The Sewerfix Wet Weather Alliance (SWWA) was established to deliver Sydney Water’s Wet Weather Overflow Abatement Program. The alliance brings together five organisations: Sydney Water, Manidis Roberts, MWH, PB and United Group Infrastructure. Primarily targeted at swimming sites, the program will provide benefits to the environment and human health, and is part of Sydney Water’s long-term SewerFix program of sewerage system improvements.
All sewerage systems around the world are designed with overflow points. They prevent wastewater from backing up into people’s homes when the sewer is full. During wet weather, rainwater enters sewers through cracks or faults in pipes and through illegal sewer connections. Once the sewer pipes are full, diluted sewage spills through designed overflow points into waterways. Prior to this project (diluted) sewage was discharged to Sydney Harbour via two overflows on the Mosman Submain approximately 185 times in 10 years. The Eaton St WWSF reduces this frequency to the catchment target frequency of 20 spills in 10 years.
The basic function of the facility is to pump excess flows from the Mosman Submain to another area of the catchment with spare capacity. To achieve this, the facility pumps flows along a rising main beneath Montpelier Street to the top of Forsyth Park. A short section of gravity sewer takes the flows to the top of the bore, and from there it flows via gravity to Brightmore Reserve where an energy dissipation chamber slows the flow down before being discharged to the receiving sewer.
Preliminary work prior to award
No gravity options were considered to be feasible for this project, hence a pumping station was the only viable means of transferring the flows. From concept stage the design of the pumping station and the rising main were relatively straightforward. The difficulty was in how to get the flow from the top of the rising main to the receiving sewer. Connections to several sewers/tunnels were considered, including The Northside Storage Tunnel (NST), the Alfred Street Submain, the Mosman Slopes Submain and Northern Suburbs Ocean Outfall Sewer (NSOOS), all of which flow to North Head Sewage Treatment Works via the NSOOS. All of the above are deep (in some cases very deep) tunnels for most, if not all, of their lengths – the one exception being the Mosman Slopes Submain, which has a short section of pipe running through Brightmore Reserve in Cremorne. However, three of the other options were shorter and had slightly more favourable grade tolerances, hence the Brightmore Reserve option was initially discounted.
Connecting to the NSOOS at the Alfred Street Submain was the shortest of the options but it posed the most difficult problems from a construction and safety perspective both during construction – including setting up drilling equipment on top of heritage-listed World War 2 diesel storage tanks – and for maintenance. The Alfred Street Submain is a 120 year old, brick-lined, 4 ft by 3 ft, ovoid sewer, over 90 m deep in places. The bore would have crossed within a few metres of the Alfred Street Submain, approximately 50 m from the connection point to the NSOOS. If it was damaged during this exercise there is no way, at present, of repairing it. The connection point, MH21, is over 65 m deep. The NSOOS runs at the base of MH21 and is over 3 m wide and almost 3 m high. Nearly 90 years old, it has constant waist height sewage flowing around 40 km per hour in an atmosphere that would require breathing equipment to operate in. → Given this, the problems with making the connection, the potential structural issues with the MH and the NSOOS, the associated problems with high pressure drilling fluid acting not only on the NSOOS walls but on the Alfred Street Submain, this option was discounted – although not until every possible way to make it feasible had been explored.
Several other options were investigated but due to local community, odour control, access, health and safety as well as many of the issues mentioned above they were discounted.
Design and award
Once all other options had been explored this left the Mosman Slopes Submain in Brightmore Reserve. This option was over 300 m longer than the other options and therefore considerably more expensive. In order for this option to be feasible some kind of saving had to be realised. The most obvious way to achieve this was to reduce the pipe size. Initial options had the bore at constant grade, with the average grade approximately 2.7 per cent. It was imperative that the bore be able to take all the flow that comes from the pumping station, approximately 300 litres per second. If there was any backing up within the pipe the additional head would act on the pumps and the facility would not be able to meet its design criteria. The varying nature of grade in pipes laid by HDD technology led to a DN560 PE100 SDR11 pipe being chosen to ensure the flows could be conveyed safely. This pushed the ream size to 28 inch. Options to reduce the pipe size had to be explored. Discussions between the design team, Sydney Water Operations, UEA and Highside Drilling Services led to the development of an alternative approach that had the potential to use a smaller pipe, a smaller ream and save the project over $A700,000.
Instead of attempting to drill at a constant grade and have the pipe run part full, the team investigated options to design the bore running full over part of its length. The idea being that if the bottom section of the bore is laid flat, so the sewage will start to back up the steeper, upper section, until sufficient head has built up to drive through the desired flow. Hydraulic calculations determined that the design flow could be met with a smaller, DN500 PE100 SDR11 pipe if the design grades of 1 per cent for the ‘flat’ section and 4.74 per cent for the ‘steep’ section were achieved. At no point could the ‘flat’ section be allowed to have any backfall, and it was imperative that over the last 100 or so metres the 4.74 per cent grade was accurate. Over-compensation for a grade deviation in either section could have forced the use of the larger pipe, pushing the project over budget.
