Journal of Geotechnical and Geoenvironmental Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Processing accelerogram data from the 2014 $M=8.2$ Iquique earthquake and an $M=7.7$ aftershock gives estimates of computed sliding displacements that are very close to those derived from sets of records from three Chilean earthquakes with $M=7.8,7.7,\text{and}\hspace{0.17em}8.2$ and confirmed by records from the Chi Chi ($M=7.6$) and Northridge ($M=6.7$) earthquakes. This supports the accuracy and robustness of the basic log-linear model.

This paper presents results from a laboratory program of 287 pullout tests of galvanized steel reinforcements used in the construction of mechanically stabilized earth (MSE) walls. Results focus on the evaluation of pullout resistance factors for ribbed steel strip and welded steel grid reinforcement embedded in gravelly backfill. This project used Texas Tech University’s large-scale MSE test box with dimensions of $3.6\times 3.6\times 1.2\hspace{0.17em}\text{m}$ and an applied overburden capacity equivalent to 12 m of soil fill. The research design evaluated pullout resistance factors for both ribbed strip and welded grid reinforcements for a variety of independent variables, including overburden pressure, reinforcement length, grid bar size, and grid geometry including both transverse and longitudinal bar spacing. Appropriate statistical analyses were used to interpret the data within the context of published design guidance for inextensible MSE reinforcements. The results show that pullout resistance factors for both ribbed strips and welded grids in properly compacted gravelly backfill are significantly higher than those obtained for reinforcements embedded in sandy backfill, as well as default pullout resistance factors. In addition, the results obtained for welded grids indicate that transverse and longitudinal bar spacing, factors not considered in the current formulation, have significant influence on the pullout resistance capacity.

Coextruded geomembranes with an inner ethylene vinyl alcohol (EVOH) layer are gaining attention as potential improved barriers to volatile organic compounds (VOCs) when used in barrier systems that would traditionally use high-density polyethylene (HDPE) geomembranes. The permeation characteristics of nine common VOCs in aqueous solutions through the EVOH layer are investigated for two thin films: a 0.015-mm-thick, 32 mol% EVOH and a 0.02-mm-thick, 44 mol% EVOH. The VOCs included aromatic hydrocarbons [benzene, toluene, ethylbenzene, and xylenes (BTEX)] and chlorinated hydrocarbons [1,2-dichloroethane (1,2-DCA), dichloromethane (DCM), trichloroethylene (TCE), and tetrachloroethylene (PCE)]. The BTEX permeation coefficients, $P_g$, range from $1.4\times {10}^{-14}\hspace{0.17em}\text{to}\hspace{0.17em}25\times {10}^{-14}\hspace{0.17em}{\text{m}}^2\cdot {\text{s}}^{-1}$ depending on the contaminant and mol% EVOH. When a 0.02-mm-thick, 38 mol% EVOH thin film layer is coextruded with linear low-density polyethylene (LLDPE) to form a 0.53-mm-thick geomembrane, the BTEX permeation values of the EVOH thin film itself are reduced to $P_g=0.4\times {10}^{-14}\hspace{0.17em}\text{to}\hspace{0.17em}0.7\times {10}^{-14}\hspace{0.17em}{\text{m}}^2\cdot {\text{s}}^{-1}$. The higher permeation coefficients for the thin films as compared with the film contained in the coextruded LLDPE/EVOH/LLDPE geomembrane are attributed to interactions between the water in the aqueous solution and the EVOH layer, an interaction that is prevented by the presence of the LLDPE in the coextruded geomembrane. These $P_g$ values are two to four orders of magnitude lower than values that have been observed for LLDPE and HDPE geomembranes. The effect of temperature on VOC permeation was also studied for the 32 mol% EVOH thin films at $23,30,40,\text{and}\hspace{0.17em}50\pm 1°\text{C}$. An Arrhenius relationship was developed that allows the evaluation of the permeation characteristics at temperatures 23–50°C and, by extrapolation, an estimation of the values for temperatures a little higher or lower than the test temperatures.

