Marcio S. S. Almeida

Dredging and disposal of fine sediments in the state of Rio de Janeiro, Brazil.

Dredging is employed quite frequently in the state of Rio de Janeiro, especially for the installation and upkeep of commercial ports and rehabilitation of the hydraulic section of silted bodies of water. Until recently, all dredged material with no economic use was destined for marine disposal or stored at the edge of the water body. Since the 1990s, however, a new approach has been adopted for dredging as a result of pressure from the environmental organisations, encouraging closer interaction in Rio de Janeiro between the local and state public authorities and the universities on issues relating to licensing of this kind of activity. The recent experiments of the Civil and Ocean Engineering Programs of COPPE-UFRJ (Federal University of Rio de Janeiro) described herein are included in this context. The state of Rio de Janeiro has three bays, several coastal lagoon systems and a number of small and medium sized rivers in or close to urban areas, with a gentle slope as they near the sea. This is, then, a region highly susceptible to silting processes of water bodies, and therefore, to maintenance and/or environmental rehabilitation. As discussed in the article, fine and almost always organic sediments prevail, which is a considerable obstacle to the end disposal and possibility of reuse.

Influence of Penetration Rate on Penetrometer Resistance

Penetration resistance in fine-grained soils varies with the rate of penetration. Considering undrained behavior as a reference, as the rate of penetration is reduced, soil resistance increases because of the effects of partial consolidation and soil strengthening immediately ahead of the probe. Many penetration tests have been performed under different rates of penetration to identify the range of drainage characteristics of the soils used, correlating these conditions with laboratory interpretations and in situ tests. A backbone curve relates the variation of the normalized point resistance with the normalized rate of penetration. This work presents an analytical approach to the backbone curve equation used to fit test data. In addition, this paper presents a set of centrifuge tests with variable penetration rates performed with a soil classified as silty tailings, which has different geotechnical behavior from most of the soils used by previous researchers.

Physical Modeling of Lateral Clay-Pipe Interaction

This paper presents results from physical model testing of lateral clay-pipe interaction. The main reason for this research was the accident in Rio de Janeiro in January 2000 when more than 1× 106  l of crude oil was spilled into Guanabara Bay due to thermal buckling of a pipeline. A set of centrifuge tests was performed for a model pipeline at shallow burial depths in order to assess the lateral resistance of the soft clay from the incident site. In addition, a set of 1-g model tests was also performed to observe the failure mechanism in more detail. These tests permitted the development of an analytical model, based on geotechnical and geometrical parameters, able to evaluate the soil resistance when subjected to lateral pipe movements for shallow burial depths. The centrifuge data are then compared with the proposed analytical model and with literature data, revealing good agreement in both cases.

Load-Displacement Compatibility Analysis of a Low-Height Column-Supported Embankment

This paper presents instrumentation results and analyses of a low-height, geosynthetic-reinforced, column-supported embankment constructed in Brazil. The approach involves back analyzing relevant properties of the embankment fill and geogrid reinforcement and applying the Load Displacement Compatibility (LDC) approach to calculate geogrid deflections in a unit cell of a square pile cap array. The results show that the LDC approach is consistent with the observed response of the embankment.

Arching and Deformation in a Piled Embankment: Centrifuge Tests Compared to Analytical Calculations

AbstractMeasured loads in centrifuge tests on piled embankments were compared with values calculated by three analytical arching models. The Zaeske and Van Eekelen models use sets of arches that give the direction of the load transfer, and result in an increasing load on the soft soil with increasing embankment thickness or surcharge load, which matches the measurements. The Svano model, however, assumes that the load on the soft soil remains constant with increasing embankment thickness or surcharge load, thereby being transferred directly to the piles. Therefore, the Zaeske and Van Eekelen models match the measurements better than the Svano model. The Van Eekelen model slightly overestimates the pile efficiency for relatively thin embankments, but matches the measurements better than the Zaeske model for increasing embankment thickness or surcharge load.

Embankments over Soft Clay Deposits: Contribution of Basal Reinforcement and Surface Sand Layer to Stability

This paper evaluates the significance of basal reinforcement and the presence of the surface sand layer in the stability. This evaluation is carried out by means of field measurements and stability analyses of three test embankments on soft clay taken to failure. Two of the test embankments were reinforced and one was unreinforced. Stability analyses were carried out taking into account measured values of reinforcement tension forces during construction. The set of analyses have shown that the top sand layer was more important to the stability of the embankments than the basal reinforcement. The cases studied have also shown that the conventional design practice that assumes for the reinforcement a fixed tension contribution may lead to unrealistic higher factor of safety.

