braun Braun Consulting Engineers ENGINEERING EVALUATION OF THE STANLEY MINE ADVENTURE PARK AREA CLEAR CREEK COUNTY, COLORADO Prepared for: STANLEY MINES ADENTURE PARK 3375 W. POWERS CIRCLE LITTLETON, COLORADO 80123 303-346-5196 Prepared by: C.A BRAUN, P.E, CPG D. MOSCH, ENGINEER BRAUN ENVIRONMENTAL, INC. 355 TELLER STREET, SUITE 200 LAKEWOOD, COLORADO 80226 Date: May 4, 20186 355 Teller Street, Suite 200, Lakewood, Colorado 80226 Phone (303) 988-7697 Fax (303)-697-2140
Braun Consulting Engineers was retained by Stanley Mines Adventure Park (Stanley) to perform a review of the stability of the hillside in area around the old Stanley Mine property, located in Clear Creek County, Colorado. The area is currently being proposed as an adventure park. This report is divided into two parts, the first being a review of previous work, and the second to field observations made by the authors of this report. I Previous Work Stanley provided Braun with a set of documents relating to surface stability in the general area of Idaho Springs. The Colorado Geological Survey produced Open File Report 75-5 titled Idaho Springs, Clear Creek County, Area Geologic Hazards Map, and a second updated Open File Report 03-02 Titled Selected Geologic Hazards, Clear Creek County, Colorado. The 1975 map identifies the map as containing Rockfall-Rockslide areas and Unstable or Potentially Slopes. The 2003 map - Plate 2 shows an increased area of coverage from the 1975 map, and the Rockfall-Rockslide designation was replaced with Rockfall Area, and Unstable, and Potentially Unstable designations were added. It is interesting that nearly all of Clear Creek Canyon in the vicinity of Idaho Springs is designated either Rockfall, or Unstable on the map. Referring specifically to the area around the old Stanley Mine, both the 1975 and 2003 maps are the same, except for the term changes. The 2003 map designates the entire north-facing slope in the general area of the Stanley Mine as Unstable with exception of two small areas that are designated Rockslide. A third document, a letter signed by Kevin McCoy, of the Colorado Geological Survey on April 26, 2018, states that he is in agreement with the risks identified by Stanley in their reports, refers to the above referenced maps, and his conclusion that he cannot confirm adequacy of any designs on the site without reviewing additional engineering analysis. It should also be noted that production of these Colorado Geological Survey maps are not generally based on engineering characteristics of the bedrock, bedrock structural features, or the make-up of surficial coverings, but are instead dominantly based on surface slope angle as interpreted from the U.S. Geological Survey Quadrangle maps. Thus, they provide more of a statistical basis, than a factual document showing where actual landslides and rockfalls occur. This is discussed in the disclaimer on Plate 2 of the 2003 map (Figure 1). While the maps generally differentiate level areas from steeper areas, they are not of much use to predict the actual conditions at some specific location on a site, other than requiring some investigation to disprove the designations. Braun Consulting Engineers Page 1
II Field Generated Information C. A. Braun and D. Mosch performed field investigations of the area during the last week of April and first week of May, 2018. The investigations included review of other published materials, field truthing the published material, and collection of additional field information on the actual surface conditions within the Stanley Mines project area. This investigation covers three specific items with respect to the stability of the area: 1 Mechanical movement as a result of failures in bedrock; 2 Rapid and creep migration of overlying colluvium, and; 3 Subsidence/collapse from past underground mining activities. A Mechanical movement as a result of failures in bedrock The bedrock in the area of the Stanley Mine property is part of the Precambrian Idaho Springs Formation. The rock is made up of granite gneisses, biotite gneisses, and amphibolite (Authors and Moench, 1964) (Figure 2). All of these rock units have approximate compressive strengths of around 30,000 pounds per square inch (psi), meaning that they are hard competent rocks. This high strength results in vertical cliffs standing 50 feet or more where unfractured rock outcrops. These cliffs can be easily observed both naturally in existing high bank cuts along Interstate Highway 70, and at cuts made in the hillsides for various buildings in the county, including casinos in Black Hawk. A second factor for rock stability is angle of foliation, and fracturing. If foliation is significant and bedding planes are closely spaced and planer, then foliation can lead to mechanical failure. At the Stanley Mine, the general foliation of the gneiss in the hillside to the south of the buildings dips steeply to the west at an angle of between 50 and 80 with a strike of about North 30 degrees East (Figure 1). This map also shows distribution of rock types that make up bedrock. As can be seen on the map, this strike direction approximately normal to the trend of Clear Creek valley. The foliation is generally widely spaced, and the planes are not necessarily completely parallel, or perfectly planar. Nearly all observed natural joints, fractures and vein structures share orientations of the gneiss foliation bedding or may have even steeper dip angles. The slope of the hill side was measured and has an average slope, just south of the Stanley Mine buildings, of 32 degrees. The lower portion of the hill side is approximately 19, with the majority of the upper portions being around 30. Very short sections are as steep as 45. This slope is less than either the measured foliation and fracture dips, thus the hillside slope would not be expected to produce failure along foliation or fracture planes, even if the fracture were oriented parallel to the hillside slope. Inspection of the area found no evidence of any existing mechanical large failures within bedrock beyond those of natural surface spalling caused by weathering. Thus, within recent geologic history, no evidence of mechanical failure has been found, and the probability of a rock future failure along the hill immediately south of the Stanley Mine buildings is low. Braun Consulting Engineers Page 2
