Abstract
Weathering of soft mudstone walls of unlined tunnels that were excavated several decades ago in the Kakinokidai Formation (middle Pleistocene) and the Kiwada Formation (lower Pleistocene) of the Kazusa Group of Chiba prefecture in Japan was investigated by mineralogical, chemical, physical and biological methods. Sulfuric acid was generated by pyrite oxidation and calcite dissolved by sulfuric acid, and gypsum crystallized at the surface of wall. The weathering rates (rates of movement of oxidation fronts) were explained by diffusion of oxygen from the wall surface. The relationship of depth of oxidation front (lcm) and elapsed years since excavation (tyear) was expressed by l=R√t with considering the diffusion of oxygen into the rock. The R values were 5 on sandy mudstone of middle Pleistocene, 2-4 on mudstone of middle Pleistocene and 0.3 on mudstone of lower Pleistocene. The diffusion coefficients of oxygen (De) calculated by the chemical compositions corresponding to the above three kinds of rocks were 10-5cm2/s, 10-7-10-5cm2/s and 5×10-8cm2/s, respectively. De may depend on coefficients of air permeability of rocks that correlate closely with coefficients of the water permeability (K). The relationship is log Decm2/s=0.91logKcm/s-0.71. Bacteria were slightly found at a part of the oxidized zone. Size of rock pore and its water quality may be hardly fitted for bacterial habitation.
