ABSTRACT:

Coral reef calcareous rock, deposits in neritic environment, is widespread in tropical ocean area. It possesses special structure and engineering characteristics due to the sedimentary environment and short depositional history. This paper, on the basis of an actual project in East Java, discusses the basic engineering properties of coral reef rock. By way of electronic microscope scanning and mineral analysis, the results indicate that weak diagenesis of calcareous rock results in its large porosity and loose structure and, further, controls the mechanics behavior of the rock. Moreover, indoor tests and in-situ tests also show that the rock is the very soft type: its SPT blow count presents the feature of non-normal distribution with wide range of fluctuation. Besides, shear wave travels in reef rock much more slowly than it does in ordinary rocks, and the ratio of elastic wave speed and shear wave speed close to 2.0, all the phenomena showing the typical loose structure of this rock. Given plenty pores, water pressure test in borehole suggests high permeability and good connectivity of the calcareous rock, and its hydrogeological characteristics also differ greatly from that of other rocks.

1 INTRODUCTION

Coral reef, deposit with rich calcium and insoluble carbonate material, is a type of calcareous rock developed mainly in tropical ocean environment under the biological (reef, seaweed, shellfish and other ocean species) formation and geological formation. It is widely distributed in the tropical waters between 30° south latitude and 30° north latitude.

Reef calcareous rock is formed after the reef deposits have gone through years of physical and chemical actions. Majority of it belongs to modem marine biogenic carbonate rock, with short depositional history, unique development environment and special material composition. Mineral composition of the rock includes mainly aragonite and calcite, and the rock mass structure retains the original skeleton of the biological pore. Characterized by high porosity, fragility, loose structure, easy cementation, and poor hardness, its engineering mechanics properties thus differ greatly from other terrestrial or marine sediments, making it a special type of rock and soil mass, whose special mechanic behavior and engineering properties deserve further studies when constructions are conducted in such calcareous rock area (Xu 1989, Zhao et al. 1997, Sun et al. 1999, Wang et al. 2008).

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