Two  vertical and orthogonal systematic joint sets are generally arrayed in a grid pattern on  the bedding surface, which are the significant features of flat-lying sandstone terrains. Although extensive researches are reported on   this topic, many fundamental  problems have still   not  been  solved. Such mutually perpendicular opening-mode fractures are an  obvious manifestation of effective tensile stresses in  two orthogonal directions in  the horizontal bedding plane. A good understanding of these orthogonal joint systems is a key  to structural analysis, landscape interpretation, and guidance of resolving a number of very practical problems in  engineering, mining and hydrologic projects. Based on  an  anatomic investi- gation on the orthogonal joints in the Potsdam sandstone of Cambrian age  at Ausable Chasm (New York State, USA) and Beauharnois (Quebec, Canada), we  proposed that the orthogonal joints may result from the auxetic effects of quartz-rich sandstone rather than local or regional rotation of the maximum tensile

stress (s3) direction by  about 90   . The  sandstone beds with negative Poisson’s ratios are so  fascinating

that, when placed under vertical burial compression and layer-parallel extension in one direction (s3), it becomes stretched in  the transverse direction (s2), producing two orthogonal sets of  mutual abutting and intersecting joints (J1 and J2 normal to s3 and s2, respectively), and both are normal to the bedding surface. Joint set J1 is more closely-spaced than J2 by a factor of w3.3, which is correlated with an average

Poisson’s ratio of     0.3  for  the Potsdam sandstone at the time of joint formation.

  2021 Institute of Rock  and Soil  Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This  is an  open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

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