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

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

  • uniaxial compressive strength
  • modulus of elasticity

Engineering Classification of Rock

Step by Step Method for Classifying Rock on the Basis of Engineering Parameters)

There are 3 steps in this method of rock classification, as given below.

Stage-I (Classification of Rocks on the basis of uniaxial compressive strength)

Step-2 (Classification of Rocks on the basis of Modulus Ratio)

Step-3 (Including both the above classifications)


Stage-I (Classification of Rocks on the basis of uniaxial compressive strength)

  1. Perform an axial compressive strength test on a cylindrical rock specimen with a length to diameter ratio of at least 2. Click here to know the UCS test procedure.
  2. Based on the result of the uniaxial compressive strength test, the rock is classified as Class . is classified as a B C D And I, The table below was prepared by Deer and Miller to classify rock based on its axial compressive strength.
class name Description Uniaxial Compression Strength (kg/cm)2, rock material
a very high power 2250. more than Quartzite, diabase, basalt, most igneous rocks, strong metamorphic rocks.
b High strength 1125 to 2250 Weakly cemented sandstones, hard shale, most limestones, dolomites
C medium power 562.5 to 1125 Many shale, porous sandstone and limestone, schistose, varieties of metamorphic rocks.
D low strength 281.25 to 562.5 porous, low density rocks, friable rocks, troughs
I very low strength < 281.25 Clay shale, weathered and chemically altered rocks of any lithology

Step-2 (Classification of Rocks on the basis of Modulus Ratio)

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

MR = Et50 , ICorrect

Where,


MR = Modulus Ratio

It50 = 50% of the final compressive strength of the tangential modulus of the rock

ICorrect = uniaxial ultimate compressive strength

  1. On the basis of modulus ratio the rock is classified as square. is classified as h, M And Took, After calculating the modulus ratio, use the table below to classify rock based on modulus ratio.
Class Description value of modulus ratio
h High >500
M medium (average) 200 to 500
Took Less < 200

Step-3 (Combining both the above classifications)

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

For example CM means mean of medium power and modulus ratio.


Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.