Rocky Mountain Geo-Conference 2016

The 3.1-mile-long Stanley Canyon Tunnel is a key component of Colorado Springs Utilities’ Northfield Water System. The 9-foot diameter tunnel is lined with cast-in-place concrete and steel, and has an operating pressure of up to 765 psi. Shannon & Wilson, Inc. performed a condition assessment of the tunnel in October 2015 using the National Association of Sewer Service Companies (NASSCO) Pipe Assessment Condition Program (PACP). A three-person team recorded more than 500 defects over a three-day period, and oversaw a non-destructive evaluation using spectral analysis of surface waves (SASW) and impact echo (IE) methods. Based on the condition assessment, repair locations were identified and prioritized; repairs consisting of steel plates and chemical grouting were constructed over a three-week period in November 2015. This paper details the execution of the condition assessment and construction of the repairs.

The presence of hydro-collapsible soils beneath the proposed Erie Police Station project site led to the successful combination of two ground modification techniques. Normally in the presence of up to 30 ft (9 m) of hydro-collapsible soils, the soils are either over excavated and replaced, or the structure is supported by a deep foundation system. A collaboration between Hayward Baker Inc. (HBI) and GROUND Engineering Consultants (GROUND) resulted in an alternative two step ground modification process. The first step consisted of water injection to pre-collapse, “treat,” the soils to a depth approximately above “perched” water levels, at about 18 ft (5.5 m). Consolidation tests were subsequently conducted on treated soils obtained from across the site to evaluate the effectiveness of the treatment process. In the second step, stone columns were installed to increase the bearing capacity at spread and column footing locations. The combination of these techniques provided the client with a sufficient foundation design that was cheaper and faster to construct when compared to conventional over-excavation and replacement or deep foundation system options.

Compaction grouting was developed over 50 years ago as a means of densifying soft and weak soils under existing structures as well as a means to re-level the structure itself once settlement had occurred. The technique can treat not only the symptom of the problem (settlement), but the cause (the underlying soft soil); thereby cost-effectively, and with limited intrusion, extend the life of the structure. This paper presents techniques and associated installation procedures as they relate to ground improvement. Settlement control will also be discussed and applications for structure rehabilitation will be summarized.

North Lake Dam is a high hazard embankment dam built in 1964 in Las Animas County, Colorado. The normal maximum pool elevation is 8586.5, which is about 70 feet above the downstream toe. The original dam did not have filters, an internal drainage system, or a grout curtain. Concentrated seepage through and around the right abutment has been observed since the 1970s. Rehabilitation of the dam was completed in stages, involving an author of this paper since 1998. This paper presents the final stage of rehabilitation, which was completed in 2014 and addressed concentrated seepage, high foundation pore pressure in the bedrock, and slope stability issues. The 2014 rehabilitation modifications included a two-stage filter and seepage collection system, a stability berm, outlet works pipe replacement, and instrumentation. Seepage modeling was calibrated based on piezometer measurements. Slope stability analysis considered artesian pore pressures in the foundation, three-dimensional influences from the irregular downstream geometry, and residual shear strength of the shale bedrock. Potential deflection and elongation of the existing outlet works pipe below the stability berm was evaluated using two-dimensional settlement analysis. Also discussed are seepage control challenges encountered during construction and presents post-rehabilitation instrumentation data. Hydraulic aspects of the rehabilitation are not presented in this paper.

Enhancing geologic and geotechnical investigation techniques is fundamental for improving the reliability of any geotechnical investigation. During a typical geotechnical drilling program, the geologist and driller work together to capture both “real” data (core logs detailing lithology, rock quality designation, samples, etc.) and “fugitive” data (driller’s notes about rig behavior or subtle subsurface variations, etc.). The fugitive data can often be as valuable to the final design and construction as the core logs or rock/soil samples. Drill parameter recorders (DPR) can be installed on drill rigs to allow for real-time monitoring-while-drilling (MWD) of important fugitive parameters, such as: penetration rate, hold-back force, torque, thrust, rotational speed, mud or water flow rates and pressures, with depth, and time. The parameters can be used to spatially correlate strata between borings, and identify areas of highly fractured rock, voids, clay seams, or weak/thin stratigraphic zones that may be washed-out or lost during drilling. Correlations also exist to specific energy, rock mass and soil strengths, and components of the rock mass rating (RMR) system. This paper provides background about DPR system and seeks to illustrate its potential uses for subsurface investigations.

