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HomeMy WebLinkAboutGeological Site Assessment for BLD2005-01356 - BLD Engineering / Geo-tech Reports - 6/24/2005 UIV IL W�'L 7-0 ILE GEOLOGICAL SITE ASSESS ENT , Prepared for: Mr. And Mrs. Shane Nagle Prepared by: Craig K. White, Inc. Geosciences Consulting P.O. Box 3398 C - ,. Silverdale, Washington 98383 �` •-� June 24, 2005 MASON COUNry Pmo-L.n4te, c.c;EI Table of Contents Property Location and Description 1 Topography and Drainage 2 Vegetation 3 Geology and Ground Materials 4 Soil Characteristics 5 Slope Stability 6 Potential Seismic Hazards g Conclusions and Recommendations g Summary and Limitations 11 Appendixes: Location Map Appendix 1 Potential Seismic Hazards Appendix 2 Detail: Area of Study Appendix 3 Craig n/' SHORELINE,BLUFF&SLOPE SPECIALISTS Vhite, Il GEOLOGICAL SITE EVALUATIONS GEOTECHNICAL STUDIES,REPORTS&ASSESSMENTS GEOSCIENCES CONSULTING LICENSED PROFESSIONAL GEOLOGIST,WASHINGTON&ALASKA June 24, 2005 GEOLOGICAL SITE ASSESSMENT Tax Prc. # 123093490030 Portion of the E1/2, SE'/4, Section 9, T23N, R01W W.M. Mason County, Washington Property Location and Description: The subject property is located at 415 Bear Creek-Dewatto Rd., near the city of Belfair, Washington, where it occupies a portion of the south-facing hillside slopes above the Union River valley. The property is bounded on all sides by other private lands, with the exception of the southeast corner of the property which fronts on Old Belfair Highway. Property size is approximately 304,000 s.f, or about 7 acres. No existing structures are present on the property, however the new owners are proposing construction of a single family residence in the northwest corner of the lot. A Location Map, Seismic Hazards Map and Detail Plat showing drainage and slope features within the Area of Study are included with this report as Appendices 1, 2, and 3, respectively. P.O.box 3398.-5ILverdaLe, Vashington 98383 (360)830-0718 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 Access to the property is from Bear Creek-Dewatto Rd., approximately 0.5 miles northwest of its intersection with Old Belfair Highway. A gravel access road leads south to the subject property and the proposed building site at the top of the slope. Access to the upper slope was achieved along a narrow logging trail that cuts across the slope face about 100 feet below the building site and afforded a good opportunity to examine the ground materials and assess the stability of the slope. Topography and Drainage: Drainage on the subject property is to the southwest, toward Old Belfair Highway and the Union River valley. In the area of the proposed building site, a minor component of drainage also occurs to the southeast into a shallow saddle, the axis of which lies approximately along the east property line (see Appendix 4). Slope angles were measured at several locations on the property using a Brunton hand clinometer and laser sighting device. In the vicinity of the proposed building site, slopes ranged from less than 10 degrees (17.5 percent) to 13 degrees (23 percent) near the west property boundary. On the slope face below the proposed building site, recorded slope angles were from 3 5 degrees (70 percent) to as much as 40 degrees (84 percent). 2 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 Measured slope angles along the axis of the drainage swale on the east side of the proposed building site, ranged from 12 to 15 degrees (21 to 27 percent). Overall relief on the slope from Old Belfair Highway to the top of the slope at the proposed building site is approximately 180 feet. This site evaluation was conducted in early summer during periods of light to moderate rainfall, however, no springs or seeps were noted along the face of the slope. In several areas, indications of minor surface erosion were present, probably due to uncontrolled water runoff from uphill sources during periods of heavy winter rainfall. This condition underscores the need for good water management in the development of the property. Vegetation; Large-growth vegetation on the subject property consists of conifers(Douglas fir, Western Hemlock and Western red cedar) as well as several varieties of broadleaf trees (Red alder, Bigleaf maple and willow). Lower-growth plants include Himalayan blackberry, Vine maple, Salal, Swordfern, Red huckleberry, Scots broom and Stinging nettle as well as a variety of weeds and grasses in cleared areas of the property. 3 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 Native vegetation in the upland portion of the property, in the area of the proposed building site, has been mostly cleared away, along with much of the surface soil. Along the upper slope, growths of relatively young, conifers, Vine maple and same-age Red alder suggest this area was logged, recently, possibly to provide a view from the proposed building site. Vegetation on the lower portion of the slope, below the logging trail, is well- established with mature conifers, deciduous trees, and a dense understory of lower-growth plants that reflect stability of the slope in this area for at least the last 50 to 75 years. Geology and Ground Materials: A review of existing geologic maps of the area(Carson, 1976) indicates that the subsoil geologic units present at the subject property are glacial and interglacial deposits of Quaternary age, most of which are less than 20,000 years old. Reconnaissance mapping indicates that the upland portion of the property is underlain by Vashon till, a layer of compact cobbles and coarse sand in a binder of clay and silt that commonly ranges from a few feet to as much as 100 feet in thickness. This unit is comprised of materials deposited directly beneath a massive layer of glacial ice that once overrode the Puget Sound region. In