Geotechnical Frontiers 2017: Walls and Slopes

Limit equilibrium is often used to evaluate global stability of retaining walls. The critical failure surfaces tend to be “V-shaped” with distinct active and passive regions separated by an abrupt point. The validity of these failure surfaces is explored in this study through comparison with bearing capacity theory and finite element analysis. V-shaped surfaces are similar to lower bound bearing capacity solutions, imply a discontinuity in stress orientation, and tend to have conservative factors of safety. The amount of conservatism depends on the accuracy of the limit equilibrium method used and the side force assumptions. Other factors, such as the bearing pads often placed below MSE walls, can also affect the accuracy of limit equilibrium global stability analyses. The factor of safety can be checked using bearing capacity theory. The proposed method produces factors of safety within 5% of finite element results for undrained conditions.

The geosynthetic reinforced soil integrated bridge system (GRS-IBS) is an alternative design method to the conventional bridge support technology. This new technology has a number of advantages including reduced construction time and cost. This paper presents a case study on instrumenting and short-term performance monitoring of the first GRS-IBS project in Louisiana. The monitoring program consisted of measuring bridge deformations, settlements, strains along the reinforcement, vertical and horizontal stresses within the abutment, and pore water pressures. Measurements from the instrumentations also provide valuable information to evaluate the design procedure and the performance of GRS-IBS bridges. The instrumentation readings showed the overall performance of the GRS-IBS was within acceptable tolerance in terms of measured strains, stresses, settlements and deformations. The magnitude and distribution of strains along the reinforcements vary with depth. The locus of maximum strains in the abutment varied by the surcharge load and time.

A correct understanding of passive force-deflection response is important for lateral load evaluations of bridges during extreme events such as earthquakes and during in-service conditions resulting from thermal expansion and contraction of the superstructure. The goal of this study was to better understand this behavior for abutments backfilled with both gravel and geosynthetic reinforced soil (GRS). Large-scale testing was performed with non-skewed and 30° skewed abutment configurations. Two tests were performed at each skew angle, one with gravel backfill and one with GRS backfill. The test abutment backwall was 3.35 m wide, non-skewed, and 1.68 m high and loaded laterally into the backfill. Both backfills exhibited greater passive resistance than sand backfills owing to increased internal friction angle and unit weight. Skew angle reduced the passive force in both cases by about 40%. The GRS backfills had reduced initial stiffness and only reached 79% to 87% of the passive force developed by the unreinforced gravel backfill. This reduction was considered to be a result of reduced interface friction at the geotextile-backwall interface due to wrapping. Reduced stiffness may be favorable for abutment configurations because it allows thermal movement without developing excessive induced stresses in the bridge superstructure.

This paper presents a numerical study on the effect of backfill compaction on the static response of geosynthetic reinforced soil-integrated bridge system (GRS-IBS). The numerical simulations were performed in stages to simulate the construction process, and considered soil-geotextile, soil-block, geotextile-block, and abutment-bridge interactions. Backfill compaction was simulated using a temporarily-applied uniform vertical stress to each lift during construction. Simulation results of the GRS-IBS behavior corresponding to various levels of compaction effort are presented and discussed, including lateral facing displacements, settlements, and maximum tensile forces in the reinforcement layers. Results indicate that greater backfill compaction leads to smaller bridge seat settlement and backfill compression, but also results in larger lateral facing displacements.

Although 2-dimensional (2D) design methods have been shown to work well in defining the longitudinal reinforcement layout in geosynthetic-reinforced soil (GRS) bridge abutments, three-dimensional (3D) effects may play a role in the design of the side walls and the associated transverse reinforcement layout. The objective of this study is to understand the deformation behavior of GRS bridge abutments considering 3D boundary effects, using finite difference analyses to simulate the deformation behavior of a hypothetical GRS bridge abutment expected during construction. Soil-concrete and concrete-concrete interactions were simulated using interface elements and soil-geogrid interactions were simulated using geogrid structural elements. Analyses were performed in stages to simulate the abutment construction process with different reinforcement vertical spacing and length. The results presented in this paper provide insight into the lower wall lateral facing displacements in both the longitudinal and the transverse directions, as well as bridge seat settlements at different sections. This information is a useful component in the development of comprehensive design guidance for GRS bridge abutments.

This paper presents an experimental study on the seismic performance of a half-scale geosynthetic-reinforced soil (GRS) bridge abutment. Experimental design of the scale model followed established similitude relationships for shaking table testing in a 1 g gravitational field. This involved scaling of model geometry, reinforcement and backfill stiffness, bridge load, and characteristics of the earthquake motions. The GRS abutment was constructed using well-graded sand, modular facing blocks, and uniaxial geogrid reinforcements with a vertical spacing of 0.15 m in both the longitudinal and transverse directions. The bridge deck was placed on the GRS abutment at one end and supported by a concrete wall resting on a sliding platform off the shaking table at the other end. The table was connected to the base of the support wall with steel beams to transmit the table motions. The measured lateral facing displacements and bridge seat settlements during application of a series of earthquake motions in the longitudinal direction are presented and indicate good seismic performance of the GRS bridge abutment.

Considering that the backfill soils of most geosynthetic-reinforced soil structures (GRS) are well-compacted granular soils exhibiting evident strain softening, it is expected that the strain softening behavior beyond the peak strength will affect the long-term response of GRS structures if the structures are subjected to large in-service load. Since granular soils generally exhibits small time-dependent behavior, it has been demonstrated by many that under small service loading, geosynthetic reinforcements generally experience load relaxation, and the service loading is redistributed to the backfill soil. However, if the service load is large, and strain softening is mobilized in some part of the backfill soil, the situation could be different. In this study, a calibrated finite element procedure for the long-term behavior of GRS structures was employed to analyze the long-term response of wrapped-face GRS walls. It was shown that in contrast to load redistribution to the backfill soil, after strain softening, the reinforcement load increased with time in the high wall with large reinforcement spacing. This issue should raise attention in the limit state design of GRS structures. Adequate margin of soil strength should be preserved in order to maintain the long-term safety of GRS structures.

