Direct Shear Tests on JSC-1A Lunar Regolith Simulant

A series of direct shear tests were conducted on the JSC-1A lunar regolith simulant in a 101.6-mm- (4-in.-) diameter container. The direct shear test provides a unique mode of failure that aids the development of excavation tools for the Moon. Relative density and normal load were varied to study the strength behavior of such granular material at peak and critical state conditions. The values of the internal friction angle ranged from 30 to 70°. A relationship between the internal friction angle of the direct shear and the published triaxial compression test results is presented. Additionally, the measured dilatancy angle is related to the difference in peak and critical state stress friction angles.     

Stress Path Testing of an Anisotropic Sandstone

The Berea sandstone used in this study is transversely isotropic with respect to elastic response, with P-wave velocities of 2,160?m/s normal to bedding and 2,290?m/s parallel to bedding, a variation of only 6%. Triaxial compression and extension tests involving failure by loading and unloading were performed along the two directions of symmetry. With axial stress applied parallel to bedding, the internal friction angle was approximately 55° for compression and extension, indicating no intermediate stress effect for the linear Mohr-Coulomb criterion. However, for axial stress normal to bedding, the friction angle in compression was 50°, whereas in extension it was 44°. This anomalous behavior was attributed to strength anisotropy of the sandstone.     

Discrete Element Modeling of Strength Properties of Johnson Space Center (JSC-1A) Lunar Regolith Simulant

This paper simulates the three-dimensional axisymmetric triaxial compression of JSC-1A lunar regolith simulant under lunar and terrestrial gravity environments under a wide range of confining pressures and relative densities. To accomplish this, the discrete element method (DEM), using Particle Flow Code In Three-Dimensional (PFC3D) software, was employed. The paper focuses on the peak and the critical state (CS) friction angles, which were predicted in the ranges of 35.4°–82.7° and 31.2°–79.8°, respectively, depending on the specimen density and confining pressure. A significant increase in peak and CS friction angles was predicted at near-zero confining pressure. The DEM results validated an empirical model that relates the peak friction angle with the CS friction angle, relative density, and mean effective stress at the CS. Comparison of DEM results with lunar in situ measurements of friction angle, from Apollo missions and other extraterrestrial laboratory experiments under a microgravity environment, shows a favorable agreement.     

Influence of Optimized Tire Shreds on Shear Strength Parameters of Sand

This paper presents the usefulness of optimizing the size of waste tire shreds on shear strength parameters of sand reinforced with shredded waste tires. A relatively, uniform sand has been mixed with randomly distributed waste tire shreds with rectangular shape and compacted at 2° of compaction. Waste tire shreds were prepared with a special cutter in three widths of 2, 3, and 4?cm and various lengths for each shred width. Three shred contents of 15, 30, and 50% by volume were chosen and mixed with the sand to obtain a uniformly distributed mixture. In order to compare the shear strength of different sand–tire shred samples, two compaction efforts in terms of sand matrix unit weights of 15.5 and 16.8?kN/m3 were considered. The results show that the influencing parameters on shear strength characteristics of sand–shred mixtures are normal stress, sand matrix unit weight, shred content, shred width, and aspect ratio of tire shreds. With the selected widths of shreds, compaction efforts, shred contents, and the variations of aspect ratios, it is possible to increase the initial friction angle ?1 up to 113.5%, that is ?1 = 67°. The average value for the influence of aspect ratio variations on increase in friction angle of the mixtures for all tests has been found to be about 25%. These average values for lower and higher compacted samples containing different widths and aspect rations were 37.6 and 17.2%, respectively. It has been investigated that for a given width of tire rectangular shreds, there is solely a certain length, which gives the greatest initial friction angle for sand–tire shred mixtures. This is the main contribution of this paper.     

