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HomeMy WebLinkAbout721043010 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 May 16, Zoos John Nisbet 4817 Eaglemont Road Chimicum, WA 9832s ltr...,..._.~~ _, ;,. ,_ __ 1 ~ 4' 6 ___- _... _..... ~, ~ ' JUN ~ 6 2~5 I ~ ~~-ri=_~SON COUP4TY .. r: .,i~FGil71' CEVELOPr.~_ Re: Geology Hazard Evaluation and Stormwater Recommendations Proposed Development Lots APN 721-043-010, -001,-009, -014, -015, and -016 Jefferson County, WA Dear Mr. Nisbet: Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced properly located on South Point Road in Jefferson County. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failut~es impactix~ development on the subject gcnperty and provide general site development and maintenance recommendations. It is my understandingthat asix-lot development is proposed on the site with three lots located east of South Point Road between the road and the shoreline and three lots located west of South Point Road. No specific home development plans are proposed at this time. The purpose of this evaluation is to provide recommendations for future home siting and development. As part of this evaluation, I completed the stonmwater site plan submittal template for the site. Stormwater recommendations are provided in this report as appropriate management of stormwater on the site is a significant requirement to prevent stonmwater from negatively impacting the stability of the slopes. This evaluation included a visual insp~tion of the property and vicinity, a visual inspection of the slopes on the properly and vicinity, review of available geologic mapping in the area, and inspection of the shoreline on the eastern side of the property, and inspection of the stream valley that flows through the property. Shallow test pits were excavated on the property as well to assess the geology of the site and for purposes of evaluating storm water impacts on the site and vicinity. The property is located on the east and west sides of South Point Road. Hood Canal is located on the east side of the property. The location of the property is indicated on Figure 1. The property is located in the southeast quarter of Section 4, Township 27N, Range 1 East W.M. May 16, 2005 Nisbet Proposed Development, Jefferson County, WA Geology Assessment SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the slope conditions, shoreline conditions, and relevant conditions in the vicinity of the ProPertY• 2) Observed surface soil conditions on the slopes and on the uplands above the slopes on the property and vicinity. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the slope stability in regards to proposed development on the property, recommendations for site development, and recommendations for further investigation, if necessary. GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject properly. The Geolo is of Surficial Deyosits in the Seattle 30' by 60' Quadt'aaszle. Washi~nsztc3n (Yount, Minard, and Dembroff,1993) indicates the subject property is underlain by Vashon Till and recessional outwash deposits. The Geologic Map of the East-CentralLJefferson Count Washin tin (Birdseye, 1976) indicates the property is underlain by Vashon glacial till and Vashon advance outwash deposits. Both maps indicate the upland area along the road and east of the road is underlain by till. The till consists of nonsorted, nostratified clay, silt, sand and gravel up to boulder size with the coarser material embedded into the finer matrix. The till was deposited directly by glacial ice and is in a very hard compact condition. Yount, Minard scut Dembroff (1993) indicate the shoreline slopes are underlain by recessional outwash while Birdseye (1976) indicates the shoreline slopes are underlain by advance outwash deposits. The tow units both consist of sand, gravel and silt deposits associated with glacial melt water. The advance deposits were deposited as the Puget lobe ice sheet advanced toward the area scan ~P Fik: ~ ~. i s.oa 2 ~__..._ while the recessional deposits were deposited as the glacier retreated from the area. Advance outwash deposits were overridden by the glacier and have been highly compacted by the weight of the glacial ice. Recessional deposits have not been overridden by glacial ice and therefore tend to be not be compacted and are in a loose condition. Based on my site observations,l concur with Birdseye (197 that the shoreline slopes are underlain by advance outwash deposits. Sand, silt and clay units observed on the slopes are highly compacted, coarsen upward, and are directly overlain by glacial till. These features are consistent with the deposits being advance outwash. It is likely that the difference between the maps was simply a transcription error as the Yount, Minard and Dembroff (1993) map is a compilation map that relied on the mapping completed by Birdseye (1976) and the notation on the Birdseye (1976) map is difficult to read. The ~,looe Stabilitv Map of East-Central Jefferson County Washington (Birdseye,1976) identifies the subject property area as having both unstable areas and stable areas that may become unstable if modified by man. The stable areas are the gentle uplaml slopes of the property and the unstable areas are the steeper slopes and slopes near the shoreline. No recent landslides or areas of past rapid movement are identified on the slope stability map on the subject property. A deep-seated landslide is located on the eastern portion of the property east of South Point Road. This landslide feature is a very old feature. Movement of the slide has taken place in the recent past as trees are twisted and fiactures are present on the ground surface of the slide area. SPECIFIC SITE