Stabilized Dredged Material. II: Geomechanical Behavior

This study presents the results of a detailed geotechnical evaluation of six stabilized dredged material (SDM) blends incorporating various combinations of lime, cement kiln dust, high alkali and slag cements, and Class F fly ash. The dredged material classified as CH/OH soil with an in situ moisture content (MC) of approximately 130% and void ratio of 3.35. Mix designs and unconfined compression strength tests were completed for each SDM blend based on 3-day mellowing characteristics. Compacted dry densities were on the order of 7.8–11.2?kN/m3 (49–71?lb/ft3), with MCs on the order of 34–73%. Peak effective friction angles ranged from 20–50° with cohesion intercepts on the order of 30–235 kPa (4–34?lb/in.2) using a maximum stress obiliquity criterion. Postpeak effective friction angles (15% axial strain) were routinely in excess of 40° with low cohesion (<40?kPa; 6?lb/in.2). One sample exhibited very strong soil-fabric effects (cohesion) having an effective friction angle of only approximately 9°, but cohesion on the order of 450 kPa (65?lb/in.2). Negligible consolidation of a 28-day cured sample was measured. Also, contrary to expectations based on the high sulfate contents (10,000–30,000 mg/kg) of the SDM blends, negligible swell (<1%) was measured in five of six SDM blends. The main finding of this research is the SDM blends exhibit the strength, compressibility, and bulking characteristics that make them favorable for large fill applications and subgrade improvement applications at costs equivalent to or less than conventional construction materials.     

Stabilized Dredged Material. III: Mineralogical Perspective

The prior two papers in this series reported on the geoenvironmental and geomechanical properties of 20 stabilized dredged material (SDM) blends using dredged material (DM) from the U.S. Army Corps of Engineers Craney Island confined disposal facility. The pozzolans included lime, cement kiln dust (CKD), class F fly ash, and two cements (portland and slag cement). This paper reports on the mineralogical evolution of the SDM blends over a 6-month curing period using techniques new to mainstream geotechnical engineering: X-ray diffraction (XRD) with Rietveld quantification analysis which allows direct quantitative mineralogical comparisons between soil samples. Despite being classified as a high plasticity clay-organic clay (CH/OH soil), XRD showed that the DM contained no montmorillonite, illite or kaolinite, and was thus mineralogically unreactive. The quartz, feldspar, and mica contents were numerically tracked and were shown to remain stable 6 months after blending. The chlorite (in DM) content decreased over time and with the fly ash served as the sources of soluble silica and alumina for pozzolanic reactions especially in the lime-based SDM blends. Lime in the lime-based blends persisted in significant quantities (3%) as unreacted portlandite [Ca(OH)2] even at 6 months curing, indicating that the solubility of silica in the DM was the limiting factor for strength development. New (ettringite and hydrocalumite) mineral formation was quantified. CKD provided high early strength (7 and 28 days) when used in combination with small amounts of lime that provided prolonged pH buffering; CKD alone or in combination with fly ash did not maintain elevated pH (>10.8) over 6 months. Overall, the unconfined compressive strength, pH, and mineralogy results at 6 months were substantially different compared to the standard curing time of 28 days, confirming similar findings of previous long-term stabilization-solidification studies.     

Laboratory Evaluation of Crushed Glass–Dredged Material Blends

A comprehensive laboratory evaluation of blending 9.5?mm (3/8?in.) minus curbside-collected crushed glass (CG) with dredged material (DM) was conducted to evaluate their potential for beneficial use as fill materials for urban applications. Tests were performed on 100% CG (USCS classification SP) and 100% DM (OH) specimens and 20/80, 40/60, 50/50, 60/40, and 80/20 CG–DM blends (dry weight percent CG content reported first). The addition of 20% CG resulted in a 10–20 point (33–67%) reduction in wopt while increasing the dry density by approximately 1–3?kN/m3 for standard and modified levels of compaction, respectively. Simultaneously, the compressibility of the DM was reduced by approximately 50% and the hydraulic conductivity was reduced by ? order of magnitude. The addition of 20% CG significantly decreased the moisture content and significantly improved the workability of the 100% DM, where workability refers to the ease of handling, transport, placement, and compaction of the CG–DM blends (compared to 100% DM). CIū triaxial strength testing indicated effective friction angles of 34 and 37° for 100% DM and CG compacted to a minimum of 95% relative compaction by ASTM D1557, respectively. A peak effective friction angle of 39° occurred for the 60/40 and 80/20 CG–DM blends which were also 1 and 3 orders of magnitude more permeable than 100% DM, respectively. Related increases in cv resulted in decreased times required for consolidation. The range of properties obtainable by the CG–DM blends offers a versatility that allows for the design of fills that can be potentially optimized to meet multiple design parameters (e.g. strength, settlement, drainage, or higher CG or DM content).     

