Journal of Geotechnical and Geoenvironmental Engineering

An evaluation is presented on the effects of scale, stress, waste segregation, and waste decomposition on mechanical creep and biocompression of municipal solid waste. Laboratory experiments were conducted in 64-, 100-, and 305-mm-diameter compression cells. A field-scale experiment (Deer Track Bioreactor Experiment) was conducted on fresh waste of the same composition and material properties. The mechanical creep compression ratio ($C_{\alpha M}^{\prime }$) and biocompression ratio ($C_{\alpha B}^{\prime }$) were not affected by scale (i.e., specimen size). The mechanical creep ratio for fresh and degraded wastes and $C_{\alpha B}^{\prime }$ for fresh wastes were not affected by stress (i.e., similar $C_{\alpha M}^{\prime }$ and $C_{\alpha B}^{\prime }$ were obtained for a given waste at two creep stresses). Variation in $C_{\alpha M}^{\prime }$ can be related to the waste compressibility index, which is a function of waste composition, dry unit weight, and dry weight water content, or to the ratio of cellulose plus hemicellulose to lignin ($[\text{C}+\text{H}]/\text{L}$). Larger $C_{\alpha M}^{\prime }$ is coincident with larger waste compressibility index and higher $[\text{C}+\text{H}]/\text{L}$. The elapsed time for onset of biocompression ($t_B$) and the first-order decay rate ($k$) are shown to be scale dependent ($t_B$ increases and $k$ decreases as experiment size increases). A dual-model approach is presented for predicting field-scale compression based on laboratory- and empirically-derived compression model parameters.

A two-dimensional numerical model, BioClog, is used to estimate the leachate characteristics and leachate-induced clogging of laboratory mesocosms permeated with real municipal solid waste landfill leachate. The model is used to examine mesocosms with 38-mm (nominal diameter) gravel subjected to different durations of leachate permeation, mesocosms run-in series, and mesocosms with 19-mm gravel. A comparison of the calculated and measured leachate concentrations indicates that the model provided reasonable predictions of the effluent chemical oxygen demand (COD) and calcium concentrations. The calculated porosities within the saturated drainage layers are general in encouraging agreement with the measured values from all experimental mesocosms. The calculated and measured hydraulic conductivities are in reasonable agreement. The modeling results indicated that reducing the mass loading for the drainage layers and increasing the particle size of granular media can be expected to extend the time before clogging of drainage layers, and therefore, to extend the service life of landfill leachate collection systems.

Three bridges supported on deep foundations that exhibited various performance levels in liquefied and laterally spreading ground are analyzed using a beam on nonlinear Winkler foundation method. The performance levels were (1) no measurable foundation deformation, (2) moderate damage, and (3) collapse. Analyses are first performed using the best available information regarding ground motions and free-field lateral spreading surface displacements. Predictions closely match observations when the inputs are well known. The cases are subsequently reanalyzed using a probabilistic forward prediction that incorporates uncertainty in the ground motion, liquefaction triggering evaluation, lateral spreading surface displacement, and structural response. Significant differences in lateral spreading displacements estimated by different methods introduced significant dispersion into predictions of structural response for cases of poor performance in which the piles moved with the spreading soil but had little influence for cases with good performance where the liquefied soil spread around a stiff pile foundation.

An analytical model is proposed to describe the filtration process applicable to a base soil-filter system. The Navier-Stokes equations for porous media are used to capture the hydrodynamic behavior, whereas, numerically, a new algorithm is proposed to solve the Navier-Stokes equation in a nonlinear form. The various mixtures of base soil particles eroded and water flow within the system are computed using the work-energy principle incorporating the constriction size of the filter. The model can assess the filtration process through the flow rate and the accumulation and redistribution of fine particles within the filter. By discretizing the base soil and filter domains into discrete elements, the model can predict the time-dependent particle gradation of the filter for each element. Laboratory tests reported in other studies and those conducted by the authors validate the model in relation to other available models.

