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HomeMy WebLinkAboutMLA17-00043 GEOTECHNICAL REPORT / `R'{°'„ NORTHWESTERN TERRITORIES, INC. .bL 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 �� 914 WASHINGTON STREET SUITE 6,PORT TOWNSEND,WA 98368 I a .: Engineers Land Surveyors Geologists Construction Inspection Materials Testing 1 NTI Port Angeles(360)452-8491 E-Mail: infopnti4u.com December 28, 2016 Karen Bianchi 1334 S. Dawson Street Seattle, WA 98101 Subject: GEOTECHNICAL REPORT—PROPOSED DOCK REPAIR 170 MCDONALD COVE P-4 BRINNON,WASHINGTON JEFFERSON COUNTY TPN:502282012 Dear Mrs. Bianchi and Others Concerned, Northwestern Territories, Inc., (NTI) is pleased to present the following report for the repair of your dock and stairs that were destroyed by a storm in 2013. This project intends to replace the original 1971 structure under a Type I Shoreline Exemption Permit. This was discussed at the Pre-Application Conference on March 16, 2016, in the May 5, 2016 NTI letter to Jefferson County Department of Community Development (DCD), and in the modified DCD outline of the March 16, 2016 Pre-Application meeting received December 13, 2016. This report includes discussion and analysis of the geotechnically-related critical areas/geologic hazards, local geology and earth materials, and, foundation design recommendations for the proposed dock and stairs repair. Based on our site assessment work, the proposed shoreline dock structure repair/ replacement is suitable provided that the geotechnical recommendations within this report are followed. Project site plan and preliminary dock repair elevation sheets are attached to this report. 1.0 BACKGROUND AND SCOPE OF WORK NTI Engineering and Land Surveying (NTI) was requested by Mrs. Karen Bianchi to perform a geotechnical investigation of Jefferson County TPN: 502282012. The property lies near Brinnon, Washington, within the Northwest Quarter of Section 28, Township 25N, Range 2W, WM, Jefferson County, Washington. The parcel is located at 170 McDonald Cove, east of Highway 101, near Brinnon, Washington. NTI's Trent Adams, E.I.T., and Glen Wade, L.E.G., completed site assessment work on April 8th, 2016. li "isa err, 4 3 ` s i.,s 3 ,-t g ,1 J!1 Y V Geotechnical Report 2016 The site plan and profile sheets showing the existing and proposed structures are attached to this report. The 1994 photograph below shows the dock and stairs before the 2013 damage. • =ate. ... .$ I {. -4'�r,Illi Eta • -.. .*Amipil ,._ ,,,,, - . . . ..... . , .. _ _silk .1 , , _,.. it r ---- w k # 0 is : � , by .. � F'� .,mot^' ►� wAF f;:kjeci --- ,.. r- S .,m$ 1994 Site Photograph Looking Southwest The photograph below shows the current conditions at the site, after the 2013 damage. t. .. �, t } s a, -''.' - ittrtti-t- -....--.,-....pc,..i....,,,, , ,... , .7, ,,, „--. ;,, -,.,- -r.. .,,k„)..,....., , , ...,,,..1,,...,„,..,. ,-:,,-z., _,.,7_,,:t1...„. ,--,-:,-.:;,. , ; ,i,-4- „ ,,.. 4,-.„-,,, ,-. ,4#0,4„ :‘,-;:7:- -,..?: .,. ._ ,,„:40,,,-,-„ . .,,,,.- --, ,-,11, „7,i , ,,,, , vo,„. ,,,,,,,..., 4 . ", , --, . n .... .4.-..,:.- -,...,p,7 -,)-x, - _, fir. ;02L- .'ti k'�-'S ," , ..- a . -. . i -- ti x �L • I 3. +� w- ttzt fir. . """YYY . . - �! 1 i yy #5 '' y^ ' 1, Y', r-:„ ,#' ,e^y s _ 4``r,}v s A"€^5�K tq y+S ,- 4 { S 2016 Photograph, Looking North : '� JEFFERSON COUT4TY 3CD Geotechnical Report 17016 2.0 SITE CONDITIONS 2.1 The Local Geologic Setting The 2012 Washington State Department of Natural Resources (DNR) Geologic Map of Brinnon by Polenz, et al, shown in Figure 1, below, indicates the site is principally in an area of Tertiary-age Crescent Formation Basalt, Evc. This geologic unit is described by DNR as fine-to coarse-grained blocky subaerial flows with rare and thin sedimentary interbeds; typically weathered and fractured. The map also indicates that the bedrock in the site vicinity strikes in the northeast direction, dipping 12 degrees to the southeast. Field observation of exposed rock along the bluff, shoreline, and extending beyond the low tide into Hood Canal at the subject parcel confirmed the presence of Crescent Formation basalt bedrock with a strike and dip similar to the DNR measurements. ,iu U91 - l 2 Eve - Ogt� Qgo� Qam (]gt {iglC _y30 Ogic ` m t�l _>Qa,n r ^c , -S21 ml Evc tflgic 6 _ Fault Trace i i Evco t { t r r, ,f Evc F s` r, / _ G13 Qgt 7-4 - _ ' tgty Qgt Eve I _ 1 `, Op Evc-_,- -- 'Evc' �,� Evc� r —� Evc %Evc'- Qgt Evc " Ecc Evc x - - / � � _ • ��+;�- pfd .. J / Ogt !) 4 y 1 1( , .1,,,,p, 7, rte` ''� - - Eve �. Q { �` Evc c 04/ ,� r Qgt . , ' 12 c :•-__ ,..e.,..___ N„ ,,./... ..,,—" "A" QEF„',( ✓OP° 'Cl, S20 Subject Site J --a{ Figure 1 - Portion of the 2012 Department of Natural Resources Geologic Map of Brinnon. 