This paper presents an efficient robust geotechnical design (RGD) approach that considers performance requirements, design robustness, and cost efficiency simultaneously. In this paper, design robustness is measured via the variation in the performance function of concern that can be evaluated using reliability analysis. Furthermore, the performance requirements of the system are also evaluated using reliability analysis. Thus, the evaluation of design robustness and the evaluation of performance requirements share common computational steps, referred to herein as computational coupling. This coupling for computational efficiency is a significant feature of the proposed RGD approach. Within the framework of the proposed RGD approach, design robustness, cost efficiency, and performance requirements can be considered simultaneously by means of multiobjective optimization. Furthermore, a practical and efficient procedure is developed for such optimization using a feature resident in a popular spreadsheet program. Through an example of the design of a drilled shaft in clay, the effectiveness of this new RGD approach is demonstrated. The results show that the hard-to-control variability resulting from construction variation, variable loading conditions, model errors, and uncertain geotechnical parameters in the design of drilled shafts in clay can be effectively considered with the proposed RGD approach.

Constructed slopes are traditionally given a planar form. However, natural slopes are more likely to be concave in cross section. In addition, laboratory and computational studies have demonstrated that concave slopes yield less sediment than planar slopes. With current autoguided construction equipment, it is now possible to construct slopes with concave profiles and a more natural appearance, yet a simple method to describe such concave slopes for a given level of mechanical stability does not exist. This article begins with an examination of concave shapes satisfying a desired degree of stability and compares results with those from the FEM and limit-equilibrium method of analysis. An erosion model is used to demonstrate that the concave slopes proposed here yield 15–40% less sediment than planar slopes with the same factor of safety. Finally, a sensitivity analysis suggests that reasonable construction deviations do not compromise the stability of typical concave slopes.

Hydraulic conductivity, swelling, and liquid sorption capacity (i.e., maximum organic liquid mass bound per mass organoclay solid) were measured for an organoclay with dimethylammonium bound to the surface. Five fuels (No. 1 fuel oil, No. 2 fuel oil, diesel, jet fuel, and gasoline), four pure organic liquids (methanol, phenol, ethylbenzene, and dioctyl phthalate), ranging from hydrophilic to hydrophobic, and Type II deionized (DI) water were used as liquids for solvation and permeation. The more hydrophilic liquids (methanol and phenol) and DI water resulted in low swelling ($\le 6\hspace{0.17em}\text{mL}/2\hspace{0.17em}\text{g}$) or liquid sorption capacity ($\le 202\%$) and high hydraulic conductivity ($>{10}^{-6}\hspace{0.17em}\text{m}/\text{s}$). The term hydraulic herein refers to liquid and applies to all permeant liquids used. The less-refined fuels composed of heavier distillates (fuel oil and diesel) and the phthalate resulted in low swelling ($10–12\hspace{0.17em}\text{mL}/2\hspace{0.17em}\text{g}$) and liquid sorption capacity ($<235\%$) and intermediate to low hydraulic conductivity (${10}^{-10}\hspace{0.17em}\text{to}\hspace{0.17em}{10}^{-11}\hspace{0.17em}\text{m}/\text{s}$). The highly refined fuels composed of lighter distillates and ethylbenzene resulted in higher swelling ($>20\hspace{0.17em}\text{mL}/2\hspace{0.17em}\text{g}$), high liquid sorption capacity ($<340\%$), and very low hydraulic conductivity (typically, $<{10}^{-11}\hspace{0.17em}\text{m}/\text{s}$). The swelling, liquid sorption capacity, and hydraulic conductivity of this organoclay are related systematically; however, none of these properties correlates systematically with the common parameters describing hydrophobicity, namely, solubility or the octanol-water partition coefficient. When the swell index is at least $10\hspace{0.17em}\text{mL}/2\hspace{0.17em}\text{g}$, this organoclay has hydraulic conductivity of less than ${10}^{-10}\hspace{0.17em}\text{m}/\text{s}$. Below $10\hspace{0.17em}\text{mL}/2\hspace{0.17em}\text{g}$, the hydraulic conductivity increases rapidly as the swell index decreases. Sand-organoclay mixtures with uniform sand require more organoclay to achieve low hydraulic conductivity and are more sensitive to the swell index. For this organoclay, a mixture with at least 50% organoclay is recommended to ensure low hydraulic conductivity to gasoline and jet fuel. Diesel and fuel oil can require at least 75% of this organoclay to achieve low hydraulic conductivity.

