Journal of Geotechnical and Geoenvironmental Engineering

To provide design guidance on the influence of soil softening on ground response, we analyzed ground motions recorded at five instrumented liquefaction sites, including the Wildlife Liquefaction Array, Calif.; the Port Island Downhole Array, Japan, the Treasure Island and Alameda Naval Air Station sites, Calif., and the Kawagiahi-cho apartment buildings, Niigata, Japan. Conclusions from these analyses are (1) where soil softening occurs early in a strong ground motion sequence, soil softening leads to reduced short period $(T<1.0\mathrm{s})$ spectral accelerations compared to those that would have developed in the absence of softening; (2) where soil softening occurs late in a ground motion sequence, soil softening has little influence on short-period $(T<1.0\mathrm{s})$ spectral response; (3) soil softening and liquefaction may lead to amplification of long period $(T>1.0\mathrm{s})$ spectral accelerations, particularly if ground oscillation develops. Comparison of actual spectra calculated from motions measured at liquefaction sites with design spectra incorporated in seismic provisions of the 1998 AASHTO LRFD Bridge Code, the 1997 Uniform Building Code, and the 2003 International Building Code indicate that code spectra conservatively envelope measured motions from liquefied sites for short-period $(T<1.0\mathrm{s})$ structures. For longer period structures, the design spectra are generally conservative if softer site profile types are assigned, but may be unconservative for stiffer site types. Site-specific response analyses are needed for critical long-period structures.

An inverse analysis procedure that uses construction monitoring data to update predictions of deformations for supported excavation systems is presented. The numerical procedure is used to optimize the finite element model of a 12.2-m-deep excavation through Chicago glacial clays by minimizing the errors between monitoring data and computed displacements. The field observations are obtained from inclinometer data that measured lateral movements of the soil behind the supporting walls on opposite sides of the excavation throughout construction. Five construction stages are defined for the inverse analysis. At every new construction stage, the inclinometer data relative to that stage are added to the observations already available to “recalibrate” the model of the excavation. The constitutive responses of the soils are represented by the hardening-soil (H-S) model. Of the six basic H-S input parameters, only one parameter per layer is optimized, while the other parameters are either kept constant or related to the updated value of the optimized parameter. The methodology is effectively used to recalibrate the model of the excavation at early construction stages, such that good “predictions” are made of the behavior of the soil at later stages.

Leachate recirculation in municipal solid waste (MSW) landfills operated as bioreactors offers significant economical and environmental benefits. Subsurface leachate recirculation in MSW landfills is commonly achieved by using horizontal trenches or vertical wells. Currently, there are no design guidelines available for leachate recirculation using a subsurface leachate recirculation system (LRS). The key objective of this study is to prepare design guidelines for LRS consisting of horizontal trenches. This paper presents a numerical study of LRS consisting of horizontal trenches. The design parameters evaluated in this study include: (1) Leachate injection pressure head; (2) hydraulic conductivity of trench backfill and MSW; (3) dimensions of trench; and (4) spacing and geometric formation of trenches. The finite-element saturated/unsaturated flow model HYDRUS-2D was used for the numerical study. The hydraulic performance of the LRS was evaluated primarily using the simulated recirculated leachate flux and distribution of flow under steady-state flow condition. The key findings of this numerical study are: (1) Logarithm of leachate flux and leachate injection pressure head have a curvilinear relationship and leachate flux is directly proportional to the hydraulic conductivity of MSW; (2) if hydraulic conductivity of trench backfill is equal to or greater than that of MSW, any further increase in the hydraulic conductivity of the trench backfill has negligible impact on leachate flux; (3) for a given cross-sectional area, horizontal trenches having width greater than depth can recirculate greater leachate flux and can wet more area of the waste; and (4) reduction in the horizontal spacing between trenches and vertically staggering the trenches reduces “dry zones” between trenches where otherwise recirculated leachate may not reach.