OBSERVATIONS A sketch map of the property with pertinent features indicated is provided in Figure 2. The property is bounded on the east by Hood Canal. South Point Road bisects the property. Three of the building lots are located between South Point Road and Hood Canal, and three of the lots are located west of South Point Road. The western side of the property is approximately 200 feet above sea level. The property generally slopes down to the east towards Hood Canal. The subject property is tree-covered with mature second growth forest. Six terrain areas are present on the subject property and are indicated on the site plan sketch (Figure 2) and are described below. In addition to the six terrains described below, this section provides a description of other relevant features regarding water flow and topography on the site. Landslide Complex A deep-seated large landslide is located on the eastern side of the property and extends a short distance to the north of the property and to the south of the property. This slide is approximately 800 feet wide with a headwall area 300 feet from the shoreline. Based on tilted silt/clay layers along the beach and the steepness of the headwall area, the failure plain extends below sea level SThdan Group File: l 1.18.04 3 nay l6, aoos Nisbet Proposed Development, Jefferson Camty, WA Geology Assessment 4. ~~ t ~ `~ ;~ ~ - - `~ ~ - E` F' ° ~ ~ 'k ~. ~~` t ~{{ r`~ t ~ !. j t JUN - 6 2Q~5 , ~~ ~.. and is within the very compact clays that underlie the area. There is no evidence of recent deep- seated movement on the headwall scarp or at the beach area of the slide. However, recent movement of broken blocks within the slide complex is evident from the twisted trees, tilted trees and fi~actures on the ground surface of the slide complex area. The failure mechanism of the slide appears to be the result of lateral weaknesses that have developed within the very compact silt/clay underlying the lower slopes of the area. These silt/clay units have been very compacted by overriding glacial ice but the lack of confinement to the east by the erosion of Hood Canal has created a stress field that has frachued the clay unit. The slide will continue on a periodic basis as the toe of the slide is removed by eoision at the beach on the east side of the property. The toe of the slide acts as a counterweight to the slide mass and, as material is removed from the toe area, the slide will rotate further and may ultimately expand. towards the west. The current headwall scarp is cuts across the three lots on the east side of South Point Road. The headwall scarp is approximately 50 feet high. The scarp is underlain by compact sand, silt and, on portions of the upper slopes to the south, glacial till or ice contact deposits. The headwall slope is tree-covered with mature cedar, fir and maple with an understory of brush and ferns. Except for the fact that the steep slope is a landslide scarp the slope is fairly stable with only shallow surficial failures present. No evidence of new headwall scarps forming is present on the subject property. North Building Area The north building area is located above the northern portion of the headwall of the landslide. The area slopes gently to the north and to the northeast. The North Building Area is bound on the east by the Landslide Complex, on the west by the Ravine and on the south by South Building Area. The slope of the North Building Area continues to the north down to the shoreline between the landslide complex and the ravine. The area is underlain by sand and silt. Except for a small cleared area covered by grass, the area is tree-covered. The ground slope of the area is soh that any surfarce water flow would flow to the north or to the northeast towards the ravine, versus flowing towards the landslide scarp. No unstable slopes are .present in this area and there are no indications that this area has been impacted by deep-seated slope failures. South Building Area The south building area is a very gentle sloping to level to undulating area between the landslide complex on the east and South Point Road on the west. The separation of this area from the Stratum GAP Eik:11.18.04 4 ,~.__. ._._ . 4 ~ ' t_._ ....._ _._._ _._ ~ ., ._. _ 4 May 16, 2005 ! ~~~ ' ~ ~i~fi'J ~ -.: Nisbex Proposed Development, Jefferson County, WA Geology Amt C`;. ~ . L .~.. ~ ........_..._ ~.. ~..: --'- North Building Area to the north is because of the slope direction. Where any surface water run off from the north building area would be directed to the north and northeast away from the deep- seated landslide, surface water run off from the South Building Area maybe directed towards the landslide area widwut directing water away from the area. In addition this area straddles the central axis of the deep-seated landslide complex and is located above the highest portion of the headwall scarp. Ravine The ravine is a steep sided ravine with a stream at its base. The stream flows along the south side of the western portion of the property from west to east. Approximately mid way along the south property boundary the stream turns sharply to the northeast and cuts through the properties. Fill has been placed into the ravine and the stream passes through a culvert under South Point Road. The ravine is approximately 40 feet :deep. Side slopes of the ravine are primarily underlain by sand with lesser amounts of silt and clay. Slopes on the sides of the ravine are as steep as 45 degrees but generally average between 35 to 40 degrees. The side slopes of the ravine are tree-covered with mature second :growth forest and old growth stumps are present as well. The slopes are generally stable with minor soil creep and small slumps in places