Loading...
HomeMy WebLinkAboutPreliminary Geologic Site Assessment 02.09.2009Hepworth-Parvlak Geotechnical, inc 5020 County Road 154 Glenwood Springs, Coloraclo 81601 Phone: 970-945-7988 HEPWORTH - PAWLAK GEOTECHNICAL Fax 970-945-8454 email: hpgeo@hpgeotech.corn RECEIVED February 9,2009 Chris Janusz 3642Highway 82 Glenwood Springs, Colgrado 81601 Job No. 109 0164 Subject:Preliminary Geolo gic S ite Assessment, Proposed 2 Lot Subdivision,, 3642 arñ3644 Highway 82, Garfield County, Colorado Dear Mr. Janusz: This report presents the results of a preliminary geologic site assessment for the proposed 2 lot subdivision located at3642 and3644 Highway 82, Garfield County, Colorado. The purpose of the study was to evaluate the geologic conditions and their potential impacts on the project. The study was conducted in accordance with our agreement for professional services to Chris Janusz, dated January 26,2009. A field reconnaissance of the project site was made on February 2,2009 to observe the geology and potentialhazards at the site. In addition, we have reviewed previous studies in the area, published regional geologic studies and looked at aerialphotographs. Based on this information, an assessment of the potential influence of the geology on the proposed development was made. This report summarizes our findings and presents our conclusions and recommendations. Proposed Development: The proposed development consists of splitting the 1.37 acre, triangle shaped lot into two roughly equal size lots and replacing the existing trailer on the south lot (3644 Highway 82) with a single family residence. At this time, no new development is planned for the home on the north lot, and plans for the single family residence on the south lot have not been developed. 'When building and grading plans have been developed for the site, we should be notified for additional geo glc co assessment and a geotechnical evaluation. H JUN 0 q 20,18 GARFIELD COUNW COM[IUNITY DEVELOPMENT Parker 303-B4l-7119 o Coloraclo Springs 719-633-5562 . Silverthorne 970-468-i989 Chris Janusz February 5,2009 Page2 Site ConditÍons: The project site is located in the northern part of the Roaring Fork River valley about 2 miles south of Glenwood Springs along the uphill, eastern side of Highway 82. The overall property is roughly triangular in shape with side dimensions of about 279 feet and 306 feet and 415 feet along Highway 82. The property abuts a single family residence to the north and a commercial development to the southeast. The steep hillside to the northeast is cut by relatively small, ephemeral drainages which slope down towards the site and terminate at the Glenwood Ditch. The ditch is located between about 40 to 60 feet uphill ofthe existing structures. An 8-inch diameter PVC pipe has recently been buried in the bottom of the ditch for service to the adjacent properties. Currently, the ditch is about 12 feet wide and 4 feet deep. The property is located in the middle part of coalescing debris fans which originate about 300 feet upslope of the property. The property grade has been terraced to accommodate atrailer, modular residence and storage sheds which are accessed by a driveway from Highway 82 to the southwest. The overall slope across the development area is about 25 percent down to the southwest towards the Roaring Fork River, located about I,700 feet southwest of the property. Above the ditct¡ the gro und surface increases in steepness to about 50 percent. Vegetation on the property consists of grass and weeds, scattered small brush and several 10 to 20 foot tall pinon pines. Sandstone cobbles and boulders up to about 4 feet in size were observed on and surrounding the site. Most of the downhill properties on the alluvial fan have been developed with single family and commercial structures. Regional Geologic Setting: The project site is located in the Carbondale collapse center of the Southem Rocky Mountains Region west ofthe Rio Grande rift and east ofthe Colorado Plateau. The Carbondale collapse center is the western oftwo regional evaporite collapse centers in western Colorado. It is an irregular-shaped, northwest trending region covering 460 square miles between the White River uplift and Piceance basin. As much as 4,000 feet of regional subsidence is believed to have occurred during the past l0 million years in the vicinity of Carbondale as a result of dissolution and flowage of evaporite from beneath the regions (Kirkham and Others,2002). The Eagle Valley Formation is located stratigraphically below the Maroon Formation and crops out on the valley side west of the project site. Much of the evaporite related subsidence in the Carbondale collapse center appears to have occurred within the past 3 million years Job No. 109 0164 cåFtecrr Chris Janusz February 5,2009 Page 3 which also corresponds to high incision rates along the Roaring Fork, Colorado and Eagle Rivers. This indicates that long-term subsidence rates have been very slow, between about 0.5 and 1.6 inches per 100 years. It is uncertain if regional evaporite subsidence is still occurring or if it is currently inactive. If still active, these regional deformations because oftheir very slow rates should not have a significant impact on the proposed development at the site. Geologically young faults related to evaporite tectonics are present in the Carbondale collapse center but considering the nature of evaporite tectonics, these faults are not considered capable of generating large earthquakes. The closest geologically young faults that are less than about 15,000 years old and considered capable of generating large earthquakes are located in the Rio Grande