John C. Stormont

In situ gas permeability measurements to delineate damage in rock salt

In situ gas permeability measurements can be used to distinguish regions within rock salt formations which have experienced disturbance or damage. High-resolution hydrologic testing reveals rock salt well removed from an excavation (i.e. undisturbed) is a low permeability, low porosity, porous medium with a significant pore (brine) pressure. In this region, rock salt is effectively impermeable to gas due to threshold capillary pressures which preclude gas from displacing the resident brine. In a region confined to a depth of less than about one effective radius of an excavation, rock salt is typically permeable to gas. The measured gas permeabilities are consistent with a partially saturated, dilated zone developing adjacent to the excavation. The extent and nature of the damaged region interpreted from gas permeability measurements is consistent with other measurements and analyses. Thus, gas permeability measurements serve as effective means for detecting and delineating the damaged region.

Simulation of geomembrane response to settlement in landfills by using the material point method

Because of the multiple layers of dissimilar materials and large deformations involved in the subsidence of a landfill system, large-scale computer simulation of the geomechanical response to subsidence with the use of conventional numerical methods are problematic. The Material Point (MPM), 1,2 which was recently developed for dynamic problems such as penetration and perforation, is a newly emerging numerical method. The MPM is modified in this paper to simulate the geomechanical response of a landfill cover system that includes a geomembrane under quasi-static loading conditions. Sample problems, for which an analytical solution is available with certain assumptions, are considered to demonstrate the proposed solution procedure. Future work is discussed based on current research results.

Evaluation of Subgrade Strength and Pavement Designs for Reliability

Reliability is an important factor in flexible pavement design to consider the variability associated with the design inputs. In this study, subgrade strength variability and flexible pavement designs are evaluated for reliability. Six existing pavement sections design data are studied using probabilistic, AASHTO, and mechanistic-empirical pavement design guide (MEPDG). Parameters such as mean, maximum likelihood, median, coefficient of variation, and density distribution function of subgrade strength (R value) are determined. Design outputs are compared in terms of reliability and thickness using these design procedures. It is shown that the AASHTO provides higher reliability values compared to the probabilistic procedure. All the existing pavements fail in the MEPDG distress reliability such as rutting and top-down cracking reliabilities. Currently, New Mexico Department of Transportation uses a single design R value to deal with variability associated with subgrade strength in flexible pavement design. It is shown in this study that single design R value for a roadway section does not yield an effective design regarding target reliability, while the subsectioning procedure based on coefficient of variance of R value is a better way to deal with the subgrade variability. An assessment of minimum R value for making the decision of subexcavation is also presented. It is shown that increasing the minimum R value for subexcavation is not always the proper solution to meet design reliability; rather it yields an inefficient design for requiring higher frequencies of subexcavation. Finally, the reliability of the flexible pavement design is evaluated by varying hot mix asphalt properties. Alternative designs are recommended for the existing pavement thicknesses by modifying material and subgrade properties to mitigate different distresses.

Characterization of a fiberglass geotextile for unsaturated in-plane water transport

The methods and results of the characterization of a woven, multifilament fiberglass geotextile are described in this paper. The moisture characteristic curve was measured with capillary rise and hanging column methods for both wetting and drying paths. In-plane flow was first measured with a simple siphon test, followed by measurement of the unsaturated transmissivity using a constant suction permeameter. The fiberglass geotextile became transmissive during wetting at a suction of 100 mm, and remained transmissive during drying to a suction of 600 mm. A lateral drainage test with the fiberglass geotextile indicated that the geotextile accepted and laterally drained water infiltrating through a soil layer under suctions in excess of 600 mm. The fiberglass geotextile is shown to contain more water and is more transmissive at greater suctions compared to other geotextiles, and has considerable capacity to drain water under suction.

Stability Evaluation of a Mine Waste Pile

Failure of a portion of a mine waste pile motivated a study to identify remaining portions of the pile that were unstable and to suggest appropriate, remedial stabilization efforts. Data collection focused on the principal parameters that control stability, namely, pile geometry and material strength. Data from a topographic survey of the pile and the interpolated bedrock surface under the pile were used to characterize the slope of the mine waste pile and the thickness of the waste material. Strength data were obtained from conventional, small-scale laboratory test methods and a large-scale test method that was used in the field and the laboratory. The small-scale tests, which did not include the relatively coarse fraction of the mine waste material, tended to overestimate the strength of the waste pile material. Strength and pile geometry data were combined in a GIS-based model that provided a first-order, conservative estimate of the stability of the pile based on the infinite slope method. Results from this model indicated three potentially unstable areas. These areas were analyzed using the method of slices. A single location on the pile was found to have an unacceptable factor of safety. A simple grading plan to remediate the problematic location was developed.