near the bottom that have been undercut by the stream. The stream flows off of till underlain areas to the west. It appears the ravine is the result of the stream having incised through the erosion resistant till into the more erodable outwash sand and gravel. The lower part of the stream appears to have eroded down to the top of hard erosion resistant clay/silt. Fill Slopes Fill slopes are present along South Point Road where the road crosses the ravine. The fill soils were likely derived by cutting into the local grade both south and north of the ravine. The fill slopes are on the order of 40 degrees and are tree and brush covered. Except for minor soil creep, the slopes are stable. Road Cut Slopes Steep slopes are located along the west side of South Point Road where the road cuts along the base of the slope in the north central portion of the property. The highest part of the road cut is approximately 20 feet high. The cut slopes are primarily underlain by very compact glacial till and except for minor shallow sloughing and soil creep are stable. The slopes are mostly brush covered. Strs4um Group File: 11.18.04 5 West Upland Area The west upland is located west of South Point Road above the South Point Road cut and the ravine. The area is tree covered and slopes very gently to the east. The outer edges of the upland area are well defir>ed by sharp slope breaks onto the steep slope of the ravine and the road cut. The upland area extends off the property to the west and north. Most of the west upland area is underlain by compact glacial till soils. No unstable slopes are t within Upland Area 1 except the potential unstable areas associated with the proximity to the steep slopes of the ravine and road cut slope adjoining the upland area. Other Relevant Features A number of other relevant features of note regarding site stability not described in the above terrain areas are present on the property and vicinity. South Poiat Road has been cut down into the topography of the upland area to the west of the landslide area in order to maintain a grade from the upland area down to the low land area north of the subject property. The removal of soil has reduced the load on the upland area above the landslide. In addition, ditches are located on both sides of the county road for drainage and storm. water. Rainwater hitting the road on the upland area above the landslide is conveyed via the ditches to ravine on the subject property. Hence, rainwater on the road area that would otherwise have impacted the slide area is conveyed away from the slide. Shallow perched ground water is intercepted by the uphill ditch on the southwest side of the road and conveyed to the ravine. The net effect of the road on the landslide complex is positive because the road has reduced the mass of soil above the landslide area and is removing water that would otherwise reach the landslide. CONCLUSIONS AND RECOIVIMENDATIONS Based on my visual inspection of the subject property and vicinity and analysis of the terrains on and in the vicinity of the subject property, I conclude that residences can be located on the gentle sloping upland areas as indicated in the site plan sketch. Residences located within these areas should not be at risk from bluff erosion or landsliding that would thmaten the structures within the expected life of the structures. Stormwater plans for the site development should also be followed. I completed a stormwater Str~un Group Fik:11.18.04 6 May 16, 2005 Nisbet Proposed Development, Jefferson County, WA Geobgy Assessment. submittal template for the proposed project. Full dispersion will be utilized on all of the sites. Lot APN 721-043-010 east of the county road will require that surface storm water run off be directed into the county ditch on the west side of the lot. Specific recommendations for each of the six lots are presented below. In addition to the six lots, specific recommendations for landslide vegetation management are provided and should be followed to ensure that the site development does not further increase landslide activity beyond the natural conditions. Figures 3 through 8 are provided for lot specific recommendations. Lot APN 721-043-014 Building can place anywhere on this lot as long as the building site is set back at least 20 feet back: from the top of the steep slope created by the county road cut. No timber harvest should take place within the setback area. Harvest of and clearing of trees for development purposes is limited to 35% of the property per the requirements of the storm water plans goals for full dispersion of stone water. No trees should be removed within the steep slope set back or on the steep slope. No debris should be placed on the steep slope on the lot. Storm water run off from the roof gutters should not be routed towards the steep road cut slope aad should be dispersed or infiltrated into the ground in a dispersed manner outside the setback areas. The septic drain field should not be located within the setback area. The addition of water from a drain field outside the setback area should have negligible impact to the mad cut slope. Lot APN 721-043-015 Building can place anywhere on this lot as long as the building site is set back at least 20 feet back from the top of the steep slope created by the county road cut. The building should also be set back at least 20 feet from the top of the ravine slope on the east side of the property. No timber harvest should take place within the setback areas on the cut slope or on the ravine slope. Harvest of and clearing of trees for development purposes is limited to 35% of the property per the requirements of the storm water plans goals for full dispersion of storm water. No