How are soils generally classified?
Geotechnical consultants generally classify soil types according to the predominating particle size in accordance with AS 1726 Geotechnical Site Investigations Code. The following table indicates approximate field and in situ test classifications of relative density for granular soils and strength for silts and clays.
 
 
GRAVEL
Description Field Test SPT CPT Resistance
Loose By inspection of voids and particle packing See sand Divide result by 2 and use sand
Dense
 
 
SAND
Description Field Test Relative Density Perth Penetrometer blows/150 mm SPT, N blows/300 mm CPT Resistance MPa
Very loose sand Easily penetrated with 13 mm reinforcing rod pushed by hand. 0 - 15 0 - 1 0 - 5 0 - 2
Loose sand Easily penetrated with 13 mm reinforcing rod pushed by hand. Can be excavated with a spade; 50 mm wooden peg can be easily driven. 15 - 35 1 - 3 5 - 10 2 - 5
Medium dense sand Penetrated 300 mm with 13 mm reinforcing rod driven ;with 2 kg hammer, - hard shovelling. 35 - 65 3 - 8 10 - 30 5 - 15
Dense sand Penetrated 300 mm with 13 mm reinforcing rod driven with 2 kg hammer, requires pick for excavation: 50 mm wooden peg hard to drive. 65 - 85 8 - 15 30 - 50 15 - 25
Very dense sand Penetrated only 25 - 50 mm with 13 mm reinforcing rod driven with 2 kg hammer. 85 - 100 >15 >50 >25
 
 
SILT & CLAY
Consistency Field Test Undrained Shear Strength
cu		 Unconfined Compressive Strength
qu		 Dynamic Cone Penetrometer blows/150 mm * CPT Resistance MPa
Torvane (kPa) Pocket Penetrometer (kPa) **
Very soft Easily penetrated >40 mm by thumb. Exudes between thumb and fingers when squeezed in hand. <12 <25 <1 <0.2
Soft Easily penetrated 10 mm by thumb. Moulded by light finger pressure 12 - 25 25 - 50 1 - 1.5 0.2 - 0.4
Firm Impression by thumb with moderate effort. Moulded by strong finger pressure 25 - 50 50 - 100 1.5 - 3 0.4 - 0.8
Stiff Slight impression by thumb cannot be moulded with finger. 50 - 100 100 - 200 4 - 6 0.8 - 1.5
Very Stiff Very tough. Readily indented by thumbnail. 100 - 200 200 - 400 7 - 12 1.5 - 3.0
Hard Brittle. Indented with difficulty by thumbnail. >200 >400 >12 >3.0
 
*     Very approximate only, better to rely on soil sample field test
**   Note pocket penetrometer may overestimate qu by a factor of 1.5 - 2.0