Avon Gorge A4 Realignment – Guidance on groundwater inflow assessment into tunnels
It is not possible to accurately calculate water inflows without detailed knowledge of the hydrogeology and in situ permeability. However, a first approximation based on some simplifying assumptions for the groundwater conditions can be made in order to gain an insight into the potential scale or extent of the problem.
For example, assume conditions for the worst case, initially. If these assumptions give water inflows which do not pose any difficulties to tunnel excavation, then the actual conditions are likely to be favourable. However, if the converse is true, and the inflows from this estimate suggest difficult tunnelling conditions, then the groundwater regime and in situ permeability would warrant further consideration and investigation. This approach eliminates those areas along the tunnel alignment that pose no groundwater difficulties and so allows resources to be targeted to problem areas.
Possible procedure – the ‘what if’ approach:
- Estimate the probable coefficient of permeability, k (m/s) using either established relationships or typical values for similar materials published in the literature, for example, Hazen proposed a relationship between D10 and k for loose granular soils. If a value cannot be defined with confidence, consider a range of likely values and proceed using the maximum and minimum values to estimate the possible range of inflows.
- Using Darcy’s law, the inflow (Q, quantity of flowing water) can be estimated (Q = k.A.h/l (m3/s), in which A is the area of the tunnel surface through which water may flow (face + perimeter), h is the water head differential (m) and l is the flow path length (m).
- The worst case would equate to vertical flow into the tunnel, i.e. where the difference in head of water is equal in magnitude to the length of the flow path or h/l = 1. Hence, Q = k.A.
- For a circular tunnel, A will be the perimeter area, defined, as .D.l (ignoring the face area) or (.D.l) + (.D2/4) (inclusive of face area). The contribution from the face area might be significant only during excavation of the tunnel, and only when excavating through high k zones.
- Inflow should be expressed in terms of litres/minute per 10m run. The values calculated may be compared with those given in rock mass classification schemes.
- If the estimated inflows are a concern, then the recharge potential should be considered to establish whether flows can be sustained.
- Consider what control measures might be utilised should inflows cause concern, eg. dewatering, grouting, compressed air, ground freezing, or a slurry supported TBM during construction.
As an alternate procedure, look at:
Goodman, R.E., Moye, D.G., Van Schalkwyk, A. and Javandel, I. (1965). Ground water inflow during tunnel driving. Bull. Assoc. Engineering Geologists, vol. 2(1), 39–56.