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.

Horizontal directional drilling (HDD) is now widely used for pipe installation by the trenchless industry. Horizontal borehole stability under excessive mud pressure can be a concern during the drilling processes. The Delft equation has been applied to calculate the maximum allowable mud pressure, but the results are usually not safe. Meanwhile, little experimental evidence is available in the literature to examine the relationship between the maximum mud pressure and different burial depths, or to explain the mud flow paths through the soil up to the ground surface. Therefore, a series of mud loss experiments using different burial depths in sand have been undertaken. An example of mud pressure history along with the development of cracks and a shear plane reaching the ground surface were observed. The manner in which maximum measured mud pressures $P_{\max }$ rose with increasing burial depth was quantified ($P_{\max }$ rises from 17 kPa with $H/D=7.5$, a ratio of the distance from the ground surface to the springline $H$ and borehole diameter $D$, to 181 kPa with $H/D=31.5$). In addition, two test sites were excavated to explore the mud infiltration zones. These show that a clear shear failure plane developed extending from near the borehole to the ground surface. Contractors and consultants often estimate $P_{\max }$ using the Delft equation, but this equation overestimated pressures compared to the experimental results by 90%–170%. These discrepancies are explained. In addition, earth pressure sensors (null gauges) were buried at three times the borehole radius below the boreholes in the experiments to measure axial, radial, and hoop stresses. These data were compared to the closed form theoretical solutions and confirmed the influence of the initial geostatic ($K_0$) stress conditions on stress histories.

This paper presents a field seismic investigation that is effective in evaluating the in situ geotechnical properties and liquefaction potential of fine coal tailings subjected to mine blasting. In this study, seismic refraction, multichannel analysis of surface waves, and seismic monitoring are performed on two coal tailings impoundments in Appalachian coalfields in the United States. Shear wave velocity, compressional wave velocity, Poisson’s ratio, small-strain Young’s modulus and shear modulus, and peak particle velocity of the fine coal tailings are derived. A relatively high rate of vibration attenuation is observed for the fine coal tailings when compared to other blasting applications. The fine coal tailings are evaluated to be potentially liquefiable when subjected to ground vibrations from mine blasting based on the simplified procedure for liquefaction evaluation with slight modifications and the input parameters from the seismic investigations. The nondestructive techniques and methodology described herein can serve as an alternative for liquefaction assessment.

Column experiments that investigate the use of calcitic limestone as a potential material for permeable reactive barriers (PRBs), as well as its clogging behavior, are conducted under conditions that involve continuous acidic flow containing Al, Fe, and acidophilic bacteria. Results show that nonhomogenous biogeochemical clogging occurred toward the outlet, resulting in a 45% reduction of hydraulic conductivity at the inlet and 10% reduction at the outlet after the bicarbonate buffering period. A mathematical model developed to capture the reductions in longevity is presented. The model, which considers the effects of time-varying porosity, hydraulic conductivity, and head at a particular point on the horizontal flow path, is used for assessing the effect of coupled clogging in a calcitic porous medium.

This work aimed at providing analytical closed-form solutions for the design of thermal piles. To this end, a model in which a cylindrical pile is attached along the shaft to a series of distributed vertical springs representing soil stiffness is proposed. The pile has constant section and elastic properties; the restraints provided by the superstructure and base stiffness are represented through concentrated springs. The model allows derivation of exact solutions for homogeneous, two-layer soil and soil with linearly increasing stiffness with depth. In addition, approximate energy solutions are derived via the principle of virtual work for more general subsoil conditions with spring stiffness calibrated through finite element results. Expressions for the axial force and shear stress at the pile–soil interface are provided for typical soil stiffness distributions. A successful comparison to literature studies, involving complex transient-coupled numerical analyses and two field tests, corroborate model reliability. The proposed analytical solutions provide insight into the behavior of thermally loaded piles and can be used as a simple tool for ultimate limit state design.