Field Evaluation of Crushed Glass–Dredged Material Blends

Based on the laboratory results reported in a companion paper, three crushed glass–dredged material (CG–DM) blends were prepared and evaluated in the field to explore the feasibility of using CG–DM blends in general, embankment and structural fill applications. A trailer-mounted pugmill successfully prepared 20/80, 50/50, and 80/20 CG–DM blends (dry weight percent CG content reported first) within a tolerance of ±5 dry % by weight of the targeted percentages. Blending criteria were routinely met at pugmill throughputs up to 1,500?m3/day. The constructed 20/80 CG–DM embankment was compacted to a minimum of 90% modified Proctor compaction, whereas the 50/50 and 80/20 CG–DM embankments were constructed to a minimum of 95% modified Proctor compaction. Twenty to 80% CG addition to DM resulted in 1.5–5.5?kN/m3 increases in field dry densities above 100% DM, densities not achievable with other DM stabilization techniques such as Portland cement, fly ash, and/or lime (PC/FA/lime) addition. CG substantially improved the workability of DM allowing construction with conventional equipment and three person crew while achieving very consistent and reproducible results during a timeline of frequent and heavy precipitation events. The 20/80, 50/50, and 80/20 CG–DM embankments were characterized by average cone tip resistances on the order of 1.0, 1.5, and 2.0?MPa, respectively. An environmental evaluation of 100% CG, DM and 50/50 CG–DM blend samples coupled with an economic analysis of a scaled-up commercial application illustrated that the CG–DM blending approach is potentially more cost effective than PC/FA/lime stabilization approaches. These features of CG–DM blending make the process attractive for use in urban and industrial settings.     

Crushed Glass-Dredged Material (CG-DM) Blends: Role of Organic Matter Content and DM Variability on Field Compaction

Trial embankments comprised of crushed glass-dredged material (CG-DM) blends and a 100% DM embankment were constructed to provide the necessary data sets to determine if a moisture content (MC) correction was required for the nuclear density (ND) gauge, as DM may contain a high organic matter content (OC). The MCs of thin-walled tube samples of CG-DM blends collected immediately below the ND gauge were compared to the corresponding ND gauge readings. A direct correlation between the MC data pairings from the tube samples and ND gauge readings showed that the ND gauge was greater than 97% accurate for MCs up to 55% and OCs up to 10% for the CG-DM blends evaluated in this study. However, the MC determined by the ND gauge was underpredicted (not overpredicted) by approximately 2.5%, contrary to theoretical expectations. A comparison of the average MC results per embankment indicated that the ND gauge was generally within 1% of the tube sample values, again on the low side. Interestingly, the rutting of the individual embankment lifts, often used as an informal metric for compaction compliance also was found to be contrary to expectations. The (re)constructed CG-DM embankments of this study were again shown to satisfy local Department of Transportation embankment construction criteria in most cases.     

