Tong Qiu

Numerical Simulations for Large Deformation of Granular Materials Using Smoothed Particle Hydrodynamics Method

AbstractApplication of the smoothed particle hydrodynamics (SPH) method to the simulation of granular materials under large deformation is presented. The Drucker–Prager constitutive model with nonassociated flow rule is implemented in the SPH formulations to model granular flow in a continuum framework. The model developed is validated by experiments on the collapse of two-dimensional granular columns as reported in the literature. Simulations of the collapse of three-dimensional axisymmetric sand columns with various aspect ratios are also conducted. Numerical results of the granular flow pattern, final runout distance, final deposit height, and nondeformed region are in good agreement with the experimental observations as reported in the literature. It is suggested that despite being a continuum-scale model, the SPH model developed can be used to effectively simulate large deformation and dense flow of granular materials, and geomaterials in general, if proper constitutive models are implemented. The mo...

Simulation of Collapse of Granular Columns Using the Discrete Element Method

AbstractIn this study, a three-dimensional (3D) numerical investigation of axisymmetric collapse of granular columns has been conducted using the discrete element method (DEM). The simulated granular columns have a constant initial radius of 5.68 mm and three aspect ratios: 0.55, 1.0, and 2.0. The columns consist of uniform spherical quartz particles with a diameter of 0.32 mm. In the DEM model, rotational velocities of particles are reduced by a factor at every time step to partially account for the additional rolling resistance due to the effect of particle shape and hysteretic contact behavior. The simple linear contact model is used; however, its performance is improved by using different stiffness values calculated by nonlinear Hertz–Mindlin contact model for each aspect ratio. The simulated final deposit heights, runout distances, and energy dissipation values are in good agreement with experimental observations reported in the literature. The effects of initial porosity and rotational resistance on...

Model Tests by Centrifuge of Soil Nail Reinforcements

Soil nailing is an in situ technique to reinforce slopes in the vadose zone where the soil is partially saturated. In this research, model tests of soil nailing on steep cuttings of an unsaturated silty clay were performed using a centrifuge. The results show that soil nailing can greatly increase the stability of cuttings. The length and density of soil nails have significant effects on the deformation behavior and failure mechanism of the soil-nailed structure. When the ratio between the maximum nail length and the excavation depth (L/H) was 0.32 or 0.34, the model slope was unstable with a prototype spacing less than 1.8 m. However, when the L/H ratio was 0.48 or 0.80, the model slope was substantially stable against global failure under the same spacing. External failure occurred in nailed structures with densely reinforced shorter nails; the prototype spacing was less than 1.29 m and the length ratios were 0.32 and 0.34. Internal failure occurred in nailed structures with sparsely reinforced longer nails: the prototype spacing was 3.6 m and the length ratio was 1.0. The failure surfaces of the nailed cuttings were deeper than that of the cutting without reinforcement.

Analytical Solution for Biot Flow―Induced Damping in Saturated Soil during Shear Wave Excitations

This paper presents a theoretical study of Biot flow–induced damping in saturated soil during shear wave excitations. The solid skeleton is treated as equivalent linear. Biot flow–induced damping is evaluated for the cases of resonant column tests and site response analysis, based on the spectral response of a soil column/layer under harmonic torsional/horizontal excitations. Closed-form analytical solutions indicate that Biot flow–induced damping is hydraulic conductivity and frequency dependant. At the first resonance in a resonant column test, Biot flow–induced damping is dependent on a dimensionless hydraulic conductivity parameter K. For K within the range of 0.01 to 100, corresponding to coarse sands and clean gravels, Biot flow–induced damping may have an important contribution to total soil damping, especially at small strain levels. For site response analysis, Biot flow–induced damping should be considered for coarse sands and clean gravels, but can be practically neglected for fine sands, silts,...

Effective soil density for small-strain shear waves in saturated granular materials

AbstractThis paper presents an experimental investigation of the concept of effective density for the propagation of small-strain shear waves through saturated granular materials. Bender element tests and resonant column tests were conducted on various granular materials in dry and saturated conditions. Values of small-strain shear modulus for the dry condition are compared with corresponding values for the saturated condition, which are calculated using saturated density and effective density. For bender element tests, the use of saturated density produced errors as high as 28% in the shear modulus, whereas the use of effective density resulted in errors generally less than 5%. For resonant column tests, errors in the shear modulus that were obtained using saturated density were smaller than those for bender element tests because of the lower range of excitation frequency and effect of mass polar moment of inertia of the loading system. A quick chart is provided to help users determine if effective densi...

