Loading...
HomeMy WebLinkAboutPettygroves 2nd Block 106 - Geology Stability Inspection 2004.04.02t fl hts STRATUM GROUP l45l Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 April2,2004 David and Laura Rinn 5840 245th Place NE Redmond, WA 98053 Re:Geolory Stability Inspection 111 S Street Lots 1 , 2, 3, 6, 7 , and 8 Block I 06 Pettigrove Addition Port Townsend, Washington Dear Mr. and Ms Rinn: I visited the property in March 2004 to qualitatively evaluate the slope stability of subject property. The purpose of the site visit was to assess the geologic stability of the site including the criteria set forth in City of Port Townsend's Environmentally Sensitive Areas (ESA) Ordinance, pTMC 19.05. Based on my inspection of the property and vicinity, the current residence is not at risk in the reasonable future from being impacted by slope failures. The property is located in a geologically hazardous area as per the Port Townsend's ESA and per my o*tt.'iew of slope stability and processes controlling the stability of the site. Portions of the steep slope are considered geologically hazardous as per PTMC 19.05(8). However, the upland ur"u ott the property is stable, has adequate space for development purposes and can be developed in a manner that will not create instability or harm to other properties or public resources as long as the recommendations made in this report are followed. GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was deeply eroded. The Surficial Geoloeic Map of Port w (Pessl, Dethier, Booth and Minard, 1989) indicates that the subject property is underlain by glacial till. Site observations on the subject property, bluff face, and in the vicinity are consistent with the above-described mapping except for the observed preglacial deposits on the slopes of the bluff below the topmost portion of the bluff. The essentially vertical uppermost approximately 15 feet of the bluff and the house site are underlain by glacial till. The till consists of a poorly sorted mixture of rock fragments deposited directly by glacial ice during the last ice age. The pebbles, cobbles and boulders are firmly embedded in a matrix of finer material consisting of a variable mixture of clay, silt and sand. Overall, the till at the site and on the upper slope of the bluff is very hard and compact. April2,2004 Lots l, 2,3,6,7 and 8, Block 106, Pettigrove Addition, Port Townsend, WA Geology Evaluation The rest of the bluff is underlain by preglacial deposits. The preglacial deposits consist of clay, silt, and sand with sand being the predominant unit on the slope below the subject property. Immediately below the till is a compact, sand and gravel unit that appears to be approximately 25 feet thick. A compact silt/clay layer that is approximately l0 feet thick underlies this unit. The remaining lower slope of the bluff below the subject property is underlain by medium to fine cross-bedded sand. These deposits appear to be advance outwash deposits deposited by meltwater from the advancing glacial ice just prior to the last glacial period approximately 20,000 years ago. The waterways of Puget Sound are primarily the result of glacial erosion and under ice glacial melt water having carved deep channels into the land surface. Since the end of the last glaciation approximately 14,000 years ago the land has completed its rebound from the weight of the glacial ice. Wave action has eroded the shoreline back to what is now the base of the bluff below the house. The steep bluff formed by waves eroding the base of the bluff. Over time the bluff is undermined and oversteepened and the upper parts of the bluff collapse down onto the beach below. Wave action removes the failed soils and the process repeats itself. This process is still taking place at the site. SPECIFIC SITE OBSERVATIONS A site plan sketch of the subject property and a schematic cross-section of the bluff slope below the subject property are attached to this report for to show pertinent information regarding the site. The upland portion of the subject property consists a level to gently sloping area underlain by glacial till at the top of an approximately 180-foot high, steep shoreline bluff. The slope of the ground at the top of the bluff slopes away from the top of the bluff towards the west. Part of the property is underlain by fiIl soils consisting of disturbed glacial till regraded to create a level building area andyard atea. This observation was confirmed by the current owner of the property, who indicated that a ridge of hard soil had been pulled back from the top of the bluff to create the level area. The house on the upland area is located at an