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HomeMy WebLinkAbout601332017 Geotech AssessmentSteve S. Luxton, Geotechnical Engineer 672 West Anderson Road Sequim, WA, 98382 (360) 797 1901 ssluxton@yahoo.com Mr. Steven Penn February 21, 2023 9136 Genesis Ln. Port Orchard, WA. 98367-9775 Subject:Geological Report - Site Assessment Jefferson County Parcel # 601 332 017 Steve S. Luxton, a registered geotechnical engineer, completed a geologic reconnaissance of Jefferson County Parcel # 601 332 017 located on the Toandos Peninsula in Jefferson County, Washington. The parcel is within the northwest quarter of Section 33, Township 26 N., Range 1 W., W.M. The western boundary of this five-acre parcel runs along an unnamed stream that flows into Dabob Bay. The steeply-sloped ravine ( see LIDAR image below) that lies along the west side of the cleared site is forested with Douglas fir, western red cedar, maple and alder trees with a thick understory of evergreen huckleberry, salal shrubs and sword ferns. The stability of the ravine’s slopes, which terminate near the edge of the building site where wastewater facilities are planned, was the focus of our office and on-site study completed in February of 2023. ( See photograph of the site on the following page. ) The current owners, Coast Guard Captain Steve Penn and Kathy Penn, intend to build a cabin at the site and wastewater facilities positioned as sketched on Attachment 1. The photograph on the following page was taken from the center of the proposed cabin site. The westerly portion of this parcel has been designated as a Critical Area by Jefferson County and it has been mapped as having a “Slight Geologic Hazard” The slopes within the ravine were first classified as “unstable” in the Washington State Coastal Atlas. That designation probably arose from the many landslides that were observed along the bluffs and within the drainage basins along the west side of the Toandos Peninsula. 1.0 SUMMARY OF FINDINGS Due to the presence of steep slopes and the frequency of landslide occurrence in the formation described below, we find that the designation “Moderate Geologic Hazard” is appropriate for the slopes on the ravine below this site. In accordance with Jefferson County Development Code, a 65-foot total geo-hazard setback line was staked at the site.( See photo below.) This total setback includes the required 25-foot native plant buffer plus forty (40) additional feet of hazard setback. The buffer and setback will protect the proposed on-site wastewater facilities and the drainfield from future landslides or damage from earth movements. The setback will reduce the impact of the drainfield on the 65-foot high steep slopes. These slopes are within the parcel boundary and they can be permanently protected by the owner. 1 BUILDING SITE - THE RIM OF SLOPE IS BEYOND THE TREE LINE AS SEEN FROM CABIN SITE The photograph above was taken from the proposed cabin footprint. The white stakes in the photograph above mark the forward ( westerly) building line. The cabin site, as now staked, is over 250 feet from the rim of the slope on the west beyond the tree line in this view. In this position, the building site is well upslope of the landslide hazard zone and it is a safe location for the construction of a single family residence. While some mostly unpredictable regional earthquake hazards exist as outlined below,in our opinion, the recommended building zone for the wastewater facilities beginning at sixty-five (65’) from the rim of the slope will be free from landslide or earth movement hazards for the foreseeable future.( See Attachment 1.) The vegetated buffer at the rim of the slope was damaged during the clearing of the site, though much of it was kept intact. A restoration plan was discussed with the owner on site and some suggestions for the use of native plants for restoration are provided below. Drainage recommendations and the local seismic hazard are discussed below for completeness. 