The world’s cities are booming, and their growth is changing the face of the planet. Rapid urbanization in developing countries—the gradual shift in residence of the human population from rural to urban areas—is both a challenge and an opportunity to steer the world towards a more sustainable trajectory.
The latest UN World Cities Report1 found that the number of “megacities”—those with more than 10 million people—has more than doubled over the past two decades, from 14 in 1995 to 29 in 2016. Projections2 show that urbanization combined with the overall growth of the world’s population could add another 2.5 billion people to urban areas by 2050, with close to 90 percent of this increase taking place in Asia and Africa.
Traffic congestion is a serious consequence of urbanization in any country, with significant negative effects on both the quality of life and the economy. In addition to the time wasted, traffic congestion results in unnecessary fuel consumption, causes additional wear and tear on vehicles, increases harmful emissions lowering air quality, and increases the costs of transport for business. Due in part to traffic, cities and megacities produce more than 70 percent of world’s anthropogenic CO2 emissions.
According to the World Bank (2018)3, as the developing world rapidly urbanizes, there is an opportunity to build safer, cleaner, more efficient, and more accessible transport systems that reduce congestion and pollution, facilitate access to jobs, and lower transport energy consumption. In emerging mid-size cities, where most of the new urban dwellers will live4, city planners have an opportunity to design sustainable and inclusive transport systems from the start, leapfrogging more polluting and costly methods of transport.
In older or larger cities, technology and big data are helping to better map travel patterns and needs, to engage citizens, and to improve the quality and efficiency of transport solutions. There are several approaches to this ranging from the provision of enhanced bus services and dedicated bus lanes, creation of pedestrian areas with access only by public transport, to the creation of new light rail and urban rail services.
One clear solution to the problem of traffic congestion is to reduce the reliance on the use of private vehicles by restricting their use in urban areas with the introduction of congestion charges, or by removing them from the streets altogether. As an alternative, adequate, modern, easily accessible public transport should be offered to the travelling public.
In a rapidly urbanizing world, the urban rail system is an efficient way of reducing traffic congestion, reducing emissions, and decreasing pollution. The high capacity offered by rail systems can serve the high transport requirements generated within and between cities. Compact, mixed-use, pedestrian-friendly development organized around a mass transit station is one of the most effective strategic initiatives to address the negative effects of motorization and identifies rail transit systems as the backbone of urban development.
The presence of railway stations in city centers enables effective intramodality through transfers to urban public transport services in addition to cycling, walking, car sharing, and city logistics. The introduction of an urban rail system can be popular and politically attractive, but the cost is high, and in many cases, improving bus services will have a greater benefit than installing street-running trams and metros. However, rail-based transport is appropriate where there is a need to move large volumes of people (in the order of 10,000 per hour) between major centres (e.g. a transport hub and the city center).
Urban rail is segregated into metro, light rail transport (LRT), and tramways. Metro rail projects typically involve below ground, at grade, and elevated sections with multiple-unit trainsets. LRT is traditionally aboveground and both LRTs and metros operate in dedicated guideways. In contrast, tramways mainly follow existing road and paved area configurations, mixed in with normal road and foot traffic. Where new urban centres are being constructed, it is possible to fully segregate the rail lines to enable services to run quickly, reliably, and safely, as the development can be designed around the transport infrastructure. In an existing city or urban development, the challenges of integrating the transportation system require careful planning. In many cases, this is achieved by constructing underground, minimising the visual and physical impact on the surrounding environment.
Construction methods can be disruptive, depending on the needs of a project and its location. The construction of underground metro systems helps preserve quality spaces above ground, particularly in urban areas; however, limitations on urban space means that deep excavations for underground metro stations often approach existing structures such as buildings, utilities, and other underground facilities. Despite the considerable effort that goes into their design, many urban metro projects still encounter problems.
