JOINT SPACING MODELS: AN ALTERNATIVE FOR JOINT PROPERTY CHARACTERIZATION

The accurate characterization of joint properties - spacing, orientation, and trace length - is a very important, perhaps the most important, aspect of site characterization because in the broadest sense, joints control slope stability and tunnel standup time. Unfortunately, data quality can vary significantly depending on the experience and knowledge of the individual collecting the data. Joint spacing is one of the most difficult joint properties to determine accurately because of the variety of data collection methods used. In order to provide some sort of standard, more than 1500 reports of joint spacing were collected from the literature for seventeen lithologies worldwide. More than 200 papers were reviewed, 53 of which contained pertinent data. The disciplines covered were primarily geology, engineering geology, and rock mechanics. Only data reported for limestone, sandstone, and dolomite are, however, addressed here.

The known relation between joint spacing and layer thickness in sedimentary rocks, i.e., joint spacing decreases with layer thickness, allowed the determination of equations relating these properties for the three lithologies. Where layer thickness was reported and where the data were reported as mean or median spacing or could be recalculated to mean or median spacing, joint spacing was regressed against layer thickness to produce a series of equations defining joint spacing variations. Although much more data remains to be "mined" from the literature, these preliminary models for joint spacing in the three lithologies may provide a more consistent approach for determining joint spacing during site characterization.