Rocky Mountain Geo-Conference 2021

An approximately 35-foot-high retaining wall supporting eastbound U.S. Highway 36 near Milepost 49.8 experienced a failure of the foundation materials below the wall system, resulting in closure of eastbound U.S. 36 on July 12, 2019. Vertical downward movement of the wall system was in excess of 8 ft by the end of July 2019. To assist the Colorado Department of Transportation (CDOT) in designing a repair option and to facilitate rebuilding the wall system, RJ Engineering and Consulting, Inc. (RJ) worked as a geotechnical sub-consultant designer to restore traffic within 12 weeks of the initial failure. This paper presents our evaluation, analysis, design, and monitoring for the rebuild of the wall system that utilized more than 23,000 cy of geofoam also known as expanded polystyrene (EPS) and drilled shafts (caissons) for global stability support. Peak and residual shear strength parameters used in the stability evaluation and analysis including the use of both phi (internal friction angle) and cohesion shear strength parameters are described and discussed.

Utility owners and design engineers working on new pipeline installations do not always realize the challenges that their project may encounter when trenchless construction of shorter tunnel crossings is required until late in the design process. While larger diameter and longer tunnel projects are typically recognized for their significance, relatively short tunnel crossings under railroads, highways, canals, and other features can be overlooked on a primarily open cut pipeline project. However, the geotechnical challenges associated with shorter tunnel crossings can be as complex as those experienced on large tunnel projects and can be exacerbated by limited subsurface investigation and design input. This paper discusses design considerations for a variety of geotechnical risks that can be present on tunnel crossing projects, including shallow cover, obstructions, mixed face conditions, and contamination. Guidelines for performing geotechnical investigations to reduce these risks are also provided. A case study is presented which highlights how several geotechnical challenges (obstructions and contamination) were addressed during project design and construction.

Proper structural design of pavement systems requires knowing the resilient modulus of the soil, as this parameter is a proven predictor of the stress-dependent elastic modulus of soil materials under traffic loading. One of the common methods for estimating resilient modulus, in lieu of laboratory testing, involves using verified prediction models that are based on a soil’s basic index properties. These models can save considerable laboratory testing time and cost for the repeated load triaxial testing process. However, similar regression models developed for different soils may not represent the behavior of regional soils, especially given the typically limited number of soil specimens systematically studied for development of the models. A multiple regression analysis and stepwise regression analysis were performed to correlate the experimentally measured resilient modulus values with the fundamental soil properties (such as standard Proctor test, sieve analysis, hydrometer testing, and Atterberg limits). The regression models for a specific AASHTO soil type for Colorado were shown in this study to provide superior performance over the existing published models. The use of these developed prediction models can be expected to save a considerable amount of time and money in testing and analyzing soil properties.

In March 2019, a combination of melting snow and rainfall events overwhelmed a roadway culvert, leading to failure of a stone masonry retaining wall on the Zion-Mt. Carmel Highway (Utah State Route 9) within Zion National Park (ZION). ZION staff requested assistance from the Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) to assess the damage, analyze possible repairs, and contract the repair work. CFLHD was able to use available staff and an existing construction contract to expedite repair work. Repairs, including a mechanically stabilized earth (MSE) wall, stone masonry façade, drainage repair, shoulder drains, sub-excavation, and paving, were substantially complete in May 2019. This wall repair work presents a successful instance of rapid response and tailoring design to take advantage of existing contracts and resources.

The analysis of consolidation curves is an important aspect of settlement analysis in soils. This is frequently done using a visual approximation of the point of maximum curvature, which can have a high degree of inaccuracy. The point of maximum curvature is found on a plot of void ratio or axial strain versus the log of normal stress found during analysis. The unevenly spaced points in a semi-logarithmic scale plot are difficult to interpret numerically; as a result, few simple or effective numerical techniques have been developed. Using the numerical Lagrange interpolate technique, a repeatable and automated numerical method is used to calculate the point of maximum curvature and calculate the pre-consolidation stress.