Due to the presence of soft soils and high ground water levels underneath a proposed mechanically stabilized earth (MSE) wall to be constructed adjacent to Denton Creek in Grapevine, Texas, the minimum required global stability factors of safety could not be achieved. In order to make the overall MSE wall system work, ground improvements under the MSE walls were essential. Various alternatives were evaluated to come up with the most effective alternative to mitigate the site conditions and meet the project requirements. The selected ground improvement system consisted of a combination of aggregate base rock layer with geogrid reinforcement and multiple rows of cast-in-place pile (drilled shaft) foundations under the wall. The MSE wall survived the severe flood event the DFW area experienced in spring and summer of 2015 that overflowed Denton Creek. This paper summarizes the overall design approach, analysis and techniques used for the pile supported MSE wall with ground improvements.

The geosynthetic reinforced soil integrated bridge system (GRS-IBS) can be used to support a variety of superstructures for single span bridges. The technology appears to demonstrate compatibility between the substructure and the superstructure, requiring no special design details to maintain serviceability of the approach due to lateral movement induced by thermal expansion and contraction of the bridge. This paper presents the behavior of the lateral abutment face movement in combination with the corresponding back wall pressures at the interface between the bridge and the approach of a GRS-IBS built along CR47 in St. Lawrence County, NY, in 2013. The 32 m long, single span GRS-IBS, with a 3.7% superelevation and 30 degree skew, was instrumented to monitor the thermally induced movement of the abutment wall face in combination with lateral backwall pressures to evaluate the response of this particular superstructure geometry for the IBS. During the initial eight month monitoring period, the data shows both the GRS abutment and superstructure moved in unison with the seasonal change in temperature. In addition, the passive pressures induced from the thermal expansion of the superstructure are minor, and similar to those seen in an IBS with no skew.

This paper presents a conceptual framework for prospective owners of mechanically stabilized earth (MSE) walls to assess and manage the risk of their project. The proposed framework is based on information about the causes of failure for MSE walls. The format of this assessment is centered on the principles of risk management necessary to meet the objectives of an owner or agency. The intent of the assessment is to manage the effects of uncertainty and poor practice centered on the main factors associated with MSE wall failures: soil type, design, communication, water, and construction. The assessment first establishes a relative consequence, or impact of failure, valued against the project requirements, cost, and significance, followed by determining the likelihood of an event based on the level of practice leading to uncertainty that could cause poor serviceability or failure. The assessment attempts to guide the owner towards best practices while also providing them with fundamental information to identify, analyze, and prioritize risk necessary to make informed decisions on the course of a wall project with respect to economics (cost), requirements, and performance.

The first use of geosynthetic reinforced soil walls for integral bridge abutment construction in North America occurred on the Greenville Southern Connector (I-185) toll road in 1999. A second bridge with a longer span and higher loads was constructed in 2000 for the same project. Each of these four bridge abutment walls were constructed over 20 ft. (6 m) high using modular concrete block wall (MCBW) facing units and geosynthetic reinforcement with a silty fine to medium sand backfill around vertically driven steel H-pile foundation elements. While the piles were designed to carry all the vertical live and dead bridge loads, the lateral loads due to momentum, braking, and thermal movement would be transferred through the integrally cast-in-place concrete abutment to the piles laterally loading the wall facing elements, located just 3 ft. (1 m) behind the MCBW facing being restrained by the geosynthetic reinforcement within the abutment wall. This paper is a 16-year update on the service life performance of these MSE integral abutment walls based on visual observations and deformed shape measurement of the wall facing blocks. Results of survey monitoring of the wall facing deformations are presented and discussed. The wall face performance indirectly evaluates the distribution of stresses into the wall system structural components. This evaluation is done within the context of the prevailing 1998 FHWA (ASD) engineering analysis and design procedures for MSE walls utilized in 1999, including distribution of the pile induced lateral loads. A brief description of the geosynthetic installation details around the piles is presented to understand the applied analytical methods.

While significant emphasis has been placed in the technical literature on the interaction between soil backfill and geosynthetic reinforcement, companion phenomena that may develop in a reinforced soil mass due to reinforcement vertical spacing may have been overlooked. This paper integrates the results of experimental and field evaluations aimed at identifying such phenomena. Both evaluations were in turn complemented with numerical simulations. The experimental program, conducted on geosynthetic-reinforced soil (GRS) cells, indicated that the soil confined between subsequent reinforcement layers acts as a monolithic block. The field evaluation, which included assessment of the behavior of two GRS walls, showed responses consistent with those in the experimental component. Numerical simulation of these walls indicated that the effect of closely-spaced reinforcement increases with increasing backfill shear strength. Overall, the effect of reinforcement vertical spacing may have a relevant impact on the behavior of GRS that is often not accounted for in design.

This paper describes the construction of a 13 m-high tieback-supported retaining wall to repair the Media Vía slope failure on the Cuenca-Molleturo Roadway in Ecuador. Construction of the reinforced concrete wall supported with 30 m-long tiebacks had to be modified during construction due to a conceptual error in the original design. During wall construction to a height of approximately 7 m, distress fissures appeared on the lower wall panels. The authors, working for the general contractor, designed and obtained the approval for the installation of lightweight fill paired with steel beam reinforcement as replacement for the design included in the contract documents. The wall was successfully completed in 2003 and shows no signs of further distress. An effort to document wall behavior under seismic loading was considered since reference points were previously installed to document permanent displacements.