Shear Strength Behavior of Fiber-Reinforced Sand Considering Triaxial Tests under Distinct Stress Paths

The results of drained triaxial tests on fiber reinforced and nonreinforced sand (Osorio sand) specimens are presented in this work, considering effective stresses varying from 20 to 680?kPa and a variety of stress paths. The tests on nonreinforced samples yielded effective strength envelopes that were approximately linear and defined by a friction angle of 32.5° for the Osorio sand, with a cohesion intercept of zero. The failure envelope for sand when reinforced with fibers was distinctly nonlinear, with a well-defined kink point, so that it could be approximated by a bilinear envelope. The failure envelope of the fiber-reinforced sand was found to be independent of the stress path followed by the triaxial tests. The strength parameters for the lower-pressure part of the failure envelope, where failure is governed by both fiber stretching and slippage, were, respectively, a cohesion intercept of about 15?kPa and friction angle of 48.6?deg. The higher-pressure part of the failure envelope, governed by tensile yielding or stretching of the fibers, had a cohesion intercept of 124?kPa, and friction angle of 34.6?deg. No fiber breakage was measured and only fiber extension was observed. It is, therefore, believed that the fibers did not break because they are highly extensible, with a fiber strain at failure of 80%, and the necessary strain to cause fiber breakage was not reached under triaxial conditions at these stress and strain levels.     

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15?kPa and a friction angle of 36° at normal stress of 1?atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.     

Influence of Particle Shape and Surface Friction Variability on Response of Rod-Shaped Particulate Media

Discrete element methods are important tools for the investigation of the mechanics of granular materials. In two dimensions, the reliability of these numerical approaches can be explored using physical tests on rod assemblies. This work highlights the importance of representing the actual distribution of rod shapes and surface friction in numerical simulations. The sensitivity of the response of hexagonally packed rods to minor changes in particle geometry and friction is investigated using a combination of laboratory tests and discrete element simulations. Laboratory test results highlight the influence of small variations in rod geometry on the global response, with the peak friction angle decreasing significantly as the standard deviation of the rod size distribution increased. Small changes in rod shape are also seen to be important. The numerical simulations indicate that the peak friction angle decreases as the standard deviation of the distribution of particle surface friction increases. This paper illustrates the way in which laboratory tests and numerical simulations can be used in a complementary manner to better understand the micromechanics of the response of granular materials.     

Sand Transport on Steeply Sloping Plane and Rippled Beds

Experiments on sand transport have been carried out in the Sloping Sediment Duct at HR Wallingford. The aim of the experiments was to investigate sediment transport mechanisms, for sand of varying degree of grading, on sloping beds. The Sloping Sediment Duct is a steady flow, recirculating duct, capable of generating mean flow speeds of up to 1 m/s and tilting to +/?30°. Twenty-two tests with two different sediments were conducted. Both sediments had a median grain size of about 0.23 mm but different standard deviations. Bed slopes up to +/?20° were used in the experiments. The results show that bedforms have a significant effect on the transport rate. Since the bedforms, in turn, are affected significantly by the slope, the relation between transport rate and slope is not a monotonic function. Maximum suspended transport rates were attained for downslope flows at angles of about 10°. The transport rate for widely graded sediment was significantly larger than that for well-sorted sediment for almost all flows and slopes.     

Influence of Stress Ratio and Stress Path on Behavior of Loose Decomposed Granite

This paper presents results from four series of triaxial compression tests of loosely compacted decomposed granite (DG) or silty sand on both isotropically and anisotropically consolidated specimens. These tests included undrained tests, drained tests with constant deviator stress, and a decreasing mean effective stress path. The silty sand possessed high compressibility during isotropic compression. The observed high compressibility is probably attributed to the loose soil structure created by using the moist tamping method and the presence of crushable feldspar in the soil. Static liquefaction behavior and the so-called “reversed” sand behavior were observed in all undrained tests. This “reversed” sand behavior can be readily explained by the high compressibility of DG leading to the nonparallel and converging nature of the initial state line and the critical state line. Preshearing resulted in a more brittle response in the postpeak behavior. The higher the initial stress ratio (ηc), the smaller the ductility. Structural collapse of DG was observed. This collapse is characterized by a sudden large increase in both the axial and contractive volumetric strains. The mobilized angles of friction at collapse range from 31.8° to 38.7°, which are smaller than the critical state angle (?col′), but higher than the mobilized friction angle of the instability line (28.1°) determined by the isotropically consolidated undrained tests. A trilinear approximate relationship can be found between ?col′ and ηc and a liquefaction potential index is introduced to provide a simple preliminary design parameter for static liquefaction and instability prone slopes.     

