A Comparison of Different Indirect Techniques to Evaluate Volcanic Intact Rock Strength

Abstract

1 IntroductionQuantifying the strength of volcanic rocks in remote areas of difficult access is achallenging task. Volcanic areas generally present an outstanding variety of rocktypes,inarelativelyrandompattern,rangingfromverystrong,coherent,weldedrocksto weak, interlocked vesicular pyroclastic units. The strength of volcanic rock massescanbeapproached,inawaysimilartoothermaterials,asacombinationofthestudyofthe strength of the rock matrix and the structure and quality of the rock mass.The difficulty in assessing volcanic rocks in remote areas lies in the relativeinaccessibility of many outcrops, which prevents traditional geotechnical samplingand testing campaigns being used, or borehole investigations being carried out.Also, due to their genesis, many volcanic rocks contain rock type-unit-specificfractures and rock mass structures which differ from most other rock types. This canmake it difficult to decide on the criteria which ensure that the samples tested arerepresentative of the intact rock. To overcome this problem, the intact rock or rockmatrix is defined here following Hoek and Brown’s (1997) observation that there isa ‘‘critical sample size’’ below which the strength is constant. To avoid biasses inproblematic volcanic rocks, only samples below the critical sample size, but largeenough to be tested, have been considered to be representative of the rock matrix.