Craig H. Benson

The effectiveness of fly ash use in the stabilization of organic soils and the factors that are likely to affect the degree of stabilization were studied. Unconfined compression and resilient modulus tests were conducted on organic soil–fly ash mixtures and untreated soil specimens. The unconfined compressive strength of organic soils can be increased using fly ash, but the amount of increase depends on the type of soil and characteristics of the fly ash. Resilient moduli of the slightly organic and organic soils can also be significantly improved. The increases in strength and stiffness are attributed primarily to cementing caused by pozzolanic reactions, although the reduction in water content resulting from the addition of dry fly ash solid also contributes to strength gain. The pozzolonic effect appears to diminish as the water content decreases. The significant characteristics of fly ash that affect the increase in unconfined compressive strength and resilient modulus include CaO content and CaO/SiO2...

Laboratory experiments were conducted on a geosynthetic clay liner (GCL) containing Na–bentonite to determine how the swell index and hydraulic conductivity of GCLs are affected by wet-dry cycling with solutions having different relative abundance of monovalent and multivalent cations. Relative abundance of monovalent and multivalent cations was characterized by the RMD of the test solution, which is defined as the ratio of the total molarity of monovalent cations to the square root of the total molarity of multivalent cations at a given ionic strength. RMD was found to control the final swell index, relative abundance of monovalent and divalent cations in the final exchange complex, and the final hydraulic conductivity of bentonite exposed to wet-dry cycling. Ionic strength affects the number of wet-dry cycles required for a change in hydraulic conductivity to occur and the rate of change in swell index. Large increases in hydraulic conductivity and loss of swelling capacity occurred for solutions having...

Water-balance predictions made using four codes (UNSAT-H, VADOSE/W, HYDRUS, and LEACHM) are compared with water-balance data from a test section located in a semiarid climate simulating a monolithic water-balance cover. The accuracy of the runoff prediction (underprediction or overprediction) was found to affect the accuracy of all other water-balance quantities. Runoff was predicted more accurately when precipitation was applied uniformly throughout the day, the surface layer was assigned higher saturated hydraulic conductivity, or when Brooks-Corey functions were used to describe the hydraulic properties of the cover soils. However, no definitive or universal recommendation could be identified that would provide reasonable assurance that runoff mechanisms are properly simulated and runoff predictions are accurate. Evapotranspiration and soil-water storage were predicted reasonably well (within ≈25 mm∕yr ) when runoff was predicted accurately, general mean hydraulic properties were used as input, and the...

This study evaluated the effects of physical characteristics and geologic factors on the shear strength of compacted sands from Wisconsin that are used as granular backfill for mechanically stabilized earth walls and reinforced soil slopes. Physical properties and shear strength were determined for 30 compacted sands collected from a broad range of geological deposits. Relationships between strength/deformation behavior, geologic origin, and physical properties were used to categorize the sands into four friction angle groups. Sands with the lowest friction angle are derived from weathering of underlying sandstones, and tend to be medium-fine, well-rounded, and poorly graded sands. Sands with the highest friction angle are from recent glacial activity and tend to be coarser grained, well-graded, and/or angular sands. A multivariate regression model was developed that can be used to predict friction angle ( ϕ′ ) of compacted sands from comparable geological origins based on effective particle size ( D10 ) ...

Predictions of surface runoff (R) , evapotranspiration (ET), soil–water storage (S) , and percolation obtained using three numerical codes (LEACHM, HYDRUS, and UNSAT-H) employed to simulate the hydrology of water-balance covers are compared to measured water-balance data from a lysimeter used to monitor a capillary barrier cover profile in a subhumid climate. All of the codes captured the seasonal variations in water-balance quantities observed in the field. LEACHM and HYDRUS predicted total R during the monitoring period with reasonable accuracy (within 18 mm using general mean parameters), but the timing of predicted and observed R events was different. In contrast, UNSAT-H consistently overpredicted R by at least 239 mm . Evapotranspiration was predicted reliably (within 60 mm ) with all three codes when data from the first year were excluded. However, all three codes overpredicted ET in late winter and early spring, when snowmelt was occurring and S was accumulating in the field. Consequently, S gener...

