# 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)**

- 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.
- 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/cm^{2}) |
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)**

- 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.
- 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

- 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.