Louis Ge

Cyclic Behaviors of Railroad Ballast within the Parallel Gradation Scaling Framework

Because of the large grainsizes typical of railroad ballast, large triaxial samples are required to assess the reactions of these materials. The parallel gradation modeling technique was originally developed by John Lowe in 1964 to allow assessment of large grain-size geomaterial properties in smaller, more typical testing facilities. Emphasis has focused on monotonic loading, in which the material is progressively loaded to failure. Cyclic testing of this model has been absent. This paper presents an investigation of the possibility of using the parallel gradation modeling technique in a cyclic triaxial testing framework. Three separate gradations of ballast material were used in this research. The largest gradation contains a top particle size of 63.5 mm (2.5 in.) and is marketed as #3 modified railroad ballast. The second two gradations contained a top size of 38 mm (1.5 in.) and 19 mm (3∕4 in.), respectively. Up to 10,000 load cycles were applied for each test. Resilient modulus, permanent axial, volumetric strain, and particle shape were determined from the test results. It is concluded that applying parallel gradation technique to cyclic behavior characterization should be cautious. If particle shape is not consistent throughout the particle sizes used in the parallel gradation model, the model is invalid in the cyclic triaxial framework. DOI: 10.1061/(ASCE)MT.1943-5533 .0000460.

Factors Influencing Crack-Induced Tensile Strength of Compacted Soil

Mode I (tensile) fracture is the most commonly observed failure in geostructures resulting from cracks in soil. Direct or indirect tensile tests have been used to evaluate the tensile strength of geomaterials. In this paper, the unconfined penetration device and experimental procedure were modified to reduce measurement errors. It was then used to determine the tensile strength of compacted soil. Factors influencing the tensile strength of the compacted soil, including the plasticity index, rate of loading, and size of specimen were discussed in detail, as well as its reliability. Experimental results indicated that the modified, unconfined penetration technique is sufficiently reliable and operator-friendly for determining the tensile strength of compacted soil.

Low Viscosity Pore Fluid to Manufacture Transparent Soil

Transparent soil has been investigated for its potential as a substitute research media for natural soil. The mechanism for manufacturing the transparent soil is through adding an appropriate pore fluid to silica gel particles with the same refractive index. Two types of high viscosity pore fluids were identified by Iskander in 1994. However, because of the high viscosity of these two pore fluids, air was easily entrapped, which made the manufacture of a large mass of transparent soil difficult. In addition, the identified pore fluids caused serious membrane deterioration during triaxial laboratory testing. This research presented herein is an experimental investigation on low viscosity pore fluids to manufacture transparent soil, including the fluid/particle interaction in the stimulant matrix. Two low viscosity pore fluids were identified with minimum interaction with latex membranes.

Chemically Stabilized Soft Clays for Road-Base Construction

AbstractSoft clays are widely distributed in Missouri, United States. Due to their relatively low strength and high compressibility, subgrade construction in soft clays has encountered many difficulties. In recent practice, the use of fly ash (FA) along with lime to tackle soft subgrade problems has shown promising results. The effectiveness of Class C FA and lime kiln dust (LKD) in clay subgrade stabilization is examined in this research. Scanning electron microscopic (SEM) analysis, proctor compaction tests, unconfined compression tests, and resilient modulus tests were carried out on the FA and LKD modified soil mixtures. Test specimens were prepared at optimum water content and tested at various curing periods. The test specimens were reconstituted by static compression. Test results revealed that the addition of Class C FA could increase the dry unit weight of the FA treated soil, enhance the unconfined compressive strength, and improve the resilient modulus. Regression equations were developed to co...

Slope creep behavior: observations and simulations

Rock slopes undergoing long–term effects of weathering and gravity may gradually deform or creep downslope leading to geological structures such as bending, bucking, fracturing, or even progressive failure. This study uses geomechanics-based numerical modeling to qualitatively explain the cause and evolution of slope creep behavior. Constitutive models used include the creep, Mohr–Coulomb, and anisotropic models. The last two models are used with the strength reduction in calculation. First, the results of field investigation around a landslide site occurring in slate are present. The causes and modes of creep structures observed on slopes and underground are studied. Second, the study investigates the influences of slope topography and anisotropy orientations on slope creep behavior. Finally, progressive failure of slopes with different shapes is examined. The simulated results show that the bending type of structures develops near slope surfaces, and the buckling type of structures is associated with the deformation or slides of a slope. The creep pattern varies with the orientation and position of an original planar structure. The shear zone involves a joint or fracture along which displacement has taken place. Moreover, creep behavior is more significant on slopes with greater height and inclination as well as on steeper portions whether on concave or convex slopes. In addition, with the same topographic conditions, consequent slopes with coinciding cleavage and obsequent slopes with steep cleavage result in greater creep behavior. Without the effects of anisotropic cleavage, concave and straight slopes develop failure surfaces from the crowns downwards, whereas convex slopes develop failure surfaces from the toes upwards.

