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Home My WebLink AboutBLD2004-00448 Geotechnical Report III a Consulting • Robert M. Pride, Inc. Engineer October 26, 2004 Mr. Raymond Stelk -. Erfinden ,-, DEC 10 2004 133A Osprey Ridge Drive Port Ludlow,WA 98365 JEFFERSON COUNTY OEPT. OF COMMUNITY DEVELOPMENT Re: Geotechnical Recommendations Proposed Residence ,yam 813 Thorndyke Road , C�' Southpoint,Washington �i Project No. 04-257-01 a� �N S � t 04/ O�NuEK Dear Mr. Stelk, 6 fl T C This letter provides additional recommendations for establishing foundation support for your residence under construction at 813 Thorndyke Road in Southpoint. It is understood that your grading contractor overexcavated a portion of the building pad III Depth the west and south walls of the house in the area of the garage and front try. Depth of this overexcavation below planned footing grade was about 2 to 3 feet.3 ' , In our geotechnical report dated July 19, 2004 we provided recommendations for footings to be placed either on native soils or on properly compacted fill. Soil bearing values were provided for various foundation conditions as discussed in that report. It is recommended that the overexcavated soils be replaced with properly compacted fill using the onsite or imported granular soils. These soils should be placed in thin lifts and compacted to 90 0 of their maximum density. Field density tests should be taken by an approved test lab to verify that adequate soil compaction has been achieved. Please call me if there are any questions concerning this supplemental letter. s pectfully, 4�,T ltit. A ^ -�� OF WAS7i ` ,o. z i,.) (1444 " Robert M. Pride, P. E. e ;�� ,�� Principal Geotechnical Engine r \ ` ''' `" / ' w "•i \� �`1 ; /c_ dist: (1) Addressee rmp: StelkResid2 IEXPIRES LO( 0t. 13203 Holmes Point Drive NE Kirkland,WA 98034 Phone: 425-814-3970 Fax: 425-814-5672 CV?G/./AL • 111 Consulting Robert M. Pride, Inc. Engineer ,.. July 19, 2004 Mr. Raymond Stelk Erfinden 133A Osprey Ridge Drive Port Ludlow,WA 98365 ► I �I I -_ I�`, ` Re: Report on Geotechnical Recommendations Ij it► AUG - 4 2004 Proposed Residence 813 Thorndyke Road �.�_; rj;orJ couN i Y Southpoint,Washington r)'_pi_ r- MUNITY DEVELOPMENT RMP Project No. 04-257-oi Dear Mr. Stelk, This report presents the results of our geologic evaluation and engineering analysis of your property(Lot#12)located on the southeasterly side of Thorndyke Road in Southpoint. It is understood that you intend to construct a new single family residence at the location shown on Drawing No. 1. Plans for the proposed structure were prepared by Mascord, and Craig Owen is the project structural engineer. The purpose of this study is to document the underlying geologic profile and to provide recommendations for the design and construction of the new residence. Available DOE Coastal Zone Atlas maps along with previous engineering reports by Northwestern Territories(NTI) and Shannon&Wilson(S&W)were used as references for this evaluation. Site Conditions A majority of the building pad is relatively flat with a gentle downward slope from Thorndyke Road to the bluff. The rear yard extends out to the top of a steep slope that descends approximately 115 feet to a flatter gradient. Overall the slope has a gradient of about 6o%. Geologically this property is underlain by Glacial Till(Qvt) consisting of very dense silty sands, sands and gravels. There has been no recent history of shallow sliding on this property, and the 1996-97 slides to the south involved only the outer colluvial soil zone situated on a steeper slope surface. Causes of that previous slope instability were attributed to uncontrolled surface water runoff, heavy rainfall and snowmelt, and flooding from uphill properties that has since been corrected. The presence of the over- consolidated glacial till on this and adjacent sites will provide for excellent support for residential foundations. 13203 Holmes Point Drive NE Kirkland,WA 98034 Phone: 425-814-3970 Fax: 425-814-5672 • • July 19, 2004 Mr. Raymond Stelk Page 2 00 Proposed Construction House drawings prepared by Mascord show a single story wood framed structure with a high roofline. Continuous bearing wall footings will provide support for this lightly loaded structure that will have a raised wood floor system. An attached garage will be located on the westerly corner of the house. Access from Thorndyke Road will be over a compacted gravel driveway. The septic drainfield area will be located between the house and the street in front of the property. CONCLUSIONS AND RECOMMENDATIONS General On the basis of our field investigation and engineering stability analyses,it is our opinion that the property is grossly stable from a potential deep-seated failure. Bluff retreat is a known condition that affects steep slopes and loss of rear yard area due to soil weathering and slope saturation of the surficial soils. Due to the steepness of this slope and the very dense nature of the over-consolidated glacial deposits,the potential for even surficial slides is considered low. The presence of this glacial till layer is the reason that the rate of slope regression has been low during the life of the existing residential structures. Slope Stability Conclusions Several failure