3 Geotechnical Report 2016 The DNR image also indicates the presence of an east-west trending thrust fault of unknown age that lies about 1 mile northeast of the site. This fault is not shown on the fault map for Washington (Faults and earthquakes in Washington State, Open File Report 2014-05, by J. L. Czajkowski and J. D. Bowman, 2014, 36" x 45" color sheet, scale 1:750,000), but it does show the "Class B" Hood Canal fault trending northeast within roughly the entire length of the Hood Canal about 2 miles to the southeast of the subject property. This fault is shown in green and has the blue number 552 next to it on Figure 2 below. The Class B designation for the fault indicates that the fault is suspected to have had Quaternary-age (within the past 2.6±million years) deformation, but that insufficient evidence has been gathered to support a more definite age. ..;----- • • -?/, --. • /) . , 41111{ Subject Site > 4 4 ,,J go • it , # .err►: 41001111111111 -,1 , 07. mit , R* •. .d114 .....A. ' ''' 77 1P:".. : . * • iill° 1241: 10. * • itfii s • • M f • # ,,. • .4, ,-/ • se • _..,./1411/ il zier 0 * 0 0 4041) •40 : ." , .1 • Figure 2—Washington Fault and Earthquake Map The Holocene-aged (activity within the past 11,700 years) Seattle Fault zone, shown with red and labeled with number 570 next to it is about 5 miles from the site at its closest point. The map also shows two earthquakes of 3.0-6.8 magnitude as purple dots near the site, one about 2/3 mile southwest of the site and one on the trace of the Hood Canal Fault where it is closest to the site. Respectively, these earthquakes were a 3.2 magnitude earthquake from 1978 at a depth of 46 km and a 3.2 magnitude earthquake from 1971 at a depth of 14 km per the USGS site https://earthquake.usgs.gov/earthquakes/map. ---' V ) MMC ,- .a W r A i .. `L 4 JEFFERSoj C°UNTY DCD Geotechnical Report 12016 2.2 The Surficial Soils The 1975 Soil Survey for Jefferson County prepared by the Soil Conservation Service (SCS) and United States Department of Agriculture (USDA) indicates Olette-Hoodsport Complex (OpD) soils are present in the vicinity of the site. The Hoodsport Series soil is described as a reddish- brown very gravelly sandy loam underlain by cemented glacial till, while the Olette Series soil is described as a reddish-brown very gravelly silt loam underlain by basalt bedrock. Soils matching the description for the Olette series were confirmed by on-site observations at the top of the bluff slope and westward through the upland portion of the parcel. However, there is no soil exposed in the shoreline area of the proposed repair, only intact bedrock. 2.3 Slope Stability The 1978 Washington State Department of Ecology (DOE) Coastal Zone Atlas Slope Stability Map classifies the basalt bedrock and surficial soils in the vicinity of the site as having intermediate slope stability. The slope stability mapping and hazard ratings shown in the Coastal Zone Atlas were derived from the 1976 DNR Relative Slope Stability Map of Brinnon by Carson. Carson indicates that areas of intermediate slope stability are believed to be generally stable under normal conditions, but can become unstable if disturbed by man's activities. Around the time period that fieldwork was taking place for the DNR study, an unstable recent slide was mapped near Highway 101, less than a quarter-mile west of the site. The map indicates that this slide is believed to have occurred within 50 years prior to the creation of the DNR slope stability map. / r .r 1 ���� �. H f , 1 22$2+..'' V z* ' / •M� ``�. TWA *,� --- r. 5022820i t ' s t �.;'l �,x s 7/1"4"C7 r , tr is `;9 f / , s{Y ` Subject Site Figure 3 - Portion of the Department of Ecology Slope Stability Map of Jefferson County. 5 Geotechnical Report 12016 3.0 GEOLOGIC AND GEOTECHNICAL SITE OBSERVATIONS 3.1 Dock Structure Area Site Observations The area where the dock pilings are to be replaced is a somewhat planar dip-slope of the hard, intact, slightly weathered volcanic bedrock, sloping about 10-15 degrees to the east. This is consistent with geologic mapping. The rock surface is locally covered with a thin veneer of algae and sporadic barnacles. This rock surface extends westward, or landward, about 12 feet to the existing row of nine deck support piers, and about an additional 3-5 feet to the base of the bluff slope. 3.2 Bluff Characteristics and Geotechnical Indicators The bluff slope is about 16-20 feet high at the subject parcel, containing several sections and features. The lowest section of the bluff slope beneath the residence and deck has a slope of about 1 horizontal: 1 vertical for a height of about 6-7 feet. We observed some raveling in this lowest section of the bluff, consisting of gravel-to cobble-sized rock fragments that appear to topple down the slope. This minor toppling likely only occurs at times of very high tides and storm-driven wave action. The rock in the bluff slope is moderately to intensely weathered and moderately fractured, which makes it susceptible to scour, in contrast to the intact rock exposed throughout the area of the dock and deck supports. We observed very minor groundwater seepage in the lower few inches of the bluff, which was likely perched on the surface of the intact bedrock. Above the lowermost part of the bluff slope, the slope beneath the residence is about 2-3 horizontal: 1 vertical, for a height of about 6-8 feet. This section of the slope consists of intensely weathered rock and some surficial soils derived from the underlying bedrock. The slope above this section is much flatter, and includes the residence's interior footings and posts that were repaired in 2014. We did not observe any significant erosion in this portion of the slope since the 2014 repairs. if ',4 4477- „„iy, °T�� t' �� d'fW,a'ik \ A� g 4, 1 L'O ej4/ 000 6 Geotechnical Report 12016 ",, A-- )740;1-4 -:',1 : .