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.

A rigorous probabilistic-based approach for load and resistance factor design (LRFD) calibration of internal stability tensile rupture and pullout limit states for geogrid reinforced mechanically stabilized earth (MSE) walls is presented. Load and resistance factors are provided for different combinations of the load model with pullout model and the tensile strength model used in current practice. The load models are the AASHTO Simplified Method and the Simplified Stiffness Method. Resistance factors are tabulated for use in LRFD foundation practice in Canada which rewards designers with larger resistance factors for greater project level of understanding. Resistance factors are also provided that are amenable to US practice where the notion of level of understanding is not explicitly considered in LRFD practice. The paper demonstrates the application of the load and resistance factors for tensile rupture and pullout internal stability limit states using a design example and shows the reduction in required reinforcement strength using the Simplified Stiffness Method. The paper is of value to practitioners and LRFD code writers in the United States and Canada for future revisions of design specifications in their jurisdictions.

On November 5, 2015, the Fundão tailings storage facility (TSF) failed, resulting in significant tailings release and 19 deaths. Following the failure, a review panel was established to investigate the cause of the failure. This investigation suggested that the failure involved static liquefaction triggered by deformations in slimes underlying loose, saturated, sandy tailings. This conclusion was reached by interpretation of cone penetration tests (CPTu), laboratory testing of deformation-induced stress paths, and numerical modeling. However, the report did not clearly indicate what method was used to infer the in situ state $\mathrm{\Psi }$ from the CPTu probes, and electronic data are not currently publicly available. To remedy both of these limitations, the CPTu data presented by the Panel were digitized to enable their more widespread use and to assess which methods likely were used to interpret the state. As part of this process, a zone of looser sandy tailings at the location where liquefaction was likely to have triggered was identified. Comparison of the state of this material to the overall sandy tailings state indicates that this loose zone may have played a role in the initiation of static liquefaction. Examination of historical data indicated a potential source for this loose material. This examination provides a slightly different perspective of the factors contributing to the initiation of static liquefaction at Fundão.

The effects of partial drainage on the cyclic strength of saturated sand in a set of dynamic centrifuge model tests were evaluated. Three models of level profiles of saturated Ottawa F-65 sand with initial relative densities of 25%, 43%, and 80% were tested using a 9-m-radius centrifuge. Models were subjected to multiple sinusoidal shaking events with acceleration amplitudes ranging from $0.03g$ to $0.55g$. The cyclic resistance ratios (CRR) obtained from inverse analyses of dense accelerometer and pore pressure transducer arrays were correlated with cone penetration resistances ($q_{c1N}$) from in-flight cone penetration tests. Time histories of volumetric strain and surface settlement due to partial drainage were determined by inverse analyses of the array data and compared with measured surface settlements. The effect of volumetric strain on cyclic strength is examined through single-element simulations using the constitutive model PM4Sand version 3. Results of these simulations are compared to prior laboratory and numerical studies investigating the effect of partial saturation on cyclic strength. The magnitude of the volumetric strains developed in the centrifuge models due to partial drainage and their effects on the centrifuge $\mathit{CRR}\text{-}{q}_{c1N}$ correlation are examined.

This paper describes a series of centrifuge model tests undertaken to investigate the load transfer and settlements that develop around a defective pile within geosynthetic-reinforced piled embankments. Models of an embankment supported using both floating and end-bearing piles were constructed with the subsoil consolidated and piles installed in-flight to ensure the load-settlement responses of the piles were accurately modeled. In each model, a defective pile was installed to a shallower depth than the surrounding nondefective piles, and as a result, exhibited a relatively softer load-settlement response. The results show that for the floating pile case, the defective pile underwent uniform settlement with the surrounding nondefective piles, and therefore no differential settlement was experienced at the embankment surface. In contrast, for the end-bearing pile case, the defective pile underwent additional settlement compared with the surrounding nondefective piles, which resulted in a localized depression forming at the embankment surface. It is shown that the area influenced by the defective pile was limited to the confines of the surrounding pile unit cell. This was confirmed by penetrometer testing and the observation that piles adjacent to the defective pile carried no additional loads and underwent no additional settlement because of their close proximity to the defective pile.

