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HomeMy WebLinkAboutBLD1998-00260 Geotechnical Report 133.7,,,,Rm--qk .c.. ,m) s „,ezz AFi,' .'TrP721,77,7.z x:....4 ':,...vim:ea�.tt,,a'YL3,... .xs _-- � 44 i W-7009-01 Geo technical Report Mats View Terrace Subdivision Port Ludlow, Washington June 1995 I A 111 A I Mr. Harold Moe 311 Mats View Drive Port Ludlow, Washington 98365 I f I SHANNON iWILSON, INC. GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS tp rt I 400 N. 34th St. • Suite 100 P.O. Box 300303 Seattle, Washington 98103 R 1 206.632 .8020 I SHANNON6WILSON, INC. J �' r. t GEOTECHiiJCAL A JO ENVIRON.IEtJ TAL CO"4SIJLYAr,TS ~r I June 12, 1995 I Mr. Harold Moe 311 Mats View Drive Port Ludlow, Washington 98365 RE: GEOTECHNICAL REPORT, MATS VIEW TERRACE SUBDIVISION PORT LUDLOW, WASHINGTON Dear Mr. Moe: This letter report presents our field explorations, conclusions, and recommendations for the proposed residential development west of Mats Mats Bay on Oak Bay Road north of Port Ludlow (Figure 1). In a letter to Mr. Moe dated March 22, 1995, Jefferson County indicated that for approval of the subdivision, a geotechnical report must be prepared for this site which addresses slope stability and provides mitigative measures, as needed. Therefore, the purpose of our work is to provide the geotechnical assessment requested in the county letter and geotechnical recommendations for the development of the subdivision. Our work was conducted in accordance with our proposal dated April 25, 1995, as authorized by Mr. Moe on May 9, 1995. Our geotechnical evaluation is based on site reconnaissances performed on April 24 and May 19, 1995, and on subsurface explorations completed on May 19, 1995. Our understanding of the proposed subdivision layout and development is based on engineering drawings (site plans, cross-sections, and conceptual drainage plans) provided by Mr. Harold Andersen of Quadra Engineering, Inc. I SITE DESCRIPTION The proposed subdivision is located adjacent to the west side of Oak Bay Road, due west of Mats Mats Bay (Figure 1). The proposed subdivision is situated on 13.5 acres. The site is roughly 1,050 feet long (north-south ) by 750 feet wide (east-west). The precise configuration and dimensions of the site are indicated on Figure 1. In general, the ground slopes from the south property line down to the north and east, toward Oak Bay Road. As _ _ _ .7; SiPEE.1 • ,SUITE on W-7009-01 • Mr. Harold Moe S,ANNON nWiLSON.INC. June 12, 1995 Page 2 I indicated on Figure 1, a north-south ravine is located in the northwest corner of the 3 property, creating a small ridge between the ravine and Oak Bay Road to the east. Elevations across the site range from a low of about 110 feet at the northeast property corner to about 260 feet along the south property line. Slopes across the site range from less than 10 percent to over 100 percent in the vicinity of the ravine. The upper sections of the slope along the south property line and the top of the ridge 11 (approximately parcels 6 to 12, drainfields 2 and 3, and well) had been logged within the past few decades. As a consequence, the vegetation in this area consists of small (1-inch diameter or less) alder, young fir, salmon berry, blackberry and associated shrubs and grasses. Slopes in this area are generally about 15 percent or less. Vegetation across the remainder of the site typically includes up to 2-foot-diameter alder, maple, and cedar, with an undergrowth of sword ferns, elder berry, and some nettles. Slopes across the remainder of the site are up to 50 percent down toward Oak Bay Road to as much as 100 percent at the base of the ravine. Growth position of the trees on the slope indicates that relatively slow soil creep is occurring on the steeper portions of the slope. Soil creep occurs on nearly all slopes and is the imperceptibly slow downslope movement of soils under the effects of gravity. Relatively small flows of surface water (less than 1/4 gallon per minute) were observed in two ditches along the south and west property lines at the southwest corner of the site, and in the road ditch along the west side of Oak Bay Road. PROJECT DESCRIPTION The subdivision will include 12, 0.3- to 0.5-acre building lots for single-family residences, one well parcel, three drainfield parcels, and two open space parcels, as shown on Figure 1. Approximately 1,200 feet of roadway is planned to provide access to the parcels. Plans indicate that road grades will not be steeper than 14 percent. Alignment cross sections indicate that cuts will be up to 23 feet vertical (station 3+00) with fill slopes up to 9 feet W-7009-01 a 1 Mr. Harold Moe S iANNON ZIW'LSON.iNC. June 12, 1995 Page 3 I vertical (station 3+50). Conceptual drainage plans call for a detention pond at the bottom 3 of the northwest ravine, and a detention tank at the northeast corner of the site alongside Oak Bay Road (Figure 1). We understand that the embankment for the detention pond will be about 6 feet high or less. We understand that a small culinary water