Advances in Transportation Geotechnics and Materials for Sustainable Infrastructure

Understanding of various natural phenomena and industrial processes such as landslides and bulk material handling requires research on the flow of granular materials. In this study, a 3-D numerical investigation of axisymmetric collapse of granular columns has been conducted using a commercial and research discrete element method (DEM) code. The simulated granular columns have an initial radius and height of 5.68 mm and 5.71 mm, respectively, rendering an initial aspect ratio (height/radius) of approximately 1.0. The columns consist of uniform spherical quartz particles with a diameter of 0.32 mm. The nonlinear particle-particle contact behavior and rotational resistance are indirectly accounted for in the DEM model. Comparison between the simulated final deposit morphology (e.g., final height and runout distance) with experimental results that are readily available in literature reveals the importance of rotational resistance in realistic simulations of granular column collapse.

In this study, a numerical code has been developed using free lossless audio codec (FLAC) 2-D to model the prefabricated vertical drain (PVD) assisted preloading process considering the smear zone, and evaluate the efficiency of the proposed equations for the conversion of permeability coefficient from axisymmetric state to plane-strain condition. A laboratory PVD assisted preloading test has been conducted employing a fully instrumented large Rowe cell to verify the developed numerical code. The results of the numerical plane-strain and axisymmetric simulations have been compared using four methods of permeability conversion from axisymmetric to plane-strain condition.

Climatic factors play a significant role in pavement design due to the impact of these parameters on pavement performance. Thornthwaite moisture index (TMI), a climatic parameter, is widely used in geotechnical engineering, as well as other disciplines, to evaluate the changes in moisture conditions in near surface soils in the unsaturated zone. This paper evaluates historical climatic data acquired from Mesonet weather stations across Oklahoma for pavement applications. Based on the TMI values calculated from the historical climatic data by applying three different models, contour maps have been created using ArcGIS software. This paper also builds the relationships between equilibrium suction and TMI values in subgrade soils and compares the results among three TMI models.

Red soils with white net-like veins occur widely along the south of middle to lower reaches of the Yangtze River in China. Up to the present few studies paid attention to hydraulic and unsaturated mechanical aspect in spite of many investigations focusing on age, distribution, and climate significance. However, the soil-water characteristics are critical for understanding the engineering features. Herein, microstructure features of undisturbed red soils with white net-like veins taken from Chenshan laterite section in Xuancheng, China were analyzed by scanning electron microscope (SEM). Also, the soil-water characteristics curves (SWCCs) were discussed by means of unsaturated GDS triaxial stress path system and numerical simulation. The SWCC test of axial pressure 510kPa and radial pressure 505kPa was conducted. The Gardner model, Van Genuchten model, and Fredlund model were used to simulate the measured drying cycle of SWCC. The results of SEM tests showed that microstructures and micropores are of obvious anisotropy, and the measured SWCC of the net-like red soil during wetting process behaves hysteretic characteristics compared to that during drying process. The computed results indicated that the simulation value of Fredlund model has good agreement with measured value.

A model is developed for computing the expansive soil crack depth in this paper. By analyzing the process of moisture content changes, it is considered that the soil suction will increase when the moisture content changes, then the expansive soil contraction deformation occurs, but cracks will not appear until the soil suction arrives at the tensile strength. Under wetting-drying cycles, the soil strength reduces, the tensile strength and crack depth change, the effect of moisture content changing rate on matrix suction is analyzed, and the crack depth computation module is attained. The computation module applicability is validated through testing. The achievement of the model is important to determine the depth of water delivery and subgrade engineering.

Electrical resistivity is measured simultaneously during unconfined compression (UC) tests of undisturbed, reconstituted, and artificially cemented loess specimens. Test results indicate that both artificially cemented and undisturbed loess specimens are of typical structural characteristics. The stress-strain relationship of structured loess during unconfined compression test is actually a multistage development process, and it can be characterized by the variation of corresponding electrical resistivity, which thereby provides a potential monitoring method of soil behavior and failure under compression. In addition, the secant slope of electrical resistivity may be adopted to evaluate the soil structure of loess specimens. The relationship between the secant slope of electrical resistivity and vertical strain can be fitted well with the function of exponential growth.

The Atterberg Limits are the most common tests specified by practicing geotechnical and pavement engineers the world over. They are used to classify soils using the framework pioneered by Terzaghi and Casagrande during their work with the U.S. Public Roads Bureau in the 1920s and 1930s and are also correlated with many fundamental soil parameters used in design and construction projects. In the 21st century the Atterberg Limits remain a key component of the testing armory of practicing geotechnical engineers, as they can be obtained easily for the large numbers of samples that are needed on major construction projects and allow for rapid assessments of key soil parameters. Their fundamental definitions are worthy of review; the thread-rolling test for plastic limit has remained largely unchanged since Atterberg first described it in 1911, but the definition and measurement of liquid limit varies across the globe. The fundamental mechanics of the Casagrande Cup liquid limit have been the subject of recent study showing clearly that liquid limit determined in this way relates to a fixed value of specific soil strength (i.e. strength per unit density) as opposed to a fixed strength value when liquid limit is measured by the fall-cone method. These findings explain the deviation between liquid limits measured by the two methods for high plasticity soils without the need to invoke different strength regimes. The brittle failure mechanism in the thread-rolling test has also been recently re-examined. It is proposed that the brittle failure observed in the plastic limit test is caused by either air entry or cavitation in the clay and plastic limit and does not correspond to a fixed strength. The Atterberg Limits are used to compute liquidity index which is widely related to clay strength variation, this is critical for many areas of construction (especially when rapid assessments of strength are required). The Russian code for the design of piled foundations, for example, uses liquidity index values to assess shaft friction. Recent research outcomes at the University of Cambridge have challenged certain assumptions pertaining to widely used correlations between liquidity index and undrained strength.

