IDENTIFYING ANISOTROPIC IN SITU STRESS CONDITION AND ITS IMPACT ON DISPLACEMENT PROFILES FOR TUNNELING WITH HIGH OVERBURDEN

<p> Anisotropic in situ stress field does induce specific patterns of tunnel displacement while tunneling through hard sedimentary rocks with high overburden. Orientation of anisotropic principal stresses, along with a tunnel axis direction corresponding to these stresses, leads to the development of nonuniform stress distribution or local stress concentration in front of the tunnel face. This asymmetric stress condition with regard to a tunnel axis direction generates displacements both ahead of and behind the face, subsequently. To elucidate these mechanisms, first, in situ principal stresses were measured on the tunneling site, excavating through sedimentary rock masses that generally exhibited elastic behavior under the depth of more than 700 meters in the Akaishi Mountains region, central Japan; they were concordant with the regional anisotropic stress field: approximate east-west compressive tectonic force acting in the study area. Then, observed imbalance in displacement vectors reciprocally suggested that the asymmetric stress condition, evolving adjacent to the excavated space, might well control tunnel displacements to a certain extent. Finally, numerical analyses helped to identify that these displacements derived from anisotropic in situ stress field in conjunction with a tunnel axis direction. This paper emphasizes the importance of incorporating an impact of anisotropic in situ stress condition with the analysis or evaluation of displacement profiles for a tunnel which passes across a known regional stress field.</p>