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HomeMy WebLinkAbout901245005 Geotech AssessmentGEOTECHNICAL REPORT Prepared For Jason Purser December 26, 2006 For the Property Described as Tax # 901245005, Spring Hill Road Section 24, Township 29 North, Range 1 West, W.M. Jefferson County, Washington Prepared by NTI ENGINEERING & SURVEYING, INC. 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com ~~~~~~~r ~~~~ December 26, 2006 Jason Purser 20488 Miller Bay Road Poulsbo, WA 98370 Geotechnical Report Tax # 901245005 Subject: Geotechnical report for Tax # 901245005 Located in Section 24, Township 29 North, Range 1 West, W.M., Jefferson County, WA Dear Mr. Purser: Background At your request, NTI Engineering & Surveying, Inc. (NTI) conducted a slope stability inspection at the above-mentioned property on December 18, 2006. The purpose of this inspection was to examine the property by visual means in order to determine the relative stability of the slopes on the property and make recommendations in regards to the proposed construction of a single family residence, in accordance with the Jefferson County Critical Areas Ordinance. Site Description The subject property is located on Spring Hill Road off of S.R. 19, south of Chimacum in Jefferson County (Figure 1). The hilly wooded property is bounded on the north by wooded timberland, on the east by developed residential property, and on the south and west by undeveloped private property (Figure 2). Existing improvements to the property include a driveway and utilities. The property is situated on a west facing slope overlooking Chimacum Valley. Three west and southwest trending ravines cut through the property such that there is a prominent spur off the main ridge which trends west then south. Both the home and the ~, drainfield will be constructed on this spur (Figure 3). The existing driveway follows the ~~ ravine that runs along the east side of the property. There are ditches and culverts along the driveway. The elevation of the spur is approximately 300' and the base of the ridge is at about 200' elevation. The downslope angles off the spur range from approximately 10° to 18° (18% to 33%). West of the drainfield area, at approximately the 280' or 290' contour, the ~~ slope angle increases to about 20° to 22° (37% to 40%). At about the 250' contour, the slope angle again changes in some locations to approximately 30° (58%). However, i ~~ these slope measurements are below the "angle of repose" which is defined as the maximum slope or angle at which loose, cohesionless material remains stable, and commonly ranges between 33 and 37 degrees on natural slopes. Figure 1 The property is well vegetated with young to mature trees and brush (Photo 1). Predominant species include fir, cedar, maple, alder, and madrona, generally in the one to two foot diameter range, with an understory of salal and sword fern. The property has been logged probably twice in the past as evidenced by old growth stumps with spring board notches as well as second growth stumps. Some of the trees exhibit curved trunks, suggesting that downslope creep of the surface soil is occurring in places. No groundwater seeps-were observed on the slopes, however, surface water runoff was running in the ravine northwest of the property and in the driveway ditch on the east side of the property. No evidence of landslides was noticed on the slopes. 2 Figure 2 Figure 3 Site Geology The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" maps the soil in the area of the property as Lodgement till (Qvt) and as Undifferentiated glacial, fluvial, glaciofluvial, lacustrine and glaciolacustrine deposits (Qu). The till is described as boulders, cobbles and pebbles in a matrix of sand, silt and clay; a compact and unsorted mixture. The undifferentiated deposits are older than the Fraser glaciation and include sediments from pre Fraser glaciations. This unit contains a variety of soil types and is generally mapped when more detailed data is not available. r r i r r According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the subject site is in an area mapped as the Cassolary sandy loam (CfD). This soil formed in glacial drift and/or marine deposits on terraces and is composed of sand, silty sand and silt/clay with gravel. Runoff is listed as medium and the hazard of water erosion as moderate. 