tank will be located on the southwest corner of the site. Current plans do not call for the construction of 0 retaining walls or rockeries. EXPLORATIONS AND LABORATORY TESTING 1 An engineering geologist from our firm conducted geologic site reconnaissances on April 24 and May 19, 1995. In addition, six test pit explorations were performed on the site on May i19 with a backhoe under contract to Mr. Moe. The test pits were designated TP-1 to TP-6, and their locations are shown on Figure 1. The location of each test pit was determined by 1 using a tape to measure distance from known survey points. The test pit locations were selected in the field to obtain subsurface information across the planned developed portion 111 of the site, in the different topographic features, and in areas where differing soil conditions might be expected to occur. ITest pit depths ranged from 6 to 12 feet. Soil exposed in the test pits was logged, and I relative densities or consistencies were estimated in the field by our engineering geologist. Representative samples were collected in the field and returned to our laboratory. The test pit logs are presented on Figures 2 through 7. I Sample classification was visually checked, and sample moisture contents were determined Iat our laboratory. The moisture contents are shown on each test pit log. A grain-size analysis was conducted on the predominant soil type that may be used for fill to aid in 1 determining its engineering properties and suitability for fill. The result of the grain-size analysis is presented on Figure 8. I I IW 7009-01 I Mr. Harold Moe SHANNON f����/I SO 1.INC. June 12, 1995 Page 4 I SUBSURFACE CONDITIONS I Geologic maps of the area indicate that the site is underlain by Vashon lodgement till. Till Iis a non-sorted mixture of clay, silt, sand, and gravel with scattered cobbles and boulders that is deposited by a glacier. Lodgement till is deposited at the base of a glacier and is subsequently overridden by the advancing glacier. The Vashon Ice sheet that deposited the I lodgement till is estimated to have been up to 3,000 feet thick in the area, and as a result, the till and underlying soils were overconsolidated to a very dense or hard state due to the great weight of the ice. The test pits confirmed the presence of the lodgement till beneath the site. Test pits TP-2, TP-3, TP-4, and TP-5 encountered a hard, slightly clayey, slightly gravelly, sandy silt with Iscattered cobbles and boulders, which we interpret as Vashon lodgement till. Similar soil was observed in the cleared ditch banks on the west side of Oak Bay Road. In general, 6 1 inches to 2 feet of relatively loose topsoil was observed above the till (although topsoil was not encountered in test pits TP-1 and TP-4). Immediately below the topsoil, the till Iappeared to be weathered to a medium stiff to very stiff consistency. The till typically became unweathered and hard at about 1 1/2 to 5 feet below the ground surface. Test pits TP-2, TP-3, and TP-4 did not penetrate through the till. Only in test pit TP-5 was the till penetrated. The till extended to a depth of 5 1/2 to 7 1/2 feet below the ground surface. 1 Beneath the till, dense to very dense, silty, fine sand was encountered. This soil is advance outwash deposited in front of the advancing glacier. It was subsequently overridden with till deposited above it. Test pit TP-1 in the southeast corner of the site did not encounter till. Bedded dense to Ivery dense sand and gravel was encountered. This material is also likely a glacial outwash, but it was not determined whether it is advance outwash or recessional outwash (i.e., Ideposited as the glacier retreated and was not glacially overridden). ITest Pit TP-6 was excavated to observe the thickness of relatively recent, soft soils deposited in the swale, or ditch, between Oak Bay road and the slope. Four feet of soft to I IW-7009-01 FMr. Harold Moe SHANNON ZiWi'_CON.INC. June 12, 1995 Page 5 medium stiff silt was encountered above a dense to very dense, gravelly, silty sand in the Ibase of the excavation. iGroundwater was observed in test pits TP-1, TP-2, TP-3, and TP-6. In test pits TP-2 and TP-3, very slight to slight seepage was observed in some of the coarser portions of the till I between depths of 5 to 10 feet. Till is typically relatively impervious, and groundwater observed in or above till at this site is probably indicative of local, shallow, perched water 1 zones, not a major aquifer or groundwater table. Moderate seepage was observed at a depth of 8 feet in test pit TP-1 in the outwash. Moderate seepage was also observed at depth of 2 feet in test pit TP-6. This groundwater level is consistent with surface water observed in the ditch alongside Oak Bay Road. No groundwater was observed in test pits TP-4 and TP-5. I 1 CONCLUSIONS AND RECOMMENDATIONS 1 Slope Stability and Project Impact 1 Based on the subsurface conditions observed at the site and our experience with similar soils in the Puget Sound area, it is our opinion that the existing slopes are