The purpose of this study is to clarify the relationship between axial point load strength and uniaxial compressive strength in hydrothermally altered rocks, which are typical of the soft and semi-hard rocks found in northeastern Hokkaido, Japan. 1,747 rock specimens were collected for the axial point load strength test along with 326 rock specimens for the uniaxial compressive strength test. These came primarily from the earth's surface in ancient hydrothermal fields. Rock specimens in the form of cores underwent axial point load strength and uniaxial compressive strength tests using a laboratory testing machine with specimens in forced-dry and forced-wet states. An axial point load strength has a strong correlation with a uniaxial compressive strength. The estimated relationship between axial point load strength (Is) and uniaxial compressive strength (qu) is qu = 12.9 Is in soft rocks with axial point load strengths below 1.5 MPa. This combines the relationship between axial point load strength and uniaxial compressive strength in the forced-dry and forced-wet states and might be applied to onsite tests of rock with natural moisture content. Using this relationship, we can calculate the uniaxial compressive strength from only an axial point load strength test for rocks with axial point load strengths below 1.5 MPa.
The strength of fresh rocks and altered rocks, including hydrothermally altered or weathered rock, is generally evaluated based on uniaxial compressive strength (UCS). However, rock core pieces for the UCS test cannot be always obtained from outcrops of faulted, jointed or cracked rock masses. In these cases, the point load strength (PLS) test is a very convenient and effective alternative to the UCS test because it can be done promptly using on-site testing equipment for small rock specimens having various shapes taken from outcrops or floats. Provided that we can calculate a UCS estimate from an axial PLS value, the PLS test can lead to cost reduction and convenience. Many researchers have already studied the relationship between the PLS and UCS of hard rocks. For example, frequently cited correlations between PLS (Is) and UCS (qu) are qu % 20–25 Is  and qu % 24 Is . The purpose of this study is to clarify the relationship between axial PLS and UCS in hydrothermally altered rocks, which are typical of the soft and semi-hard rocks found in northeastern Hokkaido, Japan (Fig. 1).
The geology of the sampling sites consists primarily of the Upper Miocene Oteshikaushinai, Hanakushibe, and Shikerepe Formations, and the Pliocene Shikerepeyama Lava in the Okushunbetsu area of Teshikaga Town; the Upper Miocene Ikutawara Formation in the Ikutahara area of Engaru Town; and the Upper Miocene Komatsuzawa Formation in the Asahi-Nishi area of Rubeshibe Town, Kitami City, northeastern Hokkaido, Japan (Fig. 1). Rock samples, which were collected primarily from the earth's surface in ancient hydrothermal fields, are hydrothermally altered volcaniclastic rocks, including fine tuff, medium tuff, pumice tuff, lapilli tuff, and welded tuff, dacite, tuffaceous mudstone, tuffaceous sandstone, and tuffaceous conglomerate.