A case study on the observed deflections and internal forces of pile-anchor retaining excavation in Jinan City of China is reported in details. Based on field measured data, the ground surface settlement, deflection of wall, axial anchoring forces, and the deformation of surround pipeline are investigated. The results indicates that the combining application of concrete piles, jet grouting columns and anchors support system can effectively control excavation-induced surface ground settlements. The field maximum lateral wall deflections are between 0.02 percent and 0.19 percent of the excavation depth due to the difficult site conditions. The axial forces in the upper layer of anchors increased gradually during the excavation, but those of the lower layer of anchors first decreased slightly and then tended to stabilize during the excavation process.

A robust geotechnical design methodology for design of cantilever retaining wall subjected to earthquake is presented in this paper. Initial geotechnical designs were performed considering possible variations in backfill soil properties and earthquake loading parameters to establish upper and lower limits for the design variables. Then, dynamic finite element simulations were conducted on models for selected design cases to obtain the wall tip deflection-time histories. Considering the maximum wall tip deflection as the response of concern, a response surface was developed to establish a relationship between the response and the variables (random and design). Finally, a design optimization was conducted considering cost and robustness as the objectives. The standard deviation of the response and volume of wall were considered as the measure of robustness and cost, respectively. The optimization yielded a set of preferred designs known as Pareto front, and the optimal final design was then selected using knee point concept.

For retaining systems involving rigid walls, the soil-structure-interaction between the side soil and the wall affects the response of the system. The relationship between lateral earth pressure and wall displacement at any given depth is expected to be complex and affected by the height of the wall, the relative density of the backfill, the non-linearity of the soil response, and the type of wall movement. The objective of this paper is to investigate the static soil-structure interaction between rigid walls and sand backfill using finite element analyses for active states of loading. Numerical results indicate that (1) active conditions are mobilized at different wall displacements along the height of the wall and not at a fixed displacement as assumed in the literature; (2) the gradual mobilization of active earth pressure (starting from the at-rest pressure) at any given depth could be expressed as a function of the distortion of the wall using a simplified trilinear model; and (3) the use of a non-linear constitutive soil model is required in lieu of the more common elastic-perfectly plastic model where numerical instabilities were observed in the solution of the soil-structure interaction.

Urban cofferdams are made of steel sheet piles that are structural sections with interlocking systems used to build a continuous wall. These systems are constructed by driving sheet piles, assembling subsequent sheet pile sections, and installing ring beam bracings as the excavation proceeds. The interlock connections between sheet pile sections tend to have a natural slack which leads to additional deflections of the support system. It is only after some deformation that the natural slack of the interlocks and sheet pile-to-steel ring beam connections close their gaps and the bracing system is fully engaged. These features on urban cofferdam behavior are presented parametrically in three-dimensional numerical analyses. Large differences on the computed lateral wall deformations are presented when ignoring these installation effects. Disregarding these pre-excavation construction effects conceal the fundamental reason for the resulting ground deformations in the construction of urban cofferdams and deep excavations.

Uncertainty in soil parameters and variation in surcharge loading applied to the ground surface often make engineers overdesign for safety, which may lead to a cost inefficient design. In order to get a design, which is safe, insensitive to the uncertainties, and also cost efficient, the concept of robust geotechnical design (RGD) is adopted. In RGD methodology, robustness and cost are the main objectives to be optimized, while satisfying the safety requirement, by only adjusting the design parameters without reducing the existing uncertainties. In this paper, the aim is to develop an efficient and practical RGD for a design of a deep excavation supported by soldier pile anchor tieback system, using a single spreadsheet in Microsoft Excel. However, the analysis of complex supported excavation using numerical software such as Deepex 2015 within the RGD framework is computationally challenging. To make this RGD approach efficient and practical, the response surface procedure is adopted. In the design optimization, the signal-to-noise ratio (SNR) is taken as the robustness measure. The optimization between cost and robustness is performed in a single spreadsheet in Microsoft Excel using Solver. To locate the best compromise between these objectives, the minimum distance approach (MD) is adopted which can be implemented easily in a spreadsheet. Finally, the results are compared to the original design constructed in Taiwan.

The use of sustainable practices in infrastructure improvements has been progressing in practice. Rain gardens are one type of sustainable stormwater control measure (SCM) used to control rainfall runoff. Rain gardens that are constructed without liners will infiltrate water to recharge the underlying aquifer. The geotechnical changes, which come with these hydrological changes, must be considered when installing rain gardens near infrastructure such as pavements, slopes, and foundations. In this study, a model was produced in GeoStudio 2012 to show how infiltration from a rain garden can affect slope stability. Decreases in slope stability from rain garden infiltration were quantifiable, implying that these types of models should be considered during the design phase of retrofit rain gardens because engineers that designed existing infrastructure likely did not consider additional infiltration from green infrastructure.

The U.S. Army Corps of Engineers, Buffalo District, performs yearly maintenance dredging in the Cuyahoga River federal navigation channel in Cleveland, Ohio. Up to the present, the Corps has disposed of dredge material in three diked rubble mound (armor and fill stone) confined disposal facilities (CDFs) in Cleveland Harbor, Ohio. In 2003, it became evident that capacity in these facilities was rapidly being depleted. To provide more capacity, the facilities were vertically expanded with raised earth dikes: the first was CDF 12, constructed in 2008; the second was CDF 9 in 2010. This paper discusses the engineering analysis performed using geotextile reinforcement for raised CDFs and stabilization. It presents a design approach for geotextile reinforced embankments considering rotational mode of failure using GEO-SLOPE International Ltd.’s SLOPE/W software.

