M. A. Gabr

Controlled low-strength material using fly ash and AMD sludge.

Controlled low-strength material (CLSM) is a cementitious material with properties similar to stabilized soil. After hardening, CLSM provides adequate strength in bearing capacity and support but can also be easily excavated. To be classified as a CLSM, the material must have a compressive strength between 450 kPa (65 psi) and 8400 kPa (1200 psi). Typical CLSM contains coal-combustion fly ash (FA), cement, water and fine or coarse aggregate. In this paper, physical and strength properties of CLSM formed by combining sludge, a by-product from the treatment of acid mine drainage (AMD), with Class F FA are investigated. The sludge is a lime-based waste product that when combined with FA, exhibits self-hardening characteristics similar to cement. A main focus of this research is to develop a CLSM mix in which by-product material utilization is maximized while satisfying workability and performance requirements. A mixture of 10% AMD sludge, 2.5% Portland cement (PC), 87.5% Class F FA (dry wt.%) with water provided unconfined compressive strength values within the range for classification as CLSM. This mixture satisfies the excavatability and walkability requirements as well as the hardening time and stability criteria.

Characteristics of geogrid-reinforced aggregate under cyclic load

The characteristics of geogrid-reinforced aggregates over soft subgrade soil were investigated through a laboratory-testing program. Nine cyclic plate load tests were conducted with varying base layer thickness and reinforcement type. Results indicated that aggregate base course (ABC) degraded under cyclic loading as manifested by an increase in stresses at the interface between the ABC and the subgrade with increasing number of cycles. The improvement in stress distribution due to geosynthetic inclusion at the interface of ABC and subgrade soil is indicated by a decrease in the measured maximum stress (under the center of the loaded area) and the measurement of a more uniform stress distribution on the subgrade soil layer. The higher modulus geogrid provided a better load-spreading effect compared with a lower modulus geogrid used in the testing. The improvement in plastic surface deformation was related to two aspects: (a) decrease in vertical deformation of the subgrade and (b) decrease in lateral spread of the ABC. A model that incorporates the mechanics of reinforcement contributions to deformation and stresses and the effect of ABC degradation as a function of number of load cycles needs to be developed for the transportation community to fully realize the benefits of reinforcement in such an application.

Experimental Evaluation of Strength Characteristics of Stabilized Dredged Soil

A composite geomaterial (CGM) is developed using dredged soil, bottom ash, cement and air foam. The engineering properties of the CGM are characterized as a function of the content of the various admixtures and the curing time in a laboratory test program. Several series of unconfined compression tests were carried out to characterize the strength and elastic properties of the CGM. Experimental results indicate that the unconfined compressive strength and modulus of CGM are influenced by the content of each component in the mixture. The unconfined compressive strength of CGM increases with an increase in curing time due to the pozzolanic reaction of the bottom ash. The strength after 28 days of curing is found to be approximately 1.5–2.3 times the strength after 7 days of curing, regardless of mix conditions. The bottom ash materials contain nearly 50% siliceous material and 13.9% CaO. With the removal of particles larger than 4.75 mm, a larger surface area is available to react with cement. It is postula...

Shear Strength of Degraded Reconsitituted Municipal Solid Waste

Relative changes in Waste shear strength parameters as a function of strain level and stress path are investigated based on the results of 16 direct simple shear (DSS) tests, one direct shear (DS) test with four stages, and three triaxial tests. The magnitudes of shear strength parameters obtained from drained DSS tests and undrained DSS tests with pore water pressure measurement were comparable. This was the case even though the effective stress path in both approaches was different. Data indicated the dependency of the mobilized strength parameters on strain, or deformation level. Generally, stress-deformation response increased monotonically with no well defined peak or ultimate stress levels. The results of the DSS and DS tests show no dependency of the strength parameters on the stress level. Results from DSS and DS indicated a range of effective strength parameters of 9 to 14 kPa for cohesion and 23°–29° for friction angle. Data from the triaxial testing showed dependency of the shear strength parameters on the initial compression stress level. Given the number of potentially confounding issues associated with the measurement of shear strength, it is rather important to also report information on sample collection methods, sample age and chemical composition, sample processing, sample composition, the size of testing equipment and level of strain (instead of ultimate or peak) at which the strength parameters are evaluated.

First-Exposure Performance of the Bentonite Component of a GCL in a Low-pH, Calcium-Enriched Environment

Testing was conducted on the bentonite portion of a Geosynthetic Clay Liner (GCL) for application to an environment characterized as having high concentrations of dissolved calcium ions. This environment presents conditions that might affect the long-term hydraulic function of the GCL as a component in a barrier system. Experiments were conducted to investigate first-exposure compatibility of a sodium bentonite GCL subject to the affects of acidic groundwater and second from the combined affects of acidic groundwater enriched with calcium. Relationships between the ionic exchange of sodium, potassium, magnesium and calcium species in the bentonite, and changes in hydraulic conductivity and electrical conductance are reported and discussed.

The effect of shredding and test apparatus size on compressibility and strength parameters of degraded municipal solid waste.

