Measuring the Hydraulic Conductivity of Soil–Bentonite Backfill

The hydraulic conductivity of soil–bentonite backfill in three pilot-scale cutoff walls was measured using laboratory tests on disturbed samples, laboratory tests on undisturbed samples, piezocone dissipation tests, and piezometer tests (also known as slug tests or single-well tests). In addition, a global measurement of the average hydraulic conductivity of the soil–bentonite backfill in one of the cutoff walls was made using the pilot-scale test facility. Two main factors distinguish these five different methods of measuring hydraulic conductivity: remolding and sample size. Remolding of samples tested in American Petroleum Institute filter press equipment significantly reduced their hydraulic conductivity compared to the hydraulic conductivity of undisturbed samples, which were of similar size. For the other tests, where the degree and extent of remolding were less significant, hydraulic conductivity was found to increase as sample size increased, with the global measurement producing the highest value. The existence of bentonite filter cakes on trench walls reduces the influence of sample size on the equivalent hydraulic conductivity of the barrier. Findings regarding locating defects with a piezocone and hydraulic fracture in piezometer tests are also presented.     

Methodology for Determining Laboratory and In Situ Hydraulic Conductivity of Asphalt Permeable Friction Course

The permeable friction course (PFC) is a layer of porous asphalt pavement overlain on conventional impervious hot-mix asphalt or portland cement concrete. The drainage properties of PFC are typically considered to be governed primarily by two hydraulic properties: hydraulic conductivity and porosity. Both of these hydraulic properties change over the life cycle of the PFC layer due to clogging of the pore space by sediment. Therefore, determination of the hydraulic conductivity and porosity of PFC can be problematic. Laboratory and particularly field tests are necessary for accurately determining the hydraulic conductivity of the PFC layer. Taking multiple measurements over the life of the pavement shows how these hydraulic characteristics change with time and the varying roadway conditions at which they are evaluated. Constant head laboratory testing has shown that PFC experiences a nonlinear flow relationship as described by the Forchheimer equation. In addition to the laboratory analysis of the hydraulic characteristics, a falling head field test is recommended to determine the in situ hydraulic conductivity. This incorporates the modeling techniques used in the laboratory testing and applies them to the falling head conditions used in the field. The result is a nondestructive test procedure for determining the in situ hydraulic conductivity which is necessary for estimating the extent to which the benefits associated with the drainage characteristics of the PFC layer will persist.     

Hydraulic Conductivity and Compressibility of Soil-Bentonite Backfill Amended with Activated Carbon

Flexible-wall permeability tests and rigid-wall consolidation/permeability tests were performed to evaluate the hydraulic conductivity and compressibility of a model soil-bentonite (SB) backfill amended with granular activated carbon (GAC) or powdered activated carbon (PAC). The tests were performed as part of an assessment of enhanced SB backfill with improved attenuation capacity for greater longevity of barrier containment performance. Backfill specimens containing fine sand, 5.8% sodium bentonite, and GAC or PAC (0, 2, 5, and 10% by dry weight) were prepared to target slumps of 125±12.5?mm. Hydraulic conductivity (k) and compressibility of backfill test specimens were measured in consolidometers as a function of effective stress, σ′ (24 ? σ′ ? 1,532?kPa), whereas flexible-wall k was measured for backfill specimens consolidated to σ′ = 34.5?kPa. The results indicate that addition of GAC has little impact on the hydraulic and consolidation properties of the backfill, whereas addition of PAC causes a decrease in k and consolidation coefficient (cv) and a slight increase in compression index (Cc). Differences in behavior between GAC-amended backfills and PAC-amended backfills are attributed primarily to differences in GAC and PAC particle size.     

