Journal of Geotechnical and Geoenvironmental Engineering

Relief well systems for dams and levees are often designed and analyzed as an infinite line using average and midwell heads. Well discharge and excess head in the foundation are calculated by adjusting the solution of a continuous slot to account for additional head losses as flow lines converge to wells. The average and midwell uplift factors allow this by representing the effects of well system geometry and foundation depth in discharge and excess head equations. For partial penetration systems, upwelling may occur landward of the well line with head exceeding that in between the wells. Therefore, without a method for estimating landward head, the average value in the plane of the wells has historically been used as a conservative design value. This approach is better than considering the midwell uplift alone, but in some cases, it can lead to costly overdesign when the landward head is substantially less than the average. Uplift beyond an infinite well line is complicated as its behavior also depends on the ratio of the effective seepage exit distance to well spacing. A third so-called landward uplift factor was developed through finite-element modeling to improve the accuracy of the design process. It allows for a more comprehensive understanding of excess head distribution and for certain scenarios, such as low well penetration and high pressure confinement, improves design head accuracy by one-third to 1 m compared to the average head-based design approach. New charts and equations for each of the uplift factors are provided, along with an example problem to aid the practitioner in efficient graphical problem solution and finite-element model verification.

Filamentous fungi grow by extending and branching hyphae through soil pores, which creates an interconnected fibrous network known as mycelium. Fungal mycelium can cross-link and entangle soil particles, which reduces pore size and alters pore structures. Fungal mycelium also can secrete hydrophobic compounds, increasing the water repellency of soils. This study investigated the effect of fungal mycelium on the hydraulic properties of sands, including the soil-water retention curve (SWRC), soil water repellency, and hydraulic conductivity. Ottawa $20/30$, $50/70$, and $100/200$ sands were treated with a filamentous, nonpathogenic, and saprotrophic fungus, Trichoderma virens (ATCC 9645). The results showed that fungal mycelia increased air entry suction by as much as 11.8 times, and increased water repellency at the sand surface from hydrophilic to extreme water repellency after 10 days of fungal growth. Hydraulic conductivities of fungal-treated sands decreased (by as much as 21 times at 20 days of fungal growth) with increasing fungal contents. The reduced hydraulic conductivities of fungal-treated sands can be maintained even under starvation condition (i.e., absence of nutrients). Scanning electron microscopy (SEM) images showed that fungal mycelia modified pore structures by cross-linking and entangling sand particles.

Abstract

Tropical residual soils are found in different parts of the world and consist of mixtures of different types of soils such as sand, silt, and clay, resulting in intricate microstructures and mechanical responses. In this context, and guided by the varying composition of these soils, a 3D discrete element method (3D-DEM) model was developed in which two different contact models are assigned among idealized spherical particles to represent the coarse and fine parts of the tropical soil with two distinct sets of numerical parameters. A simple linear rolling resistance contact model was used to represent the coarse cohesionless component, while a softer adhesive rolling resistance contact model with a linear approximation of the van der Waals attraction force was used for the fine cohesive component. The numerical coarse network is continuous in terms of interparticle contacts and represents the main skeleton of the DEM sample, whereas so-called fine contacts form a local force network between the coarse particles. After a parametric study on the effects of adopting such a numerical mixture, the model was calibrated for a drained compression triaxial test with a specific void ratio. To estimate the equivalent DEM model void ratio, a proportionality between the real soil void ratio and the DEM model void ratio was efficiently employed. During the validation phase, successful model predictions were achieved on drained and undrained triaxial tests and cyclic tests with different strain amplitudes and moderate (hundreds of kPa) confining pressures.

Practical Applications

Tropical residual soils were proposed to be simulated through a grain-based numerical model using the discrete element method (DEM), inspired from the microstructure and the physical components of those soils. The proposed model may contribute in various ways to reliable numerical modeling of existing or new earthfill structures under monotonic and cyclic loadings in tropical areas. First, with an understanding of its limitations (e.g., regarding grain breakage), the model can complement lab mechanical tests, which are often scarce, to consider additional loading conditions. This may lead to better definition of analytical constitutive relations for tropical soils, because the model outputs a wide range of macro- and microscale information (e.g., elastic properties, the influence of the fine content, etc.) on the mechanical behavior of mixed soils. Finally, with significant computational resources, the model could be directly employed for 3D multiscale discrete-continuum modeling of a structure as a boundary value problem, whereby analytical constitutive models are bypassed and the constitutive response of the material is instead derived through direct stress–strain computations in the proposed model.

