Modulus Estimation Method

The deformation modulus (Young's Modulus) of a rock mass is often not a well known or easily measured parameter. Several researchers have proposed empirical relationships for estimating the rock mass deformation modulus on the basis of classification schemes.

In the RocSupport Project Settings dialog, you may select one of three methods for estimating the rock mass deformation modulus. These methods are based on the GSI rock mass classification scheme and the Generalized Hoek-Brown strength criterion. The applicable equations are described below.

The application of the Modulus Estimation Method in RocSupport depends on the Solution Method, Analysis Type and other selections in the input data dialogs. For a summary of how the Modulus Estimation method is applied in each case, see below for details.

Simplified Hoek & Diederichs (2005)

The Simplified Hoek & Diederichs equation for rock mass modulus, (Hoek & Diederichs (2005)), only requires GSI and D (Disturbance Factor) as input parameters. The modulus is calculated in MPa.

                   Eqn.1

This equation is useful if reliable values of intact rock modulus are NOT available.

Hoek & Diederichs (2005)

The detailed Hoek and Diederichs equation includes the modulus of the intact rock (Hoek & Diederichs (2005)), as well as GSI and D. The modulus is calculated in MPa.

                   Eqn.2

This equation is recommended if reliable values of intact rock modulus are available.

Hoek, Carranza-Torres, Corkum (2002)

The Hoek, Carranza-Torres and Corkum equation for rock mass modulus (Hoek, Carranza-Torres and Corkum (2002)), uses the uniaxial compressive strength of the intact rock (sigci), GSI and D.

For sigci <= 100 MPa, Equation 3 is used:

                        Eqn.3

For sigci > 100 MPa, Equation 4 is used:

                                 Eqn.4

The modulus is calculated in GPa.

Application of Modulus Estimation Method

The application of the Modulus Estimation Method in RocSupport depends on the Solution Method and Analysis Type (in Project Settings) and other selections in the input data dialogs, as described below.

Solution Method = Carranza-Torres , Analysis Type = Deterministic

To allow Modulus Estimation:

• In the Tunnel and Rock Parameters dialog, peak strength parameters must be defined in terms of GSI, mi and D. Modulus Estimation is NOT applicable if peak strength parameters are defined using mb, s and a.

Solution Method = Carranza-Torres , Analysis Type = Probabilistic

To allow Modulus Estimation:

• In the Probabilistic Tunnel and Rock Parameters dialog, peak strength parameters must be defined in terms of GSI, mi and D. Modulus Estimation is NOT applicable if peak strength parameters are defined using mb, s and a.

• Select the Estimate Modulus from Strength checkbox.

Solution Method = Duncan Fama , Analysis Type = Deterministic

To use Modulus Estimation:

• Select the Calculate from GSI button in the Tunnel and Rock Parameters dialog, to access the Parameter Calculator dialog.

• Select the Young's Modulus of Rock Mass checkbox in the Parameter Calculator dialog.

Solution Method = Duncan Fama , Analysis Type = Probabilistic

In this case Modulus Estimation cannot be applied, because the variables GSI and D cannot be directly used by the Duncan Fama solution method in conjunction with a Probabilistic Analysis.