09_raise boring's role in two major civil tunnelling projects

IOM3 (Hong Kong Branch) Hong Kong Tunnelling Conference 2009

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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, Perth2) 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.


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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, and4. 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.


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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 (<20 – 100 MPa) quartzose sandstones and lesser shales, siltstones and laminates) to a depth of approximately 80 to 100m from surface with the underlying Newport formation rocks (comprising predominantly 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 Average reaming Notes (m) (m) deviation penetration rate (mm) (m/hr) Vent/Spoil shaft 168 4.50 26-Mar-99 29-Jul-99 164 0.48 1,3,4,5 Lift shaft 154 5.00 22-Jul-99 03-Oct-99 114 0.52 1,3,5 NSOOS shaft 160 2.10 02-Jul-99 13-Aug-99 26 1.91 1,2,3 URM shaft # 1 42 1.80 20-Apr-99 04-May-99 <1% 0.95 1 URM shaft # 2 42 1.80 06-May-99 13-May-99 <1% 1.06 1 Notes: 1) Start and finish dates refer to drilling work only 2) Lower 91m only reamed for the NSOOS shaft 3) Directionally controlled pilot hole drilled by ARD sub-contractor 4) Top 11m secant piling installed by ARD sub-contractor 5) Primary support comprising rockbolts, mesh and shotcrete installed by ARD

Table 1 – Summary data - North Head shafts


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Plates 1 and 2 show ARD’s Robbins 73R raise drill and mobile headframe respectively at the North Head sites.

Plate 1 – ARD’s Robbins 73R drill on Vent shaft site

Plate 2 – ARD’s mobile headframe on Lift shaft site

2.2.2 Main Overflow Site shafts

This portion of the NSTP development was the subject of a separate work package to the North Head Sewage Treatment Plant shafts. It was tendered in June 1999 and the contract, which provided for the drilling of four shafts, was awarded to ARD in December of that year. Site work at the first shaft commenced in late February. Varying ground conditions were encountered at each of the sites. Table 2 below shows summary data in relation to these shafts.


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Location Depth Diam. Start Finish Average reaming (m) (m) date date penetration rate (m/hr)

Lane Cove River West 50 1.8 02-Mar-00 14-Mar-00 1.88 Quakers Hat Bay # 1 70 1.8 30-Mar-00 10-Apr-00 1.73Quakers Hat Bay # 2 70 1.8 15-Apr-00 01-May-00 1.73

Scotts Creek 57 1.8 06-May-00 17-May-00 1.95

Table 2 – Summary Data – Overflow site shafts

Site restrictions at the Quakers Hat Bay site required that steps were taken to reduce the site area required by the drilling equipment and the impact on the surrounding residential areas. These included: • A layout that was specifically designed for the site. As examples, the rig was unable to use its normal

radiators for cooling (because of both noise and area restrictions), the drill rods and the required generating set were located on the road verge overlooking the drill site and 4 cubic metre capacity skips were used for recirculation of drilling water rather than the normal dams.

• Drilling hours were restricted to 07.00 to 18.00 on weekdays and 08.00 to 13.00 on Saturdays.

2.3 Discussion of NSTP work

The objective of making use of raise boring to excavate the shafts for the NSTP was to take advantage of the intrinsic merits of raise boring in shaft development, particularly in urban areas. In the case of the NSTP, it was the ability to remove the spoil from the excavation activities through the tunnel rather than using trucks in built up areas, rapid and economic completion of the shaft excavation process, minimisation of the noise and dust impact and a very high degree of safety. As can be evidenced from the narrative above, apart from ground problems encountered while installing the secant piles on the Vent shaft, the shafts were completed quickly and successfully. There were no accidents recorded during the construction of the shafts.

3 HONG KONG WEST DRAINAGE TUNNEL

3.1 Background and general description

The Hong Kong West Drainage Tunnel Project (HKWDT) was developed to mitigate flooding in the low-lying areas of northern Hong Kong Island. The heavily urbanised low-lying areas are built on reclamation that in places extends 0.5 to 0.8km from the original coastline. This then backs steeply on to mountains of +500m elevation. Hong Kong has a subtropical climate with a rainy season extending from April through to September and an average rainfall of 2,200mm/year, however, it can be subject to intense rainstorms with hourly rainfall rates in excess of 100 mm and daily maximums in excess of 300mm. Such intense rainfall can overwhelm the existing drainage system, that due to rapid urbanization, is decades old. Plate 3 shows the results of such rainfall events. To alleviate flooding the Drainage Services Department (DSD) of the Government of the Hong Kong Special Administrative Region (HKSAR) initiated the HKWDT Project.


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Plate 3 - Flooding in Hennessey Road – Wan Chai – June 2008

Initial ground investigation commenced in 2004, the design and alignment of the scheme was completed in early 2007 and the detailed design and build contract was awarded to Dragages-Nishimatsu Joint Venture (DNJV) in November 2007. The general concept is to intercept the storm water midstream and divert it to an outfall at Cyberport on the west of Hong Kong Island. Intakes will intercept the water at natural stream channels, existing culverts or storm drains. The water will enter a vortex structure and drop down a vertical shaft in a helical motion to the adits below, where it will flow until it meets the main tunnel and eventually discharges into the sea (Figure 3). The majority of the shafts will be constructed by raise boring with the remainder by mechanical excavation or reverse circulation drilling. The adits will be excavated by drill and blast and the main tunnel by tunnel boring machines. Figure 4 shows the horizontal alignment of the tunnel.