This paper investigates the deformation of buildings due to tunneling-induced soil displacements. Centrifuge model tests of three-dimensionally (3D) printed building models subject to a plane-strain tunnel excavation in dense, dry sand are discussed. The small-scale structures replicate important building characteristics including brittle material properties similar to masonry, a realistic building layout, façade openings, strip footings, and a rough soil-structure interface. Digital images were captured during the experiments, enabling image-based measurements of the building response. Results demonstrate the essential role of the building-to-tunnel position and structural details (i.e., opening percentage and building length). The onset of building cracking and cracking patterns confirms the importance of the building-to-tunnel position and structural details. The tests illustrate that an increase in the façade opening area leads to increased shear deformations while longer buildings caused an increase in bending deflections. An evaluation of the widely accepted framework of treating a structure separately at either side of the greenfield inflection point shows that this procedure can underestimate building damage.

This paper presents a laboratory investigation into the monotonic and cyclic responses of weakly cemented sand. An extensive laboratory testing program comprising unconfined compressive strength and tensile strength tests, as well as undrained monotonic and cyclic triaxial tests, was performed on clean quartz sand, with cement contents (CCs) varying from 1% to 8%. The effect of various parameters, among which are the cementation content, density, effective stress and curing time is examined. Even at the smallest CC of 1%, a distinctive enhanced behavior of the cemented sand over that of the uncemented was observed. At a given density and effective confining stress, increasing cementation results in the decrease of contractiveness of sand under monotonic shearing and in the transition of cyclic behavior from flow-type liquefaction to cyclic mobility type.

An experimental study was conducted to assess the effectiveness of lime stabilization with precompaction mellowing in stabilizing six different high-sulfate soils. Three mellowing times, 0, 3, and 7 days were studied. A test suite comprising engineering and chemical tests was performed on untreated and lime-treated high-sulfate soils. Four of the six soils considered in the research study responded positively to the lime stabilization with a precompaction mellowing period. Two soils with soluble sulfate levels exceeding 30,000 ppm were not successfully stabilized with the precompaction mellowing, resulting in the chemical treatments, and higher swell strain magnitudes. Two swell prediction methodologies were developed, one based on sulfate content consumed other based on compaction void ratio of the treated soils. Both methods rely on stoichiometric principles with mass-volume relationships of chemical reactions. Swell strains predicted by both methods were analyzed and compared with measured swell strains of all soils at different mellowing periods. Results showed that the sulfate content–based method predictions were not in agreement with the measured data, whereas the void ratio–based method provided better swell strain predictions. Better prediction by this method was attributed to accounting for ettringite crystal growth inside the void space of the treated soils. Potential implementation of this method to screen chemical treatments of high-sulfate soils for effective stabilization was described.

Suction buckets are becoming established as a viable foundation solution for offshore wind turbines. In sand, suction-induced seepage flow reduces effective stresses at the skirt tips, which decreases penetration resistance. However, layered seabeds are often encountered in areas of offshore wind farm development. The effect of the presence of a clay layer on the suction-induced seepage flow in the sand layer is not well understood. Therefore in this study, the effects of a clay layer on suction bucket installation in dense sand was investigated. This was achieved by analyzing images of a half-bucket installed against a Perspex window. The images were captured during tests performed in a geotechnical centrifuge, such that the stress levels are realistic and relevant to field conditions. Installations in sand-over-clay were unproblematic and characterized by deformation of the sand-clay interface, with no clear interruption of the seepage flow. Installations in clay-over-sand were also successful. Uplift of the clay plug was identified as the mechanism to transfer suction to the underlying sand, creating seepage flow and thus facilitating further skirt penetration rather than terminating the installation.

