Groundwater inflow to tunnels - Initial heading inflow rate estimates

Hi everyone,
we're often using FEFLOW to estimate groundwater inflow and drawdown related to excavations in fractured bedrock (eg.: tunnels). To do so, we use hydraulic conductivity values obtained from packer tests and/or long duration pump tests.
Often, our groundwater inflow estimates are used to guide contractors in pump selection. The long term steady-state estimates from numerical might be OK.
However, such estimates probably represent significant underestimate of the initial heading inflow rate coming from narrow zones of higher permeability that contractors may have to deal with.
Now, the actual question.  How do you deal with the initial heading inflow rate estimates? Is this the kind of estimate we should try to derive from numerical modelling? Do the the progressive excavation schedule (and geometry) of the excavations and discrete nature of fractures prevent us to provide realistic and timely estimates of these transient groundwater infiltration?
Are Heuer-like empirical equations the best approach for flush flow estimates?
Any thoughts greatly welcome.
Best regards,
Thomas

Hi Thomas,

I think one way or the other, you'll get better estimates using transient simulation rather than steady state.  However, the estimates depend heavily on the detail of information you have to work with.  If your packer tests miss the most permeable fracture, the model cannot predict large inflow rate from it.

Transient introduction of the tunnel into the model will also result in averaging of inflow rates and underestimation of gradients, errors which can only be reduced using a fine mesh and small time steps (which are in turn limited by the time and budget you have and computer resources).

I'm not familiar with Heuer equations. I imagine that initial inflows relate to turbulence of flow and damage induced during tunneling.

Pete

Pete,

sorry for this really late reply. And thanks for replying to my post BTW. Agreed that transient help getting a better feeling of the situation. In this case, considering other sources of uncertainty, I wonder if the extra work is worth the effort of implementing everything in a numerical model and doing simulations with very short time steps...

Thomas

Hi Thomas,  The worthiness of a numerical implementation depends very much on the overall cost of the project. Analytical and simple numerical models are fine for small-budget projects, but not, in my view, appropriate for multimillion dollar tunneling projects.  To me, the model should also be compatible with the data collected and big projects usually come with a complex (and expensive) data set.

Cheers,
Pete

Pete