Contraction, Dilation, and Failure of Sand in Triaxial, Torsional, and Rotational Shear Tests

The response of a saturated fine sand (Nevada sand No. 120) with relative density Dr ≈ 70% in drained and undrained conventional triaxial compression and extension tests and undrained cyclic shear tests in a hollow cylinder apparatus with rotation of the stress directions was studied. It was observed that the peak mobilized friction angle for this dilatant material was different in undrained and drained tests; the difference is attributed to the fact that the rate of dilation is smaller in an undrained test than it is in a drained test. Consistent with the findings of others, the material is more resistant to undrained cyclic loading for triaxial compression than for triaxial extension. In rotational shear tests in which the second invariant of the deviatoric stress tensor is held constant, the shear stress path (after being normalized by the mean normal effective stress) approached an envelope that is comparable but not identical in shape to a Mohr-Coulomb failure surface. As the stress path approached the envelope, the shear end deviatoric strains continued to increase in an unsymmetrical smooth spiral path. During the rotational shear tests, the direction of the deviatoric strain-rate vector (deviatoric strain increment divided by the magnitude of change in Lode angle) was observed to be about midway between the deviatoric stress increment vector and the normal to a Mohr-Coulomb failure surface in the deviatoric plane. The stress ratio at the transition from contractive to dilative behavior (i.e., “phase transformation”) was also observed to depend on the direction of the stress path; therefore this stress ratio is not a fundamental property. Results from torsional hollow cylinder tests with rotation of stress directions are presented in new graphical formats to help understand and interpret the fundamental soil behavior.     

Effect of Compressive Load on Uplift Capacity of Model Piles

Thirty six tests on model tubular steel piles embedded in sand were carried out in the laboratory to assess the effects of compressive load on uplift capacity of piles considering various parameters. The model piles were of 25 mm outside diameter and 2 mm wall thickness. The soil–pile friction angles were 21 and 29° in loose and dense conditions of sand. The piles were embedded in sand for embedment length/diameter ratios of 8,16, and 24 inside a model tank. They were subjected to a static compressive load of 0, 25, 50, 75, and 100% of their ultimate capacity in compression and subjected to pull out loading tests. The experimental results indicated that the presence of the compressive load on the pile decreases the net uplift capacity of a pile and the decrease depends on the magnitude of the compressive load. A logical approach, based on the experimental results, has been suggested to predict the net uplift capacity of a pile considering the presence of compressive load.     

Computation of Cavity Expansion Pressure and Penetration Resistance in Sands

A cavity expansion-based theory for calculation of cone penetration resistance qc in sand is presented. The theory includes a completely new analysis to obtain cone resistance from cavity limit pressure. In order to more clearly link the proposed theory with the classical cavity expansion theories, which were based on linear elastic, perfectly plastic soil response, linear equivalent values of Young's modulus, Poisson’s ratio and friction and dilatancy angles are given in charts as a function of relative density, stress state, and critical-state friction angle. These linear-equivalent values may be used in the classical theories to obtain very good estimates of cavity pressure. A much simpler way to estimate qc—based on direct reading from charts in terms of relative density, stress state, and critical-state friction angle—is also proposed. Finally, a single equation obtained by regression of qc on relative density and stress state for a range of values of critical-state friction angle is also proposed. Examples illustrate the different ways of calculating cone resistance and interpreting cone penetration test results.     