The effects of compression and decomposition on the water retention curve (WRC) and unsaturated hydraulic conductivity ($K_{\theta }$) of municipal solid waste (MSW) were evaluated in the laboratory and field. Laboratory measurements of the WRC and $K_{\theta }$ were made using the multistep outflow (MSO) method. Numerical inversion of data collected during the Deer Track Bioreactor Experiment (DTBE) was used to estimate field-scale unsaturated hydraulic properties. For the MSW tested in this study, data indicated that increasing dry unit weight resulted in an increase of the air-entry suction. Increasing levels of decomposition had the opposite effect, reducing air-entry suction for MSW at constant dry unit weight. Multiple mechanisms likely resulted in the observed changes in air-entry suction, including reduction in pore size during compression and changes in overall MSW hydrophobicity during decomposition. At field-scale, air-entry suction tended to remain constant as MSW compressed and decomposed simultaneously indicating that the two processes, which had opposite effects in the laboratory, also may have opposing effects the field. For fresh MSW, van Genuchten’s $n$, which is inversely related to the variance of the pore size of a porous medium, ranged between 1.3 and 2.5 and decreased as the dry unit weight increased in laboratory-scale tests. For decomposed MSW, $n$ ranged from 1.1 to 1.3 and did not vary systematically with dry unit weight. This indicated that for degraded MSW, the variance of the pore-size distribution did not change substantially with increasing dry unit weight. The value of $K_{\theta }$ decreased with increasing dry unit weight for MSW in this study, consistent with observations for saturated hydraulic conductivity in MSW. However, $K_{\theta }$ tended to increase in laboratory tests with increasing levels of decomposition at constant dry unit weight, consistent with observed increases in void ratio. At field scale, observed changes in $K_{\theta }$ varied consistently with changes in saturated hydraulic conductivity as decomposition and densification occurred simultaneously.

This paper describes finite element analyses using a generalized effective stress soil model, MIT-S1, to investigate size effects in the load-settlement response of surface foundations on deep homogeneous sand layers. Prior studies have shown that MIT-S1 can describe realistically the compression and shear behavior of sands over a wide range of confining pressures and densities using a unique set of material input parameters calibrated for a given material. The current analyses show that the model is able to represent transitions in the mechanisms of ground deformation from general to punching modes with changes in foundation size (${\gamma }^{\prime }D/p_{at}$) and sand density (initial void ratio, $e_0$). These mechanisms explain differences in the computed load-settlement responses and produce large decrements in the bearing capacity factor $N_{\gamma }$ with increased foundation size (in the range $0.1\le {\gamma }^{\prime }D/p_{at}\le 20$). Small circular foundations (${\gamma }^{\prime }D/p_{at}\approx 0.1$) have a bearing resistance of about 50% that of comparable strip foundations, but shape effects are negligible for very large foundations (${\gamma }^{\prime }D/p_{at}\approx 20$). Comparisons of computed results for two sands (Toyoura and Berlin) of contrasting particle shape and formation void ratio exhibit similar bearing capacity factors at the same relative density levels. Model predictions of the bearing capacity factor $N_{\gamma }$ are in very good agreement with data reported from centrifuge model tests of circular and strip foundations on Toyoura sand.

Soils often experience repetitive changes in pore water pressure. This study explores the volumetric and shear response of contractive and dilative sand specimens subjected to repetitive changes in pore water pressure, under constant deviatoric stress in a triaxial cell. The evolution towards a terminal void ratio $e_T$ characterizes the volumetric response. The terminal void ratio $e_T$ for pressure cycles falls below the critical state line, between $e_{\min }<e_T<e_{cs}$. Very dense specimens only dilate if they reach high stress obliquity ${\eta }_{\max }$ during pressurization. The terminal void ratios for very dense and medium dense specimens do not converge to a single trend. The shear deformation may stabilize at shakedown, or continue in ratcheting mode. The maximum stress obliquity ${\eta }_{\max }$ is the best predictor of the asymptotic state; shakedown prevails in all specimens subjected to stress obliquity ${\eta }_{\max }<0.95·{\eta }_{cs}$ and ratcheting takes place when the maximum stress obliquity approaches or exceeds ${\eta }_{\max }\ge 0.95·{\eta }_{cs}$. Volumetric and shear strains can accumulate when the strain level during pressure cycles exceeds the volumetric threshold strain (about $5\times {10}^{-4}$ in this study). A particle-level analysis of contact loss and published experimental data show that the threshold strain increases with confinement $p_o^{\prime }$.