angle from the top edge of the bluff. The northeast comer of the house is located approximately 16 feet from the edge of the bluff while the southeast corner of the house is located approximately 52 feet from the top edge of the bluff. The top approximately 15 feet of the bluff is essentially vertical and is underlain by very compact glacial till. The slope below the till layer slopes down at an angle of approximately 40 degrees. This section of bluff is approxim ately 25 feet high. This section of the slope is tree-covered with 2 Stratum Group File: 3.20.04 April2,2004 Lots l, 2,3,6,7 and 8, Block 106, Pettigrove Addition, Port Townsend, WA Geology Evaluation Douglas fir, madrona, alder and maple with an understory of Oregon grape, snowberry and grasses. The largest fir trees are approximately 12 inches in diameter. The slope is underlain by a slope colluvium consisting of the underlying compact sand and gravel and glacial till that has fallen onto the slope in the past. The slope steepens to 65 degrees for a height of approximately 15 feet. This section of slope is underlain by very compact hard clay. A few trees are present on this section of the slope with thin patches of grass and brush. The slope then becomes less steep and slopes down to the shoreline at an angle of approximately 35 degrees. This lower slope is underlain by loose sand. Grass and small fir trees are growing on the slope. The portion of the slope immediately above the beach is covered with trees with a thick root mass. This lowermost ,.riion of the slope appears to be part of a shallow soil slab that has slid as a wedge down to the top of the beach. The sandy slope is derived from the underlying compact but low cohesion sand underlying the lower part of the bluff. The sand unit is exposed elsewhere along the lower slopes of the bluif and forms cliffs in a few areas beneath the overlying more competent units. No areas of water seeps or vegetation indicating seasonal water seeps were observed anywhere on the slope immediately below the residence' The beach at the base of the slope is a sand gravel beach. During times of high water combined with storm waves the sandy soils at the base of the bluff are eroded. The wedge of failed sandy soil with trees near the base of the bluff betrow the subject property is providing some protection of the slope at the present time. The root mass and woody material within the wedge are of sufficienfsizeto withstand most wave action at the base of the bluff. Very large waves at this site are a relatively rare event because of the lack of open fetch for wind generated waves to form on this portion oithe coastline. The fetch over the water to the east is approximately 7 miles. This distance is not large enough to generate large waves and high winds from that direction are not common. Other portions of the bluff are steeper. A fairly large volume of material will need to be eroded and removed from the base of the slope below the subject property before the upper compact silt and subsequently the uppermost till layer are undermined sufficiently to fail. Failures of the till and silt layer appear to be slab failures involving only a few feet of the upper part of the bluff at time. Port Townsend Geologically Hazardous Areas Evaluation As per Port Townsendos Code 19.05.100 B I through 6 the following classification criteria were evaiuated to determine if the subject property is located in a geologically hazardous area. Based on this evaluation and the conditions described in the above sections, the shoreline bluff slope and areas near the top of the shoreline bluff slope on the subject property are located in a tl Stratum Group File:3.20'04 April2,2004 Lots l, 2,3,6,7 and8, Block 106, Pettigrove Addition, Port Townsend, WA Geology Evaluation geologically hazardous area. 1g.05.10081: Any area containing soil or soil complexes described or mappedwithin the United States Department of Agriculture/Soil Conservation Service Soil Surveyfor Jffirson County as having a severe to very severe erosion hazard potential. Soils underlying the upper part of the bluff site consist of very compact, sandy, gravely silt based on test pits and site observations. These soils are not prone to severe erosion potential. The sandy soils on the slope below the subject property are readily eroded because of the steep slope. If these soils are eroded the outer edge of the upper bluff underlain by the glacial till will be undermined and will fail. 1g.05.10082: Any area with slopes greater than l5 percent with impermeable soils (typically silt and ctay) frequently interbeddedwith granular soils (predominantly sand and gravel); and springs or gr:ound water seepage from perched water tables or potentially unstable slopes. Slopes building site are essentially under 15 percent. However slopes on the east side of the subject property and below the subject property are mush greater 15 percent. These slopes also inciudelnterbeds of sand and silt and clay. However, there is no evidence of perched water, springs or seeps seasonal or otherwise on the steep shoreline slope. 