This paper presents a case study of a large wedge failure occurred during excavation of the last bench of storage cavern with an approximate dimension of 80 m length and up to 20 m height with a depth of 8 m. The adopted intervention followed a structured approach, which included immediate rock support, geotechnical and geological investigations in the failure zone and design modifications. Back analyses of the failure zone were also carried out to assess design parameters in failure zone. Final design in the failure zone was carried out using modified design parameters, which included shorter benches, rock support installation schemes such as longer rock bolts, reinforced ribs of shotcrete and reduced construction advances. Geotechnical monitoring in and around failure zone were carried out for recording any alarming movements in the rock mass. The paper discusses the failure, investigation, analysis, design, back analysis, remedial measures and results of geotechnical monitoring carried out.

Climate change, and natural and man-made disasters cause failure of crucial geotechnical components in civil infrastructure systems, which may result in catastrophic damage not only to the civil infrastructure but also to local communities. For example, road embankments provide structural support to transportation infrastructure; thus, they greatly influence the mobility of public to access essential human needs such as food, shelter, work, and medical care. The concept of resilience, which is defined as the ability to absorb, recover from, and adapt to disruptions, introduces a new paradigm to overcome challenges with inevitable disruptive events arising from climate change, natural and man-made disasters. In this paper, a quantitative framework is proposed for evaluating the resilience of geotechnical infrastructure along with consideration of its sustainability. The framework is demonstrated through a hypothetical case study based on a selected road network in the province of Ontario, Canada.

The 2015 Gorkha Earthquake struck Nepal on April 25, 2015 leaving over 22,000 people injured and almost 9,000 people dead in addition to damaging many buildings. The earthquake was also followed by more than 400 aftershocks, the largest of which occurred on May 12, 2015. A major concern was the potential for landslides following the 2015 Gorkha Earthquake and its aftershocks. This study details a geospatial landslide database, containing over 4,000 post-Gorkha Earthquake landslides that was developed using high resolution Google Earth images and helicopter surveys conducted during the post-earthquake reconnaissance. The geological, topographical and seismological patterns of the co-seismic landslides are discussed. The results show that the majority of the landslides were observed in the area between the epicenters of the main shock on April 25th and the major aftershock on May 12th with the most landslides observed in the Gorkha, Sindhupalanchowk, and Dhanding districts. Further analysis revealed that most of the landslides occurred in gneiss, migmatite formations followed by those in quartzite materials.

A series of devastating dam failures caused significant damage to communities throughout South Carolina in October 2015. The South Carolina Department of Health and Environmental Control (SC-DHEC) reported a total of 47 dams breached, with 22 of them located in the Columbia area. South Carolina has at least 2,500 dams, of which, 205 are classified as high-hazard potential structures. Many of these dams are privately owned and often lack the attention and funding needed for maintenance, monitoring, and upgrades. This paper presents results from a post-flood reconnaissance study aimed to collect perishable data from select dams that failed during the extreme flooding in Columbia. The study included collection of extensive photographic evidence and documentation of descriptive information related to the failure; collection of soil samples, and field and laboratory geotechnical tests. These data form valuable sets of well-documented case histories that can be used in the future to support large-scale research for advancing our fundamental understanding of complex failure mechanisms in extreme scenarios.

Evaluating the bearing capacity adjacent or near slopes is an important issue as it effects design criteria not only for shallow foundations, but walls and highway structures. Past research has evaluated bearing capacity near slopes often using analytical models using a projected, two-sided failure mechanism that is based on punching or shear failure of strip footings; however, the failure mechanism for bearing capacity may transition into a one-sided slope failure when the influence of the slope is significant. This analysis evaluates both bearing capacity near slopes and coupled slope failure using a parallelized limit equilibrium procedure, evaluating not only the ultimate bearing capacity, but also the associated mechanism for failure. The proposed model is verified against both known bearing capacity equations for horizontal ground, but also compared to numerical analyses, showing good agreement. A subsequent parametric analysis is performed to evaluate the effects of slope angle, setback and friction angle of soil, presented in convenient design charts.

This paper presents a case study of a large open pit lignite mine excavation taking place on a moving landslide. The Mavropigi Mine in Northern Greece has reached depths of more than 120m from the ground surface and produces a substantial amount of lignite for an adjacent power plant. The lignite is important for the power supply of Greece and the mine must constantly operate. The southwest slopes of the mine started moving after 2011 and large tension cracks appeared in the ground surface. The landslide has reached displacement values of more than 8m until today. This paper presents the geotechnical investigation conducted for the evaluation of the sliding mass, which consisted of boreholes, inclinometers, survey measurements, inSAR and geophysical investigation. Based on the information gathered from the investigation a detailed geological–geotechnical–kinematic model of the sliding mass was proposed and validated with the execution of limit equilibrium back analysis. The verified model was then used to provide the appropriate excavation balance between the toe and the crest for the mine to continue operation even with moving slopes but with controlled displacements.

Stability of supported excavations in sands and other granular matters against deep failure is of particular importance as different modes of failure are often probable. Among all, the active/passive earth pressure failures, failure of supports and braces, hydraulic failures due to the ground heaving or piping and the bearing capacity or deep seated failure can be pointed out. In some staged-construction procedures, a sheet-pile may not be previously extended below the final level of the cut or the extended length can be insufficient to prevent such failures. In this research, the stability of vertical cuts against such failures has been studied. In particular, when the sand is below the groundwater table, a rather steady state flow of water will form towards the bottom of the cut and together with the weight of the surcharge in the proximity of the cut, it can cause several kinds of failure. For such a problem, a semi-analytical approach is implemented comprising of the solution of the flow field by complex analysis and the stress field (stability analysis) by the slip lines method. Whereas the first analytical procedure works as a background solution, the foreground numerical solution of the stress field can be used for stability analysis. A number of analyses revealed that the deep seated failure may precede the piping and/or heaving failures. Also, in absence of the groundwater seepage, a deep failure is probable. The results are presented in terms of some measure of stability, defined as a factor of safety.

