On the basis of laboratory test results a comparison of various parameters (unconfined compressive strength, CERCHAR and LCPC abrasivity indexes, mineralogy) is presented and discussed in order to outline certain correlations between them. Samples cover a wide range of lithological types coming from various sites of tunnels.

A partir de resultats d'essais de laboratoire, une comparaison de divers paramètres (resistance à la compression simple, indices d'abrasivite CERCHAR et LCPC, mineralogie) est presentee et discutee dans le but d'en tirer certaines correlations. Les echantillons couvrent un large eventail de types lithologiques provenant de plusieurs sites de tunnels.

Basierend auf Laborversuchen, werden verschiedene Gesteinseigenschaften (Druckfestigkeit, CERCHAR und LCPC Abrasionsindexe) prasentiert und Korrelationen aufgezeigt. Die untersuchten Proben verschiedenster Lithologien stammen aus mehreren Tunnelprojekten.


The 690 MW hydropower plant Karahnjúkar (Iceland), which is now in the early stage of construction, includes 55 km of underground waterways. The main pressure tunnel, between the intake at the Karahnjúkar dam and the top of the 400 m high twin shafts, is 40 km long. The 15 km long Jökulsa tunnel diverting the waters from the Fljotsdalur catchment, connects to the main tunnel some 20 km downstream of the intake.

The tunnels will be driven across a slightly inclined (2–5°), 1500 m thick pile of basalts with intercalated, generally well-cemented volcanoclastic and fluvio-glacial sediments. Three TBM's will be used for the excavation of the waterways.

This short paper presents the main properties of intact basalt samples, as determined in the laboratory and correlations with the LCPC and CERCHAR indexes.

Rock characteristics

Two main basalt types occur in the project area:

  • Olivine basalt

  • Tholeiite basalt

Each of the two types above can develop a porphyritic structure, in which case it is designated Porphyritic basalt.

The characteristics of both Tholeiite and Olivine basalts are shown in Table 1.

(Table in full paper)

Strength tests

The porphyritic basalt has the basic characteristics of one of the two types described above, with more than 5 % plagioclase (bytownite) phenocrysts. Phenocrysts of other minerals like Olivine and Augite may also occur in small amounts. When the amount of phenocrysts exceeds 7–10 %, the groundmass is usually of olivine basalt character.

The basalts are characterized by high to very high unconfined compressive strength, relatively high tensile strength

(Table in full paper)

Abrasivity tests
Cerchar abrasiveness test

In the Cerchar test, a sharp steel indenter (hardness of 200 kg/mm2) of 90°cone angle is applied to the surface of a rock specimen with a static force of 70 N. The steel point is then slowly moved on 10 mm. This procedure is repeated five times in various directions on the rock surface, always using a fresh steel tip.

The abrasivity of the rock is obtained by measuring with a microscope the resulting wear flat on the steel cone. The unit of abrasivity is defined as a wear flat of 0.1 mm diameter.

(Table in full paper)

LCPC abrasivity coefficient

The abrasivity test of the Laboratoire des Ponts et Chaussees

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