Four materials with high sorptive capacities for organic compounds [granular activated carbon (GAC), shale, benzyltriethylammonium-bentonite (BTEA-bentonite), and hexadecyltrimethylammonium-bentonite (HDTMA-bentonite)] were evaluated for their use in compacted clay landfill liners by conducting laboratory sorption and permeability experiments with both a 0.002 N $\mathrm{Ca}\mathrm{S}{\mathrm{O}}_4$ solution (to simulate the ionic strength of ground water) and a synthetic leachate. Results from equilibrium sorption experiments with 0.002 N $\mathrm{Ca}\mathrm{S}{\mathrm{O}}_4$ indicate that all four amendments have a very high sorptive capacity for the three organic solutes tested: benzene, trichloroethylene, and 1,2-dichlorobenzene. GAC exhibited the highest sorptive capacity for all three solutes, followed by BTEA-bentonite, HDTMA-bentonite, and shale. Experiments conducted with synthetic leachate indicate that GAC and BTEA-bentonite did not exhibit competitive sorption effects with the synthetic leachate constituents, while shale did exhibit some competitive sorption effects between benzene and trichloroethylene and the synthetic leachate constituents. Permeability tests conducted on specimens composed of Ottawa sand, untreated bentonite, and either 3 or 9% amendment by weight were also performed. Results indicate that all amended specimens had a hydraulic conductivity less than or equal to $1\times {10}^{-7}\mathrm{cm}∕\mathrm{s}$ , with the exception of the specimen amended with 3% GAC, which had a measured conductivity value of $2\times {10}^{-7}\mathrm{cm}∕\mathrm{s}$ . Changing the permeant fluid to a synthetic leachate had little effect on the overall conductivity of the specimens. One-dimensional benzene transport simulations were performed to more completely evaluate how effective these materials would be if incorporated into a compacted clay liner. Simulations indicated that all proposed amendments would effectively retard the transport of benzene through the liner. Amending liners with 3 or 9% GAC or 3 or 9% BTEA-bentonite effectively eliminated benzene flux through the liner over a $100\text{-}\text{year}$ period. Liners amended with HDTMA-bentonite or shale also reduced the benzene flux exiting the liner over the simulation period, but to a lesser extent.

Fracture flow of two-phase mixtures is particularly applicable to the coal mining and coal bed methane projects in Australia. A one-dimensional steady-state pseudo-two-phase flow model is proposed for fractured rock. The model considers free flow of a compressible mixture of air and water in an inclined planar fracture and is based upon the conservation of momentum and the “cubic” law. The flow model is coupled to changes in the stress environment through the fracture normal stiffness, which is related to changes in fracture aperture. The model represents the individual air and water phases as a single equivalent homogenous fluid. Laboratory testing was performed using the two-phase high-pressure triaxial apparatus on 54 mm diameter (approximately $2:1\text{height}:\text{diameter}$ ) borehole cores intersected by induced near-axial fractures. The samples were of Triassic arenaceous fine-medium grained sandstone (known as the Eckersley Formation) that is found locally in the Southern Coalfield of New South Wales. The sample fracture roughness was assessed using a technique based upon Fourier series analysis to objectively attribute a joint roughness coefficient. The proposed two-phase flow model was verified using the recorded laboratory data obtained over a range of triaxial confining pressures (i.e., fracture normal stresses).

A special large-scale torsional resonant column apparatus was used to test gravelly soils. Values of small strain elastic shear modulus $\left(G_{\max }\right)$ and modulus reduction relationships [shear modulus $\left(G\right)$ versus shear strain $\left(\gamma \right)$ ] were measured for specimens of uniform and graded crushed limestone gravel, graded river gravel, standard Ottawa and crushed limestone sands, and gravel–sand–silt mixtures. Measurements of $G_{\max }$ were used to modify existing three-dimensional constitutive equations for soil elasticity for application to gravelly soils. The value of $G_{\max }$ for relatively clean uniform and graded gravels was found to increase with particle size. Soils with a variety of gradations were tested to identify the particular particle size in a graded material that is effective in determining $G_{\max }$ . With respect to modulus reduction, the need to normalize both modulus and strain is demonstrated. Normalization of both modulus and strain ( $G∕G_{\max }$ versus $\gamma ∕{\gamma }_r$ , where ${\gamma }_r$ is defined as reference strain) leads to relationships that are approximately independent of stress level for a given material.

Remediation schemes for the treatment of liquefiable ground, such as vertical drains, have evolved over the last $30\text{years}$ largely as a result of consideration of uniform sand beds. However, many liquefiable sites also contain thin layers of silts and clays which will act as an obstacle to fluid dissipating from liquefying sands, such that the performance and requirements of a remediation scheme will be different. In particular, water films may be produced. This paper presents results from a series of dynamic centrifuge experiments on vertical drains in level liquefiable sand deposits. Centrifuge tests were conducted both with and without surface fine layers. The drains prevented sand boiling, and the thin surface fine layers did not significantly affect excess pore pressure response. A further centrifuge test on a thin layer of low-permeability fines separating two liquefiable sandy layers, showed evidence suggesting that material boiled from the lower sand to the upper sand in the absence of a drain through the fines. All of the experiments suggest that the presence of unblocked vertical drains prevent water film formation.

