The algorithm of rock mass variation with depth has been proposed on the systematic viewpoint for
tunneling in this research. At first, at proposed algorithm, rock mass data has been investigated at
different stages of geotechnical studies in a systematic process. Then, collected rock mass data has
been classified based on the index parameters of ground behavior. Finally, the ability of depth
parameter has been proved for modeling the variation of rock mass characteristics by quantitative
and qualitative method. Therefore, depth dependent variation of rock mass characteristics is
applicable phenomenon for designing the tunnels. Classification of rock mass data based on index
parameters of ground behavior produces the generalization of past results to the similar rock mass
condition. Thereupon, geotechnical studies transforms to comprehensive structure having
determined final goals which grows the data basics effectively. Performances of proposed algorithm
has been estimated for depth dependency of 11 rock mass characteristics such as: spacing, joint
persistency, joint aperture unconfined compressive strength, elastic modulus, RQD, BRMR, JCR,
deformation modulus, rock mass strength and continuity factor in 12 twin tunnels of
Khorrammabad – Pol-e- Zal. According to the results, increase of depth from 50 to more than 250
m has been coincident with improvement of rock mass characteristics and joint properties and
equivalent continuous rock shifts to range of intact to blocky rock. Therefore, variability of rock
mass characteristics with depth has to be considered for designing the support system, excavation
method and tunneling contracts.