Membrane Behavior of Two Backfills from Field-Constructed Soil-Bentonite Cutoff Walls

Two soil-bentonite cutoff-wall backfills obtained from construction sites, one in New Jersey and one in Delaware, were tested for the existence of membrane behavior. Both backfills were designed as a mixture of dry bentonite (3–4% by dry weight) and the locally excavated soil blended with bentonite water slurry to provide slumps ranging from 100 to 150?mm (from 4 to 6?in.). The results of the membrane tests indicate that both backfills exhibit membrane behavior. Further, the magnitude of the membrane behavior increases with decreasing void ratio. However, the magnitude of the increase in membrane behavior in these construction-site backfills was lower than that previously reported for model backfills prepared in the laboratory. The difference in the membrane behavior is attributed, in part, to a lower percentage of clay in the construction-site backfills relative to the model backfills. Nonetheless, based on the measured membrane efficiencies for the two field-constructed backfills, the total liquid flux (q) through the cutoff walls can be expected to be reduced relative to that in the absence of membrane behavior (qh) by 1–10% for the cutoff wall in Delaware and 7–8% for the cutoff wall in New Jersey, depending on the void ratio. Thus, the results of this study suggest that membrane behavior in field-constructed cutoff walls can be significant, depending on the void ratio and the clay content of the backfill.     

Undrained Anisotropic Monotonic Behavior of Sand from In Situ Tests

A procedure for estimating the undrained stress-strain behavior of sand from drained self-boring pressuremeter and seismic piezocone penetration tests is proposed in this paper. The procedure offers an inexpensive alternative to laboratory testing and avoids the uncertainty of the empirical methods based on index measurements such as the Standard Penetration Test blow count and the tip resistance in a Piezocone Penetration Test (CPTU). To check its validity, the proposed procedure was used to infer the undrained triaxial stress-strain curves and the results were compared with laboratory triaxial tests on undisturbed samples. The undrained limit equilibrium stability of a dike was also assessed using the inferred stress-strain behavior to illustrate the usefulness of the procedure. The result of the stability analysis was found to be in qualitative agreement with the observed performance of the dike during a recent field experiment attempting to trigger static liquefaction.     

Hydraulic Conductivity of Soil Sorptive Zones Created by In Situ Injection of a Cationic Surfactant

The sorptive capabilities of soils for organic contaminants can be greatly enhanced by treatment with cationic surfactants, and this has been suggested as a potential in situ approach for contaminant plume management. The hydraulic properties of soils modified by injection of hexadecyltrimethylammonium (HDTMA) were investigated using soil columns and a fixed-ring consolidometer. Oshtemo soil (87% sand, 10.5% clay, 2.5% silt) under two different effective stresses, was equilibrated with 1?mM NaCl and treated by recirculation of two different HDTMA soil concentrations, one above and one below the cation exchange capacity. No statistically significant changes in hydraulic conductivity occurred as a result of HDTMA treatment at any of the experimental conditions studied. These results suggest that sorptive zones created in situ with HDTMA may be hydraulically feasible.     

Classroom Activities to Illustrate Concepts of Darcy’s Law and Hydraulic Conductivity

A Darcy bottle is a water bottle that is filled with porous material and liquid, and that contains a drain assembly on the bottom of the bottle to allow liquid to drain. A collection of Darcy bottles with different cross-sectional area, different types and amounts of porous material, and different types and amounts of liquid are used in a series of classroom activities to illustrate Darcy’s law and basic concepts of hydraulic conductivity. Each activity can be completed in a few minutes, and can be conducted during a regular class period to supplement the lecture. Assessment results show that the Darcy bottle activities enhance student learning, particularly for less intuitive concepts.     

Hydraulic Conductivity and Swell of Nonprehydrated Geosynthetic Clay Liners Permeated with Multispecies Inorganic Solutions