B Potential for Rapid or Creep Migration of Colluvium Migration of colluvium has been inferred but not precisely defined in the Colorado Geological Survey reports. For the purposes of this report, the concern is in modern movement. Movement of surficial materials has historically occurred over the history of the Earth, and most recently during the last ice ages of the Pleistocene. Remnants of these materials still remain along the hillsides of the canyons at elevations above 7,500 feet. The ice movement of the last glacial period, coupled with movement of debris by high water flow rates during periods of melting have resulted in the placement of much of the colluvium that we see along the mountain slopes, as well as much of the alluvium that makes up the valley bottoms that have developed along Clear Creek. Based on field studies, there is no indication of any modern movement of the colluvium on the hillside. Two specific areas have been referenced by the Colorado Geological Survey reports are shown on a site plan (Figure 3). The rock fall area identified as being located near the east edge of the project area, appears to be an ancient debris flow, most likely occurring sometime near the end of the Pliocene Epoch. This identified area is completely overgrown with vegetation including trees as old as 75 years. Thus, it is considered to be stable and poses no threat to the Stanley Mine building area. A second rock fall area identified as being located immediately south of the Stanley Mine buildings is a true rock slide but is very limited in size. This slide is the result of a short cliff that was produced from differential erosion, caused by variations in competence along a North 30 east trending, approximately 50-foot wide, layer of granite pegmatite that is in contact with a and quartzbiotite Schist. Rock from the resulting 25-foot-high cliff of granite pegmatite falls into a gully located within the quartz-biotite schist. Since the surface angle of the adjacent hillside slope is slightly less than the angle of repose, a slide develops and moves slowly down the hillside for a short distance. Movement within the slide is just high enough that it precludes the growth of vegetation. The typical angle of repose for soils and broken fragmented rock is greater than 35 and the hill side slope is less than 33 at this location. As a result, this area, while experiencing creep, would not be expected to produce any sudden or fast movement of rock. Most fragments that might fall from the cliff would be anticipated to fall within the active slide area. Any fragments that might fall in an unexpected area would have momentum energy absorbed by vegetation. C Possible Subsidence from Historic Mining Activities The Stanley Mine produced metals from a vein. The near vertical vein has general trend of north 30 east, and is generally narrow in width, very rarely being over four feet. The miners would sometimes mine upward creating a cavities or stopes. If the vein reached the surface, sometimes the mine would daylight, creating an open hole, or in other instances, the miners might not mine completely to the surface, but the opening would be close enough to the surface that collapse would eventually occur in the future. This is common in other mines in the district. An examination of the Stanley maps indicates that the mine openings can be within 50 feet of the surface in the area between Clear Creek and the tunnel portal at Road Level. However, the historic maps show that the old mined areas have been backfilled. The mining method, relied on filling the stopes with waste rock during the mining process to provide support for the mine. Thus, the likelihood of the stoped areas caving to the surface is low. Review of the compressive strengths of the rock adjacent to the vein, as discussed previously, is high and mines in the general area can have Braun Consulting Engineers Page 3
APPENDIX A Figures Braun Consulting Engineers Page 5
Rock Fall Areas Unstable Slopes Gehrmann Shaft Building Unstable Slopes Unstable Slopes 0 500 1,000 feet Figure 1: Colorado Geologic Survey, Scale: One Inch = 500 Feet Geologic Hazards Map Contour Interval 40 feet (Stanley Mine Adventure Park Area) Source: Colorado Geological Survey produced Open File Report 75-5 titled Idaho Springs, Clear Creek County, Area Geologic Hazards Map By Beth L. Widmann Clear Creek County, Colorado Section 34, Township 3 South, Range 73 West
Road Level Gehrmann Shaft Building Whale Level York Level 0 500 1,000 feet Contour Interval 100 feet Source: Geology of Precambrian rocks, Idaho Springs district, Colorado Bulletin 1182-A, Plat 1, By: R.H. Moench, 1964 Figure 2: Geologic Map of the Stanley Mine Adventure Park Area Clear Creek County, Colorado Section 34, Township 3 South, Range 73 West By: D. Mosch 5/03/2018
Figure 3: Potential Geologic Hazard Location Map of the Stanley Mine Adventure Park Area With Park Design Clear Creek County, Colorado Section 34, Township 3 South, Range 73 West By: D. Mosch5/03/2018 Legend Possible Rock Falls Highest Potential Mine Subsidence 0 500 1,000 feet Scale: One Inch = 500 Feet