Rocky Mountain Geo-Conference 2014

Severe flooding on the Front Range of Colorado in September 2013 tested many dams, including several dams on and adjacent to the Left Hand Creek drainage north of Boulder, Colorado. A tropical-like storm drove moisture up against the foothills, dropping 200 mm (8 in) to more than 350 mm (14 in) of precipitation in a 24-hour period based on a number of weather stations within the watershed. The precipitation resulted in flash flooding in Left Hand Creek. A dam west of the main precipitation event experienced no damage. A diversion dam near the mouth of Left Hand Canyon was completely destroyed by the storm event due to scour. An off-channel valley reservoir dam experienced damage to both the service and emergency spillways. The experiences along Left Hand Creek demonstrate the need for geotechnical and structural design of dams for extreme events, especially in mountain environments where uncertain flood events can occur due to unusual weather patterns.

Post-fire debris-flows are a serious hazard in the western United States. Potential impacts of these events include loss of human life, destruction of structures, and degradation of habitat and water quality. While loss of human life is the most important concern in a geohazard assessment, potential loss of life is extremely difficult to quantify and is beyond the scope of this paper. This paper focuses instead on the analysis of economic risk from post-fire debris-flows in support of cost-optimized post-fire debris-flow hazard management strategies. Common approaches to evaluating post-fire debris-flow hazards provide either qualitative estimates of inundation zones or no inundation estimates at all. Quantitative estimates of debris-flow damage and economic risk support the selection of natural hazard management strategies following a fire via optimization approaches. The first step in this process is an analysis of damage cost estimates and related probabilities. Debris-flow hazards and associated damage costs for individual drainages can be analyzed in ArcGIS utilizing existing models and readily available GIS data. Preliminary case study results suggest that this process can be used to identify the most economically concerning drainages. These results can guide allocation of emergency management funds and optimization of debris-flow management strategies following a wildfire. This paper discusses where to acquire geologic and social input data and how to operate the GIS models in support of the post-fire debris-flow economic risk assessment. A case study from southern California is provided to illustrate the methods.

Rio Grande Dam was constructed over 100 years ago near the headwaters of the Rio Grande in the San Juan mountains of southwest Colorado. The reservoir is impounded by an earthfill and rockfill dam built in a challenging geologic environment. The left abutment consists of a large heterogeneous rock slide deposit, while the right abutment is made up of fractured volcanic rock. Historically, the reservoir has experienced high seepage through both abutments and the dam foundation. A rehabilitation project was designed in 2012 and constructed in 2013 with the goal of reducing the seepage through the abutments and the foundation of the dam. A clay slope liner and clay blanket were constructed on the upstream face of the left abutment over the rock slide deposit to decrease seepage. A grout program was developed — utilizing real-time, computer-aided grout delivery monitoring — to construct a grout curtain into the fractured volcanic rock of the dam foundation and right abutment with the intent of infilling fractures to reduce seepage. Preliminary data, to date, indicate the rehabilitation project appears to have successfully reduced seepage through the dam foundation and the abutments by more than 50 percent of the historical average.

The Dallas Floodway System (DFS) consists of 37 km of earthen levees, pump stations, gravity sluice gates, and pressure storm sewers along the Trinity River in Dallas, Texas. Construction of the original system was completed in the 1930s after the region experienced catastrophic flooding from the Trinity River in 1908. The DFS was strengthened in the 1950s by the U.S. Army Corps of Engineers (USACE). In 2009, the USACE, through the Levee Safety Program, rated the DFS as "Unacceptable" during Periodic Inspection No. 9. As a result of this inspection, the USACE rescinded their letter of support for accreditation from the Federal Emergency Management Agency (FEMA). The City of Dallas worked with HNTB Corporation (HNTB) and the USACE to assess the levees anddesign improvements to the levees, including over 5,700 m of seepage cutoff walls. In March of 2012, the City of Dallas awarded Magnus Pacific Corporation (Magnus Pacific) the 100-year Levee Remediation - Cutoff Wall Improvement project, a critical step toward regaining the FEMA 100-year accreditation. Magnus Pacific was the prime contractor for the project, supported by specialty subcontractors Layne Geo and SMB Enterprises, Incorporated. This paper presents a case study of the levee assessment, design, and construction of over 5,700 m of seepage cutoff wall.

