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HomeMy WebLinkAboutGEOtechnical Assessment 2003 & 2011 - BLD Engineering / Geo-tech Reports - 4/25/2011 Mason County Department of COMmunity Develo ment Submittal Checklist For a Geological Assessment Instructions: This checklist must t*.subrmtted with a Geological Assessment and completed, signed, and stamped by the licensed professiona)(s)who prepared the Geologies{Assessment for review UY Mason County pursuant to the Mason County Resource ordinance. if an item found to be not applicable, the report should explain the basis for the conclusion_ Applicantfowii± 1 N Parcel OCa 1 v Site Address (1) A discussion of geologic conditions in the general vicinity of the proposed developmeril, with geologic unit designation based on referenced maps Located on page .s} 4 (2) (a) A discussion of the grgand wat%_cgpditions at the site, Located on Page(s) V',r..P 3 0+ (b) A discussion of thees�imated depth to water Located on page(s) 1' ;_3 r (c) A discussion of the ci aritity of�surface seepage Located on page(s). , •r I, (d) A discussion of the tipslope geomorphology Located on page(S) 1 r (e) A discussion of location of PIA woterbodies and wetlands. Located on page(s)_ PIA (a) The approximate depth�o hard qr dense competent solt, e.g. gladul till or outwash sand. Located on pages)_ 3.r Y (4) A discussion of any geomorphic expression of past slope instability(presence of hummocky ground or ground cracks,terraced topography indicative of landslide block movement, bowed or aphod tree iny�icating downslope, movemert. o?te). Located on page(s) }� Z 0 (5) A discussion of the history of landslide actwitY in the vlclnity, as avaiiabie in the referenced maps and c�cords. 11 Lacata�rl nn page(s) l`K- 2—/'— (5) An opinion on whether the proposed development is within the landslide hazard area or its associated buffer or setback and the potential for landslide activity at the site in right of the proposed develorpe 3 # P� Located on page(s)� 0i��Y�, of (7) A recommendation by the preparer whether a Geotechnical Report should be required to further evalm#A site conditions and the proposed development of the subject property. Page i of 2 Form Effective June 2008 Located on page(S)� �- ($) if the presence of a l7azand is Geterrrtrr►ed within 300 feet of the proposed development, then the following am tietinmated on a geologic mapl site map: (a) the area of the proposed devel ptneM, Located on Map(s) (b) the tjr undaries of the Ian s i l mazaard ama (top, both sides, and toe), Located on Map(s) ------ (c) the associated buffers(t0l?. h sides, and toe) Located on Map(s) - /' - (d) building or other setbacks both sides, and toe), Looated (9) A site map drawn to scale shawmg the property boundaries,scale, north arrow,and the location and nature of existing and p sed develop ent on the site. Located on Maps) E�-c!� •^` �lc Gwn•7 hereby certify under penatty of poury tnat i am a cMl ngineer licensed in the State of Washington with specialized knowledge of geotechnicaUgeological engineering or a geologist or engineering geologist licensari in the State of Washinctton with special knowledge of the local candraans. t also certify Ail that the Geological Assessment,dated /S©°t� ��, and entitled meets all the requirements of the Mason County Resource Ordinance, Landslide Hazard Section, is complete and true, that the assessment dernomitrates conclusively that the risks posed by the landslide hazard can be mitigated ed in such ohm v inciuded to prevent harmtol design y a d public health aendatios,and nd aFI hazards are nd safety (Siggn�atu)re_and Stamp)/l✓ �'P`I� o W t kA L V-e v�$ �� Cs v�� ✓� 0 e— r✓/ /�'LP-t�j 1 J q"- O! `�! �4N yPL th/� s�� �C(J`E��G�J�1/�U'Y►�� 'C���j'vS(/�C c.G/ T� I�P�Yn"�-- n^-e 1'5 1/4 PIr-If 0 9 d cJ Disclaimer: Mason County goes not certify the quality of the work done in t olo$ + Assessirient. F Page 2 of 2 ` 04/25/11 09:24 FAX 1 360 867 9307 Bradley Group f71 Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 25 April 2011 Mrs. Kim Delaney 4190 East Agate Road Shelton, Washington 98584 Subject: Compaction of gravel borrow in the south center section of the footing for your new garage at the above address. Dear Mrs. Delaney: I am surprised that the Mason County Building Inspector did not feel that they were competent in evaluation of a thin (2-to 3-inch thick) fill section of gravel borrow placed and compacted in the center section of the south footing line. This section of footing will not support building loads as it is the future opening for the garage door and will only be supporting slab and slab loads. To have us re-evaluate this thin fill section has caused you delay and additional expenses. As I directed you, to compact this thin fill section we recommended that you use a flat- faced hand compactor typically used for compaction of asphalt concrete- The compaction criteria was the development of a surface of firm and non-yielding material. Based on our inspection,the fill as placed does meet this requirement_ Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES David C. Strong, L.E.G, ��` •III I I � 'I 03100603 Page I of I I ? 