Aging of Crushed Glass-Dredged Material Blend Embankments

Four crushed glass (CG) and dredged material (DM) [(CG-DM)] blend embankments constructed (2004) and reconstructed (2005) to local DOT specifications were subjected to cone penetrometer tests (CPT). The CPT resistance of the original set of embankments was evaluated shortly after construction and approximately 360?days later, immediately prior to being demolished for purposes of a second study. Cone tip resistances were observed to double to triple with aging. For the 80/20 CG-DM blend, a 4?MPa [40 tons per square foot (tsf)] or threefold increase in CPT tip resistance was measured. Likewise, isotropically consolidated, undrained triaxial shear tests were performed on relatively undisturbed thin-walled tube specimens of the 360-day aged CG-DM blend materials. The triaxial tests revealed that the effective friction angles of the aged materials increased by up to 8° over freshly prepared laboratory CG-DM blend specimens. The strength gains appeared to be more strongly linked to (amorphous) silica cementation rather than the formation of carbonates. Disturbance (demolition and reconstruction) generally reduced the in situ CPT behavior to that of the originally constructed embankments.     

Parametric Studies on the Behavior of Reinforced Soil Retaining Walls under Earthquake Loading

The finite element procedures are extremely useful in gaining insights into the behavior of reinforced soil retaining walls. In this study, a validated finite element procedure was used for conducting a series of parametric studies on the behavior of reinforced soil walls under construction and subject to earthquake loading. The procedure utilized a nonlinear numerical algorithms that incorporated a generalized plasticity soil model and a bounding surface geosynthetic model. The reinforcement layouts, soil properties under monotonic and cyclic loadings, block interaction properties, and earthquake motions were among major variables of investigation. The performance of the wall was presented for the facing deformation and crest surface settlement, lateral earth pressure, tensile force in the reinforcement layers, and acceleration amplification. The effects of soil properties, earthquake motions, and reinforcement layouts are issues of major design concern under earthquake loading. The deformation, reinforcement force, and earth pressure increased drastically under earthquake loading compared to end of construction.     

Support Mechanisms of Rammed Aggregate Piers. I: Experimental Results

This paper is the first of a two-part series investigating the mechanical behavior of rammed aggregate pier (RAP) groups supporting isolated rigid footings. The first paper presents the experimental test results from instrumented load tests performed on two different 2.3?m square reinforced concrete footings supported by four 0.76?m diameter RAPs of two different pier lengths—2.8 and 5.1?m. Comparisons are made to load tests performed on three isolated RAPs of the same diameter and lengths. Instrumentation consisted of total stress cells, inclinometers, and tell-tale reference plates. Soil conditions at the test site were evaluated using various in situ testing techniques and consist of relatively uniform soft alluvial clay overlain by a 1-m-thick desiccated layer. Interpretations of the test results focused on load-deformation behaviors of the isolated piers and pier groups, group efficiencies in terms of settlement and bearing capacity, stress concentrations as a function of applied load at the top of the piers, and stress transfer with depth.     

Dynamic Response of Compacted CG, DM, and CG-DM Blends

The cyclic behavior of 9.5 mm (3/8 in.) minus curbside-collected crushed glass (CG) blended with dredged material (DM), classified as an organic silt by the Unified Soil Classification System, was evaluated using a cyclic triaxial testing program. Tests were performed on 100% CG and 100% DM specimens, and 20/80, 40/60, 60/40, and 80/20 CG-DM blends (dry CG content is reported first). The specimens were compacted to a dry unit weight equivalent to 95% of the maximum dry density based on ASTM D1557. For each material, a minimum of three specimens was tested at cyclic stress ratios of 0.20, 0.35, and 0.45. The DM used in this study exhibited significant plasticity, which would be expected to display cyclic softening behavior according to liquefaction susceptibility criteria proposed by Boulanger and Idriss in 2006. However, the high density of the material resulted in transitional behavior between cyclic mobility and cyclic softening. These findings suggest that as long as the CG, DM, and CG-DM blends are compacted, they should not be susceptible to strength loss or large strain under cyclic loading.     

Screw Conveyor Device for Laboratory Tests on Conditioned Soil for EPB Tunneling Operations

Earth pressure balanced (EPB) full face tunneling machines have experienced a remarkable increase in the number of applications throughout the world due to both mechanical developments and a more effective use of additives to condition the ground. Conditioning modifies the mechanical and hydraulic properties of a soil by making it suitable for the pressure control in the bulk chamber and extraction with the screw conveyor. The extraction system plays a fundamental role during the EPB operations particularly for a correct application of the face pressure. Despite the extensive use of the EPB technique, little knowledge exists concerning the understanding of the behavior of conditioned soil, particularly for noncohesive ground (sand and gravel). This paper presents and describes a prototype laboratory device, which simulates the extraction of the ground from a pressurized tank with a screw conveyor. The results of a preliminary test program carried out on a medium sized sand show that the prototype device is efficient in verifying the effects of foam for an optimal use in EPB conditioning.     