A Large-Scale Triaxial Apparatus for Prototype Railroad Ballast Testing

Shear strength and compressibility of large grain-size materials are critical parameters for the geotechnical design of road bases, rock fill embankments, and railroad sub-base. However, due to the correspondingly large scale of triaxial specimens necessary for rock-fill and railroad ballast material testing, the numbers of facilities that are capable of testing these materials are few. In this paper, a cost effective design is documented for a triaxial apparatus capable of testing prototype railroad ballast material containing particle sizes up to 63.5 mm (2.5 in.). Unique to this testing apparatus is the use of vacuum as confinement to allow an unobstructed digital image measurement of specimen volume change during testing. The specimen preparation methodology, manufacture of latex membrane, and instrumentation are also discussed. Finally, the results of a cyclical triaxial test are presented to demonstrate the quality of the testing data from this triaxial apparatus.

Investigation of the Thermal Conductivity of Compacted Silts and Its Correlation to the Elastic Modulus

Thermal conductivity measurement is a quick and easy test for compacted soils. This study presents the results of thermal conductivity testing on silt compacted under standard and modified Proctor efforts. A comparison of moisture content measurements from both lab oven and field oven is also presented and discussed. Measured thermal conductivity was correlated to the bulk density and water content. It is further correlated to Youngs modulus and Shear modulus of the compacted silt, which were determined through ultrasonic pulse velocity measurements. Several trends were observed which may be used to correlate soil thermal conductivity of the compacted silt to compaction and elastic moduli for quality control/quality assurance of compacted soil.

Modified Time of Setting Test for Fly Ash Paste and Fly Ash–Soil Mixtures

A survey of current literature has revealed that the testing protocol for determining the time of setting of fly ash-soil mixtures is not available. Although the current ASTM standard recommends using the Vicat needle method with a water-to-fly ash weight ratio of 0.35, it was originally proposed for testing the time of setting of cement. The current study showed that the Vicat needle method yielded poor test results for some common fly ashes. Moreover, the water-to-fly ash weight ratio of 0.35 may not be appropriate for some fly ashes because the chemical components are varied in different fly ashes. Therefore, a British fall cone was adopted to determine the time of setting of fly ash paste and fly ash-soil mixtures. It suggests that the best starting water-to-fly ash weight ratio should be at its liquid limit. As the water content increases, the initial and final times of setting of fly ash paste increase accordingly. For the fly ash-soil mixture, the initial and final times of setting decrease as the ash-to-soil weight ratio increases. Finally, a linear correlation was established based upon the water content, ash-to-soil weight ratio, and the ratio of calcium oxide to silicon dioxide present in the fly ash. DOI: 10.1061/(ASCE) MT.1943-5533.0000604.

Effect of Parallel Gradations on Crushed Rock-Concrete Interface Behaviors

The response of a soil-structure system subjected to monotonic or cyclic loading is significantly influenced by the mechanical behavior of its interface. In this paper, the friction characteristics of crushed rock-concrete interface and the role of the crushed rock deformability were examined through a series of monotonic and cyclic direct shear tests under the framework of parallel gradation technique. Two parallel gradation curves of the crushed rocks were used. All the tests were carried out in dry condition and with two initial void ratios representing both loose and dense states. Two concrete interfaces of different roughness were used in the interface direct shear tests. The static test results show that parallel gradation technique can be used to characterize the residual shear strength but not the volume change. Moreover, under cyclic loading, it can neither yield the same volume change, nor the same shear strength. The critical state of the soil interfaces was also studied, which shed some light on the mechanism of pile skin friction mobilization and modeling of soil-concrete interface behaviors.

Modeling Permanent Deformation of Unbound Granular Materials under Repeated Loads

Finite-element analysis on a pavement structure under traffic loads has been a viable option for researchers and designers in highway pavement design and analysis. Most of the constitutive drivers used were nonlinear elastic models defined by empirical resilient modulus equations. Few isotropic/kinematic hardening elastoplastic models were used but applying thousands of repeated load cycles became computationally expensive. In this paper, a cyclic plasticity model based on fuzzy plasticity theory is presented to model the long-term behavior of unbound granular materials under repeated loads. The discussion focuses on the model parameters that control long-term behavior such as elastic shakedown. The performance of the constitutive model is presented by comparing modeled and measured permanent strain at various numbers of load cycles. Calculated resilient modulus from the complete stress-strain curve is also discussed.