many places, the weight of the ice lobe 4 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 compacted these sediments into a concrete-like mixture that is almost always impermeable, but considerably more resistant to erosion than the underlying, unconsolidated sands and gravels. At the base of the slope, sediments along the valley floor are mostly Quaternary alluvium, water-lain sediments deposited in the floodplain of the Union River. This unit is probably underlain by glacial till and outwash deposits from the glacier. Direct examination of the ground materials at the subject property generally supports the reconnaissance mapping of the area. A layer of buff-colored, poorly-sorted cobbles, pebbles, sands and silts is present in three soil log test pits on the property near the proposed building site, excavated to depths of 60 inches. These sediments were probably derived from the underlying Vashon till. The till layer is exposed in a number of patchy outcrops along the upper slope face where road cuts are present on the uphill sides of the logging trail. Due to poor sorting, a high percentage of interstitial clays, and compaction by the overlying ice, this unit probably has low permeability at some depth below the soil layer. However, fractures and minor zones of porosity may occur that can produce seeps and springs that"daylight" along slope face where this unit is exposed. Soil Characteristics: 5 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 The principal soil type in the upland area of the subject property as identified on Mason County's Soil Survey maps is Everett gravelly sandy loam, 15 to 30 percent slopes. The Everett series consists of somewhat excessively-drained, pale brown, gravelly soils that occur as gravel ridges on glacial moraines and outwash channels between ridges, where they developed from assorted glacial till and outwash material. Everett soils are classified as"droughty" because the loose gravel, sandy soil and substratum offer little resistance to the downward movement of water and the capacity of the surface soil to hold available moisture is low. These soils on the steeper slopes at the subject property are more variable than at lower slope angles. Depth to the substratum ranges from 12 to 36 inches which, in places, is compact and weakly-cemented. This soil has low fertility and is unsuited for raising crops. Its main use is for forestry and it is well-suited for growing Douglas firs that are cut for Christmas trees. No"Severe" classifications for this soil were identified nor were limitations described for urban development. Slope Stability: The subject property is not shown on Washington State Department of Ecology Coastal Zone Management (Slope Stability) maps which extend inland only about a mile inland from the shoreline. The presence of slopes greater than 15% on the property 6 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 supports its designation as a"Critical Area" based on Mason County building codes. Direct examination of the hillside slope at the subject property reveals several areas where minor erosion and sloughing of bluff materials have occurred. These areas are mostly along the uphill side of the over-steepened logging trail, below the proposed building site, where debris from the upper slope face has accumulated. In many cases, these areas have been re-vegetated with young, same-age alder that is quickly established after a slide occurs. On the lower slope, below the logging trail, indications of only minor erosion are present as this area appears to be well-protected by dense growths of vegetation that are effective in holding the soils in place. In several areas along the slope face, the trunks of old-growth trees are present in an undisturbed state, indicating this area has probably been stable for many years. The well-compacted nature of Vashon till makes this geologic unit somewhat more resistant to erosion than the more poorly-indurated, interglacial sediments that underlie it. However, erosion, sloughing and minor slides may still occur periodically along the steeper portions of the slope face. For this reason, establishing an appropriate setback for structures from the top of the slope and adopting a good storm water management plan are important factors in the development of this property. 7 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 Potential Seismic Hazards: The Puget Lowland has been classified as a Seismic Zone III(moderately high risk) by the Uniform Building Code (UBC). In 2003, the state of Washington replaced the UBC with the International Building Code(IBC)which provides guidelines for construction based on"Seismic Design Categories" rather than seismic zones. Architects and builders should refer to these guidelines when designing structures in this area. Crustal earthquakes, which occur along surface faults, have been documented in several areas of the Puget Sound region (see Appendix 3: Seismic Hazards Map). However, the largest earthquakes in this region have been subcrustal events, ranging in depth from 30 to 60 miles or more. These deeper-focus earthquakes are not related to surface faults but are usually caused by movements along plate boundaries or within deeper sub-crustal slabs. Although the mapping of faults and study of earthquakes in the Puget Sound region is a continuing effort, the potential effects of a large seismic event on hillsides and slopes in this area are still not completely understood. Conclusions and Recommendations: The observations and interpretations outlined in this report support the 8 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 classification of the hillside slopes in this area as potentially unstable and indications are that erosion and sloughing of ground materials along the slope face