Park to cease: outcome unknown

Members of the community are alarmed to hear that Eurobodalla Council intends to remove the standpipe on Mackay Park and stop pumping the ground water that lies only a metre below the surface. Long term locals have known that the site of the Bowling Club, swimming pool and sports fields have sat on a site that was once covered in bogs and swamp. Over time it was cleared and over time it was also reclaimed with building materials and soil from town construction.

Above: Aerial view of the Bateman's Bay Township 30 May 1937 - Royal Australian Historical Society With much speculation that the ground below the now demolished Batemans Bay swimming pool was "fill", "sand as far as you can drill" and "quicksand", with suggestions that the water table was just below the surface the community continued to pester Council for the findings of their geo-technical report wanting to determine for themselves the validity of Council choosing that location for the site of the $70 million community owned project in light of the construction difficulties and ongoing complexities found just across the road in the Village Centre that sits less than 100 metres away on the same strata. The June 2019 Geotech report was finally made available publicly and made for interesting reading. In a nut shell: The Eurobodalla Shire Council commissioned Public Works Advisory (PWA) to manage the delivery of a Regional Aquatic and Arts Centre (Centre), in Batemans Bay. On a separate commission, the Council engaged PWA (Geotechnical Engineering Section) to undertake a geotechnical investigation at the proposed site. It was proposed to increase the number of boreholes to eight (8) and increase the depths of the boreholes to 10m, in lieu of the specified 6m. At the Council’s request, the scope of work was further amended to cover the car parking areas. The final program comprised the drilling of eight (8) boreholes to 10m depth on the proposed Centre’s site; and, seven (7) boreholes to 2m depth along the access road alignment and within the proposed car parking areas. The program was further amended. Along the western perimeter of the site, the depths of the boreholes were increased up to 19.4m depth, to determine the levels at which the basement bedrock occurs. In addition, in two of the boreholes, the bedrock was diamond cored to determine the quality of the bedrock with respect to the degree of weathering, rock substance strength and defect spacing. The results of the investigation were presented in July 2018. In January 2019, the Project Management requested that an additional investigation be carried out. It included the drilling of two additional boreholes to bedrock, installation of two groundwater monitoring wells and the drilling of an additional borehole. The June 2019 geotechnical report presents the data obtained from the field investigations and laboratory testing and discusses geotechnical aspects relevant to design and construction of the proposed development. With little surprise the report states that "drilling revealed that the sand deposits in the upper 1.5m (approximately) of the profile are possibly windblown in origin, while the underlying sediments, characterised by their grey to dark grey colour and presence of shell and organics are inferred to be associated with the tidal-delta flat environment." The report goes on to note "that all deep boreholes were drilled outside the perimeter fences of the swimming pool and mini-golf complexes. Consequently, the thicknesses of any fill, associated with the construction of the existing development, are not known.

"The subsurface profiles generally comprise a thin layer of topsoil/fill overlying unconsolidated sediments. The upper strata consist of variably coloured, medium grained and fine to medium grained sands. The sands are underlain by estuarine clayey sandy silts and sandy clayey silts to variable depths and then silty sands with traces of or some clay.

The unconsolidated estuarine sediments are in turn underlain by meta-sedimentary bedrock or, locally, a sequence of residual clay and weathered bedrock to the termination depth." "Although there is a certain degree of similarity in sediment types across the site, the subsurface conditions are highly variable with respect to thicknesses of various units, and, in case of the upper sand strata, some variations in the degree of compactness of the sands." Editors Note: The above description reflects the same geotechnical observations of the Stocklands (Village Centre) project over the road. That site had CONSIDERABLE difficulties in securing piering and continues to have issues with groundwater permeation. Of interest to those who warned of the presence of ground water just below the surface, the report reveals that "Groundwater was intersected in all of the boreholes, apart from borehole BH7 showing that the table was at an average R.L of 0.8m and an average depth of just 1.5m." The report provides its findings for each bore hole and summarises by saying "The groundwater is likely to be hydraulically connected to the McLeods Creek/Clyde River system and is expected to be subject to tidal fluctuations. Given that the sands are pervious, rises in groundwater levels are also possible during extended periods of adverse weather." In regards to the foundations required for the building the report reveals "Although conditions within the footprint of the proposed structures are not known, the available data suggests that there is a high potential for differential settlements to occur if shallow footing systems are adopted. For heavy loaded or settlement sensitive structures, piles bearing in weathered bedrock would be an appropriate foundation system. Due to the presence of shallow water table and collapsible strata, Continuous Flight Auger piles would be a suitable option." "The investigation revealed the presence of groundwater table at relatively shallow depths. If

excavations more than 1.5m (approximately) are proposed, then lowering the water table will be required. Dewatering of the sand strata may best be achieved by external

dewatering techniques, such as jetted dewatering wells." This week, however, we learn that Council will be closing the ground water filling station in Vesper Street (Mackay Park) due to the "significant developments in and around Mackay Park."

This means that Council will be removing the ground water pump that was put in place to assist in limiting the impact of the ground water on the Mackay Park and Village Centre facilities whilst also providing cheap, non-potable water for water carts undertaking construction. Council instead now advise "We have investigated options for a temporary filling station nearby to no avail. The difficulty is finding a high capacity water main with safe and practical access to and from the roadway for watercarts. However, we do intend installing a new water filling station on the northern side of the Clyde River. "A new site must have safe access and egress from both the Princes and Kings Highways as well as being located near a significant water main so that we can deliver high

flow without affecting nearby customers. To this end, we have been upgrading the water

mains in conjunction with the bridge works. We have a site identified but are unable to

provide access until the bridge works are completed." From this we can assume that: 1) Council is demobilising the existing Mackay Park groundwater pump 2) Council is aware of the groundwater and that lowering of the groundwater table is required 3) Council is aware that the groundwater table is the same one that impacts on the Village Centre. 4) Council is well aware it is building on a reclaimed swamp. 5) The cost of building on a swamp adds a significant unknown to substructure costs before the building even gets out of the ground (as was the case with the Village Centre) 6) That, depending on climatic conditions, the ground water table may have an impact on the life expectancy of the building. 7) Council are advised that continued dewatering of the groundwater table and lowing of the ground water table WILL be required. This represents an additional operational cost.