analyses were performed to determine the factor of safety of this steep slope extending from the east edge of the rear yard to the toe of the slope. These analyses were conducted to confirm that the easternmost location of the house would be founded on glacial soil deposits that had a minimum factor of safety of at least 1.5 at a distance of 25 feet from the top of the slope. The results of these analyses show that the overall slope has a gross factor of safety in excess of 2.0 when considering the foundation setback of 25 feet from the top of the slope. Refer to Figure Nos. i and 2 that summarize the results of the stability analyses and shows the failure circles that extend from the upper building pad to the toe of the slope. Foundation Recommendations The new residence should be supported on continuous bearing wall footings that extend a minimum depth of 18 inches below final grade. It is recommended that the westerly foundation walls be located no closer than 25 feet to the top of the existing slope. An 13203 Holmes Point Drive NE Kirkland,WA 98034 Phone: 425-814-3970 Fax: 425-814-5672 • # July 19, 2004 Mr. Raymond Stelk Page 3 00 allowable soil bearing value of 300o psf and.2000 psf may be used for the lightly loaded house footings founded on native soils and properly compacted fill, respectively. This value may be increased to 400o psf for retaining wall footings that are at least 24 inches in width. Settlement of footings founded on adequate bearing soils will be minimal. Resistance to lateral loads on the structure can be provided by passive earth pressure on footings and the stem walls. We recommend the use of 30o pcf for passive pressure, and a coefficient of sliding friction of o.5. Retaining walls should be designed for an active earth pressure of 3o pcf,and they should have a foundation subdrain to eliminate hydrostatic pressure buildup. All retaining walls should be backfilled with soil compacted to 90%of their maximum density to reduce the potential for future settlement under proposed concrete walkways and porch slabs. Exterior Slabs-on-Grade Concrete slabs may be poured directly on the very dense glacial soils or compacted fill that are expected to be exposed at final grade. Control joints should be included on about ten foot centers across the slab sections to minimize shrinkage cracking. If the concrete is poured with a low slump,reinforcing steel is not required for any slabs supported on native or compact soils. All concrete should be placed in accordance with ACI guidelines. Drainage In addition to the foundation drains around the perimeter of the basement wall footings, there will be area drains and roof drains that will collect storm water runoff. All of this water should be tightlined to the street at th.e front of the property, or to a suitable infiltration trench that is at least ioo feet west of the top of slope. Every effort should be made to prevent surface water runoff over the top of the rear yard slope, and to prevent excessive infiltration into the upper soils near the steep slope. Summary We have discussed our recommendations for continuous bearing wall footings with the structural engineer who is in the process of revising the house drawings. Construction and consultation services should also beprovided to verifythat subsurface monitoring are similar to those described in our report. Should conditions be revealed conditions Po during construction that differ from the anticipated subsurface profile,we will evaluate those conditions and provide alternative recommendations where appropriate. The conclusions and recommendations presented in this report are based on i)our interpretation and evaluation of soil conditions on the site, 2) confirmation of the actual 13203 Holmes Point Drive NE Kirkland,WA 98034 Phone: 426-814-3970 Fax: 425-814-5672 S July 19, 2004 Mr. Raymond Stelk Page 4 subsurface conditions encountered during construction, and 3)the assumption that sufficient observation and testing will be provided during appropriate phases of the work. Our findings and recommendations of this report were prepared in accordance with generally accepted principles of geotechnical engineering as practiced in the Puget Sound area at the time this report was submitted. We make no warranty, either express or implied. Please call me if there are any questions regarding the findings and recommendations of this report. R pectfully, obert M. Pride, . E. ~" Principal Geotechnical Enginee f dist: (2) Addressee 6271 40NAL encl: Drawing No. Figures 1 and 2 l 7�� EXPIRES ) r rmp: StelkResidi 1 13203 Holmes Point Drive NE Kirkland,WA 98034 Phone: 425-814-3970 Fax: 425-814-5672 ....., • 4111 .,*"V , 3h nioarcencE moo ... -- / / .�-r 261-- l�l :-. 20 ' i 20. r r 1' 1 1 �-216 , IA.�', IV I //. 11 1 1 �/ /i PI 7/1 ---- _ t / - am • ;: ' i 'ft1J7' f, ' i ? _-- =1A bk0 1 --- _J i , :,.5)61-,-c.- lel,!� . itls.►i _ .adr .' 256 ---} 250�fJ �r - yr 244 1 z38 —4 - 23' ��l 230 224 `-------220- Ref: Site Plan and topography by'Malan Engineering, Inc. SITE PLAN Proposed Stelk Residence Project No. 04-257-oi 813 Thorndyke Road Southpoint,Washington Drawing No. 1 Robert M.Pride,Inc. consulting Engineer 7.; Pride, Inc. ..• . • ..., ,... 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