ABSTRACT:

This article studies the behaviour of a gypsiferous marl mass with a smectite content of around 3% in order to determine its expansion potential. There were plans to build a large-scale electricity generating complex in this area. Oedometric cell tests were carried out to determine the degree of swelling under different pressures. The rock showed a very slow expansion rate as compared with the usual values for expansive soils. It was then decided to analyse the caracteristics of the natural rock mass from which a long-term ceiling limit for the swelling behaviour of the marl mass could be extrapolated.

1 INTRODUCTION

During the Tertiary Era, probably in the Oligocene, fine sediments partly of clay were deposited along the River Ebro Basin which now, as a result of the river-caused erosion, appear in different areas in the form of powerful marl strata with evaporite nodules and veins iterspersed with thin layers of limestone, calcarenite and sandstone.

In one of these areas, see Figure 1, it was planned to build an important electricity power complex requiring earth stripping (Fig. 2) down to depths of as much as 70 m in the marl mass in some places, in order to reach the foundation level for the various buildings involved. In this area the marly course cropped out through the natural rock mass and extended below the foundation level to depths of over 50 m.

Local experience, together with the presence of smectites in the clay fractions of the marl, indicated that deferred heave could be expected in the foundation of the buildings and this possibility had to be quantified. Two methods were used for this. In the first place, oedometric cell laboratory tests were performed to determine the swelling pressure and the degree of swelling under different pressures. Secondly, the degree of variation in the characteristics of the rock mass with depth was analysed from the natural terrain surface down.

(Figure in full paper)

2 CHARACTERISTICS OF THE ROCK MASS

Visual inspection of the boring samples showed that the rock mass under the foundation level, to a depth of at least 50 m, consisted of over 85% marl strata while the rest was made up of thin layers of clay, limestone, calcarenite and su1phates.

(Figure in full paper)

2.1 Composition

The marls were composed of carbonates (calcite and dolomite), sulphates (gypsum and anhydrite), quartz, clay and traces of other minerals, not in a regular pattern but altering with depth in a sequence typical of a rythmical sedimentary formation (Fig. 3).

The distribution of the carbonates, sulphates and quartz, clay and other minerals in the marl samples analysed is shown in Figure 4. Chemical analysis was used to identify the limestone and dolomite, 'gypsum and anhydrite, and quartz and clay contents by determining the different anions and cations existing in the rock. X-ray analysis was used to identify the various types of minerals composing the clay fraction which were paligorskite, illite and smectite.

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