Engineering geological investigations into the border

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Technische Universität München, Engineering Geology, Munich, Germany. ABSTRACT: ... In engineering geology it is useful to relate the durability of rocks to the lifetime of a build- ing, which usually .... principles are evident: Hard rocks are ...
Geologically Active – Williams et al. (eds) © 2010 Taylor & Francis Group, London, ISBN 978-0-415-60034-7

Engineering geological investigations into the border between hard and weak rocks M. Nickmann, S. Sailer, J. Ljubesic & K. Thuro Technische Universität München, Engineering Geology, Munich, Germany

ABSTRACT: Experience gained over a number of construction projects has highlighted the need to be able to define the border between solid rocks staying unchanged for a long time, and weak rocks showing degradation over a period of months to years. Investigations have shown however, that there is no suitable method currently available to formally calculate this delineation. Nickmann (2007, 2010) has developed a methodology for the determination and classification of the rock durability working with a modified wetting-drying-test combined with a crystallisation test. Using this test, not only can weak rocks be separated into 5 classes of durability, but also hard rocks can be discerned from weak rocks. The distinction between weak and hard rocks depends on its “structural strength” G, a summary parameter describing the composition (grain size distribution, content of clay minerals susceptible to water, cementation) and porosity characteristics. More recent investigations have shown that the structural strength depends on the rock fabric and can be reduced by microscopic inhomogeneities like bedding planes or small fissures, because at such joints the bond strength of the rock is weakened and the water conductivity increases. Weathering also reduces the structural strength by weakening latent fissures or deteriorating the cementation which increases the pore volume. The investigations showed, that already a slight weathering of stage V2-V3 after ISRM, transform a formerly strong rock into a weak rock.

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INTRODUCTION

Experience gained over a number of construction projects has highlighted the need to predict the rocks long term behaviour following exposure to the surface. On one hand it is important for the estimation of the stability of the foundation, on the other hand the grade of durability affects the ability to reassemble the excavated rock material. So it is necessary, to decide if the rock is hard, staying unchanged for a long time, or weak showing degradation of a period of months to years. Although rocks on the border between weak and hard rock are encountered in many construction projects all over the world, the durability parameters are only rarely determined. One reason for this omission is the lack of a suitable testing method. An incorrect assessment of durability can lead to an increase in time and cost during the construction phase and/or decrease the quality and stability of the building. This paper therefore summarizes several methods to define rock durability and to separate weak and hard rocks and investigates the rock properties and processes causing slake durability.

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THE BORDER BETWEEN HARD AND WEAK ROCKS

Weak rocks as well as hard rocks are a part of the group of rocks, as opposed to soils. Whilst hard rocks under real conditions remain stable for a long time without a loss of strength, weak rocks tend to disintegrate within a short time period (days to several years) 2265

when being exposed to water and/or climatic changes. In this case weak rocks can show an extremely varying behaviour from spontaneous decay into small fractions (aggregates or grains) shortly after water supply, to slow degradation into aggregates over months to years. This loss of strength is not reversible under real conditions. Weak rocks and hard rocks are linked by geological processes. Thus, a hard rock can be formed from a weak rock by metamorphism heating up the clay minerals to form minerals insensitive to water. Alternatively a hard rock can be transformed into a weak rock by weathering processes. Since hard rocks also disintegrate in geological time, the transition between the two groups of rock is continuous and varies with the time scale viewed (see Figure 1). In engineering geology it is useful to relate the durability of rocks to the lifetime of a building, which usually means some hundred years. 2.1

A modified wetting-drying test for the classification of durability

Recent evaluations indicated a lack of suitable methods to determinate the durability of a rock and to differentiate weak from hard rocks. None of the commonly performed testing methods, as stipulated in National Standards (e.g. DIN EN, ISO, BS, ASTM) or Suggested Methods (e.g. IAEG, ISRM), has the ability to completely and clearly comprise and describe both the behaviour of natural rocks directly during exposure as well as their medium- to long-term behaviour (Nickmann 2007, 2010). This is generally due to poor preparation practices (e.g. oven drying) or/and unsuitable load types (e.g. rotation during the slake durability test (ASTM D4644-87)). Due to insufficient load intensities, most wetting tests (e.g. DIN EN 14689-1, ISRM 1994) and wetting-drying tests lack the ability to differentiate weak from hard rocks. The Uniaxial Compressive Strength (UCS) parameter, often used for the distinction between weak and hard rock (e.g. Taylor 1988), was shown to be unsuitable. The rock samples tested showed an overlapping of the UCS values of weak and hard rocks by testing at natural water content, as well as after oven-drying (Figure 2). Nickmann (2007, 2010) developed a new methodology for the determination of rock durability. A modified wetting-drying-test combined with a crystallisation test is defined as the standard test for the classification of durability. The 3-cycle wetting and drying test simulates the immediate response of a fresh sample with natural water content as well as the short to medium term behavior of the rock. Corresponding to the course of disintegration, weak rocks can be separated into 5 categories of durability—VK1 to VK5. To distinguish between weak rocks of category VK1 (low slake durability) and hard rocks the 3-cycle wetting-drying-test is too low. Both rock types survive the wetting-drying test without loss (maximal loss during sample preparation