Relationship between joint spacing and bed thickness in sedimentary rocks: effects of interbed slip

Abstract

This paper consists of three parts. The first part presents a critical review of previous mechanical models on the relationship between joint spacing (s) and bed thickness (t) in sedimentary rocks. The second part describes a new mechanical model dealing with the effects of interbed slip on joint spacing. The third part presents the comparison between theoretical results of the model and joint data measured from Cambrian flysch sediments at Plage Victor in the Saint-Jean-Port-Joli area of the Quebec Appalachians. This study demonstrates two formation mechanisms of tensile joints: near-end fracturing takes place in the layers with smaller tensile fracture strength (C₀<30 MPa) and smaller interbed shear strength (τ₀<20 MPa) while mid-point fracturing occurs in those layers with larger C₀ (>50 MPa) and τ₀(>30 MPa) values. For the rocks with moderate tensile fracture strength (30<C₀<50 MPa) and interbed shear strength (20<τ₀<30 MPa), the near-end fracturing and mid-point fracturing take place preferentially in the thicker and thinner brittle layers, respectively. In the regime of near-end fracturing, the correlation between s and t is linear and independent of the thickness of the bounding non-jointing layers (d). In the regime of mid-point fracturing, however, the s–t relationship can be either linear or non-linear, depending on the variation of d value. The present study also suggests that the near-end fracturing is probably the prevailing process for the formation of tensile joints in bedded sedimentary rocks and that the coefficient of joint spacing (K) defined by the ratio of s to t is considered as an indicator of C₀/(2τ₀) for the jointed layer.