Figure 3 –Schematic of the Hong Kong West Drainage Tunnel


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Figure 4 – Horizontal Alignment of Hong Kong West Drainage Tunnel

From the outset the preferred method of shaft construction was raise boring. This method has significant advantages for shaft construction within the urban environment of which the most important is to minimize disturbance to the public at large: 1. Bottom up construction method. All spoil during reaming is removed from the base of the shaft via

the adits and main tunnel to the east portal or the barging point at the west portal. This significantly reduces the impact on traffic in the vicinity of the shaft and forgoes the need for surface transportation of spoil. This is of importance as some of the shafts that are located on narrow or steep roads that would not be practical for heavy dump trucks.

2. As the shaft is not open until the completion of the raise boring there is a very significant reduction in the environmental impact in terms of excavation noise and dust.

3. Reduction in construction time compared with traditional methods. 4. The site area is severely restricted at many shafts and such challenges can be overcome by raise

boring. Plates 4 and 5 show typically restricted drill sites.

Plate 4 – BR5 drill site – steep hillside, single track road, weight restriction


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Plate 5 – MA17 drill site – hairpin bend in busy road

3.2 Vertical Development Required

DNJV has carried out ground investigation at the shaft sites which has comprised of drilling cored boreholes within a few metres of each shaft to at least the invert level of the adit. Tests conducted included: • Lugeon tests to determine ground water characteristics • Impression packer tests to determine joint orientations • Falling & rising head tests to determine groundwater characteristics in saprolite • Laboratory tests including unconfined compressive strength (UCS) and rock joint shear tests. Cores were logged in accordance with local Geoguide 3 practice, together with detailed logging of individual discontinuities and logging to the Q system (Barton 1974). All data collected was subject to geotechnical assessment by DNJV and their design consultants. This included: • Kinematic analysis – using Rocscience software Dips and Unwedge • McCracken Stacey Analysis – Probabilistic stability analysis based on a modified Q-system

(McCracken 1989) • Plaxis – finite element analysis From the analysis the suitability of raise boring was confirmed and shaft portions that require ground improvement/ grouting were identified. The geology encountered in the shafts is expected to be granite and tuff; • 11 shafts are comprised mainly of Mount Butler or Kowloon medium grained granite • 10 shafts are comprised mainly of Repulse Bay Group coarse ash crystal tuff and fine ash vitric tuff • 3 shafts are in the transition zone and are comprised of a combination of both granite and tuff. With the exception of the rock/soil interface zone, the rocks are typically slightly decomposed to unweathered with the granites having UCS strengths of 100 to 280 MPa and the tuffs of 100 to 380MPa. Figure 5 illustrates the proportion of soil and rock for each shaft and the preferred method of excavation.


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Figure 5 – Illustration of ground type & construction method

3.3 Discussion of HKWDT work

From a raise boring perspective, there are some clear similarities between these two major civil projects. • Both projects are situated in highly urbanised situations which impose restrictions on the noise, traffic

and dust emissions that are tolerable from the shaft excavation activity. This is particularly relevant to the HKWDT situation with high density housing proximate to a number of the sites. Raise boring provides an ideal method to address these issues as the spoil is not brought to surface and the shaft is not open until it is completed. Restrictions to working hours and acoustic shielding, if necessary, can be used to address the noise issues.

• A number of sites in both cases have restricted access for heavy equipment. This is severely exacerbated in the case of the HKWDT where drill sites are generally very tight and, in some cases, situated on very steep terrain with little normal road access. For the sites situated on the Bowen Road (BR4, W1 and BR5), the rig and equipment will be broken down into loads of less than 3 tonnes in weight and re-assembled on the drill site. Specifically designed gantry cranes will be used to lower equipment into the sites.

• Both projects have drill sites that are of very restricted site area. The methods by which this was managed at the NSTP site at Quakers Hat Bay have been discussed above. In the case of the HKWDT, where the site restrictions will be more acute, machine selection has been a critical consideration. ARD have selected Atlas Copco 73R hydraulic drive rigs that can be broken down into small components to meet the transport criteria and has had the powerpacks for the drill rigs specifically fabricated to suit the site restraints. Consideration of power usage has also been important as accommodation of large generating sets on the sites is not possible.

• Both projects have had to deal with poor near surface ground conditions. In the case of the NSTP, the major Vent shaft was supported by a secant pile wall for the top 11 metres and the QHB sites were stripped to bedrock and refilled with engineered fill. In the case of HKWDT, where the depth of the vortex structure is insufficient to fully remove the zones of poor ground, the parties have agreed that any remaining unsuitable ground will be removed and replaced with mass concrete backfill.

• Both projects, including the vertical shafts are long term projects. In the case of the NSTP, support was only required in the two large diameter shafts as the other shafts were sufficiently small diameter


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to be self-supporting over the required period. At the HKWDT, while the final diameter of the shafts is yet to be determined, all shafts will be fully hydrostatically lined with segmented concrete lining and grout.

4 CONCLUSIONS

The application of raise boring technology to the development of vertical shafts in the civil construction industry proved very successful at the NSTP and it is likely that the same advantages will manifest themselves in the application on the HKWDT. The advantages that were perceived for this method of shaft excavation, particularly in urban areas have been achieved. Raise boring has proven to be a safe, effective, environmentally friendly and economic methodology for the development of vertical shafts for major civil projects.

ACKNOWLEDGEMENTS

The authors acknowledge the assistance provided by Sydney Water, Atlas Copco Australia Pty Ltd. and the Drainage Services Department of the Government of the Hong Kong Special Administrative Region.

REFERENCES

Barton N, Lien R, Lunde J [1974]. Engineering classification of rock masses for the design of tunnel support. Rock Mech 6(4): 189-236

McCracken A, Stacey TR [1989]. Geotechnical risk assessment of large diameter raise-bored shafts. Shaft Engineering, Inst Min Met, pp. 309-316

09_raise boring's role in two major civil tunnelling projects

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