relatively stable, and 1 provided that the following recommendations contained in this report are implemented, the impact of the proposed subdivision on the stability of the site and adjacent slopes will be small. It is our opinion that reasonable development on site slopes as steep as 2H:1V (50 percent) Imay be accomplished with little impact on slope stability. Additional geotechnical explorations and studies may be required for structures planned on slopes steeper than 2H:1V and should be reviewed on a case-by-case basis. We note that the building lots in - the subdivision as planned are generally no steeper than 2H:1V. Areas where slopes are I steeper than 2H:1V (i.e., the ravine on the northwest and the slope to the road on the northeast) have been judiciously designated as open space and presumably will not be Ideveloped. IW-7009-01 MEI Mr. Harold Moe Si-10,NNON&WILSON,INC. June 12, 1995 Page 6 1 Please note that there is some risk of future instability present on all hillsides which the owner must be prepared to accept. Such instability could occur because of future water breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or other actions or events on a slope that may cause sliding. Excavations and Site Grading It is our opinion that permanent excavations into the very dense glacial till will be stable at slopes to about 1/2H:1V. However, permanent slopes cut this steeply will ravel. It has been our experience that a slope cut to 1H:1V in till will not ravel and maintain vegetation. Slopes cut steeper than 1H:1V in till should be protected with rockeries. We recommend that rockeries be no taller than 8 feet. Figure 9 provides additional detail and recommendations for rockery construction. Fill slopes and excavations made into other soils at the site should be sloped 2H:1V or flatter. I During construction, we recommend that the stability of the excavation slopes be made the responsibility of the Contractor, as he/she will be the one most familiar with conditions exposed in the excavation and will be at the site on a full-time basis. The Contractor should be responsible for following all current and applicable safety regulations regarding excavations, shoring, etc. The Contractor should also be responsible for the control of all ground or surface water wherever encountered on the project. All fill and/or backfill beneath pavements, slabs-on-grade, and other areas where settlements are to be minimized, should be structural fill compacted to a dense, unyielding state, and to at least 95 percent of its Modified Proctor maximum dry density (ASTM: D 1557-91, Method C or D). In areas where moderate settlements can be accepted, such as in non- structural landscape areas, the compaction requirement could be reduced to a dense, unyielding condition and to at least 92 percent of the Modified Proctor maximum dry density. We recommend that the thickness of fill/backfill layers before compaction not exceed 8 inches for heavy compaction equipment or 4 inches for hand-operated mechanical t compactors. W-7009-01 • 1 Mr. Harold Moe S;�,.'�tial€\a�'?i_�C,s,INC . June 12, 1995 Page 7 . V The non-organic portion of the on-site soils can be used for fill/backfill if suitably Ncompacted as previously recommended. However, because of the relatively high silt content and natural water content, most of the site soils are moisture sensitive, making them 0 difficult to work with and to compact when wet. The natural moisture content of these soils is often sufficiently high to require some drying/aeration before compacting. If earthwork 1 is planned during the rainy season or in wet conditions, it will likely be necessary to use imported, clean, granular fill rather than the on-site soils. 0 If imported soil is needed for fill/backfill, it should consist of clean, well-graded sand and I gravel. It should contain not more than 5 percent fines (soil passing the No. 200 sieve, based on wet sieving the minus-3/4-inch fraction). The fines should be non-plastic. It should have a maximum particle size of 3 inches, should be free of organic matter, and Ihave a moisture content at or slightly below its optimum for compaction. RPavement Recommendations IIn order to provide frost protection for pavements, we recommend that a total of at least 12 inches of pavement, base course, and/or granular subbase be provided between the native isite subgrade and the top of the pavement. Four inches of crushed rock should be placed immediately beneath the asphalt. The remainder of the base course and granular subbase I should consist of clean pit-run sand and gravel, well-graded crushed rock, or a blend of commercial rock products, and contain not more than 3 percent material finer than 0.02 I mm. Normally, soil containing no more than 5 percent fines will meet this criteria. It should conform to the following gradation requirements: Gradation for Sub-base 1 U.S. Standard Sieve Size Percent Passing By Weight _ 2 inches 100 1 inch 70-100 No. 4 35-65 INo. 200 (by wet sieving) 3-5 (non-plastic) W-7009-01 Mr. Harold