Industry groups have proposed updating AASHTO’s Simplified Method design guidance for reinforced soil to include geosynthetic strips (or straps). If implemented, the use of concrete panel facing for retaining walls would be facilitated, and the corrosion problem of steel would be avoided. This new interest results from observations of retaining walls at St. Remy (France) and Christiana, Delaware (United States). In these retaining walls, k_r/k_a of approximately 1 avoids both the settlement associated with smaller values and the capacity reduction of larger values. Parameter k_r is the horizontal earth pressure coefficient in reinforced soil and k_a is the active earth pressure coefficient. Previously, FHWA data showed that S/D determines granular stability of geosynthetic-sheet-reinforced soil, where S is vertical spacing between strips and D is diameter of the largest soil particles. Using laboratory tests performed at the University of Colorado Denver, this paper extends the FHWA data from sheets to strips. And, it validates industry observations in terms of the reinforcement coverage ratio R_c equal to b/S_h, where b is the strip’s width and S_h is horizontal spacing between strips. For 30 years, engineers have questioned high reinforcement stresses measured in a wall at Guildford Bypass (U.K.), and this paper provides an explanation.

The design of foundations using geogrid stabilized rafts typically involves the calculation of a two-layer bearing capacity analysis. Existing calculation models are empirical with imprecise input parameters, while other methods have tended to involve multiple design charts and are suited to either strip or circular foundations only. It has also been difficult to quantify the benefits of geogrid materials. The recently developed “T-Value Method” defines bearing capacity in terms of the ratio of strengths of the two layers. It allows realistic integration of the benefit of multi-axial stabilizing geogrid through the enhanced shear strength of the upper granular layer. The recently developed “T-Value Method” is leading to significant cost savings due to thinner working platforms designed in a safe and scientifically rigorous way. Additionally, the geogrid stabilized raft offers a more constructible and quantifiable alternative compared to the Colorado Department of Transportation’s (CDOT’s) woven raft standard below mechanically stabilized earth (MSE) walls and behind abutment retaining walls. This paper summarizes the development of this innovative design method and describes a case study of a multi-axial geogrid stabilized raft constructed as part of a value engineering alternative on the I-25 South Gap: Monument to Castle Rock.

In 2019, a team of geologists, engineers, and specialty drilling contractors worked for the EPA at the Bonita Peak Mining District Superfund Site near Silverton, Colorado. The team successfully installed a monitoring well in an inclined borehole drilled into the American Tunnel. A combination of historical research, careful digitization of hand-drawn maps (ca. 1970s), detailed ground surveys, gyroscopic and magnetic surveys within the borehole, and conventional and directional drilling techniques were utilized to intercept the anticipated location of the 11-foot-diameter tunnel from over 600 ft away. Drilling was completed through intensely fractured, altered, and faulted andesitic rock. Conventional geotechnical drilling methods can result in significant “wandering” of the borehole due to deflection of the drill stem within the rock. The introduction of a steerable drill bit (with downhole surveying) allowed for orientation corrections of the borehole. The program was successful, allowing for construction of a monitoring well into a sealed portion of the tunnel to determine the water pressure behind the existing bulkhead. The well will provide groundwater levels and water quality data to augment understanding of the regional hydrology, assess the safety of the bulkhead, and evaluate the hydrologic impact of work on other local abandoned mine workings.

During late winter of 2018–2019, Hinsdale County (and other areas of Colorado) experienced unprecedented avalanche activity. Massive avalanches numbering in the hundreds impacted the two main drainages in Hinsdale County. These were avalanches of historic proportions that shaved the forests and soil down to bedrock, resulting in massive amounts of debris being deposited in county waterways, leading to potential of flooding and debris flow within the town of Lake City. Governor Polis issued emergency declarations due to severe weather, including one on May 6, 2019, for Hinsdale County avalanche debris and flood risk mitigation and preparedness efforts. Henson Creek posed the primary risk for flooding of Lake City. Two historic concrete dams related to mining activity in the 1880s and early 1900s occur along Henson Creek. These are the Ute-Ulay and Hidden Treasure Dams. Analysis of the dams and the potential impacts from debris led to the partial removal of the Hidden Treasure Dam. In addition, concern about debris-flow flooding potentially impacting Lake City led to an initial evaluation of locations for debris-flow barriers. Ultimately, the natural draining characteristics of the debris/ice mix led to a “no-action” decision for debris-flow barriers.

The Lower Chinns Dam is an earthen/rockfill embankment located in Clear Creek County, Colorado. The dam was built in the early 1900s for irrigation water supply. The dam is approximately 300 ft long with a height of 16.5 ft at the maximum section. The outlet pipe is 15-in. PVC with a capacity of 16 cubic ft per second (cfs). The reservoir was placed under a zero-storage restriction by Colorado Division of Water Resources, Colorado Dam Safety, due to ongoing seepage through the dam and the accumulation of sediment on the downstream side from material piped of the dam. The primary goal was to complete a repair for the dam that addressed the seepage concerns. The repair included construction of an upstream seepage reduction blanket, fabricated from a geosynthetic clay liner (GCL) to reduce and control the seepage water that was entering the dam embankment. The design included flattening the upstream slope of the embankment to a 2.5:1 (horizontal:vertical) maximum slope, requiring that the existing outlet pipe be extended approximately 15 ft. To limit the development of preferential flow paths along the exterior of the outlet pipe, the extended outlet pipe was backfilled with bentonite-augmented soil. Design and construction challenges included a short construction season due to the high elevation mountain location, the remote site location, and a limited budget for the repair. The repair proved to be effective, eliminating the seepage and piping problems. This paper will discuss the history of the dam, the seepage and piping problems, the repair options, considered the design of the repair, and construction.