$-)Y f a g# jy y - i --4,-.14,...., -,, ,„. *....,„1- , \;.,.. I 1 T a ", l'..."':-.',„ * -,, J40,- ,, ,.., ".•.• • t'P` t { T C} Ili { 4 •„ s , f ( • .. • 11 fes : i .. n y` 4 r' '.424 ,..1,,a; a ::'bw Looking North at the Northernmost Deck Supports and the Seasonal Drainage The seasonal drainage shown on the attached site plan and in the photograph above immediately north of the residence cascades over the bluff and does not appear to be significantly scouring the bluff. This is partially due to the strength of the exposed rock and partially due to the ephemeral nature of the flow. South of the residence, the bluff is vegetated with large conifer and deciduous trees, with a thick understory. The lower roughly 3-6 feet of the bluff is unvegetated, consisting of exposed rock. ' !� f 4.0 Conclusions Regarding Normal Geologic Hazards ,1/4 ��°�� '4116").. � > *)' cfett 4.1 Landslide Hazard Large and deep-seated rotational landslides that might envelope a large width of the parcel are unlikely to occur at the proposed shoreline development site due to the strength of the underlying volcanic bedrock. Minor raveling/toppling rock fall is likely in the steep lower section of the bluff, but is unlikely to damage the proposed dock repair replacement piers. It is possible that the damaged northeastern corner deck support repaired in 2014 may have been damaged by such rock fall, but there is no sign of damage from rock fall to the other supports. Large translational block failures of the competent basalt formation are also unlikely to occur 7 Geotechnical Report 1 2016 under normal circumstances due to the low dip angle of the rock formation and high frictional resistance between the irregular surfaces of rock layers. Groundwater-driven slides are infrequent and rare due to absences of incised drainage paths at the site. This type of slide is partially mitigated by the dense canopy of tree and shrub vegetation that overlies most of the parcel above the bluff. The vegetation cover acts as an interceptor for rainfall through absorption and evapo-transpiration processes which reduces water saturation of the surficial soils near the bluff. If ground cover or drainage conditions significantly change or are exacerbated by prolonged and intense rainstorm events, rare shallow slumps and sliding may occur in the surficial soils. Surficial landslides and slow erosion along the bluff will likely be limited to episodic small-scale rockfalls and thin slumps of the surficial soils. In our opinion, the overall long term rate of retreat of the surficial soils is estimated to be 6 to 18 inches per decade. The average rate of recession from oceanic wave attack at the toe of the bluff is very low to,. none in the bedrock. 4.2 Erosion Hazard VC The Soil Survey for Jefferson County rates the erosion hazard potential for the Olette Series soil based upon soil characteristics and the local steepness of the topography where the soil type is encountered. Since the slope steepness of the surficial soils near the top of the marine bluff is between 30 and 90 percent, the erosion hazard potential at the site of the proposed shoreline development is indicated to be severe to very severe. Above the bluff for a distance of about 16 to 20 feet, the slope is less than 30 percent and the erosion hazard potential is indicated to be slight to moderate. The area under the residence has less erosion hazard as it is covered from surface water drainage. Erosion in this area is likely to be associated with the minor rockfall noted in section 4.1 above. 5.0 Conclusions Regarding Extraordinary Seismic Geologic Hazards For completeness some recent geologic research should be mentioned and considered. Brian Atwater, a scientist at the University of Washington, and other researches have concluded that the Olympic Peninsula could experience a "subduction zone" earthquake. There is evidence that an earthquake and related tsunami of this type occurred in the 1700's and several other occurrences have been tentatively identified. If such an earthquake does occur, it may be longer in duration, more energetic, and more destructive than the other earthquakes that have occurred in the Western Washington Region during the modern period. If such an event were to occur, larger landslides than seen before might occur along steep bluffs due to the longer duration of the motion and the longer 8 Geotechnical Report I 2 016 wavelengths of the ground motion. Based on observations of slide events that occurred during earthquakes of the modern period, some geologists have suggested that seismically-driven landslides could reach widths of 50 to 100 feet. At the subject site, the volcanic bedrock that makes up the lower half of the bluff provides some protection against the largest slope instabilities. Land-sliding in the weathered bedrock and surficial soils forming the upper half of the bluff should be expected in