TITLE: Morphology of the Missionary Ridge Rockfall, Durango, Co

Missionary Ridge Rockfall Hazards: Past, Present, and Future, Animas Valley, Durango, CO Ben Peterson Senior Seminar April 2007 ABSTRACT On July 5th, 1998, a 50,000m3 block of Dakota Sandstone broke loose from a west facing cliff of Missionary Ridge. The 1998 Missionary Ridge rockfall, located on the east side of Animas Valley in Durango, Colorado, is still on the move as of April 2007. In October, 2006, debris flows removed the toe of the debris pile creating an over steepened slope below the remaining debris pile. An investigation of the rockfall area was conducted to assess the new debris flow hazard and identify the controls of rockfall and debris flow hazards along this cliff. The Missionary Ridge Rockfall consists of Dakota Sandstone blocks overlying rock and soil of the Morrison Formation. The fallen blocks have been trapping moisture in the soil since 1998, accelerating the weathering process. Soil from the debris pile and debris flow channel was tested for its engineering properties and chemistry. X-ray diffraction detected no swelling clays and indicated that the soil is kaolinite rich. Atterberg limits were used to classify the soil as an inorganic clay of medium plasticity (with a plasticity index of 20), indicating that the soil lacks cohesive properties. The remaining, over steepened debris pile is highly unstable and more debris flows will likely occur during intense rainfall events. The Dakota Sandstone cliff of Missionary Ridge is also prone to other rockfall events. Another site was identified as a potential rockfall hazard. The Colorado Rockfall Simulation Program was used to estimate the outcome of this potential rockfall. The program was calibrated using the results from the 1998 Missionary Ridge rockfall. Rocks from the potential hazard were estimated to roll 320m before coming to rest on the hillside, a down slope distance which is very similar to the 1998 rockfall. Deformation bands found in the Dakota create planes of weakness that is controlling most of the joints. This highly fractured rock is unstable making the cliff prone to future rockfalls. A rockfall occurring at this potential site may play out in a morphologically similar scenario to the 1998 rockfall. INTRODUCTION Background The fireworks for the Independence Day celebration pale in comparison with the rumble heard across Durango on the next day, July 5th, 1998. Bystander’s outside directed their attention towards the cliffs along Missionary Ridge, north of Durango, Colorado. A massive 50,000 cubic meter rockfall had just occurred sending up a dust cloud that was visible from town (figure 1). Most of the debris from this fall only traveled about 300 meters down slope. The Dakota Sandstone block that had become dislodged from the cliff, fractured into mostly boulder size rocks but some of the rock blocks measured up to 40 feet in length. The fallen blocks came to rest mostly on top of the Morrison Formation and the rockfall appeared stable for approximately eight years. In October of 2006, heavy rain drenched Durango for over a week. The Animas River flooded its banks, reaching 8000 cfs. in town. The weathered rock and soil beneath the debris pile became saturated with water and lost cohesion. As a result, the overlying blocks of the rockfall began moving downslope and a slow-moving debris flow was initiated (figure 2). July 5th 1998 Figure 1- Photo of the 1998 Missionary Ridge Rockfall. Before (2001) After (Current image) Figure 2- Aerial and satellite photography showing before and after 2006 debris flows OBJECTIVES The purpose of this project was to identify and quantify the controls of rockfalls and debris flows that occur along Missionary Ridge (figure 3). These hazards put the homes built at the base of Missionary Ridge at risk. In addition, a new subdivision (The Cliffs of Durango) offers prospective buyers land parcels that are adjacent to the hazardous Missionary Ridge cliff. A new rockfall hazard site was identified near one of the new lot boundaries. In an effort to determine how far this newly identified block might travel down slope, the Colorado Rockfall Simulation Program was employed. For the debris flows, the physical properties of the soil were analyzed. Identifying the soil’s composition and quantifying the liquid and plastic limit, allowed for its classification. Knowing how the soil behaves allows for a better understanding of how future debris flow events could take place. Locatio Missionary Ridge Durango, N Figure 3- Location of Missionary Ridge rockfall. Lithology The Animas Valley is a glacial valley, with about 2000 ft. of relief. The slopes on both sides of the valley have been over steepened as a result of the glaciers that once filled the valley, during the last ice age. The cliffs along Missionary Ridge are Cretaceous Dakota Sandstone and are composed of white, light grey, and yellowish brown fine to coarse sand grains and conglomerate interbedded