RJH Consultants, Inc. (RJH) recently completed design and construction of a groundwater recovery system that included two parallel slurry trenches. The project included a soil-bentonite wall to provide a groundwater seepage barrier and an aggregate-filled recovery trench constructed using biopolymer slurry techniques to collect groundwater upgradient of the barrier wall. Both the barrier wall and recovery trench were keyed into hard sandstone bedrock up to about 45 ft below the ground surface. It was critical to define the top of bedrock at the site so the installed facilities could satisfy performance criteria of drawing down alluvial groundwater to below the top of bedrock. Subsurface data collected throughout design and construction included various phases of borings, a geophysical survey, and construction records from the trench installations. Trench-construction records included excavation data from installation of the barrier wall and pre-drilling along the recovery trench alignment prior to its excavation. This paper describes the evolution of subsurface data collection during the project and illustrates the variability of the top of bedrock over relatively short distances and how the predicted top of bedrock can vary at a given location based on different data sources.

Denver Water replaced the aerial siphon crossing of the Fraser River near the west portal of the Moffat Tunnel in Winter Park, Colorado. Development of the ski resort area, the adjacent Union Pacific Railroad, and other facilities has resulted in a large thickness of fill, which forms an artificially over-steepened slope along the west bank of the Fraser River. The near-vertical riverbank at the toe of the west slope was reinforced to prevent continued progression of toe erosion. This was accomplished with a top-down construction method to avoid a large excavation behind the current slope face by using a soil nail retaining wall. Micropiles were used for the siphon pilers, abutments, and for the pipeline anchor blocks. Due to void spaces in the fill, permanent steel casing was required for the micropiles. The micropiles were also installed on a batter, lending efficiency to their capacity for horizontal loads. Construction was completed in the fall of 2019. This case history will focus on the geotechnical aspects and challenges throughout design and construction of the project.

Becker penetration tests have long been used to evaluate the liquefaction susceptibility of gravelly deposits. Historically, only the energy measured at the head of the Becker drill string was used in this assessment. In recently described instrumented Becker penetration tests (iBPT), the energy transfer to both the head and the toe of the closed-end Becker drill string has been used to calculate a continuous, normalized blow count versus depth. This continuous blow count profile is used to identify liquefaction susceptible layers. The strain and acceleration measurements acquired at the head and toe locations in the iBPT are typically obtained using cabled connections. One drawback to widespread iBPT usage has been the limited number of available systems as well as the cable connection to the toe sensor location, which must be internally threaded through each rod section as it was added to the drill string. This paper describes three cases where iBPT data was acquired using readily available dynamic foundation testing equipment. Acquiring and processing strain and acceleration measurements from the drill string head and toe using conventional dynamic testing equipment increases the routine availability of the iBPT methodology and encourages its widespread use in the liquefaction assessment of gravelly deposits.

Each year, the Green Box Arts Festival in Green Mountain Falls, Colorado, presents an art installation for the community. The 2019 project was the 1.8 net sculpture by American sculpture and fiber artist Janet Echelman, previously displayed in London, U.K. (2016), San Diego, CA (2016), Mexico City, Mexico (2017), Beijing, China (2017), and Xian, China (2018). Spanning 145 ft by 115 ft, the 1.8 net sculpture was suspended 50 to 60 ft above the town lake. Due to the temporary nature of the structure, its helical pier foundations, stage towers, short timelines for both design and construction, precarious location, and precise artistic requirements, this project required a unique geotechnical and structural acumen to address the construction challenges. Cranes, kayaks, stand up paddle boards, lifts, and a party barge retrofitted into a helical pier drill rig were all necessary to complete the project. Extensive coordination between the engineering design team, construction manager, project manager, foundation contractor, tower designer, tower manufacturer, net installation/lighting crew, and Echelman’s representative resulted in a successful showcasing of the dynamic net sculpture in Green Mountain Falls from July 4 to September 2, 2019.