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HomeMy WebLinkAboutGeotechnical Assessment - GEO General - 5/20/2008 4 GEOTECHNICAL ASSESSMENT REPORT Prepared for: HAROLD & BA"ARA CAREY P.O. Box tW 3-?P WASHINGTON 98524 Prepared by: ROSENDAHL ENGINEERING, INC. 3388 BYRON ST SUITE 200 SILVERDALE,WASHINGTON 98383 Project No.RE 2009 May 20, 2008 S�2 0/0 '6 OS� WAsg�ycl r 42329 w� �cIS' - �`SSIONALti�G� � g Rosendahl Engineering•3388 Byron St•Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 Page No. ii TABLE OF CONTENTS Page LIST OF FIGURES W 1.0 INTRODUCTION 1 1.1 Site Location 1 1.2 Proposed Construction 1 1.3 Purpose 1 2.0 SITE INVESTIGATION 1 2.1 Site Description 1 2.2 Geologic Setting 2 2.3 Subsurface Exploration 3 2.3.1 Soil 3 2.3.2 Groundwater 3 2.4 Seismic Information 3 3.0 CONCLUSIONS AND RECOMMENDATIONS 4 3.1 Erosion Control 4 3.2 Drainage 4 3.3 Earthwork Considerations 4 3.3.1 Site Preparation 4 3.3.2 Groundwater Concerns 5 3.3.3 Excavations&Constructed Slopes 5 3.3.4 Structural Fill 5 4.0 LIMITATIONS 6 Rosendahl Engineering •3388 Byron St • Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 , Page No. iii LIST OF FIGURES Figure No. Title 1 Site Vicinity Map 2 Soils Map 3 Hazard Map Rosendahl Engineering •3388 Byron St•Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 • Project No.RE 2009 May 14,2008 Page No. 1 1.0 INTRODUCTION Rosendahl Engineering was retained by Harold & Barbara Carfy to conduct a geotechnical Assessment investigation for the logging project near Allyn,Washington. 1.1 Site Location This site covers approximately 9.79 acres near Allyn, Washington. The subject property is located North of E Wheelwright. The logging project consists of 18 parcels. According to the United States Geologic Survey(USGS),the property is located in Mason County in Section 12, Township 22 North, Range 1 West and at Longitude 122.83 degrees W and Latitude 47.38 degrees N. The approximate location of the site is shown on the Site Vicinity Map,Figure 1. 1.2 Proposed Construction Based on our review of the erosion control plan by C&C Engineering and our discussions regarding this project with the owner, we understand that the proposed project is for logging only. The current plan does not show any proposed locations houses, within the existing lots. We expect that the homes will be eventually built on these lots and that they will be typical of single family residential construction. We anticipate that the houses will be one to three story wood framed structures with shallow foundations. Due to the impact of the proposed logging on existing lots the report addresses slope stability concerns associated with the logging and impacts on future development. 1.3 Purpose The purpose of this investigation was to evaluate the relatively isolated steep slopes and their impact on the existing lot's and there future development. 2.0 SITE INVESTIGATION 2.1 Site Description The subject property for the logging is bordered by undeveloped forested area to the east,undeveloped forested area to the north,undeveloped forested area to the west and undeveloped forested area and existing residential area to the south. The property within the proposed logging is currently undeveloped and covered with trees. A section of cut is present in the east portion of the site that has the steep slopes which is the location of interest. The area of previously cut is covered with Scotch Broom, at the time of the site visits for this phase of services. The site generally slopes downward from east to west with an elevation change of approximately 160 feet across the entire site. The majority of the site slopes are less than 15 percent. A existing slope of 20 percent appears to be a borderline case for determining if slope stability analysis is required. We have included an approximate analysis as part of this evaluation. At the time of the on site exploration services, Rosendahl Engineering reviewed the site surface conditions to look for signs of slope movement. Such signs can include hummocky terrain, head scarps, curved tree trunks, tension cracks, seepage of water and strata of course grained soils over fine grained materials. During our site visit we did not observe any obvious signs of significant slope movement. Rosendahl Engineering • 3388 Byron St• Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 Project No.RE 2009 May 14,2008 Page No.2 2.2 Geologic Setting The Washington Division of Geology and Earth Resource (WDGER), Geologic Map of Washington - indicates that the site is underlain by glacial till (Qgt). The glacial till typically consists of an unsorted, non-stratified mixture of sand, silt, gravel and cobbles. The till is typically very dense or hard, as a result of compaction under the weight of the glacial ice. The soils encountered at this site are typical of advance outwash and glacial till. The advance outwash is typically present below the glacial till, since this material was deposited in advance of the glacier and then was overridden by the glacial ice. The advance outwash is typically comprised of relatively stratified sand and gravel with occasional silt and clay interbeds. The United States Department of Agriculture (USDA) Soil Survey of Mason County Area, Washington, information indicates the following soil type exists on the project site: Ab-Alderwood gravelly sandy loam, 5 to 15 percent slopes. The soil survey descriptions of these soil types are summarized in the following table. Mason County, Washington Version date: 12/5/2006 3:15:57 PM Ac—Alderwood gravelly sandy loam, 15 to 30 percent slopes Map Unit Setting Elevation: 50 to 800 feet Mean annual precipitation: 25 to 60 inches Mean annual air temperature: 48 to 52 degrees F Frost free period.