Tunneling and underground construction in urban areas is increasing throughout the world for various reasons. This requires handling of complex challenges for safe construction and to minimize impacts on existing buildings, structures, utilities and the public.
Tunneling in urban areas frequently requires construction with shallow cover, in difficult ground conditions of soft soils and mixed ground, with high groundwater infiltration, frequently in close proximity to major sensitive structures and historic buildings. The potential impact of tunneling on structures and on surface traffic, adjoining businesses and everyday life of the residents is high.
The Needs for Underground Space in Urban Areas
According to UN reports, the world population in 2017 was estimated at 7.5 billion people with anticipated growth in population by 2050 to 10 billion people. There are 22 cities with population of 10 to 20 million people and 34 cities with population between 5 and 10 million people.
The global trend of urbanization and industrialization has caused migration from rural areas to cities. Mega cities are growing in population and the number of mega cities is increasing. For example, Tokyo is the most populated city in the world with 32 million and Seoul, Delhi and Mexico City have populations of over 20 million each. As a result of the human growth and the concentration of populations in centralized urban areas, the need for infrastructure and transportation facilities is increasing.
According to the International Tunneling Association, the tunnel and underground market is estimated at $7.5 trillion. It is further predicted that the tunnel and underground construction market is expected to outpace other heavy civil/infrastructure markets.
Some of the drivers for the increase in underground construction in urban areas beside population growth and the number of large cities are:
- The growing needs for sustainable, efficient, economical and environmentally friendly transportation and infrastructure systems
- The need to reduce environmental, property and visual impacts, and to minimize surface disturbance
- Government regulations requiring cleanup of waterways by dealing with combined sewers in older cities
- The need of resiliency of cities due to climate change
- Protection of low-land areas from flooding (diversion and flood control tunnels), and
- The public demands underground construction for more noble uses of the surface area and the economy and advancement of technologies supports the underground use.
Recent advances in tunneling technologies, improvement in safety, efficiency in implementation, better risk management and the ability to build larger diameter tunnels in more complex geology enable the construction of many newer underground facilities that were previously not possible or having high risks.
Challenges of Tunneling in Urban Areas
Tunneling in urban setting presents a number of unique challenges. A typical urban setting will include the presence of major roadways, potential shallow ground cover, soft ground conditions and potentially mixed ground, existing or abandoned foundations and buried structures, and large intricate networks of wet and dry utilities. Additionally, space constraints in urban settings magnify the challenge of implementing tunneling in such a manner as to avoid inducing displacements damaging to adjacent facilities, structures and utilities.
The use of cut-and-cover construction will further impact traffic, require utility relocation and/or support in place, affect businesses and expose the public to noise, dust and vibration and impact the people quality of life during construction.
However, these challenges can be addressed with carefully designed tunneling method, the use of the latest technologies in tunneling, the use of prudent excavation and support sequencing, implementation of ground improvement and a robust instrumentation and monitoring program that will identify potential issues early and implement corrective actions. With a risk mitigated approach during the design phase, and the use of the latest TBM technologies, tunneling has proven successful in complex urban settings.
TBM Tunneling in Urban Areas
Prior to the advent of pressurized face TBMs, urban tunneling was confined to ground conditions with sufficient stand-up time to allow the tunnel to be advanced and support erected. Tunneling below the groundwater table was limited to strata that could be stabilized by the application of compressed air to control seepage through the face. Ground improvement techniques including permeation grouting and ground freezing were used to stabilize water bearing non-cohesive soils but this was confined to short drives due to their expense.
In 1964 John V. Bartlett patented the Bentonite Tunnel Boring Machine. This TBM was the predecessor of all pressurized face TBMs and allowed safe and economic tunneling in water bearing non-cohesive soils. Earth Pressure Balance (EPB) TBMs were developed in Japan in the 1970s based in part upon experience of slurry TBM operation. EPB operation was initially confined to soils with fines content of 20% or greater. Now the operational ranges of slurry and EPB TBMs overlap significantly due to the use of additives with both types. Further development of TBM cutterhead design incorporating disc cutters and soft ground cutting tools has extended the range of the pressurized face TBMs to mixed faces of soil and rock and, on occasion, full face rock.
Now pressurized face TBMs are employed under cities across the globe excavating tunnels for transit systems, rail, sewer networks, highways, power distribution and other functions.
In Malaysia, slurry TBMs tackled karstic limestone, residual soils and mine tailings to construct the Klang Valley Mass Rapid Transit (KVMRT) Red Line under highly developed area, the first use of the variable density slurry TBM. In Shanghai a 49-ft diameter EPB TBM was used to bore a twin deck highway tunnel under the historic Bund in normally consolidated sediments and fills with less than 30 ft of cover in places passing over operational subway tunnels with 5 ft of separation. In Hong Kong, a 19-ft diameter articulated shield slurry TBM bored through full face granite, mixed face and residual soil negotiating 500-ft radius curves to construct the Lai Chi Kok Storm Water Tunnel. In Pittsburgh a 23-ft diameter slurry TBM bored the two tubes under the Allegheny River for the LRT North Shore Connector designed by AECOM through fluvio-glacial deposits and coal measures at a gradient exceeding 8% and very tight horizontal curve, one of the early uses of slurry TBMs in the Americas.