trees should be removed within the steep slope-set back or on the steep slope. No debris should be placed on the steep slopes on the lot. Storm water run off from the roof gutters should not be muted towards the steep road cut slope Stratum Gnaup Fik: 11.18.04 7 May 16, 2005 Nisbet Proposed Development, Jefferson County, WA Geology Assessment and should be dispersed or infiltrated into the ground in a dispersed manner outside the setback areas. The septic drain field should not be located within the setback area. The addition of water from a drain field outside the setback area should have negligible impact to the road cut slope. Lot APN 721-043-016 Much of this lot is within the ravine that crosses through the area. The east half of the site is located within the ravine and the ravine slopes are located all along the southern edge of the lot. The slopes of the ravine are fairly stable, but are subject to surface creep and there is a risk of differential settlement along the top edge of the slope from minor surface movement. Because of the narrowness of the upland area above the ravine on the subject property, the practical building area is located in the approximately middle portion of the lot where the upland area is wider. This site is suitable for building as long as the building site is set back at least 20 feet back from the top of the ravine slope. No timber harvest should take place within the setback areas on the cut slope or on the ravine slope. Harvest of and clearing of trees for development. purposes is limited to 35% of the property per the requirements of the storm water plans goals for full dispersion of storm water. No trees should be removed within the steep slope set back or on the steep slope. No debris should be placed on the steep slopes on the lot. Storm water run off from the roof gutters should be dispersed or infiltrated into the ground in a dispersed manner outside the setback areas. Lot APN 721-043-009 Any buildings constructed on the property should be located at least 40 feet back from the landslide complex scarp and at least 15 feet from the steep slope associated with the ravine. It is my opinion that the chance of the landslide complex expanding within the 40-foot set back is very low. However, the consequences to any structures within that zone would be very high. The proposed building area envelope is consequently limited to an upland area bounded on the east/southeast by the landslide complex and on the west by the ravine. The proposed building area is located above the northwestern limb of the slide area and is not located along the centerline of the slide access. Another factor is that slopes on this upland area are towards the north and even the northwest away from the slide area. Therefore any storm water generated at the building site can readily be directed to the north or northwest away from the slide area. In addition the slope is such that any clearing in this area will not cause an increase of water Stradan Group Fik: 11.18.04 g May 16, 2005 Nisbet Development, Jefferson County, WA Geology Assessment impacting that landslide complex. ....__ __ ~_ ~ t ~ _.._..~.._.__..__ ~_ ~ ~ `t The slopes of the ravine. northwest of the building site area are on the order of 25 degrees and appear to be stable and ~ 1!~ot pose a hazard to the subject property building area. However, any structure should be set back at least 15 feet from the top edge of the slope to avoid the risk of differential settlement from foundation loads. No timber harvest should take place within the setback area above the landslide scarp or within the landslide area except that described in the Landslide Vegetation Management section of this report. No harvest should take place on the ravine slopes that are gmater than 20 degrees. Harvest of and clearing of trees for development purposes is limited to 35% of the property per the requirements of the storm water plans goals for full dispersion of storm water. No debris should be placed on the steep slopes on the lot or anywhere on the landslide hazard area or landslide hazard area buffer. Storm water run off should be dispersed or infiltrated into the ground in a dispersed manner on the forest floor to the north/northwest of the building site such that no water is directed towards the landslide area or landslide area buffer. Dispersion of storm water should be directed. onto the forested floor on the slope above the ravine or on the upper slope of the ravine. Lot APN 7Z1-043-001 This lot is very similar to Lot APN 721-043-009 in regards to recommendations. Any buildings constructed on the property should be located at least 40 feet back from the landslide complex scarp along the northern portion of the lot and SO feet back along the southern portion of the lot. Buildings should be located at least 15 feet from the steep slope associated with the ravine. It is my opinion that the chance of the landslide complex expanding within the 40.foot to 50.foot set back is very low. However, the consequences to any structures within that zone would be very high. Because the southern portion of the lot is closer to the centerline of the axis of the landslide, the setback in this area should be slightly larger. The risk of a larger portion of the upland being impacted is greater near the axis than along the limb of the slide complex. The proposed building area envelope is consequently limited to an upland area bounded on the east/southeast by the landslide complex and on the west by the ravine and county road. The slope of upland area is towards the north/northeast such that the directing of any storm water to areas away from the landslide area can be readily accomplished and the area cleared will have minimal impact on recharge of water to the landslide complex. The slopes of the ravine northwest of the building site area are on the order of 25 degrees with the slope being steeper near the southwest portion of the lot. A culvert passing under the access Stratum Group Fik: 11.18.04 9 May 16, 2005 Nisbet Proposed Development, Jefferson County, WA Geology Assessment `~ E I '~-~ ~; ~f ~~i f' JUN - 62005 ~~~ ~..:. 