Kowloon Southern Link contract KDB200 is part of a 3.8km railway project to link the operational East Rail and West Rail lines for client MTR Corporation (formerly Kowloon Canton Railway Corp, prior to a merger in Dec ‘07), in Hong Kong. The KDB200 Contract includes twin TBM tunnels, West Kowloon Station, two emergency access and egress shafts, northern cut & cover tunnels connecting to an adjacent contract and southern cut and cover tunnels connecting to the existing station at East Tsim Sha Tsui.

The US$259M (HK$2.018bn) KDB200 contract is being carried out under a design and build contract by the Link 200 JV, comprising Leighton Asia, Balfour Beatty, Kumagai Gumi and John Holland. Mott Meinhardt is the designer for the JV.

The Link200 JV bid proposed a slurry TBM for construction of the 1.1km long, twin 7m i.d. tunnels through mixed ground beneath the congested shopping, hotel and tourist area of Tsim Sha Tsui in Kowloon. This is only the second project in Hong Kong to use a Mixshield TBM, the first being the Kai Tak Transfer Scheme (T&TI, April 2004), which was also undertaken by John Holland.

Urban area tunnelling constraints

Due to the building density in Tsim Sha Tsui, the available reservation for the alignment was generally beneath public roads, however a tight 225m radius curve was required beneath the heritage listed Former Marine Police Headquarters building.

The width of the road reservation in Canton Road, due to existing basements and piled buildings, meant the tunnels were required to be stacked vertically with just 6.25m of clearance (see p19) in this section.

To achieve the stacked configuration beneath Canton Road the tunnels needed to change elevation to ensure enough clearance between the drives. This meant driving the TBM on up and down grades in the order of 3%, in conjunction with the 225m horizontal radius curve.

These grades were also required to ensure enough clearance to the other alignment constraints, such as passing beneath the Kowloon Park Drive flyover, Kowloon Park Drive Subway No.1 and clear space between the tunnels.

Due to its urban setting, a key consideration for the project was the ability of the TBM to limit settlements and ground borne noise impacts. In addition to the TBM selection, contingency plans were developed for identified risk areas, including:

• Kowloon Park Drive Flyover – contingency deck jacking plan

• Emergency access and emergency egress shafts and connection adits passing – temporary lining

• Kowloon Park Drive Subway – reduced operating pressure

• MTRC Tsuen Wan Line crossing and DSD culvert passing – ground treatment and isolation horizontal pipe piles

• Emergency temporary traffic management

Ground conditions

The tunnels encountered varying ground conditions, all below the groundwater table (with pressures up to 2.6 bar at axis level) and within close proximity of Hong Kong Harbour. Both tunnels encountered a full range of ground conditions including completely decomposed granite (CDG), mixed face of CDG and fresh granite, full face of granite, faulted zones and mixed face of CDG, alluvium and marine deposits, as identified in the geotechnical investigation report.

Areas of shallow cover included both launches (right to left, as shown in figure 1) within CDG and alluvium, overlain by marine-placed reclamation granular fill; as well as within the marine deposits at Salisbury Road, when side by side at the end of the drive.

The start and end of each of the drives were similar, however beneath Canton Road the tunnels were in the stacked configuration and therefore encountered different conditions. The first drive experienced more rock and increased water pressures at greater depth and the second drive was shallower with more soft and mixed ground (CDG/granite interface, CDG with corestones (fresh to slightly decomposed granite boulders within a completely decomposed granite matrix), alluvium and marine deposit).

TBM selection

The twin tunnels were both about 1100m in length and one TBM was used for both of the tunnel drives. The TBM was planned to excavate the Up Track (lower) tunnel first, then be recovered and relaunched to complete the Down Track (upper) tunnel.

The key TBM selection criteria were:

• Control of face pressure in soft ground, mixed face conditions and shallow cover to minimise surface settlement

• Ground water inflow control at faults and interfaces

• Hard rock production rates

• Excavation through corestones and blocky ground

• Spoil handling systems

• TBM shutdown and maintenance periods

• Wear and abrasivity

• Ground borne noise impacts

A 7.99m diameter Herrenknecht Mixshield TBM was selected by the JV for the project, as it was considered more suitable for maintaining accurate and reliable face control and groundwater control, particularly at the soil/rock interface and corestone zones. The ability to manage wear of the cutterhead and cutters and cope with blocks or boulders, by use of a rock crusher behind the submerged wall, were also considered superior with the Mixshield than for an EPBM, or slurry TBM by another manufacturer.