Present 3D limit equilibrium (LE) methods do not incorporate shear resistance from near vertical sides parallel to the direction of slide movement. Consequently, the computed 3D factor of safety (FS) is underestimated and the shear strength parameters from an inverse analysis are overestimated. The present study uses continuum mechanics to calculate the magnitude of side shear resistance along near vertical sides of a translational slide mass. Results of the parametric study show use of an earth pressure coefficient (K_τ) that is in-between at-rest (K₀) and active (K_A) earth pressure and Mohr-Colombo strength criteria provides a reasonable estimate of the side shear resistance and 3D/2D FS ratios that are in agreement with finite element (FE) and finite difference (FD) a continuum analyses. Based on these findings, charts showing the influence of shear resistance on 3D/2D FS ratios for various slope inclinations and geometries are presented herein.

The impact force from a granular sliding mass on a rigid obstruction can be used to gain insights into the granular flow; however, such data is remarkably scarce in the literature. In this study, a two-dimensional soil flume was constructed and utilized to measure the impact force from a granular sliding mass on an obstruction instrumented with dynamic load cells in order to systematically study the effects of different factors on the impact force. In this paper, the effects of initial soil density and inclination angle on the impact force were investigated. Results from these experiments showed that in gentle slopes (i.e., low inclination angle) as the initial density increases the impact force decreases. However, in steep slopes (i.e., high inclination angle) when densely packed granular mass flows downslope with high velocity, the flow is similar to the sliding of a less-deformable block and the impact force increases as the initial density increases.

The yield criterion typically employed in slope stability analyses is the Mohr-Coulomb function with internal friction angle and the cohesion as the two strength parameters. The implication of this yield function is the presence of the tensile strength. However, it is doubtful whether geotechnical design should rely on the existence of tensile strength in soils, and models such as Cam clay do not allow uniaxial tensile strength in soils. It is demonstrated that eliminating tensile strength from the yield condition leads to a reduction in the stability factor. This is because the absence of tensile strength allows a larger variety of admissible failure surfaces. Seismic excitation produces the load that amplifies adverse consequences of the absence of the tensile strength. Two types of mechanisms are considered: translational and rotational. Examples of analyses with both types of collapse mechanisms are presented using the kinematic approach of limit analysis.

A common problem in northern climates during the spring is slope failures occurring on slopes constructed the previous year. The main cause of the failure is a lack of vegetation coupled with ground freezing that prevents water drainage. Typical remediation of failed slopes is to reconstruct the slope back to its original configuration and place rip-rap over the failed section of the slope; a remediation technique that has been found to work well. This case study discusses the construction of a slope adjacent a state highway that utilized a geotextile separator and rip-rap in the construction of the slope. The following spring, however, the slope dramatically failed resulting in a large amount of soil flowing onto the state highway and a sinkhole forming on the back side of the slope. The primary cause of failure was found to be clogging of the geotextile thus allowing snowmelt and rain to generate pore water pressures behind the geotextile in excess of the rip-rap’s ability to prevent failure.

The White Point Landslide destroyed a portion of a coastal roadway and threatened nearby residences in the Palos Verdes Peninsula of Southern California. The presence of a weak bentonite layer facilitated rapid mobilization of the landslide, which moved about 20 m (60 feet) towards the ocean in about 20 minutes. This article describes the design and implementation of an emergency program intended to stabilize the coastal bluff east of the landslide area and stop slope instability from propagating towards nearby residences. The stabilization program consisted of dewatering drains installed using horizontal directional drilling (HDD) combined with steeply inclined high capacity tieback anchors. This article also describes the conventional slope stability analyses and numerical modeling which were performed to better understand the mode of failure, nature of the failure, and impact of the proposed stabilization.

The M_w 7.8 2015 Gorkha Earthquake left close to 9,000 people dead, 22,000 people injured and a loss of properties over $5B. One among the major disasters triggered by this earthquake was landslides. Moreover, many buildings, including several of modern construction, collapsed in Kathmandu due to the earthquake shaking. One of the widely covered damages was the ground failure at Lokanthali, along Araniko Highway in Kathmandu. This failure affected a 250 m long stretch of the highway with vertical movements of more than a meter and lateral displacements on the order of 0.5 m, and also seriously damaged dozens of buildings. To identify the causes of ground failure, the authors performed a site investigation including subsoil exploration with field vane shear testing and Swedish weight sounding equipment, as well as topographical mapping. Additionally, numerical analyses were also performed to understand the seismic vulnerability of the site. Our study revealed that Lokanthali was affected by seismically induced landslide movement although the site has gentle slopes and the Gorkha Earthquake produced relatively moderate ground accelerations at this location.

One of the critical design scenarios for debris-resisting structures against geophysical flows is being able to predict the overflow velocity if the retention volume is compromised. Complex barrier geometries, especially vertical barriers equipped with deflectors on an inclined slope, can alter the overflow velocity. However, there are no scientific guidelines on how to select an ideal deflector geometry to minimize the overflow velocity. Flume tests were carried out to study the interaction between dry granular flow and rigid barrier deflectors of varying angles. The physical tests were then used to calibrate a discrete element model (DEM). The DEM was then used to carry out a parametric study considering the influence of channel inclination and deflector orientation on overflow velocity. In this paper, the flume and discrete element modelling techniques are described and preliminary results are discussed.

The current study presents a framework for assessment of levees’ probability of failure under various water level and rain intensity conditions. Fragility functions are developed to demonstrate the probability of levees’ failure under various stress conditions. intensity-duration-frequency (IDF) curves of extreme precipitations are used in numerical simulation of an earthen levee in Sacramento, CA, to develop fragility curves that consider a range of performance indicators. In order to examine the factors influencing the levee performance, a fully coupled 2D stress- variably saturated flow finite element model was built to simulate the levee behavior under combined effects of water level fluctuation and extreme precipitation. The results are then used to simulate the probability response of the collapse by creating fragility curves. The performance of the levee is expressed under extreme precipitations in terms of probability of failure against a prescribed rain intensity-duration for different return periods and water levels.