at the subject property will probably continue to occur. Despite these conditions, given an adequate setback from the top of the bluff for proposed structures, proper management of storm water runoff, and preservation of natural vegetation on and in the vicinity of the slope, we see no reason why the proposed development of this property should not occur. Our specific recommendations are as follows: 1. It is recommended that a minimum setback distance of 35 feet from the upper slope edge be observed for all structures. Within this setback area, we also recommend that a 25-foot natural vegetation buffer be adopted where the native soils not be disturbed and the re-growth of deep-rooting native plants be encouraged. Larger trees that may threaten the safety of structures or impede a limited view corridor may be cut or topped, as necessary, however the stumps and roots systems of such trees should be left in place as they will help hold the ground materials in place and prevent erosion for a period of time. 2. An effective water collection system (gutters and downspouts) should be installed on all new structures with collected water tightlined downhill to the logging trail at the base of the upper slope. Buried portions of tightlines should utilize 4- or 6-inch, 9 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 smoothwall, sewer/drainpipe firmly cemented at the joints. In the event it is discovered that too much water is being discharged from the tightline system onto the logging trail, causing ponding or erosion along the lower slope face, this water should be contained and tightlined to the base of the slope near the south property line. 3. Because of the impermeable nature of the ground materials, grading and site preparation should be conducted in a manner that will allow water from uphill sources to flow away from the proposed structure. Consideration should also be given to installing a foundation drain on the uphill side of the structure to intercept surface and near-surface runoff water from higher elevations that might, otherwise, collect near the foundation. The outlet ends of this drain can be connected to the tightline system. 4. Efforts should be,made to promote and maintain a healthy growth of deep- rooting vegetation, wherever possible, along the slope face to aid in retaining surface soils and reduce the effects of erosion from storm water runoff. 5. Frequent inspections of the slope should be made, especially after periods of heavy winter rainfall, to determine the location of any new slides or areas of undercutting and to identify places where excessive water runoff or erosion may be occurring. Recording the appearance of potentially unstable areas in photographs, taken each year, 10 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 and comparing them with current conditions may assist in this evaluation. Summary and Limitations: Although the development of properties along hillside slopes in the Puget Sound region is common, it should be acknowledged by property owners that these areas may be inherently unstable and involve higher risks than other areas due to the steepness of slope faces and the unconsolidated nature of the ground materials. Frequent and severe winter storms commonly produce high volumes of water runoff that may cause extensive surface erosion, saturate ground materials and destabilize slopes, resulting in ground movements that often occur without warning. Given these conditions, it should be expected that slides, sloughing of ground materials and erosion along the bluff f F� g gr ace at the subject g J property will continue occur. While the potential for larger-scale movements, such as may be precipitated by a seismic event, are not well-understood in this area, the hazards that may be posed by such an occurrence should also not be ignored. This report has been prepared for the exclusive use of Mr. and Mrs. Shane Nagle for specific application to the referenced site, and the conclusions and recommendations presented, herein, should be applied in their entirety. Within the limits of scope and 11 Geological Site Assessment Mr.And Mrs. Shane Nagle June 24, 2005 budget, this study was conducted in accordance with generally-accepted practices employed at the time this work was done. No other warranty of conditions is expressed or implied. You should also be aware that these conclusions and recommendations are based on a general knowledge of this area and the interpretations of surface and subsurface conditions as they are believed to exist. These conditions may, in fact, be different than interpreted and events may inevitably occur that were not predicted. Clients should also be aware that there is a practical limit to the usefulness of this report and recommendations without a site re-evaluation and critical review. Although this time limit is somewhat arbitrary, it is suggested that 2 years be considered a reasonable limit for the usefulness of this report. Wash/. r6 525 `y Cr 'g te, Engineering Geologist �S e d G e°�° June 4, 2005 CRAIG K. WHlTE 12 APPENDIXES Appendix 1: Location Map Appendix 2: Potential Seismic Hazards Appendix 3: Detail: Area of Study t r r . i rYiOVA s + W dmrzdo ! Append.i*x,., , Llocf • a tion 'Map ��• � yf fD fw � Y � • y� y�,� b.•�. y�A �• VV�iV//llJll�, i 3 � Ik -f .�•��{?7�•,. ' •••�+..:• • :l'-�+fit•. .:s� ' `dqf' r. � .�� .,. •�.•."y �• ' ...--yyy rmL r 1 : Is 1 r : t �a Proposed Building Site Pro oosed Drainfield JOE Upper Slope op of ON �� •.• .`Drainage Swale Trall �. .. .. , v Subject Property Lower Slope C 7b0/lh �4~ MAP EXPLANATION Measured Slope Angle (Degrees) Arrows Indicate Approximate Direction of Drainage A Soil Log Test Pit Appendix 3 Detail: Area of Study Note:Sketch not to scale. Distances and relative positions of objects is approximate.