TBM Technologies Enable Larger Tunnels
Before the introduction of closed face pressurized TBMs, tunnel diameter was limited in part by the strength of the ground and by the need to have sufficient ground cover to mobilize ground arching. These together with the face area determined the stand-up time of the tunneled opening and allowable advance without support. The application of face pressure by a TBM has turned these requirements on their head. Ground strength and cover are important as they limit the maximum pressure that can be applied by the TBM without risk of ‘blow out’ of the tunnel, but face area is no longer as critical a consideration in determining face stability. Therefore the use of large diameter tunnel is increasing in situations where previously a twin-bore configuration would have been adopted. In some circumstances the feasibility of the single bore solution has allowed a project to proceed where lateral and alignment constraints ruled out a twin-bore configuration.
The Bund Tunnel in Shanghai could only be constructed as a single bore because a side-by-side twin bore could not fit the lateral constraints of the corridor. To date, a few single-bore, large-diameter tunnel have been constructed in urban areas, such as the Barcelona Metro Line 9 in Spain, the Alaskan Way project in Seattle, the Evergreen line in Vancouver, and the Chong Ming Tunnel connecting Shanghai with Chong Ming Island. As confidence is built in the operation of large-bore tunnels, they will become increasingly common due to the multiple advantages they hold over twin-bore construction, as can be seen in the new projects under design and or in construction such as Riyadh Metro and Paris Metro expansion.
Advantages of the single-bore option include the elimination of cross passages, sufficient space to accommodate in-line sumps and equipment rooms within the tunnel cross section and more efficient ventilation strategies. AECOM was instrumental in recommending and designing the largest diameter bored tunnel in the world that saved three years of construction, the Tuen Mun Chek Lap Kok Link (TM-CLK) Tunnel in Hong Kong at a diameter of 57 ft, 9 in. (17.6 m) which when completed provides direct connection to the airport.
Flexibility and Adaptability of SEM/NATM
Sequential Excavation Method (SEM) (or the New Austrian Tunneling Method (NATM)) has become the method of choice for tunneling in urban areas to construct complex underground structures such as metro stations, multi-track metro lines, rail crossovers, short road tunnels and underground road ramps in order to avoid cut-and-cover construction with its impacts on streets, utilities, traffic, businesses and the public.
Under these conditions and where complex and challenging ground conditions exist, underground construction requires a flexible design that can be executed effectively and safely, while minimizing impacts to existing structures. This specifically includes tunneling in running and flowing ground, tunneling under high water ingress, encountering mixed face conditions, low ground cover, presence of sensitive buildings and structures within the influence zone of the excavation, presence of boulders, abandoned foundations or uncharted utilities and complex geometrical configurations. SEM minimizes impacts on traffic and utilities/services throughout construction, reducing disruption to everyday life.
However, SEM in an urban setting presents a number of special challenges beyond what is discussed previously. Space constraints in urban settings magnify the challenge of implementing SEM tunneling in such a manner as to avoid inducing displacements damaging to adjacent facilities, structures and utilities. Such challenges can be addressed with carefully designed excavation and support sequencing, including potential ground improvement and a robust instrumentation and monitoring program.
For example, on the Northern Blvd Crossing in New York City as part of the East Side Access for which AECOM is the PM/CM, the tunnel was constructed using SEM accommodating a very large cross-section with a width of 60 ft, 4 in. (18.4 m) and height of 38 ft, 9 in. (11.8 m) under existing transit lines and with shallow cover and unfavorable geology of mixed glacial deposits below the water table. To enable the construction, the cross section was subdivided into multiple drifts and using ground freezing to stabilize the ground and enable safe construction.
For the Chinatown Station as part of the Central Subway project in San Francisco, and for which AECOM is also the PM/CM, the station was constructed in a highly developed area under a narrow street, congested businesses, residential dwellings and historic and institutional buildings. The station caverns were excavated in multiple drifts using pipe arch canopies as a pre-support. An extensive instrumentation and monitoring program was implemented and a compensation grouting system was used to deal with the settlement of buildings and utilities.
Ground Improvement and Pre-Support Measures
With respect to SEM, ground improvement measures improve soil standup time during excavation and allow the installation of optimized initial support while providing safe excavation. Ground improvements also serve to control ground water, reduce ground loss and potential surface settlements and minimize the tunnel deformations during excavation. The variety of ground improvement techniques available are diverse and include dewatering, jet grouting, cementitious or chemical permeation grouting, compaction grouting, ground freezing, etc. In the scenario where settlement would potentially occur, compensation grouting can be used as a remedial measure to overcome tunneling-induced settlements. Instrumentation and monitoring are critical for detecting ground movement and implementing corrective measures.