This paper presents a method to estimate the side resistance of a driven displacement-pipe pile in clay using the stress history and normalized soil engineering parameter (SHANSEP) concept. The side resistance is treated as an adhesion, and this adhesion is normalized to the effective overburden stress. This normalized adhesion is then related to the soil overconsolidation ratio using an adaptation of the SHANSEP concept to separate the normally consolidated behavior from the overconsolidated behavior. This approach provides a rational means of screening representative measurements of undrained shear strength from measurements that may have been affected by sample disturbance. Assessments of the soil overconsolidation ratio are developed with laboratory odometer test data and an empirical approach using laboratory undrained strength data. A database of pile load tests screened for the effects of incomplete set-up and soil-property data screened for the effects of sample disturbance is used to evaluate the normalized side resistance and the relationship of normalized side resistance to overconsolidation ratio.

To enhance the assessment of the overconsolidation ratio (OCR), this paper describes the importance of relating the soil type and soil index properties to the empirical correlations used to evaluate the preconsolidation stress and OCR using the piezocone and the importance to apply the normalized soil behavior approach. Data from a variety of sites is assembled using published and unpublished sources to develop empirical correlations for the normalized soil behavior or the stress history and normalized soil engineering parameter-based approach that vary with the plasticity index, liquid limit, median particle size, and organic content of the soil. Examples are presented to illustrate application of the correlations.

This paper examines a seismic capacity design approach for retaining structures with piled foundations, which assumes full-strength mobilization in the soil and in the foundation piles during the earthquake. The plastic mechanism activated by the seismic forces consists of a horizontal movement of the structure, involving plastic hinging in the piles. This mechanism is triggered when the seismic inertial forces acting within the structure and the soil mass equal the overall strength of the soil-pile foundation system. The paper describes an iterative procedure for evaluating the critical seismic acceleration that activates the plastic mechanism. The seismic performance of the structure is expressed by its permanent displacements and the corresponding curvature ductility demand in the foundation piles. With reference to an idealized bridge abutment, this procedure is expressed in a fully consistent nondimensional form and is applied to a reference case, to show its potentiality and to discuss the influence of a number of key parameters, such as the soil strength and the foundation geometry on the seismic performance of the structure.

Laterally loaded piles are analyzed using the Fourier FEM. The analysis is performed for piles embedded in single-layer elastic soil with constant and linearly varying modulus and in two-layer elastic soil with constant modulus within each layer. The pile responses were observed to be functions of the relative stiffness of pile and soil, and of the pile slenderness ratio. Based on the analysis, equations describing pile head deflection, rotation, and maximum bending moment are proposed for flexible long piles and stubby rigid piles. These design equations are developed after plotting the pile responses as functions of pile-soil stiffness ratio and pile slenderness ratio. These plots can also be used as design charts. Design examples illustrating the use of the analysis are provided.

This study addresses the issue of adopting constant resistance factors for the LRFD approach to the axial compression of drilled shafts in multiple strata of clay, sand, gravel, and rock. The focus is on how to achieve a uniform reliability level for a wide range of stratum scenarios with constant or nonconstant LRFD factors. The main finding is that a uniform reliability level cannot be achieved if the LRFD factors are constant and are independent of the thicknesses or relative contributions of the strata. This conclusion is confirmed in three separate ways from examination exercises: (1) by using LRFD factors recommended in a recent Federal Highway Administration manual, (2) by using tailor-made LRFD factors derived from optimization, and (3) by using LRFD factors calibrated from the first-order reliability method. A possible strategy of calibrating nonconstant LRFD factors without resulting in a proliferation of factors is proposed in this study. The use of this approach is demonstrated by a design example.

To fully implement seismic performance-based design for geotechnical structures, it is necessary to be able to correctly identify relevant earthquake records for the site in question in conjunction with advanced constitutive models that can accurately predict soil deformation resulting from these dynamic actions. In this paper, a numerical study is conducted to assess the seismic response of an earth dam using different approaches ranging from simple regression models to advanced numerical analyses. A kinematic hardening constitutive model is used to describe the mechanical behavior of the involved clayey soils. Five ground motions are chosen based on an existing hazard assessment to determine the relevant peak ground acceleration and frequency content at the dam site. Two design levels are considered in the evaluation of the seismic response of the earth dam. A comparison in terms of permanent accumulated displacements during the shaking obtained by the different approaches is presented highlighting their limitations and advantages.

