The Drainage Services Department (DSD) of the Hong Kong SAR Government commissioned the design and construction of the Kai Tak Transfer Scheme (KTTS) in the year 2000. The purpose of scheme is to relieve flooding in Mongkok, by diverting storm flows from the existing storm water drainage system via a new 1.4km long, 4.4m i.d. drainage tunnel and box culvert, running from Waterloo Road in Kowloon Tong to the Kai Tak Nullah (Figure 1).
The US$49M design-build tunnelling contract, that also includes the construction of six access shafts, was awarded to the China Harbour/Transfield joint venture (CHTJV). The JV employed Ove Arup & Partners Hong Kong for tunnel and shaft design, whilst Black & Veatch Hong Kong designed the culverts (Black & Veatch are DSD’s consultant and Supervising Officer (SO)). Design responsibility for a section of the culverts was also transferred back to CHTJV after the negotiation of a contractor’s proposed variation.
In January 2003, Transfield sold its construction business, including its tunnelling business, to John Holland Pty Ltd, a subsidiary of Leighton Holdings of Australia. Transfield retained responsibility for the contract under the terms of the sale, but engaged John Holland Tunnelling to manage and complete the project.
Geology
The geology along the alignment is highly variable and includes completely decomposed granite (CDG), highly, moderately and slightly decomposed granite (HDG, MDG and SDG), alluvium and CDG with corestones (Figure 2). Each tunnel section between shafts generally has defined geological conditions as follows:
All sections of the tunnel are beneath the water table with a maximum head in the soft ground of 1.4 bar.
Alignment
The vertical alignment falls at a grade of approximately 0.1% and is fixed by the level of the existing drainage structure in Waterloo Road and the outfall level at Kai Tak Nullah. Site B was the only location with sufficient space for the tunnelling operations. This resulted in the TBM having to be launched from Shaft B twice, tunnelling both downgrade and upgrade.
The tunnel’s horizontal alignment was dictated by the requirement to remain under Government land, such as roads and parks, utilising a number of 245m radius curves to achieve this. At Shaft D2, the alignment turns 90° to follow the surface roads and avoid historical monuments within the Kowloon Walled City Park.
TBM and equipment
Due to the varied ground conditions and strict settlement issues, CHTJV selected a refurbished 5.17m diameter Herrenknecht Mixshield TBM.
The Mixshield was chosen because of its ability to control ground pressures with low wear in the highly variable geological conditions, minimising the risk of surface settlements. Design of the cutterhead included 24 x 14″ single cutters capable of 20t per cutter, four twin cutter sets for rock conditions and 50 soft ground scrapers.
The Mixshield’s “bubble” was maintained by two alternating electric compressors, provided with high quality filtration and a chiller unit for any necessary CA interventions.
A water chiller system equiped with high-pressure supply pumps and a cooling tower were required for the TBM’s cooling water. All servicing of the TBM was carried out via the shaft using a 12.5t gantry crane, which lowered segment stacks in 0.5 ring loads onto the rolling stock.
Tunnel excavation
The TBM was launched from Shaft B towards Shaft E in October 2002. The 10.5m x 9.5m elliptical sheet pile Shaft B had its shaft eye removed following the installation of two rows of contiguous auger piles for ground stabilisation. The shaft layout included the breakout seal ring attached to a shotcrete launch stub, thrust frame, slurry pump and a window for lowering segments.
The three TBM launches (two at B and one at D2) required for the project were a major challenge and CHTJV worked closely with Herrenknecht for the development of the short, staged launch and use of umbilical lines (refer inset).
The first 100 rings were excavated in soft ground with negligible wear. However, when the TBM entered the rock section performance reduced dramatically in the slightly decomposed to fresh granite. CHTJV, with the support of Herrenknecht, focussed on improving the TBM’s performance by increasing the cutterhead RPM by 25% and replacing the main drive displacement cylinders to increase the contact force by approximately 45%. Advance rates improved after the modifications.
To negotiate the tight radius curve, the pre-gauge and gauge cutters were offset with cutter seat inserts to provide greater clearance. The disadvantage of the over-cut was the extra grout control needed to compensate for losses to the slurry circuit and potential ring movement/damage due to flotation. An accelerated high cement grout mix was pumped through the lining in the crown and shoulders to stabilise the lining (in the rock section) and followed-up with back-grouting.
