Hole Design for Rocks

These charts show recommended hole sizes and spacing for boulders and rock faces. Measure your material, pick the closest diagram, and cross-check with our general hole-design page before drilling.

Breaking boulders — layout guide

d	38 - 44 mm
d	1 1 / 2 "- 1 3 / 4 "
D	70 % of Height

(1) Soft rock
(Tensile strength: < 60 kg/cm² / ~850 psi)

(2) Medium-hard rock
(Tensile strength: 60–100 kg/cm² / 80–140 psi)

(3) Hard rock
(Tensile strength: > 100 kg/cm² / ~140 psi)

Underground and partially buried rock

d	38 - 44 mm
d	1½ "- 1¾ "
L	30 - 60 cm
L	1 '- 2 '
D	Shown in Figure

d	38 - 51 mm
d	1½ "- 2 '
L	60 - 90 cm
L	2 '- 3 '
D	90 % of Height

(2) Mostly buried boulder (3) Fully underground boulder

Splitting oversized boulders

d	32 - 35 mm	44 - 51 mm
d	1¼ "- 1 3/8 "	1¾ " - 2 "
L	30 - 40 cm	60 - 90 cm
L	1 '- 1 ' 4 "	2 '- 3 '
D	70 % of Height

Cutting stone into slabs

d	32 - 35 mm	44 - 51 mm
d	1¼ "- 1 3/8 "	1¾ " - 2 "
L	20 - 30 cm	40 - 50 cm
L	8 "- 1 '	1'4 "- 1'8"
D	90 % of Height

Fresh (unfractured) rock faces

d	44 - 51 mm
	1¾ " - 2 "
L 1	30 - 40 cm
	1 '- 1'4"
L 2	60 - 90 cm
	2 '- 3 '
D	Add 5 % of Height
	80° - 90°

Tighten hole spacing at the toe and break forward from the free face, row by row. Leaving the toe undrilled usually gives poor results. Front rows (up to the sixth) can be filled at once. For easier removal, fill one bore first, then rows two and three after waiting 6–20 hours.

Trenching and tunnelling

d	38 - 51 mm
	1½ "- 2 "
L	30 - 60 cm
	1 '- 2 '
D	1 - 1.8 m
	3 '- 6 '
	45° - 60°

Add relief holes when the trench is deep

Repeat the same relief-hole layout for greater depth

d	38 - 51 mm
	1½ "- 2 '
L 1	40 - 60 cm
	1'4 "- 2 '
L 2	30 - 40 cm
	1 '- 1'4"
D	1 - 1.8 m
	3 '- 6 '

d	38 - 44 mm
	1½ "- 1¾ "
L	30 - 60 cm
	1 '- 2 '