Design and construction of offshore wind turbines (OWT) in near-fault sites is more in demand worldwide. However, very limited studies are available that assess the effects of near-field ground motions on the response of OWTs. This paper describes a numerical study on the response of OWTs to pulse-like near-field earthquakes in liquefiable soils by performing a series of advanced fully coupled nonlinear dynamic analyses using FLAC-3D. The simple anisotropic sand (SANISAND) constitutive model was adopted for soil to consider liquefaction. Two types of foundations commonly used for OWTs, namely the monopile and suction bucket were modeled. One of the characteristics of near-field ground motions is containing a significant vertical component, which is mostly in the range of high frequencies. Meanwhile, OWTs can be sensitive to vertical ground motion in terms of having a high natural frequency in the vertical direction. Therefore, focusing on the frequency content of input seismic motion, a comparison was made between the response of OWT structures to horizontal and vertical components of earthquakes in the far-field and near-field conditions. The mechanisms behind the obtained results were elaborated, and some suggestions were made for the design of OWTs under near-field conditions. According to the results, liquefaction and tilt are the major potential risks associated with the near-field earthquakes in the horizontal direction. However, in the vertical direction, the excessive amplification of acceleration and upthrow of the rotor-nacelle-assembly (RNA), are the major issues to be considered. Under near-field conditions, a suction bucket foundation can be a better option in terms of lower amplification factor of vertical acceleration, although its aspect ratio should be selected with special care to the rotation response.

Pipe leakage can induce soil fluidization resulting in severe consequences to the urban environment where underground buried pipes are extensively used. Soil fluidization is the process of the transition of soil particles from solid-like to liquid-like behavior that can lead to the failure of the supporting ground and buried utilities. This paper applies the advanced two-phase double-point material point method (MPM) technique to investigate the soil fluidization mechanism around a leaking pressurized water pipe embedded in fully saturated soil. In the model, the inflow water velocity leading to the initiation and evolution of soil fluidization around the leaking pipe is identified based on the changes in soil porosity and soil bed expansion ratio. This study shows that the MPM results are consistent with published experimental studies. Parametric analyses are presented to investigate the influence of different parameters, including the orifice size, bed height, and soil porosity on soil fluidization. The results show that the inflow velocity required for the onset and development of fluidization decreases with the increase in orifice size and soil porosity. The bed height increases the resistance of the soil bed against fluidization. The double-point MPM formulation is shown to be an effective and promising way to study soil-water interaction resulting from a leaking pipe. The model developed in this study can be used as a prediction tool to estimate the significance and progress of the fluidization zone and to determine the critical state that leads to ground failure. Such a tool would be of significant value to asset managers that are responsible for the maintenance of buried pipes, their supporting ground, and surface transportation infrastructure.

Due to the mechanical properties of the seabed exhibiting horizontal and vertical spatial variability, an important challenge in offshore geotechnical design lies in the selection of the relevant soil strength profile for foundation sizing, called hereafter the “design line.” Design values are most often selected depth-wise using existing field data, through knowledge of the volume and distance of the field data from the designed infrastructure and considering the relevant limit state and targeted level of reliability. The cone penetration test (CPT) is well-suited to this purpose given the quasi-continuity and high repeatability of its measurements. This paper proposes a statistical CPT-based method to identify the design line for the design of open-ended piles under drained uplift loading to achieve a target reliability. Application of the proposed method reveals that the statistical procedure used to select the design line is dependent on the proximity of the nearest CPTs to the piles, and the horizontal and vertical scales of fluctuation and the coefficient of variation of the cone tip resistance. This information is used in a companion paper to inform optimization and selection of site investigation strategies for the design of uplifted piles for offshore wind farms.