Moe S ^,NON F'MLSON,INC. June 12, 1995 Page 8 Base and subbase should be thoroughly compacted to achieve a dense and unyielding surface, to at least 98 percent of its Modified Proctor maximum density. IPavement subgrades should consist of very stiff to hard native soil or compacted structural fill. All loose, soft, or disturbed soil and all soil containing organics should be removed from beneath areas to be paved. In general, this will require a 2-foot stripping depth, except along the ditch area along Oak Bay Road where the stripping depth will be 11 approximately 4 feet. We recommend that prepared pavement subgrades be proof-rolled with a loaded dump truck or scraper prior to placement of base and subbase materials. Soft or spongy materials identified during the proof-rolling should be removed and replaced with Icleaner and/or drier materials. Wet Weather Earthwork Wet weather generally begins about mid-October and continues through about May, although rainy periods may occur at any time of the year. Therefore, it would be most advisable to schedule earthwork during the normal dry weather months of June through mid-October. Earthwork performed during the wet weather months will generally prove Imore costly. I The soils at the site generally contain sufficient silt and plastic fines to produce a cohesive, unstable mixture when wet. Such soils are highly susceptible to changes in water content, and they become muddy, unstable, and difficult or impossible to proof-roll and compact if Itheir moisture content significantly exceeds the optimum. The condition of exposed hard glacial till will soften rapidly when exposed to moisture and construction activity. 1 It should also be noted that particularly during the wet weather months, groundwater levels Iwould be highest within the relatively loose/soft soils, which overlie the glacial till. Such groundwater could seep into site excavations and would need to be intercepted by drainage Iditches, trench drains, or otherwise removed. It is our experience that the presence of standing water upon the glacial till surface, along with construction activity, will result in I I W 7009-01 I I . Mr. Harold Moe S; A��i�ii i�` �i�SON.INC. June 12, 1995 Page 9 I disturbance and softening of the till. This could lead to deeper excavations than possibly anticipated. 1 The following recommendations are applicable if earthwork is to be accomplished in wet weather or in wet conditions: Ia. Earthwork should be accomplished in small sections to minimize exposure to wet weather. If there is to be traffic over the exposed subgrade, the subgrade should be 0 protected with a compacted layer (generally 8 inches or more) of clean sand and gravel or crushed rock. The size or type of equipment may have to be limited to prevent soil disturbance; 0 - b. Fill material should consist of clean, granular soil, of which not more than 5 percent by dry weight passes the No. 200 mesh sieve, based on wet sieving the fraction I passing the 3/4-inch sieve. The fines should be non-plastic. Such soil would need to be imported to the site; Rc. The ground surface in the construction area should be sloped and sealed with a smooth-drum roller to promote the rapid runoff of precipitation, to prevent surface water from flowing into excavations, and to prevent ponding of water; Id. No soil should be left uncompacted and exposed to moisture. A smooth-drum vibratory roller, or equivalent, should be used to seal the ground surface. Soils that Ibecome too wet for compaction should be removed and replaced with clean granular soil; 1 e.' Excavation and placement of structural fill material should be observed on a full- time basis by a geotechnical engineer, or his/her representative, experienced in wet- weather earthwork, to determine that all unsuitable materials are removed and suitable compaction and site drainage is achieved; and, I f Covering of work areas, soil stockpiles, or slopes with plastic; sloping; ditching; installing sumps; dewatering; and other measures should be employed, as necessary, to permit proper completion of the work. Bales of straw and/or geotextile silt fences Ishould be aptly located to control soil movement and erosion. Detention Pond Embankment I For the design and construction of the detention pond embankment, we recommend that Itopsoil at the location of the embankment be removed and that the embankment be founded IW-7009-01 E Mr. Harold Moe S; �AlC 1a&L'vI'SOiti;.INC. June 12, 1995 ( Page 10 on medium stiff or stiffer weathered till. Embankment slopes should be no steeper than 2H:1V on the downgradient side and 3H:1V on the upgradient side. The embankment fill should be considered structural fill and compacted to 95 percent of its Modified Proctor maximum density. Additionally, the embankment fill should be relatively impervious. In general, the nonorganic portion of the weathered and unweathered till that will be excavated across the site will provide a reasonably impervious embankment fill. We also recommend that the design of the embankment