he concept of ‘Time-Space Effect’ is a theory specific to underground works in soft ground. It was popularised by researchers from China based on their practical experience of tunnelling in soft ground for metros of Shanghai. It comprises two inter-related notions:
• The time effect means that the deformation of excavated surface in soft ground develops gradually over time in the absence of support. A timely window for installing required support, or reinstating support after removal, is thus provided, before the ultimate failure occurs, or deformation becomes excessive and unacceptable.
• The space effect means that the amount of deformation in soft ground is approximately proportional to the area of the total unsupported excavation face at any given time.
There are also interactions between the time and space effects – an increased area of excavation face results in a decrease in the stand-up time without support, whereas an earlier provision of support allows a larger area of exposed face.
The application of this theory can take the form of segmentation of excavation, which is well represented by the sprayed concrete lined, i.e. SCL tunnels with sequential excavation. The excavation is segmented both in time and in space. Each incremental advance in excavation is controlled to a pre-defined distance, and the excavation face is also divided into a number of divisions. The dimensions are calculated based on required level of safety and ground settlement control. In this way, during construction, the excavation face is only unsupported in a small region over a short period at a time.
This paper shows an example of using adopting the 'Time-Space Effect' theory on a real project.
https://doi.org/10.1680/jcien.18.00031
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