Dry Granular Soil Research Articles

DEM analysis of constrained modulus nonlinearity and damping factor of dry granular soils

The seismic site response analysis requires the dynamic soil properties (i.e., the modulus and damping). While it is well understood that the shear modulus and damping ratio are nonlinearly shear-strain dependent, the knowledge on the constrained modulus and damping ratio of compressional waves is still very limited due to lack of laboratory testing equipment. This study aims to simulate cyclic tests of constrained compression for granular specimens by discrete element method (DEM) to understand the dynamic soil properties of compressional waves, i.e., the nonlinearity in constrained modulus and damping ratio with the compression strain. The evolution in microstructure of granular specimens is revealed to provide micromechanical interpretations for the compressional soil nonlinearity. The results show that the dependency of constrained modulus on compression strain is different in compression and extension stages, and the modulus reduction is only observed in the extension path. The damping ratio of samples under cyclic constrained compression is smaller than that of cyclic shear. The nonlinear soil behavior is more obvious at lower confining pressure and for dense sample. The nonlinearity of constrained modulus depends on the coordination number, contact normal force and fabric anisotropy, while the associated damping ratio is only related to fabric anisotropy.

Seismic bearing capacity of shallow foundations subjected to inclined and eccentric loading using modified pseudo-dynamic method

In this paper, a comprehensive parametric study is conducted to evaluate the seismic bearing capacity of shallow strip foundations overlying dry and cohesionless granular soil under the actions of inclined and eccentric loadings. For this purpose, a systematic combination of the lower-bound theorems of the limit analysis, the finite element method, and the nonlinear programming is implemented. The seismic loading condition is simulated by the well-established modified pseudo-dynamic (MPD) approach by accounting for the significant influence of phase difference as well as the primary and shear waves propagation through applying non-uniform inertia forces along the vertical and horizontal directions, respectively. Unlike the pseudo-static approach, in the so-called modified pseudo-dynamic loading, a more realistic dynamic nature of the seismic excitation is taken into account using various soil dynamic properties and spectral parameters of the problem under study. In this way, a more viable response of the shallow foundation subjected to seismic loading could be captured by simulating a visco-elastic material through the well-defined Kelvin-Voigt model. The modified pseudo-dynamic (MPD) method considers the influences of shear and primary wave velocities, amplification of earthquake loading, and period of lateral shaking. The current MPD approach is capable of considering the local site effects and damping characteristics of the soil deposit underneath shallow footing by adopting a non-uniform and nonlinear acceleration field inside the soil medium. The results of the seismic bearing capacity are presented in the forms of spectral responses and failure envelopes for the eccentric and inclined loadings. The results show that the failure envelopes of the shallow foundation subjected to either inclined or eccentric loading significantly shrink with the increase in the earthquake accelerations and decrease in the material damping of the underlying soil mass. The amount of changes in the size of failure envelopes in the normalized V-H and V-M spaces due to the variation of seismic intensity and material damping depends directly on the non-dimensional frequency of the earthquake excitation.

Influence of Friction Angles on Earth Pressures in Dry Granular Flow Dynamics and Soil Mechanics

The earth pressure coefficient (K) determines the nature of (tendency of) deformation of the granular mass during flow or deposition. When flow velocity is increasing, K takes its active state Kact and the flow is divergent. When the flow velocity is decreasing, K takes its passive state Kpas and the flow is convergent. The mathematical relations presented here and their 2-D and 3-D plots highlight that the passive and active earth coefficients strongly depend on the internal angle (δ) and basal angle (ϕ) of frictions. The mathematical relation for dry granular mass flow is extended to find these coefficients in soil mechanics. Results further show that active earth pressure drops as the internal angle of friction increases, but passive earth pressure rises. The earth's pressure is at rest if the wall is in its natural position

Estimating the subgrade reaction at deep braced excavation bed in dry granular soil using genetic programming (GP)

Modulus of subgrade reaction (Ks) is a simplified and approximated approach to present the soil-structure interaction. It is widely used in designing combined and raft foundations due to its simplicity. (Ks) is not a soil propriety, its value depends on many factors including soil properties, shape, dimensions and stiffness of footing and even time (for saturated cohesive soils). Many earlier formulas were developed to estimate the (Ks) value. This research is concerned in studying the effect of de-stressing and shoring rigidity of deep excavation on the (Ks) value. A parametric study was carried out using 27 FEM models with different configurations to generate a database, then a well-known “Genetic Programming” technique was applied on the database to develop a formula to correlate the (Ks) value with the deep excavation configurations. The results indicated that (Ks) value increased with increasing the diaphragm wall stiffness and decreases with increasing the excavation depth.