include provisions for a spillway in the event that the outlet control becomes temporarily plugged. The spillway may consist of riprap, cobbles, or pavement, depending on the calculated velocity of water that over-tops the spillway. Drainage and Foundation Backfill The conceptual drainage plans that we reviewed appear to provide reasonable drainage for the road. We would recommend, however, that the ditches at the point where they discharge into a culvert be paved so as to minimize the potential for water to enter the fill that will be placed around the culverts. For buildings that will eventually be constructed on the site, we recommend that footing drains be installed along the outside perimeter of the structures and on the upslope side of continuous interior footings. Footing subdrains should consist of slotted, 4-inch-diameter, plastic pipe bedded in washed 3/8-inch pea gravel. Typical installation of these drains is shown in Figure 10. Figure 10 also contains subdrainage and foundation wall backfill recommendations. On-site soils will not be suitable for use as drainage sand and gravel. Note that the perimeter subdrain invert should be located at least 18 inches below the lowest adjacent grade. A drainage geotextile should not be used around the subdrain pipe. Roof drains should not be connected to the footing subdrains. The discharge from footing drains and roof drains should be routed by means of a tightline to a suitable discharge point- (i.e., road ditches, storm sewers, etc.). Water should not be allowed to discharge onto the surface of a slope. All hard surfaces around the structures should be sloped to catch basins and the collected water disposed of as previously outlined. All outside grades should be graded to slope away from the structures. W-7009-01 • Mr. Harold Moe SHANNON b\IVI SON.INC. June 12, 1995 Page 11 Foundations In our opinion, spread footings bearing in the very stiff, weathered till and dense outwash could be designed for 3,000 pounds per square foot (psf) maximum allowable bearing pressure. Stripping depths to reach these soils is about 2 feet, based on the subsurface conditions observed in the test pits. Footings bearing in the hard, unweathered till may be designed for 6,000 psf maximum allowable soil bearing pressure. Where present, the hard, unweathered till is located about 5 feet below the ground surface. Footings bearing within structural fill placed over the hard till could be designed for allowable bearing pressures up to 4,000 psf. Continuous footings should have a minimum width of 18 inches, and column footings should have a minimum width of 24 inches. Minimum footing widths may govern footing design. Footings should bear at least 24 inches below the lowest adjacent grade. The bearing pressures given above may be increased by one-third for seismic loading conditions. If footings are supported by structural fill, this fill should extend beyond the outer edges of footings a minimum distance equal to the fill thickness below the footing. If adjacent individual footings are located at different elevations, it is recommended that the horizontal distance between them be at least 1.5 times the elevation difference between their bases. Where adjoining continuous footings are at different elevations, the upper footing should be stepped down to the lower footing. Foundation subgrades should be evaluated during construction to verify the presence of competent bearing soil, and to determine that all soft or loosened, disturbed soils and all existing topsoil have been removed. This evaluation should be made by a geotechnical engineer or his/her representative. Assuming compliance with the recommendations in this report, we expect settlement of con- ventional spread footings to be no more than about 1/2 inch, with differential settlements (between adjacent footings or over a 20-foot span of continuous footing) of approximately 1/4 inch, or less. Settlements would occur almost simultaneously with load application. W 7009-01 varranirn f Mr. Harold Moe Si�r^� ;U;�&�d ,i_ 0;�,INC. June 12, 1995 C Page 12 Floor Slab Support Floors for future structures could be constructed as slabs-on-grade bearing on very stiff to hard native soil or on structural fill compacted to 95 percent of its modified Proctor maximum density. Care should be taken to compact any localized backfills, such as footing or utility excavations, to the same degree as that specified for structural fill. We recommend that a capillary break be placed beneath the slab. A 4-inch-thick (minimum) layer of washed pea gravel placed atop floor subgrades, as shown in Figure 10, is one method to provide this break. The capillary break should be hydraulically connected to perimeter footing drains downslope. As illustrated in Figure 10, the use of 2-inch- diameter weep holes is one method for providing a hydraulic connection. A vapor barrier consisting of a plastic sheet should be placed directly over the gravel. Lateral Earth Pressures and Retaining Structures Basement walls of the future