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Home My WebLink AboutGEO-Tech for BLD2001-00363, 00599 - BLD Engineering / Geo-tech Reports - 5/30/2001 -�D a Q�'a -7 ss 'IC)1 I�-jq�oo (- 00`3�-3 Slope Stability Investigation 254 Union Heights Road II Union, WA Prepared for Ralph & Jody Bruce by Geotechnical Testing Lab Olympia, WA May 30, 2001 GEOTECHNICAL TESTING LABORATORY WASHINGTON HOME CENTER,INC. P. O. Box 176 CHEHALIS,WA 98532 Re: Slope Analysis for Bruce residence 254 Union Heights Rd. Union,WA Gentlemen: As per your request, we have conducted a slope stability and setback analysis for the above mentioned property. The results of this investigation, together with our recommendations, are to be found in the following report. The site is located near Union, Washington. SUBSURFACE EXPLORATION PROGRAM The subsurface exploration program consisted of a site reconnaissance and a review of information from our files of similar work performed in this area. The slopes were observed by our geologist, who collected samples from the soils on the site. The soil strata shown on the slope profile were observed at spot locations across the site and on the slope. SITE CONDITONS The site for the proposed home is located at the top of a gentle slope. All trees had been recently removed from the building area. The vegetation on the site consists of native underbrush and some conifers. No erosion was observed on the slopes and no deep seated instability was observed. An access road is provided at the top of the slope. Under the surface, the soil was found to be a dense silty gravely sand. GEOLOGIC CONDITIONS The geology of the area as taken from the "Coastal Zone Atlas, Volume 9, Mason County" consists of a Vashon till(Qvt) which is a member of the Vashon Drift, consisting of a very compact unstratified or very crudely stratified mixture of silt, sand, and gravel with erratically scattered and generally angular boulders. It may occur as a thick deposit that cuts across older materials and forms gray vertical cliffs or as a horizontal layer a few feet thick near the top of a bluff of older sediments. The native soils can be considered to be at or near their natural angle of repose, and the factor of safety for most of the slope will be near unity. 10011 Blomberg Street SW,Olympia, WA 98512 Phone#: (206)754-4612 Fax#: (206)754-4848 GEOTECHNICAL TESTING LABORATORY HYDROLOGIC CONDITIONS The native soils are free draining and we do not expect that these soils will allow for any buildup of pore water pressures. It is assumed that the natural ground seepage of water is to the east and ultimately to county roadside ditches and the Hood Canal. SLOPE STABILILTY MODELING &ANALYSIS The slope stability of the slopes in the subject vicinity is mapped as "stable" by the "Coastal Zone Atlas, Volume 9, Mason County". Stable slopes are predominately underlain by competent material such as firm pre-glacial deposits. Some slopes in the area of our study, however, are mapped as"intermediate". Intermediate slopes are considered to have less than critical geologic, groundwater, or wave erosion factors, but which may become critical, and therefore subject to landsliding if disturbed. Slopes may fail by a number of mechanisms, depending on the nature of the soil involved and the arrangement of natural earth materials at the site in question. Slope failures occur because forces tending to cause instability exceed those tending to resist it. Generally, the driving forces are represented by a component of soil weight downslope, and the resisting forces are represented by the soil strength acting in the opposite direction. The factor of safety(FS) of the slope is expressed as the ratio of the resisting forces or"moments"to the driving forces or"moments". When the FS is 1 or less, the slope must fail. When the FS exceeds 1, the slope is theoretically stable. When estimating the FS for the slopes encountered, we considered the following types of information: 1) The soil and water profiles 2) The kinematics of the potential slope failure 3) The strength and weight of the soil 4) The proposed slope geometry 5) Static and seismic (dynamic) loading conditions The site was modeled using a slope program and we have included