This is the challenge that Sydney Water recently faced with the rehabilitation of one of its large trunk sewers at Hale Street in Botany. It required a unique solution comprising of a fully supported continuous internal HDPE lining that has proven to be successful thanks to careful and detailed planning during its implementation.
The Southern and Western Suburbs Ocean Outfall Sewer (SWSOOS) system carries sewage from as far away as Campbelltown to the Malabar STP for treatment. From Sydney Airport to Malabar, the system comprises two sewers running approximately parallel, with the older of the two sewers, SWSOOS No.1, being 90 years old while SWSOOS No.2 is 60 years old.
The above ground section of SWSOOS No.1 in Botany has had a history of problems with the leakage of effluent through the walls. The problems have stemmed from the original design of the box section which was supported on both aqueduct and a cellular base on ground. The aqueduct section was constructed with inadequate reinforcement in the walls, which led to significant longitudinal cracking from shrinkage and thermal effects particularly on the horizontal curve. The box on ground section is comprised of a containment cell with regular expansion joints constructed on a continuous base. This led to major problems with longitudinal cracking at the vertical expansion joints and horizontal construction joints.
These longitudinal cracks in the walls are active and move under thermal effects resulting in leakage. The problem arose early in the life of the structure and in spite of numerous attempts to address this problem they only provided short-term solutions.
In more recent times increased septicity in the sewer has also resulted in deterioration of internal concrete surfaces above flow level from hydrogen sulphide related gas attack.
A detailed investigation undertaken by Sydney Water found active movement of cracks which were thought to have the potential to gradually worsen increasing leakage, resulting in an unacceptable long-term risk of major leakage from an internal surcharge during wet weather. It found that previous repairs undertaken externally were not adequate and any future option using external repair methods was unlikely to provide a suitable long-term solution. Rigid internal repair systems such as epoxies were also rejected as they were unable to tolerate cyclic movements at cracks. The report recommended that the sewer be internally sealed and protected with a continuous lining.
Sydney Water engaged the services of SAS TTI, a joint venture between Sewer Assessment Specialists and Trenchless Technology International, to assist in the preparation of technical requirements and drawings for tender documentation. Sydney Water called tenders for the rehabilitation work which was awarded to Metropolitan Restorations.
The contract required an internal repair system that was continuous without joints but was still able to accommodate existing movement regimes at expansion joints and horizontal cracks in the structure. Additional requirements included retention of the internal profile of the sewer, lining material composition with a proven long term corrosion resistance in a sewer environment and the ability to negotiate two horizontal bends in the alignment.
The rehabilitation solution provided by Metropolitan Restorations consisted of a continuous HDPE lining. The lining is known as the Steuler Bekaplast system, which originates from Germany and consists of a HDPE sheet with close centred truncated conical studs welded to the rear of the sheet. The sheet is held in place by infill, grouting the gap between the rear of the lining and the cleaned internal face of the sewer. The product is provided locally by Pacific Lining System, who provide the lining in 3 m long tubes and undertake site welding of joints between segments and any penetrations through the lining.
The rehabilitation comprises of the lining of 1,100 m of a box shaped reinforced concrete section with a curved shape floor and internal dimensions of approximately 3,400 mm x 1,820 mm and 3,400 mm x 1,640 mm. Expansion joints are located at approximately 17 m centres and there are two horizontal changes in direction and two cross connecting structures to the nearby SWSOOS No.2.
Successful installation requires the inside of the lining to be fully supported during grouting. Metropolitan developed a special retractable mobile form, constructed using hinged tubular framing. Each side is bow shaped with equally spaced ribs and a movable joint that allows both hydraulic and screw jacks to adjust the height and location of the form. In order to meet the time frame for the contract, two forms were manufactured and special fast setting grouts were used so that two 3 m segments can be installed for each day’s production. The 3 m length was selected so that the retractable frame and lining could negotiate bends in the horizontal alignment.
A lot of planning and design went into the development and construction of the retractable form. The success of the form was crucial not only because it represented a significant investment before any production work could proceed, but also if problems arose with the form, it had the potential to result in major delays and cost overruns. Fingers were crossed and breaths were held unnecessarily, as after a few teething problems in the first week sorting out containment of grout at the ends and the stretch characteristics of the lining, the forms were working at full production.
A special feature of the design was the use of a continuous lining without joints. Existing expansion joints in the box structure joints are first sealed to prevent grout flowing into the existing joints. A compressible filler is then installed over a short distance either side of the joint and the studs behind the lining removed over the same area, so the lining bears against the compressible filler. This effectively enables the HDPE lining to be debonded for a short distance over each expansion joint. The length of debonding selected is so that movement can be accommodated by the elastic distortion of the debonded length of lining.
The inside of the sewer surface was thoroughly cleaned before installing the lining to remove all loose and gas attacked concrete. The high level of cleaning provided good surface for grout bond. A specially - developed sequenced grout method was used to fill the gap between the lining and inside surface of the sewer that was able to successfully eliminate the formation of voids. Cube tests were used to confirm the early strength gain and pull off tests were used to ensure grout bond strength were adequate for the wide flat roof section.
Other features of the project include the use of HDPE inlets sleeved and welded through the lining, the attachment of special PE step irons and the use of a welded fabric strip at terminations in the lining. The strip enables a bond to be formed with the epoxy mortar coatings which were installed at manholes, stop log guides at cross connections and the upstream and downstream ends.
To date 800 m of lining work between Southern Cross Drive and Botany Road has been completed and the sewer has been returned to operation. Work has commenced on the remaining 300 m section upstream of the isolation for the cross connection towards Ross Smith Avenue, Mascot.