Estimating the Geological Strength Index (GSI) in Regional Seismic-Landslide Zonation Using the Empirical Regression Model

Table 2

The field estimates of the uniaxial compressive strength of intact rock pieces, Hoek et al. [38].

Grade^a	Term	Uniaxial compressive strength (MPa)	Point load index (MPa)	Field estimate of strength	Examples
R6	Extremely strong	>250	>10	Specimen can only be chipped with a geological hammer	Fresh basalt, chert, diabase, quartzite
R5	Very strong	100–250	4–10	Specimen requires many blows of geological hammer to fracture it	Gabbro, gneiss, granodiorite limestone, marble, rhyolite, tuff
R4	Strong	50–100	2–4	Specimen requires more than one blow of a geological hammer to fracture it	Limestone, marble, phyllite, sandstone, schist, shale
R3	Medium strong	25–50	1-2	Cannot be scarped or peeled with a pocket knife specimen can be fractured with a single blow from a geological hammer	Claystone, coal, concrete, schist shale, siltstone
R2	Weak	5–25	b	Can be peeled with a pocket knife with difficulty, shallow indentation made by firm blow with a point of a geological hammer	Chalk, rocksalt, potash
R1	Very weak	1–5	b	Crumbles under firm blows with a point of a geological hammer can be peeled by a pocket knife	Highly weathered or altered rock
R0	Extremely weak	0.25–1	b	Indented by thumbnail	Stiff fault gouge