In many situations, MSW components are processed and shredded before use in laboratory experiments using conventional soil testing apparatus. However, shredding MSW material may affect the target property to be measured. The objective of this study is to contribute to the understanding of the effect of shredding of MSW on the measured compressibility and strength properties. It is hypothesized that measured properties can be correlated to an R-value, the ratio of waste particle size to apparatus size. Results from oedometer tests, conducted on 63.5 mm, 100 mm, 200 mm diameter apparatus, indicated the dependency of the compressibility parameters on R-value. The compressibility parameters are similar for the same R-value even though the apparatus size varies. The results using same apparatus size with variable R-values indicated that shredding of MSW mainly affects initial compression. Creep and biological strain rate of the tested MSW are not significantly affected by R-value. The shear strength is affected by shredding as the light-weight reinforcing materials are shredded into smaller pieces during specimen preparation. For example, the measured friction angles are 32 degrees and 27 degrees for maximum particle sizes of 50 mm and 25 mm, respectively. The larger MSW components in the specimen provide better reinforcing contribution. This conclusion is however dependent on comparing specimen at the same level of degradation since shear strength is also a function of extent of degradation.

ELASTIC MODULUS OF GEOGRID-REINFORCED SAND USING PLATE LOAD TESTS

An experimental study was conducted to evaluate the elastic modulus of sand reinforced with polymeric geogrids. A total of nine plate load tests were performed in the laboratory using a 1.52 m × 1.52 m × 1.37 m (length × width × depth) test box, and a 0.3 m square test plate. The measured test data were used to evaluate a modulus constant (E1), rather than the bearing capacity, as traditionally presented in literature. The modulus constant was estimated based on two deformation levels of 9.2 mm and 4.6 mm. These deformation levels, defined as δ1 and δ0.5, correspond to normalized settlement ratios (δ/B) of 1.5 and 3.0%, respectively, where B = width of the test plate. In general, a stiffer load-settlement response was measured when the geogrid reinforcement was included. Using SR1 geogrids with sand, the modulus constant (E1) decreased as a function of increasing u/B ratio (u = distance from plate to Eop reinforcement layer). In comparison, results indicated the presence of a critical u/B ratio when the SR2 geogrids were used. In this study, this particular ratio was estimated to be 0.65. Values of E1 from large scale model testing by Adams and Collin (1997) correlated well with E1 values evaluated from this testing program.

LATERALLY LOADED DRILLED SHAFTS EMBEDDED IN SOFT ROCK

The current design criterion for laterally loaded drilled shafts embedded in weathered Piedmont rock profiles requires a challenging effort on the part of the engineer. A substantial cost saving could be realized, while maintaining an acceptable and safe performance, if a rational method were developed for the analysis and design of drilled shafts in such a profile. In a current research project, the primary objective is to develop and validate a procedure for design and analysis of laterally loaded drilled shafts embedded in the Piedmont weathered rock profiles. A major component of this research is a field-testing program. Presented are the results of the first in a series of several lateral load tests performed on two drilled shafts 0.762 m (30 in.) in diameter embedded in Piedmont weathered rock. These shafts were instrumented with inclinometers and strain gauges. Field data obtained from the instrumented shafts were used to develop P-y curves. Field testing also encompassed the use of a borehole dilatometer to establish correlations between the rock strength and deformation parameters and potential P-y curves. A comparison is made between backcalculated P-y curves, P-y curves predicted by using Reese’s method, and P-y curves from the rock dilatometer. Loaddeformation results are presented and discussed for all methods used.

Deformation-resistance model for geogrid-reinforced unpaved road

Unpaved road sections experience degradation and accumulation of plastic deformation under repeated loading. Geogrid placed between subgrade and aggregate base course (ABC) can improve section performance through several mechanisms and lead to reduction in stresses and plastic deformation. An unpaved road design model that includes provisions for mobilization of subgrade bearing capacity under axisymmetric condition is proposed. The model incorporates base course property, mobilization of subgrade bearing capacity with rutting, degradation of base course stress attenuation with cyclic load, and the effect of reinforcement inclusion. An elastic layer method has been used to back-analyze the vertical stresses on subgrade with data from previous cyclic plate load tests performed in the laboratory. The degradation of unpaved sections was expressed as a reduction in base course-subgrade elastic modulus ratio (E1/E2) with an increasing number of cycles or a decrease in stress distribution angle of base course. ...

Inverse Analysis of Plate Load Tests to Assess Subgrade Resilient Modulus

Cyclic plate load testing is commonly used to investigate subgrade response under repetitive loads. Two frameworks for performing inverse analysis are described for backcalculating resilient moduli on the basis of measured key outputs. In the first approach, an elastic modulus is back-calculated in each selected domain; in the second, selected parameters in the resilient modulus model are estimated. The axisymmetric finite element model analysis results suggest that the second approach is more robust because it allows the modulus to be distributed in the selected domain. A series of sensitivity analyses was conducted with the second approach to illustrate how the assumed properties or model geometry affects the backcalculated parameters. Discrepancies between the back-calculated parameters and their known values were observed when the distance to the boundary–-that is, the radial distance from centerline to sidewall–-was not properly assigned. When backcalculating only selected parameters in the resilient modulus equation, it is necessary to assign the other parameters carefully (i.e., from laboratory tests or references). An example analysis shows the application of the proposed approach to an actual plate load test.