Harold W. Olsen

Use of hyperspectral images in the identification and mapping of expansive clay soils and the role of spatial resolution

Hyperspectral images were acquired along the Front Range Urban Corridor in Colorado to determine the feasibility of identification and mapping of expansive clay soils with two, high-SNR imaging spectrometers. Swelling soils are a major geologic hazard, and cause extensive damage world-wide every year. The cost of postconstruction mitigation and standard engineering soil tests for creation of regional maps are immense. Smectite is the clay mineral group that has the greatest swelling potential and is responsible for most of the severe swelling soil damage observed in Colorado. Data sets were acquired from 1997 to 1999 with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and the Hyperspectral Mapper (HyMap). Using a matched filtering algorithm, maps of exposed clay material were produced, despite a strong vegetation cover. Among those exposures, spectral discrimination and identification of variable clay mineralogy such as smectite, smectite/illite, and kaolinite, in decreasing order of swelling potential hazard, was possible. The comparison of the results from the two sensors showed that higher spatial resolution provided purer image endmembers in more heterogeneous sites, but did not exhibit more endmembers and did not identify new natural outcrops that a lower spatial resolution data set would miss in a homogeneous terrain. However, an increase in the signal-to-noise ratio (SNR) of the instrument by pixel summation made possible the identification of low reflectance exposures. This work demonstrates that, using recent instruments and well-established methodologies, imaging spectrometry can be of practical help for the detection and mapping of expansive clays.

Flow and transport through clay membrane barriers

Flux equations for liquid and solute migration through clay barriers that behave as semi-permeable membranes used in waste containment and remediation applications, known as clay membrane barriers (CMBs), are discussed. The results of a simplified analysis of flow through a geosynthetic clay liner (GCL) using measured values for the chemico-osmotic efficiency coefficient (ω) of the GCL indicate a total liquid flux that counters the outward Darcy (hydraulic) flux due to chemico-osmosis associated with clay membrane behavior of the GCL. Also, the solute (contaminant) flux through the GCL is reduced relative to the solute flux that would occur in the absence of membrane behavior due to chemico-osmotic counter advection and solute restriction. Since diffusion commonly controls solute transport through GCLs and other low-permeability clay barriers, the implicit (empirical) correlation between ω and the effective salt-diffusion coefficient of the migrating contaminant is an important consideration with respect to contaminant restriction in CMBs.

A laboratory apparatus to measure chemico-osmotic efficiency coefficients for clay soils

A laboratory apparatus for measuring the chemico-osmotic efficiency coefficient, ω, for clay soils in the presence of electrolyte solutions is described. A chemico-osmotic experiment is conducted by establishing and maintaining a constant difference in electrolyte concentration across a soil specimen while preventing the flow of solution through the specimen. The chemico-osmotic efficiency coefficient is derived from a measured pressure difference induced across the specimen in response to the applied concentration difference. The effective diffusion coefficient (D*) and retardation factor (Rd) of the electrolytes (solutes) also can be determined simultaneously by measuring the diffusive solute mass flux through the specimen until steady-state diffusion is achieved. Experimental results using specimens of a geosynthetic clay liner subjected to potassium chloride solutions indicate that the measurement of ω may be affected by soil-solution interactions, as well as by changes in the induced chemico-osmotic pressure difference due to solute diffusion. As a result, ω should be evaluated using the induced pressure difference at steady state. The time required to achieve a steady-state response in induced pressure difference is related to the time required to achieve steady-state diffusion of all solutes, and may be affected by the circulation rate at the specimen boundaries. The circulation rate should be sufficiently rapid to minimize changes in the boundary concentrations due to diffusion, but sufficiently slow to allow measurement of solute mass flux at the lower concentration boundary for evaluating D* and Rd.

