characterizing rock slope failure mechanisms using combined remote sensing and numerical modelling - 2022 spring Cross Canada Lecture
by Doug Stead, Ph.D Emeritus Professor from Simon Fraser University on June 20, 2022 at 1730.
Slope failure mechanisms vary in complexity from simple translational slides to complex multi-block/multi-slip mechanisms. Developments in remote sensing and slope analysis techniques have seen a rapid transition from air photograph interpretation and simple limit equilibrium analyses to increasingly sophisticated and readily available ground and airborne/satellite sensing methods and 2D/3D numerical models. A focus of this talk will be on improving our understanding of complex rock slope failure mechanism through remote sensing characterization and monitoring constrained against numerical models. Using case studies of natural and engineered slopes from North America, Europe and Africa the importance of structural geology, kinematics and slope damage will be demonstrated. With the continued advent of innovative technology, the need to fully optimize available data is emphasized, along with the need to ensure that the limitations of these techniques are considered and that the key role of conventional engineering geological and engineering geomorphological investigations recognized.
The January 28, 2021 highway 1 embankment failure at rat creek, bug sur, California & lessons learned
by Dr. Dimitrios Zekkos from University of California Berkeley on June 8, 2022 at 1730.
On January 28, 2021, a portion of scenic Highway 1 failed at Mile Marker 30 known as Rat Creek, in Big Sur, California. The failure resulted in complete closure of the highway at that location and a detour of 255 km for residents. The surface flow-induced erosion washed both lanes and some of the surrounding embankment material into the Pacific Ocean. An investigation under the Auspices of the Embankments, Dams, and Slopes (EDS) Technical Committee of the ASCE Geo-Institute (GI) was conducted and the findings of this investigation and subsequent studies will be presented. The failure was caused by overtopping and subsequent erosion of the roadway embankment that occurred after a debris flow triggered upstream and reached the embankment. The debris flow was triggered due to the destructive synergy of the Nolan wildfire that occurred a few months earlier, an atmospheric river that caused significant precipitation and the collapse of a natural debris-tree dam. The investigation involved on-the-ground field deployment that included UAVs, terrestrial lidar and geologic characterization as well as subsequent analyses. In addition, system-level analyses were conducted to understand exactly why the failure occurred specifically at Rat Creek and not somewhere else along the long stretch of Highway 1. The presentation will conclude with lessons learned from this failure and recommendations for enhanced system-level resiliency.
Liquefaction-induced downdrag on piles: centrifuge and numerical modeling, and design procedures
by Sumeet Kumar Sinha, PhD., Post Doctoral Scholar at University of California Berkeley on April 27, 2022 at 1730.
Pile foundations are designed to transfer superstructure loads through positive skin friction and tip resistance while undergoing acceptable settlements. For sites with liquefiable soil, estimating drag load (from soil reconsolidation) and pile settlement (from seismic loads, reduced pile capacity from excess pore pressures, and downdrag) becomes an important design consideration. Most of the challenges related to the liquefaction-induced downdrag phenomenon are the incomplete understanding of the different mechanisms that affect drag load and pile settlement leading to over-conservative or unsafely designed piles. A series of centrifuge model tests were performed on liquefiable soil deposits to assess liquefaction-induced drag load and pile settlement, understand the mechanisms of pore pressure generation/dissipation, and soil/pile settlement during and after a shaking event. A numerical modeling approach, dynamic TzQzLiq analysis, was developed incorporating the observed mechanism and validated with centrifuge test results. It consisted of the existing TzLiq and a new QzLiq material (implemented in OpenSees), which accounted for the changes in the pile’s shaft and the tip capacity as free-field excess pore pressures develop/dissipate in soil, initial drag load on piles, and sequencing of excess pore pressure and soil settlement. Results provided time-history of axially loading distribution and settlement of piles during an asking event. Finally, a displacement-based procedure was proposed for industry using a simplified pseudo-static (four-step) TzQzLiq analysis for designing axially loaded piles subject to seismic loading and liquefaction-induced downdrag. The redistribution of high pore pressures from liquefiable to adjacent non-liquefiable deposits also impacted pile performance significantly. Therefore, an analytical procedure was also developed to estimate excess pore pressure redistribution in liquefiable and non-liquefiable layers.