19.05.10083: Any area potentiatty subject to mass movement due to a combination of geologic, topographic, hydrologic factors, but not limited to those areas mapped or described by the Soil Conseriation Service, Washington State Department of Ecologt, Department of Natural Resources or U.S. Geologic Service. The classification may be based on performance standards rather than mapPing. Indications of shallow rapid mass wasting are apparent on steep shoreline bluff slope on the eastern portion of the subject property and on the steep shoreline slope in the vicinity of the property. The mass wasting is the result of wave action undermining the base of the slope and causing the slope to become over steep. The different soil types have various angles of repose based on soil cohesion. However, even though the glacial till at the top of the slope can maintain vertical angles, it too will fail when the lower slope erodes back to the point that the till is undermined. 1g.05.10084: Any area potentially unstable as a result of rapid stream incision, stream bank erosion or undercutting by wave action. Portions of the subject properlry near the steep shoreline bluff are at risk of becoming unstable 4 Stratum Group File: 3.20.04 April2,2004 Lots l, 2,3,6,7 and 8, Block 106, Pettigrove Addition, Port Townsend, WA Geology Evaluation due to wave erosion at the base of the bluff. 19.05.10085: Any slope of 40 percent or steeper. Slopes on the shoreline bluff are greater than 40 percent. 19.05.10086: Areas sub.iect to severe risk of damage as a result of earthquakes, slopefailure, settlement, soil liquefaction or faulting. These areos are identffied by the presence of poorly drained soils (greater than 50 percent silt and less than 35 percent coarse material) loose sand or gravel, peat, artificialfill and landslide materials or soils with a high organic content. The site is not underlain by potentially liquefiable soils and compact glacial till is not typically susceptible to earthquake induced mass movement. An earthquake may generate sufficient force to trigger shallow soil failures on portions of the slope that have been oversteepened by wave action. CONCLUSION AND RECOMMENDATIONS Observations along the shore and the top of the bluff indicate that the primary cause of slope instability is the undermining of the base of the bluff by wave action. Sandy deposits at the lower portion of the bluff will be susceptible to wave action erosion when exposed to waves' Otherwise the bluff appears to be relatively stable. At the present time there are no sections of the bluff slope below the subject property that are significantly oversteepened to the point that significant mass slope failures will occur. Neither 1119 1'.ry compact silt/clay section of the bluff nor the glacial till unit at the top of the bluff have been undermined in any significant manner. Both these units are capable of holding nearly vertical angles and will until undermined from below. At the present time even the lower sandy slopes below the subject property are not significantly undermined due to the wedge of landslide material with trees at the base of the slope and the well established grass and the small trees on the slope. The slope conditions at this location are significantly better than other nearby slopes. As the very steep slope at the top of the bluff weathers shallow slab type failures are likely to occur. Based on my observations, it is my opinion that these slab type failures will involve soil an areaabove the failure that will be on the order of at most five feet and more likely just a couple of feet. Hence, even the closest portion of the house is at a very low risk of being impacted by slope failures within the life of the structure (typical life of structure is defined as 75 years from present). 