a profile of the model later in this report. The native soils were modeled using 132 lbs/ft3 unit weight with a phi angle of 36°. We have calculated various"worst case" scenarios and have found that the site shows no indication of any deep seated instability of the slope under either static or dynamic loading. HILLSIDE FOUNDATIONS (DAYLIGHT DIMENSION) Structures built on sloping sites offer a number of potential problems. The basic consideration is what is called the "daylight dimension', which is the horizontal distance from the bottom of the footing to the adjacent ground surface. Many building codes require a minimum distance for this dimension in order to assure some safety against the pushing out of the footing in the downslope direction. Logically, the limit 10011 Blomberg Street SW,Olympia, WA 98512 Phone#: (206)7544612 Fax#: (206)7544848 GEOTECHNICAL TESTING LABORATORY for this distance depends on the type of soil and the angle of the ground surface. A distance of eight feet will provide the critical exposure necessary. A common problem with hillside construction is that material placed on any slope can become unstable if placed on the slope without compaction or the taking of precautions with regard to the control of runoff. If the spoils material is to be used to support any building or porch, patio, or veranda, the material must be compacted to 90% of the maximum dry density as per ASTM D-1557. Should a deck be planned, the support column pad must be placed to a depth of three feet into the ground. A close examination of the subject site showed no signs of cracking and slumping. No deep-seated instability was observed, but we recommend that the building construction continue to observe the (DAYLIGHT) principle mentioned above. HISTORY OF LANDSLIDE ACTIVITY Although landsliding has occurred along the shoreline, the subject lot is free from major earth movements. IMPACT ANALYSIS We do not anticipate that the proposed project will have any off-site impact. On-site care should be taken during construction to insure that runoff caused by wet weather is fenced to protect drainageways from siltation. OPINION OF POTENTIAL STABILITY Potential for instability is minimal. The septic is sufficiently set back to keep the slope from saturation. RECOMMENDATIONS We recommend that the proposed structure be built using a minimum horizontal distance of eight feet from the intersection of the slope to the foundation footing. Care should be taken when landscaping or moving material on the slope below the proposed home. All material excavated from the site cannot be dumped on site without adequate sub-grade preparation at the spoils location and proper fencing and runoff controls are placed. All spoils soil must be compacted to at least 90% of maximum dry density as per ASTM D- 698 and protected from erosion( i.e. straw, mulch, hydro-seed, etc.). Low shrubbery and vegetation should be encouraged to grow on the slope to mitigate erosion. Footing drains must be installed around the footing or basement with the invert of the drain placed below the bottom of the footing. In the footing area, the foundation for any building must be placed in the firm native soil or compacted fill. We do anticipate that some retaining walls may be needed to develop the slope side of the lot. These may be masonry or rockeries but we do not expect them to exceed four feet in height. However, should the walls exceed four feet, an engineer should be consulted and local building code requirements followed. In either case, the impact of the walls will not adversely affect the slope stability if drainage is directed toward controlled discharge facilities. 10011 Blomberg Street SW,Olympia, WA 98512 Phone#:(206)754-4612 Fax#: (206)754-4848 GEOTECHMCAL TESTING LABORATORY LIMITATIONS The conclusions and recommendations in this report are based on our interpretation of the site conditions, as they presently exist, anticipated future construction activities, and the expectations that our exploratory efforts adequately define the subsurface conditions throughout the site. In the event that the scope or location of the project should change, or subsurface conditions different from those encountered during our study be observed or suspected, we should be advised. At that time, a review of the changed conditions will be made and alternative or remedial recommendations given as requested. Thank your for this opportunity to be of service to you. If you have any questions,concerning this report, please contact us at (360) 754-4612. Respectfully submitted, GE(OTEr�N�IOC ESTING LAB 1�- Harold Parks, Engineering Geologist 10011 Blomberg Street SW,Olympia, WA 98512 Phone#:(206)754-4612 Fax#: (206)754-4848 ° its., '0gilill A ►.