The TBM was designed in Germany and key components, such as the main drive, cutterhead, motors and pumps, were manufactured in Europe. The steel structure of the TBM and the back-up equipment were manufactured in Shanghai and Guangzhou, China, respectively. The TBM was then assembled and commissioned in Herrenknecht’s Guangzhou factory and shipped directly to the job site.

Segmental lining

The tunnel lining design was undertaken by Mott Connell (as part of the Design JV for the project). The lining was required to be 7000mm i.d. to allow for walkways, M&E provisions and kinematic envelope. The 350mm thick reinforced concrete lining was developed for a range of loading conditions including ground, surcharge, handling, TBM operation loads, and TBM thrust loads due to steering through the 225m radius curve in rock.

The concrete mix specified for the segments was a grade 50/10 mix, including a corrosion inhibitor additive for durability and polypropylene fibre for the four hour fire rating required for the lining.

Two ring types were used for the alignment, both universal taper and six segments plus one key:

• A nominally 1500mm long ring for the general tunnel with a taper of ±12mm to achieve a 500m radius curve, and

• A nominally 1200mm ring for use at the 225m radius curve with a taper of ±25mm

The segments were sealed using a Phoenix EPDM rubber gasket with hydrophilic insert. The EPDM gasket was installed at the precast yard and the hydrophilic gasket installed on-site prior to delivery to the TBM. Spear bolts and inserts, by Sofrasar, were used on the radial and circumferential joints. The ring build teams were very experienced from previous projects in Hong Kong, and hence the ring build damage was very low.

Tunnel drives

The working site for the tunnel operation is located at the northern extent of the hotel and retail area of Tsim Sha Tsui. Two sides of the site are bound by high-rises, with the other two sides facing an expansive open area and the station construction. The site operated under strict Environmental Protection Department issued construction noise permits (CNP), hence the shaft contained an acoustic enclosure, along with all other surface fixed plant to allow tunnelling operations on a 24 hour basis (such as the MTRC crossing). Prior to completion of the noise attenuation, site activities were constrained until 11pm, after which site noise was required to be less than 55dBA at the noise sensitive receivers.

The TBM drive was launched from a 22m deep diaphragm wall box with approximately 14m of cover. The end wall of the shaft was constructed using fibreglass reinforcement (GRP) in the panels. Outside the diaphragm wall was a zone of jet grouted columns to enable the secondary seal on the launch wall to be installed. The GRP reinforcement caused minor delays due to blockages within the rock crusher grill and also at the STP during the launch. Cleaning to remove the jet grout and GRP from the slurry was carried out, in parallel with grouting of the secondary bullflex launch seal ring, prior to moving into the natural ground.

The tunnels encountered corestones within the CDG over the first 100m of tunnel until the first interface, between the CDG and bedrock, was reached. The tunnels were only 6m apart through this launch zone however the second tunnel experienced more boulders and a less cohesive matrix and this resulted in greater cutter damage. Over a similar length of tunnel at the start, almost four times as many cutters were used during the start of the second drive due to damage. The interventions to repair cutter damage were carried out under compressed air beneath the three lanes of traffic in each direction.

The TBM then encountered hard granite, in the order of 110-185MPa, beneath the Kowloon Park Drive Flyover section. Advance rates of 10-12.5mm per minute were achieved in the Up Track drive compared to a designed rate of 15-20mm per minute. During the second drive, using similar loadings and RPM, the TBM achieved 20-25mm per minute, as the extent of fracturing was greater than encountered in the first drive.

Advance rates in the soft ground, full face CDG or marine deposits, were in the range 40-55mm per minute, governed by the annulus grouting rates and spoil handling at the slurry treatment plant.