5 Stratum Group File: 3.20.04 April2,2004 Lots l, 2,3,6,7 and 8, Block 106, Pettigrove Addition, Port Townsend, WA Geology Evaluation The lower half of the slope is susceptible to erosion and slope failures. Over time this erosion will cause the further steepening of the upper sections of the bluff as the loose sand will be eroded away. However, given the slope conditions observed below the subject property and the presence of trees on areas of the lower sandy slope elsewhere along the shoreline, it appears that ih" r"ruo"nce interval between lower slope failures is on the order of tens of years and involves only shallow soil failures. Hence, it will be a very long time before the upper till layer is significantly undermined at the subject property - well beyond the expected life of the structure. Recommendations As noted above the subject property contains slopes that are geologically hazardous. I recommend that any future development avoid these areas and be set back far enough to remain outside of the geologically hazardous area. Any new development should be set back from the top of the steep shoreline bluff at least 30 feei. ttris distance is sufficiently far enough back to avoid being within a potentially hazardous area. That said, the existing home is not at risk at the present time of being impacted by landslides. Storm water management is critical to stability on this shoreline bluff. Storm water and site drainage should be directed towards the west side of the property. This is readily accomplishable as the itop. at the top of the bluff slopes away from the top edge of the bluff. Development of the site can be accomplished with typical foundation work. Roof gutters and the down spout system should either direct water into an approved storm water system or should be infiltrated into the ground in a level spreader placed in such a manner as to avoid concentrating surface water discharge. No water lines should be buried within the 30 feet of the bluff. Native vegetation on the steep slope should be disturbed as little as possible. Vegetation -unug..-nt within the 3O-foot setback and on the steep slope should be such that irrigation water will not be required. Currently grass is growing up to the edge of the steep slope. Because the area is open to the sun, invasive species such as blackberry may become established on the upper part olthe slope. Invasive non native plants such as blackberry or English ivy can crowd o.ti -or" desirable plants and lack root strength that provides good erosion control. If present, these plants should be removed before they become established. Establishing native, drought and sun tolerant plants along the top of the bluff will limit the potential of invasive, non-native species. 6 Shatum Group File: 3.20.04 April2,2004 Lots l, 2,3,6,7 and 8, Block 106, Pettigrove Addition, Pott Townsend, WA Geology Evaluation Largetrees can play acritical role in slowing bluff erosion. While large trees on the upper slopes will not prevent the slope from being undermined, they can greatly slow erosion after a slope failure because large woody debris incorporated within landslide material at the base of the slope will provide natural armoring to the shoreline for an extended period of time. This is particularly true along shorelines not subject to large waves such as this site. Therefore, trimming of trees on the bluff slope should be limited to limbing for view purposes only. No debris or yard waste should be placed on the steep slope. Yard waste eventually builds up and can form a wet unstable mass that will slide down the slope damaging the slope and increasing the rate of slope failures. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our assessment please contact our office at (360) 714-9409. Sincerely yours, Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist [3ani*! fv] lk;nlrl i,/;r 7 Stratum Group Fite: 3.20.04 Fr€w 1. ViEinityMap NAD2? 122'43'J3',\{tt22.4'I'20" W)ir 6.o ._,.- 4 // ^$--n '-., tp,.*.r'' 12 c F9t i, ,i -\ 1r :. -\ ,:..] q:"./' -.,.s 3ti OUAROA?toil a (j!. A (b s 5 U LA 22'4'1',20'w 1 +\.€(v 1} ls 'o 'a)I ,tU f.? COAST OUAI'I)nF;qgnvATl0ia Point tudson I\x{t .0 A? O'JJ' POB' NNI}' ..,1' Grry whartE TNt /MN /t'g" .'*; ?0 (o sr NAD2? 122'43'53'W MILE @1998 Wildilowr Ppdutjore (w.topo com) O IO(}O FEETI........@ Printed fmm TOPOI Figure 3Cross- Section SketchSiteGroupt.ltll eont Slt ctBtunghots, t 96225(580) 7rF91oEtum\Hoil<"\\i\--rt\<fl{'a5'(.\ta,('6.1*ee,,€ \ortl ''e{ I rrpe-.4{f.ire"*d,ralngqr€ct5IPrw" *o*f"s2\rtt9"?-4\\C.,-,4\\H$$I{**u{" o^A lota*:*-s applor"*6.}qf,o r'tl l r:lr-|lu4. P^/ F Pses,r: 5 0,McShaneDaniel ,$,o,od>fun,f i ba.il.{"nuu, **erf e t'opevery (e^ficot;ll(onoar*'s^no{ qn/(ca{u a,.*f &g#'li"*t #f $rq,?-;*rritrleof' ,*f qlacial +"ll s6,*r o* hg 'F tlopr{T tl1o{o .rg n {/ar* w)}4 *rreS;. (r{o 4a ?S"a S/op< o {u* "lreer q^o{ drrg/,r$ r*ii*l(aw,paa* s;l*clay-/- L 3r'o llotse grAls qndd Saalt :i qHnS i,rstSqna[ wiL\I'ou cahes;'"^tueo{g, o{ sour// so;/ ,.;lt, lree-g8u"*dFigu re 2Site Sketchl" = so'StratumGroup1/151 Grglt Stt GtBdllnghonr' WA 98225(360) nF94@