Innovation » Remediating the environmentally sensitive area of Waratah Rivulet
 

Remediating the environmentally sensitive area of Waratah Rivulet

Longwall Operations to the Bulli Seam of the Illawarra Coal Measures, at a depth of 400-500m below the surface, had resulted in subsidence which caused fracturing of rock in sections of the Waratah Rivulet.

The Waratah Rivulet is located 31km north of Wollongong in New South Wales and has a catchment area of 22 square kilometres which comprises approximately 29% of the total water catchment for the Woronora Dam servicing many townships south of the Georges River in Sydney and the Illawarra region.

Several remediation methods had been attempted in the past to fill these cracks in sections of the Waratah Rivulet but these had proved unsuccessful and as such Minova were contacted to investigate remediation using its special grades of Polyurethane Resins.

Robert Hawker, Minova Australia Product Manager, said Minova CarboPUR WF and Minova CarboPUR WFA were selected as the most appropriate products for use in this application due to their unique cure time, expansion properties and low toxicology levels.

The main areas of concern within the Waratah Rivulet area were geological rock bars within the stream system. Three rock bars were identified as areas for remediation, WRS1, WRS3 and WRS4. This paper describes the remediation efforts and successful results achieved on the WRS4 rock bar.

WRS4 fracture overview

The WRS4 rock bar is a relatively flat section within the Waratah Rivulet with a width of approximately 100m. The overall remediation was separated into an initial trial section of 20m followed by a further 75m after the initial trial proved successful.

The fracture network was caused by a combination of thrusting along cross beds, buckling near surface beds and tensile cracks due to upsidence. The depth and extent of voids/fractures were measured to be to a depth of 20m and voids varied between a few millimetres in width to 100mm in width.

The largest voids were identified to be within 5m of the surface. Water flow velocities within the fracture network were identified to be in the order of 5 m/s with measured permeability in the order of 10-2 to 10-4 m/s.

Environmental management and approval

Prior to any works commencing with the Waratah Catchment area an environmental review and management plan was established, and approved by the Sydney Catchment Authority, to control the onsite use of equipment and fuels, transport and storage of the Minova CarboPUR products and injection of the Minova CarboPUR products.

The injection process was identified as the most critical aspect for environmental management and as such Toxicology and Ecotoxicity tests were carried on the Minova CarboPUR product prior to and also during the injection process.

The Toxicology testing included pH, true colour, turbidity, Total Organic Carbon (TOC) and Dissolved Organic Carbon (DOC) whilst the Ecotoxicity testing examined affects on bacteria, algae and daphnia. Results of this testing identified the CarboPUR product once cured was inert with no affects on higher life forms within the stream system or to the existing water quality.

The application

During development of the Environmental Management Plan a specific drill and injection sequence was established to ensure effective overlap of the injected CarboPUR material. Boreholes of diameter 56mm were core drilled to a depth of 20m and spacing of 2m over the initial 20m trial zone.

These boreholes were then injected with CarboPUR in stages using combinations of injection pipes and borehole packers starting from the 13m-20m depth and working upwards. Injection limits relating to pressure (maximum 100bar) and volume (100L) were set to ensure injection did not cause further cracking of the rock bar and the spread of PUR was controlled within the injection zone. Injection rates of 10L/min were used throughout the injection process.

The injection equipment was setup within bunded trailers 200m downstream of the WRS4 rock bar. In excess of 7000L was injected into the first 20m trial zone. 
 
Following success of the first 20m zone, work commenced on injecting the remaining 75m across the WRS4 rock bar. A further 50,000L of CarboPUR was injected using the same staged injection process used for the initial injection. During the final stages of injection the following results could be seen.

- air bubbles being released upstream of the Minova CarboPUR grout curtain as a
consequence of their displacement from the filling cavities

- drop in downstream pool levels and subsequent rise in upstream pool levels as
the subsurface fracture network was sealed.

The results

Hydraulic conductivity tests were used to measure the success of the injection process by comparing pre-injection to post injection results.

Final conductivity measurements of between 10-7 to 10-9 m/s showed a 3 to 7 order of magnitude drop in the WRS4 rock bars hydraulic conductivity.                        
 
Water level monitoring in the various Waratah Rivulet Pool systems before and following the Minova CarboPUR injection process has show Pool F that is situated above the WRS4 rock bar has returned to water cycle levels of the downstream pools which are unaffected by mining activities. Unremediated Pools upstream of Pool F clearly show different water level fluctuations.
 
Core sampling taken within the injected zone has shown the Minova CarboPUR material to have filled fractures less than 1mm in width and up to 100mm in width.
 
It has been estimated the average thickness of the Polyurethane curtain to be 8m.
 
Water quality monitoring conducted during the injection process and for four weeks post the injection process was consistent with pre injection lab results and confirmed Minova CarboPUR’s ability to be used in an environmentally sensitive area.


For more details please contact:

Robert Hawker
Technology Manager
Email: robert.hawker@minovaint.com