Common methods of pre-support, which include spiling, pipe arch canopies and sub-horizontal jet grouting, act to improve the standup time of weak ground during and after excavation. In addition to minimizing risk during excavation, effective pre-support measures will minimize disturbance to in-situ ground during excavation, thereby limiting surface settlements. However, pre-support measures are only suitable for implementation when they have close contact with the ground. This is essential in order for the ground and pre-support elements to work effectively as a reinforcement integrated into the ground.
Instrumentation and Monitoring Measures
Instrumentation and monitoring are essential for safe and efficient tunneling. This is due to uncertainties in both the ground model and response of the ground, utilities and structures within the zone of influence of the tunneling.
It is common practice to monitor groundwater level (pore water pressure) and ground movement in addition to the movement of buildings and utilities. Short term lowering of groundwater can cause significant ‘immediate’ and irrecoverable settlement in normally consolidated soils. If pore pressures do not recover stiffer soils can consolidate for years after construction. Another potential consequence of dewatering is that timber piles supporting historic buildings can rot if exposed to air
Ground movement is a 3-D phenomenon and it is vital to monitor the ‘bow wave’ settlement in advance of the tunnel face. This allows adjustments to be made to TBM operating parameters (or to the face excavation in case of SEM) as it advances to ensure that the target volume loss is not exceeded. Standard practice is to use surface measuring points set out on a square grid along the alignment. The spacing of the monitoring points must be sufficient to define the shape of the curve as the location of the point of inflection in the settlement curve is as significant as the maximum centerline settlement. In addition to surface monitoring points multipoint extensometers installed in boreholes should be used to determine settlement at intermediate levels between the tunnel crown and ground level.
Buildings are not totally static structures and over their life they will have been subjected to movements caused by a number of external factors. These include wind loads, foundation movements, seasonal variations in temperature and adjacent construction. All of these, and other factors, may have left their mark in the form of cracks, sticking doors and structural damage. Therefore it is essential to carry out a condition survey of all structures within the zone of influence of the tunnel works. This survey should record all visible defects both superficial and structural. The objectives are to ensure that pre-existing defects are not attributed to the tunneling work and identify if the residual structural capacity is sufficient to accommodate the predicted ground movements.
It is a common practice to monitor building movement using total station systems. These comprise automatic reading theodolites that scan prism targets secured to the structure to determine changes in their position in 3-D space. The accuracy of these systems demonstrates that buildings are continuously ‘moving’ in response to seasonal effects. These include thermal expansion and contraction, and groundwater table fluctuation. The annual seasonal response of sensitive structures should be established before the start of excavation. Failure to do so could lead to false alerts, unnecessary stop work orders and remedial/protection works. The emphasis is always on monitoring movement and controlling it within acceptable limits. Where it is not possible to achieve this with good tunneling technique alone, consideration must be given to protection and mitigation measures. These include underpinning, protection walls, grouted blocks and compensation grouting.
The urban subgrade is packed with utilities ranging from power cables, communication cables to sewers and water mains. All are to some degree critical to the life of the city. The potential effect of tunneling on utilities is often the most problematic issue when planning major underground works. This is because not only are their locations and condition often unknown, but their exact locations are unknown and their ownership is diverse introducing numerous stakeholders to the project. Perversely, these stakeholders often have little interest in the project beyond ensuring ‘zero harm’ to their utilities. On occasion, this results in protection criteria that are either unnecessarily expensive to satisfy or extremely difficult to achieve in the field.
Tunneling in urban areas is becoming highly viable as a result of advancement in technologies, application of safety measures, implementation of risk mitigation strategies, and efficiency in design and construction. Properly implemented, it will avoid cut-and-cover construction and its associated impacts on traffic, utilities, businesses and the public.
Detailed geotechnical investigations, good understanding of the ground behavior during tunneling, and a robust design are essential for successful tunneling in urban areas. In addition, a comprehensive instrumentation and monitoring system with predetermined threshold limits and potential remedial measures is crucial, along with pre-qualification of all involved parties. An engaged program manager with strong technical knowledge and effective communication and collaboration between the designer, contractor and owner’s representatives is vital, and a fair and equitable risk sharing mechanism through a well-thought out Geotechnical Baseline Report are essential elements for successful implementation of tunneling in challenging settings such as urban environments.
About the Authors
Nasri Munfah is the Director of Tunneling and Underground Engineering Center of Excellence of AECOM. He oversees the firm’s tunneling and underground projects and provides leadership in project pursuit and delivery, the development of innovative solutions, recruitment and the professional development of staff. With over 30 years experience in tunnleing in urban areas globally, he provides in-sight perspective of issues related to tunneling in urban areas.
Bob Frew is a world leading tunnel specialist with over 40 years of experience. He is highly experienced with tunnel construction using soft ground and rock TBM in urban environments. Frew is based in the United States but his experience extends worldwide. He has worked on tunnel projects in Australia, Europe, North America, Middle East, China and South East Asia.