'II II i 04/21/11 08:31 FAX 1 360 867 9307 Bradley Group z01 i l Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 - FAX 360-867-9307 i 21 April 2011 Mrs. Kim Delaney i 4190 East Agate Road Shelton, Washington 98584 i Subject: Compaction of the 3-inch minus bank run gravel used for the leveling course under footings. j i Dear Mrs. Delaney; From our telephone conversation, we understand that you had placed about two to three inches of bank run gravel in a section of the footing to be cast to control the generation of"mud" on the surface of the subglacial till. We understand that the rest of the footing excavation has extended to and exposed till for foundation support To meet the intent of the International Building Code (IBC), the fill placed must be subjected to mechanical compactive effort. This compaction can be by hand tamping or use a plate compactor. In that this material is thin and coarse, standard compaction testing is not feasible. The material should be compacted to a firm and non-yielding surface. The alternative would be to remove the bank run gravel and expose the till under this section of the footing. If you have any additional geotechnical questions on these recommendations, or if we may be of further assistance to you on your project, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES David C. trong, LEG. o #� 12 03100602 Page I of I =� l s �. � .��0.t ,�.• t c;r s si i � 21. vp��IXO,I e ` 42 Xl'y .. e e„'l• 1 Kl *f 576> or S y N i Y 29 � .:. ?�Od GO Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 15 October 2003 Ms . Kim Delaney 22212 SR 410 5' Bonney Lake, Washington 98390 q Subject: Geotechnical assessment of the slope stability and erosion potential and their probable long-term effect on construction of a residence on your property located in the Church Point area of Hammersley Inlet at the end of the Agate Beach Place road, Mason County Tax Parcel Number 2400010 . Dear Ms . Delaney: As requested, we visited the above property in order to develop an opinion on slope stability and erosion potential of the site and their probable long-term influence on construction of a new single-family residence on this lot. Our scope of work consisted of a site visit, review of available geologic information, and preparation of this report based on our observations, research, and understanding of the proposed construction on this lot. The project site has gentle slopes in the northern half with slopes increasing to the moderate range in the southern half . A low marine bluff is found at the toe of the slope at the back of the beach adjacent to- Hammersley Inlet. At the time of our site visit, test pits had been excavated in the area of the proposed septic drain field. Soils exposed were examined by us as part of our field work. We found the site to be vegetated with both soft and hardwood trees . Low-growing brush and ground cover typical of the forest form the low vegetation. Examination of the property in the area between the drain field to the back of the beach did not indicate 03100601 Page 1 of 4 r 03100601 Page 2 of 4 that recent or old mass wasting events have occurred on this property. We did note that some of the trees in the lower slope area do exhibit bending of the trunks due to soil creep, the slow downslope movement of the active soil zone resulting from saturation and freeze-thaw cycles under gravitational influence. Soil creep is not an indicator of future major mass wasting events . Marine erosion of the toe of the slope appears to be slow. Soils exposed in the vertical face, about six to eight feet in vertical relief, are highly overconsolidated and weakly cemented. These soils are naturally resistant to erosion. You can expect that from time to time small mass wasting events will occur in the marine bluff area. This is caused by erosion of the bluff face by sheet washing and freeze-thaw cycles . This erosion creates unsupported soil masses with vegetation at the top of the bluff . These masses fail when sufficient material is removed and fall to the beach. These events are random in occurrence. Between events, apparent stability and slow erosion returns to the marine bluff. You indicated that you were considering the construction of a rock bulkhead. You need to be aware that the permitting process for construction of armoring structures along the beaches is at best difficult . We expect that the permitting process for a stairway to access the beach from your property is