• 180 to 220 days Map Unit Composition Description of Alderwood Map Unit SymbolMap Unit Name Acres in AOIPercent of AOI Ab Alderwood gravelly sandy loam, 5 to 15 percent slopes 21.2 50.7% Ac Alderwood gravelly sandy loam, 15 to 30 percent slopes16.4 39.2% Setting Landform: Drainageways Parent material: Basal till with a component of volcanic ash Properties and qualities Slope: 15 to 30 percent Depth to restrictive feature: 24 to 32 inches to dense material Drainage class: Moderately well drained Capacity of the most limiting layer to transmit water(Ksat):Very low to moderately low(0.00 to 0.06 in/hr) Depth to water table: About 18 to 36 inches Frequency offlooding:None Frequency of ponding.None Rosendahl Engineering • 3388 Byron St• Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 Project No. RE 2009 May 14, 2008 Page No. 3 Available water capacity: Very low(about 2.5 inches) Interpretive groups Land capability (nonirrigated): 4e Typical profile 0 to 10 inches: Gravelly sandy loam 10 to 28 inches: Very gravelly sandy loam 28 to 60 inches: Gravelly sandy loam 2.3 Subsurface Exploration No exploratory test pits were excavated at the site on May 10, 2008 as no evidence of instability was found. An existing vertical cut of 4 ft was found and examined, it was glacial till. The property owner stated the cut had been open since 1993 and has remained vertical. There was no evidence of sluffing. 2.3.1 Soil No organic material was observed in the open cut. 2.3.2 Groundwater No evidence of groundwater seepage was found on the site visit of May 10, 2008. 2.4 Seismic Information Rosendahl Engineering has reviewed table 1613.5.2 of the 2006 International Building Code (IBC). Site specific data is not available to a depth of 100 feet. No Standard Penetration Test (SPT) N-values were obtained during the field exploration work. The exploration at this site was restricted to observation of cuts which have estimated N-values between 15 and 50. Therefore, for seismic consideration of the design of any structures, the site should be considered class D, "stiff soil profile",as defined in the IBC. Ground motion accelerations for the site were obtained from the USGS Earthquake Hazards Program website and are presented in the following table. The latitude/longitude method was used to obtain the ground motions with a latitude of 47.38 degrees and a longitude of 122.83 degrees using the 2002 data tables. Probability of Exceedance 2% in 50 years Peak Ground Acceleration(PGA) 0.573 0.2 second period SS 0.844 1 second period(S,) 0.477 In addition to the sprectral acceleration values noted in the table above,we have provided the seismic design parameters Fa and F„in accordance with the 2003 NEHRP Seismic Design Provisions. Fa 1.00 From Table 1613.5.2(1)of the 2006 IBC Fv 1.53 From Table 1613.5.2(2) of the 2006 IBC Rosendahl Engineering •3388 Byron St•Silverdale,WA 98383 Office(360)692-3802 Fax(360)692-1051 Project No. RE 2009 May 14, 2008 Page No. 4 3.0 CONCLUSIONS AND RECOMMENDATIONS General Based on the findings of this investigation, it is our opinion that the proposed logging and future residential development is feasible. It is also our opinion, based on approximate global slope stability analyses, that the slope will not be impacted by logging. 3.1 Erosion Control The soils on the site slopes may erode in the disturbed state or under conditions of channelized water flow. Therefore, best management practices for erosion control including silt fences, hay bales, etc. should be used to prevent sediment from leaving the site and entering storm water sewer systems or surface waters. Water should not be allowed to flow over the slopes in a concentrated manner. Stripping of vegetation on slopes should be limited to the greatest extent possible for the proposed construction. We further recommend that vegetation be replanted on the slopes as soon as practical following completion of any grading. Stripped slope areas should be protected from weather with a plastic visqueen cover when construction will not be occurring on them for more than one to two days. The Washington State Department of Ecology (DOE) has three publications, which may be helpful in developing long- term slope vegetation maintenance/protection and landscape plans: • "Slope Stabilization and Erosion Control Using Vegetation: A Manual of Practice for Coastal Property Owners", May 1993, Publication 93-30. • "Vegetation Management: A Guide for Puget Sound Bluff Property Owners", May 1993, Publication 93-31. • "Surface Water and Ground Water on Coastal Bluffs: A Guide for Puget Sound Property Owners", June 1995, Publication 95-107. 