Some of the main soft rock characteristics are relatively rapid deterioration and degradation of strength when soft rock is exposed to atmospheric agents. For this study marl samples from Eocene flysch formation found in Dalmatia region in Croatia are selected. When weathered, marl tends to degrade into a fine-graded soil-like material with high compressibility and low shear strength. Current methods of predicting behavior of marl (i.e. slake durability index) when exposed to weathering process are sometimes unreliable, therefore there is a necessity to establish new durability index. This study is an effort to try to implement loss slake index developed by Bryson et all. on shale samples as a new index for marl. In this case samples are taken from a coastal area, therefore information about natural moisture level could not be used to make a valid correlation analysis. Results show that loss slake index, as a method of evaluating durability, could be also applied on marl.

1 Introduction

Flysch is a rock mass that is most frequently encountered during engineering constructions in the region of Dalmatia, Croatia. One of the major components of the Eocene flysch in the Split region is marl. Problems such as slope stability and settlement of embankments made of crushed marl are very common in these materials Miščević & Vlastelica 2012). The susceptibility To weathering upon exposure is the main cause of problems with clay bearing rocks. Weathering can induce a rapid change in rock material from initial properties to soil-like properties.

The sensitivity of a rock type against weatherability is usually described by a durability parameter, such as the slake durability index – SDI (Franklin & Chandra 1972). However, marl resistance is not always detected satisfactorily by SDI thus modifications of this test are often suggested. Some authors suggested additional criterions such as plasticity index (Gamble 1971), modifying the durability test by using only one wetting and drying cycle with different cycle time (Hopkins & Deen 1984), sieved slake durability test (Richardson & Long 1987), etc. Bryson et al. (2012) developed a new durability index (loss slake index – LSI) that better characterizes the durability behavior of shales which is based on Hopkins & Deen (1984) observations.

Results by Gokceoglu et al. (2000) and Miščević & Vlastelica (2011) also indicate that durability of claybearing rocks would most likely be better captured by increasing either the number or the duration of the slaking cycles for the durability test. This study is an effort to better understand and implement LSI developed on shale as a new index for marl.

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