Cassino Beach

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GEOTECHNICAL PROPERTIES OFTHE CASSINO BEACH MUD

By Claudio R.R. Dias and Antonio M.L. Alves(Federal University of Rio Grande, September 2008)

CE 530 ADVANCED GEOTECHNICAL ENGINEERINGLABORATORY


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Agenda

Introduction

Geotechnical Testing Procedures

2004 Characterization Campaign


Discussion and Conclusions


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Introduction

Marine bottom, hydrodynamic characteristics and climatic effectsdetermine dynamic behavior of coastal offshore soil deposits.

The reasons are still open to discussion. However, mud suspension is

usually attributed to the increase in shear stress and decrease ofshear strength (liquefaction) occuring in the seafloor due to dynamicloading of the ocean waves.

Cassino Project was conducted by research groups from many parts

of the world. Sampling and geotechnical characterization of theseafloor at some stations offshore the beach were made.

Atterberg limits, physical index, grain size distribution and shearresistance were determined by submission of some laboratory tests.


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Where is Cassino Beach?


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Mud Event in 1999


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SamplingSamples of the muddy bottom were manually taken by divers at

some stations offshore Cassino Beach by means of plastic

tubes of 10 cm in diameter.


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Grain size distribution

Water content (w)

Unit weight of the grains (S)

Unit weight of the soil ()

Void ratio (e)

Degree of saturation (S)

The plasticity index (IP)

The liquidity index (IL)



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Points above the A line

characterize clayey soils;

points below are typical of

silty soils.

Right of the B line shows

soils with high plasticity and

compressibility; left of it shows

opposite properties.


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Bathymetric map of the are under study with locations of the

sampling points


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The grain size distributions of the samples



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It was observed that the extreme softness of the mud is

located in the middle part of the sampling region, not

covered by the sand.

The other samples show higher water contents close to the

base of the core and lower values in the region of the top.

This is probably is an evidence of a consolidation processtaking place at the upper part of the mud, under the effect

of the overburden pressure caused by weight of the sandy

layer.


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Atterberg Limits:

The natural water content of the

mud is very high, causing aliquidity index of 1,7.

Soils with liquidity index higher

than 1,0 have a mechanicalbehavior similar to viscous

liquid.


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In 2005 a complementary set of field experiments

was carried out.

Grain size distributions and index properties were

in agreement with the results of 2004 campaign.



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A UU triaxial test was conducted with a sample

taken at a depth of 18 cm. The estimated (SU)was

equal to 4,0 kPa and (w) was equal to 85%.

The estimated (SU)for a sample taken from 31 cm

depth was 1,5 kPa and measured (w) was 131%.

The hypothesis of a consolidation process occuring

in the regions where the mud is covered by sand

was corrobarated by these shear tests.



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Geotechnical parameters of the Cassino Beach mud

are typical of very soft soils, highly plastic and

compressible, with extremely low shear strength.

Some regions of the offshore deposits seem to be on

a consolidation process, gaining shear strength on

their upper parts due to the effects of the overburden

pressure originated by the weight of a coveringsandy layer.


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The very low values of shear strength of the mud

samples with high values of liquidity index, indicatethat the exposed mud may be re-suspended by the

dynamic shear loading generated by oscillatingwaves during strong storms.

Together with all the other available information

regarding hydrodynamic and climatic behavior ofthe Cassino Beach region, it may contribute for abetter understanding of the mud dynamics andbehavior.



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Cassino Beach