Dry Granular Soil Research Articles

The kinematics of granular soils subjected to rapid impact loading

The paper reports new observations of strain localisation following dynamic impact. Two dimensional physical model tests have been performed to investigate the response of dry granular soils to the impact of a free falling steel plate. The tests have been performed to study the mechanics of the densification of soils during dynamic compaction, which is a widely used process to improve the performance of deep soil deposits by repeated dropping of large weights onto the ground surface. High speed photography and digital image correlation techniques have enabled the deformation patterns, soil strains and strain localisations to be observed. Tests have been performed on sand with a range of densities and a sand–silt mixture. The results have shown that the soil deformations are comprised of a conventional bearing capacity mechanism at the surface and a series of compaction bands that propagate downwards beneath the impacting plate. Similar patterns are observed in all tests, however as the compressibility of the soil increases the contribution from the bearing capacity mechanism decreases.

Analysis of wave propagation in dry granular soils using DEM simulations

In this paper, a three-dimensional particle-based technique utilizing the discrete element method (DEM) is proposed to study wave propagation in a dry granular soil column. Computational simulations were conducted to investigate the soil response to sinusoidal motions with different amplitudes and frequencies. Three types of soil deposits with different void ratios were employed in these simulations. Different boundary conditions at the base such as rigid bedrock, elastic bedrock, and infinite medium were also considered. Analysis is done in time domain while taking into account the effects of soil nonlinear behavior. The computational approach is able to capture a number of essential characteristics of wave propagation including motion amplification and resonance. Dynamic soil properties were then extracted from conducted simulations and used to predict the response of the soil using the widely used equivalent linear method program SHAKE and compare its predictions to DEM results. Generally, there was a good agreement between SHAKE and DEM results except when the exciting frequency was close to the resonance frequency of the deposit where significant discrepancy in computed shear strains between SHAKE predictions and DEM results was observed.

Model tests for studying the failure mechanism of dry granular soil slopes

The aim of this study is to investigate the behavior of sandy soil that has collapsed due to slope failures as well as to examine the influence area resulting from the failure. A model slope composed of three sections was employed to perform tests. The tests were conducted on various combinations of slope angle, slope height, slope surface condition, and the relative density of soil. Slope failure was induced by lifting up a retaining plate to cause the soil to collapse. Images of the sliding mass were taken by three cameras from different positions. The series of images clearly displayed the movement and development of granular flows. We then observed and measured the final profile of the soil and the shape of deposition area. The displacement of soil was measured by particle image velocimetry (PIV). Based on the measured displacement of flow, the front velocity of the flow was obtained. The slope angle and the internal friction angle of soil were found to be the most significant factors influencing the front velocity. Furthermore, a simplified equation based on Newton's law of motion is developed to predict the run-out of the flow. Comparison is then made between the measured and predicted run-out distances, and they are generally in good agreement.

Wave dispersion in dry granular materials by the distinct element method

Numerical studies are conducted to investigate the existence of wave dispersion in resonant column tests on dry granular soil. To this end, the two-dimensional distinct element method (DEM) in the time domain is employed. The investigations focus on the effect of sample width, voids ratio, viscous damping and wavelength, on propagation velocities of longitudinal harmonic waves in rectangular samples of uniform grains. It is shown that granular materials may exhibit anomalous dispersion that is, wave velocities that increase with increasing excitation frequency. This increase may exceed 20% for squatty samples, but becomes less pronounced for slender samples. Similar findings have been reported in some experiments on granular materials, but have not been systematically explored by numerical means. Results are presented in the form of dimensionless graphs and charts that elucidate the salient features of the problem. Comparisons with findings from gradient elastodynamic and viscoelastic theories are discussed.