rift to the east ofthe project site. The northern section of the \Milliams Fork Mountains fault zone Q50 is located about 50 miles to the northeast and the southern section ofthe Sawatch fault zone Q56b is located about 80 miles to the southeast. At these distances large earthquakes on these two geologically young fault zones should not produce strong ground shaking at the pdect site that is greater than the ground shaking shown on the U. S. Geological Survey 2002 National Seismic Hazards Maps (Frankel and Others,2002). Project Site Geology: The geology in the project area was observed on February 2, 2009. Our geologic interpretations also use regional geology maps by Soule and Stover (1985) and Kirkham and others (1995). The local bedrock is Maroon Formation consisting of moderate to shallow dipping, blocky red saídstone. These rocks were deposited in the central Colorado trough during the Ancestral Rocky Mountain orogeny about 300 million years ago. The regional bedding generally strikes to the north and dips to the east. Bedrock outcroppings directly northeast of the project site dip between 11 and26 degrees down to the east, into the hillside. The Maroon is overlain by dense, black resistant alkali basalt in lava-flow layers several feet thick deposited between 8 and23 million years ago. The closest basalt flows are located about 3,000 feet to the north ofthe project site. Job No. 109 0164 cåFtecn Chris Janusz February 5,2009 Page 4 Sur{icial Soil Deposits: The soil deposits in the prdect area are largely the result of erosion of red sandstone layers from bedrock outcroppings deposited as alluvial fans through a series of small drainages northeast ofthe project site. Several sandstone cobbles and boulders are visible on the surface. These fans appear active and could still be sites of infrequent rockfall and debris flows or floods associated with intense thunderstorms. A 6 to 8 foot incised drainage channel is located north of the property but appears to empty northwest of the site. The existing irrigation ditch would also appear to divert water and debris to the northwest around the proiect site and act as a catchment basin for rockfall. Geologic Site Assessment: There are several conditions of a geologic nature that should be considered as project planning and design proceeds The most significant are potential rockfall and debris flows and floods. Other geologic conditions are sinkhole potential, earthquake ground shaking and moisture sensitive foundation soils. Percolation testing performed in 1980 and 1984 for septic design reported percolation rates of 10 and 15 minutes per incl¡ respectively. A log of a domestic welldrilled in 1962 reported rocky soils to a depth of 73 feet and ground:water level at 50 feet. The alluvial fan soils above the water level are considered to be collapsible when wetted. Shallow foundations are typically used in this area for lightly loaded structures, but recommendations for bearing on the moisture sensitive soils should be addressed with a site specific geotechnical study when the building plans have been developed. The potential risks and possible mitigations to reduce the risks associated with debris flows, rockfalls, sinkholes and earthquakes are discussed below. Debris Flow and Flood Potential: The property is located in the middle part of coalescing alluvial fans that could be subiect to sheetwash flows and floods during periods of snowmelt or heavy rainfall. There is a relatively high likelihood that future floods and flows will occur along hillside drainages but flows of the main drainage currently follow an incised drainage channel which passes northwest ofthe property. Smaller drainage channels are intercepted by the irrigation ditch which appears to carry water around the property to the northwest. In our opinion, there is a risk that debris flow material could breach the inigation ditch during alarge event and expose the project site JobNo.10901óA cåttecrr Chris Janusz February 5,2009 Page 5 to signific ant building and property damage and possible harm to the building occupants. Ifthis risk is not acceptable then mitigation to reduce the risk should be considered. Rockfall Potential: The steep hillside clifß located to the northeast ofthe project site are resistant beds of the Maroon Formation that are source areas for repeated falls of rocks of varying sizes. Although rockfall occurrences have been documented in the Glenwood Springs area, we are not aware of rockfall documentation on the project site. Several cobble and boulder size rock fragments on the property do indicate a potential rockfall hazard. In our opinion,the risk is moderate that a rockfall event could reach the project site and cause significant building and property damage and possible harm to the building occupants. If this risk is not acceptable then mitigation to reduce the risk should be considered. The existing irrigation ditch has a 4 foot wall and a 12 foot wide catchment area which will prevent some rocks from reaching the project site. Sinkholes: Geologically young sinkholes are present in the westem Colorado evaporite region mostly in areas where the Eagle Valley Formation and Eagle Valley Evaporite are shallow. The Eagle Valley Formation underlies the Maroon Formation in the project area. Evidence of sinkholes was not observed at the project site during our field reconnaissance or on aerial photographs reviewed. Although geologically active in the region, the likelihood that a sinkhole will develop during a reasonable exposure time at the site is considered to be low. The property owner should be advised of the sinkhole potential, since early detection