Rock splitting solution on Hong Kong bypass 23 Mar 2017

Traffic congestion along the northern seaboard of Hong Kong Island is now chronic. Land reclamation and urban development in the area, including the new convention centre, the airport express railway terminus at Central, and many new high-rise structures, has increased east-west traffic to the point where extra highway capacity is now vital.

Fig 1. 3D cross section of the mined tunnel of the underground bypass passing underneath the southern portal structure of Hong Kong’s first harbour crossing immersed tube road tunnel

The need was recognised as part of the island’s most recent phase of land reclamation, but suggestions 10 years ago to complete further reclamation to create space for a new surface highway, had the citizens of Hong Kong enraged. Opposition to more reclamation, and perhaps eventual disappearance of Victoria Harbour all together, put a stop to the plans and had the SAR Hong Kong Government Highways Department and its consultants think again. The protests resulted in the Protection of the Harbour Ordinance, which now forbids any further reclamation either side of Victoria Harbour.

The solution, as in so many cases of limited land space, is underground with the middle 3.7km of the new 4.5km Central-Wan Chai Bypass project solution aligned in tunnel. When opened in late 2018/early 2019, a journey of 5-6km, that can take an anticipated 45 minutes today at peak, will be reduced to about 5 minutes. The road users of Hong Kong just cannot wait.

Project design

Construction is mainly by cut-and-cover and from within temporary reclamation operations for under water with the seabed of the harbour restored to its natural state once the undersea elements are completed.

Alignment of the bypass project with the Causeway Bay on-ramp tunnel and the Cross Harbour Tunnel (in light blue) and the Shatin-Central Metro Line (in grey)

One of the most complex sections of the project is the reach between the two deep temporary reclamation cut-and-cover sections in the Causeway Bay district where only a mined solution was possible. In this section the subsurface alignment passes adjacent to the Ex-Police Officers’ Club and the Royal Hong Kong Yacht Club, which are influential construction-site neighbours indeed, and, more importantly, on a wider scale, beneath the portal structure of the first cross-harbour immersed tube highway tunnel crossing between Causeway Bay and Kowloon. Tunnelling under the portal structure and through the hard granitic bedrock would be a challenging operation.

The four-lane cross-harbour immersed tube, built in the 1970s, carries an average 120,000 vehicles/day and its service cannot be interrupted in anyway. Engineer to the Highways Department for the new HK$36 billion Central-Wan Chai Bypass project is AECOM Asia. As well as preparing the conforming design for the re-measurement contracts, AECOM is also Resident Engineer on site to supervise the works.

Following planning and design, construction of the bypass was divided into ten construction packages with China State Construction Engineering being awarded the cut-and-cover sections either side, and the short 167m long mined rock tunnel section in between, in September 2010 and March 2013 respectively. Chun Wo and Leighton have cut-and-cover tunnel approach ramp sections further to the east and west of the bypass tunnel and Leighton has the M+E fit out and communications contract.

Urban rock excavation

From the earliest planning stages, the short 167m mined section of the project presented significant challenges.

Fig 2. Geological section of the mined tunnel and its path under the anti-floatation ground anchors of the cross harbour tunnel portal structure

First, when it came to investigating the original design and construction of the existing harbour tunnel portal structure (Fig 1), there was a lack of as-built details or records. Information was very limited, explained Tim Leung, Senior Resident Engineer for AECOM for the project. “One of the best we had was a technical article written by the consulting engineering company and published in a magazine in 1971. From it we gathered some information about support of the portal structure by a series of tie-down rock anchors for anti-floatation, but actual engineering detail was not available. Blasting was therefore prohibited, due to the expected sensitivity, and the lack of detailed construction records, of the existing cross harbour tunnel structure. We had to design to a high degree of safety and to the most balanced of assumptions, and then to monitor the structure intently during construction.”

As an alternative, the relatively rarely used rock splitting excavation method had to be adopted.

Working access point for progress of the mined rock tunnel heading

Added to that, the mined rock tunnel had to accommodate an on-ramp slip road for traffic into the east-bound carriageway. There are only four slip roads on to the 4.5 km long new bypass project and all are into the underground alignment: an on- off-ramp at Hong Kong Convention and Exhibition Centre and the on-ramp at Causeway Bay to entre the bypass and merge to the mined rock tunnel. With the two, three-lane carriageways either side and the slip road on-ramp in the middle, the cross-section of the mined underground structure is exceptional at some 50m wide x 11m high and 460m² in total (Fig 2). “This cross section we divided into three headings and call it our trinocular configuration,” explained Leung.

