The Mass Transit Railway Corporation (MTRC) in Hong Kong recently completed the Tseung Kwan O Extension Project, extending its high capacity urban transportation system to new towns east of the Kowloon peninsular (figure 1). Contract 603, Black Hill Tunnels, comprised one of the major rock tunnelling contracts. The tunnels were constructed at a maximum depth of 180m in granite and volcanic tuff, and connect Yau Tong, north of the Eastern Harbour Crossing, with Tiu Keng Leng to the east.
A design-and-build contract was awarded to the Dumez GTM–Chun Wo Joint Venture (DCJV) in November 1998. DCJV appointed Halcrow China Ltd (Halcrow) as its detailed design consultant. The contract comprised four running tunnels, approximately 6m diameter and 1.8km in length, a centre siding tunnel and two crossover tunnels, in addition to ventilation and emergency access shafts and adits. To provide the required cross-platform interchanges at Yau Tong and Tiu Keng Leng, it was necessary for the tunnels to change their relative positions as they passed through the hill, resulting in some complex setting-out and closely spaced excavations.
Optimisation of several aspects of the design was possible, notably the driven tunnel linings and drainage, the ventilation shaft and adits, and the double-deck cut and cover tunnels. These are described below.
Tunnel linings and drainage
All MTRC rock tunnels are required to be concrete lined, fully waterproofed around the arch, and pressure relieved, with a design life of 120 years. Waterproofing and pressure relief is achieved by a conventional arrangement of protective and drainage geotextile layer behind a fully welded and seam-tested waterproofing membrane on the lining extrados. Groundwater is collected in the geotextile and flows down to the invert drainage system. The typical cross section of the Black Hill Tunnels is shown in figure 2. Optimisation of the invert groundwater drainage and pressure relief system, using a customised HDPE void former, has been described previouslyà. This was part of an overall optimisation of the basic lining design.
The tunnel design was driven by the need to minimise both the excavation profile and the temporary support and final lining thickness requirements. However, it was also necessary to comply with the MTRC Design Standards Manual (DSM). The DSM sets out minimum design requirements to ensure that the necessary standards for this high capacity rail system are met.
The procedure for the temporary support design was for the support to be predicted in the first instance using the Q-System – with an excavation support ratio (ESR) of 1.5 – but subject to analytical checking of the following:
In general, there was little opportunity to optimise the temporary support to any large extent, although discrete element modelling was effective in demonstrating the acceptability of normal levels of support in some complex and low cover areas.
In the case of the permanent cast-concrete linings, these were required to be un-reinforced as far as possible, for durability reasons, and to be a minimum thickness of 250mm, to avoid any risk of membrane penetration by equipment fixings. The design effort was therefore focussed on ensuring that all permanent-loading conditions could be sustained by the minimum lining thickness, while at the same time minimising the excavation profile.
It is a difficult task to ensure that all possible loads, load combinations, and particularly load distributions, are catered for in the design of an arch lining in rock. A pragmatic approach has been proposedá, which considers the maximum feasible support pressure that can be installed during excavation using conventional support, and applying this as appropriate loaded lengths to critical locations around the arch. This approach was used as a check on the design resulting from application of the DSM requirements. The DSM defines rock loads in terms of Proof, derived from the Q-System, or as rock wedge loads, defined from ubiquitous joint analysis, distributed over the face area of the wedge, and earthquake loads applied as pseudo-static loads. To address the risk of local blockage of the drainage geotextile during the 120-year design life, a requirement is also included for a nominal head of 10m of water above the tunnel crown, decreasing below axis level to zero at the invert. The nominal water load was taken in combination with submerged rock loads. However, the water load was often found to be beneficial because it induced compression into the arch. Asymmetrically applied rock loads alone were generally found to be the most critical load case.
Development of the alternative invert drainage system, as noted above, allowed the arch linings to be analysed as a plane frame ‘ring’ structure, rather than as a conventional arch on footings, with compression-only spring supports to model rock stiffness. In addition to modelling the lining/rock interaction, the software also modelled self-weight, shrinkage, and an ‘analytical gap’ between the lining and the rock to allow for the compressible geotextile and membrane materials. The methodology recognised the non-linear stress-strain behaviour of concrete, which can otherwise lead to overly conservative designs. The overall approach allowed the lining thickness and excavation profiles to be optimised (figure 2).
Permanent linings were successfully sprayed onto membranes in areas of non-standard cross section. The design concepts for this form of lining were the same as for the cast concrete linings.
Ventilation shaft and adits
A ventilation shaft and connecting adits were required to connect a surface ventilation building with the running tunnels. The ventilation building and equipment were provided under other contracts.
The shaft was 9.275m in internal diameter, and approximately 40m deep. It was sub-divided into four individual ducts by a cruciform of internal dividing walls, so that each tunnel could be ventilated independently. The original concept was for the four ducts in the shaft to connect into two adits, each sub-divided into two ducts each. Each of the four ducts would then connect into the crown of its corresponding running tunnel by means of a short drop shaft. Because of the stacked arrangement of the four tunnels at this point, it was necessary for the two adits to leave the shaft at different levels, as shown in figure 3.
