Abstract: To investigate the applicability of stress release for pilot heading in soft rock tunnel, the mechanical model of stress release for soft rock tunnel excavation by pilot heading method is established based on the classical elasticplastic theory. The elasticplastic solution of tunnel excavation considering the stress release of pilot tunnel is derived by a twostage method. The stress release ratio of surrounding rock is defined to reflect the effect of stress release, and the influences of pilot tunnel radius on stress release effect under different insitu stresses, elastic modulus and strengths of surrounding rock are studied. Results show that the ground pressure applied on the support can be reduced by the excavation of a pilot tunnel, especially in high geostress environment or weak rock, the stress release effect is much more obvious by pilot heading method. However, the pilot tunnel radius is not as larger as possible. The field tests show that for a threelane large crosssection soft rock tunnel, the large section of the pilot tunnel is unfavorable to the stability of the surrounding rock. Stress release measure should be carried out under the premise of ensuring the stability of surrounding rock in tunnel construction. Thus, the primary support should be installed as soon as possible to realize the safe and rapid construction.

Key words: soft rock tunnel, pilot heading, stress release; analytical solution, field test