Applicability of Modified Slurry Deposition Method for Reconstitution of Sulphide Soil Samples

Soil’s structure is defined as the issue responsible for the differences observed in the mechanical response of natural soil samples compared to the intrinsic behaviour of reconstituted samples. The parameter that incorporates the differences in microstructure of intact and reconstituted clays is designated by stress sensitivity. The influence of microstructure and fabric on the behaviour of soils has been evaluated by comparing strength and compressibility characteristics of intact and reconstituted samples of the same material. Included on a broad characterization study of Lisbon Miocene Prazeres clay held at Porto University, some oedometer and triaxial tests were undertaken on intact and reconstituted samples. In this paper, the process of preparing reconstituted samples and consolidating them in a large oedometer to the in-situ stress is thoroughly described. The results of oedometer and triaxial tests on reconstituted samples are presented and compared with results for intact samples from the same site. A proposal for parameterisation of the intrinsic characteristics of this clay is established on the basis of test results. Concerning compressibility, oedometer test results show good adjustment to the isotropic compression line (ICL). Triaxial test results for intact samples in the light of critical states soil mechanics are similar to those obtained in s’-t space for the reconstituted samples. The intrinsic parameters show good agreement with those described in the literature for soils of the same nature.

The basic behaviour of one-dimensionally normally consolidated and lightly overconsolidated reconstituted Bothkennar soil was investigated in laboratory triaxial stress path tests. The main programme of tests established a state boundary surface, containing critical state lines and one-dimensional and isotropic compression lines, and evaluated the basic material parameters. The compressibility of Bothkennar soil was found to be similar to that for high plasticity clay but the critical state friction angles were comparable with the values normally associated with coarse-grained soils. Additional tests demonstrated that the effect of ageing, over periods up to 200 hours, was to increase the apparent overconsolidation ratio. The results of a small number of tests on intact samples were examined within the basic framework developed for the reconstituted material. They illustrate the importance of accounting for the current states in interpreting the behaviour of intact and reconstituted samples. Le comportement fondamental du sol du Bothkennar reconstitué normalement et un peu surconsolidé a été étudié à l'aide d'essais triaxiaux avec chemin de contraintes. Le programme principal d'essais établit une surface limite d'état contenant les courbes d'état critique et les courbes de compression unidimensionnelles et isotropiques, il a également évalué les propriétés de base du matériau. On a trouvé que la compressibilité du sol de Bothkennar est semblable à celle d'argiles à haute plasticité mais que les angles de frottement à l'état critique avaient des valeurs comparables à celles normalement associées des sols à grains grossiers. Des essais supplémentaires démontrèrent que l'effet du vieillissement, pour des périodes jusqu'à 200 heures était d'accroître le degré de surconsolidation apparent. Les résultats d'un petit nombre d'essais sur des échantillons intacts furent examinés dans le cadre de base développé pour le matériau reconstitué. Ils illustrent l'importance de prendre en compte l'état présent pour interpréter le comportement des échantillons intacts et reconstitués.

Mechanics of compression in the reconstituted and intact shale with a transitional mode of behaviour

An intensive experimental investigation by means of triaxial and oedometer tests was performed on a clayey loess that was retrieved from two depths at a location on the southern Chinese Loess Plateau. Intact and reconstituted samples were used to identify the effects of the natural structure on soil behavior in saturated conditions. The behavior in compression was clearly affected by structure with the intact samples reaching well-defined gross yield points outside the intrinsic compression line of the reconstituted soil, after which the compression paths converged towards the intrinsic compression lines, behavior which is consistent with destructuration. However, very high stresses were required to give complete convergence. Similarly the triaxial tests that were carried out at lower stress levels also did not give convergence of the critical states so that different critical state lines could be defined for the intact and reconstituted soils. This was consistent with qualitative observations from scanning electron micrographs that natural elements of fabric and possibly bonding persisted even after triaxial shearing. The effects of structure at the two depths on the compression and shearing behavior were slightly larger for the shallower samples. Despite the very different genesis of the soils, it was found that similar frameworks could be applied to those used for sedimentary clays and that the degree of structure was equivalent to a clay of medium sensitivity.

