Current tool estimates the deformation modulus or modulus of elasticity of soft rock based on Hoobs (1974) Mass Factor theoretical equation. It requires as an input the unconfined compressive strength of rock from UCS laboratory tests and RQD values from coring.

The equation uses the mass factor j, to account for rock mass discontinuities, fractures and weathering and the modulus ratio value, M_r to account for rock type.

E_m = j · M_r · UCS

Where:

• j is the rock mass factor to account for fractures, weathering and mass discontinuities.
• Mr is the ratio between deformation modulus.
• Unconfined compressive strength UCS is the unconfined compressive strength of intact rock as reported from laboratory tests.

Mass factor values with RQD are presented in Table 1. Typical modulus ration values with rock type are presented in Table 2.

Table 1: Modulus ratio values for various types of rock.

Group	Rock type	Modulus ratio
1	Pure limestones and dolomites Carbonate sandstones of low porosity	600
2	Igneous Oolite and marly limestones Well-cemented sandstones Indurated carbonate mudstones Metamorphic rocks, including slates and schists (flat cleavage/foliation)	300
3	Very marly limestones Poorly cemented sandstones Cemented mudstones and shales Slates and schists (steep cleavage/foliation)	150
4	Uncemented mudstones and shales	75

Table 2: Mass factor values.

Quality classification*	RQD (%)	Mass factor (j)
Very poor	0 to 25	0.2
Poor	25 to 50	0.2
Fair	50 to 75	0.2 to 0.8
Good	75 to 90	0.5 to 0.8
Excellent	90 to 100	0.8 to 1.0