According to the New Austrian Tunneling Method (NATM), after excavation of a cross section of a tunnel, shotcrete is applied onto the tunnel walls, constituting a thin flexible, closed shell. Deformations, as well as the loading of the lining, are continuously monitored during construction, serving as input for the decision making process following the closed control cycle “excavation–monitoring–parameter adaptation–excavation”. The objective of this control cycle is to optimize the tunneling process with respect to cost, crew safety, and long-term tunnel stability. In this paper, questions related to this optimization process are addressed, dealing with the influence of driving parameters and changes in the in situ geological and geotechnical conditions on the deformation and loading of the shotcrete lining. Based on realistic material models for shotcrete and ground, axisymmetric analyses, allowing for consideration of the three-dimensional nature of the tunnel excavation, were performed. The assumption of axisymmetry represents a good approximation of the static conditions of tunnels with high overburden. The obtained results are presented in dimensionless form, providing new insights into the complex ground-shotcrete interaction in NATM tunneling.

This paper highlights the mechanism of slope surface failure that occurs on highway embankments with cohesive soils. Based on systematic instrumentation in the field, the major reason for such surface failures is ascribed to the reduction of soil strength due to the seasonal variation of moisture content in embankment soils. Shrinkage cracks develop in an embankment during dry seasons. A great amount of rainfall then infiltrates into embankment soils through these cracks in subsequent wet seasons. As such, the shear strength of embankment soils progressively decreases as soils swell until a surface failure occurs. This surface failure mechanism was indirectly studied through a series of shrinkage and unconfined compressive tests on cohesive soil similar to that at the highway embankment. A limit equilibrium analysis method was also suggested accordingly for the back calculation of slope stability. Finally, a rehabilitation technique using nonwoven geotextile was recommended for highway maintenance engineers to repair such slope surface failures.

Repeated load triaxial tests are widely used to determine nonlinear behavior of unbound granular materials. However, conventional triaxial specimen preparation techniques may not be able to reconstruct the internal structure of a material which has great impact on some of the nonlinear parameters. In addition, it is difficult to reproduce in situ stress conditions in a triaxial cell. In this study, full-scale cyclic and static laboratory plate load tests were performed to investigate the mechanical properties of unbound granular materials. An algorithm was developed to backcalculate nonlinear parameters for prediction of models as a function of bulk stress. Study results demonstrate the pronounced influence of water content on the behavior of unbound granular materials and a generally linear relation between nonlinear parameter $K_1$ and the degree of saturation.

Class F fly ash and bottom ash are the solid residue byproducts produced by coal-burning electric utilities. They are usually disposed of together as a waste in utility disposal sites with a typical disposal rate of 80% fly ash and 20% bottom ash. Direct use of these materials in construction projects consuming large volumes of materials, such as highway embankment construction, not only provides a promising solution to the disposal problem, but also an economic alternative to the use of traditional materials. Representative samples of class F fly and bottom ash were collected from two utility power plants in Indiana and tested for their mechanical properties (compaction, permeability, strength, stiffness, and compressibility). Three mixtures of fly and bottom ash with different mixture ratios (i.e., 50, 75, and 100% fly ash content by weight) were prepared for testing. Test results indicated that ash mixtures compare favorably with conventional granular materials.

Geosynthetic materials such as geomembranes and geosynthetic clay liners (GCLs) are frequently used in composite liners for municipal solid waste landfills. Heat generated within such facilities due to decomposition of organic material within the waste creates thermal gradients that have the potential to cause desiccation of the mineral component of GCLs, with potential impacts on long-term performance. This paper presents the results of an experimental investigation into the potential for moisture redistribution in and around GCLs forming part of a composite liner system when subjected to thermal gradients. Large-scale laboratory testing was performed using two different subsoils and GCL materials, with emphasis placed on the spatial and temporal variation of temperature and water content within and beneath the GCLs. The influence of key initial and boundary conditions such as the applied temperature gradient, initial GCL and subsoil water content, and the type of GCL is discussed, as well as the implications of the findings for long-term GCL performance. Recommendations are made regarding aspects of the design and operation of landfill facilities likely to reduce the potential for desiccation.