ABSTRACT:

As a first step towards the characterization of deformability, resistence, post-failure behavior and dynamic properties of a gypsiferous marl of the Oligocene stage in the North East of Spain, uniaxial compression tests at low water contents have been performed using a closed-loop servo-hydraulic machine. The rock is first described and its physical properties are given. Results obtained in quasi-static tests using a linear combination of stress and strain as the feedback signal are used to classify the material according to different criteria. In a second series of tests rock samples have been tested at a frequency of 1 c.p.s. under different constant stress amplitudes. Finally, conclusions have been drawn from a comparison made between static and dynamic behaviors.

1 INTRODUCTION

The Ebro river valley located in the North East of Spain is a large triangular basin opening towards the southeast, resulting from subsidence during the lower Tertiary with synchronous infilling by continental deposits characterised by molassic tectofa cies.

The southeast part of this zone called "Catalan Sector", is constitued by sediments belonging to the Oligocene Stage made of a mixture of detrital and evaporitic rocks such as calcarenites, limestones, shaley limestones, marls and gypsiferous marls.

The neotectonic evolution, in this part of Spain, with uplift in the continental margins and subsidence in the bord of the Mediterranean sea has developped changes in its drainage patterns and erosion process.

In order to build up a power station in that zone a huge excavation, up to a depth of 90 m over an area of 2000 square metres, has been made on gypsiferous marl. This rock is extremely sensitive to the presence of water in the sense that small variations in its water content indu ce large variations in its mechanical pro- perties. Besides, the heterogeneity of its mineral composition makes the assesment of its behavior under static and dyna mic loads a very difficult task.

In an effort to shed light on the deformability, resistence, post-failure behavior and dynamic properties of the material at very low water contents, quasi-sta tic and cyclic uniaxial compression tests have been carried out in the Geotechnical Laboratory of CEDEX using a closed-loop servo-controlled testing machine.

2 MATERIAL TESTED

The material was formed in a lacustrine basin bounded by three mountain systems. Sedimentation of materials discharged by rivers together with lacustrine deposition and fluctuations of the water level, have strongly influenced the characteristics of the deposits.

From different geological studies it has been inferred that the thickness of the sediments in the past could have reached 1000 metres or more. Weathering and erosion from the Ebro river have uncovered former deeper deposits which can be expected to be strongly preconsolidated at present.

The material is a sedimentary rock with a significant amount of sulphates in diffe rent forms such as:

  • Veins, microveins and filaments of secondary type.

  • Detrital nodules.

Lithologically, strong carbonate cementa tion characterises the material.

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