Weathering rate of mudstone and tuff on old unlined tunnel walls

Abstract

The mechanisms and rate of weathering of soft sedimentary rocks exposed along unlined tunnel walls were investigated using mineralogical, chemical and physical methods. Tunneling occurred several decades ago within the Kakinokidai Formation (middle Pleistocene) and the Kiwada Formation (lower Pleistocene) of the Kazusa Group of Chiba Prefecture, as well as within the Fukuda Formation (Miocene) of Saitama Prefecture in Japan. Since then, the walls of the tunnels have been desiccated by ventilation, sulfuric acid has been generated by pyrite oxidation, calcite and glass have been dissolved by sulfuric acid, and gypsum has crystallized on the surface of the tunnel walls. The oxidized zone was formed on the surface of the walls by the weathering. The formation of this zone and weathering rates (determined as rates of movement of the oxidation fronts) in the vadose zone are explained by the diffusion of oxygen from the surface of the tunnel walls. The relationship between the depth of the oxidation front (l(cm)) and the number of years since excavation (t(year)) was expressed by l=R(oxi)root t. The R-oxi values from the tunnels ranged among 5 for the middle Pleistocene sandy mudstone, 2-4 for the middle Pleistocene mudstone, 0.3 for the lower Pleistocene mudstone and 1-3 for the Miocene tuff. R-oxi depends on the diffusion coefficients of oxygen and the chemical composition of rocks. The diffusion coefficients of oxygen (D-e), calculated using the R-oxi and chemical compositions of the four kinds of rock above, were 10(-5) cm(2)/s, 10(-7)-10(-5) cm(2)/s, 5 x 10(-8) cm(2)/s and 4 x 10(-7)-3 x 10(-6) cm(2)/s, respectively, depending on the air permeabilities of the rocks that correlate closely with the coefficient of water permeabilities (K). The relationship for the mudstone was log D-e (cm(2)/s)= 0.91 log K (cm/s)- 0.71. The values of D-e for the tuff were smaller than for the mudstone due to the smaller air porosities of 0.01-0.04. (C) 1999 Elsevier Science B.V. All rights reserved.