Field and Laboratory Investigation of Pullout Resistance of Steel Anchors in Rock

AbstractThis paper presents a field and laboratory investigation of pullout resistance of steel anchors in rock. The field testing involved pullout tests of six steel anchors installed in rock by gravity grouting and pressure grouting. The laboratory testing involved unconfined axial compression tests of six grout specimens prepared in custom-made molds using the two grouting methods to investigate the effect of grouting method on the mechanical properties of grout. The pressure-grouted specimens had a higher density and compressive strength, which was a result of their denser microstructure with significantly fewer and smaller voids from air bubble inclusions than the gravity-grouted specimens. The field pullout tests suggest a progressive failure mechanism, which was manifested through the progressive elongation of the bond zone. The pressure-grouted anchors yielded higher pullout resistance than did the gravity-grouted anchors. The increase in rock-grout bond strength from pressure grouting is likely d...

Evaluation of Granular Soil Abrasivity for Wear on Cutting Tools in Excavation and Tunneling Equipment

AbstractThis paper presents an experimental study of tool wear and abrasivity of granular soils using a testing system specifically designed for the evaluation of wear on earth-moving machines, especially soft ground tunneling applications where the impact of soil abrasion on the operation is significant. In the testing system, a propeller fitted with steel covers of different hardnesses is rotated at 60 rpm in granular soil samples in a chamber under ambient pressures of up to 10 bar. The developed system can quantitatively assess the abrasive characteristics of soils through the measurement of weight loss on the special covers of the propeller, accounting for soil gradation, mineral composition, sphericity and roundness, water content, and tool hardness. Preliminary test results indicate that water content, particle angularity, and relative hardness between the tool and soil have significant impact on tool wear and soil abrasion. For the tests conducted, it is observed that angular sands produce signifi...

Integrated monitoring of wind plant systems

Wind power is a renewable source of energy that is quickly gaining acceptance by many. Advanced sensor technologies have currently focused solely on improving wind turbine rotor aerodynamics and increasing of the efficiency of the blade design and concentration. Alternatively, potential improvements in wind plant efficiency may be realized through reduction of reactionary losses of kinetic energy to the structural and substructural systems supporting the turbine mechanics. Investigation of the complete dynamic structural response of the wind plant is proposed using a large-scale, high-rate wireless sensor network. The wireless network enables sensors to be placed across the sizable structure, including the rotating blades, without consideration of cabling issues and the economic burden associated with large spools of measurement cables. A large array of multi-axis accelerometers is utilized to evaluate the modal properties of the system as well as individual members and would enable long-term structural condition monitoring of the wind turbine as well. Additionally, environmental parameters, including wind speed, temperature, and humidity, are wirelessly collected for correlation. Such a wireless system could be integrated with electrical monitoring sensors and actuators and incorporated into a remote multi-turbine centralized plant monitoring and control system.

Field Tests and Numerical Modeling of Vehicle Impacts on a Boulder Embedded in Compacted Fill

Landscape Vehicular Anti-Ram (LVAR) systems are a group of protective barriers, which are designed using natural materials (e.g., boulders) and have proven to both effectively protect sensitive structures against threats and be aesthetically pleasing. This paper presents two consecutive vehicular crash tests hitting the same single boulder embedded in AASHTO coarse aggregate fill. A LS-DYNA model was developed to simulate the field-scale tests, which were instrumented with high-speed cameras and pressure cells. A readily available truck model from the National Crash Analysis Center was modified and implemented in the LS-DYNA model. The boulder and surrounding soil were modeled using the Mohr-Coulomb failure criteria. The model parameters were calibrated using results from the first field-scale test with a truck traveling at 48.3 km/hr (30 mph) impacting the LVAR system. The calibrated model was then used to simulate the second field-scale test, which involved a truck traveling at 80.5 km/hr (50 mph) impacting the same LVAR system without resetting the boulder or soil. The calibrated model was able to provide the global response of the system, including the time-history of the translational displacement and rotation of the boulder, and was in good agreement with field-scale test results. This suggests that the overall global response was dominated by the dynamic behavior of the truck and boulder system upon impact. Hence, a simple material model for soil and boulder is sufficient for simulating the tests conducted.

Evaluation of Nondestructiveness of Resonant Column Testing for Characterization of Asphalt Concrete Properties

The resonant column (RC) test has been used as a nondestructive test (NDT) to study dynamic properties of soils for the past few decades. With some modifications, this test can also be employed to characterize properties of asphalt concrete, especially because these properties are strongly dependent on the loading frequency. A conventional RC apparatus was retrofitted to characterize asphalt concrete properties at a range of temperatures from 10°C to 45°C. The RC test is believed to be nondestructive for most soils; however, this must be verified in case of testing asphalt concrete, especially at elevated temperatures. For this purpose, the impact resonance (IR) test, as a purely NDT tool, was used to check the integrity of asphalt concrete specimens before and after RC testing. The modulus values measured before and after RC tests, at each of the testing temperatures, were compared to evaluate the nondestructiveness of RC testing. Strain levels were also monitored to ensure that the material remained within the linear elastic range through the tests. The results show that the specimens exhibited the same modulus before and after RC testing over the full range of temperature and frequency sweep tests.