Centrifuge Modeling of Granular Soil Response Over Active Circular Trapdoors

The trapdoor problem is a useful model for providing a clearer understanding of stress distribution around basic geotechnical engineering structures such as tunnels, conduits and anchor plates. The interest is mainly directed towards the determination of soil load on the trapdoor, which can be substantially different from the initial geostatic loads, when the trapdoor is moved even slightly. Development of shear bands around the yielding trapdoor is also of interest. A series of centrifuge and 1g model tests were conducted to study active arching in dry granular soil on circular trapdoors. The study was undertaken mainly because of two reasons: (i) limited success in earlier centrifuge modeling studies of a trapdoor problem, and (ii) a lack of understanding of the load-displacement characteristics under axisymmetric conditions. A trapdoor assembly and in-flight precompression technique were developed to perform a series of tests involving different overburden soil thickness on circular doors of different diameters. Correct initial geostatic loads were measured. Greater confidence in the experimental results was obtained because modeling of models type experiments were also successful. A parametric study involving different overburden soil thickness to trapdoor diameter ratios (H/D) ranging from 0.67 to 6 was conducted. The pattern of shear bands in sand above the trapdoor observed in a centrifuge model under a high gravity field differed considerably from that in a 1g model. Maximum arching was completely mobilized at movements of only 1.5% of trapdoor diameter. The minimum load on the trapdoor became constant at H/D equal to 5 regardless of the initial overburden pressure or height of the soil model.

The indirect estimation of saturated hydraulic conductivity of soils, using measurements of gas permeability. I. Laboratory testing with dry granular soils

A comprehensive knowledge of soil hydraulic conductivity is essential when modelling the distribution of soil moisture within soil profiles and across catchments. The high spatial variability of soil hydraulic conductivity, however, necessitates the taking of many in situ measurements, which are costly, time-consuming, and labour-intensive. This paper presents an improved method for indirectly determining the saturated hydraulic conductivity of granular materials via an in situ gas flow technique. The apparatus employed consists of a cylindrical tube which is embedded in the soil to a prescribed depth. Nitrogen at a range of pressures was supplied to the tube and allowed to escape by permeating through the soil. A 3-dimensional, axisymmetric, steady-state, finite element flow model was then used to determine the value of the soil intrinsic gas permeability which produces the best fit to the pressure–air flow data. Saturated hydraulic conductivities estimated from the application of the gas flow technique to 5 granular soils covering a wide range of permeabilities were in close agreement with values determined using a conventional permeameter. The results of this preliminary study demonstrate the potential of this approach to the indirect determination of saturated hydraulic conductivity based on measurement of gas flow rates in granular and structured soils.

Automatic Volume Measuring Device for Testing Dry Soils: “Martina”

Abstract A new automatic measuring device for testing dry granular soils in laboratory is presented. The device enables direct measure of volume changes of dry specimens by measuring gas that comes out or in during testing due to soil deformability. As the soil deforms, voids (which are supposed to be hydraulically connected to each other) change their volume; if they are part of a closed hydraulic circuit (any leakage is avoided) and gas pressure is kept constant by adjusting the total volume of such circuit, the amount of gas through soil voids is transferred to the measuring device, which varies its volume. The apparatus is described in detail as to its working principle. A strong effort was made to assess errors and to calibrate such apparatus as well as to evaluate its precision when used for triaxial testing. Finally, to validate its behavior, the most significant results obtained comparing dry versus saturated granular soil triaxial tests are here presented.

Density prediction using a static cone penetrometer

This paper presents results of investigation on the use of a static cone penetrometer for predicting densities of two air dry granular soils. Cone penetration resistance values (qc) were measured for soil specimens prepared at specific densities with different surcharge loads. Linear regression techniques were utilized to develop correlations between values of (qc)' surcharge loads and depths of penetration. Calibration curves to be used for predicting densities are given on the basis of tests data. A procedure is outlined for use of the penetrometer in determining soil densities.

Micromechanical model to predict sand Densification by Cyclic Straining

It has been observed that the application of cyclic shear loading to a dry sand and other cohesionless soils results in a progressive volume reduction and a relative density increase. Although this densification is larger and takes place faster in loose sands, it occurs in both loose and dense soils, and is caused by the tendency of any sand to contract under a small shear strain. A micromechanical statistical model was developed by the writers to interpret and compute the compaction of such a dry granular soil subjected to a strain‐controlled cyclic simple‐shear test. The method postulates that an important role is played in the compaction phenomenon by the spatial statistical distribution of porosity within the soil specimen. Numerical simulations of densification are presented using the proposed model, which are in excellent agreement with observed sand compaction in actual cyclic simple‐shear tests. These simulations provide new insight into the densification process, and especially on the significanc...

