C+S March 2020 Vol. 6 Issue 3 (web)

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. Going Underground Construction methods can be disruptive, depending on the needs of a project and its location. The construction of underground metro sys- tems helps preserve quality spaces above ground, particularly in urban areas; however, limitations on urban space means that deep excava- tions 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, under- pinning, 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 plan- ning 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 disrup- tion 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. Tunnelling 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 de- pending 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 suc- cessfully. 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 excava- tion, and minimize the associated economic costs. The NATM leverages the behavior of rock masses under load and mon- itors 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 di- vided 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 con- crete 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 continu- ously 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 distin- guish 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 wa-


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