The assessment of the safety level of natural slopes, road cuts, embankments and levees require consideration of uncertainties and variability in material properties. In this study, for a number of slope geometries, including a real-life landslide case, probability of failure (PF) and the most critical failure surface are investigated with and without cross-correlation of shear strength properties. Slopes having different traditionally-defined factor of safety (FS) levels are studied. The uncertainty of soil properties are considered by different levels of coefficient of variation (COV). Limit equilibrium method is used for slope stability analyses and geotechnical material properties are considered to have normal statistical distribution. The results of this analyses show that the PF and the critical failure surface is significantly influenced by the COV level, the consideration of cross correlation of shear strength parameters, and by the traditional FS level of the slopes. The inverse relation between FS and PF is demonstrated to be nonlinear and the COV level has significant effect on this relationship. Results indicate that the deterministic slope stability analyses resulting in a single FS value is no longer sufficient to evaluate the safety of a slope in geotechnical engineering, and that the deterministic critical failure surface with minimum FS value is not always the most critical slip surface. The results presented in this study could be useful for further understanding of probabilistic slope stability and the effects of soil variability/uncertainty, with the aim of better geotechnical risk evaluation and communication.

Slopes and earth dams made of stiff clay can fail after many years of their construction. The delayed failure of these structures has been explained by: (1) the pore pressure equilibration within the stiff clay; and (2) the creep induced structural breakdown of the stiff clay. These explanations assume the stiff clays involved in the failure of the earth structures to be intact. However, the majority of stiff clays contain fissures. The role of these fissures on the delayed failure of slopes and earth dams made of stiff fissured clay has not been investigated to date. Under constant gravitational stresses (creep), the fissures in the clay could slowly propagate and interact producing as a result the failure surface that causes the delayed failure of these earth structures. In this study, the delayed failure of slopes and earth dams is explained by the creep propagation of the fissures in the clay.

Landslide is one of the major geological disaster threat to human lives. As a promising alternative to conventional remediation measures, the use of electrochemical stabilization techniques is expected to provide an effective means of remediation for shallow and intermediate depth landslide failures. Electrochemical treatment, however, demands a large amount of power access to the electrical grid. Traditionally, the electricity is provided via a mobile fuel-based generator. The fuel consumption causes high cost and negative environmental consequences due to the carbon dioxide emission. Renewable energy such as the wind and solar energy provides a promising alternative to driving electrochemical treatment. This paper provides a feasibility analysis on the electrochemical treatment concept with renewable energy. The results show that electrochemical soil improvements using a renewable generator are much more economical than the traditional gasoline generator. It, therefore, is a promising and environmentally friendly alternative for electrochemical treatment of landslide.

This study compiles the results of seismic stability analysis of an infinite slope from three methodologies, i.e. pseudo-static analysis, Newmark sliding block deformation analysis, and two-dimensional (2D) finite difference analysis. Ten different ground motions are considered. While the pseudo-static factor of safety (FS_PS) remains unchanged for constant PGA and seismic coefficient, a slope can exhibit different dynamic performances for each ground motion. The slope’s dynamic performance can be adequate even when both the conventional pseudo-static factor of safety (FS_PS), and the time-varying dynamic factor of safety (FS_dyn), drop below one. Limitations to the application of the Newmark sliding block analysis and the importance of numerical modeling are discussed. Results on the sensitivity of the numerical model to input parameters are also presented.

A 2D dynamic finite element analysis (FEA) was carried out to simulate the stress wave propagation and accumulated strains of a blasting-induced landslide in sensitive clay that took place near the city of Namsos, Norway, in 2009. The blasting triggered a large unintended transient motion of the rock mass with unfavorable weak planes and fault zones. The transient penetration of the rock mass into the clay triggered the devastating slide. A linear elastic model was used for the undrained response of soil. The interaction between soil and the blasted rock was modeled by means of an interface. The blast load was defined as a triangular velocity–time function with a peak value of 10 m/s. In accordance with the field observations after the slide, the predicted deformation of the rock face out into the sensitive clay was around one meter. The results showed a clear trend of the blast impact that diminished with distance. Stability checks using the volume of remolded clay determined from the dynamic analysis revealed that the factor of safety of the slope was reduced to less than unity following the blast.

A numerical study using the finite element method was conducted to evaluate the working mechanisms of an anchor reinforced vegetation system, which consists of a high performance turf reinforcement mat secured into a soil slope using percussion driven earth anchors. The results indicated that the anchors predominantly contributed to the relatively shallow, but wide and non-localized slope stability; while the mat played a significant role in preventing localized failure between or across anchors. The numerical analysis results are consistent with the field observations of anchor reinforced vegetation systems. Additional parametric analyses were performed to further compare the performance of different mats by varying their material properties in the numerical models. Since a slope stability analysis is usually performed using the limit equilibrium method in geotechnical engineering practice, a comparison of results between the finite element method and the limit equilibrium method is presented in the paper too.

In recent years, anchored reinforced vegetation systems (ARVS) have been developed as an option for stabilizing soil slopes prone to shallow translational failures. Such systems rely on the installation of a geosynthetic, high-performance turf reinforcement mat (HPTRM) to provide erosion protection and to enhance vegetation establishment, used in conjunction with patterned rigid pins (micro soil nails) or percussion-driven cable anchors to provide mechanical support to the shallow soil mass prone to slippage. Typical geotechnical design methods vary from professional experience tempered with subject-matter knowledge to a basic limit-equilibrium stability analysis based on the infinite-slope model. We have enhanced this model with options to include a non-linear shear strength descriptor, perched groundwater levels that may occur seasonally in the shallow slope system, a site-specific seismic analysis, a search to identify the “critical” depth of sliding, and a strategy for designing the required spacing and strength of soil anchors. The geotechnical design approach is explained, then illustrated with several examples.

