Nevertheless there is an increasing need to rehabilitate these ageing, often large and important structures buried deep beneath some of our older cities.
Although a circular cross-section provides optimum structural strength characteristics, the oviform shape has often been used so that velocity would be higher at low flows, allowing the sewer to be self-cleansing. However the straight or gently curved side of an oviform is a liability when it comes to providing a liner with high load carrying capability.
This was the problem faced when a section of oviform sewer at Daceyville, Sydney needed rehabilitation. The sewer comprised precast concrete oviform sections with nominal internal diameters of 736 mm vertical and 533 mm horizontal in the classic 3:2 shape. It had originally been constructed in a fully timbered tunnel.
The section of the sewer requiring lining could be accessed by maintenance holes with depths to pipe invert varying from 9.8 to 12.1 m. The deeper maintenance hole extended to a depth of 17 m to provide access to a deeper 1066 mm x 711 mm oviform sewer.Article continues below…
The sewer was well below water table level. While the precast concrete sections were in reasonable condition considering their age, the joints were leaking, leading to unacceptable infiltration. The liner was therefore designed to withstand hydrostatic loads.
For structural design purposes the external hydrostatic load on the liner was taken as 9 m. Design showed that this was above the structural capacity of economically feasible lining systems matching the oviform geometry, however, a system comprising two circular pipes met the design requirements.
Interflow completed the project using a double Rib Loc Expanda pipe liner. It was proposed that the bottom liner would have a fixed diameter of about 250 mm, with the top liner installed later and expanded to the maximum diameter that would fit into the sewer, estimated to be approximately 485 mm. The remainder of the ovoid cross section would be filled with cementitious grout.
For Interflow, traffic management was an important part of the project. The deepest manhole was in the centre of three traffic lanes in one of the two carriageways forming Gardners Road – a main thoroughfare not far from Sydney Airport. In agreement with the NSW Roads and Traffic Authority, traffic control devices could not be erected before 10:00 am and the road had to be clear each day by 3.00 p.m.
The first stage of the project involved cleaning and CCTV inspection. At some stage during its life, repairs had been attempted with cement mortar on sections of the pipeline. Removal of the cement mortar required the extensive use of chain flails so that a clear bore could be provided to accept the lining.
Assessment of flows in the oviform pipeline and the lines it serviced showed that up to 3 hours storage could be provided. It was considered that sections of Rib Loc Expanda Pipe could be installed within this time, so no flow control beyond plugging of upstream lines was proposed. In the event that the available storage time proved insufficient, the flow could be released without adversely affecting the partially installed liner. In particular, bypass pumping was considered unnecessary.
Distances between access chambers were 56 and 98 m. After preparation of the pipeline, the bottom liner was first wound in at a fixed diameter to fit the ovoid invert. This liner was temporarily secured in place by filling with water to stop floatation.
The larger line was then wound above the smaller liner and expanded to fill the obvert of the pipe. After expansion was completed, the ends of the oviform were sealed, then the voids cement grouted.
Despite the time restrictions imposed by the need for traffic control, and the extreme depth of the pipeline, only a day’s work was required to install each section of liner.