Cutter wear on standard 14″ rings in the rock was high, however this improved after replacing the rings with heavy duty alternatives. Cutter changes in the rock section were undertaken in free air, with consideration to achieving an initial set of the annulus grout before the pressure in the slurry system was reduced.
The high loadings and hard work in the rock section took its toll, resulting in cracking of the cutterhead, drive plate and displacement cylinder supports, which required additional stiffeners to complete the rock section between Shafts B and C, and additional refurbishment was carried out at Shaft C.
The TBM excavated successfully in difficult interface conditions, with 50% rock and corestone and 50% CDG with only 2.5 x diameter cover and high water table. Advance rates were reduced to minimise cutter and tool damage and to minimise regenerative noise issues for buildings adjacent to the alignment.
After the low cover intersection crossing at Junction Road, the TBM had a “safe” run beneath a public park in uniform CDG ground conditions, where all systems were pushed – 13 excavated rings (15.6m) in one 10 hour shift was achieved in this period.
At Shaft D1, the cutterhead was prepared for rock and corestone between D1 and D2 and this was realised within 40m of leaving D1. The granite transitions vary quickly over a narrow band, from CDG to SDG, which can result in cutter damage through impact loading.
After 10.5 months in the ground, the TBM broke through into Shaft D2 in August 2003 where it was turned through 90° and re-launched (refer inset). A stoppage of about four weeks was required to turn the TBM and recommence excavation.
The 90m tunnel section between Shafts D2 and E was controlled by planned stoppages rather than excavation rate. After re-launch, the TBM excavated for 1.5 weeks then stopped for 2.5 weeks, for turning gantry 1, and completed the drive after a further 2 weeks. Breakthrough at Shaft E was achieved in late October 2003. Close surface settlement monitoring was carried out over this length of tunnel, as the cover reduced from 1.1 x diameter down to 0.8 x diameter, under an operating road.
The TBM and back-up was transported back to Shaft B and the machine was re-launched in early December 2003, only 6.5 weeks after breakthrough at Shaft E. The final 280m drive was also complicated by varying geology (including a small rock-head), low clearance to major utilities and low cover below an existing road, adjacent to a sensitive residential structure.
TBM excavation was completed with high operating utilisation due to vigilant maintenance by the site team.
Slurry treatment plant and spoil handling
CHTJV selected Pigott Shaft Drilling Ltd (PSD) from the UK as the supplier for the slurry treatment plant (STP) and utilised its expertise for the selection of bentonite and flocculants. The STP consisted of a four-stage separation plant of primary shaker, desander, desilters and two centrifuges with a total capacity in excess of 750m³ per hour.
Bentonite was added to the slurry system using either a high shear mixer to produce fresh bentonite slurry or via a venturi jet hopper to add powder directly to the active system. During tunnelling, minor issues such as foaming due to cement contamination when excavating through ground treated zones and foamed concrete backfill were addressed quickly through additives and none of these issues limited TBM performance. A further key consideration was the STP performance for separation of the clays and silts within the CDG and maintaining an operable slurry density throughout the shift. To this end the PSD system proved particularly effective across the varying ground conditions and was very reliable with a remarkable 0% down time against TBM utilisation.
Settlement monitoring
Settlement monitoring was carried out by precise levelling using standard ground settlement points along the alignment centreline and specific utility monitoring points located on sensitive services. Surface monitoring provided feedback into the model for TBM operating and grout pressures, to minimise surface movements to generally less than 0.2% face loss. The TBM completed and passed three major tests for face control:
The Mixshield proved very effective, with vigilant operation and grouting, for settlement control in areas of low cover, utilities and most importantly through rock and soft ground interfaces beneath sensitive structures.
