Mernda Rail Project Factsheet: Geotechnical investigations
26 May 2016
The State Government is extending the South Morang train line to Mernda to cater for the significant population growth in Melbourne's north. To develop a robust design and inform the planning process, geotechnical drilling investigations were undertaken at approximately 130 locations along the proposed rail corridor.
What we have found
The results have shown that there is a high water table and basalt rock profile more commonly between the Plenty Gorge Park and Bridge Inn Road. Basalt rock is a naturally occurring mineral, which can be several times stronger than concrete.
Basalt rock was encountered at almost all locations and in certain areas was found to be as high as 400mm below the existing surface level. Special excavation equipment is required to excavate through hard basalt rock which is completed at a much slower pace than excavating regular soil, sand or clay. Excavating through extremely hard basalt rock is also very noisy.
The natural groundwater level varies in depth along the proposed rail corridor, with a general fall towards the south. Testing of groundwater levels around McDonalds Road and Plenty Road in South Morang have shown that the groundwater level is considerably deep, at approximately 10 meters to 11 meters. This means that the excavation works required in these areas will be done above the water table level, avoiding the need to continuously dewater the trenched area during construction and operation of the rail utilities.
The groundwater level around Hawkstowe Parade, The Parkway and Bridge Inn Road is much closer to the surface, between 2.8 meters to 5 meters.
The image below shows a cross section of the ground profile at Hawkstowe Parade
Geotechnical investigations are essential to test the types of geological features beneath the ground which influence the design and construction methods of the project.
A truck mounted rig was used to drill boreholes of around 100mm diameter to a depth of up to 21 metres below the surface.
Core samples were taken and analysed in a laboratory to provide detailed information about existing underground conditions and materials such as soil, rock, clay and sand.
Groundwater monitoring wells were installed at approximately 50 locations to measure the depth of the natural groundwater levels. Groundwater was also sampled to determine existing underground water quality which can influence the design.
Confirming underground service depths such as gas, power, water, electricity and communications was also undertaken as part of the geotechnical investigative process using ground penetrating radar and non-destructive digging techniques. The location of underground services influences the design of the rail but also ensures the project avoids causing damage to any such infrastructure.
Areas disturbed during these works were reinstated to their original condition at their completion. In some areas a small monitoring well cap was left at the location of the borehole to allow further testing to be done over time.
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