Interpreting seabed properties for future offshore wind farm development appears challenging given the requirement to investigate very large areas. Current approaches, where significant numbers of geotechnical boreholes and cone penetration tests (CPTs) are conducted—often at the location of each foundation or anchor—may prove prohibitive given the scale of modern wind farms (typically over 100 turbines). This paper presents a framework for the refinement of the design of piles under axial tension [for example, to anchor floating offshore wind turbines (OWTs)] in seabeds where the spatial variability of soil properties exhibits isotropy or anisotropy in the horizontal and vertical directions. The framework relies on the approach to the rational selection of design lines representing the soil resistance for achieving a target probability of failure ($p_f$) presented in the companion paper. The framework is validated via application to the design of piles for OWTs in both artificially generated (synthetic) and real seabeds using standard deterministic design methods, and then comparing the achieved $p_f$ values to the target. The framework is also implemented jointly with a cost model to investigate the overall project cost for different CPT layouts for an example floating wind farm layout anchored in synthetic seabeds. The spatial variability levels of cone tip resistance for these seabeds are assumed to be the same in the vertical direction but different in the horizontal direction. The optimum CPT layouts that achieved the minimum total project cost are shown to depend on the per-CPT cost and the horizontal spatial variability of cone tip resistance, so that a generally applicable “best” CPT layout cannot be identified. However, CPT layouts that include clustered CPTs and/or have small spacings between CPTs and piles (compared to the underlying actual horizontal spatial variability scale of soil properties) resulted both in a lower total project cost and require fewer total CPTs, suggesting significant potential in this approach.

Aeration is an effective in situ remediation technique to accelerate the stabilization process and reduce the odor’s concentration of municipal solid waste (MSW) landfills. The distribution of oxygen in a landfill is crucial to the effective operation and performance of a landfill aeration project. In this study, field-scale tests of air injection via vertical wells were conducted at an MSW landfill in Shanghai, China. The spatial and temporal distributions of oxygen during single- and multiple-well injections were measured and compared. The results showed that the methane volume fraction decreased from 60% to less than 10%, and the oxygen volume fraction increased to approximately 15% only after 2 h of air injection. The rapid change in gas concentration over time could potentially be attributed to the preferential flow that occurred in the heterogeneous MSW. Further, a dual-porosity model was proposed to simulate the rapid advection and diffusion of air in the macropores, thus reflecting the fast increase in the oxygen concentration in the total pore space. During multiple-well injection, a “competitive injection” phenomenon was observed; specifically, the air injection volume of different vertical wells greatly varied even under the same injection pressure due to the large difference in the gas permeability of waste in different areas, and different degrees of aerobic degradation of waste would occur in different areas. Electrical resistivity tomography images showed that only part of the leachate was driven away by the air, leading to an uneven increase in waste resistivity.

Pile drivability predictions require information on the pile geometry, impact hammer, and the soil resistance to driving (SRD). Current SRD prediction methods are based on databases of long slender piles from the oil and gas industry and new, robust, and adaptable methods are required to predict SRD for current offshore pile geometries. This paper describes an optimization framework to update uncertain model parameters in existing axial static design methods to calibrate SRD. The approach is demonstrated using a case study from a German offshore wind site. The optimization process is undertaken using a robust Bayesian approach to dynamically update uncertain variables during driving to improve simulations. The existing method is shown to perform well for piles with geometries that reflect the underlying database such that only minimal optimization is required. For larger diameter piles, relative to the prior best estimate, optimized results are shown to provide significant improvements in the mean calculations and associated variance of pile drivability as more data is acquired. The optimized parameters can be used to predict SRD for similar piles in analogous ground conditions. The demonstrated framework is adaptable and can be used to develop site-specific calibrations and advance new SRD methods where large pile driving data sets are available.