The influence of multispecies inorganic solutions on swelling and hydraulic conductivity of non-prehydrated geosynthetic clay liners (GCLs) containing sodium bentonite was examined. Ionic strength and the relative abundance of monovalent and divalent cations (RMD) in the permeant solution were found to influence swell of the bentonite, and the hydraulic conductivity of GCLs. Swell is directly related to RMD and inversely related to ionic strength, whereas hydraulic conductivity is directly related to ionic strength and inversely related to RMD. RMD has a greater influence for solutions with low ionic strength (e.g., 0.05?M), whereas concentration effects dominate at high ionic strength (e.g., 0.5?M). No discernable effect of cation species of similar valence was observed in the swell or hydraulic conductivity data for test solutions with similar ionic strength and RMD. A strong relationship between hydraulic conductivity and free swell was found, but the relationship must be defined empirically for a particular bentonite. A regression model relating hydraulic conductivity of the GCL to ionic strength and RMD of the permeant solution was developed. Predictions made with the model indicate that high hydraulic conductivities (i.e., >10?7?cm/s) are not likely for GCLs in base liners in many solid waste containment facilities. However, for wastes with stronger leachates or leachates dominated by polyvalent cations, high hydraulic conductivities may occur.     

Evolution of the Hydraulic Conductivity of Reclamation Covers over Sodic/Saline Mining Overburden

The evolution of the field saturated hydraulic conductivity of four covers located on a reclaimed saline-sodic shale overburden from oil sands mining is presented. Three covers consisted of a surface layer of peat/glacial topsoil over a mineral, soil. and one cover was a single layer of mixed peat and mineral soil. Measurements of the field saturated hydraulic conductivity of the cover and shale materials were made with a Guelph permeameter between 2000 and 2004. The hydraulic conductivity of the cover materials in the multilayered covers increased by one to two orders of magnitude over the first few monitoring seasons. The hydraulic conductivity of the single-layer cover system, which was placed three years before the multilayered covers, marginally increased from 2000 to 2002 and then remained relatively unchanged. The hydraulic conductivity of the shale underlying all four covers increased approximately one order of magnitude. Soil temperature measurements indicated that one freeze/thaw cycle occurred each year within all cover soils and the surficial overburden. This suggests that freeze/thaw effects were the cause of the observed increases in hydraulic conductivity, as previously observed by other researchers working on compacted clays.     

Near-Infrared Spectroscopy for In Situ Monitoring of Geoenvironment

This paper demonstrates the usefulness of near-infrared optical fiber analysis for sensing moisture and liquid hydrocarbons in soil. Through experiments we have carried out sensing probes which have been developed, comprising optical fibers that use the evanescent field of the guided energy. The movement of water through dry sand was simulated in the laboratory and the sensors were used in situ to measure the variation of soil moisture in real time. A similar experiment simulated the movement of an organic liquid (mineral oil) through water-saturated sand, and the sensors were used in situ to monitor the hydrocarbon movement. We found that a hydrophobic polymer-coated waveguide can amplify the hydrocarbon signal while minimizing that of water, making it possible to detect a dissolved hydrocarbon. Tests show that the second derivative transform of the absorption spectra could be used to distinguish classes of hydrocarbons.     

Mass Transfer of Ozone in a Novel In Situ Ozone Generator and Reactor

A porous tubular reactor that also served as an electrode for ozone generation was studied in this research to determine the effects of in situ ozone generation on mass transfer and reaction rates. Experimental data over a range of gas flow rates and ozone generation rates gave KLa values in the range 0.77–1.14?min?1. These values are more than double the values typically reported for bubble columns, and about 30% higher than that for packed beds. The specific power requirement for the laboratory-scale in situ reactor is an order of magnitude lower than that for bubble columns and stirred tank reactors that are used for ozone dissolution. A compartments-in-series fluid flow model was developed to describe the reactor system, and this model provides a good comparison to the experimental data for dissolved ozone and off-gas concentrations in the reactor. Sensitivity analyses indicate that the dissolved and off-gas ozone profiles are most sensitive to the gas–liquid partition coefficient and the overall mass transfer coefficient.     

