Hybrid metro station for mitigation of construction disturbance
Capital cities of the world have made a common observation that large population increases have led to traffic congestion, unpleasant conditions of life, downgraded economy and unsustainable development. They have invested in public transport systems constructed under tight urban constraints in order to maintain activity and mobility. The so-called quality of life has been exacerbated by competition and rankings by various journals and organization such as The Economist.
This article presents the concept of hybrid stations as developed or constructed in large cities to keep mobility and business unaffected during construction works. Large bored tunnels, large rock excavations, under-river construction provide interesting alternatives to traditional open-air construction. This article reviews a 20-year experience at Bouygues Travaux Publics of alternative station concepts to minimize construction disturbance and loss of mobility and economy during construction.
City competition
The Economist ranked 140 major cities on a global liveability index based on five criteria:
- Stability — In relation to crime, threat and conflict.
- Healthcare — Availability and quality.
- Culture and environment — Climate, corruption, censorship and sporting.
- Education — Availability and quality.
- Infrastructure — Auality of road and public transport, telecommunication and water.
In the world, Australia, Central Europe, Japan and Canada take the prime positions. In the United States, Honolulu takes the lead, followed by Atlanta, GA; Pittsburgh, PA; Seattle, WA and Washington, D.C.
Public mobility creates comfort and quietness for inhabitants, provides free time and opportunities for culture and education, and reduces stress and pollution in relation to healthcare. It is a prerequisite for liveability and many cities invest massively in public transport to provide superior conditions for business, communication, research and innovation and finally leadership.
Hybrid station concept has come from a mix of cavern and open-air excavation to minimize surface footprint and keep city activity unaffected.
Many interesting examples can be found in New York City, NY (USA), Barcelona (Spain), Stockholm (Sweden), and Hong Kong (China). As a leader in large bored tunnel and complex underground construction, our company has promoted such schemes in tender and development phases to provide better value for money for the inhabitants.
Amsterdam Metro, initial concept (2002)
Amsterdam is the capital city of the Netherlands with a population of 1.5 million. The city lays on a vast sedimentary plain filled with a series of loose sandy soils and soft clays. The majority of the country is near sea level and protected from flooding by a long sea defense line along the shore. The North-South metro line was launched in 2001 on strict requirements for no road closure and no impact on buildings. As a consequence, the program of works was extended and a new concept tested with large tunnels housing train platforms and requiring mini access boxes as a substitute for large stations.
Figure 1 compares three alternatives:
- Single-track tunnel and central platform with TBM near 6.5 m (21 ft) in diameter and street more than 20 m (65 ft) wide.
- Double-track tunnel and side platform with TBM near 10.45 m (34 ft) diameter and street more than 22 m (72 ft) width.
- Stacked-track tunnel and integrated platforms with TBM near 13.6 m (45 ft) diameter and same street width.
Stations built around traditional single-track or double-track tunnels have their surface dimensions governed by length and width of platform. Surface dimensions of modern stations built around superposed track tunnels are governed by vertical circulation and ventilation box but not any more by platform sizes (Fig. 2). Passenger access shaft can be limited to 50 m (160 ft) in length and 12 m (50 ft) in width and located in side streets keeping traffic in the central artery with minimum impact. The surface area reduces from 4,650 m2 (50,000 sq ft) down to 1,100 m2 (12,000 sq ft) or a ratio nearly 4 to 1.
At that time, the worlds largest TBM was 14.9 m (49 ft) as developed by Bouygues for the construction of the Groene Hart tunnel in the Netherlands and superposed track tunnel was at the edge of knowledge. Today, the worlds largest TBM is 17.7 m (58 ft) as constructed also by Bouygues for the recent Tuen Mun Check Lap Kok tunnel in Hong Kong. Superposed track is no longer near the edge of knowledge and technology. It is open to almost all soft soil conditions.
