2003 Volume 13 Pages 23-30
In tunnel construction, the phenomenon in which displacement becomes large is the result of plastic deformation of the surrounding natural ground of tunnel. We reported that the case is remarkable case where a competence factor (the competence factor is the ratio of unconfined compressive strength to initial stress) is less than 2 and an internal friction angle is less than 30 degrees, according to arrangement of the past example of construction by NATM and numerical analysis etc.
In this paper, as a method of evaluation the generating conditions and area of a plastic deformation in NATM, the function that uses the internal friction angle, cohesion, etc. as a factor was drawn. Then, the ratio of ground stability that is the reciprocal of a coefficient of stability is shown the effective factor that evaluates the grade of tunnel deformation. In the natural ground where a hard part, soft part and weak part are intermingled, due to the quality of soft and weak parts mainly influences the grade of the displacement at tunnel construction, it is important to evaluate as the whole of natural ground, from carrying out a tri-axial compressive test for estimating the factors (internal friction angle, cohesion, etc.), in order to evaluate the existence and area of plastic deformation.