Hon-Yim Ko

A Substitute Pore Fluid for Seismic Centrifuge Modeling

A time-scaling conflict exists between dynamic and dissipative phenomena in seismic centrifuge experiments. Thus, the interpretation of the test results, such as the extrapolation of model performance to prototype scale, is difficult unless the conflicts in the time-scaling relations are first resolved. One way to accomplish this is by slowing the diffusion event by employing a substitute pore fluid, which is more viscous than water (the assumed prototype pore fluid). In this paper, a substitute pore fluid consisting of powdered methylcellulose (known commercially as Metolose) was studied. The suitability of the methylcellulose-water mixtures (metolose) as a substitute pore fluid was examined in an experimental program using water- and metolose-saturated sand specimens. The program included triaxial compression tests, permeability tests, and a seismic centrifuge experiment on level ground models. In addition, modeling of models type experiments were conducted on metolose-saturated embankment and retaining wall models. Results form the triaxial tests indicated that the constitutive behavior of the saturated sand specimens was not significantly altered with metolose as the pore fluid. Results from the permeability tests showed that the scaling requirements of the centrifuge environment were satisfied. The centrifuge experiments demonstrated clearly that the conflict between the dynamic and consolidation time scales exists and reinforced the need for a substitute pore fluid in tests designed to model prototype behavior. Based on this experimental program, metolose was found to be an acceptable substitute pore fluid.

Impact of effective stress on the dynamic shear modulus of unsaturated sand.

The dynamic shear modulus of soils is needed to predict soil behavior in response to cyclic loading. Even though the effective stress has been shown to have a significant impact on the dynamic modulus of water-saturated and dry soils, its effect on the dynamic shear modulus of unsaturated soils has not been evaluated. Specifically, studies on the dynamic response of unsaturated soils have characterized variations in small-strain shear modulus (Gmax) as a function of the degree of saturation or matric suction alone. In contrast, this study evaluates the use of the suction stress characteristic curve to characterize the impact of mean effective stress (’m) on the dynamic shear modulus of unsaturated sand. A fixed-free resonant column test device was adapted with a hanging column setup so that the small-strain dynamic shear modulus could be measured for sand specimens under different confining pressures and matric suction values. Trends between the small strain shear modulus and effective stress for unsaturated sand were found to be different from those reported in the literature, where Gmax varied linearly with the square root of ’m.

Elastic properties of two coals

Abstract The elastic finite-element analysis of coal mine openings requires knowledge of the mechanical behavior of the coal, which is best supplied in the form of material stiffness matrices, or their inverse, the compliance matrices. The elastic properties of two different coal types are presented in the form of compliance matrices in this paper. Coal samples from two sources, one in the Pittsburgh seam of Pennsylvania, the other in the Herrin No. 6 seam of Illinois, were tested by two different laboratory methods to determine complete compliance matrices for three-dimensional stress states. The data are analyzed statistically, results obtained for the two coals by different test methods are compared, and possible idealizations for analysis are discussed. It appears that modeling of coal as an orthotropic material is a reasonable engineering approximation. Although the principal compliances are sufficiently similar to suggest isotropic behavior, the shear compliances are much larger than those obtained from the relations for an isotropic elastic material. Evidence is also presented which casts doubt on the applicability of the elastic model for other than monotonic load histories.

Effectiveness of vibrating bulldozer and plow blades on draft force reduction

A computer-controlled “Vibratory Bulldozer and Plow” testbed was designed and built to evaluate the effectiveness of vibratory earth moving and tilling tools with emphasis on draft force reduction. The testbed consisted of a mechanical bulldozer and plow unit with vibrating blades, sensors and actuators located in a steel box containing the soil sample. The testbed’s control system had the capability to regulate the forces, the frequency of the vibrating actuators, and the longitudinal velocity of the unit. The forces required to push the blade through soil with and without applied vibrations were measured and compared. Tests were conducted on: a scaled bulldozer blade, a moldboard plow, and a chisel plow. Experimental results showed 71 to 93% draft force reductions while applying vibratory motion in the longitudinal direction. These results were verified on several soil types and conditions ranging from dry (0% moisture d.b.) sands to highly cohesive wet clays. The significant force reduction factors suggest that the vibrating blade reduces soil strength by decreasing cohesiveness and effective stress for dry to ductile soils. The frequency dependency of the soil resistance indicates that the mechanical power delivered to the soil is also a function of the frequency.