such an earthquake, especially if it happens in the winter or spring when soil saturation is high. Fortunately, the probability of such a large seismic event occurring within the economic life of the shoreline structure appears to be low, perhaps 1 chance in about 350 to 400 for each year of existence. This may also be expressed as one chance in ten during the next 35 to 40 years. This repair is not intended to protect the existing structure from the shaking associated with such an earthquake, and damage to the structure should be expected. The replacement dock and stairs should decrease the damage from such an earthquake, if having any significant effect. Tsunami waves from a Cascadia Subduction Zone earthquake are indicated to be less than 1 foot in height at the subject parcel, based on a simulation (Myers, E. P., III; Baptista, A. M.; Priest, G. R., 1999, Finite element modeling of potential Cascadia subduction zone tsunamis: Science of Tsunami Hazards, v. 17, no. 1, p. 3-18), for which a video is available online here https://www.youtube.com/watch?v=YhjsAjZQZeg from the Pacific Northwest Seismic Network. Tsunami models for earthquakes from the Seattle fault and other nearby faults are not available. This structure is intended to resist typical storm energy and not an extraordinary //4,p, seismic event or an associated tsunami. 'A • 6.0 Recommendations and Design Parameters for Dock Repair s y� )77 The proposed repair is to consist of two new concrete piers, cast into the existing 14-inch ), diameter round excavations in the rock. The existing wood piers and concrete will be removeeh. The existing foundations extend an unknown depth into the rock. The rock excavations should extend at least 3.5 feet into the intact volcanic bedrock, or deeper if necessary to remove the existing foundation. The rock excavation is to be completed with hand tools, such as a pneumatic chipping gun or jackhammer. Care must be taken by the contractor to avoid fracturing that might weaken the surrounding formation. The piers will extend about 6 feet above the rock line in a 16-inch by 16-inch square. Illustrations of the foundation recommendations described above, which are preliminary, are provided on sheet 2 of the plans attached to this report. A structural engineer will need to design the beach access structure and deck per current regulations under the International Building Code (IBC) adopted by Jefferson County. Rock design parameters for foundation design is provided below. 9 • Geotechnical Report 1201.6 Concrete pier foundations within the intact bedrock may be designed using the following prescriptive soil parameters which were selected based upon visual examination of rock quality at the site. We ask that we have the opportunity to review the final foundation design for conformance with the geotechnical recommendations provided herein this report. Table 1 - Rock Design Parameters for Foundations in Intact Volcanic Rock Rock Description Gray/ black intact volcanic bedrock Rock Unit Weight 150 pounds per cubic foot Vertical Bearing Capacity 4,000 pounds per square foot Lateral Bearing Capacity 400 pounds per square foot per foot of depth Coefficient of Friction 0.4 Total Compression Settlement Expected Less than 1/4 inch Total Differential Settlement Expected Less than % inch PrP.., At the time of rock excavation during construction, geotechnical inspection / monitoring will be needed to verify soil and rock conditions are as anticipated and suitable for footing construction. COL, 7.0 Erosion &Sediment Control >� fr The rock excavations and concrete work should only be conducted during low tides, when the work area is free of water. All debris and excavated rock fragments should be contained and removed from the shoreline. No sediment or debris associated with the proposed repair should be allowed to enter the marine environment. Best Management Practices (BMP's) will need to be considered and implemented as necessary to comply with Jefferson County protection standards for geologically hazardous areas. 9.0 The Limitations of Our Assessment and This Report This reconnaissance assessment was completed without the use of deep subsurface borings and without measurements over long periods of time. The observations and conclusions were derived principally from surface observations and inferences from surface observations. Undiscovered and unseen conditions in the subsurface could exist that would affect the observations and conclusions of this report. Users who require a higher degree of reliance on this assessment may wish to obtain more detailed studies. The observations and conclusions of this report apply only to the subject property and they are not transferable to nearby or adjoining property. This report is the sole property of Mrs. Karen 10 Geotechnical Report I 2016 Bianchi and may be used by others only with her permission. We appreciated the opportunity to prepare this report for you. Please contact us if you have any questions regarding the findings or recommendations of this report. For NTI Engineering and Land Survey Sincerely Yours, ,e, o Wash4" �m 4it % allillihil I,*el ,,,irl.fr .1 __ I nOi umin.Gxaltp. ,. ��� ♦41 �•q�,}� Mfr'-�N GLEN G. WADE k r - �� X"41 ) �,� - 'r; Glen G. Wade, L.E.G. C '�':�,' !, /, Engineering Geologist o s ' --fly Attachment: % Site Plan and Dock Repair Elevations, 2 Sheets 11 ' `` , NORTHWESTERN TERRITORIES, INC. Nile_ 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 11 11 ' #. 