The concept arrangement was seen to be difficult to construct and an alternative optimised arrangement was designed so that both adits connect to the shaft bottom, and to the invert of a pair of running tunnels, as shown on figure 4 (see over). One of each pair of ducts continues over the crown of the running tunnel nearest to the shaft. This arrangement simplified shaft and adit construction and allowed access during construction between the shaft bottom and both pairs of running tunnels. The internal dividing walls were precast to further simplify construction.
The upper 20m of the shaft was located in completely or highly decomposed granite (CDG/HDG). Below rockhead the granite quickly improved to slightly weathered or fresh rock. The groundwater table was close to the excavated ground surface.
Although no permanent drawdown of groundwater was permissible, local drawdown was considered to be acceptable during construction because it was estimated that the drawdown cone would not extend sufficiently to influence adjacent property or services.
This allowed the shaft to be excavated through the overburden using steel arch rings and shotcrete. This is believed to be one of the few times that this simple method has been used in Hong Kong for shaft construction, and avoided the need for diaphragm walling or driven sheet pile walls etc. Careful monitoring of the drawdown cone during construction was carried out to confirm the design assumptions.
Below rockhead the shaft was excavated by careful blasting, and was supported temporarily by rock bolts and shotcrete.
The final shaft lining was designed as a reinforced concrete structure. A continuous waterproofing membrane was required. Due to the high groundwater table in the overburden, it was necessary for the lining to be designed as an ‘undrained’ lining above rockhead (i.e. fully waterproofed and subject to full hydrostatic pressure), but below rockhead, and in the adits, the linings could be ‘drained’ structures as in the running tunnels, with drainage connecting into the tunnel groundwater drainage system. However, to minimise the amount of water that might find its way down the extrados of the membrane and the blast damaged zone around the shaft excavation, a circumferential grout barrier and sealing arrangement were provided below rockhead (figure 5).
In addition to hydrostatic pressure and earth pressure loads, the design of the shaft lining had to allow for a large eccentric surcharge loading from the ventilation building. This effect was significant on the design. Because of the depth of overburden, the shaft had to be considered as a large vertical cantilever socketted into rock. In order to achieve reliable fixity below rockhead, it was necessary, with the agreement of MTRC, to delete the compressible waterproofing membrane over a length of approximately 5m. Waterproofing over this length was achieved by designing the reinforced concrete to watertight standards.
Cut and cover tunnels
At either end of the Contract, the four driven tunnels connect to reinforced concrete double-deck cut and cover tunnels. These in turn connect to the Yau Tong and Tiu Keng Leng double-deck station boxes, constructed under other contracts. The cut and cover tunnels are referred to as the West Portal Cut and Cover (WPCC) and the East Portal Cut and Cover (EPCC). The WPCC is approximately 60m long, whereas the shorter EPCC is a maximum of 13m in length. In order to optimise the designs, the four separate tunnels were integrated into single double-deck structures at both the WPCC and the EPCC.
At the time of the design, the site above and surrounding the WPCC was expected to be developed, probably as an integrated transport interchange and commercial development. The foundations were anticipated to be constructed between, and to either side of, the tunnel boxes, and likely to result in significant lateral loads on the tunnel structures. The design provided for this by allowing the lateral load to be transferred through the WPCC structure and into the surrounding rock mass.
As shown in figure 6, the WPCC was cast into rock below the base slab with shear keys. These provide the required lateral sliding stability, as well as limiting potential lateral movement.
Overturning stability is provided by connecting the two tunnel boxes with vertical transverse diaphragms. The diaphragms are located at approximately 12m intervals along the tunnels, enabling the two boxes to act as a single structural unit. Other principal concepts for the WPCC design were as follows:
The design concepts for the smaller EPCC were similar to the WPCC, except that headwalls were included at either end to ensure a composite structural action between the two tunnel boxes.
Movement joints were provided between the cut and cover tunnels and the running tunnel linings and station boxes, in order to accommodate longitudinal and lateral movements. Vertical movements were not anticipated due to the structures being founded on hard rock.
A stray current collection system was provided in the trackform, and electrical continuity in the cut and cover structural reinforcement.
In general, the WPCC and EPCC structures were constructed using bottom-up techniques. Construction joints were provided longitudinally at approximately 10m centres, and vertically at the underside and the top of each slab level. To simplify construction, intermediate and roof slabs were precast, with in-situ reinforced concrete toppings. This allowed valuable working space to be maintained in each of the tunnel tubes by avoiding the need for temporary propping of formwork.
Conclusion
The generally fair to good rock conditions, combined with the incentives of a design-and-build contract and a constructive partnering environment established by MTRC, made it possible for many aspects of the design of the tunnels and key structures to be optimised. Some of the design concepts developed for the Black Hill Tunnels could be generally applicable to future projects both in Hong Kong and elsewhere.
Related Files
Fig 1 – Map of Hong Kong showing the location of Contract 603, Black Hill Tunnels
Fig 5 – Cross section of the ventilation shaft
Fig 3 – Original concept for the ventilation shaft and adit arrangement
Fig 2 – Typical cross section of the Black Hill Tunnels
Fig 4 – Optimised ventilation shaft and adit arrangement
Fig 6 – Cut and cover tunnel cross section (west portal)
