Strength and dilatancy of sands from their image-based intrinsic properties

It is known that density and pressure enhance the strength and dilatancy of sand. The effect of density was first recognised by Taylor (1948) and later by Skempton & Bishop (1950). Taylor investigated the mechanical behaviour of dry Ottawa sand and explained the increase in strength as a consequence of sand particles interlocking which causes the specimen to increase in volume. Rowe (1962) suggested a stress-dilatancy theory based on energy considerations and is able to explain the increase in strength through dilatancy considering friction terms only (De Josselin De Jong 1976). The effect of pressure was later recognised and formulations were proposed to predict the peak dilatancy rate (Been & Jefferies 1985, Bolton 1986).

The effects of stress path and loading direction in the triaxial test on strength and dilatancy of sand are investigated. It is shown that the unique relationship observed between peak friction angle and dilation rate at peak in conventional triaxial tests is followed regardless of stress path, confining stress at failure, relative density, and the mode of loading (compression or extension). Key words : sand, peak friction angle, dilatancy, stress path, triaxial test.

This paper focuses on the grain size distribution and shape effects on shear strength of three sand - gravel mixtures from Switzerland. A total of 28 direct shear tests in a large direct shear box apparatus have been performed to investigate the strength and dilatancy of sand - gravel mixtures. A relation between the grain size characteristics, the shape and the shear resistance has been found. For each mixture, the void ratio extent (emax−emin), and angle of repose, ϕcvare determined following the standard ASTM procedure. The experimental results are analysed in terms of the frictional and dilatant contributions to the strength of mixtures as a function of the grain size distribution, shape effects and their relative density. The particle shape effects are evaluated using the criteria used by Cho et al. (2006) for natural and crushed sands. The results show the dependency of the dilation gradient on the grain size distribution and the shape of the particle. The minumun and maximum void ratios are also dependent on the grading and the shape of the particles. The shear tests are interpreted in terms of friction angle as function of the dilation angle both evaluated from the tests at peak value and at the end of the shear phase. A useful empirical equation has been developed to evaluate the friction angle at constant volume and the gradient of dilation.

A unified hypoplastic model is formulated by incorporating the anisotropic critical state theory to describe the fabric effect in sand under both monotonic and cyclic loading conditions. An evolving deviatoric fabric tensor that characterizes the internal microstructure of sand is introduced into the hypoplastic model in conjunction with the intergranular strain concept. A scalar-valued fabric anisotropic variable indicating the interplay between the fabric and the loading direction is employed to account for the impact of fabric anisotropy on both the dilatancy and shear strength of sand. The model is demonstrated to be capable of simulating the anisotropic behavior of sand, using a single set of parameters under both monotonic and cyclic loading conditions, as evidenced by the satisfactory match with experimental results from various sources. In particular, by considering the influence of fabric evolution on the dilatancy of sand, the model adequately accounts for the fabric change effect and accurately captures the deviatoric strain accumulation, cyclic mobility, and the flow liquefaction phenomenon under cyclic loading condition.

Fiber-reinforced sand strength and dilation characteristics

The development of self-healing soil emerges as a possible solution to provide an autonomous and sustainable geomaterial to ground infrastructure with enhanced durability. This study aimed at developing a capsule-enclosed healant to enhance the shear strength of sand. Encapsulated tung oil was released in sand from calcium alginate capsules when subjected to void ratio changes under compaction. The released tung oil will harden and bond sand grains after a 30-day drying period. The release amount of tung oil was tracked at different void ratios (0.754–0.976) and capsule dosages (0.99%–3.85%), with its effect on strength evaluated by drained triaxial compression tests. The inclusion of capsules with fresh (liquid) tung oil decreased the peak strength and dilatancy of sand. However, enhanced peak strength/stress ratio compared to natural sand at effective confining stress of 50–100 kPa was achieved after stabilization with aged (hardened) tung oil. The critical state line of the sand-capsule composites located below that of clean sand in e-p′ plane, with the lines almost converging in the q-p′ plane. The role of tung oil bonding was separated and explained in terms of bonding energy. The combined effect of capsule dosage and bonding on peak strength were quantitatively captured. The soft capsule decreased friction angle while the bonding contributed to cohesion increase, leading to peak strength enhancement at mean effective stress lower than 500 kPa.

The strength and dilatancy of sand: Reply

Three-dimensional fractional plastic models for saturated sand using Caputo derivative and R-L derivative: A comparative study

Fabric anisotropy has a significant influence on the mechanical behavior of sand. An anisotropic plasticity model incorporating fabric evolution is formulated in this study. Information on the overall stress–strain relationship and micromechanical fabric states from DEM numerical tests is used in the development of the constitutive model, overcoming the difficulties of fabric measurement in physical tests. The framework of the model and its formulations for fabric evolution, plasticity, and dilatancy enables it to capture the strength, shear modulus, and dilatancy of sand under both monotonic and cyclic loading. The model is validated against DEM numerical tests and physical laboratory tests on samples with different initial fabric, showing good agreement between the simulation and test results for the anisotropic stress–strain behavior of sand. The use of DEM test data also allows for the validation of the model on the micromechanical fabric level, showing that the model can reproduce the fabric evolution and its influence on key constitutive features reasonably well. The model is further applied to analyze the liquefaction behavior of sand, exhibiting the significant influence of fabric anisotropy on both liquefaction resistance and postliquefaction shear deformation.