Fig 3. Additional steel weights on the cross harbour tunnel portal structure

To add to the complexity of the Causeway Bay section of bypass highway, the site also has TBM tunnelling works as neighbours. “In fact we built our temporary reclamation section on the east of the mined tunnel section large enough for construction of our cut-and-cover tunnel and also to provide the working access site for launch and operation of the two Herrenknecht TBMs used on the approaches to the immersed tube harbour crossing of the new underground Shatin-Central metro line. The metro contractor is working alongside us and they will remove the temporary reclamation and restoring the sea bed to its natural state as required after completing their work.”

Work on the bypass project contracts started from 2009, and excavation of the rock tunnel from within the temporary reclamation cut-and-cover works either side started in March 2013 and completed in May 2015. This section of the new underground bypass highway runs about 40m below ground surface at invert level and ground surface is about +3.7m above mean sea level. For the excavation process of its mined rock tunnel, China State Construction Engineering (CSCE) engaged Atkins as its temporary works engineer.

Protection of the portal structure

Ahead of excavation, and starting in 2011, extra measures were installed to protect the cross harbour portal structure. “Because we were not sure the exact depth of the tie-down ground anchors or their exact position,” said Leung, “nor of the installation of ground anchors to prevent floatation of the structure, the first operation was to add additional weight to the structure to counter balance any potential of encountering ground anchors during the new excavation as precautionary measures” (Fig 3).

Operation of the drilling jumbo with its added guide rod or plunger

Secondly, a wide array of instrumentation to monitor any movement of the cross harbour tunnel portal structure was installed. Prisms on the portal structure itself were monitored by three automatic monitoring stations on the roof of the Ex-Policy Officers’ Club building next door to provide constant real-time data of any movement every hour and minute of every day. Instrumentation included strain gauges in the invert of the existing highway tunnel.

Also installed was a grout curtain along either side of the cross harbour tunnel portal and a standby system of dewatering and recharging groundwater wells as contingency measures. “This was an anticipation of dewatering/recharging needed during excavation of the new rock tunnel,” explained Leung. “Recharging of the watertable might be needed at the same time to prevent any settlement of the soils above and prevent any damage to the harbour tunnel portal within them” (Fig 3).

Excavation setup

To get started with excavation, CSCE equipped itself with drill jumbos from Atlas Copco and hydraulic splitter units from Yamamoto of Japan mounted on Hitachi EX120 chaises and hydraulic breakers from Sandvik, Montabert and Soosan.

The hydraulic splitters operated at up to 500 bar

The rock splitting method combines the drilling of a continuous row of interconnected drill holes around the tunnel profile to provide the necessary as free face slot or cut holes, a pattern of holes in the face for inserting the hydraulic splitter tool for forcing the rock to break back and be removed. With the core of each round excavated, with the Sandvik, Montabert and Soosan breakers for tunnel excavation and prepare the heading for installation of immediate support temporary shotcrete and and the final in-situ concrete lining.

Getting to grips with the rarely used excavation method proved a challenge for the CSCE crews and required much trial and error of the many different operations and their integration and interrelationship with the rock conditions and its behaviour under the excavation forces.

Fig 4. Multi-drift excavation of 50m wide, 460m² face

“Initially there were issues with breaking the splitters,” said Leung. “Understanding exactly why that was took much research and experimentation and the engagement on site of method experts from Yamamoto. It became evident that understanding the rock characteristics and of each new face proved vital as was the drilling of very straight holes. Slight deviations in the long splitter holes would damage the splitter. Higher breaking pressures on the splitter tools also damaged the seals and caused contamination of the hydraulic operating oils and added to significant downtime in the early days.”

As part of the design strategy and ahead of excavation start, AECOM specified two horizontal directional drill holes through the full 167m length of the tunnel. From this, geologists confirmed rock strengths of up to 120MPa and tough, massive rock conditions with few joints or cracks. This also confirmed that the potential for high water ingress through fissure connecting to overlaying sedimentary deposits or to the surface would be less than anticipated or predicted. NGI Q-mapping of each new face however became a vital requirement for planning the pattern of splitter drill holes. Repeating the same drill pattern into each new face, regardless of Q face mapping, was not the way forward.

After many rounds spent perfecting all parts of the process, an optimum was achieved that limited the length of the split holes to 3m and using a 102 mm diameter drill bit. The optimum splitter tool was 1.5m long and slightly less in diameter than the drill hole. Hydraulic pressures on the splitter tool ranged for averages of 350 bar up to a maximum of 500 bar.

Vital also to optimum splitting was true parallel alignment of the interconnected slot cut-holes around the perimeter. The holes had to connect along their full 3m. To keep the drill boom steady and on-line, a guide rod or plunger was attached to the drill-jumbo booms and inserted deep into parallel cut holes. This helped prevent the natural tendency of long drill holes to deviate with depth. The same guide rod or plunger was used to help maintain true parallel and horizontal splitter holes into the face as deviation of these holes proved equally detrimental to the splitting tools and effective splitting.