914 WASHINGTON STREET SUITE 6,PORT TOWNSEND,WA 98368 Engineers Land Surveyors Geologists Construction Inspection Materials Testing r /1/TI Port Angeles(360)452-8491 E-Mail:info(a�nti4u.com damor +*. , c::.a,s„,, December 28, 2016 t J jiii , v nef Karen Bianchi LIE'C 1334 S. Dawson Street d Seattle, WA 98101 JEFFEI o CO, N1 r DCD Subject: GEOTECHNICAL REPORT—PROPOSED DOCK REPAIR 170 MCDONALD COVE BRINNON, WASHINGTON JEFFERSON COUNTY TPN: 502282012 Dear Mrs. Bianchi and Others Concerned, Northwestern Territories, Inc., (NTI) is pleased to present the following report for the repair of your dock and stairs that were destroyed by a storm in 2013. This project intends to replace the original 1971 structure under a Type I Shoreline Exemption Permit. This was discussed at the Pre-Application Conference on March 16, 2016, in the May 5, 2016 NTI letter to Jefferson County Department of Community Development (DCD), and in the modified DCD outline of the March 16, 2016 Pre-Application meeting received December 13, 2016. This report includes discussion and analysis of the geotechnically-related critical areas/geologic hazards, local geology and earth materials, and, foundation design recommendations for the proposed dock and stairs repair. Based on our site assessment work, the proposed shoreline dock structure repair/ replacement is suitable provided that the geotechnical recommendations within this report are followed. Project site plan and preliminary dock repair elevation sheets are attached to this report. 1.0 BACKGROUND AND SCOPE OF WORK NTI Engineering and Land Surveying (NTI) was requested by Mrs. Karen Bianchi to perform a geotechnical investigation of Jefferson County TPN: 502282012. The property lies near Brinnon, Washington, within the Northwest Quarter of Section 28, Township 25N, Range 2W, WM, Jefferson County, Washington. The parcel is located at 170 McDonald Cove, east of Highway 101, near Brinnon, Washington. NTI's Trent Adams, E.I.T., and Glen Wade, L.E.G., completed site assessment work on April 8th, 2016. Geotechnical Report The site plan and profile sheets showing the existing and proposed structures are attached to this report. The 1994 photograph below shows the dock and stairs before the 2013 damage. r k, -"i,"; ',ki _,..4iimiiii 1994 Site Photograph Looking Southwest The photograph below shows the current conditions at the site, after the 2013 damage. ,# x . .1'.r * .7.1111*". i 7f•v Y. 1—.1.,,,,,,,:. 1.1/4'k s, ,i --,.-;11, --. ;,.1? t' .; R ,tib v,„, ,,.,...7 _ _,. . , _ ,,„ ,,, , _ , ,:,'ter:4.-t4,, ;Si - s - '.'*rilkit • , i O' 2016 Photograph, Looking North r 2 JEFFEISZOi.4 COUNTY DCD Geotechnical Report 2.0 SITE CONDITIONS 2.1 The Local Geologic Setting The 2012 Washington State Department of Natural Resources (DNR) Geologic Map of Brinnon by Polenz, et al, shown in Figure 1, below, indicates the site is principally in an area of Tertiary-age Crescent Formation Basalt, Evc. This geologic unit is described by DNR as fine-to coarse-grained blocky subaerial flows with rare and thin sedimentary interbeds; typically weathered and fractured. The map also indicates that the bedrock in the site vicinity strikes in the northeast direction, dipping 12 degrees to the southeast. Field observation of exposed rock along the bluff, shoreline, and extending beyond the low tide into Hood Canal at the subject parcel confirmed the presence of Crescent Formation basalt bedrock with a strike and dip similar to the DNR measurements. r Evc I , 1 62 -Clgt .� - _ • Qam r , -"' -...-,__ - ____, _/-- '' O-01sgic C gic K• � m- S21 S IC Evc + , # I Fault Trace — Ev c .u ©go i --Qgt y Evc Gil qt ;:,._ ,, Qgt ,;, (- " Qgt __ Evc Cep r E- E I ,W--. '' .. ---, vc, � ,'` Evc , � ; rt �/ 1'C t� 1 Eve' Qgt. f -- _ :` ,, . vc j' / , Evc ? E.,,-;, ,. 7 Evc '' 77 ft `` Ogt 1 J! 17a 9 f Y :y t syr;' / DEC 2 3 2, 48 . ..1 EvC `�=- Evc JEFFERSON COUNTY DDD Cgt '1/4.--- -/ • ' , 12 ga r °Pd b�: �;2c Subject Site Figure 1 - Portion of the 2012 Department of Natural Resources Geologic Map of Brinnon. 3 Geotechnical Report 12016 The DNR image also indicates the presence of an east-west trending thrust fault of unknown age that lies about 1 mile northeast of the site. This fault is not shown on the fault map for Washington (Faults and earthquakes in Washington State, Open File Report 2014-05, by J. L. Czajkowski and J. D. Bowman, 2014, 36" x 45" color sheet, scale 1:750,000), but it does show the "Class B" Hood Canal fault trending northeast within roughly the entire length of the Hood Canal about 2 miles to the southeast of the subject property. This fault is shown in green and has the blue number 552 next to it on Figure 2 below. The Class B designation for the fault indicates that the fault is suspected to have had Quaternary-age (within the past 2.6±million years) deformation, but that insufficient evidence has been gathered to support a more definite age. • • • iii /, * • # • Subject Site • • • • .y i i' • • • 1 • • •r • • ( 411111 • efI /rj >00..