�rr!�i �,�� _ •. � �. � � err'•~�t ��(fj/J(1�;(t sfjijrf ��•'it-.-...�• f 7r�ry)Y�., T I: � � +R'�� I M IN HUM MW • �i.. 01 urrrrui r,�,�»�.91�l�i�llll� ��l�m�ifi�"��Je...,.�� a�l`iIIN�1.�Z1MA 'J' MINOR 1MAP 911t_ F//g, IVA ` `' G EOTECHNICAL TESTING LABORATORY GEOLOGIC MAP Qvt Qa2 Oal Tahuya vt O � ' Sisters fr/ Lse Dal `^�r � may/ 'f' _ !i ■� � ^��►-� fs�'�'s��,.�ia l�=rJ['!~'-.��r-�''T"��''� .- /- /. � I_�..��+ ■■.//��... .'•�..f: _ �/sue` `� � -.`\ -`i! ��- J 10011 Blomberg Street SW, Olympia, WA 98512 G EOTECHNICAL TESTING LABORATORY SITE PLAN ' Z�• U.rv�. W� _ _ -rlri.ple .sue. 7g, A.. FiL .L! u'.ti� 10011 Blomberg Street SW, Olympia, WA 98512 G EOTECHNICAL TESTING LABORATORY SLOPE STABILITY t t .. Ups - Tahuva • �"`/<'Y�^�..-..ter ��-_• Jt.. Sisters Pt. Ll J J .QO 10011 Blomberg Street SW, Olympia, WA 98512 Ralph Jody Bruce angle: 36.0° Unit Weight: 132 Critical Slope Angle,Saturated: 21.0° Horizontal Sectional Sectional Angle of Total Saturated Saturated Uusaturste<Unsaturated X Y Z Elevation Distance % Slope Factor Total Total Total Total Internal Factor Section Total Section Total Sectional Grid Station Elevation Change From Last Slops Angle d Safety Horizontal Change Slope Angle Friction of Safety )1> P> > )1> Factor Line Point Feet Feet Stu.pt.(it) P F Feet Feet % P V F Critical Critical or Safety From To 1 0 275.00 0.00 0.00 0 0.0° Ida 0 0 0 0.0° 18 o/a F Stu. Sta. 1 1 275.00 0.00 35.00 0.0% 0.0° n/s 35.00 0 0.0% 0.01 18 n/a Safe Safe Safe Safe a/a 0 1 1 2 270.00 5.00 30.00 16.7% 9.5° 1.95 65.00 5 7.7% 4.4° 18 4.22 Safe Safe Safe Safe 1.95 0 2 1 3 270.00 0.00 25.00 0.0% 00° n/a 90.00 5 5.6% 3.2° 18 5.85 Safe Safe Safe Safe 3.57 2 3 1 4 261.00 9.00 5.00 180.0% 60.9° 0.18 95.00 14 14.7% 8.4° 18 2.20 Yes Safe Yes Safe 0.18 0 4 1 5 256.00 5.00 1.00 500.0% 78.71 0.06 96.00 19 19.8% 11.2° 18 1.64 Yes Safe Yes Safe 0.06 0 5 1 6 251.00 5.00 5.00 100.0% 45.0° 0.32 101.00 24 23.8% 13.41 18 1.37 Yes Safe Yes Safe 0.19 4 6 1 7 251.00 0.00 57.00 0.0% 0.01 n/a 158.00 24 15.2% 8.6° 18 2.14 Safe Safe Safe Safe 1.16 4 7 1 8 246,00 5.00 15.00 33.3% 18.4° 0.97 173.00 29 16.8% 9.5° 18 1.94 Safe Safe Safe Safe 0.97 0 8 l 9 245.00 1.00 15.00 6.7% 3.8° 4.87 188.00 30 16.0% 9.1° 18 2.04 Safe Safe Safe Safe 1.27 4 9 1 10 240.00 5.00 12.00 41.7% 22.6° 0.78 200.00 35 17.5% 9.91 18 1.86 Yea Safe Safe Safe 0.78 0 10 1 11 235.00 5.00 15.00 33.3% 18.4° 0.97 215.00 40 18.6% 10.5° 18 1.75 Safe Safe Safe Safe 0.88 10 11 1 12 230.00 5.00 20.00 25.0% 14.0° 1.30 235.00 45 19.1% 10.81 18 1.70 Safe Safe Safe Safe 1.02 10 12 1 13 225.00 5.00 20.00 25.0% 14.0° 1.30 255.00 50 19.6% 11.1° 18 1.66 Safe Safe Safe Safe 1.09 10 13 1 14 220.00 5.00 13.00 33.3% 18.4° 0.97 270.00 55 20.4% 11.5° 18 1.60 Safe Safe Safe Safe 0.97 0 14 1 15 215.00 5.00 15.00 33.3% 18.4° 0.97 285.00 60 21.1% 11.9° 18 1 54 Safe Safe Safe Safe 0.97 0 15 1 16 210.00 5.00 15.00 33.3% 18.4° 0.97 300.00 65 2L7% 12.2° 18 1.50 Safe Safe Safe Safe 0.97 0 16 1 17 205.00 5.00 15.00 33.3% 18.4° 0.97 315.00 70 22.2% 12,51 18 1.46 Safe Safe Safe Safe 0.97 0 17 1 18 200.00 5.00 15.00 33.3% 1 SA° 0.97 330.00 75 22.7% 12.8° 18 1.43 Safe Safe Safe Safe 0.97 0 18 1 19 195.00 5.00 15.00 33.3% 18.4° 0.97 345.00 80 23.2% 13.1° 18 1.40 Safe Safe Safe Safe 0.97 0 19 1 20 190.00 5.00 18.00 27.8% 15.5° 1.17 363.00 85 23.4% 13.2° 18 1.39 Safe Safe Safe Safe 1.00 14 20 Slope Profile 280.00 loc tion of home MOO elevation 240.00 aa«► 220.tio 200.00 tao.00 0 too 200 300 Nmftortr l Oblaiise(Fat) GEOTL+ CHNICAL TESTING LABORATORY /r dayhyltl ,r FIGURE-" 1 Critical exposure condition for a hlllside•1•outing. � i i l� FIGURE- 2 Problertts of foundations on graded hillside sites. !Il#€�II�IIN I IIIN 10011 Blomberg Street SW, Olympia, WA 985J2 Phone#: (206)754-4612 Fax#: (206)754-4848