structures should be designed for an equivalent fluid pressure of 55 pounds per cubic foot (pcf), plus 1 pound per cubic foot for each degree of upward inclination of the backslope above the wall (this is valid up to 20-degree inclinations; pressures for inclinations greater than 20 degrees will require further calculations). Cantilevered retaining walls that are not connected to a structure and can yield at the top an amount equal to 0.001 times the wall height could be designed for an equivalent fluid pressure of 35 pcf, plus 1 pound per cubic foot for each degree of upward inclination of the backslope above the wall. These pressures assume the walls are drained so that hydrostatic pressures cannot develop. Recommendations for wall drainage and backfilling are presented on Figure 10. Lateral forces would be resisted by passive earth pressure against the buried portions of - structures and by friction against the bottom. In our opinion, passive earth pressures in backfill could be estimated using an equivalent fluid pressure of 280 pounds per cubic foot (pcf) above the groundwater table and 140 pcf below the water table. These values assume that the structures extend at least 18 inches below the lowest adjacent grade, and that the W 7009-01 Mr. Harold Moe SH".NNO N&Ws LSO N,INC. June 12, 1995 Page 13 back ill around the structure is a compacted granular fill. The above values include a factor of safety of 1.5. We recommend that a coefficient of friction of 0.5 be used between cast-in-place concrete and soil. An appropriate factor of safety should be used to calculate the resistance to sliding at the base of footings. Erosion Control Long-term erosion control can be achieved through adequate control and discharge of surface and subsurface drainage. Implementing the conceptual drainage plan and following the drainage recommendations contained in the following section of this report will mitigate potential long-term erosion. Re-vegetating cut and fill slopes with grasses will also provide long-term erosion control. An appropriate grass seed mixture for this area includes: Red fescue 40% Colonial bentgrass 10% Perennial ryegrass 25% Orchard grass 15% White dutch clover 10% Other seed mixtures may also be appropriate. For slopes of 2H:1V or greater, hydro- seeding is recommended. An appropriate hydro-seed mixture on a per acre basis may include: 100 pounds of grass seed 2,000 pounds of wood fiber mulch 250 pounds of 12-24-24 fertilizer 40 gallons of liquid soil bonding agent If the application is done in summer add: 80 pounds moisture retention agent 500 pounds extra of wood fiber mulch (2,500 pound total) W-7009-01 • Mr. Harold Moe SHANNON&WILSON,INC. June 12, 1995 Page 14 Summer applications must be irrigated. Seeding should be accomplished by October 15 to provide any erosional control for the following rainy season In our opinion, erosion at the site during construction can be minimized by implementing the recommendations in the Wet Weather Earthwork section, and can be controlled through the judicious use of fabric silt curtains and/or straw bales. Temporary erosion control should be the responsibility of the Contractor since he is also responsible for the excavation, the ditching, the sumps, etc. Construction Monitoring and Plans Review We recommend that we be retained to review portions of plans and specifications pertaining to earthwork and foundations to determine whether they are consistent with our recommendations. We also recommend that we be retained to monitor earthwork, including structural fill placement and compaction and subgrade preparation. Additional Consideration Test pit excavations were loosely backfilled with the excavated material. If such a test pit is located in a proposed building or pavement area, the loose material should be removed and replaced with appropriately compacted structural fill. Alternatively, the excavation could be structurally bridged. Limitations The conclusions and recommendations presented in this report are based on site conditions as they presently exist and assume that the explorations are representative of the subsurface conditions throughout the site; i.e., the subsurface conditions are not significantly different - than those encountered in the test pits and site reconnaissances. If, during construction, subsurface conditions different from those encountered in the explorations are observed or appear to be present, we should be advised at once so that we can review those conditions and reconsider our recommendations where necessary. If there is a substantial lapse of time W 7009-01 I Mr. Harold Moe S ,��NON E'J!.