Bate

Effect of total organic carbon content and structure on the electrokinetic behavior of organoclay suspensions.

This experimental investigation measured the zeta potential of the clay mineral, montmorillonite, which was modified with six different quaternary ammonium cations. The organic cations were chosen to quantify the effect of cation functional groups, including chain length and cation size, on the resulting zeta potential; each of the six cations were exchanged onto the clay surface at three levels of total organic carbon. The zeta potential of the unmodified and the organically modified clays was measured as a function of pH, and in all cases, became less negative as the total organic carbon was increased and as the length of the attached carbon chain was increased, indicating that the organic cations were more strongly bound within the particles shear plane as total organic carbon content was increased. Measured zeta potential was also less negative for all clays tested (including unmodified montmorillonite) as pH was decreased. When compared on the basis of total organic carbon content, increasing the length of one carbon chain in the quaternary positions was a more effective method of neutralizing surface charge than was increasing the overall size of the cation (i.e., increasing the chain length in all quaternary positions).

Measurement of Stiffness Anisotropy in Kaolinite Using Bender Element Tests in A Floating Wall Consolidometer

A new floating wall consolidometer-type bender element testing system was developed to study the stiffness anisotropy of clays at applied vertical stresses up to 800 kPa. One-dimensional slurry-consolidated Georgia RP-2 kaolinite samples, prepared with 0.005 and 1 mol/l NaCl solutions, were tested in this system. A floating wall design eliminated the detrimental bending moment that acts upon the horizontally installed benders as a result of soil settlement in a traditional fixed wall setup, which significantly improved the signal quality and bender reuse. Floating wall–soil interface resistance was quantified with pulling tests. Analytical equations were then derived to calculate the wall resistance-corrected vertical effective stress. As a result, stresses applied to the soil were more accurately determined. The bender element (BE) test was used to measure kaolinites shear wave velocity (Vs), thereby quantifying the small strain stiffness of soils. Average Vs results for RP-2 kaolinite were lower than those for other kaolinites reported in the literature. This was postulated to be primarily due to the longer and more tortuous chains of particle contacts associated with the smaller median diameter (d50 = 0.36 μm) of RP-2 kaolinite samples. BE test results indicated that Vs increased with stress, density, and concentration. The hierarchy of Vs in three orthogonal directions (i.e., hh > hv > vh) agreed with results in the literature. It was also illustrated that Vs anisotropy increased with applied stress and decreased during unloading. In addition, a comparison was made between the BE test in the new floating wall consolidometer and the BE test in a triaxial testing setup.

Impact of Organic Coatings on Frictional Strength of Organically Modified Clay

AbstractOrganic matter is frequently encountered in both naturally occurring and engineered particulate media. Charged functional groups in the organic matter can lead to cation exchange within the clay interlayer, which results in the formation of an organic coating on the clay surfaces and alters the interfacial frictional regime in the soil mass. This study investigated the triaxial shear frictional behavior of montmorillonite particles coated with a controlled organic phase composed of quaternary ammonium cations. Through cation exchange, organic cations were loaded onto the clay’s interlayer exchange sites, with control on the density of organic coverage and structure of the organic cation. Results demonstrated that increasing the total organic carbon content of the clay resulted in increasing frictional resistance regardless of whether the increase in carbon content was attributable to increased density of organic loading, increased cation size, or increased cation tail length. Concentrating organic...

Closure to “Discussion of ‘Measurement of Stiffness Anisotropy in Kaolinite Using Bender Element Tests in a Floating Wall Consolidometer’ by X. Kang, G.-C. Kang, and B. Bate” by Coffman, Salazar and Zhao

The authors appreciate the interest of the discussers in this paper. The main arguments of the discussers were: (1) comparing the back-pressure saturated, constant-rate-of-strain consolidation device that incorporated bender elements (BP-CRS-BE device) developed by the discussers to the floating wall consolidometer based bender element testing system in the original paper (Kang, X., Kang, G.-C., and Bate, B., 2014, “Shear Wave Velocity Anisotropy of Kaolinite Using a Floating Wall Consolidometer-Type Bender Element Testing System,” Geotech. Test. J., Vol. 37, No. 5, pp. 869–883. [DOI: 10.1520/GTJ20120205]); (2) requesting the quantification of both the system lag and the machine deflection; (3) suggesting the authors use cross correlation method to determine the first arrival time because Vs values obtained by time domain method are “only approximate,” while the latter was the only method used by the discussers in their referenced work (Salazar, S. E. and Coffman, R. A., 2014, “Design and Fabrication of End Platens for Acquisition of Small-Strain Piezoelectric Measurements during Large-Strain Triaxial Extension and Triaxial Compression Testing,” Geotech. Test. J., Vol. 43, No. 2. pp. 1–11. [DOI: 10.1520/GTJ20140057]); and (4) making misleading arguments regarding the compression and shear waves measurements. In this closure, the authors first briefly compared the BP-CRS-BE device to the floating wall consolidometer based bender element testing system, and pointed out that (1) the well-documented wavelength ratio (Rd ratio) consideration in designing a bender element device and the seemingly unsatisfaction of such consideration in the BP-CRS-BE device, and that (2) both the lack of details in B-value checking to examine saturation and the lack of shear wave velocity resulted from the BP-CRS-BE device to substantiate the arguments of the discussers. Then the authors provided the requested quantifications of both system lag and machine deflection to address some postulations by the discussers. The authors disagreed with Arguments 3 and 4 made by the discussers, and respond accordingly.

Influencing Factors on the Dynamic Properties of Organobentonites

The dynamic responses of soils are important engineering properties, especially for soils with high organic contents, because these soils can underlay critical support structures such as highway bridges or levees. Consequently, the initial tangent shear modulus (G max ), secant shear modulus ratio (G/G max ), and damping ratio (D) are important parameters in evaluating the dynamic soil behavior for soils subjected to earthquake, wind, waves, traffic, and equipment vibration. In this study, these dynamic properties of organobentonites were quantified experimentally using resonant column tests. The results from this study were evaluated in terms of the effect of shear strain, organic content, and plasticity index. The organobentonites tested in this study were engineered with an organic phase that was controlled in terms of structure and density of loading. The results indicate that the total organic carbon content reduced the net surface charge on clay particle, which in turn increased the bonding between the organic cations and clay surface. The resistance is then increased, leading to increase in stiffness.

Shear Strength Behavior of Bentonite Modified by Tetramethylammonium Cations

Over the past decades, bentonite exchanged with quaternary ammonium cations to form organobentonite has gained increasing attention. Due to its high sorption capacity for organic contaminants and low friction angles, organobentonite has many applications, including in the areas of contaminant site remediation, landfill clay liners, and as a pipe jacking lubricant and drilling mud. Although many properties of organobentonites have been studied in detail, such as sorption capacity, swelling, and hydraulic conductivity, little data are available on the strength of this engineered clay. This study presents the results of the shear strength behavior of tetramethylammonium cations exchanged bentonite (TMA-bentonite) as a function of organic loading. TMA-bentonites had higher shear strength and higher hydraulic conductivity than the unmodified bentonites. The initial shear modulus increased with the amount of TMA loaded on the bentonite surfaces, i.e. as the organic loading was increased. The tested samples were normally to lightly overconsolidated, but always exhibited peak behavior and positive pore pressures in the stress strain curve. This study sets the stage for further investigation of more complicated organic cations, such as tetramethylammonium (TEA) and hexdecyltrimethylammonium (HDTMA).