Table 2   Approximate strength classification of rock				
De script ion	Uniaxial	Point load	Schmidt	Characteristic rocks
	compressive	strength ls(50),	Hammer	
	strength, MPa	MPa	N-type, 'R'	
Very weak rack -	1-25	0.04-1.0	10-35	Weathered and weakly
crumbles under sharp				compacted sedimentary
blows with geological				rocks - chalk, rock salt
pick point, can be cut				
with pocket knife				
Weak rock - shallow	25-50	1.0-1.5	35-40	Weakly cemented
cuts or scraping with				sedimentary rocks - coal,
pocket knife with				siltstone, also schist
difficulty, pick point				
indents deeply with firm				
blow				
Moderately strong	50-100	1.5-4.0	40-50	Competent sedimentary
rock - knife cannot be				racks - sandstone, shale,
used to scrape or peel				slate
surface, shallow				
indentation under firm				
blow from pick point				
Strong rock - hand-held	100-200	4.0-10.0	50-60	Competent igneous and
sample breaks with one				metamorphic racks -
firm blow from hammer				marble, granite, gneiss
end of geological pick				
Very strong rock-	>200	>10	>60	Dense fine-grained igneous
requires many blows				and Metamorphic rocks -
from geological pick to				quartzite, dolerite,
break intact sample				gabbra, basalt
Modified from Sel by. 1993: Table 5.3.