Gas permeability ($k_a$) and gas diffusion coefficient ($D_p$) are vital for controlling gas transport through earthen landfill cover overlying municipal solid wastes or acid mine tailings. Soil clods generated during the construction of landfill cover can result in various soil microstructures. However, the effects of soil microstructure on $D_p$ and $k_a$ are unclear. This study aimed to investigate and compare effects of soil microstructure on measured $k_a$ and $D_p$ of compacted manufactured sand tailings (MST)-bentonite mixtures, considering the effects of specimen preparation method, compaction water content ($w_{\mathrm{comp}}$), and wetting path. The related pore size distributions (PSDs) were measured by mercury intrusion porosimetry. The measurements show that the average $k_a$ of as-compacted specimens increased and then decreased as $w_{\mathrm{comp}}$ increased (i.e., a lower soil air content $\epsilon$—gas-filled volume per unit soil volume), while $D_p$ reduced monotonically. It was because the compacted MST-bentonite mixtures exhibited a multimodal PSD, in which macropores significantly increased the average advective flow velocity of gas. However, the average diffusive flow velocity of gas was independent of pore size, according to the kinetic theory of gas molecules. Conversely, $k_a$ and $D_p$ both decreased as $\epsilon$ decreased when the water content of the as-compacted specimen was increased along the wetting path by spraying water, due to the wetting-induced collapse of macropores. These illustrated that $D_p$ was mainly controlled by the $\epsilon$ and tortuosity of gas-filled pores, while $k_a$ was dominated by well-connected gas-filled macropores. Moreover, the PSD of specimens was affected by the specimen preparation method, bentonite content, and sieving treatment, thus affecting $k_a$ and $D_p$.

Gentle driving of piles (GDP) is a new technology for the vibratory installation of tubular (mono) piles that aims to achieve both efficient installation and low noise emission by combining axial and torsional vibrations. To provide a preliminary demonstration of the GDP concept, onshore medium-scale tests in sand were performed in late 2019 at the Maasvlakte II site in Rotterdam (Netherlands). Several piles were installed using both impact and vibratory driving methods (including GDP), with the twofold aim of comparatively assessing (1) the effectiveness of GDP; and (2) the presence of installation effects in the pile response to lateral loading. This work focuses on the latter aspect and presents a quantitative analysis of the installation effects observed in the pile loading test data recorded in the field. Due to soil inhomogeneity across the field, a purely data-based analysis would have not supported objective conclusions, which led to adoption of an alternative approach based on one-dimensional (1D) numerical modeling. To this end, an advanced cyclic p–y model was calibrated for the simulation of the reference pile loading tests, and the values of key parameters were compared to infer quantitative information about relevant installation effects. The results presented herein inform about the promising performance of the GDP method, particularly in comparison to traditional impact hammering. Although the cyclic lateral pile behavior proves affected by the installation process, certain important aspects of installation effects gradually diminish as more loading cycles are applied.

Identifying sands is an important requirement in many geotechnical exploration projects. Knowledge of the sand type can help in estimating the physical and mechanical properties of a soil. Previous studies revealed that machine learning approaches using neural networks (NN) can correctly identify up to 75% of individual sand particles using size and shape descriptors. This study explores the efficacy of deep learning methods for automatically classifying sand types from individual images of sand particles. Dynamic image analysis (DIA) was employed to generate a large data set of sand particle images, which were then used for training a deep learning model known as convolutional neural networks (CNN). The analysis was based on 40,000 binary particle images for twenty types of sand. The work demonstrates that computer vision has a remarkable ability to automatically classify 64% of individual sand particles among 20 types of sand, the accuracy for sand clusters can reach up to 100%, when a CNN model augmented with size and shape data was employed. The effects of domain size, size and shape parameters, and the selected CNN model on the classification accuracy were also investigated. The results demonstrate that sand classification using a larger number of sand types resulted in a lower classification accuracy. Classification accuracy of individual particles achieved using CNN were 10%–15% better than those achieved using NN. CNN can automatically and adaptively learn the spatial hierarchy of features which is superior to the handcrafted size and shape parameters, used by NN for identifying sand particles. The study suggests that classification accuracy benefits from data augmentation use of more particle orientations, even at the cost of trimming some particles, and suffers from reduction in image resolution. While model training requires a lot of computational work, a pretrained CNN model may potentially be tuned to run on mobile phones, which points to the potential for real-time field deployment to enable automatic soil classification on-site.

The seismic performance assessment of earth slopes and systems requires an estimate of the seismically-induced slope displacement ($D$). Modified Newmark-type analyses are useful to develop an estimate of $D$. Most simplified seismic slope displacement procedures were developed for shallow crustal earthquakes. In contrast, few procedures are available for subduction tectonic settings, which is especially the case for intraslab earthquakes. This study uses the comprehensive NGA-Sub ground motion database to formulate new seismic slope displacement models for subduction zone interface and intraslab earthquakes using a mixed random variable framework, which allows a straightforward implementation in engineering practice. The models are formulated in terms of the sliding mass properties (its yield coefficient and fundamental period), the spectral acceleration at the degraded period of the slope, and earthquake magnitude with peak ground velocity as an optional parameter. The models reveal that $D$ scales differently for interface and intraslab earthquakes, which is contrary to what has been often assumed. The models also estimate displacement-compatible seismic coefficients for pseudostatic slope stability analyses. This study advances the performance-based seismic design of slope systems affected by subduction zone earthquakes.

