Rock Drillability and Hardness Classification

Drillability refers to a rock’s resistance to penetration and fragmentation during drilling; it is also an indicator of drilling efficiency. Drillability is a composite attribute that reflects many physical and mechanical properties of the rock and serves as a primary measure of drilling difficulty. It is typically expressed as penetration rate (meters per hour). Using this scale, rock drillability is commonly divided into twelve grades.

Because different rock types have different physical and mechanical characteristics, they affect drilling speed in different ways. Under given technical conditions, the measured drilling speed for a rock is referred to as its drillability—that is, how easily the rock is broken by the drill bit.

Core-drilling drillability grades

Grade 1 Very loose, very soft soils — Representative materials: secondary loess, secondary red soil, very soft sand without gravel or clasts, diatomaceous earth, peat-humus layers without plant roots. Penetration rate: 7.50 m/h; length per trip: 2.80 m/trip.

Grade 2 Soft, loose materials — Representative materials: loess layers, red soil layers, soft peat, clayey and sandy soils containing 10–20% gravel or cobbles, soft kaolinite-type soils, humus layers with plant roots. Penetration rate: 4.00 m/h; length per trip: 2.40 m/trip.

Grade 3 Soft rock — Representative materials: strongly weathered shale, slate, phyllite and schist; weakly cemented sand layers; sandy soils with ~20% gravel/cobbles; loess containing ~20% calcareous or nodular inclusions; gypsiferous soils; marl; talc schist; coquina limestone; lignite and bituminous coal. Penetration rate: 2.45 m/h; length per trip: 2.00 m/trip.

Grade 4 Slightly soft rock — Representative materials: shale, sandy shale, oil shale, carbonaceous shale, calcareous shale, interbedded sandstone and shale; relatively dense marl, argillaceous sandstone; massive limestone and dolomite; strongly weathered olivine-bearing rocks and olivine rocks; serpentinite and phosphatic rock; medium-hard coal seams; rock salt; crystalline gypsum; kaolin layers; tuffaceous mudstone; frozen water-bearing sand layers. Penetration rate: 1.60 m/h; length per trip: 1.70 m/trip.

Grade 5 Slightly hard rock — Representative materials: pebble, gravel and cobble layers; talus/colluvial layers; argillaceous slate; sericite-chlorite schist; phyllite and schist; fine-grained crystalline limestone; marble; relatively soft sandstone; serpentinite; pure olivine rock; weathered hornblende porphyry and diorite; hard bituminous coal and anthracite; frozen coarse sand and gravel layers; frozen soil. Penetration rate: 1.15 m/h; length per trip: 1.50 m/trip.

Grade 6 Medium-hard rock (group A) — Representative materials: chlorite, mica, sericite schist; phyllite, schist; slightly silicified limestone; calcite-bearing rocks; green schist; calcareously cemented conglomerate; feldspathic sandstone, quartz sandstone, quartz diorite, hornblende porphyry; pyroxenite, gabbro; frozen gravel layers. Penetration rate: 0.82 m/h; length per trip: 1.30 m/trip.

Grade 7 Medium-hard rock (group B) — Representative materials: quartz, hornblende, mica-bearing oxidized slate; phyllite and schist with slight silicification; feldspathic quartz sandstone; quartz diorite; weakly schistose sodic feldspar porphyry; diorite; hornblende porphyry; gravel and cobble layers; mildly weathered coarse-grained granite, syenite, porphyritic rocks, gabbro and other igneous rocks; siliceous limestone and chert-bearing limestone. Penetration rate: 0.57 m/h; length per trip: 1.10 m/trip.

Grade 8 Hard rock (group A) — Representative materials: silicified sericite schist, phyllite, schist, gneiss; green schist; quartz-bearing carbonate rocks; quartz-bearing barite rocks; quartzites with magnetite/hematite; calcareously cemented conglomerates; basalt, dolerite, andesite, pyroxene-rich rocks; quartz-andesite porphyry; medium-grained sodic feldspar porphyry and amphibole porphyry; fine-grained siliceous-cemented quartz and feldspathic sandstones; cherty limestone blocks; mildly weathered granite, granitic gneiss, pegmatite, diorite, gabbro, etc. Penetration rate: 0.38 m/h; length per trip: 0.85 m/trip.

Grade 9 Hard rock (group B) — Representative materials: highly silicified slate, phyllite, limestone and sandstone; coarse-grained granite, granodiorite, gneiss, syenite; mildly weathered quartz diorite, pegmatitic granite, silicified tuff, argillaceous tuff with chlorite/sericite, fine-grained quartzite, cherty phosphatic limestone, pegmatite. Penetration rate: 0.25 m/h; length per trip: 0.65 m/trip.

Grade 10 Very hard rock (group A) — Representative materials: fine-grained granite, granodiorite, granitic gneiss, rhyolite, microgranite, quartz diorite, quartz-sodic feldspar porphyry, very hard quartz pegmatite, flinty rocks. Penetration rate: 0.15 m/h; length per trip: 0.50 m/trip.

Grade 11 Very hard rock (group B) — Representative materials: corundum-bearing rocks, quartzite, jasper, massive quartz, the hardest iron-rich argillite, jasperoid silicified schist, flint. Penetration rate: 0.09 m/h; length per trip: 0.32 m/trip.

Grade 12 Extremely hard rock — Representative materials: unweathered, extremely dense quartzite, jasper, hornfels, pure sodic pyroxene‑corundum rock; quartz, chert, jasper. Penetration rate: 0.045 m/h; length per trip: 0.16 m/trip.