Thomas C. Sheahan

Coupling of electrochemical and mechanical processes in soils under DC fields

Abstract Direct current (DC) electric fields have been applied in several geotechnical and geoenvironmental engineering applications, including electro-osmotic dewatering and consolidation, ion injection and contaminant removal. Such applications cause electrochemical effects in the soil, leading to changes in the soil’s physical, chemical and mechanical properties. Some of these changes have been evaluated extensively (such as those occurring under electro-osmotic consolidation) based on the assumption that electrochemical conditions are uniform between the electrodes. However, recent studies have shown that nonlinearity in these conditions develop between the electrodes leading to changes in the chemical, physical and mechanical properties of the soil. This paper evaluates the nonlinear effects on these properties as measured in some specific applications. In particular, the application of DC fields to soft soils is shown to alter the soil in both its mechanical and physical properties. The soil’s undrained shear strength is increased by interparticle cementation brought about by the electrochemical injection of ions into the soil under the DC field. In addition, soils can consolidate or swell under these fields. In both of these cases, the development of a nonlinear voltage distribution across the soil is clearly measured, and can affect the progression of any electrochemical soil treatment as well as the accuracy with which it is modeled.

A large permeameter for study of internal stability in cohesionless soils

Results are reported from the commissioning of a large rigid-walled permeameter that was designed to examine hydromechanical conditions prevailing at the onset of seepage-induced failure in soils with a potential for internal instability. A technique of slurry mixing and discrete deposition is used to reconstitute a homogeneous, saturated test specimen. The test specimen is consolidated and then subject to seepage flow, under head-control, in either a downward or upward direction. Two arrays of pressure transducers, located on opposite sides of the specimen, establish the variation of hydraulic gradient along the specimen. The device is configured with a top and bottom load cell, and frictionless loading ram, in order to assess the influence of side-wall friction and thereby establish the distribution of vertical effective stress along the length of the specimen. Observations of hydraulic gradient and effective stress enable a characterization of the onset of instability, which is localized within the specimen. Results of multi-stage tests on glass beads are reported that illustrate novel features of the permeameter and instrumentation, the utility of the specimen reconstitution technique and a novel approach for quantifying the onset of internal instability.

Stabilization of dune sand using foamed asphalt

Foamed asphalt technology has increasingly gained acceptance as an effective and economical soil improvement and stabilization technique, mainly because of its improved aggregate penetration, coating capabilities, and handling and compaction characteristics. This laboratory research program was carried out to investigate the feasible use of foamed asphalt technology in Saudi Arabia to improve the prevalent dune sands for possible use as a base or subbase material. Several variables were investigated to evaluate the relative improvement of dune sand as well as to permit the development of design procedures for the future use of foamed asphalt technology in the harsh climatic conditions of eastern Saudi Arabia. Statistical analysis of the results was employed to verify the effects of emulsified asphalt and foamed asphalt treatment, with and without the addition of Portland cement, on the strength characteristics of the treated mixes. The results displayed significant improvement in the performance of dune sand foamed asphalt mixes, as compared to that of the emulsified asphalt mixes.

S-wave velocity tomography: Small-scale laboratory application

Arrays of bender elements are combined with simple, yet robust inversion algorithms to develop a device for S-wave tomography. A fixed frame configuration complemented with a new versatile bender element installation permits reducing measurement errors. System design involves optimal selection of transducer separation, a frame design that prevents wave transmission, and an adequate calibration procedure. Reliable tomographic images are obtained by combining data preprocessing and the regularized least squares solution. Given the small size of the data sets, inversion techniques based on a parametric representation of the medium are implemented as well. The tomographic system is tested at low confinement and within a true triaxial cell. Results show the potential of tomographic imaging in the characterization of geotechnical systems and in the monitoring of subsurface processes. In particular, shear wave velocity tomography permits monitoring changes in the velocity field, which is related to the average effective stress in freshly-remolded uncemented soils. A minimum anomaly size and velocity contrast are required for detection. Diffraction healing hinders the detection of low velocity anomalies.

Computer Automation of Conventional Triaxial Equipment

This paper provides objective guidance on the conversion of existing manual triaxial testing equipment to computer control. An approach called adaptable automation is introduced to facilitate the conversion process. This approach consists of modifying the four basic building blocks of the triaxial system for integration into a computer-automated system and adding components needed for automation which were not part of the manual system. Recommendations are made on necessary changes to the triaxial building blocks, and examples are given from the actual conversion of a largely manual triaxial apparatus. New components required for the automated system are an analog-to-digital conversion device, digital-to-mechanical hardware, and a personal computer with test control software. Performance guidelines for selecting equipment are reviewed as are considerations when developing the software. Methods are described for evaluating the converted systems performance, and typical evaluation results are shown. The paper concludes that, while development of a prototype converted system can be costly and require considerable knowledge of system components, cost savings will be realized as additional prototype-based systems are brought online and system capabilities expanded. More importantly, the converted system allows existing customized testing procedures and hardware to remain in place.