The most commonly used approach for evaluating liquefaction triggering is via stress-based simplified models. Proposed herein is a model for evaluating liquefaction triggering where the imposed loading and ability of the soil to resist liquefaction are quantified in terms of normalized dissipated energy per unit volume of soil ($\mathrm{\Delta }W/{\sigma }_{vo}^{\prime }$), computed within a total stress framework. The proposed model overcomes limitations of many previously proposed energy-based triggering models. Additionally, the proposed energy-based model unites concepts from both stress-based and strain-based procedures, overcoming some of their limitations, and in its simplified form is implemented similarly to the simplified stress-based models. An updated field case history database is used to develop probabilistic limit-state curves. These limit-state curves express $\mathrm{\Delta }W/{\sigma }_{vo}^{\prime }$ required to trigger liquefaction as a function of corrected cone penetration test tip resistance ($q_{c1Ncs}$) for different probabilities of liquefaction ($P_L$) and have comparable predictive abilities to stress-based limit-state curves in terms of number of correct predictions for the cases analyzed. However, because dissipated energy is a scalar quantity, multidirectional shaking and other effects such as soil–structure interaction, nonvertical wave fields, and topographic site effects can readily be accounted for. Additionally, the applicability of the proposed triggering curve is not limited to earthquake loading but, rather, can be used in relation to other sources of vibrations (e.g., construction vibrations and explosive loading, among others).

Traditional bearing capacity theories assume that a foundation element is supported by a thick, uniform soil deposit. However, natural soil deposits are often stratified, i.e., a soil layer of finite thickness could be underlain by another soil layer with different material properties or shallow bedrock. In this paper, we investigate the effect of the presence of a stiff base on the load–settlement response of a half-strip model footing placed on the surface of Ottawa 20–30 (OTC) sand samples prepared inside a half-cylindrical calibration chamber. The model footing load tests were performed for different values of sand relative density $D_R$ and sand layer thickness-to-footing width ratio $H/B$. A series of high-resolution images collected during model footing loading were analyzed using the digital image correlation (DIC) technique to obtain the displacement and strain fields in the sand domain. The bearing capacity factor $N_{\gamma }$ was back-calculated from the model footing test results as a function of $D_R$ and $H/B$. The magnitude and extent of the cumulative maximum shear strains, horizontal displacements, and vertical displacements in the sand layer depend on the depth at which the stiff base is located below the footing. The results obtained in this study show that (1) the attainment of the peak unit load for the model footing does not coincide with the formation of a fully-developed collapse mechanism, (2) the presence of a stiff base below the sand sample produces a stiffer footing load–settlement response than that obtained in the absence of the stiff base, and (3) the unit bearing capacity of the footing increases when the stiff base lies within the footing’s depth of influence.

Chalk is encountered under large areas of Northern Europe and other locations worldwide, and a wide range of onshore and offshore structures are founded on piles driven in chalk. The safe and economical design of these structures is challenging because of current uncertainties regarding their axial capacity and load-displacement behavior. This work builds on recent research into the axial capacity of open-ended driven piles by proposing new shaft and base load transfer models for chalk that employ geotechnical properties measured directly in laboratory and in-situ tests. First, this study set out a new closed-form elastic analysis of the initial tension loading response. Then, a new nonlinear shaft model was proposed that captures the radial variation in properties induced by pile installation in chalk and uncouples the piles’ stiffness responses from their ultimate local shaft resistances, which are predicted independently. A new base model was also outlined that predicts the loading response based on the chalk’s small-strain stiffness and cone penetration test (CPT) cone resistances. The models are shown to offer good load-displacement predictions for 0.139-m to 1.8-m diameter piles tested between 20 and 600 days after driving at several sites. Improved reliability and accuracy are demonstrated through comparison with existing load-transfer methods.