As a typical form of internal erosion, suffusion refers to the detachment and migration of fine particles through voids among coarse particles driven by seepage flow. This paper studies the suffusion-induced evolution of mechanical and microstructural properties of granular soils using the coupled computational fluid dynamic–discrete element method (CFD-DEM). The full suffusion process is reproduced by imposing an upward seepage flow on a gap-graded specimen under the designated effective confining pressure. A series of drained triaxial tests are then performed on the eroded and noneroded specimens to obtain their stress-strain responses. The results reveal that suffusion leads to large amount of fines loss and considerable volumetric contraction, accompanied by substantial changes in the soil fabric. The microstructure alterations within the specimen, such as intermittent formation of local piping and spatial evolution of fines concentration, are continuously tracked and quantified during the simulations. It is found that suffusion significantly reduces the peak strength of the specimen under drained shearing while its impact on critical shear stress is negligible. The critical void ratio of the specimens increased after suffusion. The microstructural characteristics (i.e., coordination numbers, connectivity, void fraction distribution, and contact network statistics) are found to be responsible for the shear strength variations. Comparison with results from the eroded specimens prepared by particle removal indicates that the two procedures can generate drastically different soil microstructures and hence distinct macroscopic responses. Therefore, the preparation method for eroded specimens in DEM studies must be carefully validated, as it can lead to qualitatively different conclusions about the mechanical consequences of suffusion.

For large embedded structures, soil-structure interaction (SSI) plays a major role in dictating the overall seismic response. In light of recent strong seismic excitation affecting such structures, three-dimensional (3D) response as well as nonlinear (NL) soil behavior are among the areas of increased interest. As such, this paper presents a series of 3D NL numerical studies conducted to shed more light on the involved SSI mechanisms. In this parametric study, consideration is given to factors such as the effect of soil own-weight stress state, the structure-ground interface properties, and the intensity of seismic excitation. For comparison, additional time-domain simulations explore the use of linear soil (LS) properties derived from an equivalent linear (EL) site-response analysis. For the purposes of this study, the structure is taken to be cylindrical and fully embedded in the ground. Depending on the level of attained nonlinear response, the influence of the following modeling considerations is discussed: (1) employing the NL versus LS formulation, (2) initial own-weight lateral earth pressure stress state, and (3) the soil-structure interface characteristics. Accelerations along the profile of the structure as well as earth pressure on the walls and floor are among the main parameters of interest. In the free field it was observed that the LS representation adequately matches the NL acceleration response up to frequencies of about 10 Hz. Furthermore, both formulations generally resulted in remarkably close estimates of the structural response. Nevertheless, exceptions include the following: (1) the wall lateral earth pressure and floor pressure were noticeably different, and (2) the potential change in the soil initial stress state due to seismic excitation was manifested only in the NL modeling scenarios.

Four centrifuge model tests were conducted to investigate the effect of convex corners on the deformation of soil-nailed walls and evaluate the influence of the soil-nail layout on wall behavior. Excavations with equivalent convex geometries, but different soil–nail arrangements including a perpendicular pattern and parallel pattern, were considered, and the results were compared to those of a two-dimensional plane strain model. The results indicate that the deformational pattern of the convex corners was affected by the soil nail horizontal inclination angle and that the wall facing plays a major role in controlling wall deformation in models with a three-dimensional geometry. Three-dimensional finite element limit analysis was used to verify the validity of the observations. The results can be used to determine the appropriate soil–nail horizontal splay at the convex corners to improve the performance of the soil-nailed reinforcing system.

