Abstract
The ‘Old’ Beacon Hill Tunnel was constructed through granite from 1906 to 1910 as part of the Kowloon Canton Railway in the then British colony of Hong Kong. It was the first railway tunnel to be constructed in this region and, at 2.2 km long, longer than any Chinese railway tunnel. Prior to Japanese occupation in December 1941, the British detonated charges to collapse the tunnel to deny its use as an invasion route. Following Japanese defeat in 1945 and return to British government, the tunnel was re-excavated and the damaged sections were supported using steel arches based on Terzaghi's rock mass classification system, which uses geological descriptions to classify loading onto arched tunnel supports. The railway was subsequently realigned during the 1980s and placed in a new tunnel running alongside and west of the ‘Old’ Beacon Hill Tunnel, which was converted to accommodate a pipeline connecting gas supplies. Ground investigations implemented after return of the territory to Chinese sovereignty in July 1997 established that groundwater flow and tunnel stability were influenced by a dyke within the granite, joint set orientation, faulting and weathering as well as the wartime detonations.
The ‘Old’ Beacon Hill Tunnel, part of the Kowloon Canton Railway, was the first railway tunnel to be constructed in the former British colony of Hong Kong, now the Hong Kong Special Administrative Region of the People's Republic of China, on China's SE coast (Fig. 1). At the time of opening in 1910, at 2.2 km in length it was the longest tunnel in China and the fifth longest tunnel in the world outside Europe. Its route (Fig. 2) presently runs NNW–SSE beneath Lion Rock Country Park and the portals are located immediately adjacent to densely populated areas at Hin Keng Housing Estate to the north and City University to the south.
Map of part of Hong Kong Special Administrative Region, SE China, showing the ‘Old’ Beacon Hill Railway Tunnel (circled) and route of the present East Rail Line from East Tsim Sha Tsui north to the Sheung Shui Border Crossing. The distance across the map is 50 km. Adapted from Mackay et al. (2015, fig. 1).
The ‘Old’ Beacon Hill Tunnel alignment from north (left) to south (right), c. 2.2 km long and adjacent to the present East Coast Rail Line, showing chainages (numbered above, from north to south), positions of tunnel collapse and fault intercept (from Mackay et al. 2015, fig. 3).
The Kowloon Canton Railway was expanded to form a network during the 1980s, which included upgrading the railway alignment through the ‘New’ Beacon Hill tunnel, parallel to the old tunnel alignment (Fig. 2). The present Kowloon Canton Railway, renamed the Mass Transit Railway, East Rail Line, runs for 41.5 km between east Tsim Sha Tsui and Sheung Shui passing through three underground sections (Fig. 1).
The ‘New’ and ‘Old’ Beacon Hill Tunnels pass beneath the densely vegetated Lion Rock Country Park, which rises to a maximum elevation of 450 m at Beacon Hill (Fig. 2). The topography above the tunnel crown rises to about 400 m, 375 m above the crown about midway along the alignment (Fig. 3).
‘Old’ Beacon Hill Tunnel long section, showing concrete lined sections installed at the portals in 2008, areas for defect reparation, and window panels.
History
Construction and usage of the ‘Old’ Beacon Hill Tunnel
The ‘Old’ Beacon Hill Tunnel was excavated using standard drill and blast techniques to form an arched profile (GEO 2013). The portals (e.g. Figs 4 & 5) were located in poorer quality ground with water inflows up to 45 l/min. (Eves 1908, 1911). The initial excavation comprised a 2.6 m tall by 5.5 m wide tunnel. This was followed by multiple-face excavation that increased the outer excavation periphery to 6.8 m tall by 5.5 m wide.
Current south portal entrance (2011) to the ‘Old’ Beacon Hill Tunnel (from Mackay et al. 2015, fig. 5).
The south portal under construction in 1909 (from Mackay et al. 2015, fig. 7).
A permanent brick lining was installed upon completion of every 4.5 m full-face advancement. Three ventilation shafts were constructed: 91 m from the south portal to 28 m depth; close to the north portal to 15 m depth; and 396 m from the north portal to 81 m depth. The shafts were backfilled upon tunnel completion.
During the Second World War, the Kowloon Canton Railway was dismantled by the British to deny this as an access route to Japanese forces that invaded from the north early in December 1941. Charges were detonated within the tunnel, resulting in two main collapses within its southern portion, between chainages 1615 and 1635 (20 m length) and at chainage 1857 (Figs 2 & 3). After defeat and final surrender of the British garrison on 25 December 1941, the territory was occupied by Japanese troops who carried out temporary repairs to re-open the tunnel, including installing wooden props to support the roof. After the Second World War and reoccupation by the British, the tunnel was reinstated over an 18 month period. Work was completed by August 1947. The two collapsed sections, which became referred to as the ‘Japanese Arches’, were supported using steel arches embedded in concrete. An additional 0.8 km of tunnel was also reinstated, which included a crude, water-resistant membrane that was installed between the ribs and brick lining to divert water from the crown and walls into side drains to keep inflow away from the ballast and tracks.