• t •! , a • • J/ 0. !• t • . • ff . • ,A► ,, �r • �• •• • . • Y • '1 s • •i • • ••• • • 0 16 • • • 041) Figure 2—Washington Fault and Earthquake Map The Holocene-aged (activity within the past 11,700 years) Seattle Fault zone, shown with red and labeled with number 570 next to it is about 5 miles from the site at its closest point. The map also shows two earthquakes of 3.0-6.8 magnitude as purple dots near the site, one about 2/3 mile southwest of the site and one on the trace of the Hood Canal Fault where it is closest to the site. Respectively, these earthquakes were a 3.2 magnitude earthquake from 1978 at a depth of 46 km and a 3.2 magnitude earthquake from 1971 at a depth of 14 km per the USGS site https://earthquake.usgs.gov/earthquakes/map. 7T7' x.,,,..>`\. iP , M •-....,...:' V r J L 4 DEC 2 8 21.116 ,.*. JEFFER N utn Y 1 Geotechnical Report 12016 2.2 The Surficial Soils The 1975 Soil Survey for Jefferson County prepared by the Soil Conservation Service (SCS) and United States Department of Agriculture (USDA) indicates Olette-Hoodsport Complex (OpD) soils are present in the vicinity of the site. The Hoodsport Series soil is described as a reddish- brown very gravelly sandy loam underlain by cemented glacial till, while the Olette Series soil is described as a reddish-brown very gravelly silt loam underlain by basalt bedrock. Soils matching the description for the Olette series were confirmed by on-site observations at the top of the bluff slope and westward through the upland portion of the parcel. However, there is no soil exposed in the shoreline area of the proposed repair, only intact bedrock. 2.3 Slope Stability The 1978 Washington State Department of Ecology (DOE) Coastal Zone Atlas Slope Stability Map classifies the basalt bedrock and surficial soils in the vicinity of the site as having intermediate slope stability. The slope stability mapping and hazard ratings shown in the Coastal Zone Atlas were derived from the 1976 DNR Relative Slope Stability Map of Brinnon by Carson. Carson indicates that areas of intermediate slope stability are believed to be generally stable under normal conditions, but can become unstable if disturbed by man's activities. Around the time period that fieldwork was taking place for the DNR study, an unstable recent slide was mapped near Highway 101, less than a quarter-mile west of the site. The map indicates that this slide is believed to have occurred within 50 years prior to the creation of the DNR slope stability map. /° �, / - — : J X11 �,' r, `�.' / _ +oto„/ .---/-;?� — A ..---,,,, .- ,„ 'J 1 �', -`-'`` :-.4, _, / i ,,, ,...„,. .. - ,,, v _.4.00 / , ,, _ _ .0 ? , / //. _ . ,...,4 ..,„ „-‘ :„ t { __':,..2820,,, / __ i 1,\ msµ �A ,., ,, � h�� °t�- 77: ;c '� t _ _ ,, ,, /I "'x' y •yi_�1a. `• fir , .�arc. ,_ b ,,,,,-- Q ) -!_ 4---- 00 --�,-' , Ji 0 .. Subject Site Figure 3 - Portion of the Department of Ecology Slope Stability Map of Jefferson County. 5 Geotechnical Report 12016 3.0 GEOLOGIC AND GEOTECHNICAL SITE OBSERVATIONS 3.1 Dock Structure Area Site Observations The area where the dock pilings are to be replaced is a somewhat planar dip-slope of the hard, intact, slightly weathered volcanic bedrock, sloping about 10-15 degrees to the east. This is consistent with geologic mapping. The rock surface is locally covered with a thin veneer of algae and sporadic barnacles. This rock surface extends westward, or landward, about 12 feet to the existing row of nine deck support piers, and about an additional 3-5 feet to the base of the bluff slope. 3.2 Bluff Characteristics and Geotechnical Indicators The bluff slope is about 16-20 feet high at the subject parcel, containing several sections and features. The lowest section of the bluff slope beneath the residence and deck has a slope of about 1 horizontal: 1 vertical for a height of about 6-7 feet. We observed some raveling in this lowest section of the bluff, consisting of gravel-to cobble-sized rock fragments that appear to topple down the slope. This minor toppling likely only occurs at times of very high tides and storm-driven wave action. The rock in the bluff slope is moderately to intensely weathered and moderately fractured, which makes it susceptible to scour, in contrast to the intact rock exposed throughout the area of the dock and deck supports. We observed very minor groundwater seepage in the lower few inches of the bluff, which was likely perched on the surface of the intact bedrock. Above the lowermost part of the bluff slope,the slope beneath the residence is about 2-3 horizontal: 1 vertical, for a height of about 6-8 feet. This section of the slope consists of intensely weathered rock and some surficial soils derived from the underlying bedrock. The slope above this section is much flatter, and includes the residence's interior footings and posts that were repaired in 2014. We did not observe any significant erosion in this portion of the slope since the 2014 repairs. A Z - FFF�Ra Oco 6 Geotechnical Report 2016 —2011 f474,:„..: Fe... fltrAY?