1. INTRODUCTION
The field measurement of phenomena that control groundwater flow in rock masses has received increasing attention in recent years.1 In media that are known to be homogeneous, it suffices to consider a single representative sample of the material, measure its properties and apply these to the entire medium in order to predict the overall behaviour. Natural geological processes generally introduce a variety of heterogeneities that will often have a major influence on the groundwater regime, provided they are continuous over a significant area. (It can readily be shown from theoretical studies that heterogeneities that are limited in extent do not significantly affect the overall flow characteristics through the mass. Drastically changing the permeability of a single element in a multi-element analysis has little effect on the potential distribution.)
Of primary importance is the effect on the groundwater regime of what are suspected to be major heterogeneities within a rock mass. Bearing in mind that one normally only knows of the existence of such features at a limited number of drillhole or tunnel intersections, a major objective is to establish if these features are hydraulically continuous within the medium. A method for meeting this objective is discussed below with particular reference to its applicability in assessing slope stability, leakage through dam abutments, and foundations, and water inflow into surface excavations and underground openings.
2. CLASSES OF HETEROGENEITIES LIKELY TO INFLUENCE THE GROUNDWATER REGIME
In many large scale engineering studies it is usually feasible to consider only large scale structures, (heterogeneities that will influence the overall groundwater regimes) when evaluating the stability or seepage characteristics of rock masses. Typical heterogeneities can be classified as follows:
Sedimentary systems where the strata is composed of layers of material of different permeability, i.e. sandstone/shale or sandstone/coal/clay.
Gouge-filled faults and seams tending to form impermeable barriers within rock masses.
Shatter zones forming highly permeable aquifers within a rock mass. These are often found adjacent to faults.
Dykes or other intrusive zones having a different permeability than the host rock.
) Solution channels.
Man-made heterogeneities in the form of grouted cut-offs, stressed foundations, etc.
These types of heterogeneities fall into two classes as follows:
Class I: Permeable relative to rock mass (water carrier)
Class II: Impermeable relative to rock mass (water barrier).
3. GENERAL IMPLICATIONS IN SLOPE, FOUNDATION, AND TUNNEL ENGINEERING
Class I:
Provide good zone of natural drainage.
Hazardous in tunnel construction causing high in-flows with associated stability problems.
May form undesirable leakage channels through dam abutments or foundations.
Can be effectively used as impermeable barriers to reduce flow into excavations or through foundations.
Are potentially hazardous to slope stability.
Can lead to dangerous face bursts in tunnelling operations.
The problems caused by groundwater flow influenced by heterogeneities will usually be the result of either high groundwater pressures causing instability or high flows leading to excessive discharge and flooding.