In Situ Performance Testing of Deteriorating Water Tanks for Durability Assessment

Reports of failure of existing concrete structures due to a lack of durability, rather than a deficiency in structural strength, has made concrete technologists, engineers, and researchers focus research on the parameters influencing durability performance with respect to time. Systematic performance monitoring, with respect to chosen durability parameters of existing concrete structures, will decide the direction of future research in this area. Inferences based on laboratory simulations and testing need to be confirmed by in situ field measurements and studies. In situ condition rating and performance monitoring surveys have been conducted by many researchers, scientists, and professional associations, and reported in literature. Inferences of few such studies are summarized and discussed. Deterioration of concrete structures constructed in recent times is observed at relatively faster rates, and has been mainly attributed to cracking. Cracking is associated with the use of faster-hydrating portland cements with increased fineness and the tricalcium silicate (C3S) content to support the high speed of modern construction. In the present research, a case study of deteriorated water tank structures situated in the semitropical region of India is presented. Some selected parameters—such as concrete cover, carbonation depth, chloride concentration, compressive strength, etc. which influence long term durability of structures—have been measured.     

Data Interpretation for In Situ Measurements of Cohesive Sediment Erosion

Methods for interpreting data from in situ flume measurements of cohesive sediment dynamics are evaluated and a technique for estimating various erosion parameters using in situ measurements is proposed. There is currently a lack of uniformity in analysis techniques for cohesive erosion data collected in flumes and with in situ instruments and the proposed technique resolves some of these inconsistencies. The data set used in this study was derived from field experiments conducted with a straight benthic in situ flume in different aquatic environments in New Zealand. The experiments with stepwise increases in flow velocity revealed that peaks in the erosion rate at the beginning of each velocity step are most likely associated with heterogeneous bed structure, as transient hydrodynamic effects due to the experimental procedure were found to be insignificant. The field data showed an exponential decay of the erosion rate with time that is indicative of depth-limited erosion. These data are used to illustrate methods for the parameterization of the proposed semiempirical erosion equation, taking into account the time dependency of the erosion process.     

CPT-Based Probabilistic and Deterministic Assessment of In Situ Seismic Soil Liquefaction Potential

This paper presents a complete methodology for both probabilistic and deterministic assessment of seismic soil liquefaction triggering potential based on the cone penetration test (CPT). A comprehensive worldwide set of CPT-based liquefaction field case histories were compiled and back analyzed, and the data then used to develop probabilistic triggering correlations. Issues investigated in this study include improved normalization of CPT resistance measurements for the influence of effective overburden stress, and adjustment to CPT tip resistance for the potential influence of “thin” liquefiable layers. The effects of soil type and soil character (i.e., “fines” adjustment) for the new correlations are based on a combination of CPT tip and sleeve resistance. To quantify probability for performance-based engineering applications, Bayesian “regression” methods were used, and the uncertainties of all variables comprising both the seismic demand and the liquefaction resistance were estimated and included in the analysis. The resulting correlations were developed using a Bayesian framework and are presented in both probabilistic and deterministic formats. The results are compared to previous probabilistic and deterministic correlations.     

Assessing of Natural Attenuation and Intrinsic Bioremediation Rates at a Petroleum-Hydrocarbon Spill Site: Laboratory and Field Studies