Will it stay or will it go?: use of lidar to assess slope instabilty
by Ben Leshchinsky, PhD., Associate Professor at Oregon State University on March 30, 2022 at 1730.
Lidar is a promising tool for evaluating the hazard and behavior of unstable slopes due to its resolution, accuracy, and the ability to process away visual obstacles, such as vegetation. Elevation models processed from lidar are particularly useful as they enable quantitative and enhanced qualitative interpretation of landslide features, and in the case of repeat data collection, evaluation of kinematics and changes that are not easily discernable by eye. Most of all, when integrated with principles of slope stability analysis, lidar data serves as a robust foundation for understanding landslide behavior at multiple scales. This presentation will discuss (1) use of lidar for deriving first-order estimates of landslide volumes and strength from forensic analysis of inventories, (2) use of digital elevation models and slope stability analyses towards creating landslide susceptibility maps for seismic and precipitation disturbance under a variety of remotely-sensed antecedent moisture conditions, and (3) interpretation of lidar to reveal drivers and change for landslides in Oregon’s coastal environment. The increasing availability of lidar presents us with a unique opportunity to better assess the risk stemming from geohazards, enhance asset management, and understand geomorphic and geologic processes at a more refined level.
efficiency of ground motion intensity measures with earthquake-induced earth dam deformations
by Richard J Armstrong, PhD., P.E., Associate Professor at California State University on February 23, 2022 at 1730.
Earthquake ground shaking characteristics have profound and varying impacts on civil engineering infrastructure. Traditional descriptors of earthquake shaking, such as peak ground acceleration and pseudo-spectral acceleration, have a strong history of use in seismic hazard assessment and post-earthquake damage prediction. More recently, however, other earthquake ground shaking characteristics (often called ground motion intensity measures, or IMs), which may be able to better characterize the relationship between ground shaking and civil infrastructure response, have gained traction in research and practice. This presentation will focus on the results and conclusions of a multi-year research project that involved evaluating the best ground motion intensity measures for embankment dam response. In this work, data from strong ground motion recordings during the 1989 Loma Prieta earthquake were used to evaluate the reasonableness of nonlinear deformation analysis (NDA) models of Lenihan and Anderson dams. These models were subsequently used to assess the efficiency of ground motion IMs with embankment dam deformations. A suite of 342 recorded ground motions was used with these NDA models to assess the relationship between ground motion characteristics and embankment dam deformations. The article begins with a summary of the NDA of Lenihan and Anderson dams during the 1989 Loma Prieta earthquake. Subsequently, the ground motion database used in the analysis is described, followed by the presentation of the results in the context of the efficiency of each IM.
Mallkuchusi suspended bridge project
A joint talk from the Tunneling Association of Canada (TAC) and VGS presented by Natalia Skomorowski (BGC Engineering) on January 18, 2022 at 1730.
During the summer of 2019, Notre Dame Students Empowering through Engineering Development (NDSEED), a student chapter of Engineers in Action (EIA), constructed a suspended footbridge in the rural community of Mallkuchusi, Bolivia. As part of the bridge design, they partnered with BGC to consult on the geotechnical conditions of the bridge site.
NDSEED began the Mallkuchusi Bridge Project in the spring of 2018. Upon arrival to site during the 2018 Build Season, the team realized that excavations of the in-situ rock would be exceedingly difficult and posed significant design and construction challenges. Due to the remote site conditions and limited ability to communicate with stateside resources, the team was unable to redesign the bridge abutments to reduce the excavations in during the 2018 Build Season. They proceeded to excavate a standard gravity abutment on the right side of the bridge, which took six weeks with the help of a local Bolivian miner utilizing both dynamite and manual excavations.