2.0 SUMMARY OF LOCAL LANDSLIDE AND EARTH MOVEMENT HAZARD 2.1 Soils The soils of this area were investigated by the Soil Conservation Service in 1974-5 and mapped here as part of the Hoypus Series. Observations of gravelly sand soils at the site suggest that these are Hoypus Gravelly Sand Loam soils that were formed on glacial outwash late in the Fraser Glaciation of the most recent Ice Age. It is likely that the gravels arrived at the site by joining the copious flow of meltwater coming from thick stagnant glacial ice wasting 2 on the upland areas of the Toandos Peninsula. Test pits at the site showed roughly 40 inches of gravelly sand with silt and sandy gravel with silt underlain by cemented hardpan that is likely to be glacial till. Both are part of the “Qgic” surficial geologic unit described below. A RECENT LIDAR IMAGE OF THE RAVINES AND THE SUBJECT PROPERTY 2.2 Local Occurrence of Landslides In Similar Geologic Setting Slopes of over 50 degrees from the horizontal ( 120% ) were noted in the walls of the ravine lying directly west of the development area with some parts of the slopes perhaps 10 degrees steeper. A second ravine on the north has less severe slopes. No active landslides or exposures of the parent formations were noted during our reconnaissance of the ravines, but block slumps were seen on the north in a spur ravine. Micheal Polenz and others, working in 2017, noted a significant slide on the west side of the ravine and slide debris in a spur ravine on the north. Light detection and ranging radar ( LIDAR ) images and mapping history of the slope and the region were studied for clues on the local landslide probability Both of the slides occurred in the Qco geologic unit and both are visible on high resolution LIDAR images. The “ancestral” ravines were carved out by water erosion late in the ice ablation period at the close of the Fraser Glaciation when huge volumes of ice meltwater were flowing in them. Stream flow and erosion are both much lower now. The relevant portion of the 1 to 24,000 Seabeck Geologic Map (Polenz and others; 2017) is shown below. The “Qgic” designation of the green-gray areas on the map are the youngest and most recent surface deposits. The Qad and Qco units are sediments from the Olympic Mountains that were deposited before the Fraser Stade ice advanced over the Toandos Peninsula from the north some 10,000 years ago. The Qad deposits include glacial till and glacial outwash from the alpine glaciers of the Olympic Mountains. Many of the landslides on the Toandos Peninsula occur in the Qco unit. The Qco sediments include some lacustrine (lake-bottom) silts, clays, sands and gravels, materials that lack the strength that is typical of the overlying glacial till and related ice-contact deposits. These Qco 3 sediments were probably when alpine glaciers in the Olympics had advanced into the Puget Sound Lowland during a long period when the Fraser Ice sheet was advancing from the north. Landsliding, slumping and soil creep have found favorable condiitons in Qco deposits. Jim Yount and others, working for the DNR, reported that at least 25 landslides were observed along the west side of the Toandos Peninsula during their mapping work. (See Commentary on DNR’s Seattle Quadrangle Geologic Map of 1993.) Compact silt deposits within the Qco geologic unit impedes the vertical drainage of percolating groundwater adding the instability. The undersigned geotechnical engineer has observed this in several locations on the Toandos Peninsula. His work has taken him to a dozen landslides along the shoreline and ravines. A mass wasting site is visible on the north where slump blocks appear on mid-slope positions. A moderately sized landslide occurs on the opposite side of the ravine on the north. Although many slopes in the geologically similar region show significant landslides, no large slumps or landslides or their head scarps were seen in the images of the proximate area. Field examination of the slopes did not reveal unusual moisture, springs or seepage that would suggest the presence of an impermeable silty stratum within the slopes west of the cleared site. A PORTION OF THE GEOLOGIC MAP OF THE AREA OF PARCEL ( DNR; POLENZ, 2017) The occurrence of many landslides in the geologically similar region and the steepness and the 60 to 70-foot height of the slopes adjoining the cleared building site are the main indicators in this assessment. Also considered was the possibility of sudden slope failures in the event of regional or locally driven seismic activity, discussed below. Thus, the undersigned Geotechnical Engineer assigned a moderate landslide hazard to slopes lying west of the cleared building area.A total setback of sixty-five (65’) feet made up of a 25-foot vegetated buffer plus an additional forty (40’) feet of hazard setback is recommended and was staked at the site. 4 3.0 SUGGESTIONS FOR RESTORATION OF THE VEGETATED BUFFER Some damage occurred to the vegetated buffer during the clearing work. The owners intend to restore these areas in the near future. Dry soil conditions should be expected along the rim of the slope at this site in summer and early fall due to its exposure. Thus, November through March, when