Critical Review of Coupled Flow Theories for Clay Barriers

Coupled flow theories and systems, introduced from the 1960s to the 1990s, are critically reviewed in relation to their applicability for investigating the coupled migrations of groundwater and dissolved species through clay barriers. Most of these theories and systems are based on irreversible thermodynamics. Katchalsky and Curran developed theories for noncharged and electrolyte solutes in discontinuous systems consisting of a membrane separating solutions of a single solute dissolved in water. Olsen developed an experimental system to investigate the applicability of the force-flux relationships in Katchalsky and Curran’s electrolyte theory for soil. Greenberg et al. and Yeung and Mitchell derived theories for continuous systems with noncharged and electrolyte solutes that are based on most of the thermodynamic concepts and assumptions in Katchalsky and Curran’s theories. Alshawabkeh and Acar developed a theory based on the kinetics of ion diffusion, electrodiffusion, and advection; a phenomenological relationship for advection in response to hydraulic and electric gradients; and the mechanisms by which electric charge is transferred between carbon electrodes and solutions. This review shows that the abilities of these theories to analyze the migration of groundwater and dissolved species through clay barriers are limited. A more general coupled flow theory is needed that takes into account multiconstituent pore fluids, ion exchange, and all the coupling mechanisms involved. This model should be developed from irreversible thermodynamics and should consider the simultaneous fluxes of liquid, solutes, and electric charge in response to hydraulic, solute concentration, and electric potential gradients.

Comparison of Chilled-mirror Measurements and Filter Paper Estimates of Total Soil Suction

A comparison of chilled-mirror total suction measurements with those estimated from filter paper total suction data show general agreement. However, there are significant discrepancies that are similar to those previously reported by others. The nature and magnitude of the discrepancies are consistent with: (a) possible errors in chilled-mirror total suction measurements due to incomplete equilibration in the sealed test chamber of the chilled-mirror device, and (b) possible errors in estimated filter paper total suction values due to natural variations of the zero-water-content intercept in the log total suction versus water content relationship. The possible errors in chilled-mirror measurements are easily avoided. The possible errors in estimated filter paper measurements can only be minimized by avoiding the need for an assumed zero-water-content intercept. This can be accomplished far more easily with the chilled-mirror device than with the filter paper method.

Mineralogy-swelling potential relationships for expansive shales

This study examines the extent to which mineralogy and swelling potential can be correlated in the expansive clays and shales along the Colorado Front Range Urban Corridor. 182 undisturbed samples were collected from 20 sites from Boulder to Pueblo. Sites were selected in Cretaceous shales, including the Pierre Shale, that have been uplifted into steeply dipping strata near the foothills of the Rocky Mountains, and that are well known to be hazardous to residential and light commercial developments in this region. For each sample, mineralogy was determined by x-ray diffraction and swelling potential was obtained from moisture content (w), suction (h), and clod volume (V) measurements in terms of the suction potential (dh/dw) and suction-compression index (dV/dh) parameters used in the classification scheme McKeen proposed in 1992. Swelling potentials were also obtained on more limited suites of samples with conventional and labor-intensive schemes including Seed and Chens schemes based on tests for grain-size distribution and Atterberg limits, and with swell-consolidation measurements in response to saturation, consolidation, and rebound in an oedometer. The results show the percent total smectite provides a useful index of the swelling potential concept defined by Seed and correlates reasonably well with the swelling potential indices developed by Seed, Chen, and McKeen. However, the percent total smectite does not correlate well with conventional swell-consolidation test indices. The causes for this lack of correlation appear to be placement and environmental factors such as the initial moisture content, stress history, and surcharge loading that are beyond the scope of Seeds swell potential concept and Seed, Chen, and McKeens swelling potential indices.

Graphical Method for Determining the Coefficient of Consolidation cv from a Flow-Pump Permeability Test

A graphical method has been developed for determining the coefficient of consolidation from the transient phases of a flow-pump permeability test. The flow pump can be used to infuse fluid into or withdraw fluid from a laboratory sediment specimen at a constant volumetric rate in order to obtain data that can be used to calculate permeability using Darcys law. When the initial transient-response curve (hydraulic head as a function of time) generated by this test is examined analytically in terms of a one-dimensional consolidation process, representative type-curve solutions to the associated forced-flow and pressure-decay models are derived. These curves provide the basis for graphically evaluating the permeability k, the coefficient of consolidation cv, and the coefficient of volume change mv. The curve-matching technique is easy and rapid, and it can be applied to results of forced-flow tests, both infusion and withdrawal, as well as to subsequent pressure-decay records. Values of k, cv, and mv for a laterally confined kaolinite specimen were determined by this graphical method and appear to be in reasonably good agreement with numerically derived estimates (within 20%). Discrepancies between the two sets of results seem to be largely a function of data quality rather than of method of analysis. Where responses of hydraulic head as a function of time are apparently unaffected by experimental sources of error, agreement is excellent (within 4%). Application of this graphical method to triaxial testing has inherent uncertainties, because the solution curves that describe one-dimensional deformation are used to analyze a three-dimensional process.