Having more time and resources during the 2018-2019 Academic year, the team proposed a new bridge abutment design, utilizing nonstandard rock anchors to reduce the necessary excavations from 65 m3 to 10 m3. As the abutment design relied upon the strength of the in-situ bedrock, NDSEED partnered with BGC to analyze the site’s geotechnical conditions. BGC engineers consulted for the team on a variety of issues, from safe abutment location to excavation feasibility and construction safety and monitoring practices. In January 2019, two BGC engineers conducted a site assessment trip to Mallkuchusi with one of the 2019 NDSEED team members to collect geotechnical data and assess the rock conditions exposed by the completed excavations on the right side, as well as to assess the rock stability on the left side. BGC provided NDSEED with the geotechnical information that was vital to redesign the bridge’s rock-anchored abutments.
During the 2019 build season, NDSEED was able to construct the non-standard anchor and the rest of the bridge superstructure with the assistance of the local community. The completed pedestrian bridge has a span of 96.2 meters. The bridge now serves the 63 people of Mallkuchusi as well as the 80 people of Cobre Villa and Janco Marca. They now have access to schools, groceries, and a community centre without having to cross the river on foot.
THE EFFECTS OF SOIL GRADATION ON THE LIQUEFACTION TRIGGERING AND DEFORMATION RESPONSE OF EMBANKMENTS
by Trevor Carey, Assistant Professor at the University of British Columbia on December 8, 2021 at 1730.
The liquefaction case-history database was primarily established from observations at sites consisting of relatively clean, poorly graded sands. These case histories serve as the basis for design practices but do not represent all liquefiable soil gradations found in the built environment. This has led to the use of poorly graded sand-based analysis procedures during the design and retrofit of embankments, which are typically constructed with well-graded soils. A poorly graded sand-based design procedure ignores the lower void ratios and higher peak strengths of well-graded soils. There is also an implicit assumption that the pre-and-post-liquefaction triggering behaviors of well-graded soils are the same as poorly graded sands.
Described in this presentation is a centrifuge test program undertaken to investigate how sand gradation affects the system-level performance of embankments subjected to strong shaking. The experiment design consisted of two identically instrumented submerged 10-degree embankments positioned side-by-sidein the same model container. One of the embankments was constructed with poorly graded sand, representative of the sand in the case-history database, and the other with well-graded sand. The embankments were dry pluviated to the same relative density, but the absolute densities of the sands were different. Results showed that embankments constructed at equal relative densities would both liquefy (i.e., the excess porewater pressure ratio (ru) reach 1.0). However, the post-triggering consequences were less severe for the embankment constructed of well-graded sand. Greater resistance to the generation and faster dissipation of excess porewater pressures coupled with stronger dilatancy of the well-graded sand increased stability, curtailing deformations. This work demonstrates that soil gradation properties should be accounted for in the liquefaction evaluation procedures to improve the accuracy of deformation predictions, required for performance-based design
Shared experience of value engineering case studies from middle eastern major projects
by Benoît Latapie, ing, Eur. Ing., Msc., on Tuesday, November 16, 2021; 1730 to 1830 PST
The goal of this presentation is to illustrate, via multiple examples from the speaker’s experience, the positive impact that the integration of the geotechnical engineer to multidisciplinary engineering teams has on the success of major construction projects in terms of cost, schedule and quality. Case studies, such as, significant optimization of foundations design for several skyscrapers in Dubai (UAE) and the reduction of temporary supports for the underground construction of the Doha Metro (Qatar) and Riyadh Metro (Kingdom of Saudi Arabia) projects were possible due to the integration of geotechnical engineering in the design stages, which will be presented. The value-add of embedding the geotechnical discipline into the design process is demonstrated in different markets (transportation, infrastructure, and property) and with different clients (P3 Contractors and property developers). The importance of investing in geotechnical field investigations, scoped to support advanced geostructural analyses, is highlighted and the value-add of soil-structure interaction modelling is integral to the presentation.