SUMMARY:

An extensive laboratory testing programme has been carried out to determine the significant mechanical properties of a soft Keuper sandstone. The material is characterized by nonlinear behavior. In the range of stresses which are of interest in civil engineering, the sandstone shows various modes of failure, depending on the stress conditions. The application of the laboratory test results to practical engineering analyses is shown.

RESUME:

Un programme etendu de tests en laboratoire a ete execute pour determiner les proprietes mecaniques les plus significatives d'un grès faible. Le materiau a un comportement constitutif non lineaire. Dans la domaine des contraintes interessant le Genie Civil le gres presente des mecanismes de rupture differents qui dependent de l'etat de contraintes. L'application des resultats de tests en laboratoire à des problèmes pratiques d'engenierie est montre.

ZUSAMMENFASSUNG:

Ein umfangreiches Laborversuchsprogramm ist zur Ermittlung der wichtigsten mechanischen Eigenschaften eines schwach verfestigten Keupersandsteins durchgefuehrt worden. Das Material wird durch nichtlineares Verhalten charakterisiert. Im Bereich der Spannungen, die fuer das Bauwesen von Interesse sind, zeigt der Sandstein unterschiedliche, vom Spannungszustand abhangige Bruchmechanismen. Die Anwendung der Laborversuchsergebnisse fuer praktische Ingenieuraufgaben wird aufgezeigt.

INTRODUCTION

The rocks of the Keuper formation are encountered in extensive areas of Western and Central Europe. The Keuper Marls of the Midlands in Great Britain as well as most of the rocks of the Keuper formation in South West Germany consist predominantly of clay - and mudstones. Because of their particular engineering properties they have been given great scientific and professional attention in recent years (Davis 1967, Chandler 1969, Meigh 1976, Smoltczyk 1972). The Keuper rocks in Northern Bavaria, however, are dominated by coarse grained facies in many regions, for example in the relatively densely populated industrial environment of the city of Nuremberg, where most of the engineering structures are founded on Keuper sandstone. Originating mainly from terrestrial sand and gravel deposits, the Keuper sandstones show the erratic variation in gradation which is typical of many sandy soils. This nonhomogeneity combined with very low strength may be the reason why no systematical studies of the geological material have been presented so far. Since advanced engineering problems had to be solved for a number of major construction projects, such as subway tunnels, there was a need for a profound understanding of the mechanical properties and behavior of the soft Keuper sandstone. So, an extensive laboratory testing programme was undertaken, and some of the results obtained so far are briefly summarised in the present report.

STRUCTURE OF THE SOFT KEUPER SANDSTONE

The horizontally bedded sandstone layers do not show many traces of tectonic influences. There are no significant faults the majority of the joints are oriented vertically to subvertically, and they are closed. The lower the strength of the rock material, the less pronounced is the jointing. Close to the ground surface where the material has been subjected to some weathering and leaching effects, no discontinuities can be detected, and the material appears as a cemented, well compacted sand rather than as a rock. There is a gradual transition from rock of low strength to sand in the weathered zone. The cementing agent of the Keuper sandstone consists mainly of illitic clay minerals which are more or less aggregated and reinforced by small amounts of calcium carbonate, dolomitic components or secondary quartz crystals up to 0.1 mm size were observed on electron micrographs in the voids of the very porous fabric as can be seen on Fig. 1. The mineralogical composition of the tested sandstone is given on table

1. LABORATORY TESTS

Block samples were taken in open cuts from unweathered zones with rock saws (fig. 3) and delivered to the laboratory where various techniques were employed to obtain cylindrical specimens from the blocks. In order to determine the significant mechanical properties over the wide range of stress conditions encountered in practical civil engineering problems, the testing programme included point load-strength tests, Brazilian and uniaxial compression tests, as well as triaxial tests with cell pressures from 50 kN/m2 to 26 MN/m2. Specimens for uniaxial and triaxial compression tests had 75 and 80 mm diameter and 150 to 200 mm length. Point load tests were carried out on samples of 80 mm diameter and 80 mm length. The triaxial compression tests were carried out either under drained conditions, or as undrained tests with pore pressure measurements. Loading and unloading stress paths were studied, multiple stage tests with varying confining pressures, cyclic loading and unloading tests were also carried out. A special triaxial testing apparatus which allows the measurement of the vertical stress inside the cell, had to be designed for the test series on Keuper sandstone, because the commercially available triaxial cells for rock testing do not allow the measurement of stresses with the required accuracy in the range of low confining pressures (fig. 4).

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