Strength and Compressibility Characteristics of a Soft Clay Subjected to Ground Treatment

The compressibility properties, undrained shear strength, and stress history are essential for reliable calculations of settlement and bearing capacity of soft soils. However, it is sometimes a challenge to determine representative parameters for very soft and high plasticity clays, which are often found in Brazilian coastal areas. In this study an extensive site investigation was planned aiming to interpret the behaviour of a trial embankment on stabilized soft ground. The site investigation was carried out in a test area located in the west of city of Rio de Janeiro, consisted of three clusters in which standard penetration tests, vane shear tests, and piezocone tests (CPTu) were performed. A number of correlations was developed and compared with empirical equations in order to verify their reliability. The results of these tests made it possible to define geotechnical parameters of the soft clay to use in the numerical and analytical computations of the embankment on reinforced ground.

Strength Behaviour Analysis of an Offshore Brazilian Marine Clay

The main purpose of this paper is to describe the geotechnical behaviour of a deep water marine clay obtained from Campos basin oil field located offshore the state of Rio de Janeiro in Brazil. The soil samples were obtained at a depth of about 1500 m using a Kullenberg piston corer. Characterization tests were undertaken on the clay samples, including water content, Atterberg limits, particle size distribution and specific density. Isotropic and anisotropic undrained triaxial tests were also carried out on both normal and over-consolidated samples to assess both the conventional strength and Cam-clay parameters. Additionally, oedometer tests were performed to evaluate the compressibility of both undisturbed and reconstituted soil samples. Finally, a series of T-bar penetrometer tests were performed on the mini-drum geotechnical centrifuge at COPPE, the University of Rio de Janeiro to establish the undrained shear strength profile. The results obtained from the T-bar tests were compared with the theoretical strength curves that were established based on the Cam-clay parameters derived from the laboratory tests. The results obtained in this study indicated a useful methodology to assess and capture the behaviour of marine clay. The assessment of strength behaviour of marine clay is quite essential in modelling of wide range of soil-offshore structure interaction problems such as pipelines, mudmats and anchors.Copyright

Embankment on Encased Columns: Numerical Study of the Influence of Working Platform on the Development of Geosynthetic Basal Reinforcement and Encasement Tensile Forces

conventional stone columns are widely used to improve soft soils in which high compressibility and low shear strength are observed. In very soft soils, granular columns may undergo excessive bulging due to lack of lateral support provided by the surrounding soil. Wrapping the granular columns with appropriate geosynthetic material can reduce the total and differential settlements while improve the load carrying capacity of the composite ground. Using a compacted sand or gravel mat (known as working platform) placed below the embankment in common to prevent excessive lateral deformation of the foundation soft soil. In the circumstance of very high applied load, this granular mat may be further reinforced with a geogrid layer to enhance its effectiveness and to control the overall stability of the embankment. This paper presents the results of a series of three-dimensional numerical analyses performed to study the development of hoop forces in geosynthetic encasement under different combinations of working platform thickness and horizontal geogrid stiffness. The results showed that for a constant working platform thickness the hoop forces increased with the height of the embankment. The maximum values of the encasement hoop forces were also observed to reduce significantly as thickness of the working platform increased

Physical Modelling of Offshore Structures Founded on Sea Bed

This work presents an experimental study using centrifuge modelling on mudmat shallow foundations which are mainly used to support offshore subsea equipment. An optimized design of these foundations requires the maximizing of the ratio between the installation and the pull-out resistance of the foundation. International standards, based on classical theories, are often limited and do not meet the complexity of the projects. The bibliography is not yet conclusive about the load capacity factors for this particular problem. In this work, physical modelling has been performed on both perforated and solid mudmats subjected to vertical load and simulating both installation and extraction scenarios.The soil used in this study is a marine clay taken from the Roncador field at Campos basin located offshore of the state of Rio de Janeiro, Brazil. The physical modelling experiments have been carried out on the mini-drum geotechnical centrifuge at COPPE/UFRJ. The technique of lumps was adopted for the preparation of the soil models and a temporary surcharge using a sand layer has been applied to develop a soil strength profile similar to the one found in the field. The results obtained in this study provided a better understanding of the soil-structure interaction problem with regard to the installation and pull-out resistance of the studied type of foundation focusing on their bearing capacity factors.Copyright