HOW TO CLASSIFY ROCKS? (ENGINEERING CLASSIFICATION OF INTACT ROCK)

This method of rock classification was introduced by Deer & Miller in the year 1996. Two parameters are considered while classifying rocks on this method, which are

• Uniaxial compressive strength
• Modulus of elasticity

Step by Step Method to Classify Rock Based on Engineering Parameters)

This method of rock classification consists of 3 steps, as given below.

Step-1 (Classifying the rock based on uniaxial compressive strength)

Step-2 (Classifying the rock based on modulus ratio)

Step-3 (Combining both the above classification)

Step-1 (Classifying the rock based on uniaxial compressive strength)

1. Perform uniaxial compressive strength test on a cylindrical rock specimen having length to diameter ratio of at least 2. Click here to know the U.C.S. Test procedure.
2. Based on the uniaxial compressive strength test result, the rock is classified as Class A, B, C, D & E. the table given below was prepared by Deer & Miller to classify rock based on uniaxial compressive strength.

 

Class Name	Description	Uniaxial Compressive Strength (kg/cm2)	Rock Material
A	Very high strength	Over 2250	Quartzite, diabase, basalt, majority of igneous rocks, strong metamorphic rocks.
B	High strength	1125 to 2250	Weakly cemented sand stones, hard shales, majority of limestones, dolomites
C	Medium strength	562.5 to 1125	Many shales, porous sand stones and limestones, schistose, varieties of metamorphic rocks.
D	Low strength	281.25 to 562.5	Porous, low density rocks, friable stone, truff
E	Very low strength	< 281.25	Clay shale, weathered & chemically altered rocks of any lithology

Step-2 (Classifying the rock based on Modulus Ratio)

1. From the stress-strain curve obtained from uniaxial compressive strength test, draw a tangent at 50% ultimate compressive strength of rock & calculate the tangent modulus at this point.
2. Then using the following formula, calculate modulus ratio

MR = E_t50 / σ_ult

Where,

MR = Modulus Ratio

E_t50 = Tangent modulus at 50% ultimate compressive strength of rock

σ_ult = Uniaxial ultimate compressive strength

3. Based on modulus ratio, rock is classified as class H, M & L. After calculating modulus ratio, use the table given below to classify rock based on modulus ratio.

Class	Description	Value of Modulus Ratio
H	High	>500
M	Medium (average)	200 to 500
L	Low	< 200

 

Step-3 (Combining both the above classifications)

In this step, we use both the above class designation to classify rock as AM, BH, CM etc.

For example CM means medium strength and average of modulus ratio.