After completing the profile cut holes, working from the outside edges of the invert towards the apex crown position, the two booms of the jumbo completed the face splitter holes. The splitter had the face to itself for breaking out the core of the new round of advance.

NATM excavation sequence

To progress excavation of the wide span and section, the cross section was excavated on top heading and bench drifts with the top headings advancing with a maximum of 5m of unsupported length per round and the bench following at a distance of about 30m to accommodate a ramp and space for the drilling jumbo on the bench (Fig 4).

Fig 5. Estimated and as-mapped Q-values of the better than expected rock conditions

In sequence, the centre slip road tunnel (SR8) and the outer sections of the eastbound and westbound (EB and WB) carriageways were excavated first and supported with steel ribs at the portals and layers of steel-fibre reinforced shotcrete. A single round of 10m-long x 273mm diameter pipe umbrella pre-support was also installed at each portal beneath the shallow rock cover to overlaying sedimentary deposits (Fig 2).

Advance of the first three headings was separated by rock pillars of about equal span of the headings. Once excavation of the SR8 heading was through and its permanent base slab and rebar reinforced in-situ concrete lining installed, excavation of the inner rock-pillar sections of the eastbound and westbound carriageways was then advanced.

“Under this arrangement, multiple headings and installation of temporary rock support could take place simultaneously,” said Leung. “Early completion of the central permanent lining of SR8 also acted as temporary support to the trinocular structure and facilitated excavation of the inner eastbound and westbound tunnels to full span, with the maximum axial load and shear force at the inner end of the temporary lining of the eastbound and westbound tunnels transferred to the permanent SR8 lining.”

Excavation of the mined tunnel was a first in Hong Kong for such a large span and passing at an oblique angle beneath the portal of the cross-harbour tunnel portal. “As excavation progressed there was about 20m clearance between the invert of the portal structure and the crown of our tunnels,” said Leung. “Taking into account anti-floatation ground anchors of the portal structure, the clearance was much less” (Fig 2).

As excavation of the mined rock tunnel progressed, production reached a peak of 465m³ per day in April 2015 when excavation was progressing on the six fronts of the east- and westbound inner headings, the east- and westbound outer benches, and the east- and westbound inner bench concurrently (Fig 4). This was equivalent to achieving a pull length of about 1m/day if drill+blast had been adopted.

Observational method savings

By adopting the observational construction method and of all observational and monitoring systems in place, the design of temporary rock supports was reviewed and adjusted as necessary (Fig 5). “Because the actual ground conditions were than that assumed during the design stage, we were able to optimize the conservative temporary rock support elements and to relax the maximum distance unsupported face advance,” said Leung, “and this resulted in significant benefits to the overall project” (Fig 6).

The drill+break excavation lasted about two years, excavating a total of some 80,000m³ of hard rock and reaching completion ahead of programme. “This was only possible by using the proper plant and equipment, good site planning and tight site coordination and control for working on multiple work fronts concurrently,” said Leung.

Fig 7. Daily excavation progress rates illustrating that advance rates could have improved had excavation continued into a longer tunnel length

As multiple-heading excavation was progressing in front, the final lining installation progressed also on a rolling programme about 60-70m behind. The rolling sequence was led by installation of a full hydrostatically sealed plastic membrane water proofing system, followed by a station for fixing steel rebar reinforcement, and closed out behind with concrete casting into 5m long shutters.

After stating excavation of the trinocular mined tunnel from both ends in March and August 2013, CSCE completed all excavation and lining works and reached substantial completion of the mined tunnel in September 2015 and cut-and-cover in March 2016. With M&E and TCSS installations now progressing, a grand opening of the new underground highway in late 2018/first quarter of 2019 will be a tremendous celebration.

The experience of building this short, intricate and challenging section of mined rock tunnelling has been the subject of several technical conference papers and presentations including at the 2016 World Tunnel Congress at San Francisco last year. Greater details of the full process are available in these records.

Tim Leung, Senior Resident Engineer for the client’s consulting engineer AECOM, hosted the visit by TunnelTalk

To recognise the expertise required to plan, design, manage, excavate and build the mined tunnel, the parties entered the project into the category for best tunnel project to a value in excess of €500 million and US$500 million in the 2016 industry excellence awards and was awarded highly commended in the ITA Tunnelling Awards in Singapore in November 2016 and took top honours in the same category Tunnelling Project of the Year at the UK international tunnelling awards in London in December 2016.These are the leading categories in the two prestigious series of internationally recognized tunnelling awards. In addition, the project was shortlisted in the UK Awards in the Specialist Tunnelling Project of the Year and the Community Engagement category.

The parties involved take pride in the safe, well managed, and well executed construction of the project as a whole, and the trinocular mined tunnel section in particular.