•The drilling rate index does not give the drilling rate directly but rather is a relative measurement of the penetration rate. DWI is determined on the basis of two parameters: friability value S20 and the Siewers J-value. Friability is measure in the Brittleness test and gives a measure of the rock’s resistance to repeated hammer drops.
•Brittleness Test
In the Brittleness test,
A 14 kg hammer is dropped 20 times on a 550 gm sample.
The sample material is 11.2 –16 mm in size.
Brittleness value S20 is the percentage of undersize that passes the 11.2 mm screen
Value determined from 3 to 4 parallel tests.
•Siewers J-value
The Siewers J-value is obtained from a test carried out with a small drill.
- The depth of a hole in the sample is measured after 200 turns of the drill
- Value expressed in 1/10 millimetres
- An average of 4-8 tests is taken