�;r SO�y.1�9C. June 12, 1995 { Page 15 between submission of our report and the start of work at the site, we recommend that this report be reviewed to determine the applicability of the conclusions and recommendations, considering the changed conditions and/or elapsed time. This report was prepared for the use of the Owner and/or Engineer in the design of the development and structures. With respect to constructions, it should be made available for information on factual data only and not as a warranty of subsurface conditions, such as those interpreted from the test pit logs, site reconnaissances, and discussion of subsurface conditions included in this report. Unanticipated conditions are commonly encountered and cannot be fully determined merely by taking soil samples or making explorations. Such unexpected conditions frequently require that additional expenditures be made to achieve a properly constructed project. Some contingency fund is recommended to accommodate such potential extra costs. Please note that the scope of our services did not include any investigation for the presence or absence of wetlands or environmental assessment for the presence or absence of hazardous or toxic material in the soil, surface water, groundwater, or air on or below or around this site. We are able to provide these services and would be happy to discuss these with you as the need arises. Shannon & Wilson has prepared the attached "Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our report. W-7009-01 Mr. Harold Moe SHANNON \Af, SO .INC June 12, 1995 Page 16 I If you have any questions regarding the observations, conclusions, or recommendations Icontained in this report, please call us. We appreciate the opportunity to be of service. ISincerely, SHANNON & WILSON, INC. 1 �,�n PRoF� c� o Z,7 f f A 44, ,, 1 ,1,4 c...D -,t,,_:.A.,:t fr I I I ►sl/! ' __.3�ip,No. 1650 fA r ,. /jam` 69 ► Si g s i ^ oEo L 0 G\s- , f. EXPtRFS l'a' 95 IWilliam J. Perkins, R.P.G. Gregory R. Fischer, P.E. Geologic Engineer Principal Engineer IWJP:GRF:WPG/wjp IEnclosures: Figure 1 - Site and Exploration Plan Figure 2 - Log of Test Pit TP-1 Figure 3 - Log of Test Pit TP-2 IFigure 4 - Log of Test Pit TP-3 Figure 5 - Log of Test Pit TP-4 Figure 6 - Log of Test Pit TP-5 IFigure 7 - Log of Test Pit TP-6 Figure 8 - Grain Size Distribution I Figure 9 - Typical Rockery Detail Figure 10 - Subdrainage & Backfilling "Important Information About Your Geotechnical Report" Icc: Harold Andersen, Quadra Engineering _ Jerry Smith, Jefferson County Permit Center I W 7009-01.LTR/W 7009-lkd/ect W-7009-01 REC r , SEA' LE ® GEOTECHNICAL AND ENVIRONMENTAL CONSULTANT �� ( � SC cOUS 11 SHANNON 6WILSON, INC SOS ON J.C. PERMIT CENTER August 5, 1998 gu Z 1'V1aTs v i � c Mr. Norman Coote 231 Mats View Road Port Ludlow, Washington 98365 RE: GEOTECHNICAL REVIEW OF BUILDING SITE AND PLANS, 92 MATS VIEW TERRACE, PORT LUDLOW, WASHINGTON (JEFFERSON COUNTY CRITICAL AREA REVIEW CASE NO. CAR98-0113) Dear Mr. Coote: In accordance with our proposal dated August 3, 1998, and as required by Jefferson County (letter to Mr. Norman Coote from Jefferson County Permit Center, dated July 9, 1998), this letter provides our geotechnical review of the plans and building site at the above address. As part of the process for creating the Mats View Terrace subdivision, Shannon& Wilson, Inc., prepared a geotechnical report ("Geotechnical Report,Mats View Terrace Subdivision, Port Ludlow, Washington," dated June 12, 1995)for development of parcels within the subdivision. In this report, we indicated development on slopes of 50 percent or steeper may require additional geotechnical explorations and studies. As lot 5 (also known as 92 Mats View Terrace) includes slopes of 50 percent or more, it was subsequently set forth by Jefferson County that a geotechnical review of the final proposed building site and plans be completed as a condition of approval of the subdivision. The purpose of our work is to provide this required review. This geotechnical review is based on a visit to the site on August 3, 1998, by Mr. William Perkins of Shannon& Wilson, Inc., and a review of building plans prepared by Norman Coote and Lynne Teveliet, dated April 15, 1998. SITE AND PROJECT DESCRIPTION We understand that the proposed residence will consist of an approximately 1,200 square-foot structure that includes a basement that will partially daylight out to the east. The location of the proposed building on the site is shown on the building plans provided by Mr. Coote and is staked in the field. The proposed building site is located over an area of the lot with relatively gentle slopes. Based on measurements made with an Abney level during our recent site visit, the slopes 400 NORTH 34TH STREET•SUITE 100 P.O. BOX 300303 W-8443-01 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.633.6777 TDD: 1.800.833.6388 ! Mr. Norman Coote SHANNON FiWILSON.INC. August 5, 1998 FrPage2 immediately above (east) of the building site slope down to the east at about 23 percent and less. The slopes immediately below(west) and in the swale to the south range between about 23 and 34 percent down to the east. The building site itself is relatively level. At the time or our site visit, subsurface conditions at the building site were exposed in a test pit that had been excavated at the building location by Mr. Coote. Soils observed in the test pit included 1 foot of topsoil, underlain by approximately 1 foot