soil moisture is assured, is the best time for planting of the trees and shrubs due to the availability of moisture during root development. November or December planting gives the tree saplings and shrubs the longest possible time for root development before the onset of the next summer’s heat and drought. The disturbed portion of the buffer can be planted with Western Red Cedar and Big Leaf Maple saplings. Cedar is superior in soil stabilization. Incense Cedar trees will do well here and they can produce a striking visual result, especially when planted in attractive groups. A ground cover of sword ferns may be planted along with evergreen huckleberry to reproduce the damaged portion of the understory. Sword ferns will do well in holding the soil here if some shade is available when they are first transplanted. Additional sword ferns may be planted over a period of years as more shade becomes available. Flowering Red Currants may add color and attract hummingbirds. Salal shrubs will take time to establish themselves, but it can provide excellent soil anchorage. The Department of Ecology’s publication “Slope Stabilization and Erosion Control Using Vegetation”includes detailed suggestions for plantings.This publication on soil stabilization is online at:https://apps.ecology.wa.gov/publications/documents/9330.pdf STAKED GEOHAZARD SETBACK LINE WITH BUFFER AND RIM OF BLUFF BEYOND 5 4.0 SUMMARY OF REGIONAL EARTHQUAKE HAZARD Sudden rupture of geologic faults is a common source of earthquakes. Rupture and seismic shaking occur when the accumulated stresses due to tectonic movement of the earth’s crust are suddenly released by sudden rupture along the fault zones. Earthquake hazard at this site is significant and is outlined below. 4.1 Shallow Crustal Earthquakes Shallow crustal earthquakes are the most common type of earthquake. They are usually relatively low in magnitude but shaking near earthquake sources can be strong enough to damage buildings. There are at least ten different crustal faults in the Puget Sound and Olympic Peninsula regions that may generate earthquakes. Geologists suggest there is a 15% chance that a magnitude 6.5 or greater shallow crustal earthquake may happen sometime in the next 50 years. At least two of these earthquake-generating faults are located close to the subject property and are briefly discussed below. ( See fault location map below. ) The recently discovered Dabob Bay Fault Zone is exposed along the beach about a mile north of the outlet of the creek flowing below this parcel. ( See map below.) The fault has also been traced by imaging the deeper bedrock. This reverse fault is thought to have resulted in several feet of sudden uplift along the fault line. Preliminary evidence from the geometry of the fault suggests that the fault’s rupture and the related earthquake occurred sometime within the last 100,000 years. No firm information on earthquake magnitude or probability is available yet. The Southern Whidbey Island Fault is a significant reverse fault running along the west side of Whidbey Island a dozen miles northeast of the subject site. There is evidence that this crustal fault has produced energetic earthquakes at least four times since the ice retreated from the region some 12,000 years ago. The magnitude of earthquakes resulting from some of these ruptures of this fault has been estimated to have been from 6.0 to 7.5 with the most recent earthquake about 2700 years ago. A rupture of this fault could lead to significant earth motion at the subject property due to its proximity and shallow depth. The Seattle Fault Zone is a compressional thrust fault that caused the south end of Bainbridge Island to thrust upward. The fault is believed to run along a line south of the site as shown on the map below. Geologists think that a major earthquake of magnitude around seven occurred about 1100 years ago on this fault. The abrupt change from glacial sediments to Crescent Basalt rock at Restoration Point on Bainbridge island gives a clear indication that a sudden uplift has occurred there. 