Soil–Nail Pullout Interaction in Loose Fill Materials

A laboratory study was conducted to investigate the behavior of soil nails embedded in loosely compacted sandy fills. By varying the overburden pressure, the peak pullout force and the load–displacement behavior were determined by carrying out pullout tests in a displacement-rate controlled manner. The test results were compared to other published ones. The present results show that the pullout resistance can be interpreted with conventional soil parameters. The effect of retrained dilatancy, which is considered to be the reason for high pullout resistance in dense materials, is negligible in loose fill materials except under very low stress level. Furthermore, pullout resistance increases with overburden pressure opposed to some field test results reported in the literature which show no systematic trend in pullout resistance with overburden pressure. A numerical model was developed to simulate the mobilization of pullout force in soil nails. It has been shown that a simple one-dimensional spring model can be used to simulate the pullout load–displacement relationship.     

Laboratory Testing Material Property and FE Modeling Structural Response of PAM-Modified Concrete Overlay on Bridges

Portland cement concrete overlay on bridge deck is subjected to distresses of cracking and interface debonding under the effects of repeated vehicle loading and temperature cycling. In order to improve the overlay performance, this research used the polyacrylamide (PAM) polymer to modify the mechanical properties of concrete. The direct shear and impact resistance tests were designed to measure the interface bonding strength and dynamic performance, respectively. The comprehensive and flexural strength and three-point bending fatigue tests were conducted following the standards. Meanwhile, the three-dimensional finite-element (FE) models of the T-girder and box-girder bridges under the moving traffic loadings were built to analyze the stress response and improve the structural design. An analytical model of flexural stress was developed and validated the FE modeling results. A rubber cushion was designed in the FE model to “absorb” the flexural stress. Laboratory testing results indicate that PAM can significantly improve the flexural strength, bonding strength, impact resistance, and fatigue life of concrete. The modified concrete with 8% PAM by mass of cement poses higher flexural strength and impact resistance than concretes with other PAM percentages. FE simulation results indicate that there exists a critical overlay thickness inducing the maximum interface shear stress, which should be avoided in the structural design. The rubber cushion can effectively relieve the flexural stress.     

Clean up of Petroleum-Hydrocarbon Contaminated Soils Using Enhanced Bioremediation System: Laboratory Feasibility Study

The objective of this study was to assess the potential of applying enhanced bioremediation on the treatment of petroleum-hydrocarbon contaminated soils. Microcosm experiments were conducted to determine the optimal biodegradation conditions. The control factors included oxygen content, nutrient addition, addition of commercially available mixed microbial inocula, addition of wood chip and rice husk mixtures (volume ratio = 1:1) as bulking agents, and addition of organic amendments (chicken manures). Results indicate that the supplement of microbial inocula or chicken manures could significantly increase the microbial populations in soils, and thus enhance the efficiency of total petroleum hydrocarbon (TPH) removal (initial TPH = 5,500?mg/kg). The highest first-order TPH decay rate and removal ratio were approximately 0.015?day?1 and 85%, respectively, observed in microcosms containing microbial inocula (mass ratio of soil to inocula = 50:1), nutrient, and bulking agent (volume ratio of soil to bulking agent = 10 to 1) during 155 days of incubation. Results indicate that the first-order TPH decay rates of 0.015 and 0.0142?day?1 can be obtained with the addition of microbial inocula and chicken manures, respectively, compared with the decay rate of 0.0069?day?1 under intrinsic conditions. Thus, chicken manures have the potential to be used as substitutes of commercial microbial inocula. The decay rate and removal ratio can be further enhanced to 0.0196?day?1 and 87%, respectively, with frequent soil shaking and air replacement. Results will be useful in designing an ex situ soil bioremediation systems (e.g., biopile and land farming) for practical application.     