Strength loss of sensitive clay slopes is a common concern for soft soil sites. A useful method for studying this problem is through physical model tests. However, a problem that deters more widespread use of such tests is the difficulty of obtaining the large samples of natural sensitive soil that would be required. The work described in this paper was motivated by the need to develop practical procedures to produce artificial sensitive clay for use in model tests. To reach this goal, some fundamental experiments were performed to explore the effects of cement content, water content, aging, and remolding after aging on the undrained shear strength. Two soils [San Francisco Bay mud (SFBM) and Yolo loam (YL)] with several different cement and water contents were studied. After mixing and aging and measurement of the aged strength, the soils were again remolded to determine the aged-remolded undrained strength. The sensitivity of the aged clay could then be calculated by dividing the aged strength by the aged-remolded strength. The SFBM, a high-plasticity clay, developed a sensitivity of approximately 3–3.5 after 7 days as a result of thixotropic hardening alone. Sensitivities of 4–8 were obtained by addition of a small percentage (2–5%) of cement to the soil. The YL, a low-plasticity lean clay rich in silt content, produced much higher sensitivities of 20–27, with a small addition of cement (2–3%). A procedure for production of physical model test specimens with the desired strength and sensitivity is also outlined.

Monte Carlo simulation (MCS) provides a conceptually simple and robust method to evaluate the system reliability of slope stability, particularly in spatially variable soils. However, it suffers from a lack of efficiency at small probability levels, which are of great interest in geotechnical design practice. To address this problem, this paper develops a MCS-based approach for efficient evaluation of the system failure probability $P_{f\text{,}s}$ of slope stability in spatially variable soils. The proposed approach allows explicit modeling of the inherent spatial variability of soil properties in a system reliability analysis of slope stability. It facilitates the slope system reliability analysis using representative slip surfaces (i.e., dominating slope failure modes) and multiple stochastic response surfaces. Based on the stochastic response surfaces, the values of $P_{f\text{,}s}$ are efficiently calculated using MCS with negligible computational effort. For illustration, the proposed MCS-based system reliability analysis is applied to two slope examples. Results show that the proposed approach estimates $P_{f\text{,}s}$ properly considering the spatial variability of soils and improves the computational efficiency significantly at small probability levels. With the aid of the improved computational efficiency offered by the approach, a series of sensitivity studies are carried out to explore the effects of spatial variability in both the horizontal and vertical directions and the cross-correlation between uncertain soil parameters. It is found that both the spatial variability and cross-correlation affect $P_{f\text{,}s}$ significantly. The proposed approach allows more insights into such effects from a system analysis point of view.

