Rocky Mountain Geo-Conference 2022

A known landslide along Larimer County Road 29 in Larimer County, Colorado, failed in the spring of 2021, forcing an immediate single lane road closure and resulting in a hazard to the traveling public. The landslide measured approximately 100 ft in length and 30 ft in depth and coincided with unusually high and prolonged flows in an irrigation ditch situated above the roadway. Larimer County was able to secure a design and construction team within days of the failure and utilize emergency funding to reopen the roadway in a rapid two-month design and construction schedule. The selected team of Lithos Engineering (Designer) and Harrison Western Construction (Contractor) had previous similar experience, which allowed for an abbreviated design approach prior to mobilization and a progressive design-build to reopen the roadway. The stabilization solution ultimately included a soil nail wall, micropile, shear pins, and an MSE wall to re-establish the roadway. The three-phased approach was more economical and easier to construct in comparison to other considered alternatives, including large-diameter drilled shafts or a regraded site. This case history intends to compare the selected design concept to independently developed preliminary design concepts from 2014, discuss the design origin and methodology (including back calculation) used to determine geotechnical parameters at the site, the selected design solution, construction schedule and laydown difficulties, and celebrate the ability to deliver a unique design and construction project rapidly given the critical nature of the roadway and failed slope.

Validation exercises for computational analyses in geotechnical engineering are essential. Complex computational analysis methods are increasingly implemented in engineering practice with too little validation data. Since about 2012, the Center for Geotechnical Engineering Science (CGES) at the University of Colorado Denver (UCD) has attacked the validation problem by developing large-scale testing systems for soil-structure interaction problems in concert with finite element analyses. This paper presents a validation exercise for an isolated footing, under vertical loading, on Leyden Clay fill, behind an at-rest retaining wall. The validation experiment was performed in a large-scale, soil-structure interaction, testing system known as the “Tiger Cage”. This paper presents and compares the execution of the validation experiment and the laboratory testing for the Leyden Clay fill with three-dimensional finite element analysis, using the TrueGrid and LS-DYNA software programs.

As the population of the front range continues to increase, the growth continues to stimulate development requiring new utility installations in challenging conditions around already existing infrastructure. Trenchless boring of utilities across, next to, and under existing infrastructure is done to minimize disruption to existing infrastructure and the public in such urban environments. In larger diameter tunnel applications, ground improvement can be performed before or simultaneously during tunnel advancement. Although less common, ground improvement techniques are also utilized in smaller diameter trenchless boring where ground conditions dictate. Pre-tunneling ground improvement can stabilize raveling or weak soils which serves to reduce or eliminate settlement potential, promote the use of more economical tunneling methodologies, control water infiltration, and protect adjacent infrastructure. Ground improvement can also be utilized during and post tunneling to address actual settlement. This paper will review some available and common ground improvement techniques for tunneling and discuss two recent smaller diameter case studies completed along the front range to illustrate the benefits of ground improvement as implemented in challenging and/or unexpected geotechnical conditions. The paper will conclude with considerations that should be evaluated when approaching similar projects prior to and during the RFP and contracting process.

The Second Creek Interceptor is a 17.5-mile pipeline extending from south of the Denver airport to Brighton, Colorado. The project required construction of 15 tunnels ranging in length from 130 to 1,500 feet long at diameters between 48 and 78 inches. Due to the length and wide range of geologic conditions, numerous tunnel design and construction techniques were necessary to balance the needs of the stakeholders and design requirements across such a long project alignment. This talk will focus on the challenges and lessons learned throughout the design and construction process of this challenging project.

A combination of Standard Penetration Testing (SPT) sampling and laboratory testing, Cone Penetrometer Testing (CPT), and geophysics are typically used for sites with liquefaction issues. Geotechnical studies for water dams in Colorado do not typically include a CPT investigation or continuous SPT sampling. The dataset size from the SPT or CPT has a paramount role in estimating the extent and strength of liquefiable soils. A preliminary geotechnical site investigation and SPT-based liquefaction screening analysis to support a design of a new earth dam near the Denver area indicated the presence of continuous liquefiable soils in the foundation soils for a design earthquake with a return period of 2,475 years. The SPT results, however, were inadequate to provide a reliable estimate of liquefiable zone thicknesses and post-liquefaction residual shear strengths. The CPT investigation was subsequently performed to better evaluate the thickness and post-liquefaction strength of liquefiable soils. The thickness of liquefiable zones with a reasonable precision was obtained from the CPT-based liquefaction triggering analysis. Additionally, large CPT dataset supported a use of higher post-liquefaction residual shear strengths for liquefiable soils resulting in a satisfactory stability factor of safety for the post-earthquake scenario. This study highlights the importance of data collection methods and procedures with respect to the dataset size during the geotechnical site investigation planning in support of liquefaction studies among water dam practitioners in Colorado.

Twenty-five roadway tunnels are integral to allow for travel in geologically isolated portions of the Rocky Mountains throughout the state of Colorado. Every 24 months, each of these tunnels, according to Federal Highway Administration (FHWA) National Tunnel Inspection guidelines, are inspected by a team of engineers and geologists that report defects in the tunnel’s critical systems that include Structural, Civil, Mechanical, Electrical, and Fire/Life Safety elements. These inspections help the Colorado Department of Transportation (CDOT) to manage their tunnel infrastructure assets by improving the life cycles and safety for upcoming fiscal years. This paper illustrates that inspections can locate critical defects affecting the strength of the unlined tunnels. The inspection and following rehabilitation of the tunnel is discussed in this paper. The use of a handheld LiDAR scanner during this inspection significantly decreased the time and budget required to evaluate and rehabilitate the existing unlined rock tunnel and allowed the design team to maintain traffic clearance.

The demographic composition of the geotechnical engineering and broader geoscience industry reflects an ongoing cultural shift toward workforce diversification. While diversification across sex, gender, race, ethnic, economic, and other cultural backgrounds is largely recognized as a beneficial force for increasing innovation, collaboration, and depth of experiential knowledge, these differences can also be potential sources of tension and do not necessarily reflect the concept of “equity”. The focus of this paper is to provide a case history and corresponding takeaways from the Lithos effort to develop our Diversity, Equity, and Inclusion course throughout 2021. This paper will briefly identify a basis of need for exploring this topic, introduce working definitions to deepen understanding of concepts, identify bias and its impacts to decision making, and propose actions for both individuals and groups to implement practices that broaden inclusion within the industry. We hope to highlight how bringing awareness to our own cultural and unconscious biases contributes to a more dynamic workplace, and by extension, fosters a stronger and more inclusive geoscience industry.

What happens when wildfires burn across retaining walls, landslides, bridges, power lines, utility trenches, rockfall draped mesh, reinforced slopes, or other geotechnical assets? The intent of this paper is to raise awareness of the risks posed to geotechnical assets due to wildfires and encourage a proactive approach to protect structures before they are impacted. In recent years, the United States has experienced a surge of large wildfires burning through locations with critical transportation assets. Case histories will include fire damage to Mechanically Stabilized Earth (MSE) walls in California’s Sequoia National Park, destroyed geohazard monitoring instrumentation in California’s Ferguson Fire, and bridges burned in multiple California National Forests and National Parks. Nationally, wildfires are burning with higher intensity and year-round instead of previously assumed spot locations during only dry seasons. This means a potential increased risk of wildfire damage to geotechnical assets should be considered in design. Depending on location, topography, and climate, engineers should evaluate potential wildfire damage with other extreme event criteria for existing and future geotechnical structures.