Tetra Tech, Inc. (Tetra Tech) provided engineering support to Anadarko Petroleum Company (Anadarko) for water storage alternatives proposed for use in hydraulic fracturing operations. Tetra Tech conducted a geotechnical investigation to determine the suitability of an Anadarko site located south of Greeley, Colorado. The property, which is located near the geologic floodplain of the South Platte River, is underlain by clayey sand and low plasticity clay, gravelly sand, and claystone bedrock. An earthen impoundment was designed to maximize storage capacity and balance earthwork quantities. The footprint of the facility was 6.3 hectares (15.5 acres) with impoundment dimensions of 107 m by 235 m (350 ft by 770 ft). The embankment crest ranged from 1.5 m to 2.9 m (5 ft to 9.5 ft) above existing grade, and the capacity was approximately 79,500 m³ (500,000 barrels or 21,000,000 gal). During construction, coarse gravel that would not be a suitable liner subgrade was encountered. The material was removed and stockpiled; clay material from other Anadarko sites was trucked in and used as liner subgrade. The gravel was screened on site and used by Anadarko as road base at various other sites. Construction of the impoundment was completed in summer 2013 and has performed well; it was not damaged during the flooding of September 2013.

The Southern Delivery System's (SDS) 189,300 m³/d (50-mgd) water treatment plant and finished water pump station in southeast Colorado Springs is the largest and most complex component of the regional project. The project is also being built on some of Colorado's most complex geology. Foundation components of the project are constructed and construction continues on the various facilities. The geology of the water treatment plant site consists of eolian deposits over alluvium. Two large site structures were founded within the eolian deposits at depths ranging from 9.1 m (30 ft) below to 3 m (10 ft) above existing grade. The eolian soils are windblown soils that were deposited in a loose condition and later the individual grains were fused with weak water-soluble "cement." When these soils become wetted, particularly under conditions of increased loading, they collapse. Design, construction, and operation measures were necessary to mitigate the potential for water to collect below grade or to pond above grade, as well as minimizing the potential for pipe or tank leakage. The design-build project team mitigated the collapse potential of the site soils. Ground improvement included deep dynamic compaction (DDC) for areas of the site. A test pilot program was successful in demonstrating that DDC could be used as a cost effective method to mitigate the soils across large areas of the site. Using DDC saved 1,704,957.3 m³ (2,230,000 yd³) of over-excavation that would have otherwise been required.

The Needle's Eye Tunnel is located at elevation 3536 m (11,600 ft) near the top of Rollins Pass on Boulder County Road 117/US Forest Service Road 149 approximately 32 km (20 miles) west of Rollinsville, Colorado. The location of the Needle's Eye Tunnel is shown on Fig. 1. The curved tunnel is approximately 46 m (150 ft) long, 6.7 m (22 ft) wide, and 7.3 m (24 ft) tall. A sketch of the plan and profile is found in Fig. 2. The tunnel was blasted through a mixture of granite and gneiss, typical in the Front Range of the Rocky Mountains. Current ground support consists of rock bolts and wire mesh. During the 20^th century, the tunnel was part of a railroad and vehicle road alignment. Since 1990, the tunnel has been closed to traffic. In 2012, Deere and Ault Consultants, Inc. (D&A) was hired by the Rollins Pass Restoration Association (RPRA) to evaluate the feasibility of repairing the tunnel so that it could be re-opened for low volume vehicle and pedestrian traffic between Rollinsville and Winter Park. The evaluation by D&A showed that the tunnel is currently unsafe for traffic, but could be rehabilitated with a mixture of shotcrete, rock bolts, and drains.

