The aim of this paper is to determine the geotechnical properties of clastic sedimentary rocks and especially sandstone which constitute a great part of the flysch formation. Laboratory tests were conducted in samples collected from a tunnel site (near portal, depth up to 30-40m) in Manipur state, India. Physical and mechanical properties like, dry density (ρd), sound velocity (Vp), Brazilian tensile strength (σt), point loading strength (Is50) and uniaxial compressive strength (σci), were determined as per the relevant Indian standards. Additionally, the material constant (mi) and uniaxial compressive strength (σci) of intact rock were determined by fitting method in the RocDatav5.0 software. Further, the effect of geological strength index (GSI) and disturbance factor (D) on rock mass strength or minor principal stress(σ1) for particular given value confining stress (σ3) were investigated. To illustrate the advantage of Hoek-brown strength criterion, a tunnel is analysis is involved with numerical analysis with Phase2v9.0 software Hoek-Brown (HB) strength criterion followed the non-linear failure envelope, which is nearly applicable for all types of rock mass with the link of GSI. Therefore, HB strength criterion can predict the accurate deformation and plastic zone formation of surrounding rock mass for the tunnel excavation.