A biogeochemical model describing the rate at which calcium carbonate (${\mathrm{CaCO}}_3$) is precipitated from the pore water and biogas (carbon dioxide and nitrogen gas) is generated by dissimilatory reduction of nitrate (denitrification) through microbially induced desaturation and precipitation (MIDP) has been developed. Both ${\mathrm{CaCO}}_3$ precipitation and desaturation via biogas formation can improve the static and cyclic mechanical properties of granular soil. ${\mathrm{CaCO}}_3$ precipitation can improve the static and cyclic stiffness, shear strength, and volume change characteristics of granular soil. Desaturation via biogenic gas generation suppresses excess pore pressure development and thereby improves the cyclic shear resistance of granular soil. MIDP represents the combined effect of these two mechanisms. Effective implementation of MIDP for ground improvement demands quantitative understanding of the rate at which both mechanisms occur. The biogeochemical model developed herein is an upgrade of the model presented in earlier research; it includes additional features, such as inhibition terms, an expanded number of organic substrates, and a greater number of chemical constituents. It predicts the rate of ${\mathrm{CaCO}}_3$ precipitation and gas production by integrating stoichiometry, thermodynamics, microbial growth kinetics, and chemical reaction kinetics for a continuously stirred batch reactor. The model was calibrated using results from laboratory test columns. Sensitivity analyses conducted using the calibrated model identified a molar ratio for acetate:calcium:nitrate of $0.9:1.0:1.0$ as the preferred ratio for maximizing ${\mathrm{CaCO}}_3$ precipitation and gas generation while avoiding excess residuals in the modeled batch reactor system.

The aim of this paper is to study the face stability of earth pressure balance (EPB) shield tunnels in dry granular soils considering the influence of the dynamic excavation process. The analysis was performed using three-dimensional discrete-element method (DEM). In the DEM model, the EPB shield considering the cutting-tool and opening configuration of panel was established, and the support of soil in the chamber was simplified as normal pressure acting on the excavation face particles. The present work has two advantages: (1) it can consider the soil disturbance induced by the dynamic interaction between the tunnel face and shield cutterhead; and (2) it can consider the support provided by the cutterhead panel. The simulation results show that the face stability in granular soils is overestimated without considering the dynamic excavation process. The face failure mechanism is affected by the buried depth of the tunnel and the friction angle of soil. For shallow tunnels with $C/D=0.5$ ($C$ = buried depth and $D$ = tunnel diameter), the failure zone resembles a wedge directly propagating to the ground surface; for tunnels with $C/D=1.0$, the failure zone resembles either a wedge with a chimney on top outcropping the ground surface or a closed bulb, depending on the friction angle of the soil; for deep tunnels with $C/D=2.0$, the failure zone resembles a bulb regardless of the friction angle of the soil.

Parametric analysis of the effects of soil relative density on earthquake surface fault rupture interaction with foundations having different contact pressures and positions was performed with the discrete-element method (DEM). DEM simulations provide insights into particle responses not possible with continuum methods. The relative density of soil has a significant impact on the interaction of surface fault rupture with a building foundation. The number of horizontally oriented contacts achieves a peak value before decreasing during reverse fault rupture through a dense particle assemblage due to its strain-softening response, and the number of horizontally oriented contacts tends to monotonically approach a constant value during reverse fault rupture through a loose particle assemblage. The mobilization of stresses reflects the trends seen with the contact forces and shows quantitatively the more brittle response of denser particle assemblages. Heavy foundations consistently alter the path of reverse fault rupture propagation while undergoing less rotation than light foundations. During normal fault rupture, heavy foundations can undergo more rotation when positioned on the footwall side of the free-field surface outcrop location.