Sandton Station Gautrain (2006-2010)
Johannesburg is the financial capital of South Africa with a population of more than 4 million. In 2005, Bouygues Travaux Publics was awarded the Gautrain rapid rail link project on a concession basis with geological risk to the contractor. The project was a standalone 75-km (46-mile) long, 10 stations and one depot international gauge project with a 15-km (9-mile) long underground section from the center of Johannesburg to the city of Marlboro on its outskirts. Located in the business district, Sandton station was the masterpiece of the project with highest passenger exchange and specific service direct to the airport terminal. Gautrain project was presented at conferences in Chicago, IL in 2008 and Moscow in 2013 for its interest in both structural and architectural expression.
The station was located in fresh granite with faulted areas either side of the station and variable topography. The detailed design and risk analysis concluded in deepening the excavation level of the station to 42 m (140 ft) below ground, drastically increasing rock excavation and adversely delaying the construction program. A review of cavern alternatives was introduced to minimize rock excavation and keep program within contract.
Six cavern options were developed in parallel from a single large double-floor cavern with span of 25 m (82 ft) in length, to a dual medium size-double floor cavern with unit span of 15 m (49 ft) in length and a triple narrow single-floor cavern with unit span of 9 m (30 ft) in length. Option six, shown in Fig. 3, consisting of a dual cavern with single and stacked-track cavern configurations together with central platform access. It was considered most appropriate in keeping cavern dimensions reasonable, rock excavation minimum and easy access to platforms.
It was further developed and constructed. Surface works were reduced from a traditional 165 m (540 ft) width by 30 m (100 ft) box down to two access shafts of 56 m (220 ft) length, 17 m (56 ft) width and 20 m (65 ft) length, 15 m (50 ft) reducing the surface area from 4,950 m2 (53,000 sq ft) down to 1,440 m² (15,000 sq ft) or a ratio of 3.5 to 1. The program was met and Sandton station opened for the Soccer World Cup in June 2010.
Aerial photos in Fig. 4 provide an overall impression of the station in the city and traffic. The two sideplatforms are distance of 14 m (46 ft) and the dedicated airport service on the lower deck is only separated by 4.25 m (14 ft) height difference with the other platforms from which means that rapid interconnection between the lines is achieved. It is a great functional design and a popular success.
Doha West Bay people mover concept
Doha is the capital city of Qatar with a population of around one million and West Bay is its financial and business district. The city lays on a series of surficial fill, dolomitic limestone, shale and chalky limestone. It is proper for traditional and mechanised construction.
In 2012, options were developed for connecting West Bay and compared for construction volumes and impact to the city. The project was 9.6-km (6-mile) long and included 19 stations in three consecutive loops. Due to the extreme heat during the day, the surface project became underground and located under the main roads of the district. The construction of the stations impacted both utilities and road traffic.
Four tunnel options, (Fig. 5), were compared in terms of construction, surface impact, program and cost:
- A traditional single tunnel including side-to-side tracks.
- A traditional double tunnel including single track in each.
- A vertically stacked double tunnel with single track in each.
- A single tunnel with stacked track inside. The later tunnel is big enough to incorporate the station platforms.
The comparison is summarized in Table 1 in terms of quantities, surface impact, program and final estimate.
Option four, single stacked-track tunnel with integrated platform was ranked highest with economies in excavation and concrete volume of 20 to 40 percent, in construction program of 8 to 12 months and cost of 20 percent. Surface impacts were divided by a ratio of four with respect to base single tunnel.
On the particular configuration of Doha, the comparison of data was clear because the 500-m (1,640- ft) spacing between stations was short and the increase in tunnel diameter did not over pass the reduction in station volume. Figure 6 gives architectural views and shows the interest in terms of sustainability with direct sun access and natural ventilation and also in terms of passenger experience with a through view inside the stations.
Cairo Metro Alternative Station (Concept 2016)
Cairo is the largest and busiest city in Africa with more than 10 million inhabitants. Its metro was initiated in the early 1970s, its development has been uninterrupted since and construction of line 3 phase 3 and line 4 is ongoing. The city is located around the Nile River in a deep alluvial basin principally sandy layers with impervious clay layers around 100-m (330-ft) depth. A smaller derivation of the river has created Zamalek Island.