The paper aimed to present the stress–strain and yielding response of dry and wet molded calcium fly ash in connection with the deformation characteristics at small strains for different stress paths and stress states. Moreover, these responses were compared with the characteristics of reference soils: quartz sand and kaolin. The physical properties of fly ash and natural soils were presented, then tests of $K_0$ and isotropically consolidated drained triaxial extension and compression stress paths were performed on all three types of soils to investigate the elastic and yielding response of soils. The results of laboratory tests confirmed that the fly ash had higher shear and bulk modulus values than reference soils and brittle damage of fly ash occurred at lower strains, independently of the sample forming and consolidation method. In addition, there were visible differences in the shape and size of ${\mathrm{Y}}_1-{\mathrm{Y}}_4$, the yielding surfaces of the tested soils. The ${\mathrm{Y}}_4$ zone was not identified in all paths in each of the soil samples. The fly ash samples exhibit stiffness anisotropy. It was also proved that the method of preparation for the fly ash samples had an impact on the stiffness of this material, which was shown based on triaxial tests and scanning electron microscope (SEM) micrographs.

Soil classification systems help geotechnical engineers anticipate soil properties and provide early guidance for engineering analyses. Current soil classification systems recognize the central role of particle size and inherent differences between coarse- and fine-grained fractions. However, they adopt fixed classification boundaries irrespective of a broad range of fines plasticity and particle shape, disregard the distinct fines thresholds for mechanical and hydraulic properties, and overlook pore-fluid chemistry effects on fines behavior. The Revised Soil Classification System (RSCS) addresses these limitations and benefits from published data and physical insights gained during the last century. By comparison, the classification boundaries in the prevailing Unified Soil Classification System (USCS) resemble those in the RSCS only for the case of angular sands and gravels mixed with low-plasticity fines; in all other cases, extensive data sets corroborate the transition thresholds adopted in the RSCS. The complete logic tree for the RSCS facilitates its implementation; it is available as user-friendly Excel macro and a mobile application that automatically produce the soil-specific classification charts and show the soil classification in terms of the controlling fraction for both mechanical and hydraulic properties. Multiple studies have demonstrated the predictive power of the RSCS in terms of soil properties (e.g., compressibility, strength, hydraulic conductivity, and capillarity), soil phenomena (e.g., fines migration and bioactivity), and the preliminary selection of geotechnical solutions (e.g., soil improvement).

A semiempirical method to determine ground deformations and vibrations induced by impact pile driving in sandy soil conditions is presented in this study. Field data during installation of precast prestressed concrete piles with impact hammers were obtained in terms of ground deformations and peak particle velocities. Semiempirical equations are proposed using a combination of field measurements and numerical analyses to consider the following triggering factors for the ground response due to impact pile driving operations: (1) rated energy of the hammer, (2) scaled distance from the pile, (3) pre-drilling depth, and (4) relative void ratio, which is closely related to the relative density. The numerical component of this framework was developed adopting a continuous pile driving modeling approach coupled with the Updated Lagrangian approach to deal with large deformations and an advanced constitutive soil model (i.e., hypoplasticity for sands enhanced with the intergranular strain concept) capable of reproducing changes in soil void ratios during pile installation. The model parameters were adopted by computationally matching published nonlinear shear modulus degradation curves of the granular layers. A highly disturbed zone close to the pile was computed arising from pile driving-induced soil liquefaction causing large variations in computed void ratios. It was concluded that even if vibration levels are below typical vibration limits defined by regulatory agencies, large levels of ground deformations can still occur. The proposed method is validated in terms of ground vibrations and deformations induced by impact pile driving using field measurements, published vibration attenuation curves, and vibration-induced ground surface settlement prediction methods.

This study examines the load-deformation response of sands subjected to high- and low-stress cycles, i.e., both ends of the Wöhler’s fatigue curve. At high peak cyclic stress ${\sigma }_f$, the terminal void ratio decreases with ${\sigma }_f$ due to crushing-dependent densification, and it can be smaller than $e_{\min }$ when the peak stress approaches the yield stress ${\sigma }_f\to {\sigma }_y$. When ${\sigma }_f\ll {\sigma }_y$, the soil retains memory of the initial fabric even after a very large number of cycles, and the terminal void ratio correlates with the initial void ratio $e_o$. Data show that the maximum change in relative density leads to simple strategies to estimate the maximum settlement for first-order engineering analyses. In agreement with Wöhler’s fatigue, tipping points in void ratio and stiffness trends occur at a small number of high-stress cycles or after a large number of small-stress cycles. During repetitive loading, sands stiffen with the number of cycles to reflect increased interparticle coordination following crushing, as well as contact flattening due to asperity breakage and fretting. The strong correlation between the resilient modulus $M_r$ and the maximum shear modulus $G_{\max }$ suggests the possible application of geophysical methods based on shear wave propagation to monitor geosystems subjected to repetitive loading cycles.

