Estimation of the drop modulus using the brittleness index of intact rock and geological strength index of rock mass, case studies: Nosoud and Zagros tunnels in Iran

  • M. Soleiman DehkordiEmail author
  • H. A. Lazemi
  • H. R. SaeedModaghegh
Original Article


The drop modulus is defined as the slope for the softening stage of the stress—strain curve of rock mass. Estimation of the mentioned parameter is very difficult because it is related to many parameters such as intact rock properties, quality of rock mass, confinement stress and etc. In this study, based on the actual collected data from Nosoud and Zagros tunnels in Iran, new empirical equations to predict the drop modulus of rock mass using the brittleness indexes is proposed. The results show that there is a direct relation between both parameters and the best correlation between them is achieved using the Altindag’s brittleness index, BI3, to estimate the drop modulus. Finally, the relation between the drop modulus with the brittleness index of intact rock and geological strength Index of rock mass is estimated and a new equation to estimate the drop modulus using both mentioned parameters is proposed.


Drop modulus Brittleness index GSI Strain softening behavior 



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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • M. Soleiman Dehkordi
    • 1
    Email author
  • H. A. Lazemi
    • 1
  • H. R. SaeedModaghegh
    • 1
  1. 1.Department of Civil Engineering, Bafgh BranchIslamic Azad UniversityBafghIran

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