Groundwater flow and geochemical reactive transport models were used to assess the effectiveness of two strategies that are used to enhance the long-term hydraulic behavior of continuous-wall permeable reactive barriers (PRBs) employing granular zero valent iron (ZVI). The flow model accounted for geological heterogeneity and the reactive transport model included a geochemical algorithm for simulating iron corrosion and mineral precipitation reactions that have been observed in ZVI PRBs. The two strategies that were evaluated are (i) adding pea gravel equalization zones up gradient and down gradient of the reactive zone and (ii) placement of an up gradient sacrificial pre-treatment zone containing a mixture of pea gravel and ZVI. Results of simulations show that installation of pea gravel equalization zones results in a more uniform distribution of residence times within the PRB, slightly greater average porosity reductions, and smaller maximum porosity reductions. Sacrificial pre-treatment zones elevate the groundwater pH and consume many of the mineral forming ions that form secondary minerals, reducing mineral precipitation in the reactive zone. However, mineral fouling by Fe(OH)₂ still occurs within the PRB in response to iron corrosion reactions.

This paper describes a study of sustainable roadway construction where cementitious fly ashes (Class C and off-specification) were used to stabilize road surface gravel (RSG) in situ to form base for asphalt paving during reconstruction of a roadway in Minnesota. In situ stabilization of the existing RSG provided significant cost savings compared to traditional reconstruction where road surface materials are replaced with new aggregate, was more expedient than traditional construction, and enhanced sustainability by re-using existing roadway materials (RSG) and an industrial byproduct (fly ash). California bearing ratio (CBR) and resilient modulus (Mr) tests were conducted on the RSG alone and on fly-ash stabilized RSG (S-RSG) prepared in the field and laboratory. After 7 d of curing, CBR and Mr of the S-RSG were at least 2 times higher than those of the road surface gravel alone. Falling weight deflectometer surveys have shown that the in-situ elastic modulus of the S-RSG varies seasonally, with the lowest moduli occurring in spring when the pavement profile is wet and the highest occurring in late fall when the profile is drier. The findings indicate that fly ash stabilization of RSG renders it a suitable base material for paving.

This paper describes a case study where asphaltic recycled pavement material (RPM) blended with Class C fly ash was used as base course during reconstruction of a 0.5-km section of asphalt pavement in Waseca, Minnesota. The RPM was blended with cementitious fly ash to increase its bearing resistance and stiffness. California bearing ratio (CBR) and resilient modulus (M_r) tests were conducted on the RPM alone and RPM blended with fly ash (referred to as stabilized RPM, or SRPM). Falling weight deflectometer (FWD) tests were also conducted on the as-built pavement. Addition of fly ash improved the bearing resistance and stiffness of the RPM significantly. After 7 d of curing, CBR of the SRPM (13 to 94) was three to nine times the CBR of the RPM alone (3 to 17) and M_r of the SRPM ranged between 50 and 120 MPa, whereas the M_r of the RPM was 47 MPa. The FWD tests demonstrated that the SRPM maintained its stiffness even after exposure to freeze-thaw cycling during the first winter after construction.

This paper describes the contribution of geosynthetic reinforcement to improved stiffness of a granular layer originally constructed as a working platform but serves as a subbase layer after pavement structure is constructed. Large-scale model experiments were conducted that demonstrate the improved stiffness of granular layers reinforced with 4 types of geosyntetics. The load-elastic deflection data is analyzed by dividing the granular layer into sublayers including a reinforced sublayer with a thickness of 100 mm to isolate the effect of reinforcement on elastic modulus. The comparison of the elastic modulus parameter of this layer to that of the unreinforced sublayers indicate that there is an increase of 1.1 to 3.0 times in the elastic modulus depending on the type of geosynthetic. This improvement is also a function of the overall layer thickness because of the effect of shear strain amplitude that is not accounted for in the elastic modulus model.

This paper describes findings from leach testing, total element analyses, and kinetic batch tests conducted to evaluate the feasibility of using iron foundry slag (an industrial byproduct of iron casting) as a reactive material in permeable reactive barriers used to remove arsenic (As) from groundwater. Tests were conducted on four slags from foundries in Wisconsin using solutions containing arsenite (As-III) and arsenate (As-V) having initial total As concentrations ranging from 100–2500 μg/L. Comparative tests were also conducted on zero valent iron (ZVI) and sand as a non-reactive control. Water leach testing and total elemental analyses suggested that leaching from slags is not appreciably different from ZVI, at least in the context of environmental regulations in Wisconsin. For the same liquid-to-solid ratio, As removal was slower with slag than ZVI. Arsenic removal rates also varied considerably from slag to slag, and did not depend on the quantity of iron in the slag. The removal rate typically decreased with increasing concentration, and was slower for As-III than As-V. Computations made using the rate coefficients measured in this study suggest that a common 1-m-thick PRB containing iron foundry slag could effectively treat groundwater contaminated with As-III or As-V at a concentration of 1000 μg/L, although other factors such as slag heterogeneity and competition with other species might influence effectiveness in a field setting.