Page 113 - Applied Petroleum Geomechanics

Rock strengths and rock failure criteria 105

Table 3.1 Field estimates of rock uniaxial compressive strength (UCS).
UCS
(MPa) Field characteristics Examples
Extremely >250 Specimen can only be Fresh basalt, chert,
strong chipped with a geologic diabase, gneiss, granite,
hammer quartzite
Very 100e250 Specimen requires many Amphibolite,
strong blows of a geological sandstone, basalt,
hammer to fracture it gabbro, gneiss,
granodiorite,
limestone, marble,
rhyolite, tuff
Strong 50e100 Specimen requires more Limestone, marble,
than one blow to fracture it phyllite, sandstone,
schist, shale
Medium 25e50 Cannot be scraped or Claystone, coal,
strong peeled with a pocket knife. concrete, schist, shale,
Specimen can be fractured siltstone
with a single blow from a
geological hammer.
Weak 2e25 Can be peeled with a Chalk, rocksalt, potash
pocket knife with
difficulty. Shallow
indentation made by firm
below with point of a
geological hammer.
Very weak 1e5 Crumbles under firm Highly weathered or
blows with point of a altered rock
geological hammer, can be
peeled by a pocket knife
Extremely 0.25e1 Indented by thumbnail Stiff fault gouge
weak

3.2.4 Field methods for estimating rock uniaxial
compressive strength

When laboratory tests are not possible, field estimates of rock strength can
be conducted by examining rock samples and drilling cuttings. Table 3.1
presents the field methods to estimate rock UCS proposed by Hoek and
Brown (1997).

3.3 Rock strength anisotropy

In general, rocks or rock masses always present certain degree of anisotropy,
particularly in fractured (jointed) rock masses or slates, shales, and schists.