The TBM was designed with tail shield grouting using a traditional grout mix and pump system with grout excluder plates on the outside of the tail shield to stop grout travelling forward to the shield. During launch of the TBM in the first drive the grout excluder plates became entrapped in the secondary launch seal. It then became difficult in hard rock to contain the grout within the annulus surrounding the rings and as a result of grout loss in the crown some ring flotation was experienced, mainly after free air interventions in rock for cutter changes. Although the floatation issues were overcome, the grout system was changed late in the first drive to a two-component grouting system, with an on-site batch plant, from Meiwa and this provided greater flexibility for grout supply to the TBM for extended working hours. The product was pumped from the surface batch plant via two 50mm lines for the grout mix and two 25mm lines for the accelerator. The mix was injected through the segments in the shoulders of the rings.

The 225m radius curve in hard rock presented issues for the lining design and TBM steering, however these were overcome by providing additional clearance around the shield, offsetting the outer cutter positions using packer plates. The 5mm-10mm additional over-cut enabled the TBM to negotiate the curve successfully.

The TBM survey was carried out by the JV, utilising a guidance system supplied by VMT with ring selection software.

Cutter changes were undertaken in compressed air for maintenance in mixed ground conditions using the Hong Kong Compressed Air Regulation tables and an emergency medical lock was provided on the site surface during the project. The maximum compressed air pressure used in the lower section of tunnel was 2 bar gauge pressure and a project limit working time of up to 3 hours.

Owing to the adjacent sensitive structures, a number of “no planned intervention zones” were identified. In these zones it was preferred to tunnel without undertaking maintenance on the cutterhead to reduce the risk of slurry or compressed air losses during the intervention. This further risk mitigation/ control was employed prior to commencement of the second drive after an Up Track performance review (T&TI, June 2007, p6).

Slurry treatment plant

The excavated spoil was transported in a slurry suspension through 350mm diameter pipes to a slurry treatment plant (STP) provided by Pigott Shaft Drilling (PSD), with a 1350m3 per hour flow capacity. The STP consisted of a primary screening unit, three desander units, two centrifuges with a flocculation system, bentonite mixing system and storage tanks. The primary screen had a steeply declined deck to separate all material greater than gravel size. The three desander units were two-stage, each incorporating a single 660mm hydrocyclone and a bank of 18 x 5” hydrocyclones. The two centrifuges were parallel in the system to separate the fines. A flocculation unit was operated with the centrifuges to remove fine particles and process waste slurry for disposal.

The slurry treatment plant system was sized to suit the maximum advance rate of the TBM based on the grading curves for CDG and marine deposits. However, during the Up Track drive it was determined that delays between excavation cycles could be further reduced by the addition of another centrifuge, to increase the plant’s fine particle processing capacity. An additional centrifuge and flocculation system was mobilised for the second drive enhancing performance in the soft ground sections.

The spoil discharged from the STP was transported by conveyor to an adjacent spoil handling area. In the soft ground sections, where 24hr weekend excavation was required, the on-site spoil bin area was extended, as under the CNP trucks were not to be used between 11pm and 7am, Monday to Saturday, or at all on Sundays. From the bin area, spoil was loaded onto trucks and transferred to a barge loading ramp for disposal at both marine dumping and land based filling areas.

Settlement monitoring and control

An internet based monitoring system was selected for use on the project, providing both SMS and email notifications in the event of exceeding the preset alert, action or alarm (AAA) values. The monitoring system utilised information from over 1300 monitoring points, 300 ‘real time’ and 1000 manually input, including:

• Surface and utility settlement markers

• Vibrating wire piezometers, piezometers and standpipes

• Tilt meters and inclinometers

• Automatic deformation monitoring systems (ADMS)

• CCTV cameras in MTRC tunnels

The tunnelling passed through soft ground and mixed ground conditions at shallow cover along both Canton Road and Salisbury Road and the design face loss for the settlement performance using the Mixshield was estimated per drive as:

• CDG: 0.3% face loss

• Mixed face (soil/rock): 0.5% face loss

• Marine deposit: 0.7% face loss

It was envisaged that settlement measured for the second tunnel, when in the twin stacked tunnels configuration, may produce higher than anticipated levels. The actual settlement measured over a length of 400m beneath Canton Road have been less than the equivalent of 0.1% face loss in CDG, interface alluvium and marine deposits. Some minor heave was recorded along the alignment due to annulus grouting.