Interaction of Foundry Sands with Geosynthetics

Excess foundry sands from gray-iron casting are a mixture of sand, bentonite, and additives that can have properties desirable for structural fills and hydraulic barriers, depending on their bentonite content. To facilitate beneficial reuse of foundry sands, typical strength parameters need to be available so that designers can make comparisons with designs employing virgin earthen materials. To provide typical design parameters, a testing program was conducted to characterize the strength of foundry sands and their interaction with geosynthetics. Small-scale direct shear tests, large-scale multistage interface shear tests, and pullout tests were conducted using foundry sands with bentonite contents representing the range normally found in the casting industry and three geosynthetics (geotextile, geogrid, and geomembrane). The results indicate that foundry sands can be used effectively in geotechnical construction. Friction angles of the as-compacted foundry sands generally ranged between 39° and 43°, and the as-compacted cohesions ranged between 17 and 28 kPa. Drained friction angles were similar to as-compacted friction angles except at high bentonite content. Typical interface friction angles ranged between 25° and 35°, with efficiencies ranging between 0.5 and 0.9. Interaction coefficients from the pullout tests ranged between 0.2 and 1.7.     

Plasticity Model for Stress-Release Induced Damage

Stress-release damage in rocks may result in a permanent alteration of the rock properties, which has to be quantified and modeled in order to describe the in situ behavior of the rock based on tests from core specimens. A dual yield surface elastoplastic model is introduced to describe stress-release damage in a synthetic sandstone manufactured as an analogue to a reservoir sandstone. The synthetic sandstone, composed of sand and quartz cement, is cemented under in situ stress conditions. This process allows the quantification of damage during a stress-release that simulates the coring process. In the model, one yield surface describes the behavior of the sand matrix and the other the behavior of the cement, while the total stresses are given as in mixtures theories. The model is calibrated on synthetic sandstone test results and used for back analysis of the experiments.     

Aging of Crushed Glass-Dredged Material Blend Embankments

Four crushed glass (CG) and dredged material (DM) [(CG-DM)] blend embankments constructed (2004) and reconstructed (2005) to local DOT specifications were subjected to cone penetrometer tests (CPT). The CPT resistance of the original set of embankments was evaluated shortly after construction and approximately 360?days later, immediately prior to being demolished for purposes of a second study. Cone tip resistances were observed to double to triple with aging. For the 80/20 CG-DM blend, a 4?MPa [40 tons per square foot (tsf)] or threefold increase in CPT tip resistance was measured. Likewise, isotropically consolidated, undrained triaxial shear tests were performed on relatively undisturbed thin-walled tube specimens of the 360-day aged CG-DM blend materials. The triaxial tests revealed that the effective friction angles of the aged materials increased by up to 8° over freshly prepared laboratory CG-DM blend specimens. The strength gains appeared to be more strongly linked to (amorphous) silica cementation rather than the formation of carbonates. Disturbance (demolition and reconstruction) generally reduced the in situ CPT behavior to that of the originally constructed embankments.     

Effect of Loading Mode on Strain Softening and Instability Behavior of Sand in Plane-Strain Tests

Experimental data to study the effect of loading mode on the strain softening and instability behavior of sand under plane-strain conditions are presented in this paper. A new plane-strain apparatus was adopted to conduct K0 consolidated drained and undrained tests under both deformation-controlled and load-controlled loading modes. The drained behavior of very loose and medium dense sand and the undrained behavior of very loose sand under plane-strain conditions were characterized. The test results show that the loading mode affects the postpeak behavior and controls whether strain softening or instability will occur in the postpeak region. Shear bands occurred in tests conducted on medium dense sand, but not in tests for very loose sand. The failure line and critical state line are not affected by the loading mode. The study also shows that the concept of a unique “ultimate state” for both dense and loose sand as previously established based on conventional drained triaxial tests is not supported by the plane-strain data.     

