Finding the ‘Golden Ratio' | FTI Consulting

Finding the ‘Golden Ratio’: Selecting the Right Retaining System for Metro Deep Excavations

Construction | Forensic & Litigation Consulting

February 1, 2021

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Metros have been an integral part of cities since 1863, helping to reduce commuting costs and create an eco-friendlier environment. Despite being mainly underground excavations (tunnels and caverns), these necessitate deep excavations for the stations and entrances and are rarely risk and problem-free.

What are the key risks associated with metro construction?

The main risk associated with metro construction is related to settlements and lateral displacements during the excavation works (either during the open excavations or tunnelling), which can cause damage to buildings and utilities, whilst others are related to the underground water management1.

To reduce the risk of settlements and displacements and avoid potential damages to the excavation and adjacent structures, the thorough investigation of the following is required for the designs to be developed:

  • underlying geotechnical conditions
  • the loading from the surrounding structures
  • the type and depth of the adjacent building’s foundation
  • the location of the adjacent structures from the excavation.

What are the types of retaining structures in deep excavations?

For deep excavations, either in urban or rural environments, the necessity of retaining structures is always considered to ensure safe environments during temporary (construction) and permanent (operation) conditions. The main differences between urban and rural excavations are the existence of adjacent buildings and the design life of the excavations (permanent or temporary). The first difference is linked to the loading conditions, whilst the second with the material to be used for the support of the excavations’ slopes, and eventually the design life of the excavation.


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