7.2 Multiple Joint Sets‌

In low-porosity sedimentary rocks, two sets of joints at right angles to each other form an orthogonal pattern. The simplest cases of these have grid and ladder-like patterns, as idealized in the sketches of Figure 23. These patterns are inherited from changes in the stress regime through time, involving a switch between the intermediate and minimum stresses σ3 and σ2, respectively. This switch may be related to changes in the local regional stress field, or to stress release on newly formed systematic fractures.

Figure 23 – Ladder and grid patterns of orthogonal joints (Dippenaar, 2022).

Figure 24a shows an orthogonal joint pattern in a siltstone/mudstone outcrop. Figure 24b shows an orthogonal joint pattern produced in a glass plate by sequential bending in the laboratory. Some members of each set may initiate from, and terminate at, members of the other set as marked by yellow circles. The critical factor is their abutting geometry indicating their temporal relationship (i.e., the order of fracture genesis). The fluid flow behavior may be affected by the abutting styles of the joints. If all the joints are open, flow paths along second-generation joints have a higher tortuosity compared with flow paths along first-generation joints (Figure 23). This imparts a hydraulic conductivity anisotropy to the fractured rock.

Figure 24 – a) Photograph showing orthogonal joints in siltstone/mudstone exposure at the Pacific Coast, north of Santa Cruz, California, USA. b) Orthogonal joint sets in a glass plate produced in a laboratory by sequential bending. Yellow circles mark the initiation points of individual joints. Photograph courtesy of Haiqing Wu of Stanford University in 1999.