3D DEM investigation of the resistance of ice and frozen granular soils

Frozen soils are mixtures of ice and solid grains. Since the mechanical properties of ice and dry granular soils are completely different, the behaviour of the ice-soil mixture is highly dependent on the ice content. In this paper, a 3D model based on the distinct element method (DEM) is introduced and used for evaluating the resistance of frozen soil. In the DEM model, two groups of elements are used for representing soil particles and ice. The modelling of granular soils by DEM has been widely treated in the literature. However, few researches are available regarding DEM modelling of ice, and no DEM simulations have been published regarding soil-ice mixtures. For these reasons, preliminary simulations are performed to calibrate the DEM ice model and assess the relationship between the micromechanical parameters and the macroscopic behaviour. Then, several specimens are built by mixing different proportions of the two constituents to reproduce different ice contents, and appropriate parameters for the ice-soil interface are introduced. A series of triaxial compression and shear simulations are performed and analysed. The obtained macroscopic behaviour of the ice-soil mixtures is qualitatively and quantitatively described, and compared to existing experimental data in terms of resistance parameters.

Performance of a cyclic miniature cone penetration test in dry granular soil: experimental approaches

Shaft friction degradation in geotechnical problems occurs alongside large uncertainties. Under cyclic loadings, offshore foundations can episodically suffer the loss of stiffness and strength. In such cases, initial in situ soil state is lost as soil is disturbed during cyclic loading. In this sense, it is important to develop a testing procedure to assess the lateral friction degradation. Considering that a cone is a miniature driven pile, the use of this testing technique allows monitoring the degradation of sleeve friction during cyclic loading. This paper aims to improve the knowledge on the feasibility of using cone testing for the recognition of lateral friction degradation of a dry granular soil. Cyclic miniature cone penetration tests were performed in a calibration chamber considering a range of loading conditions. The effect of cyclic displacement amplitude and the confining pressure were investigated. Lateral friction, fs, degrades while repeating the load cycling, resulting in loss of lateral strength due to soil–cone interface friction degradation. After about ten load cycles, degradation tended to stabilise for all tests.

The local meshless numerical model for granular debris flow

The article deals with the possibilities of using local meshless methods for modeling the movement of avalanches and rapid slopes movements. These are dangerous phenomena that can cause extensive damage to the infrastructure of mountainous areas and numerical models are therefore an important tool in their prediction. Today, meshless methods are increasingly developed in many different areas for their potential advantages over conventional numerical methods. For the development of the model of debris flow, the article used the local method of weighted squares, as it proved suitable for addressing this type of differential equations. To verify the possibilities of this method, in the article, we focused on the models of dry granular soil movement and compared the results of the solution to several known cases.

Face Stability Analysis of EPB Shield Tunnels in Dry Granular Soils Considering Nonuniform Chamber Pressure and a Dynamic Excavation Process

Abstract The support pressure at the tunnel face of earth pressure balance (EPB) shield machines is usually assumed to be uniform from the crown to the invert, which is a notable simplification of ...

Influence of the Cutterhead Configuration and Operation Parameters on the Face Stability of EPB Shield Tunnels in Dry Granular Soils

This paper presents a study on the face stability of earth pressure balance (EPB) shield tunnels in dry granular soils using the discrete element model (DEM) method to replicate realistic excavation processes of the EPB machine. Analyses were conducted at different buried depths to determine the limit face pressures and the face failure model induced by the EPB shield machine with varying open ratios and rotation speeds of the cutterhead. The failure kinematics of the soil and the soil arching effect were also determined. The results showed that both the open ratio and the rotation speed of the cutterhead have significant effects on the stability of the face in terms of the failure kinematics, limit face pressures, and the soil arching effect. The failure zone was highly dependent on the open ratio, but it depended less on the rotation speed of the cutterhead. The stability of the face is overestimated when the excavation is conducted with a cutterhead that has a large open ratio, but it is underestimated when the excavation is conducted with a cutterhead that has a smaller open ratio. Ignoring the effect of the configuration of the cutterhead and the rotation speed results in unexpected and unrealistic results.

Evaluation of the Hydrodynamic Pressure Effect of Cylindrical Liquid Storage Tank on The Granular Soil Behavior Under Seismic Excitation

Many liquid storage tanks around the world have been affected by earthquakes, and the seismic analysis of such tanks is much more complicated due to the Fluid-Structure Interaction. Besides, when the soil properties are taken into the consideration, the analysis with the Fluid-Soil-Structure Interaction becomes very complicated. In this paper, a series of shaking table tests are conducted on a model of the cylindrical water tank rested on dry granular soil and the equivalent load is considered to study the effect of hydrodynamic pressure generated in the storage water tank on the soil behavior. An experimental investigation of (1:100) scale model has been carried out on a shake table that was manufactured with specific mechanical parts and flexible laminar shear box. The preparation of the test included three cases in different relative densities (medium, dense, medium-dense). Three earthquake histories (Kobe, El-Centro and Ali Al-Gharbi) were implemented to study a wide range of the acceleration. The results showed that the acceleration at the bottom-depth of the soil column is slightly higher than at the mid-depth, while at the top portion of the soil column, the acceleration comparatively becomes less than that at the mid-depth. The settlement due to hydrodynamic pressure in the storage base tank is significantly reduced compared with results of nonhydrodynamic pressure in all cases of the acceleration history. In addition, the lateral stress at the surface gives higher than the stress at a depth equal to the diameter of the base tank.

