Schist is highly variable in terms of intact strength, mineralogical composition and structure due to the provenance of parent rock and metamorphic conditions during formation. This paper explores the mineralogical and structural control on the intact strength properties of schist. The analysis is initially based on laboratory tests (compressional and petrographic analysis) carried out on quartz-mica schist specimens taken from Central Otago in New Zealand, and is subsequently compared to international experimental studies on schist. The results show that the foliated fabric produces directionally dependent strength anisotropy which shifts with compressional confinement exceeding 10 MPa. Furthermore, the significant variability of intact strength data of up to 48 MPa for the same loading orientation can be constrained if the mineralogical composition and degree of mineral segregation are considered. Review of the petrography of the schist shows that the relative melanocratic (mica, biotite) and leucocratic (quartz, feldspar, calcite, epidote) content of the schist strongly controls its strength due to failure mechanisms associated with the crystal lattice structure. The study results highlight the importance of detailed mineralogical estimates and descriptions of mineralogical segregation during mapping and borehole profiling.
Schist is the product of metamorphism of sedimentary, igneous or metamorphic sequences resulting in a strongly foliated anisotropic rock. The wide range of protoliths compounded by the variability of metamorphic facies conditions (different pressures and temperatures) results in significant variations in the physical, compositional and mechanical properties of schist. Metamorphic differentiation through mineral recrystallization and segregation is influenced by the metamorphic grade, and results in compositional layering into foliations with a lepidoblastic texture of preferentially aligned elongate or lamellar minerals [1].
The penetrative foliated fabric of schist produces an inherently anisotropic mechanical behaviour where the strength of the rock is directionally dependent on the orientation of the schistosity in relation to applied stress, as illustrated in figure 1 [2]. The mechanical strength and anisotropic nature of schist has been highlighted in multiple studies, such as: the laboratory characterization of the DanBa quartz mica schist from Sichuan, China [3], anisotropic laboratory data obtained from Angers Schist in France [4], the study of schistose anisotropy of the Athens schist [5], evaluation of the strength anisotropy of four different lithological variations of schist from the Himalayas in India [6–10], strength variation of the Hamedian schist from Iran under variable confining stresses [11], laboratory characterization of the anisotropic nature of the Yeoncheon Schist from Korea [12], characterization of the strength of the Otago schists in New Zealand [13–17], and the compression testing of schists from the Neelum Valley in Pakistan [18].