RELATIONSHIP BETWEEN JOINT SPACING AND BED THICKNESS IN SEDIMENTARY-ROCKS - EFFECTS OF INTERBED SLIP

This paper consists of three parts. The first part presents a critical review of previous mechanical models on the relationship between join t spacing (s) and bed thickness (t) in sedimentary rocks. The second p art describes a new mechanical model dealing with the effects of inter bed slip on joint spacing. The third part presents the comparison betw een theoretical results of the model and joint data measured from Camb rian flysch sediments at Plage Victor in the Saint-Jean-Port-Joli area of the Quebec Appalachians. This study demonstrates two formation mec hanisms of tensile joints: near-end fracturing takes place in the laye rs with smaller tensile fracture strength(C-0 < 30 MPa) and smaller in terbed shear strength (tau(0) < 20 MPa) while mid-point fracturing occ urs in those layers with larger C-0 (> 50 MPa) and tau(0) (> 30 MPa) v alues. For the rocks with moderate tensile fracture strength (30 < C-0 < 50 MPa) and interbed shear strength (20 < tau(0) < 30 MPa), the nea r-end fracturing and midpoint 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 in dependent 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 prob ably the prevailing process for the formation of tensile joints in bed ded sedimentary rocks and that the coefficient of joint spacing (K) de fined by the ratio of s to t is considered as an indicator of C-0/(2 t au(0)) for the jointed layer.