09_Raise Boring's Role in Two Major Civil Tunnelling Projects

IOM3 (Hong Kong Branch)

Hong Kong Tunnelling Conference 2009

Raise Boring’s Role in Two Major Civil Tunnelling Projects D. Aldridge1, D.R. Clayton2, R. Burke1, K. Anderson1 1) Australian Raise Drilling Division, Macmahon Mining Services Pty Ltd, Perth 2) Dragages-Nishimatsu Joint Venture, Hong Kong

ABSTRACT The Northside Storage Tunnel Project (NSTP) was a AUD460 million project comprising approximately 20km of tunnel constructed between 1998 and 2000 to collect wet weather overflows from four major effluent overflow structures situated around Sydney, Australia. The overflows so collected are then transported in a controlled manner for processing and subsequent deep ocean discharge. The NSTP required the construction of ten vertical shafts and their associated works and raise boring was selected as the preferred method of construction for all but one of these shafts. The shaft construction included the use of directionally controlled techniques to ensure pilot hole accuracy, the installation of secant piling to support the upper part of the one of the two major shafts, the raise boring of the shafts at diameters from 1.8m up to 5m with depths of up to 170m and the subsequent support of the major shafts. The Hong Kong West Drainage Tunnel (HKWDT) is a major project currently being constructed by the Dragages-Nishimatsu Joint Venture which is to collect storm water runoff from the northwest part of Hong Kong Island. The main tunnel and adits will total approximately 18.4km and will collect storm water through 32 intake shafts along the alignment. Of these shafts, 24 will be excavated by raise boring, with shaft depths varying between 32m and 172m and diameters of around 3m. Australian Raise Drilling (ARD) successfully constructed the NSTP shafts and has been selected as the sub-contractor to undertake the raise boring of the HKWDT shafts. The two projects share some common problems from a raise boring perspective – many of the site working areas are restricted in size, access is difficult and sites are in close proximity to residential areas. The upper portions of shafts, in most cases, comprise poor ground requiring lateral support. The paper considers the two projects, compares the various techniques used for the shaft construction and comments on the success of the methodologies. 1 INTRODUCTION Raise boring is a commonly employed technique in the hard rock mining industry, normally applied to drilling vertical or near-vertical holes to provide access, ventilation and for ore or waste passes between underground levels in the mine. It has also been applied more rarely, in drilling horizontal holes. Shaft depths in excess of 1,000m have been bored with shafts varying from approximately 1m up to 6m in diameter. Figure 1 shows a diagrammatic representation of the process.

71

IOM3 (Hong Kong Branch)

Hong Kong Tunnelling Conference 2009

Figure 1 – Diagrammatic representation of the Raiseboring process

While raise boring does require that there is access available at both ends of the shaft and lateral support work cannot be undertaken in the shaft until the reaming process has been completed, the technique has a number of significant advantages over other methods of shaft excavation, the principal ones being: 1. It is a non-entry method as opposed to conventional shaft sinking or raising and is thus intrinsically safe, 2. Mechanical cutting of the rock renders it, under most conditions, a much more rapid method of advance and cheaper than conventional methods, 3. Spoil from shaft reaming falls to the bottom of the shaft, obviating the need for surface removal, which is an advantage in urban areas where traffic flows are a consideration, and 4. The shaft is not open until reaming has been completed which reduces surface noise and dust emissions from the excavation process very significantly. The civil construction industry has perceived it as worthwhile to try to exploit these advantages in respect of vertical or near-vertical development used for access into underground tunnelling projects and there have been a significant number of projects on which raise boring has now been utilised. 2 NORTHSIDE STORAGE TUNNEL PROJECT 2.1 Background and general description The Northside Storage Tunnel Project (NSTP) was developed in the period 1998 to 2000 to improve the water quality in Sydney Harbour. Previously, during periods of heavy rainfall, cracked sewage pipes and illegal stormwater connections had caused water to enter the sewerage system. When the system reached capacity, these sites acted as “relief valves” releasing diluted sewage to the environment. The NSTP was developed to collect the wet weather overflow at four main sites, retain the overflows in a large deep tunnel and transport them to the North Head Sewage Treatment Plant for treatment and subsequent controlled deep ocean discharge. Generally, the tunnel is in operation for 30-40 days each year during periods of wet weather. It has a storage capacity of approximately 500 Ml. The tunnel commences west of the Lane Cove River and runs a distance of about 16km east to finish approximately 160m below surface at the North Head Sewage Treatment Plant. The main tunnel has a diameter of 6.6m. From a junction at Tunks Park, a second tunnel runs approximately 3.7km at 6m diameter to Scotts Creek. A third, 1.4km long tunnel was constructed at 3.8m diameter from North Head to Little Manly Point for the sole purpose of managing spoil during the construction phase. Figure 2 below shows the general horizontal alignment of the tunnels.

72

IOM3 (Hong Kong Branch)

Hong Kong Tunnelling Conference 2009

Figure 2 – Horizontal Alignment of Northside Storage Tunnel System

2.2 Vertical Development Required The NSTP required the development of a number of vertical shafts. They were drilled in two stages. 2.2.1 North Head Sewage Treatment Plant shafts The NSTP carried out geotechnical investigations at the North Head shaft sites, including the drilling of cored boreholes proximate to the shaft locations, in-situ water pressure tests and laboratory testing of the rock core from the boreholes. The underlying strata in the area is Triassic Hawkesbury Sandstone (comprising medium to high strength (30 – 100 MPa) sandstones with some siltstone laminae) making up the lower portion of the shafts. The data was assessed by the NSTP geotechnical consultants and it was determined that the sites were suitable for raise boring of the shafts. Following a tendering process that was commenced in November 1998, Australian Raise Drilling (ARD) was selected as the preferred contractor and a contract was awarded on 12th February 1999. Site work commenced on March 1999 and five shafts were raise bored at this location. Table 1 below summarises relevant data from these shafts. Shaft Depth Diam. Start date Finish date Pilot hole (m) (m) deviation (mm) Vent/Spoil shaft 168 4.50 26-Mar-99 29-Jul-99 164 Lift shaft 154 5.00 22-Jul-99 03-Oct-99 114 NSOOS shaft 160 2.10 02-Jul-99 13-Aug-99 26 URM shaft # 1 42 1.80 20-Apr-99 04-May-99