The mechanisms of force transmission in granular materials is a classic physics problem that has been addressed since the 19th century, when Heinrich Rudolf Hertz investigated the interaction between two similar objects that were in contact under compression. However, the study of force transmission mechanisms in assemblies of more particles has proven to be a formidable problem due to the complex nature of granular materials. In recent years, synchrotron microcomputed tomography (SMT) and three-dimensional X-ray diffraction microscopy (3DXRD) have been employed to study the mechanics of granular materials experimentally. Combining SMT and 3DXRD offers unique three-dimensional (3D) experimental measurements of the internal structure, kinematics (such as rotation and translation), and lattice strains of individual sand particles. In this paper, in situ SMT and 3DXRD scans were acquired at multiple load steps for a specimen composed of 2,705 natural Ottawa sand particles that were subjected to one-dimensional (1D) confined compression. An algorithm was developed to combine SMT images and 3DXRD lattice strain measurements and used to characterize the constitutive behavior of sand particles. The results were used to identify the crystal structure and the evolution of the stresses and lattice strains of individual sand particles. Another algorithm was developed to characterize the force structures within the specimen. Force structures were identified, and their properties (such as length) and evolution through the experiment were examined. The contact number of particles is a particle-scale property that affects the mechanics of granular materials. The effect of the contact number of the sand particles on the onset and evolution of the force structures was also investigated and discussed.

This paper evaluated the seismic performance of hypothetical tall buildings by estimating intensity measures, engineering demand parameters, and earthquake-induced losses using a soil–structure interaction (SSI) numerical framework. Numerical models for 40-story buildings were developed using OpenSees to study their seismic performance under the following modeling and building configuration conditions: (1) fixed-base structural model, (2) model including SSI effects, (3) fixed-base model with shear walls, and (4) model including shear walls and considering SSI effects. The buildings were assumed to be supported on subsurface conditions typical of downtown Los Angeles. The natural period of the soil profile was parametrically studied; the larger its natural period, the lower were the seismic demands of the building. The inclusion of shear walls caused a reduction of the natural period of the building and computed settlements in relation to the buildings without a shear wall system. Considering SSI effects in the modeling approach changed the computed seismic demands of the tall buildings in terms of maximum interstory drifts, peak story horizontal accelerations, and seismic-induced settlements. Computed median direct economic losses for the 2,475-year mean return period increased as much as 33% by considering SSI effects in the numerical analyses in relation to building losses ignoring those effects.

Seismic compression of unsaturated soils occurs due to particle rearrangement during large-strain cyclic shearing, which may be resisted by interparticle stresses that depend on the matric suction and degree of saturation. Due to the high rate of shearing in earthquakes, seismic compression is expected to be an undrained phenomenon with changes in total volume, matric suction, and degree of saturation along with an evolution in soil hydromechanical properties during cyclic shearing. To simplify this problem and better understand the mechanisms of seismic compression, this study seeks to isolate the effect of matric suction through a series of drained cyclic simple shear tests on unsaturated sand subjected to different shear strain amplitudes. These tests were performed in a cyclic simple shear apparatus with suction-saturation control using a hanging column and suction monitoring using an embedded tensiometer. Matric suction values in the funicular regime had the greatest effects on the magnitude and rate of development of seismic compression with cyclic shearing, and values in the capillary regime were similar to those in dry and saturated conditions. The volumetric contractions also caused the soil-water retention curve and suction stress characteristic curve to shift toward higher suctions during cyclic shearing.

One-g shake-table tests were conducted to characterize seismic demands and reveal failure mechanisms of pile-supported bridges in liquefiable soils with crusts subjected to different scour scenarios, namely no scour, partial scour, and complete scour of the crusts. To this end, reinforced concrete bridge models, each consisting of a single pier supported by a $2\times 2$ pile group, were constructed and embedded into liquefiable soil profiles with a full crust or with a partial crust or without crusts to represent the studied three scour scenarios. Physical observations and characteristic soil and structural responses are interpreted first, followed by the primary focus on seismic failure mechanisms and kinematic and inertial effects on pile responses. Test results indicate that scour tends to strongly affect failure mechanisms of the studied bridges via shifting damage positions from pier bottoms to pile heads and from piles at crust-sand interfaces to pile heads. This tendency is against the current capacity design principle that piers should fail before pile foundations. Special attention should be paid to future seismic design of bridges undergoing scour and liquefaction. Furthermore, the kinematic effect is found more prominent before scour, whereas the inertial effect itself becomes more influential after scour.

