Keywords

• Marine sediments;
• Mud banks;
• Geotechnical characterization

1. Introduction

Dynamic behavior of coastal and offshore soil deposits depends on interconnected aspects related to physical and mechanical properties of the marine bottom, hydrodynamic characteristics and climatic effects on the region. The interaction between oscillating waves and marine soils has attracted attention of many marine geotechnical and coastal scientists (e.g., Maa and Mehta, 1990; Foda et al., 1993; Wit and Kranenburg, 1997; Jeng and Lee, 2001; Inoki, 2000). In this context, soil mechanics play an important role in analysing and understanding many of the questions related to marine soils response under static and dynamic loading, once the key geotechnical parameters of the seabed are known.

Cassino Beach, southern Brazil, is located adjacent to the estuarine region of the Patos Lagoon (Fig. 1), where some major harbour facilities are installed (composing the Port of Rio Grande). Events in which fluid mud in suspension is deposited over the sandy beach have episodically happened in this area (Calliari et al., 2001; Martins et al., 2003), causing environmental and beach trafficability problems. Fig. 2 and Fig. 3 (Martins et al., 2003) show pictures of a famous mud event occurred in 1999, in which the desiccated mud layer with some cracks may be seen covering the surface of the beach.

The reasons for the presence of mud in Cassino Beach are still open to discussion, although the phenomenon of mud suspension is usually attributed to the increase in shear stresses and decrease of shear strength (liquefaction) occurring in the seafloor due to the dynamic loading of the ocean waves. One of the visible effects of the mud in suspension is the dissipation of the wave's energy and the consequent damping of its oscillatory behavior, which mechanism may be physically related to mud viscosity (Gade, 1958; Suhayda, 1984).

During the years of 2004 and 2005, an extensive research program aimed to study the wave attenuation on muddy bottoms took place in Cassino Beach, as a multidisciplinary field study called Cassino Project. This effort, conducted by research groups from many parts of the world, included sampling and geotechnical characterization of the seafloor at some stations offshore Cassino Beach. The soil samples were submitted to various laboratory tests, including determination of physical index, grain size distribution, Atterberg limits and shear resistance by both the triaxial and shear vane tests. Results have been interpreted in the light of classical soil mechanics, and form an important database for further applications.

2. Description of sampling and geotechnical testing procedures

2.1. Sampling

Samples of the muddy bottom were manually taken by divers at some locations offshore Cassino Beach, by means of plastic tubes, 10 cm in diameter. After extracting, the cores were carefully taken to the FURG geotechnical laboratory and maintained in a wet chamber, to avoid loosing of water content. Undisturbed and remolded sub-samples were then extracted from the cores for the geotechnical characterization tests (Fig. 4).