4,, UEf'1'. Q!~ C~C~~tinr:.~rt,s c, ~'~:.QPE~EPIY road to the lot discharges storm water from the county ditch onto the ravine slope with only minor erosion. The ravine slopes appear to be stable and do not pose a hazard to the subject property building area. However, any structure should be set back at least 15 feet from the top edge of the slope to avoid the risk of differential settlement from foundation loads. No timber harvest should take place within the setback area above the landslide scarp or within the landslide area except that described in the Landslide Vegetation Management section of this report. No harvest should take place on the ravine slopes that are greater than 20 degrees. Harvest of and clearing of trees for development purposes is limited to 35% of the property per the requirements of the storm water plans goals for full dispersion of storm water. No debris should be placed on the steep slopes on the lot or anyvvhere on the landslide hazard area or landslide hazazd area buffer. Storm water run off should be dispersed or infiltrated into the ground in a dispersed manner on the forest floor to the north/northwest of the building site such that no water is directed towards the landslide area or landslide area buffer. Dispersion of storm water should be directed onto the forested floor on the slope above the ravine or on the upper slope of the ravine. Lot APN 721-043-010 Lot APN 721-043-001 is located near the centerline of the axis of the landslide complex. Any buildings constructed on the property should be located at least 50 feet back from the landslide complex scarp. It is my opinion that the chance of the landslide complex expanding within the 50-foot set back is very low. However, the consequences to any structures within that zone would be very high. The proposed building area envelope is consequently limited to an upland area bounded on the east by the landslide complex and on the west by the county road. The upland area is nearly level. The road to the west is slightly lower so that it would be very feasible to direct any storm water to the road ditch on the west side of the lot. No timber harvest should take place within the setback area above the landslide scarp or within the landslide area except that described in the Landslide Vegetation Management section of this report. Harvest of and clearing of trees for development purposes is limited to 35% of the property per the requirements of the storm water plans goals for foil dispersion of storm water. No debris should be placed on the steep slopes on the lot or anywhere on the landslide hazard area or landslide hazard area buffer. Storm water run off should be directed to the ditch on the west side of the property. The discharge of access mad run off and roof drainage to the ditch will not cause a significant Shalom Group Fik:11.18.04 10 ,;:.. ~ i 'i i _ ~ 2v05 ~ May 16, 2005 = : '~. ~ ~ ~ ~! ~ E ...._,:. Nisbet Proposed Development, Jefferson County, WA ' Geology Assessment '~"._.. - _.. increase in volume to storm water flow in the ditch and this is the o io~caRilanrsit~"° where the ditch will be utilized for storm water. Landslide Vegetation Management As described above a daep-seated landslide is located on the lots east of the county road. At the present time the headwall scarp of the landslide area is stable and erosion rates at the toe of the slide area appear to be very slow. However, increased hydrologic loading can increase the rate and frequency of landslide movement. Management of vegetation on the deep-seated landslide and the landslide buffer can be done in a manner that will have minimal impact on the hydrology of the landslide area. Tree removal on the landslide is acceptable as long as the tree removal is limited to deciduous trees and all evergreens are left on the landslide. and landslide buffer. Deciduous trees have negligible effects on hydrologic recharge during the winter months when long wet periods would tend to cause soil saturation. The removal of deciduous trees will have the added benefit of increasing the .growth of small shaded evergi~een trees such that overtime the amount of rainwater impacting the slide area will be decreased. Limbing and topping of evergreens for the creation of view corridors may cause a slight increase in water reaching the slide surface, but actions outside the slide area described below will more than offset the increase of water from limbing and topping of evergreens. Once again, the limbing and topping may increase light to the forest floor and stimulate additional growth of the smaller tress on the site. Some actions in the vicinity of the landslide have inadvertently decreased the amount of water reaching the landslide area. The construction of the road also directs potential surface water and shallow ground water from reaching the slide area. The proposed building on the lots will also direct surface water run off from rainfall away from the landslide area Strawm Group Fik: 11.18.04 11 May 16, 2005 Nisbet proposed Development, Jefferson County, WA Geology Asaaa~ent CLOSURE Please note that there are inherent risks associated with lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 714-9409. Sincerely yours, Stratum Group an McShane, M.S., L.E.G. 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