Yuri D. Costa

INFLUENCE OF MATRIC SUCTION ON THE RESULTS OF PLATE LOAD TESTS PERFORMED ON A LATERITIC SOIL DEPOSIT

The performance of foundation systems on unsaturated soil deposits is considerably influenced by variations of the negative pore-water pressure (i.e., matric suction) distribution within the soil mass due to local microclimate conditions. Although significant understanding has been gained in the last few decades on the behavior of unsaturated soils, the use of unsaturated soil mechanics concepts in the interpretation of field tests has not been incorporated into the state-of-practice. As a consequence, conservative foundation engineering principles are used in practice in regions where unsaturated soil conditions prevail. For example, direct use of field test results (e.g., from plate load tests) obtained during a dry season in unsaturated soil deposits may lead to the selection of unconservatively high design parameters at the site. On the other hand, ignoring altogether the beneficial impact of matric suction on the bearing capacity of soils may lead to unnecessarily expensive foundation systems in tropical, arid climates. This paper investigates the influence of soil suction on the results of plate load tests conducted at a depth of 1.5 m on a structured, naturally occurring lateritic soil. Ten tests were carried out under different soil suction conditions. Matric suction was monitored during plate load testing using tensiometers installed at the bottom of the testing pit up to a depth equal to one plate diameter (0.80 m), which is generally recognized as the influence zone in which significant stress variation occurs (Terzaghi and Peck 1948). The results provide insight into the influence of soil matric suction on ultimate bearing capacity and settlement rate of plate load tests performed on lateritic soils. Background Although the notion that the presence of negative pore-water pressures (matric suction) in the soil influences the behavior of foundations is not new, there is only limited information reported in technical literature dealing with the quantification of this problem. A brief description of previous research on this topic regarding shallow footings is presented below. Bearing capacity loss associated with the soil saturation has been commonly accounted for through the use of different values of unit weight of the soil (� ) that arise due to total or partial submersion. Other soil parameters such as the effective cohesion ( c� ) and the effective internal friction angle ( �� ) are usually assumed to play a minor role. Considering the submerged unit weight of the soil is about 50 % of the moist unit weight, Terzaghi and Peck (1948) stated that the bearing capacity of shallow footings could be approximately reduced by 50 % if the water level rises from a depth equal to the footing width below the footing to the surface. Meyerhof (1955) proposed an analytical model for estimating the bearing capacity of shallow foundations based on the variations of the soil unit weight and taking into consideration the possibility of partial submersion. The groundwater table is assumed to lie between the foundation base and the lower portion of the soil failure surface. The value of unit weight of soil to be used is defined as:

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.

Estabilidade de agregados de um latossolo amarelo distrocoeso de tabuleiro costeiro sob diferentes aportes de resíduos orgânicos da cana-de-açúcar

The potential yield of sugarcane depends on factors related to climate, soil and the plant variety. Management systems that provide the addition of organic waste to the soil can cause changes in physical attributes of the soil, enhancing the growth of the root system and sugarcane yield. The objective of this study was to evaluate the effects of different management systems at several levels of organic residue addition on the distribution and indices of aggregate stability of dystrophic cohesive Yellow Latosol of coastal plains, in the State of Alagoas. Plots were selected in the following sugarcane-growing areas: area under irrigation management system, area fertigated with vinasse, and an area under application of vinasse + filter cake. These management systems were compared to each other and to a control (native forest). To evaluate the distribution and stability indices of soil aggregates, soil samples were randomly collected in the different areas, at the depths 0-0.20; 0.20-0.40 and 0.40-0.60 m, at points between two neighboring sugarcane rows. The results were subjected to analysis of variance and the mean compared by the Tukey test at 5 % significance. Simple correlation analysis was performed between some measured variables. Results showed that the different management systems induced changes in the aggregate distribution and that the stabilization of soil aggregates in the surface layer dependeds on the organic matter content and the action of successive cycles of soil wetting and drying. The studies of simple correlations showed that total organic C and aggregate stability indices were significantly and positively correlated.

Compressibilidade de um Argissolo Amarelo distrocoeso submetido a diferentes manejos

The objective of this paper was to evaluate the compressibility of an Ultisol submitted to different managements of sugarcane cultivation. The investigation was carried out at Triunfo Mill, in the State of Alagoas, Brazil. The testing area was composed by four distinct sub-areas: a preserved forest (NF), an area without irrigation (MWOI), an irrigated area (MWI), and an area with vinasse (MWV). All investigated areas, except the forest, were prepared using conventional equipment for soil preparation. Soil samples were collected within depths ranging from 0 to 20 cm, 20 to 40 cm, and 40 to 60 cm. A trench was dug in the four selected areas and four undeformed samples were collected in each depth. The samples were collected with metallic rings with dimensions of 6.4 cm in diameter and 2.5 cm in height. In the laboratory, the samples were previously saturated and submitted to soil suctions raging from 0.1 to 1500 kPa in order to build the soil-water characteristic curves. Confined compression tests were performed with saturated samples. The results obtained from the compression tests were used for the determination of the void ratio (e) and the pre-consolidation stress (σp) of the tested samples. Data were submitted to regression analysis and the differences among the obtained means were compared using the test of Tukey at 5% of probability. Regression analysis for pre-consolidation stress in function of water content (Ug) and correlations among the several variables were carried out at 5-% significance. Within all investigated areas, the depth range from 0 to 20 cm showed to be more susceptible to compaction in relation to the depth range from 20 to 40 cm. Management MWV was found to modify the compressive behavior of the soil in both depth ranges. In all investigated areas, the soil bearing capacity was higher in the depth range of 20 to 40 cm.

Compaction of a Ultisol submitted to different managements.

The objective of this work was to evaluate the susceptibility to compaction of an utisoil under different soil managements and use. Evaluations were based on both maximum density (MD) and critical humidity (CH) through Standard Proctor compaction tests, and the degree of compaction in different soil managements and use. The study was carried out at the Triunfo Mill, State of Alagoas in Brazil. The testing area consisted of four distinct sub-areas: a preserved forest (MA), an area without irrigation (MSI), an irrigated area (MCI), and an area with vinasse (MCV). Undeformed soil samples were collected for soil density and the deformed ones for organic matter and Standard Proctor tests in order to obtain MD, CH and the compaction grade (CG). Plots (50 × 50 m) were randomized within the four areas at depths of 0-20, 20-40 and 40-60 cm, with three replications. Data were submitted to variance analyses, and means compared through the Tukey test at 5% of probability. MD data followed the grade MSI > MCI > MCV > MA, in contrast to CH which presented the inverse sequence. Therefore MSI and MCI can cause soil degradation, even at lower CH. Only MCI presented CG higher than 4.3%, the critical value reference, at 0-20 cm.

Performance of prototype embankment built with tire shreds and nongranular soil

The mechanical response of a prototype embankment fill built with tire shreds and nongranular soil was evaluated. The test embankment consisted of three distinct sections, each 10 m (33 ft) long and 1.5 m (4.9 ft) high. Specifically, the embankment included a layered section composed of successive layers of soil and tire shreds, a soil-tire shred mixed section with 10% tire shreds by weight, and a pure soil section. The embankment was exposed to heavy-truck traffic immediately after construction. At 120 days after construction, the settlement rate in the two sections containing tire shreds converged to a rate similar to that observed in the section of pure soil. However, the section constructed with soil-tire shred mixture exhibited a better overall long-term behavior than the layered section, as it showed smaller differential settlements. The results collected in this study also provide insight into the in situ compression and compaction procedures and preparation characteristics of soil-tire shred mixtures and soil-tire shred layered systems.

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.