Soil scouring occurs frequently due to water flowing around bridges piers and abutments. This investigation aims to simulate pier scouring process applying numerical approach. The CFD software package FLUENT is applied to model pier scouring. The Eulerian multi-phase model in FLUENT is used to simulate soil-water interaction. The numerical modeling results are compared with measured scours at the sites and common scour prediction methods including SRICOS, old and modified Hydraulic Engineering Circular No. 18 (HEC–18). Measured scour depths occurred at two bridge sites are also available to provide a comparison. Numerical scour predictions are in a reasonable agreement with old HEC-18 method which is compatible for non-cohesive soil. The effects of various pier width and nose shape on scour were also investigated.

Prediction of levee erosion rate during floodwall overtopping for materials with various plasticity indices is critical. Levee soil properties, and intensity or duration of flood are the contributing factors in erosion rate and scour depth predictions. In this study, over 15 laboratory scaled levee-floodwalls constructed of soils with plasticity indices of 0%, 6%, 9%, 30%, and 40% were overtopped. The floodwall scale of 1:20 is utilized assuming that a typical full-sized floodwall is 2.1m high. The erosion rates at the levee crest are measured and used to propose an erosion rate prediction model which considers the plasticity index (PI) and compaction level (CL) of soils.

In this paper, the stability of a 40 m high cut slope was analyzed using both deterministic and probabilistic approach. According to the subsurface investigation, sand and clay layers existed at the site, however, the thickness of the layers varied across 1.2 km of the slope. The most critical thickness and order of layers were identified to develop representative cross section. The groundwater effect along with variations in soil shear strength properties were considered in the analysis. The results of in-situ tests and laboratory tests were used to extract soil properties. Consequently, from deterministic analysis, the safety factor at different ground water conditions in the representative cross section was adequate. However, the probability of failure was high and drainage trenches along each of the slope benches were installed to bring the ground water level down permanently. The probabilistic analysis provides an insight to slope design and ground water level adjustments to mitigate the associated risk when there are uncertainties in subsurface investigation.

Recycled materials are often selected for transportation construction for their environmental or sustainability benefits. But they can also be used to create projects that are low cost, simple, and efficient to build. This paper describes development of standard details for creating retaining walls from recycled high-strength drilling pipe and used metal beam guard rail. Construction of these walls is inexpensive and efficient because they use only readily available material, require no specialized equipment or labor, and can often be done without requiring road closures. During el-Niño fueled rainy seasons Santa Barbara County is often inundated with drenching rainstorms that cause landslides, debris flows, and embankment failures. These failures close miles of roads and isolate people from lifeline services. The County needed a way to quickly and safely stabilize and reopen roads with a supply of contractors, equipment, and materials that are limited during large storm events. Out of these emergency response efforts Santa Barbara developed standard details for several slope stability repair options using low-cost, readily available equipment, material and labor. An emphasis was placed on designs that would get roads open quickly. Using recycled high-strength drill pipe for piles and used metal beam guard rail elements for lagging, the county created standard design details for soldier pile walls up to 4 feet tall, and anchored walls up to 8 feet tall. These walls can be built in days, not weeks or months. And they only require these materials and a few other readily-available components. As a result, roads can be opened and back in service within days of an embankment failure, restoring access for emergency services and residents.

Terrestrial lidar and photogrammetry have significant potential for as-built verification and displacement monitoring of earth retaining structures. This paper focuses on application of a method for measuring the position of the wall facing to a tall segmental retaining wall (SRW) constructed in Conshohocken, Pennsylvania. The 3D point cloud data obtained using lidar or photogrammetry is processed by software developed in the Python programming language to produce vertical profiles of the wall alignment at user-specified locations. Profiles developed using point cloud data obtained at a single time are compared to the specified alignment and to measurements of alignment performed using a plumb line and total station. Profiles developed using point cloud data obtained at different times are compared to observe the displacement of the facing. Settlement of the wall is measured by comparing the position of horizontal block joints at different times.

This paper presents a study on the probabilistic seismic hazard analysis of a slope at a given site in a specified exposure time. This study is part of the long-term performance assessment of a slope. For the long-term performance assessment, the factors that may lead to failure of the slope over time, such as the seismic ground motion, spatial variability of soil strength, and fluctuation of groundwater level, should be considered. Here, the earthquake-induced ground motion at a given site in a specified exposure time is simulated by a random variable based on the USGS National Seismic Hazard Maps, the spatial variability of the shear strength of the soil is modelled by a random field, and the fluctuation of the groundwater level is modelled by a random variable. To estimate the total failure probability of the slope at a given site in a given exposure time, a recently developed sampling technique is adopted for the propagation of uncertainties, and a 2D finite difference program is used as the deterministic model for the slope stability analysis. To demonstrate the proposed approach, an illustrative example of a two-layer earth slope is studied; and, a parametric study is undertaken to investigate how the long-term performance of the slope is affected by the influencing factors. With the results presented, the versatility and effectiveness of the proposed probabilistic framework are illustrated.

Nearly all states nationwide have implemented geosynthetic reinforced soil–integrated bridge system (GRS-IBS) technology. This construction method has proven to save time and money over traditional abutment construction. The I-70 over Smith Road in Aurora, CO multi-span bridge included design methodology changes that were classified as “experimental features” and thus additional project funds were included for evaluation. In an attempt to understand the geostructural performance of the multi-span GRS-IBS, the abutment was instrumented with a vertical chain of accelerometers, vertical and horizontal pressure cells and geotextiles with embedded fiber optic strain sensors. All of the data is remotely accessed combining traditional geotechnical monitoring strategies with a cutting edge fiber optic technology. Data has been collected over the past 18 months and analyzed to identify the importance of each monitoring element and to assess the performance of the GRS-IBS. An excavation along the toe of the abutment occurred towards the end of the project which induced additional stresses. The performance of the structure during this excavation stage is discussed. Since GRS-IBS has typically been implemented for single-span bridges, the results of this monitoring program can be used in the development of future GRS-IBS technology.

