In open-pit mining, subdrill (also called subgrade drilling) plays a critical role in ensuring proper rock breakage and achieving the designed floor level. * What is Subdrill? Subdrill is the extra depth drilled below the desired floor level to account for: -Incomplete breakage at the bottom of the hole. -Toe formation due to stemming and burden resistance. -Ensuring a smooth, level bench floor for the next mining cycle. *How to Calculate Subdrill The required subdrill depends on rock characteristics, bench height, and blast design. A common formula is: Subdrill (m)=(0.2  to  0.3)×Burden * Typical values: Hard rock → 20–30% of burden Medium rock → 15–20% of burden Soft rock → 10–15% of burden ------ Typical Subdrill Values (based on hole diameter): Hard rock → 10–15 × hole diameter Medium rock → 8–12 × hole diameter Soft rock → 6–10 × hole diameter ------ Typical Subdrill Values (based on bench height): Hard rock → 8–12% of bench height Medium rock → 6–10% of bench height Soft rock → 4–6% of bench height * Why It Matters - Prevents high toes and uneven floors. - Improves fragmentation and shovel/truck efficiency. - Reduces the need for costly secondary blasting. * Example: 1- (Based on Burden): If the burden is 4.0 m and the rock is hard, subdrill should be around: 0.25×4.0=1m. 2- (Based on Diameter): If the hole diameter = 165 mm (0.165 m) and the rock is hard:10×0.165=1.65m to 15×0.165=2.48m 3- (Based Bench Height ): If the bench height = 10 m, and we apply 8–12% =0.08×10=0.8m to 0.12×10=1.2m