feasible, though this process can also be lengthy. We consider stairways to be low-value structures . Stairways are subject to damage from either drift material moving along the beach or from mass wasting events from the bluff face. We have no objection to their construction at this site. The foundation should be anchored and designed to resist damage from logs and other large floating material. You said that the Mason County Planner who visited the site indicated that a geotechnical assessment was appropriate for this site. Based on our field observations and research, we agree with the planner, and we do not consider the more rigorous geotechnical report necessary for development of this lot. We also understand that a setback from the top of the marine bluff to the building footprint using the common setback line from adjacent structures will be imposed. 03100601 Page 3 of 4 We have consulted three geologic references that present information on area geology and opinions on slope stability. The Coastal Zone Atlas Volume 9 , March 1980, Map MA 23 presents information on site geology and an opinion on slope stability. Site geology for this area is indicated as being either Vashon drift (Qv) or Vashon subglacial till (Qvt) . Soils in the toe area are indicated as Pre Frasier undifferentiated drift (Qdr) . This map suggests that the upland area is stable and that the bluffs and area immediately upland are unstable. We concur with this opinion on slope stability. Plate 1 of the Water-Supply Bulletin 29 for Southeastern Mason County, Washington by John B. Noble and Dee Molenaar was also consulted. This map indicates the upland area as Vashon subglacial till (Qvt) . The bluff in the Church Point area is mapped as Salmon Springs Drift (Qss) . This is a correct geological mapping of the Pre Frasier of the Coastal Zone Atlas . The Relative Slope Stability of Southern Hood Canal Area, Washington by Mackey Smith and R.J. Carson, Map I-853-F, published by the Washington Department of Natural Resources 1977 does not extend mapping to this area. The site geology. in this area would suggest that if the mapping had extended to this area, the slopes on this property: would be assigned a Class 2 which are believed to be stable under normal conditions . With the area being underlain by highly overconsolidated soils, deep rotational type of failure is unlikely to develop, and no mapping of this type of mass wasting event is indicated in the reference in this area. Construction of the residence at the location using the common setback line from adjacent structures is considered by us to be reasonable and prudent. We understand that you are proposing to step the structure down the slope to minimize site excavation. With the relatively thin outwash soils at this site, deep excavation is likely to encounter the subglacial till . Subglacial till has many engineering properties similar to those of lean concrete . It is for all practical purposes impermeable. If site grading exposes the contact between the recessional soils and the subglacial till, you should expect seasonal flow of ground water to develop on its surface during periods of heavy or prolonged rainfall . 03100601 Page 4 of 4 Collection and disposal of storm water from impervious surfaces must be addressed in the site plan. We recommend that all water be collected into tightlines with the water carried to the toe of the slope for disposal . Footing drains may be connected to this system at an elevation such that backup into the footing zone cannot occur. No uncontrolled water is to be directed over the slope between the residence and the toe. Concentrated flows of water can create areas of localized saturation and induce slope instability or accelerated erosion. Sloping of the foundation backfill to provide a positive slope away from the structure of at least 2% for five feet is recommended to prevent ponding of water against the foundation. The ground water table will not influence slope stability at this site. Based on our field observations and research, it is our opinion that a residence may be constructed on this lot at the location indicated to us with no risk of slope failure or erosion of the slope affecting the soil foundation support for at least the next 50 years . Attention to drainage is required to be included in the site development plans . Natural slope angles are less than the expected angle of internal friction for the soils forming the slope. When the slope angle is less than the angle of internal friction of the soils, slope stability is inferred. If you have any questions, or if we may be of additional service to you on this project, please contact us at our Olympia office. Cordially, `eo{ Wash; BRADLEY-NOBLE GEOTECHNICAL SERVICES David C. Strong, L.E.G. oP 1129 �Sed Geoff 0 DAVID C. STRONG