A fluid cushion, multiaxial cell for testing cubical rock specimens

Abstract The development of a mutliaxial test cell is described. The device utilizes fluid cushions to apply a three-dimensional state of compressive stresses to a 4-in. specimen of rock materials and determines the state of deformation by means of proximity detection devices mounted exterior to the specimen. The completely flexible cushions transmit uniform boundary loadings to the surface of the specimen and allows it to develop unrestrained deformations. Thus, if the cube can be considered as a macro-element of material, the load-deformation charateristics observed are the true material properties without reflecting boundary constraints. This concept was verified in the test cell by loading a dummy specimen with an embedded polariscope including a photo-elastic sensor sheet. The uniformity of loading generated in the interior of this specimen, as determined by the photoelastic fringe patterns, demonstrated very homogeneous boundary loadings. The cell was used in testing of coal materials which has anistropic mechanical properties. The lack of boundary constraints makes the cell especially advantageous in testing obliquely orientated specimens which undergo both linear and distortional deformations.

Three-Dimensional Mechanical Characterization of Anisotropic Composites

The problem of complete characterization of a general linearly elastic, anisotropic material is discussed. An experimental procedure of testing a normally oriented and an obliquely oriented cubical specimen of the material is proposed, from which the 36 constants in the compliance matrix can be determined. A multiaxial test cell for applying homogeneous compressive stresses to the cubical specimen is described and the procedure is applied to Scotchply, a reinforced plastic. The compliance matrix for Scotchply is determined and it is shown to be orthotropic.

Analysis of time-dependent deformations of openings in salt media

Abstract An analysis of the time-dependent deformations of openings in salt media is described. The procedure involves (1) deriving three-dimensional constitutive equations for a Carlsbad potash material by means of laboratory tests; (2) developing a method of analysis capable of handling these constitutive equations and the geometries encountered in mining structures; (3) application of the method to particular field problems for which data are available. From results of uniaxial compression tests of potash specimens carried out in this investigation, and data obtained by other investigators from multiaxial testing of a similar material (rock salt), a three-dimensional viscoelastic-viscoplastic model is postulated. The finite-element method is chosen to handle the constitutive equations of this material model in the analysis of plane strain and axisymmetric problems. Good comparison with existing closed-form solutions is demonstrated for this approximate finite-element analysis, which is then used to calculate the deformations around a horizontal circular opening in an underground mine. The predictions are compared with field measurements.

Experimental developments for studying static and seismic behavior of retaining walls with liquefiable backfills

The effects of earthquakes on cantilever retaining walls with liquefiable backfills were studied. The experimental techniques utilized in this study are discussed here. A series of centrifuge tests was conducted on aluminum, fixed-base, cantilever wall models retaining saturated, cohesionless backfills. Accelerations on the walls and in the backfill, static and excess pore pressures in the soil, and deflections and bending strains in the wall were measured. In addition, direct measurements of static and dynamic lateral earth pressures were made. In some tests, sand backfills were saturated with the substitute pore fluid metolose. Modeling of model type experiments were conducted. The experimental measurements were found internally consistent and repeatable. Both static and dynamic earth pressure measurements were determined to be reliable. It was also observed that for the test configuration adopted, a special boundary treatment such as the use of duxseal is optional. Static and seismic modeling of models were also successful, which indicated that the assumed scaling relations were essentially correct.

Air Entrapment Effects on Hydraulic Properties

The purpose of this paper is to show that the soil-water characteristic and unsaturated hydraulic conductivity functions derived for soils with continuous air channels, in which the air is assumed to exist at constant atmospheric pressure, should not be used for simulating infiltration process. Ongoing experimental work is oriented towards investigating the influence of the entrapped air on the hydraulic constitutive relations of a compacted fine-grained soil imbibing liquid. Testing is being conducted in two phases to determine (1) soil-water characteristic function and (2) unsaturated hydraulic conductivity function for systems involving discontinuous air phase. Hydraulic constitutive functions for soils with discontinuous air phase are proposed using the obtained experimental results.

Centrifuge Test to Assess the Seismic Compression of Partially Saturated Sand Layers

A new testing approach for characterization of the response of partially saturated sand layers to cyclic loading is described in this paper. The approach involves basal shaking of a soil specimen within a laminar container using a hydraulic servo-controlled shake table in a geotechnical centrifuge. Infiltration of water was used to control the profiles of matric suction and degree of saturation, and thus the effective stress state, in the partially saturated sand layer during centrifugation. At steady state infiltration, relatively uniform profiles of degree of saturation and matric suction developed with depth in the sand layer. By varying the infiltration rate, different initial unsaturated conditions were obtained for cyclic testing. Instrumentation was incorporated into the setup to measure the accelerations induced in the shake table and soil profile, surface settlements, volumetric water content profiles, and pore water pressure. Cyclic tests were performed on sand layers having degrees of saturation ranging from 0.00 to 0.55 to assess the impact of effective stress on the layer’s deformation response. A nonlinear trend was observed in the variation of surface settlement with degree of saturation, with a minimum value obtained for sand having a degree of saturation of 0.28. This trend is consistent with the relationship between small strain shear modulus and degree of saturation for this sand.