This paper presents experimental results from physical model tests on four half-scale geosynthetic reinforced soil (GRS) bridge abutment specimens constructed using well-graded angular backfill sand, modular facing blocks, and uniaxial geogrid reinforcement to investigate the effects of applied surcharge stress, reinforcement vertical spacing, and reinforcement tensile stiffness for working stress, static loading conditions. Facing displacements increased for the upper section of the walls after the application of surcharge stress and were greater for larger reinforcement vertical spacing and reduced reinforcement tensile stiffness. Bridge seat settlements were proportional to the applied surcharge stress, strongly affected by larger reinforcement vertical spacing, and only slightly affected by reduced reinforcement tensile stiffness. Measured vertical and lateral soil stresses generally were lower than calculated values for static loading conditions. The maximum tensile strain in each reinforcement layer occurred near the facing block connection for lower layers and under the bridge seat for higher layers. A companion paper presents experimental results for the same GRS bridge abutment specimens under dynamic loading conditions.

This paper presents experimental results from shaking table tests on four half-scale geosynthetic reinforced soil (GRS) bridge abutment specimens constructed using well-graded angular backfill sand, modular facing blocks, and uniaxial geogrid reinforcement to investigate the effects of applied surcharge stress, reinforcement vertical spacing, and reinforcement tensile stiffness for dynamic loading conditions. Similitude relationships for shaking table tests in a $1g$ gravitational field were used to scale the specimen geometry, applied surcharge stress, soil modulus, reinforcement tensile stiffness, and characteristics of the earthquake motions. Reinforcement vertical spacing and reinforcement tensile stiffness had the most significant effects on the maximum dynamic and residual wall facing displacements and bridge seat settlements. Acceleration amplification increased with elevation in the reinforced and retained soil zones. Residual vertical and lateral soil stresses were lower than the calculated values for static loading conditions. The maximum tensile strain in each reinforcement layer occurred near the facing block connection for lower layers and under the bridge seat for higher layers. The vertical seismic joint between the bridge beam and bridge seat closed during the Northridge motion, resulting in contact force. A companion paper presents experimental results for the same GRS bridge abutment specimens under static loading conditions.

This paper presents three-dimensional (3D) numerical analyses of a column-supported embankment case history using the finite-difference method. An undrained end-of-construction analysis is followed by a long-term dissipated analysis, in which all excess pore pressures generated in the undrained loading phase were manually dissipated for the calculation of long-term deformations. The two analyses examined limiting cases for lateral spreading, providing results that envelop case history recordings at the end of construction and 125 days after construction, respectively. Numerical calculations were performed for a unit cell and a half-embankment model. Calibration of large-deformation soil arching behavior in the embankment was achieved by reducing the Young’s modulus and friction angle of loosened zones whose dimensions were modeled after bench-scale and field-scale tests. Numerical results are in good agreement with case history recordings for vertical load transfer, settlements, and lateral displacements. In addition, numerical results are provided for foundation incremental lateral earth pressure profiles, geogrid strains, column bending moment profiles, and column stresses due to combined effects of bending and axial loading so that they can be related to the fundamental aspects of lateral spreading resistance. The increment of lateral earth pressures in the foundation soil was largest at the undrained end-of-construction condition, geogrid strains were largest in the long-term dissipated condition, and column bending moments and column tensile stresses were very large in both scenarios. Furthermore, the geogrid contributions to increasing vertical load transfer to the columns and reducing lateral spreading of the embankment were insignificant, likely due to the subgrade support provided by the top coarse-grained fill layer in the foundation.

This study establishes the drained bearing capacity of pipelines embedded up to one diameter into the seabed subject to combined vertical-horizontal loading. Nonassociated flow finite-element analyses are used to calculate the peak breakout resistance in a frictional Mohr–Coulomb seabed. Critical state friction angles and dilation angles ranging from 25° to 45° and from 0° to 25°, respectively, are considered. Analytical expressions are fitted to the results as a function of embedment depth and soil properties, and compare well with experimental measurements from previous studies. The horizontal bearing capacity at small vertical loads is also predicted well via upper-bound limit analysis using the Davis reduced friction angle that accounts for the peak friction and dilation angles. The analytical relationships presented in this study provide simple predictive tools for estimating the bearing capacity of pipelines on free-drained sandy seabeds. These fill a void in knowledge for pipeline stability and buckling design by providing general relationships between drained strength properties and pipeline bearing capacity. The insight gained through the good comparison with limit analysis techniques also gives confidence in the use of simple numerical techniques to predict the bearing capacity of pipelines for more wide-ranging (i.e., nonflat) seabed topography.