A Model Study of the Influence of Stress on the Thermal Conductivity of Dry Sand

Abstract An equivalent resistor model was investigated to determine its suitability for describing the thermal conductivity of dry granular soils. The suitability of the model for incorporating the influence of stress on thermal conductivity was also examined. The parameters necessary to specify the model were determined experimentally by fitting the model to measured values of thermal conductivity from two different granular soils. Measurements were made with a thermal probe located in the center of cylindrical specimens of dry sand which were subjected to isotropic compressive stresses in a modified triaxial cell. It was found that the model provides a convenient and accurate method to determine the thermal conductivity of dry granular soils.

A field screening technique for evaluating rice germplasm for drought tolerance during the vegetative stage

A rapid method of testing direct-seeded rainfed rices ( Oryza sativa L.) for drought reaction, concurrently with, but separately from, evaluation for agronomic traits and reaction to various stresses, is needed for early identification of desirable genotypes. Thus, a technique which quantifies soil moisture stress for screening thousands of cultivars and breeding lines for drought tolerance during the vegetative stage was developed. Seeds were drilled on dry granular clay soil (Andaqueptic Haplaquoll) in the field, then sprinkler-irrigated every 4 days for 37 days. As the soil dried, each entry was visually scored for drought reaction at 0.1–0.2, 0.4–0.5 and 0.8–1.0 MPa soil moisture tension ( smt) at 20-cm soil depth. Entries were visually scored for drought recovery 10 days after the field was reirrigated. A total of 38 691 rices was tested in the dry seasons from 1978 to 1985. The 8-year screening test showed reproducibility of results in the field as indicated by the consistent scores of drought-tolerant and susceptible control cultivars within and across years. Ten germplasm accessions repeatedly performed similar to, or better than, the drought-tolerant control. There were ten outstanding drought-tolerant breeding lines, six of which were progenies of moderately tolerant Nam Sagui 19 from Thailand. The 20 outstanding selections likewise had consistent drought-tolerant scores across years, irrespective of duration of stress, which ranged from 31 to 54 days.

Theory and experiments on centrifuge cratering

Centrifuge experimental techniques provide possibilities for laboratory simulation of ground motion and cratering effects due to explosive loadings. The results of a similarity analysis for the thermomechanical response of a continuum show that increased gravity is a necessary condition for subscale testing when identical materials for both model and prototype are being used. The general similarity requirements for this type of subscale testing are examined both theoretically and experimentally. The similarity analysis is used to derive the necessary and sufficient requirements due to the general balance and jump equations and gives relations among all the scale factors for size, density, stress, body forces, internal energy, heat supply, heat conduction, heat of detonation, and time. Additional constraints due to specific choices of material constitutive equations are evaluated separately. The class of constitutive equations that add no further requirements is identified. For this class of materials, direct simulation of large‐scale cratering events at small scale on the centrifuge is possible and independent of the actual constitutive equations. For a rate‐independent soil it is shown that a small experiment at gravity g and energy E is similar to a large event at 1 G but with energy equal to g3E. Consequently, experiments at 500 G with 8 grams of explosives can be used to simulate a kiloton in the field. A series of centrifuge experiments was performed to validate the derived similarity requirements and to determine the practicality of applying the technique to dry granular soils having little or no cohesion. Ten shots using Ottawa sand at various gravities confirmed reproducibility of results in the centrifuge environment, provided information on particle size effects, and demonstrated the applicability of the derived similitude requirements. These experiments used 0.5–4 grams of pentaerythritol‐tetranitrate (PETN) and 1.7 grams of lead‐azide explosives. They were placed at zero depth of burial and were detonated at gravities up to 450 G. These results provide rules for scaling crater dimensions in Ottawa sand over a range of more than 10 orders of magnitude in total energy release.