The engineering behavior of nonplastic silts is more difficult to characterize than is the behavior of clay or sand. Especially, behavior of silty soils is important in view of the seismicity of several regions of alluvial deposits in the world, such as the United States, China, and Turkey. In several hazards substantial ground deformation, reduced bearing capacity, and liquefaction of silty soils have been attributed to excess pore pressure generation during dynamic loading. In this paper, an experimental study of the pore water pressure generation of silty soils was conducted by cyclic triaxial tests on samples of reconstituted soils by the slurry deposition method. In all tests silty samples which have different clay percentages were studied under different cyclic stress ratios. The results have showed that in soils having clay content equal to and less than 10%, the excess pore pressure ratio buildup was quicker with an increase in different cyclic stress ratios. When fine and clay content increases, excess pore water pressure decreases constant cyclic stress ratio in nonplastic silty soils. In addition, the applicability of the used criteria for the assessment of liquefaction susceptibility of fine grained soils is examined using laboratory test results.

Importance of sedimentation process for formation of microfabric in clay deposit

The mechanics of a saturated silty loess and implications for landslides

An extensive laboratory investigation was made of the mechanics of Thanet Sand, testing the soils from a number of locations and depths in the triaxial apparatus over a wide range of stress levels. Comparisons between tests on intact and reconstituted samples and between vertically and horizontally trimmed intact samples highlighted the effects of structure on the behaviour of this uncemented sand, in particular a significant effect of anisotropy. Two distinct substrata have previously been identified with different fines contents, and the key difference in their behaviour has been found here to be the location of their Critical State Line in the v:lnp′ plane, which has a significant effect on the engineering behaviour of the two substrata and can explain recent unusual observations of decreasing pile capacity with depth. Tests in which the glauconite content was removed from the sand showed that it did not affect its behaviour.

Natural soils underlying the East-West road are mainly clay and silt of poor quality as subgrade and subbase materials. However, dredged fine grained sandy soils stockpiled at river banks along the road as wastes may be modified for strength gain to replace the excavated natural soil during road construction. The optimum stabilization conditions of the dredged soils were determined in this study to evaluate the strength improvement for beneficial use as road construction materials in view of their large quantity and nearness to the point of use. The laboratory tests carried out on the dredged soils included particle size distribution, compaction and California Bearing Ratio. Cement stabilization was performed on the dredged spoil. Results of the particle size distribution analysis showed that about 80 % of the natural soils, but less than 2 % of the dredged spoil particles are finer than 0.075mm. The moisture content and plasticity indices of the natural soils are 37 to 57 % and 21 to 33 % respectively. They are A-7-6 to A-6-7 soils which indicate poor quality as subgrade materials. Tests on the dredged sand revealed the following range of geotechnical properties: coefficient of uniformity, 2.21 - 2.89; coefficient of gradation, 1.11 - 1.28; maximum dry density, 1.79-1.84 g/cm3; optimum moisture content, 14-15 %; and CBR of 17-20 %. To improve the engineering properties and hence suitability in pavement construction of the spoils, 4 to 7 % addition of cement for stabilization resulted in significant strength increase, with optimum results obtained on addition of 6 % cement. The natural soils constitute very poor pavement construction materials based on their index properties. They may be excavated and replaced with the dredge spoils which must be stabilized before use. Keywords: Stabilization, dredge spoil, pavement, Niger Delta, cement

This paper examines the mechanical behavior of a sensitive marine clay. Various laboratory tests on intact and reconstituted samples of Guinea Gulf marine clay were performed under isotropic compression and drained triaxial compression at constant confining stresses. Microstructure analysis on intact and reconstituted samples was also carried out under different loading conditions. The effect of inter‐aggregates bonding on mechanical properties is discussed. Based on experimental analysis, a new modeling method is proposed. In this approach, the clay is regarded as an assembly of aggregates of clay particles. An inter‐aggregate contact law is introduced relating contact forces to aggregates relative displacements. The deformation of the assembly can be obtained by integrating the movement of the inter‐aggregate contacts in all orientations. Thus, the effect of inter‐aggregates bonds and debonding is considered in a direct way. The model is evaluated through comparisons between the predicted and measured results on Guinea Gulf marine clay. The evolutions of local stresses, strains, and bonds in inter‐aggregates planes are discussed to explain the anisotropy induced by the applied loading. Copyright © 2010 John Wiley & Sons, Ltd.