- - --....v elo,„ 1/..-4,, ,,,,,,tx „ ,,, -.--, - -.1 1,,,„ 'is., -',` '`-1 0. �l _ .'`4.41,-;*':::4,Fit. 4471,—.1m..'\ -,.. g yry vim„ e, •. t y ,,,, l" '",:',c-t '7^. k �"' a 411111,1 ":4 1 1 i ti — ''';' l ''4144k.'1 h' ''' ) , 4 .. , . , , p „ a. aa''X„..,y x ..;. t.,1: i.:-11:4:.,. .%p ,„-_-:.: 4-.-m ....., - f x PYA \ , .,. Looking North at the Northernmost Deck Supports and the Seasonal Drainage The seasonal drainage shown on the attached site plan and in the photograph above immediately north of the residence cascades over the bluff and does not appear to be significantly scouring the bluff. This is partially due to the strength of the exposed rock and partially due to the ephemeral nature of the flow. South of the residence, the bluff is vegetated with large conifer and deciduous trees, with a thick understory. The lower roughly 3-6 feet of the bluff is unvegetated, consisting of exposed rock. }} li�'i' � 4.0 Conclusions Regarding Normal Geologic Hazards x 4.1 Landslide Hazard JEFFERSON COUNTY DCI) Large and deep-seated rotational landslides that might envelope a large width of the parcel are unlikely to occur at the proposed shoreline development site due to the strength of the underlying volcanic bedrock. Minor raveling/toppling rock fall is likely in the steep lower section of the bluff, but is unlikely to damage the proposed dock repair replacement piers. It is possible that the damaged northeastern corner deck support repaired in 2014 may have been damaged by such rock fall, but there is no sign of damage from rock fall to the other supports. Large translational block failures of the competent basalt formation are also unlikely to occur 7 Geotechnical Report 12016 under normal circumstances due to the low dip angle of the rock formation and high frictional resistance between the irregular surfaces of rock layers. Groundwater-driven slides are infrequent and rare due to absences of incised drainage paths at the site. This type of slide is partially mitigated by the dense canopy of tree and shrub vegetation that overlies most of the parcel above the bluff. The vegetation cover acts as an interceptor for rainfall through absorption and evapo-transpiration processes which reduces water saturation of the surficial soils near the bluff. If ground cover or drainage conditions significantly change or are exacerbated by prolonged and intense rainstorm events, rare shallow slumps and sliding may occur in the surficial soils. Surficial landslides and slow erosion along the bluff will likely be limited to episodic small-scale rockfalls and thin slumps of the surficial soils. In our opinion, the overall long term rate of retreat of the surficial soils is estimated to be 6 to 18 inches per decade. The average rate of recession from oceanic wave attack at the toe of the bluff is very low to none in the bedrock. 4.2 Erosion Hazard The Soil Survey for Jefferson County rates the erosion hazard potential for the Olette Series soil based upon soil characteristics and the local steepness of the topography where the soil type is encountered. Since the slope steepness of the surficial soils near the top of the marine bluff is between 30 and 90 percent, the erosion hazard potential at the site of the proposed shoreline development is indicated to be severe to very severe. Above the bluff for a distance of about 16 to 20 feet, the slope is less than 30 percent and the erosion hazard potential is indicated to be slight to moderate. The area under the residence has less erosion hazard as it is covered from surface water drainage. Erosion in this area is likely to be associated with the minor rockfall noted in section 4.1 above. 5.0 Conclusions Regarding Extraordinary Seismic Geologic Hazards For completeness some recent geologic research should be mentioned and considered. Brian Atwater, a scientist at the University of Washington, and other researches have concluded that the Olympic Peninsula could experience a "subduction zone" earthquake. There is evidence that an earthquake and related tsunami of this type occurred in the 1700's and several other occurrences have been tentatively identified. If such an earthquake does occur, it may be longer in duration, more energetic, and more destructive than the other earthquakes that have occurred in the Western Washington Region during the modern period. If such an event were to occur, larger landslides than seen before might occur along steep bluffs due to the longer duration of the motion and the longer 8 Geotechnical Report 12016 wavelengths of the ground motion. Based on observations of slide events that occurred during earthquakes of the modern period, some geologists have suggested that seismically-driven landslides could reach widths of 50 to 100 feet. At the subject site, the volcanic bedrock that makes up the lower half of the bluff provides some protection against the largest slope instabilities. Land-sliding in the weathered bedrock and surficial soils forming the upper half of the bluff should be expected in such an earthquake, especially if it happens in the winter or spring when soil saturation is high. Fortunately, the probability of such a