Abstract

This paper presents the results of an investigation of weathering characteristics of heterogeneous mudstone and shale sediments encountered during corridor X motorway construction, crossing Republic of Serbia. Red Permian braided river deposits are prone to weathering after exposure to natural climatic conditions. As a consequence of deterioration the cuttings suffered from over ten instabilities, along 12 km long motorway section. In order to investigate the slaking properties of studied rock material slake durability indices along with disintegration ratio were determined. Five cycles of slake durability tests were performed. Disintegration ratio was determined in laboratory and field conditions. Field samples were exposed to natural climatic conditions during the period of one year. Slake durability index after second and fifth cycle has values between 95% < Id(2) <98% and 92% < Id(5) < 97%, respectively. Disintegration ratio determined after second and fifth cycle has values between 0.92 < DRL < 0.98 and 0.88 < DRL < 0.97, respectively. Field value of disintegration ratio of representative samples J-1(7), J-2(12) and J-3(2) after 2, 5 and 12 months of exposure amounts to 98–96–96, 92–91–86 and 92–82–70, respectively. Laboratory testing showed that material has high resistance to disintegration, thus could not be matched with field rate of slaking. Weathering of samples exposed to natural climatic conditions showed somewhat better agreement with weathering of exposed cut slopes. Observations of deterioration of samples during one year exposure gave better understanding of mechanical processes involved in rock mass deterioration.

1. Introduction

When dealing with cuttings constructed in clay bearing rock the risk of delayed failures exist due to possible degradation over time. These failures although usually shallow in nature have negative consequences on time extension for project completion, increase in cost for repair, maintenance, contractor's claim requests and on stability of facilities at the toe and crest of the slope. The cause of such instabilities is attributed to the weathering processes that occur immediately after excavation. Natural climatic conditions cause changes in moisture content, temperature and physical condition of free water in the rock mass. This leads to mechanical and chemical weathering of the material. Weathering of rock mass decreases its shear strength and increases deformability. Specific type of weathering when wetting and drying cycles are involved is termed "slaking". The slaking is characteristic for fine grained rock mass (mudstone, clay shale, marlstone, siltstone, claystone). Softening by means of slaking is closely related to strain softening i.e. progressive failure. For progressive failure to play a role the rock must exhibit brittle properties and nonlinear strains must be mobilized along any potential shear surface. Time to failure is closely related to pore pressure equilibration [1].

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