Evaluation of Smear Zone Extent Surrounding Mandrel Driven Vertical Drains Using the Cavity Expansion Theory

In this study, an attempt is made to analyze the extent of the smear zone caused by mandrel driven vertical drains, employing the cavity expansion theory for soft clay obeying the modified Cam-clay model. The predictions are verified by large-scale laboratory tests, where the extent of the smear zone was estimated based on the indications, such as the pore pressure generated during mandrel driving, change in lateral permeability, and the water content reduction. This study reveals that the radius of smear zone is about 4–6 times the equivalent vertical drain radius, and the lateral permeability (inside the smear zone) is 61–92% of that of the outer undisturbed zone. Finally, the predicted size of the smear zone using the undrained cavity expansion solution is incorporated in the finite-element code PLAXIS to study the performance of a test embankment selected from the Sunshine Motorway, Queensland, Australia. A good agreement between the predicted values and field measurements was found.     

Use of Material Interfaces in DEM to Simulate Soil Fracture Propagation in Mode I Cracking

Mode I fracture is common in geomechanics in desiccation cracking, hydraulic fracture, and pressuremeter testing. The cohesive crack model has been used extensively and successfully in numerical modeling of such fracture in concrete and steel but has not been applied in modeling of soil fracture to the same extent. It is argued that the cohesive crack model may be more appropriate than linear elastic fracture mechanics (LEFM) for soils because it takes into account finite tensile strength and any likely plasticity during fracture. With special reference to the Universal Distinct Element Code (UDEC) computer program, a methodology of using interfaces in the distinct element method (DEM) of analysis to model fracture has been validated herein, and this approach is considered to be useful in geomechanical modeling applications. The methodology is based on the cohesive crack approach and shows how softening laws could be back-calculated from load-displacement curves of test specimens. It has been validated using three geometries: a tension test with a rectangular cross section, a notched three-point bend beam, and a compact tension test. Approximate softening laws for St. Albans clay from Canada are proposed.     

Methods for Quantifying Lime Incorporation into Dewatered Sludge. I: Bench-Scale Evaluation

The addition of alkaline material (usually lime) to treated municipal sludge can be used to raise the pH to ≥ 12 and generate Class A or B biosolids. When lime is added to dewatered sludge, it must first be made into a slurry before the pH can be measured to demonstrate regulatory compliance. In this study, pH 12 was achieved in slurries prepared from lime-amended dewatered sludge, even when the lime was poorly incorporated and relatively high fecal coliform levels were detected. Thus, quantitative indicators of lime incorporation are needed to complement slurry pH measurements and ensure that sufficient contact occurs between lime and sludge particles to achieve adequate stabilization. In this study, the usefulness of several potential measures of lime incorporation—pH, CO2 consumption, distribution of calcium, fecal coliforms, NH3 and reduced sulfur compound production, and ATP—was systematically evaluated using a bench-scale system. Sludge pH and CO2 consumption were not influenced by the extent of lime incorporation. The distribution of calcium and fecal coliform levels appear to be useful measures of lime incorporation. NH3 and reduced sulfur compound emissions and ATP levels can also be used to assess lime incorporation provided recommended experimental techniques are used.     

Geotechnical Properties of Cemented Sands in Steep Slopes

An investigation into the geotechnical properties specific to assessing the stability of weakly and moderately cemented sand cliffs is presented. A case study from eroding coastal cliffs located in central California provides both the data and impetus for this study. Herein, weakly cemented sand is defined as having an unconfined compressive strength (UCS) of less than 100 kPa, and moderately cemented sand is defined as having UCS between 100 and 400 kPa. Testing shows that both materials fail in a brittle fashion and can be modeled effectively using linear Mohr-Coulomb strength parameters, although for weakly cemented sands, curvature of the failure envelope is more evident with decreasing friction and increasing cohesion at higher confinement. Triaxial tests performed to simulate the evolving stress state of an eroding cliff, using a reduction in confinement-type stress path, result in an order of magnitude decrease in strain at failure and a more brittle response. Tests aimed at examining the influence of wetting on steep slopes show that a 60% decrease in UCS, a 50% drop in cohesion, and 80% decrease in the tensile strength occurs in moderately cemented sand upon introduction to water. In weakly cemented sands, all compressive, cohesive, and tensile strength is lost upon wetting and saturation. The results indicate that particular attention must be given to the relative level of cementation, the effects of groundwater or surficial seepage, and the small-scale strain response when performing geotechnical slope stability analyses on these materials.     

