Sandvik reaming heads used for raise bore dropshafts on Hong Kong drainage scheme

Published 2011-08-16 09:25:02 | Updated 2011-08-16 09:46:53

Contractor Macmahon is using four Sandvik reaming heads for boring 23 dropshafts through hard granite on the Hong Kong West Drainage Tunnel scheme.

 

Four Sandvik reaming heads are being used by Australian raise boring specialist Macmahon to bore 23 dropshafts on one of the world's most complex drainage systems, the Hong Kong West Drainage Tunnel (HKWDT) project.

Raise boring was chosen because of the extremely cramped area of most of the dropshaft sites, which are situated in the Mid-Levels residential district on Hong Kong Island.

As of the middle of May, Macmahon had reached the halfway point with 12 shafts bored and is due to complete the project in early 2012.

The Drainage Services Department of Hong Kong is building the HKDWT to relieve the flooding problem in Northern Hong Kong Island.

Designed by consulting engineers Ove Arup & Partners and with the main contract being undertaken by the Dragages–Nishimatsu Joint Venture, the project includes construction of a main tunnel, 32 adits, 32 dropshafts, 34 intakes and two tunnel portals. 

The Dragages-Nishimatsu JV is responsible for constructing the tunnel in two sections. The first section, 4.5 km long and 6.25 m in diameter, runs from Tai Hang and under the Aberdeen tunnel. The second, 6 km long and 7.25 m in diameter, is from the Aberdeen tunnel to Cyberport. The contractor is using two tunnel boring machines for this project, which arrived on the site in March 2009.

The adits have a total length of about 8 km and a width of 2.3 to 3.5 m, and will direct captured rainwater to the main tunnel. The contractor is excavating the adits mainly by the drill and blast method. 

The 34 intakes will intercept rainwater from open channels and underground drainage pipes, the water being conveyed to the main tunnel via the dropshafts and adits. The water discharges via the main tunnel by gravity to the sea at Cyberport. 

Raise boring is being used for 23 of the dropshafts, the remainder being either in shallow ground that can be excavated in a conventional manner, or in unstable soil that is not suitable for this method.

This method was selected due to the isolated shaft intakes on the hillsides across the Mid-Levels, with difficult access and the restricted weight loadings of the adjacent pathways.
It was also considered to be the safest shaft option, with no people required in the shaft during boring, and ensured the least environmental disturbance. 

The JV appointed Perth-based construction and mining contractor Macmahon to undertake the raise boring for the 23 dropshafts being constructed by this method.

The company’s project manager on the HKDWT, Hayden Clarke, says that once the adit has been excavated to the stilling basin, a pilot hole is drilled through the ground down to the basin. 

A Sandvik reaming head is then fixed to the end of the drill string at the bottom of the dropshaft. The drill rig then rotates and pulls the reaming head upwards towards the drill rig, excavating the ground. The excavated material falls to the bottom of the dropshaft and is trucked out through the adits, main tunnel and portals. 

“This is one of the most logistically challenging jobs we have worked on, as the majority of the raise bore sites are in highly urbanised areas with restrictions on noise, traffic and dust emissions, and sites that are extremely tight,” says Mr Clarke.

“Not only is there very little room to manoeuvre and operate the equipment, in many cases the sites are on very steep terrain with little normal road access.

“The project does however illustrate the benefits of raise boring within an urban environment, because the disturbance to the public is minimal. There is virtually no dust, and the spoil is carried out of the tunnel far away from where the shaft is bored.

“Noise is kept to the minimum because the rigs are very quiet, and the sound of boring is contained underground.”

Macmahon has four raise bore drill rigs working on the project - three  Atlas Copco  and one Sandvik Rhino unit - allowing the excavation of four dropshafts simultaneously during an 18-month project period. 

The contractor is starting each shaft with a 35 cm pilot hole using Sandvik Pilot bits and 28.6 cm drill pipes. Four Sandvik CRH8E extendable reaming heads are then being used to bore two sizes of shaft, one of 2.441 m and the other of 3.154 m.

The Sandvik reaming heads can be adapted to different rock conditions by re-arranging the cutters, in effect changing the spacing between the rows of cemented-carbide buttons that break the rock. The CRH8E consists of a base-head to which four segments can be bolted on, which means the head can be built to different diameters. 

Mr Clarke says that the shafts vary in depth between 44 to 172 m across varying degrees of ground quality and stability, with hard granites and volcanic tuff being encountered, some of which has a compressive strength of more than 250 MPa.

“There is some exceptionally hard ground here, but overall we are achieving very good penetration rates,” he says. 

“We are also not finding any problems with drift, and are meeting the client’s preferred guide range of 1% tolerance.”

The Sandvik reaming heads and cutters are supplied by Sandvik’s raise boring team in Newcastle, NSW Australia who are working closely with Macmahon to advise on the most effective use and configuration of the equipment.