Estimation of Pore Pressure in Disturbed Mudstone from Natural Water Content with Special Reference to Landslide and Squeezing-Swelling Tunnel

Abstract

ABSTRACT To elucidate the engineering significance of natural water content of mudstone disturbed into clay from the view point of Critical State Soil Mechanics, CD triaxial test (with top and bottom drainage only) and CD direct shear test were carried out and the unique relationship between log qmax (or log τmax) and w and that between w and logp' (or log σv′) were obtained. Comparing the experimental results thus obtained, p' was found to be nearly equal to σv′ in fully softened shear zone. Thus, if wn at the depth of h is known, the effective confining stress p' at that depth can be known. Therefore the pore pressure acting at that depth can be calculated as u = γt ·h–p'. Two example case histories, Mizunashi landslide and Nabetachiyama tunnel in Neogene mudstone of Shiiya formation in Niigata Prefecture are presented. In the former, by measuring wn of boring core of 90 m, several number of fully softened shear zones were found to exist where artesian pressure was acting and, in the latter, in the section where tunneling was very difficult due to squeezing pressure, wn of fault clay was found to correspond either to p' induced by the overburden weight or to p' decreased by the amount of methane gas pressure.