large seismic event occurring within the economic life of the shoreline structure appears to be low, perhaps 1 chance in about 350 to 400 for each year of existence. This may also be expressed as one chance in ten during the next 35 to 40 years. This repair is not intended to protect the existing structure from the shaking associated with such an earthquake, and damage to the structure should be expected. The replacement dock and stairs should decrease the damage from such an earthquake, if having any significant effect. Tsunami waves from a Cascadia Subduction Zone earthquake are indicated to be less than 1 foot in height at the subject parcel, based on a simulation (Myers, E. P., III; Baptista, A. M.; Priest, G. R., 1999, Finite element modeling of potential Cascadia subduction zone tsunamis: Science of Tsunami Hazards, v. 17, no. 1, p. 3-18), for which a video is available online here https://www.youtube.com/watch?v=YhisAjZQZeg from the Pacific Northwest Seismic Network. Tsunami models for earthquakes from the Seattle fault and other nearby faults are not available. This structure is intended to resist typical storm energy and not an extraordinary seismic event or an associated tsunami. 41:14 ° L444gii 6.0 Recommendations and Design Parameters for Dock Repair �� ? , '14•V The proposed repair is to consist of two new concrete piers, cast into the existing 14-inch diameter round excavations in the rock. The existing wood piers and concrete will be removed. The existing foundations extend an unknown depth into the rock. The rock excavations should extend at least 3.5 feet into the intact volcanic bedrock, or deeper if necessary to remove the existing foundation. The rock excavation is to be completed with hand tools, such as a pneumatic chipping gun or jackhammer. Care must be taken by the contractor to avoid fracturing that might weaken the surrounding formation. The piers will extend about 6 feet above the rock line in a 16-inch by 16-inch square. Illustrations of the foundation recommendations described above, which are preliminary, are provided on sheet 2 of the plans attached to this report. A structural engineer will need to design the beach access structure and deck per current regulations under the International Building Code (IBC) adopted by Jefferson County. Rock design parameters for foundation design is provided below. 9 Geotechnical Report 12016 Concrete pier foundations within the intact bedrock may be designed using the following prescriptive soil parameters which were selected based upon visual examination of rock quality at the site. We ask that we have the opportunity to review the final foundation design for conformance with the geotechnical recommendations provided herein this report. Table 1 - Rock Design Parameters for Foundations in Intact Volcanic Rock Rock Description Gray/ black intact volcanic bedrock Rock Unit Weight 150 pounds per cubic foot Vertical Bearing Capacity 4,000 pounds per square foot Lateral Bearing Capacity 400 pounds per square foot per foot of depth Coefficient of Friction 0.4 Total Compression Settlement Expected Less thanY4 inch Total Differential Settlement Expected Less than 1/4 inch At the time of rock excavation during construction, geotechnical inspection / monitoring will be needed to verify soil and rock conditions are as anticipated and suitable for footing . construction. 1 itki, i5 r 7.0 Erosion&Sediment Control .fir. ,,,� `��r) The rock excavations and concrete work should only be conducted during low tides, when�e- work area is free of water. All debris and excavated rock fragments should be contained ar3d removed from the shoreline. No sediment or debris associated with the proposed repair should be allowed to enter the marine environment. Best Management Practices (BMP's) will need to be considered and implemented as necessary to comply with Jefferson County protection standards for geologically hazardous areas. 9.0 The Limitations of Our Assessment and This Report This reconnaissance assessment was completed without the use of deep subsurface borings and without measurements over long periods of time. The observations and conclusions were derived principally from surface observations and inferences from surface observations. Undiscovered and unseen conditions in the subsurface could exist that would affect the observations and conclusions of this report. Users who require a higher degree of reliance on this assessment may wish to obtain more detailed studies. The observations and conclusions of this report apply only to the subject property and they are not transferable to nearby or adjoining property. This report is the sole property of Mrs. Karen 10 Geotechnical Report 2016 Bianchi and may be used by others only with her permission. We appreciated the opportunity to prepare this report for you. Please contact us if you have any questions regarding the findings or recommendations of this report. For NTI Engineering and Land Survey Sincerely Yours, %ij 01 p„...3, . .., .....,...74., _ ` gineerin red'lot* olisod Ge°\°� GLEN G. WADE Glen G. Wade, L.E.G. Engineering Geologist Attachment: Site Plan and Dock Repair Elevations, 2 Sheets 11