The effects of soil plasticity and penetration rate on cone penetration resistance in low-plasticity fine-grained soils are evaluated. A series of centrifuge tests with in-flight variable rate cone penetration soundings was performed on models of four slurry deposited mixtures of nonplastic silica silt and kaolin clay (0%, 2.5%, 5%, and 20% kaolin by dry mass) with plasticity indices ranging from 0 to 6. Cone penetration resistances for an effective overburden stress of 100 kPa ranged from 26 to 40 MPa (260 to 400 atm) in the nonplastic silica silt to 0.4–1.8 MPa (4–18 atm) in the silt-clay mixture with a plasticity index of 6. The addition of a small amount of clay (as little as 2.5% by dry mass) to nonplastic silt resulted in an order of magnitude decrease in drained penetration resistance. Faster penetration rates produced partially drained and undrained conditions, with negative excess pore pressures developing in the nonplastic silica silt and positive excess pore pressures developing in the mixtures with 5% and 20% kaolin.

A program of centrifuge model tests conducted on the cyclic lateral behavior of a tetrapod piled jacket (TPJ) foundation for offshore wind turbines (OWTs) in medium dense sand was reported. The key parameters investigated were the lateral loading direction, cyclic load amplitude, and cyclic loading type (i.e., one- or two-way loading) in terms of the overall response of the TPJ and the more detailed pile-soil interaction. Under small- and moderate-amplitude loads, that is, 20% and 40% of lateral bearing capacity, respectively, loading along the diagonal direction had a more pronounced impact on increasing the accumulative displacement and unloading stiffness of the foundation than along the orthogonal direction. Both suggest less favorable conditions for the former regarding the serviceability of OWTs. Throughout the loading, a progressive transfer of the lateral load carried by the upwind piles to the downwind ones was identified and a degradation factor model was derived based on cyclic $p-y$ curves to assess the cyclic lateral pile-soil interactions of TPJ foundations in preliminary design. The complementary two-way loading test showed less hysteresis and degradation behavior of the soil than the one-way test, and hence limited accumulative displacement of the TPJ despite the load being applied in an asymmetrical form. Thus, the degradation model developed in the scenario of one-way loading is considered conservative.

Partially saturated soils associated with the presence of occluded air bubbles are commonly encountered in seismically active zones. Yet, there is a lack of deep understanding of the mechanisms of settlement that partially saturated soils suffer during seismic events. Consequently, a reliable seismic design approach for use in engineering practice is lacking. The aim of this paper is to show that seismically induced settlement of a level deposit of partially saturated soil can be estimated by summing the settlement related to excess pore pressure (EPP) generation/dissipation and increased soil mass compressibility in the presence of air bubbles, which are calculated separately. Predictions of the proposed effective stress-based methodology are found to be in good agreement with geotechnical centrifuge measurements. It is shown that the variation of degree of saturation is of significance. The proposed methodology utilizes the degree of saturation as a parameter and can satisfactorily predict the experimentally observed settlement.