Shear strength, compressibility, and hydraulic conductivity of recycled asphalt shingles (RASs) mixed with bottom ash (BA) or stabilized with self-cementing fly ash (FA) were evaluated in a systematic manner at temperatures ranging from 5 to 35°C, representing seasonal field temperature variation. Increasing temperature reduced the shear strength and increased the compressibility and hydraulic conductivity of compacted RAS-BA and RAS-FA mixtures. When the temperature increased from 5 to 35°C, the effective friction angle (${\varphi }^{\prime }$) of the compacted RAS-BA mixture containing 25% RAS decreased from 41 to 35°. The mixture containing 50% RASs decreased from 41 to 29°. The ${\varphi }^{\prime }$ of the compacted RAS-FA mixture containing 20% FA decreased from 46 to 26°; however, the effective cohesion ($c^{\prime }$) increased from 45 to 70 kPa, and the compressive strength remained higher than that of compacted sand. In contrast, the secondary compression ratio ($C_{\alpha \epsilon }$) increased with temperature. The $C_{\alpha \epsilon }$ of the RAS-BA mixture is an exponential function of temperature. Thermal cycling induced thermal precompression to the compacted RAS-BA and RAS-FA mixtures and significantly reduced the compressibility. Thermal precompression reduced $C_{\alpha \epsilon }$ for the compacted RAS-BA mixture containing 25% RASs from 0.0078 to 0.0004 and that of the RAS-FA mixture from 0.0016 to 0.0002, which are comparable with $C_{\alpha \epsilon }$ for compacted sand ($C_{\alpha \epsilon }=0.0003$). Therefore, to reduce long-term settlement of an embankment made with materials containing RAS, construction is recommended during warm seasons. In this way, most settlement occurs during construction, and settlement during service life of the embankment becomes negligible. The hydraulic conductivity of the compacted RAS-BA and RAS-FA mixtures increased with increasing temperature, which is beneficial to drainage capacity of structural fills containing RASs.

This paper provides an interpretation of the phenomenon of wetting collapse in fine-grained soils based on principles of material stability. For this purpose, classic experiments displaying the accumulation of irreversible compaction have been reinterpreted through a plasticity model for unsaturated soils. The resulting simulations have been inspected mathematically, with the goal of detecting a possible loss of control of the wetting process. The reanalysis of the experiments suggests that, if the wetting-induced deformations are interpreted as homogeneous, the considered compaction events do not tend to be affected by a loss of material stability. As a result, such well-known phenomena have been explained as controllable modes of plastic deformation. This feature has been found to be valid regardless of the mode of saturation (i.e., it is valid for both controlled suction removal and water volume injection). Indeed, parametric analyses have shown that the compaction process tends to become unstable only in the presence of highly water-sensitive media exposed to a drastic loss of suction (e.g., soaking or injection), i.e., circumstances during which the soil undergoes large strains and suffers a marked deterioration of its mechanical properties. In these cases, the theory predicts a limit threshold of the water content at which the control of the injection process is lost, the applied stress is no longer sustainable, and even infinitesimal alterations of the moisture content can cause a sharp accumulation of compaction. These findings impact the interpretation of wetting-induced settlements predicted via computational models, and are of general relevance for the design, maintenance, and safety assessment of unsaturated earthen systems interacting with hydrologic processes.

The objective of this paper is to demonstrate how the spatial variability of random soil affects the failure mechanism and the ultimate bearing capacity of foundations buried at various depths. A nonlinear finite-element analysis combined with random field theory is employed to explore the vertical capacity of foundations embedded at different depths in random soil. Different possibilities of shear failures resulting from spatial patterns of soil are demonstrated and are used to explain the significant discrepancy between the bearing capacity of the random soil and that of uniform soil. The effect of the spatial pattern of the soil on the development of shear planes is also investigated, with the coefficients of variation for the bearing capacity demonstrated to be closely related to the shear plane length. The results of the statistical variation in the bearing capacity are provided for different embedment depths, and these are also reported as the failure probability of the footing compared with the established uniform soil bearing capacity. Safety factors are proposed for foundations at different levels of failure probability. This study provides a thorough understanding of the failure mechanisms of footings in random soil, especially where structures can penetrate deeply into soil.

Mine tailings, or mill tailings (MTs), are highly susceptible to wind erosion (dust) and have brought about different environmental and safety concerns. Many efforts have been attempted to stabilize MTs for dust control, but all have drawbacks. The current study investigates the feasibility of using two natural and renewable biopolymers, xanthan gum and guar gum, to stabilize MTs for dust control. Moisture retention and wind tunnel tests were performed to evaluate, respectively, the enhancement of water retention capacity and the improvement of resistance to wind dispersal after MTs were treated with biopolymer solutions of different concentrations. Because the resistance to the formation of dust is closely related to how easy the particles can be detached from the surface, a flat-ended cylindrical penetrometer was manufactured and used to evaluate the surface strength (maximum penetration force) of dry MT specimens treated with biopolymer solutions of different concentrations. Scanning electron microscopy imaging was also performed to investigate the evolvement of the microstructure of MTs after biopolymer treatment. The results indicate that both xanthan gum and guar gum are effective in enhancing the moisture retention capacity, improving the dust resistance, and increasing the surface strength of MTs beyond that of water wetting. This is mainly because the biopolymers form coatings on MT particles and create bonding between them. The results also demonstrate that the flat-ended cylindrical penetrometer is a promising technique for characterizing the dust resistance of MTs.