The critical state approach, based heavily on the use of the moist tamping (MT) reconstitution method, is finding increasing application in tailings engineering practice. However, the available studies comparing the behaviour of intact and reconstituted samples of silt tailings have indicated significant discrepancies. This paper outlines a comparison of triaxial tests carried out on samples of tailings trimmed intact from high-quality blocks and reconstituted using both MT and slurry deposition (SD) methods. Particular focus was placed on the potential for layering within the intact block samples, which indicated that while they were relatively uniform, some minor layering was present – an aspect that has not been considered in detail in previous similar studies. The SD samples tended towards, but did not reach, the critical state line (CSL) identified with MT specimens. Intact block samples generally tended towards the MT CSL, contrary to most previous comparisons. However, greater scatter was apparent in the intact block sample end states than those of the MT specimens, which is hypothesised to result from layering seen in the block samples. A direct comparison of the shearing behaviour of MT and intact block samples from the same void ratio and stresses was inconclusive owing to the inherent variation in intact block sample behaviour.

An intensive investigation is described into the London Clay units from Heathrow Terminal 5. Intrinsic properties and composition were established, and relating the behaviour of intact and reconstituted samples allowed the effects of the clay's natural structure to be identified at all depths. Structure varied between units, but some general features emerged that have not been seen in other stiff clays. In particular, intact samples follow paths under isotropic or K0 compression that fail to provide well-defined gross yield points, or converge with the unit's intrinsic compression lines. Structure contributes to the enhanced shear strength of intact unfissured clay and affects the initial stiffness relationships. Some identifiable features of the intact units’ fabric correlated directly with their behaviour. Natural fissures within the clay, the most important mesofabric feature, had an important impact on shear strength and led to an unusual pattern of directional dependence, and particle orientation trends identified by scanning electron microscopy governed the strong elastic stiffness anisotropy. The potential for destructuration through swelling to low effective stresses was also studied and found significant for subsequent volumetric compression behaviour, but not for shearing. Hight et al. synthesise the data from this and companion papers presented by the authors, and discuss the practical consequences of the results obtained.

A study of the effects of weathering on soils derived from decomposed volcanic rocks

Slopes in flysch deposits areas wide within Europe are highly prone to landslide occurrence. Depending on the material properties and climate conditions, instabilities in a form of earthflows, shallow and deep-seated landslides were observed in these formations. Typically, slope instabilities occurred after prolonged periods of rainfall. The Rječina River Valley, Croatia, built in flysch formation, is well known by several landslides occurred in the past. The weathering process of flysch rock mass and local climate conditions resulted in a specific engineering geological profile of the valley, with the unsaturated residual soil covering the bedrock. Although the behaviour of residual soil is important for a landslide triggering both through the rainfall infiltration process and (unsaturated) shear strength, hydro-mechanical properties of this material in unsaturated conditions were not investigated in the past. This paper summarizes the results of different laboratory tests performed on intact samples for hydro-mechanical characterization of the residual soil from flysch rock mass. It was found that the unique shear strength envelope could be used to determine failure conditions both for saturated and unsaturated conditions. The results obtained from strain-controlled and wetting tests performed in conventional and modified direct shear apparatuses indicated high values of the apparent cohesion that the near-surface soil can experience due to the increase of matric suction. The hysteresis effects and hydraulic paths to which soil was exposed to in the past were found to affect the soil behaviour, while the soil formation process results with a complex soil structure that imposes the necessity of using intact soil samples for proper hydraulic characterization of the soil.

The effect of the fines content in silty sands on the location of the critical state line (CSL) has been investigated by performing standard drained and undrained triaxial tests on reconstituted samples containing Sydney sand, feldspar fines and mixtures of the two at 20, 40 and 60% fines content. In the e-log p′ plane, CSLs of silt–sand mixtures shifted parallel to one another. The transitional fines content of silt–sand mixtures was 40%. Transitional soil behaviour was also observed in silt–sand samples that contained 40 and 60% fines as densely and loosely prepared samples with these fines contents failed on two different and parallel CSLs.