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Rinderle Construction Surficial Geology Investigation, High Desert AFT Subdivision D & 35 Roads, East Orchard Mesa, Palisade, CO May 4, 2001 Page 5 GEOLOGIC I-IAZARDS The prelhninary study otGeologic Hazards of this site wrs used to position lot lines, access roads and building envelopes beyond the limits of the identified Hazards of Flood Flows and Slope Stability. Utilizing recent events and standard evaluation techniques, this tract is not considered to be within with an active debris flow hazard area. The tract is located adjacent to a potential Flood hazard area and during either large or short duration/high intensity precipitation events, may be subject to flooding. The actual hazard area appears to be limited to the gully bottoms of Sink Creek and associated tributaries. S inic Creek will affect the southwest comer of this tract. An on-site drainage plan will be required to properly define and control storm waters which may enter this site from the east and which are generated on-site. Such a study is beyond the scope of this report. The tract contains a significant amount of steep slope which are associated with Sink Creek and its tributaries. Most of the slopes are either Mancos Shale or the Shale is present on the lower portions of the slopes. These slopes range from very gentle to near vertical in some areas. These slopes also range from only a few feet high to in excess of 80 feet high. Much of these slopes, particularly the steeper and higher slopes must be considered as unstable. The less steep and lower slopes should be considered as Potentially Unstable. The limits of these problematic slopes are indicated on the attached diagrams. The surface soils are an erosional product of the upper Mancos Shale and the Grand Mesa Formations which are exposed on the west slopes of the Grand Mesa. The soils contained within these Alluvial fan features normally exhibit a metastable (collapsible) condition which can range from very slight to severe. The surface soils are normally soft and may consolidate when loaded but, Geo[echnical and Structural engineering practices and construction techniques normally employed in the Clifton area should be sufficient to properly address the possibility of building settlement on this site. The Mancos Shale Formation is exposed on portions of the site and is `bedrock' beneath the entire site. The Mancos Shale Formation exhibits shrink swell characteristics in this climate, the primary concern is swelling of the clays. Geotechnical and Structural Engineering practices and construction techniques normally employed in the Clifton area should be sufficient to address the swelling clays. Underground mining is not known to have occurred in this area and ground subsidence due to man made activities or excessive amotmts of natural solution of rock minerals was not observed or expected on this site. The site is underlain by the Mancos Shale, at depths believed to be in excess of 20-40 feet. The Mancos Shale is known to produce radon gas. The presence or absence of radon gas has not been confirmed by Grand Junction Lincoln DeVore and is not a part of this particular investigation. Most measurements of radon gas in the Grand Junction area is associated with the use of radioactive mill tailings, often times utilized for man-made fill. It is recommended that construction to be used for residential or commercial purposes be completed in a manner which would discourage the collection or trapping of naturally occurring radon gas. No evidence of mass wasting processes or other geologic hazards not listed in [his report were observed on this site which would affect the development.