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Home My WebLink AboutSlope Stability - BLD Engineering / Geo-tech Reports - 7/26/1984 Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. 2401 Bristol Court SW, Olympia, WA 98502 PO Box 12267, Olympia, WA 98508-2267 Phone 360-357-7883 FAX 360-754-4240 10 August 1998 LeRoy and Janet Hansen E1281 Timbertides Drive Union, Washington 98592 Subject: Discussion of slope stability issues and other site restrains that will affect the proposed replacement of your residence on Lot 29 in the Timber Tides Development, Mason County, Washington. Dear Mr. & Mrs . Hansen: As requested, I met with you to observe the slope and site conditions on your lot that will affect the replacement of your existing residence, a single-wide mobile home. From our discussion, we understand that the single-wide was originally moved to this lot some years back to provide you with a week-end cabin. Since your retirement, this has become your permanent residence. Because of deterioration of the structure, you are proposing to replace it with either a modular residence or a site-built home. Lot 29 is a long, but narrow lot. Width of the lot in the east-west direction is about 83 feet and the north-south direction about 550 feet. Most of the lot is over the slope down into Hood Canal and is not available for construction. The nearly level upland area is now occupied by the exiting mobile home, drainfield, garage, and water well. With the configuration of these structures, building a new residence will have some challenges . The least preferred replacement would be to move the existing mobile home off the site and reoccupy the building footprint with a new mobile home. We understand that the building and planning requirements �PQeo�� would allow you to do this, though we expect variances would still be required. We are not in favor of toh "N. option. The existing home is located immedi �e��°� J \� 98080101 \J\lo�o �C-) 0 x, Page 1 of 3 98080101 Page 2 of 3 adjacent to the break in slope. The structure is not sited at the minimum distance as required in Figure 18-1-1 of the Uniform Building Code. While the slope is apparently stable, there is always a risk of shallow slope failure removing soil support for foundations of a residence relocated on the existing building footprint. The setback from the slope not being in conformance with the Building Code might also adversely affect the marketability of the residence. Our evaluation of the slope, based on visual observations and our research, suggests that the slope is presently stable. If the slope is left in an undisturbed state, we do not expect major mass wasting events will develop. Typical failures on these slope are shallow mass wasting events carrying the upper weathered soil unit and vegetation downslope on the relatively unweathered soil unit. These events can vary in size and volume. They are usually triggered when construction on the slopes removes toe support for the soil mass or when concentrated flows of storm water saturate the soil mass. In the Coastal Zone Atlas for Mason County, published by the Department of Ecology, map MA-9, the slope between the upland area and Hood Canal is classified as an intermediate stability slope. We are in concurrence with this classification at this site. Construction of a new residence or replacement with a modular home should observe the UBC 40-foot setback from the average break in slope. In order to site the new structure, tR—e existing mobile home will have to be removed. The existing drainfield will have to be abandoned and relocated. Based on our site observations, we suggest that the new residence be located at the recommended minimum setback of the north wall line in conformance with the requirements of Figure 18-1-1 (a copy is enclosed) . This will protect the foundation from any foreseeable slope movements over the next 50 years and bring the �. structure into conformance with code requirements. �o��k Construction of decks between the residence and the top of slope is permissible, as these are low value o�\. structures that do not add appreciable loadings to th soils . The septic tank and drain field can be ' �,`��°e X\ reconstructed between the new residence and the to e�� °Q� " �G 98080101 Page 3 of 3 slope. We suggest the laterals be constructed within 25 feet of the north building line. At this location, essentially under the existing building pad, the 100 foot protection radius from the well can be observed. The existing free standing garage is already inside of the protection circle of the well. The new residence may also extend into this zone. Construction of the new residence under current rules may require a variance to allow for the encroachment. We would support the variance based on the following observations . The existing well is steel cased with a bentonite seal and is deep. The boring logs indicate at least 225 feet of cemented gravels in the upper soil zone the well penetrated; these gravels are for all practical purposes impermeable. The existing well head is located on a topographic high of the property, and surface flow of water to the casing cannot develop. We would not consider the well to be at risk of contamination from the proposed construction. Another option that could be pursued is the use of deep drilled foundation piers for support of a new residence at the location of the existing mobile home. The piers would have to extend• to a depth such that the 40-foot horizontal setback from the slope face would develop. These piers would also have to be designed as having the potential of becoming free standing columns for the upper 10 to 20 feet in the unlikely event that slope failure exposed them. We do not expect that this is an economical solution for your project as you would have the additional cost of preparing a geotechnical report, special structural engineering cost for the piers and grade beams, plus construction cost. If you have any questions, or if we can be of further assistance to you in your project, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES P �. David C. S rong Engineering Geologist Enclosure: Figure 18-1-1 L1 .9 c C 18-1-C-18-1-D,18 I-1 1994 UNIFORM BUILDING CODE TABLE 18-1-C—WEIGHTED EXPANSION INDEXI DEPTHINTERVALI 304.8 for mm WEIGHT FACTOR 0- 0.4 1-2 0.1 2-i 0.? .l-4 0.I Below 4 (1 I'llfe weighted expansion index for nonunililml soils is deiennined by multiplying file expanslon index f beach depth interval by the weight factor for that interval:old summing file pnxlucls. 21)epol in feel(305 Butt bdlow the groutxl surface. TABLE 18-1-D—FOUNDATIONS FOR STUD BEARING WALLS—MINIMUM REOUIREMENTSI,2,3 THICKNESS OF — FOUNDATION nches WALLWIDTH OF THICKNESS OF DEPTH BELOW (Inches) FOOTING FOOTING UNDISTURBED GROUND NUMBER OF FLOORS _ x 25.4 for min (Inches) — _—(Inches) — SURFACE(Inches) SUPPORTED BY THE unit FOUNDATION4 Concrete Masonry - 25.4 for rnm -- 1 -- 11 1c I --—�--- 7 Im i 10 9- I(J---_—I K — 24 — 1 Where Ullusual colxlllions or from c4Hklltl(Hls arc lound.Itxunlgs and Iilundatioos shall ts!as required in Section 11106.1. 'Ille groulm 111Rier Ile 114141r II►ay be cKcavalc4l 141 file elevation of life top o!till'Bening. ' 'Inferior sled hearing walls Illy be%upikilled by istllatell ltxllnlg Wkilh and fellglll%hall IV twice Illy width shown in this table alul the fixilings shall Ix spaced not more eha11 0 fleet(I1129 mm)on center. 41'UtIINIJ114HIS may supimi-I a rtmf in addtnon N4llte stipulated number of Iltxlrs.1-4iulltlalions supluerling rtxlls only shall he as required for supporting tHlc 11(mr. Face of footing Top of slope Face of structure H Toe of slope 1 H/'.i but need n01 exceed 40"12 192 mm)max. Hit but need not emceed 15'(4572 mm)max FIGURE 18-1-1 2-58 F 9 oG 5