One-Dimensional Transient Wave Propagation in a Dry Overlying Saturated Ground

While the propagation of stress wave generated by dynamic compaction in dry or saturated granular soil has received much coverage in the research literature, attention to situations with dry soil overlying saturated soil, or mixed-phase ground, is limited. In such cases, the compressional waves have to propagate from a dry layer above the groundwater table to saturated soil below the groundwater table. In this paper, the transient wave propagation characteristics in a mixed-phase ground with an idealized interface between the dry and saturated layer is studied. As the time domain solutions for such problems are often unavailable using analytical methods, a numerical approach based on a dual-phase coupled finite element method implemented on a commercial software platform is adopted. The wave behaviour across the interface is studied and the energy transmission and reflection mechanism from dry to saturated layer is examined. The amplitude, speed and attenuation of the compressional waves and their dependencies on the soil permeability, skeleton stiffness and load duration are quantitatively evaluated via a comprehensive parametric study. As a precursor to the numerical investigation of wave propagation in a mixed-phase ground due to dynamic compaction, the results presented in this study are likely to help in providing a better understanding of the ground improvement effect of dynamic compaction in soil involving a groundwater table.

Face Stability Analysis of EPB Shield Tunnels in Dry Granular Soils Considering Dynamic Excavation Process

AbstractThe aim of this paper is to study the face stability of earth pressure balance (EPB) shield tunnels in dry granular soils considering the influence of the dynamic excavation process. The an...

Instability of loose dry granular slopes observed in centrifuge tilting table tests

Landslides in loose saturated granular materials can experience significant deviatoric strain softening leading to static liquefaction and flow failure. This research study aimed at understanding the instability conditions leading to softening has been conducted under triaxial or plane strain conditions on loose granular soils brought to failure under undrained or drained stress paths. In this paper, it is hypothesised that the onset of granular instability should also be reproducible in tilt-table tests of loose dry granular soils. A centrifuge tilt-table apparatus was developed to test this hypothesis in which the stress path imposed on the soil layer could be inferred from an infinite slope analysis and the shear and volumetric strain be measured through the transparent side walls. Experiments performed at 1g, 20g, 40g and 60g illustrate that this approach can capture the constitutive behaviour of the soil layer. Critically, as the tilt-table test is a load-control test, instability was observed as a rapid acceleration in shear strain rate accompanied by a rapid volumetric collapse of the pore space. The tilt-table test therefore not only enables a measurement of the angle of repose, but provides a method to characterise the instability behaviour of granular materials at low stress levels found in shallow landslides.

Direct Shear Test for Coarse Granular Soil

A large direct shear apparatus was used to test coarse granular soil. In shear tests with a small shear box, deformation in shear band is homogeneous and stress–dilatancy relationships at pre-peak and post-peak phases of shearing are similar. However, in tests with a large shear box, the pre-peak and post-peak stress–dilatancy relationships are different. Using frictional state theory, it has been shown that the stress–dilatancy relationship for the large shear box is different from that for the small one conventionally used in direct shear tests, which suggests higher non-homogeneity deformation in the shear band in a large shear box in comparison to that in a small one. Parameter α of frictional state theory equals zero for the small and large shear boxes at failure. Parameter β = 1.4 for the small shear box and is a function of initial moisture and compaction for the large shear box. Average values of β range between 2.27 and 2.52 for dry coarse granular soil and between 2.17 and 2.30 for wet one. The value of critical frictional state angle Φ°= 41.2° of tested coarse granular soil is independent of soil moisture and compaction. The influence of the box’s size on stress–dilatancy relationship has also been observed in the previous studies.

Estimation of terramechanics parameters of wheel-soil interaction model using particle filtering

Accurate estimation of the parameters affecting the wheel-soil interaction terramechanics of an extraterrestrial rover is key to the success of its mission. Traditional approaches to estimating the relevant parameters based on laboratory tests lead to predictions that show significant deviation from experimental observations. The objective of this article is to apply dynamic Bayesian estimation techniques on the measurements from simple single wheel tests to estimate the terramechanics parameters. This ensures that the parameter estimation takes into account the scatter that invariably exists in physical measurements. A mathematical model for a rigid wheel driven on a dry (0% moisture content) granular soil medium is considered to model the planetary regolith. It is demonstrated that adopting Bayesian techniques for terramechanics parameter estimation leads to good predictions for the drawbar pull, torque and the wheel sinkage. This bypasses the need for using more complex models which in turn require additional parameters to be estimated.