This paper presents a case study describing the effectiveness of multiple measures implemented to repair overdeformed metro shield tunnels in Shanghai. The changes in horizontal diameter were up to 3.6% of the tunnels’ diameters due to unexpected surcharge at the ground surface. The surcharge was removed, and then structural reinforcements were added. The tunnel sections that had extremely large convergences were reinforced by bonding aromatic fiber–reinforced plastic sheets to the tunnel crown and bonding steel plates to the inner surface of the full lining rings. The tunnel sections that had relatively large convergences were reinforced only by bonding fiber-reinforced plastic sheets. In addition, instead of installing steel plates at these sections, cement-sodium silicate grouting was performed at the two sides of the tunnel springline three years after reinforcement by fiber-reinforced plastic sheets, because a gradual increase in convergence was observed at these sections. The grouting resulted in a 30% reduction in horizontal convergence. However, less than 5% of the grouted volume contributed to the horizontal movement of the tunnel cross sections. Cone penetration tests were conducted at the grouted sections as well as at green field sections. The results showed an increase in cone tip resistance of about 110%–130% in the grouted zones compared to the nongrouted zones. However, the increase was localized and had a high coefficient of variance, indicating that the improvement from grouting was highly variable.

Many conventional inspection practices for dams and levees place heavy demands on limited human resources. Furthermore, traditional inspections of riprap slope protection material rely on qualitative evaluations that involve a degree of subjectivity. The combination of these factors can result in incomplete and inaccurate assessments of the riprap condition over time. This paper presents a novel method for inspecting riprap slope protection material using three-dimensional (3D) point cloud data captured by lidar or photogrammetry. The method has been validated using two full-scale studies, a laboratory-scale study using gravels, and extensive two-dimensional (2D) numerical simulations. The validation shows that the small-scale topography of the slope face, known as roughness, is correlated to the median size of the riprap. Thus, as the riprap weathers and breaks into smaller pieces, the roughness of the slope will decrease. The roughness is a quantitative parameter that enables spatial analysis of the riprap condition over time. The studies performed to date indicate that this method can be used on slopes having an approximately planar face, clear of significant vegetation and debris, and armored using end-dumped riprap with a coefficient of uniformity not exceeding 2.5. The slopes of many operational dams and levees meet these criteria, making this a practical inspection tool that can target limited inspection resources to where they are most needed. The basics of the method are demonstrated using a field trial conducted at a reservoir in the northeastern United States.

Levees and floodwalls providing risk reduction to major urban cities along the Mississippi River are prone to sand boils due to underlying alluvial sand foundations. This paper presents a case study for a levee embankment in Baton Rouge, Louisiana, where field measurements were used to determine the hydraulic properties of unsaturated soils incorporated into a soil-atmosphere coupled finite-element saturated-unsaturated transient seepage model. In particular, piezometer data and geoprobe cores collected from a levee embankment at Duncan Point, Louisiana, during the 2011 Mississippi River flood event were used to calibrate substratum sands and unsaturated soil embankment parameters. The results illustrate the influence of climate coupling on phreatic surface migration and pore-water pressure development. In particular, the antecedent suction pressures are more uniformly distributed within the embankment and lower in magnitude than assuming a linear interpolation of matric suction. The soil–atmosphere boundary condition was critical to recreating the in situ phreatic surface, which indicates that precipitation plays an important role in maintaining a saturated embankment even after the flood event resides.

Two-dimensional nonlinear dynamic analyses (NDAs) of Austrian Dam in the 1989 ${\mathrm{M}}_w=6.9$ Loma Prieta earthquake are presented using the finite-difference program FLAC with the user-defined constitutive model PM4Silt and following engineering procedures common in practice. This relatively homogeneous, 55-m high embankment dam is comprised primarily of low-plasticity clayey sands and clayey gravels. The dam experienced estimated peak ground accelerations of 0.4–0.6g during the earthquake and developed extensive cracking with crest settlements up to 859 mm. The engineering properties of the compacted embankment materials are estimated based on the available isotropically consolidated undrained triaxial compression and resonant column test data. NDAs were performed using two alternative calibrations for the PM4Silt model and four different input motions. Sensitivity of the computed deformations to various modeling parameters and assumptions was examined. The computed responses are shown to be in reasonable agreement with the observed crest settlements, embankment deformation patterns, and excess pore pressures. Limitations in the material characterizations and analysis procedures and their possible effects on the analysis results are discussed. The results of this study provide support for the use of these analysis methods and engineering procedures in seismic evaluations of compacted earthfill embankments.