Deep excavations in densely populated urban areas impose specific challenges, especially the potential impact on adjacent structures from induced ground and structural movement. They can also be a nuisance to the community with site entry and exit challenges, shoring, underpinning, alterations to operations, dust, noise, vibrations and traffic congestion. Key to addressing these issues is the early engagement of key stakeholders and the early identification and resolution of critical issues that may have an adverse effect on the locality. Thoughtful planning and effective design solutions can minimize the impact on the built environment.
It is important to consider the social impact on residents and project-affected persons (PAPs). Project promoters and designers often carry out special studies to find design solutions that will minimize disruption and other impacts to the surrounding communities. The promoters and the contractors should establish robust grievance mechanisms to receive constant feedback from the community to help ensure that the risk mitigation plans are minimizing impacts. Robust stakeholder engagement throughout all the project phases helps to ensure that feedback is constantly collected from all relevant stakeholders. Thus, executives and managers for underground metro projects are able to make informed decisions with the wellbeing of the community in mind.
Tunneling is the least disruptive construction activity in most ground conditions. Apart from the insertion points of the tunnel boring machine (TBM), if used, and the sites necessary for the disposal of excavated material, there is minimal disturbance to the urban environment. On the other hand, the method a client selects for tunnelling can vary depending on the ground conditions and location of the works. Selecting the right method for the project will optimize costs and minimize impact.
Tunnel Boring Machines
The TBM is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. These machines can bore through anything from hard rock to sand. Tunnel diameters can range from 1 meter (done with micro TBMs) to 17.6 meters. Tunnel boring machines are used as an alternative to drilling and blasting methods in rock and conventional hand mining in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel and makes TBMs suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct and can be difficult to transport. The longer the tunnel, the less the relative cost of a TBM per kilometer versus drill and blast methods. This is because tunneling with TBMs is much more efficient and results in shortened completion times, assuming they operate successfully. Drilling and blasting however remains the preferred method when working through heavily fractured and sheared rock layers.
New Austrian Tunneling Method
The alternative to tunnel boring is the New Austrian tunneling method (NATM), also known as the sequential excavation method (SEM) or sprayed concrete lining method (SCL). NATM is a tunneling method that deliberately and purposefully uses the load-bearing properties of the advance core to optimize the mining process, secure the excavation, and minimize the associated economic costs.
The NATM leverages the behavior of rock masses under load and monitors the performance of underground construction during the project. NATM is not a set of specific excavation and support techniques. It has often been referred to as a “design as you go” approach to tunneling, providing an optimized support based on observed ground conditions.
While excavating a tunnel in urban areas, the face of the tunnel is divided into a number of temporary drifts in order to reduce the surface settlements and deformations and to help ensure the stability of the face. This is known as sequential excavation method. This method is based on understanding ground behavior as it reacts to the creation of an underground opening. During the construction of tunnels, the stability of the excavation is usually ensured by the primary lining. The definitive construction of the tunnel tube (secondary lining) is built only after the stress-strain state stabilization around the excavation.
The main structural elements of the primary lining are sprayed concrete and the anchorage system. An integral part of the NATM is geotechnical monitoring based on deformation measurements of the tunnel excavation. NATM belongs to a group of observation methods based on a geotechnics, in which the course of construction is continuously monitored, and the method of mining and excavation securing by the primary lining are adjusted according to the actual behavior of the excavation and the advance core.
This technique first gained attention in the 1960s based on the work of Ladislaus von Rabcewicz, Leopold Müller, and Franz Pacher between 1957 and 1965 in Austria. The name NATM was intended to distinguish it from the old Austrian tunneling approach. The fundamental difference between this new method of tunneling and earlier methods comes from the economic advantages made available by utilizing the inherent geological strength available in the surrounding rock mass to stabilize the tunnel.
A variation of this process incorporates a slurry TBM, a specialized version of the TBM, which includes a plenum chamber that is filled by a slurry made from the water and bentonite, a closed chamber in which pressure is applied to the slurry to balance the pressure of ground water, and a cutting wheel that used for the excavation of ground. This machine is frequently used in ground that consists of gravel and soil, but it has a limited use in clayey ground mass5. It provides support to the face of tunnel in front of the machine by using the pressurized fluid, applied on the basis of surrounding ground permeability6.
