Carina Maia Lins Costa

Failure Mechanisms in Sand over a Deep Active Trapdoor

An experimental testing program was undertaken to investigate failure mechanisms induced by the active movement of a deep rectangular trapdoor underlying a granular soil. Reduced-scale models were tested under normal gravity as well as under an increased gravitational field using a centrifuge facility. Some models were used to evaluate the performance of both flexible and rigid pipes undergoing a localized loss of support. Failure mechanisms in the longitudinal direction of the models were characterized by a single, well-defined failure surface that developed within the limits of the trapdoor. However, failure mechanisms in the transverse direction of the models were characterized by multiple failure surfaces extending outside the limits of the trapdoor. Significant dilation of the soil located immediately above the trapdoor was identified in the failure of the models. The pattern of the failure mechanisms was found to be affected by the stress level and backfill density. Higher stress levels were found to lead to well-developed failure zones. The influence of backfill density was found to be more relevant in models involving flexible pipes. Pipes embedded within loose backfill were severely damaged after loss of support, while pipes embedded in dense backfill experienced negligible deformations. These results indicate that damage to pipelines caused by ground loss of support can be significantly minimized by controlling the compaction of the fill.

Analysis of Soil Arching in Pipeline Installation Subjected to Uplift

The vast extension of a buried pipeline can lead to many specific situations involving longitudinal interactions with the surrounding soil, such as landslides, expansive soils, sinkholes, collapsible soils, just to name a few. In this study, three-dimensional numerical simulations were carried out to evaluate the mechanical behavior of a buried pipe undergoing a localized elevation. Parametric analyses were performed considering soil relative density, pipe-soil relative stiffness and vertical surcharge loading. The results of the parametric study are relevant to the design of buried pipes under the action of uplift forces.

Geotechnical Study on the Use of Sanitized Sewage Sludge in Cover Layers of Solid Waste Landfills

Giving the current trend of recycling and reutilization residues, sludge reuse has become an environmental, social and economic necessity. A possible destination would be the utilization of sludge as part of cover layer in solid waste landfills. This paper aimed at investigating the geotechnical implications of the addition of domestic sanitized sewage sludge on clayey-silty sand. The sludge was sanitized with hydrated lime, in the proportion of 25% lime and 75% sludge, and held for 90 days in an agricultural greenhouse for its total stabilization. Then, geotechnical tests were conducted using pure soil, pure sanitized sewage sludge and mixtures of these two materials. Three mixtures were tested: 50% soil and 50% sludge, 66% soil and 33% sludge and 90% soil and 10% sludge. The results showed that the sanitized sludge is a coarse-grained material and, its Atterberg’s limits showed the lack of plasticity. Generally, the greater the sludge percentage in the mixture, the bigger the decrease on the shear strength parameters, it also leaves them lighter and more permeable. At the end, the conclusions were that the 10% sludge and 90% soil mixture has shown geotechnical characteristics that allow its utilization as daily and intermediate cover layers. The sludge and the 50% sludge mixture have shown geotechnical properties adequate for their utilization as component in draining sub-layer in final cover layer.

Soil Stresses on Carbon Steel Pipe Undergoing Uplift

This paper presents a three-dimensional numerical study to evaluate the variations in stresses in the soil mass surrounding a carbon steel pipe class API 5L X60 submitted to uplift due to ground elevation. Analyses were carried out for soil relative density, pipe stiffness and surficial surcharge loading. Results have shown that stress variations due to uplift are lower for looser backfill soils and flexible pipes. Stress variations in pipe invert are meaningful in the vicinity of the region between stable and unstable soil masses.

Optimum Blend Ratio of Tropical Soil and Porcelain Tile Residue to Highway Applications Based on Statistical Analysis of the Results of Unconfined Compression Tests

Ceramic materials are used worldwide due to offer important technical and economic advantages. The ceramic industry, however, has to deal with production losses that are wasted, such as the residue of polishing porcelain tiles. This paper presents the results of laboratory tests performed using different proportions of a lateritic soil and residue of polishing porcelain tile, in order to verify the possibility to use them for highway applications. The main focus of this paper is to propose the optimum ratio of soil and residue based on statistical analysis of the results of unconfined compression tests. Grain size distribution test, X-ray fluoresce analysis and compaction tests were also conducted. An analysis of the data indicates that the addition of residue increases the unconfined compression strength, in comparison to specimens with pure soil. A statistical analysis of the data showed that the best proportion is approximately 92% of soil and 8% of residue.

Failure of geotextile-reinforced walls in centrifuge model tests

This paper presents the results of centrifuge tests performed to investigate the behavior of geotextile-reinforced soil walls beyond stress conditions. The models were built using nonwoven fabrics as reinforcement layers and dry sand as backfill. Digital image analysis techniques were used to determine the displacement of sand markers placed along the reinforcements. The models were loaded until failure by increasing centrifugal acceleration, and the movements of the sand markers were used to determine the strain distributions along the reinforcement layers. The results revealed that stresses redistribute among reinforcement layers as models approach failure. Current design methods for GRS walls were found to be conservative when applied to predict the behavior of the reduced-scale models.

Effect of Cement Content on the Behavior of Dune Sand-Cement Admixtures

The dune sands of the coast of Rio Grande do Norte State, northeastern Brazil, have serious engineering problems, namely low bearing capacity for shallow foundations and are not suitable as material for sub-base and road base. The use of this material in engineering practice needs improvement of their geotechnical properties. This paper investigates the effect of adding Portland cement to stabilize dune sand from the region of Natal, Brazil. Undrained triaxial tests were carried out to evaluate the effect of cement content on the geotechnical behaviour of dune sand-cement admixtures. Tests with admixtures with cement contents of 2.5, 5 and 10% of the dry mass of soil were carried out. Results showed that soil cohesion increased with increasing cement content. The friction angle of the admixtures was lower than that of the uncemented sand for cement content of 2.5 and 5% while It was larger than that of the uncemented sand for cement content of 10%.

Failure Mechanisms of Pipelines Subjected to Loss of Support Under Deep Burial Conditions

This paper investigates the failure mechanisms of pipelines subjected to a localized loss of support. An experimental program was conducted, which consisted of a series of four centrifuge model tests containing an aluminum tube embedded in a pure dry sand backfill that was placed over an underlying rectangular rigid base moving downwards during the test. All models were built taking advantage of the longitudinal symmetry of the problem. The prototype pipe had a diameter (D) of 1.1 m and a soil cover height of about 5 D, characterizing deep burial conditions. Failure patterns were observed within a vertical section comprising the central axis of the pipe and also in four distinct vertical transverse sections along the length of the pipe in the region of ground loss. The influence of pipe stiffness and backfill density on the behavior of the system was assessed. The transverse sections showed fully developed slip surfaces starting in the vicinity of the edge of the void towards the adjacent soil mass. The mode of failure of the flexible pipes took the form of a severe deformation at the region of the shoulder and a reversal of curvature at the invert due to over-deflection. This situation was more critical in the central section. The damage experienced by the flexible pipes was noticeably more pronounced when using the looser backfill, whereas only negligible deflections were observed when using the denser backfill. The experimental results were compared with analytical predictions, which showed to be highly unconservative for the case loose backfill.Copyright