Soil suction is one of the stress state variables describing the behavior of unsaturated soils. Soil suction has found its applications in all branches of the unsaturated soil practice, from volume change predictions of expansive soils to slope stability problems. There are different methods to measure the total suction in a soil. Chilled mirror dew point technique is one of the methods to measure the total suction. The WP4 device utilizes the chilled mirror dew point method and measures the suction by determining the relative humidity of the air above a soil sample. This study investigates the capability of the particular WP4 device by developing a characteristic curve. The characteristic curve is developed by measuring the osmotic suctions of salt solutions of NaCl and KCl at different concentrations covering a wide range of suction. The results show that the device is good only in measuring suctions more than 4 pF as the measured values at lower suctions is always a constant value (1.79 pF). A comparison study is also conducted on soil samples using the filter paper method and WP4 device. The results of the WP4 and filter paper method are in good agreement at high suctions. Because the proposed one-point calibration check by the manufacturer might not be sufficient, there is a need to develop a unique characteristic curve for each device.

This paper presents an experimental investigation of internal sand deformation around a scaled tunnel boring machine using transparent soil. Soil deformation and its control is always a critical issue, particularly in urban environments, for protecting adjacent properties and services during tunnel construction. Visualization of an internal soil deformation will improve our understanding of the influence of tunneling as most deformation measurement available are limited to ground surface settlement due to the opacity of natural soils. A new kind of transparent soil is used in this study, which is made of fused silica and a calcium bromide solution. An optical setup is developed to consist of a laser, a camera, and a computer. The laser is used to illuminate the targeted section around the scaled shield machine. A series of laser speckle images are captured during shield driving. The digital image correlation method is used to calculate the relative displacement between two consecutive images. Two model tests are performed with an overburden cover varying from once to twice the tunnel diameter. The results show that soil deformation changes with tunnel depth increases. The influence zone is changing from a rectangle over a reversed trapezoid shape in the shallower tunnel to a bell over a trapezoid shape restrained within soil mass in the deeper case. The longitudinal deformation extends to ground surface in the shallow cover case, whereas the influence zone is confined within the soil mass in the deep cover case.

Frost heave significantly affects the graded gravel used to construct passenger-dedicated railway lines. For safety reasons, such lines have strict demands on the amount of deformation that can occur to tracks. The aim of this study was to investigate the geotechnical properties of graded gravel that was grouted with slurry. After grouting, scanning electron microscopy showed that the slurry bound well to the gravel particles. With increasing iterations of freeze-thaw cycles, the frost heave ratio increased, but it did not change significantly after 20 freeze-thaw cycles. In addition, changes in the unconfined compression strength were also negligible after 20 freeze-thaw cycles, and they tended to stabilized after 10 dry-wet cycles. These results show that graded gravel samples stabilized with slurry exhibited high resistance to the effects of freeze-thaw cycles and dry-wet cycles. We concluded that slurry can be successfully used to improve frost heave associated with the graded gravel of passenger-dedicated railway line.

A new methodology called fault-implied composite element is used to simulate complex geological faults in large-scale underground caverns to solve the difficulty in modeling. In the process of modeling, the rock and soil mass is meshed first without considering the influence of geological faults, then faults are plotted according to their locations and occurrences in the space and implicitly comprised in solid elements. During simulation and computation, the composite elements comprising rock material and fault material are equally treated according to the influence degree of faults, which not only can make the numerical discretization of underground caverns simpler but also effectively reflect the effect of faults on the stability of surrounding rock of underground caverns. The elements passed by faults are regarded as a transversely isotropic medium, so the anisotropic elastoplastic damage finite difference method is used to calculate the element stresses. A secondary development is carried out on free lossless audio codec (FLAC) 3-D software platform, and applied in the underground caverns with complex faults in a hydropower station. The calculation result shows that this method provides a very effective idea of modeling and analysis for complex faults.

Hydraulic properties of unsaturated soil are the key factor for reservoir landslide seepage analysis. Conventional method is mainly depended on time-consuming, laborious, and expensive hydraulic tests. A methodology by back analysis was proposed in this paper according to real-time monitoring data of pore water pressure of No.1 Tangjiao Landslide in the Three Gorges area. A series of numerical simulation experiments were designed by orthogonal experiment theory. And the hydraulic properties in Fredlund model were optimized and selected, which was a=10kPa, n=2.4, m=1.5, k_s=0.5×10^-6m/s. To validate the proposed method, the four parameters were taken as inputting hydraulic properties to simulate the landslide seepage with reservoir water fluctuation. And the range of relative error was -0.39% ~ 1.05% between the monitoring and simulated data. That proved that this back analysis method could be used as a feasible and effective technique for predicting hydraulic properties of landslide soil.