An approximate linearization method using the familiar concept of $G\text{-}\gamma$ and $D\text{-}\gamma$ curves is presented for determining the dynamic impedance (stiffness and damping) coefficients of rigid surface footings accounting for nonlinear soil behavior. The method is based on subdivision of the soil mass under the footing into a number of horizontal layers of different shear modulus and damping ratio, compatible with the level of strain imposed by an earthquake motion or a dynamic load. In this way, the original homogeneous or inhomogeneous soil profile is replaced by a layered profile with strain-compatible properties within each layer, which do not vary in the horizontal sense. The system is solved in the frequency domain by a rigorous boundary-element formulation accounting for the radiation condition at infinity. For a given set of applied loads, characteristic strains are determined in each soil layer and the analysis is repeated in an iterative manner until convergence in material properties is achieved. Both kinematic and inertial interaction can be modeled simultaneously by the method, which thus encompasses primary and secondary material nonlinearity in a single step. The results are presented for a circular footing resting on: (1) a half space made of clay of different plasticity index and (2) a half space made of sand of different density, excited by a suite of recorded earthquake motions. Dimensionless graphs are provided for the variation of foundation stiffness and damping with frequency and excitation level in vertical, swaying, rocking, and torsional oscillations.

The paper presents diagrams of the (1) drained and undrained effective stress friction angles of sand; (2) dilatancy angle of sand; (3) parameters in a nonlinear constrained modulus expression for virgin loading, unloading, and reloading on sand and silt; and (4) diagrams with the coefficient of permeability for sand, silt, and clay. The database parameters are suitable for preliminary assessments or applications when assumed parameters are acceptable. The database can also be helpful in planning and interpreting site-specific tests and in reducing the number of site-specific tests. For important projects the parameters should be determined by site-specific tests.

One of the critical issues associated with plate anchor performance and design relates to the reduction of the loss of embedment during the keying process. As deep water offshore sediments typically exhibit an increasing soil strength with depth, the loss of embedment results in a reduction in anchor bearing capacity. A keying flap hinged to the main plate has been developed and adopted by industry with the aim to reduce the loss of embedment by limiting the vertical motion of the anchor. However, uncertainties remain regarding the behavior and the performance of the keying flap. This paper presents a series of numerical analyses performed to investigate the flap rotation mechanism and the condition of activation of the flap. They are compared with existing centrifuge modeling. The numerical results validate the centrifuge observations and demonstrate the nonactivation of the keying flap for typical anchor pull-out conditions.

Bridge scour can cause damage to bridge foundations and abutments. Bridges with foundations that are unstable for calculated and/or observed scour conditions are termed scour-critical bridges. There are approximately 17,000 scour-critical bridges in the United States. This designation comes in part from the use of overly conservative methods that predict excessive scour depths in erosion-resistant materials. Other methods capable of overcoming this overconservatism are relatively uneconomical because they require site-specific erosion testing. This paper proposes a new bridge scour assessment method. The new method, termed the observation method for scour (OMS), was developed for the Texas Department of Transportation’s statewide bridge scour assessment program. The proposed method does not require site-specific erosion testing and accounts for time-dependent scour in erosion-resistant materials. OMS was developed for use as a first-order assessment in combination with a routine bridge inspection program. OMS uses charts that extrapolate or interpolate measured scour depths at the bridge to obtain the scour depth corresponding to a specified future flood event. The scour vulnerability depends on the comparison between the predicted and allowable scour depths. This paper also includes a new hydraulic-hydrologic analysis procedure for the determination of flow parameters required in OMS. This procedure was developed specifically for the state of Texas. The new hydraulic-hydrologic analysis procedure could possibly be applied to other regions that have sufficient flow gauges. The nine case histories used to validate OMS showed good agreement between predicted and measured values. OMS was then applied to 16 bridges, 10 of which were scour-critical bridges that had sufficient information for OMS to be carried out. Six out of these 10 bridges were found to be stable according to OMS.

The slug test is a viable method in estimating the hydraulic conductivity of the slurry trench wall backfill because of its ability to consider a more representative volume of the backfill and to reflect the in situ performance of the construction. A three-dimensional numerical model is developed to simulate the slug test in a slurry trench wall with the presence of bentonite cake on the interface boundary between the wall and the surrounding soil formation. Influential factors such as wall width (i.e., proximity of wall boundary), well deviation, vertical position of the well intake section, and compressibility of the wall backfill are taken into account in the model. The experimentally obtained hydraulic properties of the bentonite cake are also incorporated in a series of slug test simulations. The simulation results are then examined to evaluate the bentonite cake effect in analyzing practical slug test results in the slurry trench wall. The simulation results show that the modified line-fitting method can be used without any reduction factor for the slug test in the slurry trench wall with the presence of bentonite cake. A case study is reanalyzed with the assumption of existing bentonite cake. The results are compared with the previously reported results by the approaches used for the case of no bentonite cake (constant-head boundary) and upper-bound solution (no-flux boundary). The modified line-fitting method and the type curve method produce similar results for slurry walls with bentonite cakes. The case study results demonstrate the importance of the bentonite cake effect in estimating the hydraulic conductivity of the slurry wall backfill.

