Term | Uniaxial Compressive Strength (MPa) | Point Load Index (MPa) | Schmidt Hardness (Type L - hammer) | Field Estimate of Strength | Examples* | R5 Extremely Strong | >250 | >10 | 50-60 | Rock material only chipped under repeated hammer blows | fresh basalt, chert, diabase, gneiss, granite, quatzite | R4 Very Strong | 100-250 | 4-10 | 40-50 | Requires many blows of a geological hammer to break intact rock specimens | Amphibolite, sandstone, basalt, gabbro, gneiss, granodiorite, limestone, marble rhyolite, tuff | R3 Strong | 50-100 | 2-4 | 30-40 | Hand held specimens broken by a single blow of a geological hammer | Limestone, marble, phyllite, sandstone, schist, shale | R2 Medium Strong | 25-50 | 1-2 | 15-30 | Firm blow with geological pick indents rock to 5mm, knife just scrapes surface | Claystone, coal, concrete, schist. shale, siltstone | R1 Weak | 5-25 | ** | <15 | Knife cuts material but too hard to shape into triaxial specimens | chalk, rocksalt, potash | R0 Very Weak | 1-5 | ** | | Material crumbles under firm blows of geological pick, can be scraped with knife | highly weathered or altered rock | Extremely Weak | 0.25-1 | ** | | Indented by thumbnail | clay gouge |
* All rock types exhibit a broad range of uniaxial strengths which reflect the heterogeneity in composition and anisotropy in structure. Strong rocks are characterized by well interlocked crystal fabric and few voids.**Rocks with a uniaxial compressive strength below 25 MPa are likely to yield highly ambiguous results under point load testing. |
