INTRODUCTION

In Iceland an increasing engineering activity, especially construction of hydroelectric power plants at sites containing volcanic rocks in subaquatic facies, has neccitated better understanding of their geotechnical behavior. The subaquatic volcanics, which to a great extent most probably formed subglacially beneath Pleistocene glaciers are very heterogeneous, grading from highly jointed pillow-lavas and intrusions to much weaker pyroclastic rocks, the hyaloclastites.

The lack of systematic geotechnical work on these rocks stimulated the undertaking of a research project dealing with their geotechnical properties. In this rock group one usually has several different rock types forming a volcanic unit (= eruption) and this project was aimed at evaluating how the petrographic factors of these various components control the mechanical properties. In this paper we are concerned only with the hyaloclastites the weakest of these rocks.

The experimental work which still is in Progress was done at the Institutes for Civil Engineering, the petrographical studies at the Geological Institute, both ETH Zurich. Rock mechanic tests include uniaxial compressive-, indirect tensile-(Brazilian-) and triaxial compressive tests performed on two series of hyaloclastites from different localities. On the weakest varieties only soil mechanic triaxial tests were possible.

GEOGRAPHICAL GEOLOGICAL CONTEXT

Iceland is almost entirely built up of volcanic rocks which constitute approximately 90 per cent of the surface of the island.

The exposed volcanic pile consists predominantly of basaltic material.

The rock sequence is traditionally divided into four stratigraphic groups based on age determinations and environmental changes resulting in different facies of volcanism. These are the following (Saemundsson 1980) (see figure 1):

  1. Tertiary(16–3.1 m. y.)consisting mainly of lithologically monotonous sub-aerial plateau basalt series and associated dyke intrusions with minor intercalated terrestrial sedimentary rocks.

  2. Rocks of Plio-Pleistocene age(3.1–0.7 m. y.)show more variety with a considerable amount of pillow-lava, hyaloclastite, fluvioglacial and glacial deposits related to the onset and development of a glacial environment.

  3. The Upper Pleistocene(0.7 m.y.-9000 y.) consists of essentially the same rock types as the Plio-Pleistocene but with a much smaller proportion of subaerial lavas relative to the subglacially formed rocks. The latter is often referred to collectively as the "Palagonite Formation".

  4. Postglacial deposites(younger than 9–13000 y.)consist of sediments and the products of active volcanism, mainly in the form of lavaflows which spread out over the older rocks to partly bury them. Volcanism also took place below sea level and beneath the glaciers which amount 1/9 of the total area of the island.

Whereas the Upper Pleistocene series consists to a great extent of subglacially generated rocks the Plio-Pleistocene can be estimated to consist of roughly equal halves of subglacial and terrestrial volcanics in accordance with equal lengths of glacial and interglacial time periods (Kjartansson 1943). The youngest two groups are merely a continuation of these cycles, the Postglacial being comparable to a start of an interglacial period. The rocks which form during a volcanic eruption in a subaquatic environment accumulate in restricted areas near the vents because water and ice act as barriers.

(Figure in full paper)

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