Sand–tire shred mixtures are useful as thermal backfills due to their lower unit weight and thermal conductivity than those of most soils. In this study, a series of thermal conductivity tests on sand–tire shred mixtures and pure sand were performed to investigate the effects of volumetric mixing ratio and tire shred particle size. A volumetric mixing ratio of 40% was found to yield the greatest decrease in thermal conductivity from that of pure sand, with a maximum percentage difference of 72%. Using tire shreds with a larger relative size ratio was found to result in higher thermal conductivity, and the maximum variation in the thermal conductivity percentage difference with the relative size ratio reached about 20% at a volumetric mixing ratio of 40%. An empirical model proposed to predict of the thermal conductivity of quartz sand–tire shred mixtures captured trends in the experimental data.

This paper investigates the effects of three different lignocellulose fibers [jute, coir, and water hyacinth (WH)] on the soil water retention curve (SWRC) and desiccation potential of compacted clayey silt. Samples of compacted clayey silt mixed with an amount of discrete natural fibers were exposed to the natural environment and controlled events of $15\mathrm{wetting}/\mathrm{drying}$ cycles for 105 days. Matric suction and moisture content of near-surface soil were monitored along with allied surface crack formation, quantified by the crack intensity factor (CIF). Adding lignocellulose fibers moderately increased the water retention capacity of the soil but had no significant effect on the rate of desaturation. The presence of fibers could reduce CIF effectively, by at least half the amount compared with bare soil. Among the fibers tested, coir provided the most significant resistance against desiccation cracking due to its comparatively higher lignin content and multifilament nature, which increase the fiber ductility and surface coarseness, both of which are favorable to resisting crack formation. The shape of the CIF-suction relation of selected soils mirrors that of the SWRC and may be represented by a simple modified van Genuchten fitting approach.

A case study of a widened highway embankment is presented, in which prestressed tubular concrete (PTC) piles and geogrids were used to reinforce the embankment and to reduce construction time. In situ measurements of earth pressures, settlements, and lateral displacements were reported and analyzed to evaluate the effectiveness of the reinforcement technologies. The instrumentation data confirm that a large portion of the embankment load is supported by the PTC piles, as quantified by a large stress concentration ratio of 6.6. Consequently, the resulting total settlement at the embankment surface and lateral displacement during and after the embankment construction were significantly reduced by using geogrids and two groups of PTC piles at different elevations of the embankment. The measurement data will be valuable for numerical simulations on pile-supported embankments and highway widening projects in the future. Lastly, a design procedure was proposed for highway widening projects based on the literature studies, and its validity was verified by this case study.

In this study, the effect of the bucket installation method on the subsoil movements was investigated in sand by using centrifuge model tests. In order to directly observe the soil behavior around the bucket installed by suction and jacking, half buckets were installed for each installation method along a box fitted with a transparent window in the centrifuge. Consecutive images were captured during the installation process and subsequently analyzed by an image analysis tool to quantify the soil movements. Direct comparisons were performed to understand the differences in soil behavior around the bucket installed in sand by suction and jacking. The results indicated that a large soil plug heave occurred along the skirt wall interior during the suction installation, whereas no significant soil displacements were observed during jacking installation, and this significantly affects the bucket penetration behavior.

Microbially induced calcite precipitation (MICP) has attracted significant attention as a promising in situ ground reinforcement method, particularly for the improvement of existing structure foundations. Prior to its widespread application, further study is required to answer questions that seek to improve understanding of the fundamental mechanisms of cementation. This paper focuses on the influence of particle shape on the stiffness and strength of MICP-treated glass beads through a series of unconfined compression tests. The concept of combined overall regularity was used to quantitatively evaluate the particle shape of five mixtures with different proportions of rounded and angular glass beads. Increases in the combined overall regularity resulted in significant decreases in the unconfined compressive strength and secant modulus at 50% unconfined strength, particularly for specimens with greater cementation, but a slight increase in the calcite precipitation. This validates that it is the calcite-bridging formation between particle contacts that primarily contributes to the strength increase observed in MICP-treated glass beads.