Two stations were located either side of the secondary Nile River bed and an alternative proposal was made to relocate one station under the Nile River with passenger accesses from both sides as shown in Fig. 7. Marine access, site installation on river banks, distance from sensitive receivers and limited utility and traffic disruptions were of significant interest.
The alternative involved the construction of an artificial island and closure of secondary Nile River bed with precautions to ensure water overpass in case of flooding, and soil treatment to allow excavation of diaphragm walls. Construction was intended to proceed on shore with diaphragm wall sealed into watertight clay layers and excavation down to tunnel level to let the TBM pass through. The platform was incorporated into the underwater trench in such a way that permanent configuration of the secondary river bed could be reestablished at the end of construction. The under-river box was terminated on the banks with access boxes running along the shore and letting people go in and out from either side of the river.
Melbourne Metro CBD Stations (2017-2022)
Melbourne is the second largest city in Australia and houses the largest universities of the country. Its population exceeds four million and the city anticipates a major growth due to immigration. The historic public transport combines a traditional tram service in the city and a train network extending far outside the city. Melbourne has been named the most liveable city in the world for several years in a row and the standard of living is clearly a priority.
In 2016, the city launched a modern underground metro concept in increasing capacity in the North-South corridor and relieving stress on the other lines as well. The project includes 6 miles (9 km) of dual single-track tunnel and accommodates five stations of which two are built in the city center with severe constraints and three are built in the student districts with traditional open excavation under the major artery roads (Fig. 8).
State Library and Town Hall stations are the two central stations, they are built under Flinders street with underground side access from shafts located under future development. The architectural style is green and generous.
The stations are built in sedimentary rock called Melbourne formation. The cavern is excavated with rock support and shotcrete from three shafts located two on one side and the third on the other side of the road. The caverns houses the concourse and central platform and offers 20 m (66 ft) width between platform screen doors. Passengers exit from the platform up then sideway to the shafts. The cavern is 29 m (95 ft) wide and 14 m (46 ft) high. It is supported by two rows of columns either side of the vertical circulation. It is constructed in three temporary stages namely a central excavation, central temporary support and side excavation (Fig. 9) and three permanent stages namely concreting of the base slab and permanent columns followed by the transfer beams and central roof and then side walls and roof.
The connection between cavern and shaft, shown in Fig. 10, is the key element that governs the whole design and construction methodology. Its space is split into passenger access and dedicated fire brigade intervention, into fresh-air supply and smoke extraction ducts, into corridors for HV feed and LV distribution cables and into any other services required in the cavern. The crossadits are 17 m (56 ft) wide and 24 m (78 ft) high, they are larger than the platform cavern and also more critical for rock stress during construction. They extend 6 m (20 ft) above cavern top which reduces rock cover and also 4 m (13 ft) below the track that increases vertical span.
Conclusions
The global demand for mass transit projects in large cities has affected both historic and business centers. Elevated stations are complicated and underground alternatives have become the norm where space is constrained. Keeping major roads unaffected and local business alive have become common and have promoted the development of alternative station concepts departing from traditional braced and open-air excavation.
Hybrid stations have benefited from the ongoing development of large bored tunnels in soft soil and large caverns in rock. Both provide interesting options to adjust to available land and necessary continuity of the city during the construction stage.
Large tunnels with stacked tracks and integrated platforms have been tested successfully and have provided acceptable solutions for railway systems and for public building safety. Under-waterway stations are interesting options. Large caverns accessed by side shafts are valid alternatives for station in rock. Anticipation and care is required on the functionality and construction of adits.
Construction technology and numerical simulations have allowed to go further into friendly construction, minimizing impact during construction while enhancing safety during operation.
References
Martin O., Chauliac A., Funck P., 2008, Gautrain, a modern rapid transit system in a progressing country, South Africa, in Creating and renewing urban structures, 17th Congress of IABSE, Chicago, United States of America
Martin O., 2013, Stations of Gautrain, proximity, architecture and services, in Next station, Moscow, Russia