6 MAP SHOWING NEARBY DABOB BAY AND SEATTLE FAULT ZONES ( DNR; 2017) 4.2 Deep Earthquakes In the last 100 years, six deep earthquakes with magnitudes greater than six have occurred about every 10-30 years in the Pacific Northwest region. The most recent significant deep earthquake to occur was the 2001, magnitude 6.8, Nisqually Earthquake. Geological scientists currently believe there may be an 84% chance that a magnitude 6.5 or greater deep earthquake will happen sometime in the next 50 years. The epicenter of this type of earthquake is likely to be at some distance from this parcel. Located about 70 miles offshore of the west coast of North America, the Cascadia Subduction Zone is a major tectonic boundary that spans a distance of 620 miles (1,000 km) from northern California to British Columbia. This plate boundary is currently in a locked state and is building up stress until the next subduction zone earthquake is triggered and releases the stress. Unlike shallow crustal earthquakes and most deep earthquakes, subduction zone earthquakes are believed to have magnitudes greater than 8 and they can cause continuous shaking that lasts several minutes. The last time a Cascadia Subduction Zone earthquake occurred is believed to be 1700 A.D. and historical records of large tsunami waves in Japan strongly suggest that it was a magnitude 9 earthquake. The recurrence interval for magnitude earthquakes of this very large magnitude varies along the length of the subduction zone, but in northern Washington big earthquakes probably occur about every 500 years or so. Geological scientists currently believe there is a 10% to perhaps 14% chance that a large earthquake will happen in the Cascadia Subduction Zone sometime in the next 50 years and about one chance in three that this earthquake could be magnitude 8 or greater. With the subject property being located 180 miles away from the subduction zone, ground shaking from a Cascadia Subduction Zone earthquake would be less intense compared to 7 places located directly on the Pacific coast. Nevertheless, the event could be significant on the Toandos Peninsula. On the Modified Mercalli Intensity (MMI) scale, ground shaking at the property is predicted by the Washington Geological Survey to be ‘very strong’, or with an MMI rating of 7. Moreover, the seismic motion could last for 5 minutes and thus be damaging to structures. 4.3 Liquefaction Susceptibility Of Soil At the Subject Property Due to the presence of relatively compact deposits below the site and the absence of sand deposits with a shallow water table, soil liquefaction potential is very low to non-existent at this site. 5.0 DRAINAGE CONSIDERATIONS The soil at this site has excellent drainage capacity and water from driveways and roof downspouts may be routed to shallow infiltration trenches. The infiltration trench is a shallow perforated pipe run at a low slope that is embedded in a gravel-filled trench. The trenches need only be 2 to 3 feet in depth and should be run in a north-south direction when feasible. Some typical designs are easy to find using a Google search. 6.0 GOOD DESIGN CAN IMPROVE OUTCOMES IF AN EARTHQUAKE OCCURS The current International Building Code ( IRC ) provides a good measure of protection from earthquake damage to modern lightweight residential structures. The small cabin whose footprint is staked on the subject site should do well in most earthquakes if it is constructed in accordance with the IRC as required by Jefferson County. The following design features and processes can improve the seismic resistance and safety of other houses built at this site or in this region: ○Comply to the IRC code on anchorage of the building frame to its foundation, ○Avoid large expanses of glass such as high and wide glass walls, ○Avoid large rooms with vaulted ceilings with few or no crossing walls, ○In addition to required shear walls, consider providing strongly framed transverse walls with drywall overlying plywood for enhanced earthquake resistance, ○Avoid unreinforced masonry construction. If masonry is used, ensure that it is adequately reinforced with steel rebars, fully grouted and that good connections are made from all veneer to the supporting structural elements, ○Have your construction plans reviewed by a structural engineer. 8 7.0 CLOSURE AND LIMITATIONS This reconnaissance study and report concerning the subject parcel was economically completed with the collaboration of the owner. This report and its recommendations apply only to the subject property. The undersigned Geotechnical Engineer warrants that the work was conscientiously performed in accordance with the professional practice of geotechnical engineering and the principles of geologic science. No other warranty, expressed or implied, is provided with this study or report. If you have questions, you may call the undersigned engineer at the phone number above. We appreciated the opportunity to do this work for you. Sincerely yours, Steve S. Luxton, MSc PE Senior Geotechnical Engineer Signed 2/22/23 Attachment 1 - Sketch map of site showing geohazard setback zone 9 10 11