of medium, dense to dense, gray- brown with orange mottling, slightly gravelly, silty sand. At a depth of about 2 feet,the subsurface soil was a very dense, gray-brown with orange mottling, trace to slightly gravelly, silty sand. This very dense soil extended to the base of the test pit (9 feet) and became gray in color at a depth of about 5 feet. While the soils were moist, no groundwater seepage was observed in the test pit. SLOPE STABILITY ASSESSMENT AND RECOMMENDATIONS Based on the relatively flat slopes in the immediate vicinity of the proposed building site and the very dense, subsurface conditions encountered at a depth of about 2 feet in the test pit, it is our opinion that the site is relatively stable and provided that the recommendations contained in our 1995 geotechnical report are implemented, the proposed building will not have a significant adverse impact on the stability of the site or adjacent lots. A copy of the 1995 geotechnical report is attached. Please note that there is some risk of future instability on all hillsides, such as the one on this site, which the owner must be prepared to accept. Such instability could occur because of future water breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or other actions or events on a slope that may cause sliding. We note that some slope movement occurred on the steep slope portion of lot 1 of this subdivision some time between June 1995 and March 1998. In accordance with recommendations provided by Mr. Brian Dorwart of Shannon & Wilson, Inc., the soils that moved were excavated and replaced with a rock buttress and drains. We understand that no subsequent movement of soils on this or other lots in the subdivision have been observed. It is likely that the slope movement on lot 1 was due to the extensive clearing and grading that occurred on this and adjacent lots. Therefore, in addition to the recommendations outlined in our June 1995 geotechnical report, we also recommend that all vegetation not be removed from the entire lot. While some clearing will be required to construct the residence, yards, and W8443-01_Ltr/W8443-pec/ikd W-8443-01 ;I Mr. Norman Coote SHANNON i;WILSON.INC. August 5, 1998 Page 3 drive/utility access, this clearing should be done judiciously and not require removal of all vegetation on the lot, especially on the steeper portions of the parcel. We have reviewed the geotechnical aspects of the building plans. While you should review the attached geotechnical report so that you are aware of the site conditions and recommendations for development on the site, the following items should be incorporated into your building p lans. Minimum Footing Widths The plans we reviewed indicated that continuous spread footings supporting the wall are less than 18 inches wide. As indicated on page 11 of the attached report, continuous footings should have a minimum width of 18 inches, and column footings should have a minimum width of 24 inches. Drainage The drainage recommendations provided on page 10 and Figure 10 of the attached geotechnical report should be incorporated into your plans. While no groundwater was observed in the test it at the time of our site visit (in August), perched groundwater zones may develop in the P subsurface soils during the wet seasons of the year. In addition, we recommend that weep holes (see Note 1 on Figure 10) be used through the footings on the down-slope (east) side of the residence to provide a drainage path for water that may occur beneath the floor slab. With regard to drainage, we recommend that water collected in footing, roof, and other drains that may be constructed on the site(e.g.,yard or driveway drains) not be allowed to discharge onto the face of the slope. This water should be carried via a tightline to a suitable discharge point (e.g., the road ditch on the west side of Oak Bay Road located at the base of the slope). By discharging the water at a suitable point and not allowing it to flow across or into the soils on the slope, it is our opinion that site drainage will not have a significant adverse impact on yours or adjacent lots. Please read the"Limitations" section (page 14 and 15) and the "Important Information About Your Geotechnical Report" enclosed in the attached geotechnical report. This section and attachment were prepared to help you and others understand the use and limitations of the W8443-01_Ltr/W8443-pedlkd W-8443-01 Mr. Norman Coote SHANNON&WILSON,INC. August 5, 1998 Page 4 geotechnical report and the current review. Please call if you have any questions or if we can be of further assistance. Sincerely, SHANNON & WILSON, INC. ply11 DR PROFS � �'� lyC�� C., OREGON �0 x "� w R/Vi/APAJ.PP, K,A�1r .�i y i,/ $ p 9095 Noofo50 1</S-A`6 oc, FCIMr?, :: 1 r (' SS:SIONALeIG\ 8 OFOLOG\S' EXPIRES 7/23J jg t William J. Perkins, R.P.G. Herman H. (Tex)Druebert, P.E. Principal Engineering Geologist WJP:HHD/wjp Enclosure: "Geotechnical Report, Mats View Subdivision, Port Ludlow, Washington" dated June 12, 1995 W8443-01_Ltr/W8443-pec/1kd W-8443-01