This paper reports on results from a series of 1-g, reduced-scale shake table tests of a 216-m-long portion of an onshore steel gas transmission pipeline embedded in horizontally layered soil. A set of first-order dynamic similitude laws was employed to scale system parameters appropriately. Two sands of different mean grain diameter and bulk density were used to assemble a compound symmetrical test soil consisting of three uniform blocks in a dense-loose-dense configuration. The sand-pipe interface friction coefficients were measured as 0.23 and 0.27. Modulated harmonics and recorded ground motions were applied as table excitation. To monitor the detailed longitudinal strain profiles in the model pipe, bare Fiber Bragg Grating (FBG) cables were deployed. In most cases, the pipe response was predominantly axial while bending became significant at stronger excitations. Strain distributions displayed clear peaks at or near the block interfaces, in accord with numerical predictions, with magnitudes increasing at resonant frequencies and with excitation level. By extension to full scale, peak axial strain amounted to ${10}^{-3}$, a demand half the yield strain, but not negligible given the low in situ soil stiffness contrast and soil-pipe friction.

Geosynthetic-reinforced column-supported (GRCS) embankments partially transfer fill loads through stiff vertical elements to competent ground at depth, reducing the embankment stress applied to weaker, more compressible soil. Previous studies have focused on load transfer via soil arching and engagement of geosynthetic reinforcement above the columns. The role of the subgrade response to system efficacy—including the influence of the downdrag mechanism to load transfer—has received less attention. Vertical deformations were measured with settlement plates and multipoint borehole extensometers beneath full-scale GRCS embankments constructed with drilled-displacement columns at the Council Bluffs Interchange System (CBIS). The subgrade response was analyzed via the load-displacement-compatibility (LDC) method in combination with measured subsurface vertical deformations, a proxy for load transfer at depth, and measured values of interface friction from full-scale column load tests. A stiff upper layer of overconsolidated clay transferred embankment stresses imparted at the base of the fill to the columns via downdrag, shielding the underlying softer clay. This mechanism limited the efficacy of the geosynthetic reinforcement. Stiff shallow clay and crust that partially adheres to the columns via interface friction at shallow depths can effectively function like a subsurface load transfer platform. Based on these observations a design sequence to determine the necessity of geosynthetic reinforcement and to optimize column spacing—using familiar GRCS concepts—is recommended.

Particle shape is known to affect the mechanical behavior of sands because it influences packing density and particle contacts. Even though the thermal conductivity of sands also depends on packing density and particle contacts, the effects of particle shape on thermal conductivity are not well understood. A series of thermal needle tests were conducted on five granular soil mixtures with different proportions of rounded and angular glass particles having the same mineral composition and gradation. The maximum and minimum void ratios and the packing density of these mixtures were found to depend on the overall regularity, defined as the average value of the particle’s aspect ratio, convexity, and sphericity. For a given overall regularity, the thermal conductivity increases with decreasing void ratio or increasing relative density. Interestingly, the overall regularity has a small effect on the thermal conductivity at a given void ratio but has a significant effect on the thermal conductivity at a given relative density. A particle shape–dependent empirical equation is proposed to quantify the effects of relative density and overall regularity on the thermal conductivity of the tested sand.

Quantifying the effect of reduced gravity on the behavior of granular matter is essential to understanding the evolution of planetary morphology and will likely affect the design of future extraterrestrial habitats. Yet despite recent research, the effect of reduced gravity/confining pressure on strength remains undetermined, with scarce results ranging from no effect to opposing trends. In this study, we employ high-fidelity discrete element simulations (DEM) of passive failure experiments to measure the influence of gravity on the peak and steady-state friction angle, and the angle of repose of sand. The results are compared against recently reported physical experiments, lending the latter support based on micromechanical information, that is unattainable experimentally. We find that the friction angles experience a small increase with decreasing gravity, while the angle of repose remains almost constant.