This paper focused on the disintegration behavior of remolded granite residual soil. A new apparatus was designed to do the disintegration tests. By conducting a series of disintegration tests with different dry densities and different water contents, the disintegration features and basic patterns were observed and measured. It was found that there were two stages in the disintegrating process. In the first stage, soil disintegrated rapidly, lasting minutes. In the second stage, soil was further disintegrated at a remarkably low velocity. We also learned that the increase of initial water content can accelerate the collapse of the soil. The basic features and patterns of the granite residual soils at different conditions were summarized based on the test results.

This paper investigates change of cyclic and postcyclic shear behavior of low-plasticity silt due to addition of clay. Bentonite was added into low-plasticity Mississippi River Valley (MRV) silt with plasticity index (PI) of 5.8, and a series of triaxial tests were conducted in the laboratory. With a higher PI, the excess pore pressure developed more slowly with cyclic loading. Opposite to MRV silt, the silt-bentonite mixture (PI = 6.2) showed cyclic softening rather than initial liquefaction. It showed higher cyclic shear strength rather than MRV silt. After cyclic loading, the reconsolidation behaved more like a compression process for both MRV silt and its mixture with bentonite. With reconsolidation, the shear strength of postliquefaction specimen was increased largely compared to that without previous cyclic loading. Without reconsolidation, there was a more obvious drop of shear strength for MRV silt than for MRV silt-bentonite mixture. This paper suggests that the behavior of MRV silt transforms from intermediate material to clay-like material at a PI of 6.0. However, for engineering practice, the PI seems to be not a good factor to identify the transformation of soil behavior, because small changes in PI produces significant transformation of soil behavior and is difficultto capture in engineering practice and even the laboratory.

The diffusion coefficient is an important unsaturated soil parameter for the estimation of the moisture movement in the soil mass. The diffusion coefficient can be measured in a laboratory on Shelby tube soil specimens subjected to specified boundary conditions using thermocouple psychrometers. This parameter can also be estimated by using the soil water characteristic curve (SWCC) and saturated permeability coefficient of the soil. This research paper presents the results of the SWCC and permeability tests on an unsaturated soil and predicts the diffusion coefficient. Although there is significant difference between the diffusivity parameters obtained using the SWCC and permeability test results, the difference between the column drying test and the SWCC and permeability test results is substantial. The diffusivity coefficients obtained from the psychrometer is 10³ times higher than those from the SWCC and permeability tests.

Vacuum preloading is a common method to improve soft, fine-grained soil. While traditional surcharge preloading increases effective stress by increasing total stress, vacuum preloading increases effective stress by reducing pore pressure. This reduces the risk of shear failure, which can be a concern when using surcharge preloading on very weak soil. This paper presents a numerical model for large strain consolidation under combined vacuum and surcharge preloading. The model is based on a readily available radial consolidation model, known as RCS1, which is able to account for vertical and radial flow, soil self-weight, unload/reload effects, time-dependent loading, hydraulic conductivity anisotropy, soil smear, and partial prefabricated vertical drain (PVD) penetration. Additional capabilities of vacuum-induced consolidation are added to the original RCS1 to consider combined vacuum and surcharge loading. The developed model is utilized to simulate a well-published case history of combined vacuum and surcharge preloading at the Second Bangkok International Airport. Good agreement is observed between the simulated and recorded settlement vs. time and excess pore pressure versus time.

In this paper, mechanics characteristic of whole weathered granite improved soil is studied through an indoor experiment. The changes of fluid plasticity index before and after improvement in different compaction conditions are analyzed. The full weathering granite is improved through adding different proportions of lime and cement. By liquid plastic limit test, the rules of changes on improved soil's liquid plastic limit are analyzed in different mixed ratio conditions. It can be concluded that the improved soil's plasticity index decreases with lime or cement dosage increasing and changes over the time. The liquid limit, plastic limit and the plasticity index of improved soil changes with the addition of lime and cement dosage. The relationship of unconfined compressive strength with the lime and cement dosage, plastic limit, and the plasticity index is the polynomial relationship.

With the development of the construction of hydropower engineering, the change rule of physical and mechanical properties of rock under water-rock interaction become a hot research topic. However, the change rules of longitudinal wave velocity and rebound value of rock were less involved in the past. Based on that, the test was designed to analyze longitudinal wave velocity rebound value and uniaxial compressive strength of rock sample under different stages of water-rock interaction. Test results showed that degradation trend of longitudinal wave velocity and rebound value of rock sample were obvious and consistent and had good correlation with uniaxial compressive strength under circulating action of immersion-air dry, based on which the comprehensive empirical formula of uniaxial compressive strength, longitudinal wave velocity, and rebound value was established. Test results show the said empirical formula is credible. Therefore, changes of mechanical parameters and properties can be speculated by changes of physical parameters and those are easily and conveniently tested.