under certain technical conditions, rooks are graded according to its drillability. Depending on the drilling method, core drilling can divide into 12 grades, details please consult “Rock 12 grade scale “issued in 1958 by Ministry of Geology of China
· Rock drillability classification
Usually we take Ⅰ、Ⅱ、Ⅲ as soft rock， Ⅳ、Ⅴ、Ⅵ as medium hard rock, Ⅶ、Ⅷ、Ⅸ as hard rock; Ⅹ、Ⅺ、Ⅻ as rigid rock for convenient use.
· Drillability of rock is classification of rock resistance to stress and breakage during drilling, that is the whether the rock is easy to break. According to drillability can divided as below:
Representative: redeposited loess, lateritite, loose sandy soil, diatomite and peaty humus layer. (drillability：7.50 m/h)
Representative: Strong weathered rock, slate, phyllite and schist, slight agglutinating sand layer, sandy soil with 20% gravel, loess layer, gypsum soil layer, marlstone, talc-shcist, lignite, bitumite. ( drillability：2.45 m/h
· Representative: shale, sandy shale, oil shale, macker, calcareous shale, marlstone, argillaceous sandstone, limestone, dolomite, strongly weathered peridotite, serpentite and limestone, medium hard, coal bed, halite, clay layer, volcanic marl stone and frozen sandstone.
· Representative: pebble, detritus, gravel layer, argillite, green mud slate, phyllite, schist, fine crystalline limestone, marble, soft sandstone, serpentinite, dunite, olivine rock, trachyte, splint coal, anthracite, frozen coarse grained sand, gravel and frozen earth. (Drillability：1.15 m/h)
· Ⅷ一一Ⅸ:硬岩 Hard rock
Representative: Silicified sericite slate, phyllite, schist, gneiss, epidosite, carbonated rock, quartzite containing magnetite and hematite, conglomerate rock, basalt, diabase, andesite, quartz porphyry.Ⅹ--Ⅺ：坚硬岩 Rigid rock
Representative: fine granite, granodiorite, granitic gneiss, rhyolite, corundolite, blout, the hardest iron horn shale, silexite.
Factors affecting rock drillability
Rock drillability is not an inherent property, it depends on rock characteristic, drilling technology and technical conditions
（1) 、Rock characteristic; include mineral composition, structural characteristics, physical properties and mechanical properties
（2）、The direct influencing factor is the mechanical property.Rock hardness levels
Rock
level
Solidness
Representative
Ⅰ
Most
solid
The most solid, compact and ductile quartzite,
basalt and other
various solid rock. （f＝20）
Ⅱ
The
most solid
The most solid granite, quartz porphyry, siliceous schist,
relative rigid quartzite,
Most rigid sandstone and limestone. (f=15)
Ⅲ
Solid
Dense granite，very solid sandstone and limestone，quartz
vein，
solid conglomerate rock, very
solid iron ore. (f=10)
Ⅲa
Solid
Solid
sandstone、limestone、marble、dolomite、pyrite，unsolid
granite.（f=8)
Ⅳ
More
solid
Ordinary
sandstone、iron ore（f=6)
Ⅳa
More
solid
sandy shale，shaly
sandstone（f=5)
Ⅴ
Medium
solid
Solid
muddy shale ，unstable sandstone, limestone，soft gravel.（f=4)
Ⅴa
Medium
solid
Various
unsolid shale，compact marlstone (f=3)
Ⅵ
Soft
软弱页岩，很软的石灰岩，白至，盐岩，石膏，无烟煤，破碎的砂岩和石质土壤。 (f=2)
Ⅵa
Soft
Gravel
soil，crush shale, bond grave, detritus，solid coal，sclerotic clay（f=1.5)
Ⅶ
Soft
软致密粘土，较软的烟煤，坚固的冲击土层，粘土质土壤。（f=1)
Ⅶa
soft
soft
sandy clay、gravel，loess（f=0.8)
Ⅷ
Earthy
humus，peat，soft sandy soil，green sand（f=0.6)
Ⅸ
Granular loose
Sand,
gravel pile, fine gravel, loose soil, mined coal (f=0.5)
Ⅹ
· Quicksand
shape
· quicksand，marshy soils，water-bearing loess
· and other water-bearing
soil (f=0．3 ) A