3.2 Drainage Runoff from any residential buildings and impervious surfaces should be directed into an appropriately designed stormwater disposal system. Ground surfaces should be sloped a minimum of 3 percent for a minimum distance of 10 feet away from structures in accordance with Section 1803.3 in the 2006 International Building Code(IBC). 3.3 Earthwork Considerations We expect that any additional on site grading will be limited. During wet weather conditions, which are typically present from October through April, subgrade stability problems and grading difficulties may develop due to high moisture content in the soil, disturbance of sensitive soils and/or the presence of perched groundwater. Therefore, we recommend that any proposed earthwork activity be performed during the dry season. 3.3.1 Site Preparation Any additional clearing in required areas should include removal of vegetation, trees and associated root systems, wood, pavement, retaining walls, rubble, and rubbish. Site stripping should extend to a minimum depth of 6 inches, or until all organics in excess of 3 percent by volume or other unsuitable soils are removed. These materials will not be suitable for use as fill for roadway or building areas. However, stripped topsoil may be stockpiled and reused in landscape or non- structural areas. Rosendahl Engineering • 3388 Byron St•Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 Project No.RE 2009 May 14,2008 Page No. 5 Any buried structures encountered during construction should be properly removed and backfilled. Excavations, depressions,or soft and pliant areas extending below planned finish subgrade level should be cleaned to firm, undisturbed soil and backfilled with structural fill to the planned finish subgrade elevation. 3.3.2 Groundwater Concerns We do not expect that significant groundwater will be encountered during construction. Perched groundwater in the near surface soils, particularly at the interface with less permeable underlying soils, if present, could develop during the wetter portions of the year. If groundwater is encountered, we should be contacted for further recommendations. Significant groundwater flow, if encountered during construction,would require modifications in the completion of excavation work. 3.3.3 Excavations & Constructed Slopes We expect that future construction will involve limited excavation depths. It is our opinion that the soils encountered in the test pits are a Type C material as defined by the Washington Industrial Safety and Health Act's(WISHA)regulations on excavation, trenching and shoring. The Type C classification in the medium dense to dense fill material is given, primarily due to the fact that these soils are in place as a constructed fill section. Temporary slopes excavated in Type C material should be inclined no steeper than 1.511:1V (horizontal: vertical). A representative of our firm should evaluate temporary and permanent slopes to verify that they are appropriate for the soils encountered during the construction. In areas where it is not possible to maintain the recommended slopes due to space constraints, temporary shoring may be required. Such shoring would need to be properly designed by an engineer. The Contractor should be familiar with applicable local, state, and federal safety regulations, including the current WISHA regulations on excavation, trenching and shoring. Construction site safety is the sole responsibility of the Contractor, who shall also be solely responsible for the means, methods, and sequencing of construction operations. Rosendahl Engineering is providing this information solely as a service to our client. Under no circumstances should the information provided above be interpreted to mean that Rosendahl Engineering is assuming responsibility for construction site safety or the Contractor's activities;such responsibility is not being implied and should not be inferred. The soils to be penetrated by the proposed excavations may vary significantly across the site. Rosendahl Engineering's preliminary soil classification is based solely on the materials encountered in the test pits. The Contractor should continually classify the soils that are encountered as excavation progresses with respect to the WISHA system. 3.3.4 Structural Fill We expect that any fill placement will be limited in depth and aerial coverage. The majority of the on site soils will likely be suitable for use as structural fill, however the fines content does indicate moderate moisture sensitivity. We recommend that any earthwork be performed during the normally dry period of the year. If earthwork takes place during wet weather periods,the on site soils may not be suitable to achieve compaction and moisture content requirements. If the earthwork is to take place during the normally wet period of the year,provisions should be in place for the possible export of wet, moisture sensitive soil and import of granular structural fill material. Imported structural fill should consist of well-graded gravel and/or sand with a maximum