Mission impossible on Yarra River with Soilmec SR-100

Soilmec SR-100 Piling Rig

3 m diameter new siphon shaft drilled in basalt close to Walmer street bridge in Melbourne, Australia The Yarra River flows from the pristine flanks of Mt Baw Baw in Victoria’s West Gippsland region. For 242 kilometers the main water course – and 24 tributaries – drains an area of 4060 m2, about half the area of metropolitan Melbourne. On its journey through Melbourne CBD (Central Business District) and finally to Port Phillip Bay, it supplies 9 catchments dams and provides drinking water to about 2.6 million households. Today, the upper Yarra is a green and residential area with golf greens and bicycle tracks. Close to Walmer Street Bridge an old sewage siphon built of bricks in 1911 which crosses under the Yarra River has to be replaced.

The project
The siphon brings the sewage water across the river from Yarra Bend Park downstream to Walmer Street Bank where a new inspection shaft (new Man Hole) 3m diameter, 20m deep has to be drilled. The siphon, 150m long, to be executed by Horizontal Directional Drilling, is of 450 mm diameter, it will be fitted with two pipes of 160mm and 190mm diameters. Main contractor, Fulton Hogan has entrusted the shaft drilling work to BRC piling. The geological conditions of soil to be drilled are very difficult especially on the Walmer Street Bank. On the top, fill and residual intact soil areas present up to 100MPa U.C.S values. Five to ten meters of slightly weathered basalt, sand, basalt gravel and cobbles are following up to a weathered siltstone horizon located at the shaft bottom.

Soilmec SR-100 Piling Rig

Drilling methodBecause of the shaft diameter and soil difficulties, BRC Piling & Foundations have chosen the most powerful rig of the Soilmec range, the SR 100. First of all, a 3,050 mm diameter hole was drilled in the 1 m thick clay level overlaying the basalt at the drilling spot. At the top of the shaft, in order to stabilize the clay and establish an external fall protection at the hole mouth, a 3 m internal diameter, 2 m long casing section was placed in the 3,050 mm diameter drilled shaft section. During drilling, the casing acted as a safety barrier for the piling crew. This diameter has been chosen to allow the placement of a 2,600 mm diameter, 20m long permanent casing which will become the working area for construction of the sewer shaft. Unlike traditional shaft construction methods, the major advantage of using the SR 100 to excavate the shaft is that there will be no need for men down the hole until the shaft is completely shored and safe. After the top casing was placed and the top clay layer secured, coring of the basalt commenced.

Soilmec SR-100 Piling Rig

Due to the rock strength, a 1,200 mm diameter pilot hole was first drilled all the way to the toe of the rock. Then a 2,400 mm and finally a 2,900 mm diameter hole was cored all the way through the basalt. Once the rock had been cored at 2,900 mm, the 2,600 mm diameter permanent casing had to be inserted. It was vibratory driven in two sections of 12 m and 8 m having a 20 mm Wall thickness. The 12 m casing was inserted first and vibrated down to a workable level. The SR 100 was then used to drill out the inside of the casing using a 2500 mm diameter digging bucket, ensuring not to drill past the casing toe. After the casing was cleaned out, the 8 m length was welded to it and then vibrated to the shaft toe. Due to the weathered siltstone at the toe of the shaft, BRC Piling & Foundations were able to seal the casing in this material achieving a dry bore. The casing was pre prepared with a series of threaded holes so that grout lines could be attached after completion. The outside of the casing was pressure grouted to stop the inflow of any water or material when the Horizontal bore comes through. In respect to the traditional method with an oscillator used in combination with the drilling rig and heavy double wall casing sections, the BRC Piling & Foundations method including casing vibratory driving has brought a significant advantage. In fact, vibratory driving used in combination with the powerful SR 100 capable of delivering high torque and great crowd force has brought BRC Piling & Foundations to reduce the shaft execution time by 50% in respect to the preliminary evaluations.