Soft ground tunnel/trenchless designers and contractors have used the Tunnelman's Ground Classification system (Heuer, 1974 after Terzaghi, 1950) for over 60 years to describe potential ground behavior for tunneling. Geotechnical baseline reports (GBRs) utilize the Tunnelman's Ground Classification system to baseline ground behavior for trenchless construction. More recently, GBRs are being used to describe soil behavior for shafts and open-cut excavations utilizing the same method. Use of this system for shafts and open-cut construction is outside the original intent of the classification scheme and subsequent modifications. While many ground types behave similarly in tunnels, shafts, and open-cut excavations, there are considerations that are unique to shaft and open-cut excavations. This paper introduces a modified ground behavior classification system for use specifically in describing ground behavior for trench, shaft, and sloped excavations in soils based on the Tunnelman's Ground Classification system. This system provides classification of different ground behaviors, descriptions of the ground behavior, typical soil types associated with those behaviors, and impacts of groundwater on the behaviors. The proposed classification system could be used in not only GBRs, but also incorporated into traditional geotechnical evaluation and recommendation reports which commonly refer to OSHA guidelines for minimum slopes, but do not address ground behavior.

Landslides pose hazards along many stretches of Colorado highways. To assist in quantifying hazards and risks from known, existing landslides and to help prioritize their importance, the Colorado Landslide Hazard Rating System (CLHRS) was developed as a companion to the existing rockfall rating system used by CDOT. A preliminary version of the CLHRS was generated based on a review of the current body of technical literature regarding the factors that contribute to landslide hazard, consequence, and risk. It consisted of eleven Hazard Factors and eight Consequence Factors to calculate an overall risk score. The preliminary CLHRS was used to evaluate 69 landslides distributed throughout western Colorado. The resulting scoring distributions were then subjected to a suite of statistical analyses in order to identify the factors that possessed the greatest statistical merit. The statistical screening led to a final functional version of the CLHRS consisting of six Hazard Factors (geology, vegetative cover, slope aspect, surface water influence, failure frequency, and slope angle) and six Consequence Factors (depth to slide plane, length of highway affected, average daily traffic, detour options, worst-case scenario detour time, and annual maintenance cost). The CLHRS total risk scores clearly identify a subset of the landslides with both highest hazard characteristics and potential consequences.

A record snowfall throughout the Rocky Mountains during the winter of 2010/2011 combined with torrential spring rainfall led to infrastructure damage throughout Montana, Wyoming, and North Dakota. Many roadways were closed due to flooding and landslides. Interstate 90 - between Hardin, MT and Buffalo, WY - experienced multiple deep-seated landslides and several shallow slips. One of the landslides occurred at MM 16.3 on the westbound on-ramp, a critical access point to I-90 due to the vicinity of the Wyoming Port of Entry. A 3-m to 4-m (10-ft to 12-ft) scarp near the guardrail led the Wyoming Department of Transportation (WYDOT) to look for innovative techniques to quickly stabilize the ramp to permit safe traffic flow. Safety and right-of-way concerns also complicated the design and construction of any earthwork structures. WYDOT chose launched (ballistic) soil nails faced with reinforced shotcrete to provide temporary stabilization of the near-vertical scarp. The temporary stabilization allowed for stability of the roadway platform and also permitted construction of a large earthen buttress. Due to the time and safety constraints involved, as well as the innovative technology selected, unique approaches in the slope stability modeling, onsite design, and construction planning were required.

The use of rockfall barrier systems is common in the United States in mountainous areas. Many of these rockfall systems are placed along the shoulder of roadways to stay within the right-of-way owned by the various transportation agencies. Many issues have arisen with the design, certification, testing, and application of barrier and post systems as most of the rockfall barrier systems marketed in the United States are tested and approved in Europe. The rockfall barrier testing guidelines generally follow the Guideline for European Technical Approval of Falling Rock Protection Kits, otherwise known as ETAG 027. This guideline provides a basis for documenting full-scale testing of rockfall barrier systems for energies ranging from 100 kJ to 4,500 kJ. When specifying and/or designing post foundation systems for transportation agencies, two major limitations become apparent using ETAG 027: (1) no specified limit for outward panel deflection resulting from a rockfall event, and (2) no protocols or guidance for post foundation testing or design. This paper provides observations, results, and summaries of the 17 vertical-drop tests in which concrete "boulders" were dropped into various barrier fence systems for energies up to 1,000 kJ and, separately, 29 post and post foundation direct impact tests that were conducted for energies up to 220 kJ.