Roadways subject to seasonal load restrictions are located in remote areas where collection and transmission of real-time subsurface data is problematic. A demonstration project used wired underground sensors connected via cell/satellite transmissions. The present project developed radio frequency wireless sensors and transmission systems to measure subsurface data. A major advantage with wireless is installation simplicity, eliminating the need to trench out the roadway for cables from the sensors to a roadside data collection and transmission system. Real-time wireless sensor data and environmental data are integrated into a decision support system designed to assist state Department of Transportation (DOT) officials in setting winter weight premiums (WWP) and seasonal load restrictions (SLR). WWP and SLR decisions are made using forecasting models using computed environmental and subsurface parameters. Collected and computed information are displayed on a web browser, alerting Maine and New Hampshire DOTs when critical threshold values are reached.

Streambank erosion is an important component of geomorphic responses to channel changes that affect a wide range of physical, ecological, and socio-economic issues in the fluvial environment including water quality and on- and near-stream infrastructure. A number of direct (e.g., longitudinal surveys, erosion pins, remote sensing) and indirect (e.g., process-based models) methods have been used to quantify streambank erosion. These methods are not only very resource intensive, but many are feasible and appropriate only for site-specific studies and not suitable or practical for erosion estimates at larger scales. Recent developments in unmanned aircraft systems (UAS) provide opportunities for rapidly and economically quantifying streambank erosion and deposition at variable scales (from site-specific to river network), and is the focus of this paper. At the site-specific scale, the capability of UAS to quantify streambank erosion was assessed by comparing it to terrestrial laser scanning (TLS) and RTK (real time kinematic) GPS for validation. At the individual site level, the estimation of bank erosion using UAS was within 4% of the actual erosion at a surveyed cross-section. At the river network-level scale, initial results indicate even bank retreats of less than a meter can be detected, provided banks are not completely obscured by dense vegetation.

Many engineering professions have adopted asset management procedures to properly construct, monitor, maintain, and support physical assets through the full service life-cycle of all assets within a network. All asset management programs, whether structural, geotechnical, or transportation, have one common goal: to achieve life-cycle performance goals (e.g., safety, preservation, economic and environmental sustainability, etc.) by cost-effectively managing physical structures. Monitoring deformation rates across an asset can be used as an indirect method of obtaining initial condition assessment information, which is vital for understanding an asset’s current life-cycle stage. Persistent scatterer interferometry (PSI), an interferometric synthetic aperture radar remote sensing stacking technique, is capable of measuring displacement rates at 1 mm/year accuracy on anthropogenic infrastructure not undergoing immediate, catastrophic failure. Geographic information systems (GIS) allows storing, processing, analyzing, and displaying geographic data. By combining PSI and GIS capabilities, this paper will illustrate how these techniques can be utilized to spatially and temporally map deformation rates on a variety of assets, and how an initial condition assessment can be made on each asset. Structure mapping can be conducted in four steps: (1) digitization of geographic location for all structures; (2) processing of radar imagery, which results in displacement rate data for all viewable structures; (3) spatially analyzing displacement rates and assigning PS points to individual structures; (4) producing maps, including both spatial and temporal information (e.g., displacement-time series analyses). This procedure will be demonstrated using 40 COSMO-SkyMed satellite radar data, 3 m resolution images acquired between July 2012 and September 2014, over urban infrastructure in San Pedro, California.

A small unmanned aerial vehicle (sUAV) is used with structure from motion (SfM) computer vision to measure the amount of settlement that is induced by deep dynamic compaction at a site of a new casino near Lake Havasu City, AZ. Details of the project and field operations are provided, and comparisons are made between induced settlement measurements from traditional techniques and induced settlement measurements from the new sUAV/SfM approach. Results of the study show that the sUAV/SfM approach estimates an average induced settlement of 38 cm across the site, which straddles within 2.5 cm of the average induced settlements that were measured with other traditional techniques. Additionally, the sUAV/SfM technique is shown to provide significant detail of the distribution of induced settlements across the site. This distribution of settlements could be indicative of the distribution of subsurface soils that were more affected by the DDC such as looser or cleaner sands. Implications of the findings of this study are briefly discussed.

The integrity and reliability of levees are essential components of homeland safety. The failure of such systems due to a natural or manmade hazard can have monumental repercussions, sometimes with dramatic consequences on human life, property and the country’s economy. This paper presents some results of integrated monitoring and modeling to assess the performance-based response of a levee section. The modeled levee is part of the Whale’s Mouth section on Sherman Island where satellite images and in-ground GPS sensors are used for displacement measurements. The Whale’s Mouth levee is modeled using the large deformation option of the finite element program PLAXIS 2D. The model is used to establish a deterministic performance response under maximum water level loading and to investigate the effect of peat decomposition on the deformation response of the levee section. The remote sensing and in situ data are used to calibrate the numerical model. The results are compared to the pre-defined limit state and illustrate how the peat layer decomposition affects the modeled levee section performance. The concept of performance limit states of these critical structures provides a means to quantitatively assess the functionality of an earth structure under severe storm loading events. The probability of exceeding a prescribed limit state is defined based on the strain or gradient levels in potential emerging failure zones. The variation in strength properties and hydraulic conductivity of the levee embankment, as well as the rate of rising water level and duration of flooding, may lead to the progression of the structure state from a low probability of exceeding adequate functionality to the probability of exceeding a high limit state, i.e., imminent failure. The displacement data collected during these loading and unloading events is used to establish the levee condition assessment on the basis of the performance limit states.