A study was conducted at 12 sites across the United States to evaluate field-scale hydrology of landfill final covers using water balance methods to control percolation. The sites were located in climates ranging from arid to humid, with annual precipitation varying from 119 to 1,263 mm. Fifteen test sections were constructed with large ($10\times 20\hspace{0.17em}\text{m}$) drainage lysimeters for continuous and direct monitoring of the water balance over a period of 3–6 years. Monolithic and capillary barrier designs were used for water storage, and plant communities consisting of grasses, grasses and shrubs, or grasses and trees were used to promote evapotranspiration. Data from these test sections are analyzed along with data from 10 other sites in the literature to draw general inferences regarding the hydrology of water balance covers. Percolation ranges from $0\hspace{0.17em}\text{to}\hspace{0.17em}225\hspace{0.17em}\text{mm}/\text{year}$ (0–34% of precipitation) on an average annual basis and is shown to be affected by annual precipitation, preferential flow, and storage capacity of the cover. Evapotranspiration is the largest component of the water balance ($>60\%$ of precipitation) and varies with water availability from precipitation, energy demand as characterized by potential evapotranspiration, and type of plant community. Surface runoff is the smallest fraction ($<16\%$ of precipitation) and depends on the saturated hydraulic conductivity of the surface soils, intensity of precipitation, and the occurrence of snowmelt and frozen ground.

The Kansai International Airport was constructed in Osaka Bay in 18- to 20-m-deep seawater to avoid noise pollution and land acquisition disputes. Construction of the 511-ha Island I began in 1987 and Runway I began operation in 1994. Construction of the 545-ha Island II began in 1999, and Runway II began operation in 2007. Using more than 2.2 million vertical sand drains fully penetrating into the 17.3- to 24.1-m-thick Holocene clay layer and 430 million cubic meters of fill material, the project is viewed as an engineering marvel. On the basis of a detailed review of the geology of Osaka Bay, construction of the Airport Islands, and the permeability and compressibility of the Holocene and Pleistocene subseabed deposits that reached a depth of 400 m below the seafloor at the Kansai Airport site, settlement analyses were conducted assuming the uniqueness of end-of-primary void ratio–effective vertical stress relationship and the $C_{\alpha }/C_c$ law of compressibility. Airport Island I has already settled below the 4-m above sea level surface elevation required by the design specification, and the surface elevation of Island II is predicted to be 4 m above sea level by 2023–2036. Airport Islands I and II will be at sea level, respectively, by 2067 or sooner and by 2058–2100. By the end of the 21st century, Island I and Island II are predicted to settle, respectively, 17.6 and 24.4 m.

Practical undrained shear strength $s_u$ test results, such as the $s_u$ values from field vane (FV) and unconfined compression (UC) tests, cannot be used directly as design values. However, they are correlated with the design value of $s_u$, referred to as the mobilized undrained shear strength $s_u\left(\mathrm{mob}\right)$. This study derives the transformation equations that convert practical $s_u$ results (e.g., FV and UC test results) into the mean value and the coefficient of variation (COV) of $s_u\left(\mathrm{mob}\right)$. The derivations are based on the multivariate probability distribution previously constructed by the authors that models the correlations among $s_u$ values from various $s_u$ test modes. An important feature of the transformation equations is that they accept multiple practical $s_u$ test results, so the transformation uncertainty of $s_u\left(\mathrm{mob}\right)$ can be reduced further by incorporating more practical tests. This reduction of transformation uncertainty can be exploited to economize design in a reliability-based design framework. The effectiveness of the resulting transformation equations is demonstrated and verified using real case histories.