Integrated Shear and Flow Parameter Measurement

Studies on the variation of permeability and the specific storage of bentonite-sand mixtures with shear deformations that may be caused by earthquakes and/or other geological events are of fundamental importance for long-term safety assessments of radioactive nuclear waste disposal facilities. This paper presents a recently developed and improved method for integrated shear and flow parameter measurements on a mixture of Kunigeru V1 Bentonite and D-Sand. This material will be used in low-level radioactive nuclear waste disposal facilities in Japan. The results of this study show that: (1) temperature control is important for measuring hydraulic parameters of low-permeability materials with the flow pump method; (2) shear strains up to about 3% did not significantly influence either the permeability or the specific storage of the bentonite-sand mixture; and (3) permeability and specific storage values interpreted from different time intervals during the transient rise and transient decay phases of the flow pump permeability tests were almost the same, which suggests that the reliability of both the experimental results and the newly derived theoretical analysis used to interpret the hydraulic parameters.

Spectral Reflectance as a Rapid Technique for Field-Determination of Soil Engineering Properties

This study examines the extent to which reflectance spectroscopy and swelling potential can be correlated in the expansive clays and shales along the Colorado Front Range Urban Corridor. The study involved trenching in the steeply-dipping Pierre and Smoky Hill Shales to expose bedrock for the acquisition of approximately 70,000 reflectance spectra that were developed into spectral images of the trench walls, and extensive sampling for geotechnical and mineralogical analysis. Regression models were developed which span a large range of swell potential indices, and provide a means to determine swell potential magnitudes from reflectance spectra in the field in seconds. These models also have the potential to extend swell potential measurements beyond the time and labor-intensive laboratory testing methods used in current practice. Useful correlations for prediction were obtained for percent clay, Atterberg limits, the swell potential indices proposed by Seed, Chen, and McKeen, and the percent swell obtained from consolidation-swell tests. For the sake of brevity only liquid limit and swell percent models are discussed. Slab performance risk categories can be predicted using results of partial least squares regression models based on spectral reflectance. 300–500 samples per day can be analyzed in the field using these spectral methods.

GEOTECHNICAL CHARACTERIZATION OF THE STEEPLY DIPPING PIERRE SHALE

This study compares alternative approaches for differentiating and characterizing the swelling potential of steeply dipping strata in the Pierre Shale along the Colorado Front Range Urban Corridor. The hazard associated with this geologic setting is commonly referred to as heaving bedrock because adjacent strata expand to different degrees and may cause damaging differential deformation to residential and light commercial structures. This study is part of a larger investigation aimed at determining the feasibility of using reflectance spectroscopy, a remote sending technology, to identify and characterize the swelling potential of expansive soils and bedrock. A trench in the Upper Pierre Shale was logged and 252 samples were taken horizontally with thin-walled brass tubes. Grain size, Atterberg Limits, filter paper suction, and clod tests provided, for each of the samples, the swell potential indices and rankings according to schemes proposed by Seed, Chen, and McKeen. Swell-consolidation tests provided percent swell pressure indices under in-situ surcharge pressures. All the indices are consistent in that they suggest that trench wall can be differentiated into two zones of steeply dipping strata overlain by a horizontal and shallow overburden soil. However, the schemes differ in that McKeens indices vary over a significantly broader range of categories compared with the Seed and Chen indices. McKeens indices and also the swell-consolidation indices have two advantages compared with Seeds and Chens indices. They are obtained on relatively undisturbed materials and they are properties involved in quantitative heave prediction models. These advantages suggest that McKeens scheme is providing the most sensitive and useful basis for differentiating semi-quantitative categories of swelling potential at this site.