In this study, a hypoplastic model is developed to describe the mechanical behaviors of cemented sand under both monotonic and cyclic loading conditions. A state variable is proposed to qualify the bonding strength, and it is incorporated into the model to reflect the influence of cementation on the strength, stiffness, and dilatancy of sand. To reflect the bonding degradation, this variable evolves during the shearing following a simple evolution rule and may vanish after large deformation. The critical void ratio and friction angle are related to the initial cemented content to consider the variation of the critical state induced by the cementation. The model is subsequently extended to account for cyclic loading by incorporating the intergranular strain, fabric change effect, and semifluidized state. The capability of the model is demonstrated by simulating the behavior of cemented sand under both monotonic and cyclic loading conditions.

Extensive data of the strength and dilatancy of 17 sands in axisymmetric or plane strain at different densities and confining pressures are collated. The critical state angle of shearing resistance of soil which is shearing at constant volume is principally a function of mineralogy and can readily be determined experimentally within a margin of about 1°, being roughly 33° for quartz and 40° for feldspar. The extra angle of shearing of ‘dense’ soil is correlated to its rate of dilation and thence to its relative density and mean effective stress, combined in a new relative dilatancy index. The data of ø′max – ø′crit in triaxial or plane strain are separately fitted within a typical margin of about 2°, though the streneth of certain sands is underpredicted in the 1000–10000 kN/m2 range owing to the continued dilation of their crush-resistant grains. The practical consequences of these new correlations are assessed, with regard to both laboratory and field testing procedures. L'auteur analyse de nombreuses données concernant la résistance et la dilatance de 17 sables sous déformation plane ou axisymétrique pour différentes densités et pressions d'étreinte. L'angle de résistance au cisaillement dans l'état critique d'un sol soumis au cisaillement à volume constant est principalement une fonction de la minéralogie et peut se déterminer facilement à 1° près, comme ayant une valeur d'environ 33° pour le quartz et 40° pour le feldspath. L'angle supplémentaire de cisaillement d'un sol dense dépend à sa vitesse de dilation donc de sa densité relative et de la contrainte effective moyenne, combinées dans un nouvel indice de dilatance relative. Les données de ø′max – ø′crit en déformation plane ou triaxiale sent separées par une marge de 2° approximativement, bien que la résistance de certains sables soit sousestimée dans une fourchette de 1000 – 10000 kN/m2 en raison de la poursuite de la provoquée par l'écrasement de leurs grains résistants. L'article évalue les conséquences pratiques de ces nouvelles corrélations en ce qui concerne les méthodes d'essai en laboratoire et in-situ.

To evaluate the effect of stabilisation by using a colloidal silica aqueous gel on the subsequent behaviour of sand, the dilatancy and peak and ultimate strength characteristics of M31 sand were investigated before and after stabilisation. Triaxial compression tests were performed in drained and undrained mode, at effective stresses ranging from 100 to 6000 kPa. Important changes in the sand's mechanical behaviour were observed after stabilisation, including a significant increase in stress ratio and dilatancy rate at peak and a relocation of the treated sand's critical state line in the e–p′ plane, substantially above that manifested by the untreated sand; however, the two lines converged at high stresses. The results confirm a state-dependent behaviour for the sand that is not applicable to the treated sand, which exhibits predominantly stress-dependent behaviour. A modified state parameter was used to normalise the treated sand's behaviour at peak failure.

Discussion: The strength and dilatancy of sands

Discussion: The strength and dilatancy of sands

In the present study, an attempt is made to evaluate the strength and dilatancy parameters of sands mixed with jute fibres. A series of direct shear tests were performed on sample of sands with mixture of 0, 0.25 and 0.50% of jute fibres at three different relative density states, namely loose, medium dense and dense, the effect of stress level is also bought out by varying the effective normal stress. The tests were conducted on dry sand having different relative densities (i.e. 20, 50 and 80%) subjecting them to different constant values of vertical normal stress ranging from 50 to 400 kPa. At each stress level and density state for each case of sand fibre mixture, peak frictional angle and dilatancy angle were found out by conducting direct shear tests. A series of the direct shear tests were conducted up to shear strain value of 40%. The stress-strain response was observed, and the shear strength and dilatancy parameters were obtained for sand–jute fibre mixture with each relative density and normal stresses. Also, a correlation between peak friction angle, dilatancy angle and critical state friction angle was obtained for sands mixed with jute fibre. The present data was also compared with those of the previous established correlations by Bolton (Geotechnique 36(1):65–78, 1986) and Kumar et al. (Indian Geotech J 37(1):53, 2007).