Investigation of Consolidation-Induced Solute Transport. I: Effect of Consolidation on Transport Parameters

This paper presents an experimental investigation of the effect of clay consolidation on parameters that govern the advective-dispersive transport of an inorganic solute. Batch, diffusion, dispersion, and solute transport tests were conducted using kaolinite clay and dilute solutions of potassium bromide (KBr). Batch tests produced the highest levels of K+ sorption and indicated that equilibrium sorption was achieved in approximately 10–30 min. The increase in sorption observed in the batch tests, as compared to the dispersion or solute transport tests, reflects the significantly lower solids-to-solution ratio and more efficient mixing process. By comparison, kaolinite consolidation had little effect on sorption due to the relatively small change in porosity. Values of hydrodynamic dispersion coefficient (Dh), effective diffusion coefficient (D?), and apparent tortuosity factor decreased with decreasing porosity. Values of D? obtained for Br? were generally larger than for K+, whereas Dh values for Br? were significantly smaller than for K+. Values of longitudinal dispersivity (α) were larger for K+ than Br? and showed no clear trend with decreasing void ratio. In general, the experimental results suggest that changes in D? and Dh should be taken into account during clay consolidation whereas the sorption isotherm and α may be considered as unchanged during the consolidation process.     

Slope Stabilizing Piles and Pile-Groups: Parametric Study and Design Insights

This paper uses a hybrid method for analysis and design of slope stabilizing piles that was developed in a preceding paper by the writers. The aim of this paper is to derive insights about the factors influencing the response of piles and pile-groups. Axis-to-axis pile spacing (S), thickness of stable soil mass (Hu), depth (Le) of pile embedment, pile diameter (D), and pile group configuration are the parameters addressed in the study. It is shown that S = 4D is the most cost-effective pile spacing, because it is the largest spacing that can still generate soil arching between the piles. Soil inhomogeneity (in terms of shear stiffness) was found to be unimportant, because the response is primarily affected by the strength of the unstable soil layer. For relatively small pile embedments, pile response is dominated by rigid-body rotation without substantial flexural distortion: the short pile mode of failure. In these cases, the structural capacity of the pile cannot be exploited, and the design will not be economical. The critical embedment depth to achieve fixity conditions at the base of the pile is found to range from 0.7Hu to 1.5Hu, depending on the relative strength of the unstable ground compared to that of the stable ground (i.e., the soil below the sliding plane). An example of dimensionless design charts is presented for piles embedded in rock. Results are presented for two characteristic slenderness ratios and several pile spacings. Single piles are concluded to be generally inadequate for stabilizing deep landslides, although capped pile-groups invoking framing action may offer an efficient solution.     

Laboratory Study of Successive Soil Saline Leaching and Electrochemical Lead Recovery

This research is related to a laboratory study on the performance of a successive soil saline leaching and electrochemical lead recovery process for soil decontamination. Erlenmeyer leaching assays showed that the addition of 5.5 mol NaCl/L in 25% (w/w) of soil pulp density maintained at pH 3.0 was found the most effective condition for Pb leaching. Under these conditions, 65% of Pb was leached from soil. Electrochemical treatment using an iron–monopolar electrode system operated at a current intensity of 3.0 A was able to reduce Pb content in soil leachate from 650 to 0.15 mg/L and this without production of metallic residue. Then, a leaching tank reactor and electrolytic cell coupled in a closed loop showed that the Pb mass balance of extrants/intrants ratio indicated 99.0±1.6% of Pb was recovered. Likewise, 94.1% of chloride ions were reused in the leaching tank reactor after electrochemical treatment.