However, rapid development and limited urban space in most cities means that deep excavations for underground metro stations are often very close to existing structures such as buildings, surface roads, underground facilities, tunnels and services.
Warren Dou talks about the effective risk management and innovative technical solutions that address the challenges of constructing underground stations in urban environments.
Deep excavations in densely populated urban areas impose specific challenges, especially to the safety of and impact on adjacent structures through potential damage from induced ground and structural movement.
Effective risk management in the design and construction of deep excavations in the urban environment must consider a range of issues:
the specification of responses to construction difficulties;
pre-construction surveys of adjacent properties and utilities;
construction impact assessment on surface roads; and
the implementation of effective monitoring systems.
Deep excavation works in urban areas can also be a nuisance to adjacent property owners and the community with site entry and exit challenges, shoring, underpinning, alterations to operations, dust, noise, vibrations and traffic congestion.
Despite the considerable effort that goes into the design and construction planning and risk management, many urban deep excavation projects encounter problems such as damage to existing structures, interruption of services, delays and cost overruns.
Our experience in the design and construction of underground stations has taught us that a multidisciplinary approach and consideration of all major stakeholders is key to the early identification and resolution of critical issues.
Some innovative solutions that we have adopted to address the specific issues of constructing stations in urban environments include:
Diaphragm walls combined into a single walling and foundation unit, which provides a temporary retention system and permanent basement walls, groundwater cut-off and vertical support elements. This combination of the diaphragm wall has proven to be an economic option in many circumstances, particularly in soft ground conditions.
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. Excavation works can be achieved quickly without a complicated construction sequence or coordination between the excavator and the shoring or anchor installer.
Another benefit of the inherently stable circular diaphragm walls lies in the much reduced need for embedment to provide wall stability. It is recognised that as the hoop force provides a stiff continuous support to the wall, the bending moment and shear force in the wall remain generally small, leading to llight' reinforcement ratios. Circular shaft walls can also be constructed by contiguous steel pipe piles, bored concrete piles and sheet piles. In these cases inner ring beams are often used to support the walls. Grouting treatment is used to seal the gaps between the piles to provide groundwater cutoff.
The top-down construction method builds the permanent structure members of the basement and station structures 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 minimised.
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