ofbuilding distress or settlement of graded areas may help with timely remedial action which is important in reducing the cost of repairs should an undetected subsurface void start to develop into a sinkhole. Earthquake Potential: Historic earthquakes at the project site have typically been moderately strong with magnitudes of M 5.5 and less and maximum Modified Mercalli lntensities ofVI and less (Kirkham and Rogers, 1985). Modified Mercalli IntensityVl ground shaking should be expected during a reasonable exposure time for the site, but the probability of stronger ground shaking is low. Intensity VI ground shaking is felt by most people and causes general alarrry but results in negligible damage to structures of good design and construction. JobNo.109016A cåStecrr Chris Janusz February 5,2009 Page 6 Future buildings on the site should be to withstand moderately strong ground with little or no and not to co under stronger ground For firm rock sites with shear wave velocities of 2,500 fps it the upper 100 feet the U. S. Geological Survey 2002 National Seismic Hazard Map indicates that a peak ground acceleration of 0.069 has a 10% exceedence probability for a 50 year exposure time and a peak ground acceleration of 0.23ghns a2o/o exceedence probability for a 50 year exposure time at the project site (Frankel and Others, 2002). This corresponds to a statistical recrrrence time of about 500 years and 2,500 years, respectively. The soil profile at the building site is estimated as Class D, stiff soil site, as described in the 2006 International Building Code. Radiation Potential: Regional studies by the Colorado Geological Survey indicate that the closest radioactive mineral occrürences to the project site are located along the Grand Hogback near New Castle about 14 miles to the northwest (Nelson-Moore and Others, 1978). Regional studies by the U. S. Geological Survey (Dubiel, 1993) for the U. S. Environmental Protection Agency (EPA) indicate that ttrg-projecl.q"-1!g_"i.s-ll1m9-q-9f?!-e- radon gas potential zone. The 1993 EPA regional radon study considered data from: (l) indoor radon surveys, (2) aerialradioactivity surveys, (3) the general geology, (4) soil permeability estimates, and (5) regional architectural practices. It is not possible to accurately assess future radon concentrations in buildings before they are constructed. Accurate tests of radon concentrations can only be made when the buildings have been completed. Because ofthis, new buildings in moderate to high radon areas are often designed with provisions for ventilation ofthe lower enclosed areas should post construction test ing show unacceptable radon concentrations. Surface Drainage/Grading: The grading plan for development of building sites should consider runoffthrough the project and not New grading should not impact existing drainages or reduce mitigation provided by the irrigation ditch and should consider potential flooding impacts.for the project prlor to construction. Job No. 109 01óA We should review site grading plans cåStecn Chris Janusz February 5,2009 PageT Limitations: This study has been conducted according to generally accepted engineering geology principles and practices in this area atthis time. 'We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the field reconnaissancer revie,w of published geologic reports, aerial photograph interpretation, and our experience in the area. This report has been prepared for the exclusive use for our client and is an evaluation of the potential influence of the geology on the proposed development. We are not responsible for technical interpretations by others of our information. Additional study will be needed if additional debris flow and flood, and rockfall mitigation is proposed. As the project evolves, we should provide continued consultation, conduct additional evaluations and review and monitor the implementation of our recommendations. Significant design changes may require additional analysis or modifications to the recommendations presented herein. If you have questions or if we may be of further assistance, please let us know. Respectfully Submitted, HEPV/ORTH - PAWLAK GEOTECHNICAL, INC E.I. Engineering Geologist Reviewed by: Steven L. Pawlak, P SWR/vam (tg¡.0 e aa I 9.ao cçt 5 222 q /" 3 JobNo.109016A cåEtecrr Chris Janusz February 5,2009 Page 8 References Dubiel, R. F., 1993, Preliminary Geologic Radon Potential Assessment of Colorado in Geologic Radon Potentiøl EPA Region 8, Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming: U. S. Geological Survey Open File Report 93- 292-H. Frankel, A. D. and Others,2002, Documentationfor the 2002 Update of the National Seismic Hazørd Møps; U. S. Geological Survey Open File Report 02-420. Kirkharr¡ R. M. and Rogers, W. P., 1985, Colorado Earthquake Data and Interpretøtions 1867 to 1985: Colorado Geological Survey Bulletin 46. Kirkhar4 R. M. and Scott, R. 8., 2002,Introduction to Late Cenozoic Evaporite Tectonism and Volcanism in West-Central, Colorado, in Kirkham R. M., Scott, R. Kirkharn, R. M., Streufert, R. K., and Cappa, J. 4., 1995, Geologic Map of the Glenwood Springs Quadrangle, Garfield County, Colorado, Colorado Geological Survey Open File Report 95-3. Nelson-Moore, J.L., Collins, D.B., and Hornbaker, A.L.,1978, Radioactive mineral Occurrences of Colorado: Colorado Geological Survey Bulletin 40 (reprinted as a CD-ROM, 2005). Soule, J.M. and Stover, 8.K., 1985, Sufficial Geology, Geomorphology, and General Engíneeríng Geology of Parts of the Colorado Ríver Valley, Roaring Fork River Valley, and Adjacent Areas, Garfield County, Colorado: Colorado Geological Survey Open-File Report 85-1. JobNo. l09016A cåStecrr