SUMMARY: Tests were carried out on samples of a blue marly rock in the context of a soil investigation at the site of a deep highway cut. The results of these tests are presented and discussed. The main observations are that the blue marl was strain softening, exhibiting an almost brittle behaviour, that saturation resulted in a large reduction of the peak strength: and that despite the wide scatter of the test results, as a general trend, strength values increase with an increase in overburden and a decrease in void ratio and water content.
RESUME: Dans le cadre d'une recherche congernant l'etude des problèmes associes à une excavation profonde pour une construction routière, un nombre d'essais a ete execute sur des echantillons de marne bleue. On present les resultats de ces essais et on procède à une brève discussion de leurs caracteristiques. Les essais ont montre que la marne bleue se caracterise d'un comportement du type "strain softening", avec une fragilite importante. La saturation de cette marne conduit à une grande reduction de la resistance de "pic". Malgre la dispersion notoire des resultats experimentaux, on se conduit à l'etablissement d'un aspect qualitatif de ce comportement; la resistance accroit lors de l'augmentation de l'epaisseur de la couche superposante (overburden) et la diminution de l'indice des vides et de la teneur en eau.
ZUSAMMENFASSUNG: Im Rahmen einer bodenmechanischen Untersuchung eines Autostrassenschnittes, wurden Versuche in blauen Mergelstein ausgefuehrt. Es werden die Versuchsergebnisse gegeben und kritisiert. Als Hauptbemerkungen der Ergebnisse sind dass der Mergelstein Dehnungserweichend (strain softening) und brueckelig ist, dass die Wassersattigung eine grosse Reduzierung der maximaler Festigkeit bringt, und dass trotz der breiten Verteilung der Versuchsergebnisse, die algemeine Tendenz ist dass die Festigkeitswerte mit der Vergrosserung der Überlagerungsmachtigkeit und der Abnahme der Porenziffer und des Wassergehaltes zunehmen.
1. INTRODUCTION A new highway is being built in northwestern Greece, connecting the town of Preveza with the port of Igoumenitsa. During the excavations for the highway several problems arose and several slides occurred (Cavounidis-1979). One of the problems that had to be faced was the crossing of an area, where the designed depth of the cut was more than 40 m. The excavation of such a deep cut required geotechnical investigation in order to check the feasibility of the design and to propose measures which would ensure the stability of the slopes. During the investigation it was found that the main part of the excavation would be in a thick layer of blue marly rock. Laboratory tests were carried out to determine the strength and other properties of the marl. The main results of these tests together with observations and correlations that were made are presented herein.
2. GEOLOGICAL AND MORPHOLOGICAL CONSIDERATIONS The general area is a hilly to almost mountainous part of Epirus, in northwestern Greece, a few kilometers south of the ancient Aheroussia lake and about 1 km. inland from the coastline of the Ionian sea. The upper formations are sedimentary layers of the Pliocene period. They consist of marls and conglomerates (or sand-gravels). The bedrock consists of massive limestone, cars tic and weathered in the upper part. In its cavities and on its surface there is terra rossa due to chemical weathering of the limestone. The region belongs to the Ionian zone which was folded in the late stages of the alpine orogenesis. It has faults running in a N-S or NW-S direction, that is, parallel to the Ionian coast. The joints in the Pliocene formations run N-S and NE-SW. The region has not yet equilibrated and thus earthquakes of a tectonic nature are not uncommon. It belongs to Category II according to the Greek antiseismic regulations (Royal Decree-1963) with a seismic coefficient ε= 0.06 for rocks and ε=0.08 for soils.
3. SOIL PROFILE Four boreholes were drilled by rotary drilling in the initial phase of the investigations, followed by another three in the final stage, of 282 m. total length. The investigation showed that the top part of the soil consists of a greyish brown stiff marly clay having some sand content. In one borehole much black organic material was also found. Fig. 1 shows a typical cross section of the soil as well as the level of the highway and the slopes of the cut in the initial design.
4. TEST RESULTS Laboratory tests were carried out on a large number of samples of the blue marly rock. Table I lists the average values of the Atterberg limits and the physical properties of the marl, where wL is the liquid limit, wp is the plastic limit, Ip is the plasticity index, w is the natural moisture content, Yd is the dry unit weight, Yt is the total unit weight and e is the void ratio. The degree of saturation of the samples tested varied from 80% to 100%.