An Osterberg cell (O-cell) load test was performed on a drilled shaft (typically referred to locally in Colorado as a caisson or drilled caisson) to evaluate the side and tip resistance, and validate the values used in design. The drilled shaft was constructed as part of the Colorado Department of Transportation (CDOT) project for the replacement of the Colorado State Highway 82 Bridge and adjacent pedestrian bridge located in Glenwood Springs, Colorado. Due to concerns of potential impact to nearby geothermal resources the shafts were not designed to penetrate into bedrock, and all of the resistance was developed in the overburden soils. The overburden soils consist primarily of gravels and sands with cobbles and boulders. Strain gauges attached to the rebar cage were used to monitor strain during the test and to evaluate load transfer. The results of the O-cell test are presented and compared to the values that were used in design. A back analysis was also performed to see what design parameter values produced the best fit when compared to the test results. The current AASHTO design methods agreed reasonably well with the O-cell results.

Geosynthetic reinforced soil (GRS) structures are now widely used and it has become important to estimate their capacity and deformation. This investigation found that accurate calculation of deformation must include both elastic as well as plastic strain. Although vacuum-wrapped GRS structures are impractical outside the laboratory, they do provide a practical means for determination of facing pressure. The relationship between capacity and facing pressure is examined. This paper revisits the generic soil generic composite (GSGC) tests, which were vacuum-wrapped tests performed at the U.S. Federal Highway Administration (FHWA) and reported by Wu et alia (2010). This investigation furthers an understanding of behaviors, both elastic and plastic, within AASHTO’s guidance for reinforced soil. In particular, it enables a quantitative relationship between capacity and facing rigidity.

Seepage analyses are routinely performed as part of levee analyses. While finite-element analysis is being used with increasing frequency, semi-analytical solutions such as “blanket theory” are still commonly used, especially for simplified reliability analysis in conjunction with first-order methods. Unfortunately, the simplified approaches do not properly account for the spatial variability inherent in geologic materials when considering the blanket thickness. In this paper, random field theory is used to determine the influence of a confining layer of random thickness on levee under seepage and exit gradients. Results indicate that neglecting spatial variation in blanket thickness does not cause significant error in computed exit gradients and flow quantities obtained from simple reliability analysis using blanket theory.

This is a case study of Fruitgrowers Dam, which is an earthfill embankment constructed in western Colorado in 1938-1939. The left abutment of the dam was constructed on an active block landslide founded in the Mancos Shale. Movement of the landslide has been documented since construction and represents a potential risk to the performance of the dam. An extensive instrumentation monitoring program was initiated in 1987 due to movements of the landslide. The risk presented to the dam by the landslide was evaluated in 2014. In order to define the risks, instrumentation data were used to define the landslide extent, movement directions, and movement rates, and laboratory testing and back analyses were used to evaluate the material shear strengths of the slide plane. The results of these analyses were used to develop and evaluate potential failure modes related to the landslide movement and to assess the risk to the dam.

The Michigan Ditch is located in Jackson County, Colorado and is part of the City of Fort Collins water supply system. It conveys water from the west side of the Never Summer Mountains to Joe Wright Reservoir on the east side of the range. The 5.2 mi long ditch was constructed in the early 1900’s and has been plagued by slope instability since it was built. An area known as the “Mudslide” has experienced accelerated slope instability in recent years and the piped portion of the ditch through the landslide was destroyed in 2015 when the landslide moved over 20 ft horizontally and 7 ft vertically. Following an extensive geotechnical investigation utilizing vertical and horizontal borings at the site, several mitigation alternatives were evaluated, the associated risks were weighed, and a tunnel behind and under the landslide was selected. The project was designed and will be constructed utilizing the City of Fort Collins utilities alternative product delivery system (APDS). APDS is a collaborative approach unique to the City of Fort Collins utilities and focuses on delivering quality projects that are built on time and on or under budget. The total construction cost for this project is estimated at $8.5 million dollars and will be constructed between May 2016 and October 2016.