A reliable assessment of earthquake-induced permanent displacements is a fundamental step for both displacement-based and force-based methods applied to the seismic design of embedded anchored walls. Acceleration reduction factors recommended by the codes for a force-based design of these structures derive from parametric studies of the performance of gravity walls or slopes. This may lead to either conservative or unconservative results and is certainly misleading from the point of view of the physics of the problem. Based on the results of an extensive numerical study, this work clarifies the mechanisms by which anchored walls accumulate permanent displacements during earthquakes, showing that full mobilization of soil passive strength requires displacements of the order $u_{\mathrm{r}}/H\approx 1\%–2\%$, making the assumption of rigid-perfectly plastic behavior of the system unsuitable. Moreover, both the critical acceleration and the pattern of final displacements depend on the plastic mechanism effectively activated within the soil-wall-anchor system. The issue of the proper choice of performance factors to be used in a force-based design of anchored walls is also addressed, and a new method is outlined for a preliminary calculation of the permanent displacements of the wall. Finally, the paper critically reviews two methods recently proposed in the literature to compute earthquake-induced wall displacements. All three methods account for the soil-wall system nonlinearity and hardening during the earthquake. The results discussed herein, together with the complementary work presented by the same authors in a companion paper, provide a thorough conceptual framework for the performance-based seismic design of anchored walls.

Flow-like landslides are recognized to be the most destructive slope movements and to threaten human life. They may occur in several geological settings, as is the case of the pyroclastic soil covers resting on either igneous, or carbonate, bedrocks. The pyroclastic soil strata involved in these shallow landslides typically are partially saturated over the whole year. Intense rainfall, considered at hourly time scale, represents the triggering factor because it causes the decrease in matric suction and the consequent decrease in soil shear strength. However, it is recognized as the role of the hydromechanical slope behavior, the vegetation cover, and the geomorphological irregularities on shallow landslide hazard. These factors should be properly simulated in physically based predictive models of the failure onset to set up a reliable early warning system (EWS). This paper presents a new coupled thermohydraulic modeling of a pyroclastic soil cover in Campania, accounting for several slope factors that may predispose landslide activation, including the geomorphological local irregularities. The Mount Faito test site, in the Lattari Mountains (Southern Italy), has been adopted as a prototype slope for the geomorphological and hydromechanical scenarios of reference because of the extensive field and laboratory characterizations of the soil strata already available from previous studies. Once validated with these data, the numerical model has been used to estimate the slope response to critical rainfall scenarios. Such numerical estimations have been then compared to the instability predictions currently provided by empirical approaches, defined in terms of the intensity and duration of rainstorms threshold of shallow landslide activations. By comparison between the empirical and physically based approaches, the crucial role of antecedent slope hydraulic conditions and the geological setting for implementing reliable EWS, reducing false alarms, is proved.