Penetration Resistance of Offshore Skirted Foundations and Anchors in Dense Sand

Penetration of skirts is an essential design issue for offshore skirted foundations and anchors in sand. Skirts may not penetrate far enough into dense sand by the available submerged weight alone. It may therefore be necessary to apply underpressure inside the skirt compartment to produce an increased driving force and to reduce the penetration resistance. This paper recommends procedures to calculate penetration resistance and required underpressure for skirts penetrated in dense sand with and without interbedded clay layers. The recommendations are based on interpretation of skirt penetration data from prototypes, field model tests, and laboratory model tests in dense sand. The paper first presents a model to calculate the penetration resistance of skirts penetrated by weight, or other external vertical load that does not cause flow of water in the sand. Two models are considered; one based on bearing capacity equations with friction angles from laboratory tests, and the other one based on empirical correlations with CPT tip resistance. The bearing capacity model gives more consistent correlations with the empirical data than the CPT model. Thereafter, a model to account for the effect of underpressure applied inside the skirt compartment is proposed. This model is developed based on interpretation of available prototype and model test data from skirts penetrated by underpressure. The results show that underpressure facilitates skirt penetration in sand considerably by providing both an additional penetration force and a reduced penetration resistance. It is also shown that interbedded clay layers can prevent flow of water through the sand and eliminate the beneficial reduction in penetration resistance.     

Properties and Performance of a Pulverized Fly Ash Grout

A laboratory investigation was conducted in order to develop a new grout based on fly ash produced in Greece. Ptolemaida fly ash was selected because of its hydraulic properties and was pulverized (Blaine specific surface over 8,300 cm2/gr, D15 = 1.3?μm, D50 = 6?μm, and D85 = 20?μm) in order to improve its groutability and its hydraulic activity. Pulverized fly ash (PFA) suspensions with selected additives have properties comparable to those of ordinary and microfine cement suspensions. Clean sands were injected using two specially constructed devices. Hydraulic conductivity, unconfined compression, and UU and CU-PP triaxial compression tests were conducted on grouted sand specimens. Coarse sands can be grouted effectively with PFA suspensions. Conventional groutability ratios were found to overestimate the groutability of these suspensions. Grouting with PFA suspensions reduces sand hydraulic conductivity by up to seven orders of magnitude and yields unconfined compression strength values up to 3,000 kPa. The Mohr–Coulomb failure criterion represents the behavior of grouted sand with cohesion values ranging from 280 to 450 kPa and angle of internal friction slightly higher than that of the sands.     

Direct Shear Test of Sandstone-Concrete Joints

Understanding the shear behavior of sandstone-concrete joints is important for the design and analysis of concrete structures interacting with sandstone, for example rock socketed piles, rock anchors, and dam foundations. This paper presents the results of a laboratory investigation into the shear behavior of Sydney Hawkesbury sandstone-concrete joints with unbonded interfaces. Joint roughness has been simulated using regular triangular asperities and fractal profiles. The tests were carried out in a large direct shear machine (with sample size up to 600 mm in length) under a range of constant normal stiffness and initial normal stress conditions. The tests showed that significant wear of the sandstone surface occurs during shear displacement, and this wear has a significant affect on the behavior of the joints. The laboratory modeling and test results are briefly described, followed by a discussion of the shear behavior of the joints.     

State-Dependent Strength of Sands from the Perspective of Unified Modeling

This paper discusses the state-dependent strength of sands from the perspective of unified modeling in triaxial stress space. The modeling accounts for the dependence of dilatancy on the material internal state during the deformation history and thus has the capability of describing the behavior of a sand with different densities and stress levels in a unified way. Analyses are made for the Toyoura sand whose behavior has been well documented by laboratory tests and meanwhile comparisons with experimental observations on other sands are presented. It is shown that the influence of density and stress level on the strength of sands can be combined through the state-dependent dilatancy such that both the peak friction angle and maximum dilation angle are well correlated with a so-called state parameter. A unique, linear relationship is suggested between the peak friction angle and the maximum dilation angle for a wide range of densities and stress levels. The relationship, which is found to be in good agreement with recent experimental findings on a different sand, implies that the excess angle of shearing due to dilatancy in triaxial conditions is less than 40% of that in plane strain conditions. A careful identification of the deficiency of the classical Rowe’s and Cam-clay’s stress–dilatancy relations reveals that the unique relationship between the stress ratio and dilatancy assumed in both relations does not exist and thereby obstructs unified modeling of the sand behavior over a full range of densities and stress levels.