Scour of the soil by flowing water around bridge supports is the number one reason for bridge collapse. Predicting the depth of the scour hole is an integral part of the bridge foundation design, as it impacts the depth of the piles. Indeed, the scour depth must be ignored in the vertical and horizontal resistance of the piles. This paper presents a method to calculate the maximum depth of the scour hole around bridge supports when subjected to a constant water velocity. Most existing methods take into account the water velocity and the geometry of the obstacle but not the soil type. The method presented in this paper keeps those important variables and adds the paramount influence of the soil erosion characteristics. It is developed on the basis of 94 flume tests, some of them very large laboratory-scale tests, as well as dimensional analysis and experience. It applies to pier scour, contraction scour, and abutment scour. The method is evaluated by comparing the maximum scour depth predictions against measured data from 10 databases of pier, contraction, and abutment scour depths.

Scour of the soil by flowing water around bridge supports is the number one reason for bridge collapse. Predicting the depth of the scour hole is an integral part of bridge foundation design, as it impacts the depth of the piles. Indeed, the scour depth must be ignored in the vertical and horizontal resistance of the piles. This paper presents a method to predict the progression of the scour depth around bridge supports as a function of time when subjected to a velocity hydrograph. It also addresses the issue of layered soils. The equations make use of the commonly used water velocity and dimensions of the obstacle, but add a new and important component: the soil erosion function to characterize the degree of soil resistance to erosion. The equations apply to pier scour, contraction scour, and abutment scour. The predicted output is the scour depth as a function of time for a given velocity hydrograph and soil layer stratigraphy. A comparison between measured and predicted scour depth is presented, but more comparisons are necessary, particularly at full scale.

The long-term performance of three different high-density polyethylene (HDPE) geomembranes (GMBs) is investigated at 85°C using immersion tests. By comparing the degradation behavior of the three GMBs in different synthetic leachates, it is shown that different chemical constituents in the leachate affected different stages of the degradation, with surfactant having the greatest effect on antioxidant depletion (Stage I) and salts having the greatest effect on the degradation after antioxidant depletion (Stages II and III). The magnitude of the effect of these chemical constituents differed from one GMB to another. Thus, for the purpose of comparing the relative long-term performance of the three GMBs for municipal solid waste (MSW) landfill applications, the GMBs were immersed in a synthetic leachate (Leachate A), which contained the primary constituents (i.e., salts, volatile fatty acids, surfactant, and trace metals under reduced conditions) present in the leachate from a large MSW landfill leachate located in Canada. At 85°C, the longest antioxidant depletion stage was for the GMB with the highest resistance to antioxidant depletion in Leachate A, even though its initial oxidative induction time values were not the highest of the three GMBs. After antioxidant depletion, the greatest resistance to degradation was for the GMB with the highest initial stress crack resistance and the lowest melt flow ratio.

Determining the consolidation coefficient of clay is necessary for estimating the consolidation behavior of stratigraphic units during and after construction. Use of piezocone dissipation tests is one in situ method that is advantageous to estimate consolidation behavior because it uses equipment typically mobilized as part of the site investigation effort. The routine use of cone dissipation tests in practice remains limited because of the time required to reach 50% dissipation of excess pore pressures. In low permeability clays, the $t_{50}$ value may be on the order of 6–24 h, rendering it costly and difficult to complete a cone sounding in 1 day. This paper presents a method for fitting incomplete piezocone dissipation tests using a hyperbolic function to estimate the $t_{50}$ value and therefore $c_h$. Results are presented from evaluation of the simplified approach applied to 20 dissipation tests reaching at least 50% dissipation in soils ranging from clayey silts to high-plasticity clays. Results show that only 30% dissipation may be necessary to reasonably estimate $t_{50}$ and $c_h$, resulting in potential time savings, which can motivate the increased use of pore-pressure dissipation tests in site investigation programs.