4 Visual observations made in the drain field test pits and in road cuts along the driveway revealed unsorted silty sand with gravel and cobbles consistent with the Lodgement till described above. Discussions with the septic system designer also indicate that the soil in the test pits became very hard at about 40", which further indicates compacted glacial till soil. Conclusions and Recommendations The subject property appears grossly stable and the project seems feasible from a geotechnical perspective. The proposed location of the house and drainfield as shown on the site plan (provided by client) in the appendix of this report are acceptable from a geotechnical perspective. We recommend that the drainfield lines be at least 20' back from the 20° to 22° (37% to 40%) slope that is located west of the drainfield area, which approximately corresponds with the 280' contour line on Figure 3. We understand from the septic system designer that this is the case. We further recommend that building foundations comply with Section 1805.3.1 & 3.2 of the International Building Code (IBC) which deals with footings on or adjacent to slopes. That text and related illustration can be found in the appendix of this report. The following recommendations should also be considered with regards to the proposal: 1. We understand that trees will be removed in the home site area to accommodate the house, driveway and yard area. Once this site. work has been completed, some form of ground cover should be utilized to reduce the erosion potential of bare soil. 2. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 3. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage control device such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. It may be feasible to control release this water to the existing natural drainage corridors with quarry spalls or perforated pipe. 4. Surface water should not be allowed to flow freely down the face of the slopes and cause erosion of the slopes. If this occurs, the water should be directed away from the slopes and/or conveyed to a safe discharge area, possibly into the natural drainage corridors mentioned above. 5. Silt fences or other sediment control devices may be needed during construction so that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 6. Drainage control devices should be maintained in good working order and inspected at least once a year. 5 1. 7. All drainage control features should conform to the current regulations that apply to the development of this property and be constructed in accordance with standard engineering practice in order to .insure that surface water discharge or sedimentation to adjacent properties does not exceed predevelopment r conditions. Based on the findings, recommendations and limitations of this report, with the control of post development runoff to predevelopment rates and the application of standard erosion and sediment control BMPs, it is our opinion that: 1. There should be minimal landslide hazard as suggested by a lack of evidence of recent landslide activity in the vicinity in the past. 2. Observations of slope stability indicate that the proposal should not increase the risk of landslide. 3. The proposal should not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. 4. The proposal should not decrease slope stability on adjacent properties. 5. The proposal should be stable under normal geologic conditions. For further information please review the three enclosed publications published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs". These publications are now out of print but can also be obtained from the DOE website at: http://www.ecy.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains additional useful information regarding slope stability and site development; this reference is highly recommended. Limitations This report has been prepared for your exclusive use in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. Others may use it only with the expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other warranty, expressed or implied, is made as to the conclusions and professional advice included in this report. 6 The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes and/or surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NORTHWESTERN TERRITORIES, INC. Robert A. Leach, P.E., MBA Principal Engineer ~-P~ Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports\PURS0601.slope stability.24(29-1).Chimacum Valley.doc J J tz(zz~Dc ~~~ ~~ ~~~ ~q ~' ~ f ~~,~~8~91~D`~~ William C. Payton Jr. EXPIRES 12/30/2008 i SOILS AND FOUNDATIONS tance shown in Table 1804.2 unless data. to substantiate the use of higher values are submitted for approval. For clay, sandy clay, silty clay and clayey silt, in no case shall the lateral sliding resistance exceed one-half the dead load. 18043.1 Increases in allowable lateral sliding resistance. The resistance values derived from the table are permitted to be increased by the tabulaz value for each additional foot (305 mm) of depth to a maximum of IS times the tabulaz value. isolated poles for uses such as flagpoles or signs and poles used to support buildings that are not adversely affected by a '/Z-inch (.12.7 mm) motion at the ground surface due to short-term lateral loads are permitted to be designed using lateral-bearing values equal to two times the tabular values. SECTION 1805 FOOTINGS AND FOUNDATIONS 11805.1 General. Footings and foundations shall be designed and constructed in accordance with Sections 1805.1 through 1805.9. Footings and foundations shall be built on undisturbed soil, compacted fill material orCLSM. Compacted fill material shall be placed in accordance with Section 1803.5. CLSM shall be placed in accordance with Section 1803.6. The top surface of footings shall be level. The bottom surface of footings is permitted to have a slope not exceeding one unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation aE the top surface of the footing or where the surface of the ground slopes mare than one unit vertical in 10 units horizontal (10-percent slope). 