Geotechnical Mapping of Underground Exposures

Mapping of exposed rock faces allows for the direct assessment of several rock mass characterization parameters that cannot be established by routine drillhole logging for orebody delineation purposes. Geotechnical mapping of direct accesses such as stope drill drives, and access crosscuts can be used to determine the orientation, linear frequency, size, and surface strength of the geological discontinuities.

In addition, observation, interpretation and description of faults and shears, precise determination of the number discontinuity sets, trace lengths, and observation of the large-scale planarity and joint roughness characteristics can all be achieved by direct geotechnical mapping. Importantly, the need for oriented core is minimized if unbiased data from direct mapping can be used to establish reliable joint set orientation boundaries leading to the geotechnical description of each geological discontinuity set.
Several methods are available to determine the geological discontinu ity set characteristics including line sampling (Call, 1972; Priest, 1985), cell sampling techniques (Mathis, 1988), and strip mapping (Landmark and Villaescusa, 1992). The data collected can be divided into two classes (Call et al., 1976): major structures and minor geological features. Major structures, such as faults, dikes, contacts, and related features, usually have a size of the same order of magnitude as that of the site to be characterized. They are usually continuous, have low shear strength, and sometimes can be seismically active. The position in space, physical properties, and geometrical characteristics are usually established deterministically for each of those main discontinuities.


Major structures are characterized by routine geological mapping carried out by the geologists, who usually gather data on orebody boundaries, rock types, alteration, and location of the main structural features using at a 1:500 or 1:1000 scale. Following the completion of mapping along several drives and elevations, the mine geologists undertake data interpolation to determine which structures are continuous across several drives and levels, thus forming a large-scale structure.

The interpretation is usually based on structure type, orientation, alteration, and infill type and thickness. Figure 4.21 shows an interpretive longitudinal section featuring the position of the major discontinuities with respect to an entire stope block area.

For practical purposes, minor features represent and infinite population in the area of a stope design. As a result, their geometrical characteristics and physical properties must be estimated by measurements of a representative sampled (smaller) population using the methods described later.

Cell Mapping

This is a form of areal sampling or two-dimensional mapping in which an area interception criterion is established in order to collect the field data. Rectangular or square windows, which are called cells are defined along excavation walls (Figure 4.22). A statistical value based on the properties of the geological discontinuities found within the boundaries is assigned to each cell.

In this method, the individual discontinuity sets are defined visually within the cell boundaries. This process requires the grouping by eye of a family of discontinuities with similar orientational properties in order to form a geological design set. For each discontinuity set, the orientation, location, and end points of all the discontinuities within the cell boundaries are recorded. A sampling line can be used to calculate the average apparent discontinuity spacing.


Mathis (1988) developed a quick areal sampling method, in which the discontinuity properties are sampled from a reduced number of observations that appear to represent the mean values for each set. Nevertheless, cell mapping methods are time consuming compared with routine geological mapping or the more conventional geotechnical mapping based online sampling.

Line Mapping

This is a systematic, one-dimensional spot sampling technique, which can be extended to two dimensions if the line is located inside a sampling window. The method consists of stretching a measuring tape along an exposed face and recording the measurements and features of interest of every discontinuity that intersects the tape (Figure 4.23). Ideally, the sampling sites should be randomly selected in three equal-length, mutually orthogonal directions.

In this way, any discontinuity ignored by one line, because of its orientation, will be sampled preferentially by one or two lines. In practice, however, the sites are determined by the availability and accessibility of the rock exposures. For example, vertical sampling lines are very important in determining the properties of any flat-lying discontinuity sets. However, vertical lines are difficult to obtain due to the absence of vertical development within an area of interest. A recommended compromise is to use several randomly located, short ladder-based lines within the drives or face walls where heights approaching 3–4 m are available.

Experience has shown that approximately 2 days are required to choose an appropriate mapping site, establish the line, and record the data required. Mapping should be undertaken on clean (washed) or newly exposed rock surfaces, which allow for a better exposure of the discontinuity characteristics (Figure 4.24). The length of the sampling line is usually extended until a prerequisite number of observations are obtained. Savely (1972) determined that at least 60 observations are required to stereographically define the discontinuity sets found along a particular sampling line. Villaescusa (1991) found that at least 40 observations per set are required in order to construct experimental histograms of spacing, trace length, and discontinuity orientation. In practice, however, depending upon the complexity of the discontinuity network (a rock mass generally contains between three and six discontinuity sets), and the number of sampling lines used, between 200 and 300 observations are required to establish a structural domain for design.

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