Coal Ridge Waste Dam, located in Weld County, Colorado is a small homogenous earthen embankment dam. The dam is approximately 2,720 ft long, has a maximum jurisdictional height of 28 ft, and a normal storage volume of 600 acre-ft. In January 2014, during repairs to the reservoir’s control gate, contractors removed some poor quality concrete at the invert of the outlet vault structure, which allowed saturated embankment material and groundwater to flow into the vault, resulting in a significant sinkhole on the upstream slope, adjacent to the vault. To prevent further damage and possible failure of the dam, emergency measures were taken to drain the reservoir and stabilize the control vault. This paper discusses the emergency response and mitigation alternatives considered, along with the slip lining and grouting program selected to install a new outlet pipe through the dam with a new control gate on the upstream face of the dam. A construction manager at risk (CMAR) contract was selected for the design and construction of the repairs. The project was completed ahead of schedule and under budget due to the cooperative environment between the owner, designer, contractor, and regulator created by the CMAR contract.

In Erie, Colorado as well as globally, disturbed earth materials are commonplace, extensive, and permanent. Using land areas from 37 discrete, residentially developed areas in Erie, covering 2,655 ha (6,560 ac), residential development alone has created 80 to 119 million tons (Mt) of human-altered and human-transported (HAHT) soil. Transformed soils lack the systematic physical and chemical properties, and hence predictability of the original soils, which poses significant challenges in terms of end use. Newly built housing developments may be at risk to natural and anthropogenic hazards, such as vapor intrusion, shrink/swell potential and mine subsidence, and natural landscapes are lost. However, the risks can be significantly reduced through adequate planning and mitigation strategies, and the new landscapes include positive aspects, e.g., areas of enhanced absorptive capacity for stormwater and various created habitats. Geotechnical engineers, geologists, and scientists working in transformed landscapes and HAHT soils should understand not only the geologic origin of earth materials, but also how anthropogenic processes contribute to a new geotechnical and geological fabric of disturbed earth materials and transformed landscapes, as well as field identification and characterization of soil properties evidencing human action.

In 2009, a possible dam deficiency was identified. Hyrum Dam was built over a relatively thin unconsolidated alluvial deposit that could be at risk for strength loss during a large earthquake. If large deformations were to occur because of the strength loss, the crest height might be reduced or cracking of the embankment may occur, and could potentially lead to failure. In order to assess the situation, a field investigation was completed to measure the in situ properties of the dam embankment and the alluvium; and a seismic hazard analysis was performed. A comprehensive analysis was then undertaken to estimate potential deformation and overtopping. Using the data from updated seismic loading, field investigation, and analysis, a risk analysis was performed for potential failure modes. The annualized failure probability was estimated, using an event tree approach. The risk analysis revealed that the likelihood of dam failure during a seismic event was below the Bureau of Reclamation’s (Reclamation) standards.

For decades compaction grouting and low mobility grouting (LMG) have been utilized extensively to stabilize structures, remediate variable soil types, enhance bearing capacity, cut off water, and relevel structures and roadways throughout the Rocky Mountains and beyond. The popularity and diversity of this geotechnical solution will be demonstrated through a multitude of case histories in cohesive, granular, collapsible, and voided ground conditions as well as in wet and dry environments. Though the fundamentals of the process have primarily remained the same since inception, advancements have been made in the form of innovative applications, data acquisition systems, and computer modeling to assist engineers and owners in assessing unique and challenging geotechnical sites and conditions.

Mitigation of landslides and failing embankment fills in low strength clay materials and over-steepened slopes has been an ongoing problem for both the Colorado Department of Transportation (CDOT) and the Wyoming Department of Transportation (WYDOT). Yeh and Associates, Inc. (Yeh) has been involved with mitigation of failing slopes using expanded polystyrene (EPS), otherwise known as rigid cellular polystyrene geofoam, on three separate corridors in Colorado and one corridor in Wyoming. The projects generally entailed providing a mitigation system to stabilize failing roadway embankments. The roadway pavement sections were generally in distress, with loss of one or more travel lanes after widening of the roadway from two lanes to three lanes. The paper describes the overall geotechnical conditions, design concepts, constructability issues, and long-term effectiveness of the mitigation systems.