The cut-and-cover technique, in which a trench is excavated (cut) at a shallow depth and then backfilled (covered), is often used for the construction of sub-surface, shallow tunnels. At a depth of 18 meters and more, the cut-and-cover method is commonly used for the construction of rapid transit tunnels. At a depth of 10 to 14 meters, this method can be more practical and cheaper than underground tunneling7. However, this method has the significant disadvantages of longer construction duration, construction easement requirement, and high surface distortion8. It is also limited in its route, as it cannot pass under surface structures and buildings, and can only be used in locations where there is clear space above, such as roads and greenfield sites.
Drill and Blast
The drill and blast method dates back to the early 1600s and is suitable for both weak-strength rocks (e.g. chalk, clay, and marl) as well as high-strength rocks (e.g. quartz, basalt, gneiss, and granite). It is suitable for non-circular cross sections and tunnels of comparatively shorter length, where a TBM is not considered suitable for use.
The drill and blast method consists of several steps such as drilling blastholes, charging boreholes, tamping, blasting, fumes extraction by ventilation, mucking, and support installation9.
The main drawbacks of the drill and blast method are the vibrations and shockwaves from the blasting process. These make it an unpopular choice in heavily populated urban locations. The drill and blast technique has the added disadvantages of intense noise, gases, dust, and flying debris. As a result, both workers and machines must be evacuated from the working area10.
Selection of Tunnelling Methods
In selecting a method of tunneling, various factors need to be taken into account11. The following chart details the relative advantages and disadvantages of the various tunneling methods.
The movement of materials frequently has a significant impact on the people who live in and travel through the affected project areas. Disposal of excavated material in urban locations is problematic. Delivery of construction materials creates traffic congestion during construction, no matter how well this is managed. Delays make people late for school, work, appointments, and other important everyday activities. Businesses suffer when clients and customers find it hard to access them. Everyday emergencies turn critical when ambulances, rescue crews, and fire trucks are not able to travel regular routes.
Restrictions on the time of travel for trucks bringing in construction materials and taking away excavated material can help alleviate the impact. In some cities such as Doha, Qatar, overhead conveyors have been used to transport excavated material from the tunnelling operations in the heart of the city to dump sites. Depending on the physical location of the sites, a range of options may exist.
In the case of Crossrail in London, most excavated materials were transported by barge and ship due to the proximity to the River Thames and the access to the waste recovery and landfill site by sea. The approach to material movement will vary by location. Project executives must prepare adequately to respond to those needs in ways that limit impact to communities.
Construction of underground stations results in areas of disruption whilst the station is excavated and built. Modern techniques such as top-down construction, when practical, reduce the period of disruption and the amount of temporary works needed.
The top-down construction method builds the permanent structural portions of the basement and station along with the excavation from the top to the bottom. The top-down method is particularly suited to the construction of underground stations beneath busy roadways and provides significant savings on overall construction time. This is an important technique for major projects, in which time is of primary importance and surface disruption needs to be minimized.
Circular shaped shaft excavations, supported either by diaphragm walls or secant pile walls, provide significant advantages over plane walls. They do not need supports such as struts or tie-back anchors. Such excavation works can be achieved quickly without a complicated construction sequence or coordination between the excavator and the shoring or anchor installer.
For elevated guideways (structures that support tracks in the air), the main disruption is the construction of the support piers. The guideway is usually constructed using precast segments that are lifted into position using a crane mounted on the piers and then post-tensioned to form the span.
This has a distinct advantage over more traditional methods of construction such as casting concrete in situ, which relies on extensive support scaffolding to support the concrete deck during construction.
In elevated sections, the station is also constructed above ground. In many cases, station construction can be achieved without significant disturbance to the local environment.
No significant construction project, such as an urban metro, can proceed without a thorough evaluation of its impact on the environment. An Environmental and Social Impact Assessment (ESIA) is a formal process used to predict the environmental consequences of any development project. It ensures that the potential problems are foreseen and addressed at an early stage in the projects planning and design.