with dark to medium grey siltstone (figure 4). Within the Dakota Sandstone, composing the Lower Cretaceous, is the Burro Canyon Formation. This layer is a sandstone, chert-pebble conglomerate and greyish-green, non carboniferous claystone. This layer is weaker than the upper Dakota and results in differential weathering, undercutting the more resistant sandstone layers above. The Jurassic Morrison Formation consists of a greenish-grey, bentonitic mudstone and claystone with thin beds of very-fine grained sandstone in the upper Brushy Basin Member. The lower Salt Wash Member contains silicified light grey to white, fine to medium grained sandstone interbedded with thin beds of greenish grey mudstone (Carroll et al). The Morrison is the layer on which the debris pile now rests. It is the source of most of the clays found in the soil under the debris pile. Figure 4- Stratigraphy of the Animas Valley DEBRIS FLOWS HAZARDS Soil Chemistry X-ray Diffraction (XRD) was used for chemical analysis of the soils within the debris flow area and shows that it is a kaolinite clay rich soil with some noticeable amounts of quartz, and traces of muscovite. Although the Morrison Formation contains bentonitic clays, they are not as abundant in the soil at this location. This XRD data is useful in determining that no swelling clays are present in the soil at this site. Atterberg limits Atterberg limits were determined in order to classify the soil. Four samples were collected in the field for analysis. Of the four samples, one contained high amounts of sand and was not useful for this study since no limits could be determined from it. Of the remaining three samples, an average value was determined since each of their limits were very similar to each other. The liquid limit for the soil was determined to be 31% and the plastic limit was 11%. This gives the soil a plasticity index of 20. When plotted on a plasticity chart, this soil is classified as an inorganic clay of medium plasticity (figure 5). It should be noted that it lies on the cusp of being an inorganic clay of low plasticity. This soil lacks any cohesive properties, making it highly unstable especially when wet. Figure 5 - Atterberg’s Plasticity Chart used to classify the soil as an inorganic clay of medium plasticity. Morphology This debris pile has undergone a metamorphosis since 1998. Moisture that percolates through the overlying rocks has been trapped in the soil and rocks of the Morrison below, since it can no longer be reached by the sun’s powerful rays. This increase in moisture has locally accelerated the weathering process, leading to the clay rich soil that is found under the debris pile. Only the bottom portion of the debris pile failed last October, leaving most of the debris pile that still remains to be completely over steepened. This over steepened slope in combination with the soils cohesion properties is what really constitutes the increased hazard. ROCKFALL HAZARDS Deformation Bands Slicken lines are abundant on the wall of the 1998 rockfall scarp. They’re also abundant around the potential rockfall site. These slicken lines are bands of deformation that occur throughout the Dakota Sandstone. They create a plane of weakness that is controlling most of the jointing that occurs along the cliff. These bands are mostly vertical and strike in every possible direction. One dominant direction trends east to west and may be related as conjugate fractures. Areas that exhibit the most deformation bands are also the areas with the most fractured rock. These deformation bands only occur in the upper section of the Dakota Sandstone and do not continue into the conglomerate layers of Burro Canyon. The density of these deformation bands is uniform along the cliff at about .1 in/in-1. Differential Weathering The Burro Canyon Formation directly beneath the Dakota is not well articulated and is very prone to weathering, as the chert pebbles easily crumble out of the claystone matrix. This combination of highly fractured rock on top of easily weathered claystone makes this cliff prone to more rockfall events in the future. Potential Rockfall Hazard One location along the cliff has been identified as a potential rockfall hazard (figure 6). This area features a block of similar size to the one that failed in 1998. The potentially hazardous block is estimated to be around 52,000 cubic meters. This block was chosen as a hazard due to a large crack, up to two feet wide in some places, that completely separates this block from the cliff. This crack extends, almost due north, for 185 ft. from end to end and has several large ponderosa pines growing in it. Evidence of prior smaller rockfalls scatters the area below this block. Colorado Rockfall Simulation Program Using a program developed by the Colorado Geological Survey, called the Colorado Rockfall Simulation Program, estimates could be made on how far this potential rockfall block could fall.

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