Transmissivity of a nonwoven polypropylene geotextile under suction

A permeameter has been developed for measuring in-plane transmissivity of geotextiles under a nearly constant value of suction along its length. The permeameter is capable of imposing gradients in excess of 10% and normal pressure up to 240 kPa, and permits the monitoring of the suctions within the geotextile during testing. To demonstrate the capability of the permeameter, a series of transmissivity measurements were made on a nonwoven polypropylene geotextile subject to different suction heads, normal pressures, and gradients. Transmissivities were up to two orders of magnitude less than the saturated value depending the on the magnitude of the suction head and whether the geotextile was being wetted or dried (hysteresis). Transmissivity values were independent of the gradient for these measurements. Increasing the applied normal pressure decreased the transmissivity at all values of suction head.

Evaluating Parameters for Characterization of Cracking in Asphalt Concrete

This study evaluates parameters derived from laboratory tests to characterize crack initiation and propagation within the semi-circular asphalt concrete samples in the laboratory. Asphalt samples are compacted using a Superpave gyratory compactor, and then sliced and notched using a laboratory saw. The notch acts to facilitate crack initiation. Notched samples are subjected to a strain controlled compressive load in three point bending. Crack opening displacements (CODs) and loads are recorded in real time using four linear variable displacement transducers. Ultimate load, COD at ultimate load, cracking potential, fracture load, crack velocity, and slope of the crack propagation curve are determined from laboratory test results and evaluated for their suitability in characterizing crack initiation and crack propagation. In essence, these parameters are evaluated through examining the effects of notch tip location, moisture condition, and void ratio on crack initiation and propagation. It is shown that crack initiation can be best characterized by the ultimate load. Crack velocity and slope of the crack propagation curve show promise in characterizing crack propagation in the notched asphalt samples.

Conduct and interpretation of gas permeability measurements in rock salt

Abstract A series of gas permeability measurements have been conducted surrounding excavations in a bedded rock salt formation. The resolution of the measurement system is on the order of 10−20 to 10−21 m2, depdending on the test zone pressure and to a lesser degree on the test duration. Two types of tests were conducted: constant pressure flow tests for permeabilities above 10−18 m2, and pressure decay tests for smaller permeabilities. The gas flow data are interpreted in terms of transient flow through a compressible, porous medium. A finite difference solution was used to develop estimates of permeability and porosity which match the measured data. The gas permeability of the undisturbed rock salt (1 to 5 m from an excavation) is very low, corresponding to a gas permeability of less than 10−21 m2. Between the undisturbed region and the excavation, the interpreted permeabilities range from 10−16 to 10−20 m2. Generally, immediately above and below excavations, relatively great permeabilities are measured in the interbed layers. These measurements substantiate the existance and nature of a limited disturbed rock zone sorrounding excavations in a bedded rock salt formation.

Mixed waste landfill design report

An Evapotranspiration (ET) Cover is proposed to be deployed over the Mixed Waste Landfill (MWL) at Sandia National Laboratories in Albuquerque, New Mexico. This design report details a potential ET Cover design with all supporting information. The report is intended to serve as a supporting document to the more extensive submission for disposition of the landfill prepared by Sandia National Laboratories. The design that was evaluated and proposed in this report is a soil cover 4 feet 3 inches thick (3 feet 6 inches of compacted native soil overlain by 9 inches of gravel admixture). The cover is to be seeded with a mixture of native grasses. The design assumes the soil will act similar to a “sponge” in that it will store all moisture from the worst case infiltration event until it can be removed via the combination of evaporation and transpiration known as ET. The design is based on supporting field data and calculations. A first order estimate of the required soil thickness was based on the soil’s measured storage capacity. It was Revision 1. 113 1l2000 refined using the combination of numerical modeling, relevant natural analog data, and available field data. The gravel admixture surface layer was designed to assist any established vegetation minimize surface erosion. A very detailed set of calculations complimented by computer simulations was used to determine a 9 inch thick layer composed of one part gravel to two parts soil by weight would be adequate for the final cover design slopes to minimize surface erosion. Revision 1 1/3 1/2000

Total Soil Water Evaporation in a Riparian Environment: Model Development and Application

A total soil water evaporation model that includes both transient (event based) evaporation and water table evaporation was developed by modifying an existing water balance type model incorporated in the widely used FAO-56 method. Using results from a parametric numerical study, the method applies a predictive equation for soil water table evaporation as a function of distance to the water table, soil properties, and climatic conditions. The model, which can be easily implemented in a spreadsheet, can account for the effects of water management decisions relating to tree canopies and surface mulch. Applied to a location in the Middle Rio Grande riparian zone over the course of a year, the model provides insight into how soil water evaporation may be impacted by restoration and management activities. Soil hydraulic properties were shown to greatly affect the predicted amount of total soil water evaporation. With a relatively shallow water table and an exposed soil surface, the amount of soil water evaporation can be comparable to water consumption by riparian vegetation. Canopy shade reduces the amount of total soil water evaporation by more than 50%. As expected, the model indicates that surface mulch yields an even greater reduction in total soil water evaporation. Mulching bare soil surfaces can effectively reduce evaporative losses after exposing a soil surface by removing undesirable vegetation. This study shows that riparian tree removal in areas of shallow ground water may result in no water savings.