1805.2 Depth of footings. The minimum depth of fooeittz~ be- fow the aralisturbed getrt-nd:surface shall be 12 inch (305 mm). Where applicable, the depth of footngs shall also con- form to Sections 1805.2.1 through 1805.2.3. 1805.2.1 Frost protection. Except where otherwise pro- tected fmm frost, foundation walls, piers and other. perma- nent supports of buildings and stuucGues shall be protected from frost by one or matt< of the following metltodst 1. Extending below the frost line of the locality; 2. Constructing in accordance with ASCE-32; or 3. Erecting on solid rock. ~tception; Feee-standing buildings tneetiag all of the following conditions shall nor lie required to fie pro- tected: 1. Classified in importance Category f (see Table 1604.5); 2. Area of 4f10 square feet (37 m2) or less; and. 3. Eave height of 10 feet (3048 tttnx) or less. Footings shall not bearnn frozen sail unless such frozen con- dition is of a permanent character: 364 1805.2.2 Isolated footings. Footings on granular soil shall f 1 be so located that the line drawn between the lower edges of ~.~"' adjoining footings shall not have a slope steeper than 30 degrees (OS2 rod) with the horizontal, unless the material supporting the higher footing is braced or retained or other- wise laterally supported in an approved matter or a greater slope has been properly established by engineering analysis. 1805.2.3 Shifting armovingsails. Whereit is known thatthe shallow subsoils are of a shifting or moving character, foot- ings shall be carried to a sufficient depth to ensuie stability. 1805.3 Footings on or adjacent to slopes. The placement of buildings and structures on or adjacent w slopes steeper than one unit vertical in three units hori2ontal(33.3-percent slope) z. ~ $ 5 , shall conform to Sections 18053.1 through 1805.3.5. 1$053.1 Building clearance frotn:.aseendfng slopes. In general, buildings below slopes shall be set a sufficient dis- tance from the slope to provide protectiou from slope drain- age, erasion and shallow failures. Except as provided for in Section 1805.3.5 and Figure I 805.3.1, the following criteria will be assumed to provide this protection. Where the exist- ingslope issteeper than one unit vertical in one unit horizon- ~ 5 k tal f 100-percent slope), the toe of the slope shall by assum~i to be at the intersectionof a horizontal place draws-from the top of the foundation and a plane drawn tangent to the slope . at an angle of 45 degrees (0.79 rod) to the horizontal. Where _ _ a retaining wall is constructed at the toe of the slope, the height of the slope shall be measured from the top of the wall ~.~' to the tap of the slope. 18053.2 Footing setback from descending slope sur- face. Footings on or adjacent to slope surfaces shall be ffouaded in f rm material with an embedment ajtd set back from the slope surface sufficient to provide vertical and lat- eraisupport far the footing without detrimental. settlement. Except as provided far in Sect%on 1805.3.5 and Figure 1805.3.1, the following setback is deemed adequate to meet thecriteria_Wheretfneslopeissteeperthap 1 ututver-- tical ire l unithorizontal (lOf}-percent slope), the requited setbac& shall ire tneasuted"feotat an "iYnaginar} Plane 45 degrees (Q:-~~i 9~d) eo the haxizootal; Fsnjected upward from tine toe of the slope.- 180533Puo1s.The setbackbetweenpaxnls eegulati'd bythis code. and stapes shall be egnal to one--half the buiidi ng foot ing setback distance required by this section. That portion of the poo•1 wall within a horizontal distance of 7 feet (2134 mm) from the tap of the slope shall be capable of supporting the water in the gaxrl without. soil suppor[_ 1$f153 41?onndatlon elevation. On proses sites, the top of any exterior foundation shall extend above the elevation of the stt>«et gutter at paint off discharge or the inlet of an approved drainage deviate a minimum of 12 inches (305 mm) plus 2 peeoent. Alternate elevations are permitted sub- ject to the appmvafl of the building official, provided it can ._,. fie demansirated drat required dra%roage_ to the point of dis- charge and away faom fire steucture is provided at all laca- cions an the site. 2603 INTERNATIONAL BUIL1)IPIf CODE® 2003 International Building Code ~~ s~a~ FacFOF ~srnucrrx~ rr~oF se_o~ nr~n nror©te~m re Fr. xaax. For SL• 1 foot = 3Q4.8 mm. r~ a~ Foormta ~saorn~onrcrr ~xc~n4uFz max. N FIGURE 18Q5.3.1 FOUNDI4TION GLEARANGES FRO1A SLQRES ~G peualf !~'". ' ~ 't'~fvu +' '~`~r, ~~~~ ~~ -: . ~ . .,~WPaJ :. ~. ~"",~,~. ~ s, L ~ i., . F ~ ~ = ~{q 1 ~ r ! _ ~""~''''''''~~~~~111' / A ' 5 4j ~ -. r ~, i < a V a y 1 f asf ~ ~ ~ ~.~ ~~ ~~~ i t 9~~ ~ ~~ ~,~ ~ ~ . ~cn ~,i~ ~~ ~ T ~ ?56Y` ~Jry'. Fl>~'i~r~..~t x~~u~~ I ~S'~ ~ ~~{ ~.` ,4 y~x 4 2 C1 ^~ 4'~ ~~: [a-i l ~- 4 ~' b