Abstract

Characterizing gravelly soil using in situ penetration testing procedures is a significant challenge for geotechnical engineers. The interaction between the penetrometer and large gravel particles can obscure the penetration resistance of the soil matrix, which is the target of the investigation, and lead to significant uncertainty regarding the properties of the soil. Hence, the size of the probe relative to the maximum particle size of the soil matrix is an important factor to consider when performing interpreting and penetration tests. In this respect, the standard penetration test (SPT) and cone penetration test (CPT) can have potential issues in measuring the penetration resistance in certain cases depending on the size and percentage of gravel particles. Recently, the Chinese dynamic cone penetration test (DPT) consisting of a larger-diameter probe with higher hammer energy has been used to develop probabilistic liquefaction triggering curves for gravelly soil. In the present study, an instructive comparison is presented between the performance of the DPT and CPT in evaluating the liquefaction potential of gravelly soils based on in situ testing at the port of Wellington in New Zealand. Gravelly reclamation fill at the port liquefied during the 2016 $M_{w}7.8$ Kaikōura earthquake, but only limited parts of the same fill deposits manifested liquefaction during the $M_{w}6.6$ Cook Strait and Lake Grassmere earthquakes that occurred in 2013. Triggering analyses have been performed using both DPT- and CPT-based triggering procedures to estimate the potential for liquefaction in these gravelly deposits for these three earthquake events. The CPT-based cyclic resistance ratio (CRR) profiles showed several intermittent spikes with depth due to the interaction of the small-diameter cone with large gravel particles. However, the lower range of values excluding the spikes in the CPT-based CRR profiles primarily governed the liquefaction potential of the reclamation fill, and they are in good agreement with the DPT-based CRR profiles. Both the CPT and DPT-based triggering analyses successfully estimated liquefaction manifestation during the Kaikōura event, some liquefaction manifestation during the Cook Strait event and very limited manifestation during the Lake Grassmere event, which is largely consistent with observations.

Practical Applications

This case history clearly shows that sandy gravel can liquefy. In this case, the sand content was high enough ($>30\%$) to fill the pore space and reduce the permeability so that it could liquefy. For sandy gravel at Centerport, New Zealand, with $D_{50}$ (50% finer particle size) between 3 and 10 mm, the ratio of $D_{50}$ to penetrometer diameter ($D_p$) would occasionally exceed 33% to 50% for the CPT, and some artificial increase in penetration resistance would be expected. In contrast, the $D_{50}$ to $D_p$ ratio for the DPT would not exceed 15%, and gravel particles would not affect the DPT blow count. This assessment is borne out in the comparison between the CRR from the CPT and DPT. For example, the CRRs from the DPT were relatively constant in the sandy gravel, whereas the CRRs from the CPT showed several spikes with depth, but were otherwise consistent with those from the DPT. In some cases, the CPT cannot penetrate layers with larger or denser gravel particles. The DPT cannot identify nonliquefiable cohesive layers; therefore, samples from a companion borehole are needed to identify them. The good agreement between the CRRs from the CPT and DPT for three earthquakes confirmed the reliability of these two independently developed methods.

Colloidal silica (CS) grouting is a soil improvement technique for seismic liquefaction risk mitigation consisting of the injection of a time-hardening, colloidal silica–based grout into potentially liquefiable sandy soils. A published laboratory test database—consisting of cyclic undrained triaxial tests on liquefiable clean untreated sand and on sand stabilized by 5% CS under different levels of initial stress anisotropy—is here discussed following an energy-based approach. The test results are presented and analyzed in terms of dissipated energy during cyclic loading. The obtained results show that (1) the development of dissipated energy is clearly related to the buildup of residual extra pore water pressure only for untreated sand samples; (2) under the same testing conditions, the dissipated energy at failure is significantly higher in stabilized sand than in untreated sand; and (3) the dissipated energy at failure is found to be independent of the cyclic stress amplitude and dependent on the degree of initial stress anisotropy for the untreated sand, while opposite trends are observed for stabilized sand. Therefore, contrary to the untreated sand, there is no evident advantage for using the dissipated energy at failure to assess the capacity of the stabilized sand.

Backfilling is critical to deep and high-stress mining in order to minimize stress redistributions in the host rock, which can lead to mining-induced seismicity and underground excavation collapse. Cemented paste backfill (CPB) is the most popular backfilling material in the mining industry; it is a mixture of mine tailings, binder, and water, which tightly fills the mined-out space, providing optimum regional ground control. This paper presents the results of a laboratory study of the evolution of the coefficient of at-rest earth pressure ($K_0$) of a typical CPB material. The impact of parameters such as cement content and curing time on $K_0$ and one-dimensional behavior of a CPB in stresses up to 1.8 MPa was studied. In general, $K_0$ behavior is characterized by three regions: elastic, transitional, and posttransitional. Fitting functions are proposed to predict evolving $K_0$ based on cement content, specimen curing time, and unconfined compressive strength (UCS) as the most common laboratory test in geomechanical engineering practices. Although the calibrated functions are specific to the material tested, the testing approach can be used to characterize other mines’ backfills.