Durmeková T., Frankovská J. & Kopecký M., 2014: Correlation between point load index and uniaxial compressive strength for soft and weak rocks. Acta Geologica Slovaca, 6, 1, 61–69. (in Slovak with English summary)

Korelácia medzi pevnosťou pri bodovom zaťažení a pevnosťou v prostom tlaku pre poloskalné horniny

Correlation between point load index and uniaxial compressive strength for soft and weak rocks

Tatiana Durmeková1, Jana Frankovská2 & Miloslav Kopecký2

1Katedra inžinierskej geológie, Prírodovedecká fakulta, Univerzita Komenského v Bratislave, Mlynská dolina, 845 15 Bratislava; durmekova@fns.uniba.sk
2Katedra geotechniky, Stavebná fakulta STU Bratislava, Radlinského 11, 813 68 Bratislava; jana.frankovska@stuba.sk, miloslav.kopecky@stuba.sk


For weak and soft rocks, i.e. rocks with the uniaxial compressive strength less than 50 MPa, the Point Load Test is often the only way to determine their strength properties. Despite the indisputable advantages of this test, the value of the conversion factor K between the results of this test, the point load index, and the uniaxial compressive strength, which is used as classification property, is permanently analysed and discussed. The experiences from the field engineering geological investigations show that the conversion factor K for soft and weak rocks is usually less than 22, what is the value stated in EN 1926, Slovak Standard STN 72 1001 or in suggested methods of ISRM. The necessity of individual approach to determine geotechnical characteristics of weak or soft rocks is documented on some examples from the ground investigation for the Highway D1 in Slovakia. The value of the conversion factor K for weathered marlstones and marly or sandy limestones in the section Turany – Hubová of the D1 Highway in the route of the designed tunnel Havran was about 14, in the section Jánovce – Jablonov for weathered sandstones it reached about 19. In another case the conversion factor 18 or 20 should be used for weathered and altered granite, granodiorite and pegmatite rocks from the Bratislava territory for the purpose of correct classification of rocks.

Key words: soft and weak rocks, point load index, uniaxial compressive strength, conversion factor K

Manuscript received: 2014-01-22

Revised version accepted: 2014-04-04

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