In addition to the surface monitoring, in-tunnel monitoring, using prism arrays, was installed in in three specific areas of the completed Up Track tunnel: 1) Tunnels side-by-side in close proximity in CDG; 2) Tunnels stacked vertically in close proximity in CDG; 3) Tunnels side-by-side in close proximity in marine deposit and jet grout.

The in-tunnel monitoring did not reveal any significant movement in the first tunnel as a result of the driving of the second tunnel.

Ground borne noise and vibration

The alignment passed numerous noise sensitive receivers (NSRs) including the Hong Kong Cultural Centre (HKCC) facility.

Early GBN monitoring was undertaken to determine the impacts and validate the initial modelling and the levels measured were generally as predicted. The HKCC impact was limited to +3dB increase and the studio theatre levels reached +7dB. For the closer but less sensitive buildings, the impacts were limited to relatively short exposures as the TBM passed.

Completion of drives

The KDB200 TBM tunnels have overcome a range of tunnelling issues. The alignment required steep grades to achieve the twin stacked tunnel configuration and a tight radius curve to achieve the rail track design. The cover was variable between the two drives but was shallow in difficult ground conditions beneath public roads for the majority of the second drive. A number of instances of close proximity TBM passing were required, including adjacent to shafts, existing buildings and above the MTRC Tsuen Wan Line tunnels.

The second tunnel commenced in October 2007 and was completed in early March 2008. The best week achieved was 102m. The TBM has since been removed from the reception shaft, and the back-up through the tunnel, completing a technically challenging project through one of the most congested areas of Kowloon. T&T


Salisbury Road and MTRC crossing

The final section of both tunnel drives included shallow tunnelling under Salisbury Road in marine deposits, the MTRC Tsuen Wan Line crossing with less than 2m of clearance, and 2m of clearance beneath a DSD culvert (connected to the harbour).
In this zone where cover reduces to less than one diameter and clearance between the tunnels reduced from 3m to under 1m, jet grouting in advance of the drive was employed. The JV installed horizontal pipe piles above the MTRC tunnels to isolate the TBM drive from the existing running tunnels. This isolation was required as the tunnels were built through mixed ground in mid-1970’s and were subsequently refurbished by removing the inner face of the rings and reinforcement, replacing these with steel fibre shotcrete.
The 23.5m crossing of the MTRC tunnels on both drives commenced at 10pm on a Saturday evening, in June 07 and March 08. This meant that the passing of the TBM’s cutterhead above the crown of the MTRC tunnels occurred outside normal traffic hours.
In addition to the instrumentation in the MTRC tunnels (see Settlement Monitoring), traffic hours train cabin inspections were undertaken and non-traffic hours manual surveys were undertaken. Monitoring within the tunnels was linked to a project control centre, which included the TBM operational data, so that full monitoring could be undertaken.
The TBM drives successfully crossed the MTRC tunnels on both occasions with negligible movement recorded.

The launch shaft in West Kowloon The launch shaft in West Kowloon Finished tunnel lining Finished tunnel lining Alignment of the drives, as seen from the southern East Tsim Sha Tsui Station interface Alignment of the drives, as seen from the southern East Tsim Sha Tsui Station interface Final breakthrough Final breakthrough The cutterhead is removed from the reception shaft The cutterhead is removed from the reception shaft TBM disassemby and recovery

The TBM reception shaft is located at the corner of Salisbury Road and Nathan Road, in Tsim Sha Tsui. The site is bound by three traffic lanes on two sides and a major intersection on the third side. Therefore road closures and diversions were required to mobilise the 500t mobile crane and trailers for the removal process.
The 10 main TBM components were planned for removal over a 14 night period, with an operational window from midnight to 6am. Each night the road closures commenced at midnight, then the crane and counterweight was set up, the lifts completed, load secured to the trailer, transported under police escort to site whilst the crane was demobilised and the road re-opened. The back-up trailers were removed through the tunnel using purpose built bogie cars and removed from the launch shaft over six days.
Due to congestion of utilities at the shaft, there was no direct lifting available for the Up Track recovery. To enable the components to be placed under the crane hook, the shaft floor was plated with steel and the cradle fitted with Teflon plates prior to pushing the TBM into the shaft.

Figure 1. Alignment and geology Fig 1 – Alignment and geology