The main purpose of the ESIA is to inform decision makers of the likely impacts of a proposal before a decision is made. ESIA provides an opportunity to identify key issues and stakeholders early in the life of a proposal so that potentially adverse impacts can be addressed before final approval decisions are made.
The ESIA should be prepared initially by the project promoter and built upon by the designers and contractors as the project progresses.
The Goals of an ESIA are:
- to predict environmental, social, economic, and cultural consequences of a proposed activity.
- to assess and review plans to mitigate any adverse impacts resulting from the proposed activity.
- to support the goals of environmental protection and sustainable development.
- to integrate environmental protection and economic decisions at the earliest stages of planning and activity.
A Risk Worth Taking
Traffic congestion is a serious consequence of urbanization in any country, with significant negative effects on both the quality of life and the economy. In a rapidly urbanizing world, the urban rail system is an efficient way of reducing traffic congestion, reducing emissions, and decreasing pollution. The high capacity offered by rail systems can serve the high transport requirements generated within and between cities.
The construction of rail in an urban environment will entail numerous environmental and social impacts that require careful management and monitoring, particularly during the construction phase. It is imperative that they are understood, managed, and monitored extensively through a variety different means.
Construction methods will vary from location to location, but their overall aim must be to minimise disruption to the urban environment during construction where possible. Deep excavations in densely populated urban areas impose specific challenges, especially the potential impact on adjacent structures from induced ground and structural movement.
Tunnelling is a good option for constructing metros in older and larger cities. It is the least disruptive construction activity in most ground conditions. And, depending on the ground conditions and depth of excavation, a range of construction methods exist. But it can be expensive. Care must be taken to select the right tunnelling option for a particular project to optimize cost and minimize impact.
In considering any project and in particular one where there is inevitably significant disruption to daily life during the construction phase, conducting an ESIA and engaging early with stakeholders is essential. Lines of communication with the local community must be open throughout, as well.
When the construction starts, the promoter will keep track of the implementation of the management plans addressing social risk and identify any further issues through establishing a grievance mechanism and robust stakeholder engagement. After the work is finished, the final impact on the surrounding area is minimised and often greatly improved by the creation of passenger transit and access areas, in-station retail outlets, and infrastructure improvements to the local road network.
Ultimately, the disturbance to the daily routine of the local inhabitants during the construction phase is worth tolerating, especially when it will be these local inhabitants who reap the ultimate benefits of a modern public transport system that reduces congestion and impact on the environment.
Case study: Crossrail, London
Crossrail is a new UK railway that runs for over 100 KM from Maidenhead and Heathrow in the west, through new tunnels under central London to Shenfield and Abbey Wood in the east. It is currently Europe’s largest construction project. Work started in May 2009 and over 10,000 people were working across over 40 construction sites. The project is planned to open in 2021.
Within Central London, Crossrail is routed through 42 KM of running tunnels. The works in Central London will generate 6,000,000 T of excavated material. 4,500,000 T of this material will be shipped to be deposited at a ‘Waste Recovery’ facility at Wallasea Island approximately 60 KM east of London, where it will be used to create a coastal nature reserve.
Material unsuitable for deposition at Wallasea Island or unsuitable for shipping to Wallasea will be transported by road, rail, and/or barge to a number of licensed landfill sites to the east of London.
The excavated material is derived from the tunnel boring machines (TBMs), sprayed concrete lining (SCL) tunnels, and excavations for boxes, shafts and portals. Crossrail is utilizing eight TBMs, six earth pressure balance machines (EPB) and two slurry machines.
Transportation was mainly by rail and river barge, then by ship to the landfill, avoiding the use of heavy trucks wherever possible.
1 UN Habitat (2016). World Cities Report; 1.2 World Cities: A Gathering Force, available at http://wcr.unhabitat.org/wp-content/uploads/sites/16/2016/05/Chapter-1-WCR-2016.pdf
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