Natural attenuation is a passive remedial approach that depends upon natural processes to degrade and dissipate contaminants in soil and groundwater. Intrinsic bioremediation is believed to be the major process among the natural attenuation mechanisms that account for the reduction of contaminant concentrations. In this study, a mass flux approach was used to calculate the contaminant mass reduction at a petroleum-hydrocarbon spill site. The mass flux technique is a simplified mass balance procedure, which is accomplished using the differences in total contaminant mass flux across two cross sections of the contaminant plume. The mass flux calculation results show that up to 86% of the dissolved total benzene, toluene, ethylbenzene, and xylene (BTEX) isomers removal was observed via natural attenuation at this site. Evidence for the occurrence of natural attenuation was the decreased contaminant mass flux through the plume cross sections along the transport path and limited spreading of the BTEX plume. Evidences for the BTEX biodegradation include: (1) decreased BTEX concentrations along the transport path; (2) depletion of dissolved oxygen, nitrate, and sulfate; (3) production of dissolved ferrous iron, sulfide, methane, and CO2; (4) deceased pH in the spill source area and increased pH in iron-reducing area; (5) increased alkalinity and microbial populations; and (6) preferential removal of certain BTEX components along the transport path. The effectiveness of intrinsic bioremediation on BTEX removal was evaluated by the in situ tracer method. Results reveal that approximately 74% of the BTEX removal was due to the intrinsic biodegradation process. The first-order decay model was applied for the natural attenuation and intrinsic bioremediation rates calculation. Results show that the biodegradation capacity (34.5 mg/L) for BTEX was much higher than the detected contaminants within the plume. The calculated total BTEX first-order natural attenuation and intrinsic bioremediation rates were 0.025 and 0.017% 1/day, respectively. Results of polymerase chain reaction, denaturing gradient gel electrophoresis, and nucleotide sequence analysis reveal that some petroleum-hydrocarbon degraders (Flavobacterium capsulatum, Xanthobacter sp., Xanthobacter flavus, Xanthomonas codiaei, Pseudomonas boreopolis, Methylobacterium sp., Reichenowia pictae) might exist at this site, which might contribute to the BTEX biodegradation. Results suggest that the natural attenuation mechanisms can effectively contain the plume, and the mass flux method is useful in assessing the occurrence and efficiency of the natural attenuation and intrinsic bioremediation processes.     

In Situ Pore Pressure Responses of Native Peat and Soil under Train Load: A Case Study

Settlement and formation of piping holes on surfaces were observed along a rail embankment subject to normal traffic load. Piezometers were installed in the native peat and soil underneath the embankment inside and outside problematic area to measure the pore pressure responses during train traffic. Peculiar pore pressure responses were observed. Cyclic pore pressures were only measured during the first 60–80?s of the 6-min train passage, and thereafter the pressures decayed rapidly to the initial values. The pore pressure changes in the shallow peat layer were lower than those in the deep soil layer. Possible mechanisms causing such peculiar pore pressure responses, surface settlement, and piping holes were explored and identified. It was found that the stiffness contrast between the stiff, upper granular fill and the soft, native peat material could lead to a redistribution of tensile stress in the granular fill layer to the peat layer due to the moving train load. This stress redistribution promotes the propensity of vertical piping in the peat layer.     

Development of a Field Design for In Situ Gaseous Treatment of Sediment Based on Laboratory Column Test Data

A testing methodology is presented that supports the development of a field design for in situ gaseous treatment of sediments with diluted hydrogen sulfide. This approach involves the collection of column breakthrough test results at various flow rates, allowing a relationship to be developed between pore velocity of the carrier gas and velocity of the hydrogen sulfide reaction front that permits sizing to the field scale. A regression fit of a set of laboratory column breakthrough test data collected in this study is utilized to illustrate the development of a field design based on a two-dimensional radial flow analytical model. Information regarding treatment time and hydrogen sulfide consumption characteristics associated with in situ gaseous treatment can then be obtained from this model and used as a basis for estimation of treatment schedule and costs. The regression relationship can also be utilized in numerical models in more complex geometries to support the field design of in situ gaseous treatment operations.     

Effect of Uncertain Hydraulic Conductivity on the Simulated Fate and Transport of BTEX Compounds at a Field Site

Monte Carlo simulations were conducted to investigate the effect of uncertain hydraulic conductivity (K) on the natural attenuation of BTEX compounds (benzene, toluene, ethyl benzene, and xylenes) through aerobic respiration, denitrification, Fe(III) and sulfate reductions, and methanogenesis observed at a field site on Hill Air Force Base, Utah. First, the uncertainty in the ln?K field was represented by multiple realizations of spatially correlated random fields. The simulated BTEX plumes resulting from a light nonaqueous phase liquid source were analyzed for mass distributions and the relationships among various factors such as dissolved BTEX mass, plume spreading, and depletions of electron acceptors or productions of Fe2+ and methane. Second, additional K realizations with the same mean but different variances and correlation lengths were used to determine how the model responds to varying degrees of uncertainty in the K field. The methodology and insights are of general interest and applicable to fuel-hydrocarbon natural-attenuation sites.