During 1984 the Kowloon Canton Railway was diverted into twin tunnels running alongside the original alignment (Fig. 1). The ‘Old’ Beacon Hill Tunnel was converted to accommodate a gas pipeline supply (Mackay et al. 2015). As the demand for gas supply has increased, an additional gas pipeline installation was proposed during 2011 (Mott Macdonald 2011).
Support
After the Second World War, the tunnel support was assessed using the Terzaghi rock mass classification system (Terzaghi 1946). This was the first classification to relate geological descriptions to rock mass loading and the corresponding support requirements. The tunnel support during this period comprised steel sets and brick lining installed into an arched profile excavation and the degree of support was estimated from loading from the surrounding ground allocated from seven Terzaghi rock mass categories, ranging from the best quality ‘intact rock’, Categories D1 and D2, through to ‘blocky and seamy’ ground, Categories C3–C5 and B6, to the poorest quality, ‘swelling’ ground, Category A7.
The 1984 tunnel diversion also used Terzaghi's rock mass classification system to provide an economical tunnel upgrading approach and comparison with the support used for the ‘Old’ Beacon Hill Tunnel. The geological descriptions used in this system were adapted to include specific weathering conditions and geological features encountered during the tunnel excavation and updated during the 1980s (GCO 1988). Terms such as ‘partial weathering conditions’ (GCO 1988; Whiteside 1988; GEO 2007; Fig. 6) were used instead of ‘blocky’ for Support Categories C3 to C5, ‘faulting’ and ‘dyke intrusions’ for ‘stratified’ for Support Category B6, and ‘extensive zones of completely decomposed granite’ for the poorest Support Category A7.
Granitic weathering: PW, partially weathered; UW, unweathered. Adapted from Geotechnical Engineering Office (2007), with permission of the Head of the Geotechnical Engineering Office and the Director of the Civil Engineering and Development Department, the Government of the Hong Kong Special Administrative Region (SAR). The Government of the Hong Kong SAR does not accept responsibility for the accuracy, completeness or up-to-date nature of any reproduced versions of the material.
Following the railway realignment, the ‘Old’ Beacon Hill Tunnel was upgraded by installing cast in situ concrete lining for c. 1.5 km. This included relining the ‘Japanese Arches’ and extending weep-holes from the brick lining through the cast in situ lining (Mott Macdonald 2011). Because the brick lining continued to deteriorate with increased water inflow, further upgrading works were carried out during 1998. Despite this, the water inflow increased up to 2700 l min−1 (GEO 2013), requiring concrete lining reinstatement for the remaining tunnel sections along the initial 0.35 km of tunnel inward from each portal during 2008 (Fig. 3) and drip-shed installations to divert water inflow into the side drains. Further site investigation works were carried out at this time to verify the tunnel's suitability to accommodate the additional pipeline installation (Mott Macdonald 2011). Recommendations from the site investigation included concrete lining installation along the entire tunnel length, and localized grout injection and concrete reinstatement within the central portion of the tunnel to arrest water inflow and infill voids behind the brick lining.
Geological conditions along the tunnel
The Hong Kong terrain is dominantly of igneous rocks, volcanic or granitic (Fig. 7). The majority of the ‘Old’ Beacon Hill Tunnel passes through the medium- and coarse-grained Kowloon Granite and a lesser part through the typically coarser-grained Shatin Granite, intruded by the Kowloon Granite to the south and by quartz monzonite to the north of the route. There is a geological interface between the Kowloon and Shatin Granites intercepting the tunnel alignment towards the north which has been intruded by a subvertical, porphyritic quartz monzonite dyke that trends ENE to WSW. Faults run sub-parallel and perpendicular to the dyke, NW–SE and NE–SW, dipping subvertically and intercepting the central portion of the tunnel alignment. Photo lineaments, linear surface depressions potentially representing faults (possibly the documented faults) identified by aerial photographic interpretation, intercept the central portion of the tunnel. The faults and dykes intercepting the tunnel, interpreted from Sewell et al. (2000), are shown in Figure 3.
Geology of the Beacon Hill area, along the ‘Old’ Beacon Hill Tunnel alignment. The distance across the map is 10 km and north is to the top of the map. Adapted from Mackay et al. (2015, fig. 9).
These geological features were not recognized during the initial tunnel construction, and only became apparent from geological observations recorded during construction of the East Railway Line in the 1980s and increased understanding of the engineering geological conditions during the recent decades. The effects of each of the geological features intercepting the tunnel and the detonations during the Second World War and their impacts on the tunnel stability are detailed in the following sections, with inferred rock mass categories.
Quartz monzonite dyke
The dyke is composed of finer-grained constituents and therefore weathers to a finer-grained soil with a reduced permeability. The surface expression of the dyke is situated c. 200 m above the tunnel crown, in a depression at the foot of a 150 m vertical, steeply dipping hillside. Owing to the relatively decreased elevation and the presence of finer-grained constituents, the dyke forms an aquitard for groundwater flowing from higher to lower elevations, increasing the groundwater levels beneath the higher-level terrain. The dyke was allocated a C3 rock mass category.