grain size of 1'/z inches and less than 5 percent fines(material passing the U.S. Standard No. 200 Sieve). All material proposed for use as structural fill should be approved by a representative of the geotechnical engineer. Structural fill should be placed in loose lifts no more than 12 inches thick, moisture conditioned as necessary (moisture content of soil should be within 2 percent of optimum moisture)and compacted to 95 percent of the maximum dry density as determined by ASTM Test Method D-1557. Additional lifts should not be placed if the previous lift did not meet the required dry density or if soil conditions are not stable. Note that, although in place density testing of fill is frequently used as the primary criterion for acceptance of fill, it should not be the only criterion. If, in the judgment of the geotechnical engineer or his representative, placed fill is not suitable it should be rejected regardless of in place density test results. As an example, fill that is compacted wet of the optimum moisture content may exhibit"pumping"behavior Rosendahl Engineering - 3388 Byron St- Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 Project No.RE 2009 May 14,2008 Page No. 6 even if in place density test results indicate greater than 95 percent compaction has been achieved. In such a situation,the fill should be removed and replaced with drier material. 4.0 LEVIITATIONS This report has been prepared for Harold & Barbara Carey regarding the subject project. Information presented in this report has been collected and interpreted in a manner consistent with the level of care and skill ordinarily exercised by members of the profession currently practicing under similar conditions, and in accordance with sound and generally accepted principles consistent with normal consulting practice. No other warranty, expressed or implied, including (but not limited to)any warranty or merchantability or fitness for a particular use has been made. Harold& Barbara Carey and Rosendahl Engineering discussed the risks and rewards associated with this project, as well as Rosendahl Engineering's fee for services. Harold & Barbara Carey and Rosendahl Engineering agreed to allocate certain of the risks so that, to the fullest extent permitted by law, Rosendahl Engineering's total aggregate liability to Harold & Barbara Carey is limited to $5,000 or the fee, whichever is greater, for any and all injuries, claims (including any claims for costs of defense or other incurred costs), losses, expenses, or damages whatsoever arising out of or in any way related to Rosendahl Engineering's services for this project, from any cause or causes whatsoever, including but not limited to, negligence, errors, omissions, strict liability, breach of contract, breach of warranty, negligent misrepresentation,or other acts giving rise to liability based upon contract tort,or statute. In the event that change in the nature, design,or location of the proposed construction is made, or any physical changes to the site occur, recommendations are not be considered valid unless the changes are reviewed by Rosendahl Engineering and conclusions of this report are modified or verified in writing. The exploration and related information depicts conditions only at the specific locations and at the particular time designated. The passage of time may result in a change of subsurface conditions at these exploration locations. Subsurface conditions at other locations may differ from conditions occurring at the exploration locations. The nature and extent of variations of subsurface conditions between explorations are not known. If variations appear during additional explorations or construction,reevaluation of recommendations in this report may be necessary. Stratification lines designating the interface between soil types in exploration logs represent approximate boundaries. The transition between materials may be gradual. Analyses and recommendations provided in this report are based in part upon the data obtained from the explorations. The scope of Rosendahl Engineering services did not include an environmental assessment for the presence or absence of hazardous and/or toxic materials, in the soil, groundwater, surface water, or atmosphere. Any statements or absence of statements in this report on any subsurface exploration log regarding staining or odor of soil, groundwater, or surface water,unusual or suspicious items,or conditions observed are strictly descriptive information for Harold&Barbara Carey Rosendahl Engineering • 3388 Byron St• Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 APPENDIX A Rosendahl Engineering•3388 Byron St•Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 Ea a � W � �Q h J 20 h � f y 3� WWJ Olt r 3 Q W J CASE INLET N� i Zp fj E BR44 y Lapp 0 469ft f /dpael e j py /2 'el /7 — C76'1736 Rosendahl Engineering•3388 Byron St• Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 A �> T y OOW • ... k t F f' Additional Information Rosendahl Engineering•3388 Byron St•Silverdale,WA 98383 Office (360)692-3802 Fax(360)692-1051 tnr rwr TFIr tt i! 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