A Consolidation and Contaminant Transport Device for Assessing Reactive Mat Effectiveness for Subaqueous Sediment Remediation

This paper describes the design and fabrication of a new laboratory testing column to assess the effectiveness of a permeable reactive mat for in situ sediment remediation. After the composite mat, which consists of top and bottom filtering geotextile layers and a middle reactive core, is placed on the sediment surface, hydrodynamic dispersion and pore fluid flow carry contaminants through the mat. In the reactive layer, the contaminants react with one or more amendments. A thin cap of new sediment material can be placed on top of the geocomposite to promote a new, healthy benthic community. The processes controlling these geocomposites have not been physically modeled to assess their effectiveness. The new device is fundamentally a sediment column, but with the capability to apply a constant, overlying stress to the sediment-reactive mat column to cause its consolidation. Pressure-volume controllers at the top and bottom of the sediment column allow in situ pressure conditions to be imposed and allow for chemical sampling of inflow/outflow fluids.

The Effect of Gypsum Cementation on the Mechanical Behavior of Gravely Sands

The behavior of a cemented gravely sand is studied using triaxial tests. Drained and undrained tests were performed on dry and saturated specimens, and stress-strain characteristics of the soil, along with volumetric and pore pressure changes, were identified. The gypsum plaster was used as the cement agent and was mixed with the soil in different percentages. The tests were done in the usual range of confining pressures, from 25 to 500 kPa. Test results show that dilation occurs even at the highest confining stress and the least cement content. The behavior of the cemented soil is found to be more brittle in drained condition than the undrained one. However, the brittleness of soil decreases with increase in confining stress. The ratio of cemented soil shear strength to the uncemented one decreases as the confining stress increases. The failure envelopes are curved and the drained failure envelopes are above the undrained ones. The friction angle of soil increases slightly with cement content, but the cohesion intercept increase is more noticeable. The principal stress ratio at failure decreases with increase in confining stress.

A MULTISLEEVE FRICTION ATTACHMENT FOR THE CONE PENETROMETER

Accurate knowledge of the strength of soil-geomaterial interfaces is becoming of increasing importance in geotechnical engineering. Systems whose performance is heavily dependent on soil-geomaterial interfaces include deep foundations, synthetic impervious liners, trenchless technologies, and an assortment of earth retaining structures. The strength of the interface is typically estimated by applying adjustment factors to values of soil or interface strength measured in laboratory tests. These adjustment factors are intended to correct for differences between the test and anticipated operating conditions such as variations in soil type and density, strain rate, surface roughness, or confining stress and are often empirically based with little theoretical underpinnings. Of these adjustment factors, the surface roughness is considered to be of utmost importance in that it has the potential to alter the interface strength by 100% or more. This paper describes the development of a new multisleeve Friction attachment for the cone penetrometer that allows for direct in situ measurement of the relationship between interface strength and surface ronghness. As discussed herein, the ability of the attachment to quantify this relationship, in conjunction with additional ongoing research, provides the opportunity to improve the design of friction dependent systems, as well as site characterization. A key characteristic of the penetrometer attachment is the ability to obtain four individual sleeve friction (fs) measurements at each elevation within a sounding, in addition to the conventional Cone Penetration Test (CPT) fs measurement. This allows for direct in situ analysis of the effects of sleeve roughness on the fs measurement. Considerations pertinent to the development of the device including assessments of the conventional CPT fs measurement and soil geomaterial interface mechanisms are first presented. A description of the new penetrometer attachment including key characteristics and capabilities follows. Finally, validation of the operation of the device through laboratory and field tests is described, and future applications of the attachment are discussed.

Volume Change Effects on Solute Transport in Clay Under Consolidation

This paper evaluates the effect of volume change due to consolidation on one-dimensional transport of contaminants. The rate of consolidation and excess pore pressure dissipation result in a transient, nonlinear advective component of transport through clay. Consolidation experiments were conducted on kaolinite samples prepared at 50% water content. Bromide, a non reactive tracer, was mixed with the upper half of the kaolinite sample at concentration of 960 mg/L. Flux of bromide as a result of consolidation was monitored in the water surrounding the kaolinite sample. The results show continuous flow of bromide as a result of water drainage by consolidation. The bromide mass flux compared well with consolidation rate and ceased at the end of the consolidation process. A formulation for the simultaneous consolidation and diffusion of solutes in clays, solved by finite difference, is also presented. A hypothetical case is solved to demonstrate the effect of consolidation under single and double drainage on contaminant transport and breakthrough. The results show that consolidation in doubly drained clay impacts concentration profiles, but does not significantly impact breakthrough of the diffusive flux. Consolidation under single drainage conditions significantly impacts the diffusional flux.

Time-Dependent Triaxial Relaxation Behavior of a Resedimented Clay

The paper describes the results of K0 consolidated-undrained triaxial compression relaxation tests on resedimented Boston Blue Clay (BBC) using a computer-automated triaxial apparatus. Specimens were either normally consolidated (overconsolidation ratio, OCR = 1) or had an OCR = 4, and they were sheared at different axial strain rates to axial strains (ea) from 0.1 to 15%. Each strain level was maintained until the monitored shear stress and pore pressure reached equilibrium levels. The results show that a single specimen can be used reliably for multiple relaxation tests. During relaxation, the rate of normalized shear stress decay with log time, (−Δq/σ′vm)/Δlogt, is approximately constant, regardless of ea or OCR. Changes in shear-induced pore pressure during relaxation were negligible except when specimens were rapidly sheared. Independent of OCR, end-of-relaxation equilibrium stress states for relaxation strains ea ≤ 1.5% lie on a line of obliquity defined by K = σ′h/σ′v = 0.50, compared to K0 = 0.49 for OCR = 1 BBC; whereas, for ea ≥ 2.5%, the equilibrium states lie on a steeper obliquity line defined by K = 0.40.