This paper summarizes some case histories indicating the unpredictable nature of groundwater inflow into manels and the important geotechnical and geologic parameters to identify and bracket for more accurate prediction. Methods that have been used to estimate groundwater inflows are also summarized.
The joint ASCE-AIME Underground Technology Research Council (UTRC) and the Association of Engineering Geologist have formed a subcommittee on groundwater inflow into manels. Another UTRC subcommittee on Research and Development Needs surveyed 50 practitioners of underground engineering including independent consultants, contractors, designers, manufacturers and suppliers, and academics. This subcommittee, in compiling a list of 15 topics of research needs for tunnel engineering found that the number one problem was predicting groundwater inflows into tunnels CLITRC 1991). The topic was on the top of everyone's list, except contractor's and academics, who scored the topic about sixth. The study indicated:
"Groundwater is probably the cause of more rock (and soil) tunneling difficulties and cost overruns than any other single factor. Really practical and reliable methods to predict inflow into tunnels from fractured rock masses have not been developed and there appears to be little research oriented in this direction. There is a need to define simple parameters defining this problem, to find ways to determine these parameters, and to implement ways to predict the inflow and effects of water, including how inflows diminish with time. Just as the Qand RMR rock mass classification methods brought some semblance of order in the art of ground support selection, one could hope for similar order in the art of groundwater inflow prediction."
Part of UTRC's Subcommittee for the investigation of Groundwater Inflows into Tunnels is to compile illuminating case histories from owners, consultants, and contractors to bring a practical perspective on the subject. About 50 case histories on the subject have been gathered thus far by the subcommittee.
Groundwater inflows in tunnels are dependent upon the local geologic and hydrologiconditions. There is not a standard method for estimating exact volumes and locations of groundwater inflow that may be encountered in rock manels. Where the geologic conditions are not complex, and where the rocks are unfractured, predicting water inflow (or water losses) may be more amenable to simple analysis. The volume of water may be more confidently estimated when there is subsurface information from either pump tests, packer tests and/or a knowledge of the fracture characteristics and spacing from subsurface borings. In runnels with low intact-rock permeability and porosity, rock inflows of water are associated with joints and fractures, with the heavier inflows associated with shear or shatter zones. Minor fractures and joints may be quite different in spacing, number, and character at the surface than underground due to weathering, location, and depth (geostatic load) effects. Except for major shear zones or other geologic structures, which can be identified through geologic exploration or projections, locating where inflows may occur depends on how accurately geologic structures can be projected below ground. The exact flow-paths followed by water within a fractured rock mass are often very intricate and unpredictable.