Mobilisation
Given the complexities of steering a bore with such a flat grade it was believed that the other aspects of the bore would have been relatively straightforward. However, upon completion of service location and design of the energy dissipation chamber in Brightmore Reserve, it became apparent that the bore needed to commence on grade from the existing surface level, dictating the need to either lower the entire drill rig by 3 m or remove the rail and secure it in a purpose-built sheet piled hole. Due to restricted working hours and days it was decided that the best option was to install the rail into a sheet piled hole, eliminating the risk of having the entire rig below surface level for the duration of the project.
Movement of plant and equipment to site presented numerous challenges through the tight one-way systems of Cremorne, with the Vermeer D300x500 and maxi rig spread not usually seen in this sort of suburban setting.
Pilot bore
Geotechnical information provided by the client indicated that the majority of the borehole was to be within Sydney Sandstone, with high likelihood of encountering shale bands at various points along the bore. Accurate geo-tech information helped forewarn areas of potential grade deviation as well as prepare for the expected groundwater infiltration.
Twice weekly updates from site kept the design team informed as to the progress of the pilot bore. The information was used to update and assess the hydraulics as work progressed. Agreement was reached between all parties that should the bore go out of tolerance, drilling would stop, the hydraulics would be assessed and corrective action taken. The Lead Civil Engineer Kenny Bowie said “Discussions between the engineers helped each other understand the drivers and constraints of their respective challenges, outline risk mitigation strategies and, in the end, produced a better than expected result.”
At the completion of the pilot bore the bit within the exit pit was surveyed and all as-drilled information used to calculate expected hydraulic performance and determine required pipe diameter. Fortunately, the pilot bore was a complete success and, due to the accuracy of the drilling, the bore can take more than the design flow rate, therefore providing Sydney Water with an asset that has room for future expansion beyond the design requirements.
Tolerances for the final surveyed bore are as follows:
‘Flat’ Section Ch 0.00 to 850.00
Design Grade = 1 per cent
Constructed Grade (ave) = 0.89 per cent
Minimum Grade = 0.41 per cent
(0.59 per cent less than design grade)
Maximum Grade = 1.75 per cent
(0.75 per cent more than design grade)
‘Steep’ Section Ch 850.00 to 1,403.00
Design Grade = 4.74 per cent
Constructed Grade (ave) = 4.67 per cent
Minimum Grad = 4.36 per cent
(0.38 per cent less than design grade)
Maximum Grade = 5.43 per cent
(0.69 per cent more than design grade)
Reaming
Due to the close proximity of residents to the drill site considerable effort was made to reduce noise impact, including the installation of sound attenuation around plant and equipment, the use of electric pumps and limited working hours and days in order to successfully undertake the project.
Information gained on the pilot bore was used to develop tooling, with the decision made to forward ream straight to final borehole size of 24 inches. Reaming of the borehole commenced in early December 2009 with good progress made through to the planned Christmas period shutdown.
As the borehole size increased so did the ingress of groundwater, requiring the need for further onsite processing of drilling fluid. By utilising its DFE 700 gal/min cleaning system and DFE 600 x 900 high volume centrifuge, UEA were able to easily process and recycle the large volumes of drilling fluid and groundwater encountered on site. The use of specialised solids control equipment not only limited the volumes of liquid waste being removed from site but provided sandstone cuttings that were easily able to be reused off site.
Reaming works recommenced early January 2010 with forward reaming out to 1,130 m successfully completed. The decision was made to backream the remaining 270 m in two passes with the bore hole swabbed in preparation for pipe pull. In order to confirm grade and suitability, CCTV inspection of the borehole confirmed a clean, cylindrical borehole ready to accept pipe.
Pipe welding and installation
Due to the limited access at the exit side and the inability to pre-weld over 1.4 km of pipe, several options were investigated. After lengthy discussions and testing it was agreed that ten strings of pipe could be pre-welded and joined during the pipe pullback without compromising the integrity of the pipeline. Limited working hours prevented round the clock pulling, but due to the borehole condition this was not an issue. From connecting the swivel to the pulling head to watching it arrive in the launch pit, pullback pressures were less than calculated and everything went according to plan.
Hydrostatic testing of the pipeline was successfully completed after installation, confirming a usable pipeline and successful outcome for all involved.
Completing this project has been a proud achievement for all involved including drilling more than 67 m below Military Road, successfully installing pipe on such a flat grade via HDD all without a MTI, LTI or non conformance.