Joint set orientation within coarse-grained granite
The joint sets generally have an orthogonal pattern consisting of three sets: two subvertical sets that are generally aligned parallel and perpendicular to the main fault orientation, and one subhorizontal joint set aligned sub-parallel to the ground surface. Joint orientation is therefore influenced by the proximity to the ground surface and major fault locations. Joint orientation influences the water flow, either increasing or decreasing the flow depending upon the permeability of the weathered materials along the joint traces and the connection with water run-off from the ground surface. The rock mass category improves away from the dyke and fault locations and was allocated D1 to D2, ‘tight joints, slightly loosened’.
Faults
Faults are generally weaker and more fissile than the adjacent rock. Within the Kowloon Granite, joint frequency and persistence increase significantly along the fault traces with joint spacing decreasing in closer proximity to the fault trace, causing an increased susceptibility to weathering which, in turn, influences the hydrogeological regimes (Fletcher 2004). As the faults dip subvertically, this also allows connection to water run-off from the ground surface. Furthermore, the presence of the faults intercepting the tunnel allows localized increases in groundwater inflow. The approximate location of the fault in relation to the tunnel is shown in Figure 3. Owing to the ‘seamy and blocky’ characteristics of the faults, the rock mass was allocated to Category C5.
Partial weathering effects
Increased weathering intensity was observed along the joints during the initial 100–140 m of excavation from the portals for the East Railway Line. This is termed partial weathering (GCO 1988) and generally increased in intensity as the excavation approached the ground surface. Typically, corestones of moderately to slightly decomposed granite are present within a completely to highly decomposed granite soil matrix (Fig. 6).
Detonations during the Second World War
These detonations opened up discontinuities and, over time, allowed the finer-grained, looser constituents to be removed from within the joints by the increased water inflow. The geological features and impacts during the Second World War increased the water inflow from the 45 l min−1 recorded in the early 1900s tunnel construction to the current inflow of 2400 l min−1.
Twenty-first century site investigations and findings
Site inspections and condition surveys carried out during 2000 and 2003, prior to the additional tunnel lining placement in 2008, revealed an increased water inflow during the wet season. This increased water inflow exacerbated deterioration, through loss of mortar from the brick lining joints and in the ground behind the outer brick lining. The voids thus created were subsequently filled with accumulated debris.
During 2010 and 2011 a ground investigation consisting of further condition surveys was carried out. Forty-eight cored boreholes were drilled to a maximum length of 3.5 m and 28 window panels (each 0.5×0.5 m) were opened through the tunnel side walls (Fig. 3). The data obtained were used to characterize the inner concrete and outer brick tunnel linings and to identify the voids, debris accumulation and in situ ground conditions. The concrete lining was unreinforced and averaged 0.25 m thick and the brick lining ranged from 0.34 to 0.57 m thick, averaging 0.5 m. Water-resistant geotextile installed in the late 1940s was identified locally between the concrete and brick linings. Moderately to slightly decomposed granite blocks had accumulated behind c. 25% of the lining and the void thickness ranged from 0.12 to 0.6 m.
The increase in void volume behind the tunnel lining generally corresponded with an increased water flow, increased groundwater pressure towards the central portion of the tunnel and an increase in the degree of weathering towards the portals. The voids also increased towards the ‘Japanese Arches’ where the early 1940s detonations had probably opened existing discontinuities surrounding the tunnel. Weathering intensity also increased in the vicinity of the quartz monzonite dyke along which completely to highly decomposed weathered bands up to 1.75 m thick are present.
The ground investigation and extrapolation of the Terzaghi rock mass classification from the East Rail Line tunnel were used to provide additional geological detail, to describe the tunnel lining conditions, and to verify the support requirements along the ‘Old’ Beacon Hill Tunnel. The poorer ground conditions, allocated C5 to C3 and B6, generally corresponded with the increased water inflow associated with the poorer geological conditions and the ‘Japanese Arches’. The recommended reinstatement (Fig. 3) included back-grouting to infill voids behind the lining, mainly towards the ‘Japanese Arches’, and localized lining reinstatement.
Conclusions
The ‘Old’ Beacon Hill Tunnel was originally constructed over a century ago and, despite the militarily induced tunnel collapses during the Second World War, and following reinstatements during and since the war, is still in operation to facilitate a gas supply network beneath the Lion Rock Country Park. The work described here shows how the use of precise engineering geological descriptions related to rock mass classification support and associated groundwater inflow can guide recommendations for an economical method of tunnel support. The tunnel has been in almost continuous use for over a century and is therefore now approaching the end of the 120 year design life typically specified for tunnel construction projects.
Acknowledgements
The author would like to thank Mr Tymon Mellor for his assistance in providing archived data and editorials.
Funding
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
- © 2019 The Author(s). Published by The Geological Society of London. All rights reserved