Tunnel lining
The lining is a single-pass “universal taper” lining, 4400mm i.d., 250mm thick, nominally 1200mm in length with a 28mm taper. The six piece precast ring has a two-part EPDM and hydrophilic gasket to achieve water tightness. The circumferential joint uses galvanised dowels, whereas the radial joint has spear bolts (removed behind the back-up). The precast lining was manufactured in China using French-made (CBE) moulds and shipped to Hong Kong. The lining has all lifting sockets and bolt pockets backfilled with a cement based mortar to ensure the hydraulic smoothness for high flow in operation. The invert has a secondary insitu concrete roadway with an asymmetric low flow channel, cast using standard steel formwork panels.
Compressed air and safety
Compressed air interventions were carried-out for cutterhead maintenance in soft ground below the water table. The TBM has a standard interconnected twin-lock arrangement, certified up to 3 bar. The CA work was undertaken in accordance with the HK Labour Departments requirements Chapter 59 Factories and Industrial Undertakings Ordinance Regulations, which uses the Blackpool tables for decompression.
The Fire Services Department (FSD) set up an Emergency Pressurisation Team (EPT) capable of entry into the working chamber. This required ongoing pressurisation training of FSD personnel in the surface re-compression chamber, on a monthly basis for a total of over 60 FSD personnel. There were less than 180 man interventions and exposure time on average was less than 2.5hrs. Fortunately the tunnel is shallow and the water table was not high and hence 95% of the compressed air work was undertaken at less than 1 bar above atmospheric pressure.
The TBM used FSD approved fire resistant hydraulic oils, fire resistant vent duct and was fitted with fire alarms, automatic fire detection/suppression systems and continuous gas monitoring.
Shaft construction
There were six shafts constructed along the tunnel alignment (Figure 1) to allow DSD access to the permanent works. The methods of construction for each shaft were varied to suit the conditions and constraints at each shaft location. The temporary works used a vibro-hammer to install sheet piles 20m deep at shaft B, a drop hammer to install sheet piles 21m deep at shaft D1 and a Super Crush Silent Piler (auger and hydraulic insertion) to install sheet piles up to 24m long in one pass at shafts D2, E and A. Due to the presence of rock and corestone at Shaft C, 406mm diameter steel pipe piles were installed 25m deep using a down the hole hammer with compressed air flushing.
The circular shafts were completed using permanent shotcrete lining placed in two layers, a primary SFRS layer and a secondary hand finished plain layer. Joints incorporated hydrophilic water stops and injection grout tubes to ensure water tightness. The permanent shotcrete lining was a first for Hong Kong and installed with the shaft excavation, limiting ground movements and providing programme and cost savings.
The mid-drive shafts C and D1 were backfilled with foamed concrete to above tunnel crown level, to suit the permanent works intermediate slab level and for the TBM to excavate through. After the TBM was safely beyond the shafts, the foamed concrete was excavated down to the segmental lining and prepared with epoxy fixed dowels for pouring in-situ walls and intermediate slab. After curing, the segmental lining was removed using wire rope saws, completing the tunnel opening.
Shaft D2 was enlarged to 9.5m i.d. for turning the TBM and also to enable construction of the TBM launch chamber. The 5.5m diameter shotcrete lined launch chamber was 9m in length and excavated by hand using NATM methods through a jet grouted CDG block.
Environment and community
The main tunnelling site was located in an exclusive residential area in Kowloon Tong and was subject to strict environmental considerations regarding truck movements and noise, restricting TBM excavation to dayshifts only. The nearest sensitive receiver was only 20m from the main working shaft and a residential university building overlooked the site compound, approximately 40m from the STP. Therefore extensive noise attenuation works were required to minimise the risk of complaints and also to extend available working periods, including enclosing the STP within an acoustic building, enclosing the compressors, placing barriers around the water chiller unit, etc.
Despite the difficult conditions, the works were completed with minimum disturbance and CHTJV’s efforts were recognised with the Environment, Transport and Works Bureau’s Considerate Contractors Site Award and Site Cleanliness & Tidiness Grand Award.
Conclusion
The KTTS project provided the first use of a Mixshield TBM in Hong Kong and delivered exceptional settlement control in the most difficult of conditions. The project also employed the first use of permanent shotcrete lined shafts in Hong Kong.
Final breakthrough of the TBM into Shaft A was achieved in late February 2004 and marked the successful completion of a technically and logistically challenging mixed ground TBM tunnelling project.
Related Files
Figure 2
Figure 1