Equations are derived to calculate one-dimensional (vertical) consolidation settlement in aquifers caused by groundwater extraction. The settlement equations capture the effect that the decline in pore-water pressure caused by groundwater extraction has on increased vertical effective stress. The settlement equations for single-layered confined or unconfined aquifers and multiple-layered aquifers are derived by linking the increase in vertical effective stress to the reduction of the void ratio using the reconstructed field consolidation curve of aquifer sediments. This paper’s approach blends groundwater hydraulics with the classical theory of one-dimensional consolidation widely used in geotechnical engineering. Closed-form consolidation settlement equations are presented for single-layer, homogeneous, isotropic confined aquifers with steady-state or transient groundwater flow and for single-layer, homogeneous, isotropic, unconfined aquifers under steady-state flow. The consolidation equations for consolidated settlement in heterogeneous, anisotropic, single-layer or multilayer aquifers are expressed as the numerical integration of the vertical strain induced by groundwater pumping. The numerical-integration settlement equations require the implementation of a groundwater simulation model to calculate the pore pressure decline within aquifer layers, followed by the calculation of the increase in vertical effective stress and the reduction in pore volume. One numerical example confirms the accuracy of this paper’s approach to aquifer consolidation by comparing with the solution obtained with the three-dimensional poroelastic theory.

The Rankine classic earth pressure solution has been expanded for the calculation of seismic earth pressure on rigid retaining walls supporting cohesionless backfill. The expanded solution is based on the conjugate stress concept, without employing any additional assumptions. A single formulation is used for static and pseudostatic seismic analyses of active conditions. The equation accounts for sloping backfill, as well as wall inclination, and can readily accommodate layered soil profiles and the presence of groundwater, which require tedious analysis in commonly used methods. A closed-form expression, as opposed to the constant value commonly used in practice, has been derived for the soil-wall friction angle as a function of the inertial forces and problem geometry for a given pseudostatic acceleration. Seismic lateral pressure increases with depth, and the point of application of the resultant is calculated based on stratigraphy. Comparison of the formulation with the widely used Mononobe-Okabe solution revealed a perfect match for the proposed predictions, provided the same soil-wall friction angle is used in both formulations.

The effect of hydraulic hysteresis on the analysis of seepage in unsaturated soils is evaluated stochastically. A hysteretic seepage model is introduced by coupling the governing Richards equation with a hysteretic soil-water retention curve. The first-order second-moment (FOSM) method is used to study the effect of uncertainty in the input parameters on the one-dimensional seepage behavior of a soil column. A performance function is established in terms of the capillary-barrier effect in a multilayered capillary soil cover system. The contribution of variations in the flux at the top boundary on the performance of the soil cover is first investigated with both the traditional nonhysteretic model (NHM) and the hysteretic model (HM). Comparisons show that the HM model is comparatively conservative in the seepage analysis of unsaturated soils and that stochastic seepage behavior is closely associated with the hydraulic history of the soil. In addition, the effects of key input parameters in the HM model on the output statistics are studied in detail, including the saturated hydraulic conductivity $k_s$, the exponential index in the unsaturated hydraulic conductivity function ${\lambda }_k$, the fitting parameters in the van Genuchten soil water characteristic curve (SWCC) equation $\alpha$ and $n$, the residual volumetric water content ${\theta }_r$, the slope of the scanning SWCC curve ${\kappa }_0$, and the top-boundary flux condition $Q$. The SD of hydraulic head is sensitive to the uncertainty inherent in these random input parameters, and for the specific case studied here, the significance of those parameters is found to decrease in the following order: $n$, $Q$, $\alpha$, ${\kappa }_0$, $k_s$, ${\lambda }_k$, ${\theta }_r$. Hydraulic hysteresis is suggested to be considered in the seepage analysis of unsaturated soils.