Pullout behavior of polymeric strip in compacted dry granular soil under cyclic tensile load conditions

Assessment of the reinforcement behavior of soil under cyclic and monotonic loads is of great importance in the safe design of mechanically stabilized earth walls. In this article, the method of conducting a multistage pullout (MSP) test on the polymeric strip (PS) is presented. The post-cyclic behavior of the reinforcement can be evaluated using a large-scale pullout apparatus adopting MSP test and one-stage pullout (OSP) test procedures. This research investigates the effects of various factors including load amplitude, load frequency, number of load cycles and vertical effective stress on the peak apparent coefficient of friction mobilized at the soil-PS interface and the pullout resistance of the PS buried in dry sandy soil. The results illustrate that changing the cyclic tensile load frequency from 0.1 Hz to 0.5 Hz does not affect the pullout resistance. Moreover, the influence of increasing the number of load cycles from 30 to 250 on the peak pullout resistance is negligible. Finally, the effect of increasing the cyclic tensile load amplitude from 20% to 40% on the monotonic pullout resistance can be ignored. The peak apparent coefficient of friction mobilized at the soil-PS interface under monotonic and cyclic load conditions decreases with the increase in vertical effective stress.

Clarification of capillary rise in dry sand

The extent of capillary rise into initially dry granular soil, that is exposed at the surface and subjected to wetting at the base, is not easy to predict. Simple rising head column experiments were performed using pure water in medium-grained poorly graded and unconsolidated sand to shed light on the extent to which water rises in initially dry soil and its distribution. Rising head experiments have been conducted in initially dry sand to explore these phenomena in columns instrumented with Time Domain Reflectometry (TDR) probes, the data from which are compared with water contents ultimately found gravimetrically from sample slices. Water was found to reach much higher than theoretical estimates based on an equivalent capillary tube, suggesting that capillary rise is responsible for fluid transport well into the zone of unsaturated soil above the saturated capillary zone. This remained the case even when the sand was subjected to strong rates of evaporation at its surface. This study challenges the conventional understanding of capillary rise in unsaturated soil and has particular relevance in desert soils subject to rapid evaporation.

Influence of Structural Stiffness on Ratcheting Convection Cells of Granular Soil under Cyclic Lateral Loading

In granular soils, long-term cyclically loaded structures can lead to an accumulation of irreversible strain by forming closed convective cells in the upper layer of the bedding. The size of the convective cell, its formation and grain migration inside this closed volume have been studied with reference to different stiffness of the embedded structure and different maximum force amplitudes applied at the head of the structure.This relation was experimentally investigated by applying a cyclic lateral force to a scaled flexible vertical element embedded in a dry granular soil. The model was monitored with a camera in order to derive the displacement field by means of the PIV technique. Furthermore, the ratcheting convective cell was also simulated with DEM with the aim of extracting some micromechanical information. The main results regarded the different development, shape and size of the convection cell and the surface settlements.

Effect of vibration on the critical state of dry granular soils

In order to investigate the effect of vibration on the critical state of dry granular soils, eighty samples of four different granular materials were tested at four normal stresses under different vibrational accelerations. The experiments were performed on a modified vibrational direct shear apparatus with vibrational loading in the horizontal (shear) direction. Strength loss versus vibration acceleration plots at different normal stresses, as well as the peak, residual and vibro-residual shear strength diagrams have been obtained for the four granular materials at different intensities of vibration. It has been determined that increasing the intensity of vibration reduces the friction angle of the granular materials. The effect of particle shape on the strength loss due to vibration has been observed as well as the effect of vibration on the normal stress.

Micromechanical behavior of granular materials in direct shear modeling

Direct shear testing is among the most frequently performed laboratory tests for determining the engineering properties of soil, particularly dry granular soils. This study employed the discrete element method to simulate a direct shear test to examine the microstructure and micromechanical behaviors of granular materials in dense and loose states. The analysis results show that a relationship exists between the microscopic and macroscopic properties of the simulated dense granular assemblies. Dilation of the dense granular material resulted from an increase in volume in the area along the shear plane. By contrast, contraction of the loose granular material resulted from a decrease in volume decrease throughout the granular sample. The dense materials exhibited axial compression and lateral extension stress paths in the middle of the shear plane. The loose materials exhibited stress path variations that were markedly more complex and varied. For the dense materials, the initial orientation of the major principal stress plane was approximately horizontal. During the test, the formation of a contact force chain caused the major plane to rotate counterclockwise until it was at approximately 45°–55° to the original, horizontal position.