Estimate the relative strength of intact rock in the field with a use of a geological hammer or pocket-knife and record in the boring logs. Field identification methods should be confirmed by laboratory uniaxial compressive strength tests performed on representative rock core sample(s) within the stratum and presented in the boring logs. Perform uniaxial compressive strength tests in accordance with ASTM D7012, Method C.

Table 4-4 provides relative strength descriptions of intact rock based on field identification methods

and laboratory uniaxial compressive strength tests of rock

. Use Table 4-4 in combination with the field observations from Table 4-3 for boring log strength classification of rock.

Table 4-4: Criteria and Description for Relative Rock Strength

Grade Designation	Strength Description	Field Identification	Approximate Compressive Strength (psi)
R0	Extremely weak rock	Specimen can be indented by thumbnail	35 – 150
R1	Very weak rock	Specimen crumbles under sharp blow with point of geological hammer and can be peeled by a pocketknife	150 – 725
R2	Weak rock	Shallow cuts or scrapes can be made in a specimen with a pocketknife. A firm blow with a geological hammer creates shallow dents.	725 – 3,500
R3	Medium strong rock	Specimen cannot be scraped or cut with a pocketknife. Specimen can be fractured with a single firm blow with a geological hammer point.	3,500 – 7,250
R4	Strong rock	Specimen requires more than one firm blow of the point of a geological hammer to fracture.	7250 – 14,500
R5	Very strong rock	Specimen requires many blows of geological hammer to cause fracture	14,500 – 36,250
R6	Extremely strong rock	Specimen can only be chipped with firm blows from the hammer end of a geological hammer.	> 36,250