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HomeMy WebLinkAbout954600117 Geotech Assessment_ r : 4 3ameS B, Sc©tt, P.E. oEcaa. roxsuiarAr~T 36Q-293-6044 ' FAX 3C~1-293-E~i144 ~ ~ ~ J. B. SCtJT~I' & ASS~I~~TE~ A~~g 3601 vw~c s~ s Anac~orbes, WA 98221 November 19, 2006 Mr. John Voigt 4150 S.W. Southern Street Seattle, WA ~l~~;IEIV~+ ~y APR 1 0 2007 JEffEH1UNCUUNI~OCD Project File 1309-B RE: Geotechnical evaluation of Project File 06-13096 or Lot 27 which is located in Sec. 20, T. 27 N., R. 1 E.W.M., Jefferson County, WA Dear Mr. Voigt: At your request I have made a geatechnica! evaluation of the subject lot as shown in Figure 1. Conditions exist that are classed as critical within the limits of the lot and also the immediate project area, both as defined by the county and as confirmed during my field inspection. Therefore, a geotechnical evaluation and a report is required before permits can be issued for building purposes. This critical condition by definition of the county, while currently valid, can be modified if certain improvements can be made. These critical conditions are related to setback limits as dictated by the county and also setback limits from what they have classed as a stream. As a result, of these standards imposed by the county, currently this lot is not considered as being buildable. These conditions will be discussed in some detail in this re~rt. FIGURE 1-TOPOGRAPHIC MAP AND 1_IMITS OF LOTS 26 AND 27 LOG ITEIV! 3r ~age_ i ,~©f . e 2 As shown in Figure 1, one of three or more possible building sites are shown if certain changes can be made in regard to the collection and disposal of surface wa#er. The field reconnaissance and evaluation was first made on Augus# 15, 2006 of both this lot. and Lot 26 which is looted just to the south. Photographs were taken to document field conditions. Then a second field inspection made in n~gard to some of the data shown in the Gounty's information and aerial photos. Some of this data was discussed in Project File ~-1309, dated October 23, 2006. In addition to the field inspection, review was made of (1} Project File 06-1309 {Lot 26), dated October 23, 2006; {2} Project File 99-625, G~technical assessment of slope failure conditions on Lot 25, Block 1, Goodfellows ~llanhattan Beach Tract located in NW'/~, Sec. 20, T. 27 N., R. 1 E. R.1 E.W.M., Jefferson County, WA, 1999; (3) Coastal Zone Alas of Washington, volume 11, Jefferson County, Washington Departmen# of Ecology, 1978; (4) Soil Survey of Jefferson County, Washington, USDA Soil Conservafion Service, 1975; and {5) the field reconnaissance. PHOTO 7 -VIEW OF POSSIBLE BUILDING SITE AND CREEK GOING UP SLOPE One of the better. locations for building is shown in Figure 1 and Photo 1 by the. blue arrows. This site has been selected because of easy access and also having a near level gone that would greatly reduce grading. There are other similar sites except for one factor ,..... the °setback limits as imposed by the county'. Based on set back requirements in regard to both the "creek" and setback from the property lines, the only building sites that currently meet County standards, are up slope in the western portion of the lot on much steeper slopes and having dense vegetation. Because of this, the creek which the cx~unty has classed as a ~.OG ITENf #3f Page z~of~ 4 3 "Type 5° stream, means that this site is not suitable as condi#ions exist. This will be discussed later in this report. Many county roads and other facilities in Jefferson County, have areas where surface and even subsurface water is collected and then removed from the site by buried pipe. This is a standard practice, not only in Jefferson County, but in ell the counties in Washington State. In my opinion, the only limiting factors for the subject lot is the surface area of the water shed, the steepness of the surface, and also the erosion factor. As you will also note in Figure 2, the location of the "creekA and. topography rather limit the development in the up slope portion of the lot. If you look at Figure 1, which was made available by Jefferson County, the location of the "creek" in places does not match some of the overlay of the topography which is also shown in red in Figure 1. This suggests that a portion of the "creek" is not natural and is the result of grading done by either the county or other land owners in the area. As seen in the field, Thomdyke Road intercepted some or much of the surface water coming down slope from the west including directed surface water flows both from the north and south. Based on the topographic map, that I have added to Figure 2 as a red line, is where !suspect the surface drainage course used to .run. As aln3ady discussed in n=port Ofi-1309 on Lot 2fi, #hat portion of the access road going down to the exis#ing house ne~ct to the shore line is quite steep in places. Yet erosion does not seem ~ to be a very serious problem except where concentrated flows exist or existed in the past. Assuming that a building could be placed on this lot, then the following improvement or modification should be made. 1 -- Placement, as discussed in the report on Lot 26, of a curtain drain to collect both suraface sheet flow water and subsurface concentrated flows as a result of Thomdyke Road. This water would then be placed into a tightline for disposal at or adjacent to the high tide zone to the east 2 -Placement of a curtain drain to collect subsurface and surface water just up slope of the proposed building sites. This water would be directed #o flow down to a collection point where it would then be placed into a tightline for disposable at or near the high tide line. This water would be directed into an "energy dissipating system as shown in Attachment A. Attachment A is Standard Design that has been.. in use for over 15 years in many counties in Washington State. There has .never been a failure of this design. LOG ~~EI ~# 3 I t As current conditions exist, the directed flows as a result of Thomdyke Road, are now in a concentrated flow cxmdition down slope via the "txeek9 to the shoreline zone. This directed concentrated flow, during a major storm could result in excessive erosion. That should not be allowed. 3 -All collected water from the roof of any proposed structure and/or impervious surfaces such as roads should be collected. This water would be taken by a tigh#line down to the same point where collected water from the curtain drains, acxess roads and other impervious surface zones is located for disposal as shown in Attachment A. Therefore based on the improvements of both the collecting water and disposing of that same water, the effective size of the "water shed, and amount of wa#er that will be left to flow down the slope is estimated to be classed as having a water shed area of one about 0.5 acres. Allowing for a 20 year storm runoff and the suggested improvements, that should provide a rather good factor of srafety in regard to storm flow erosion on the lower portion of the lot. This also includes a moderate seismic event that will happen in the future. The proposed setback from the shore Brie, should allow for both global warming and a rise in sea levels, which would provide a project life of a# least 300 years or more. PHOTO 2 - BACKHOE TRENCH TESTING SOIL CONpfTIONS Foundation conditions appear to be good. A series of test holes to determine depths for placement of septic drain fields was document by the photo as shown in Photo 2 My field inspection confirms the USDA report which classes this soil zone as Cassolary sandy loam, 0 to 15% slopes. They classed this soil as having a moderately slow infiltration rate. 1 classed the soil as fine clayey sand and silt. LC~G aTEN~ #3( . Pa~e~_of~ 5 Based on the soil and how the trench walls have been standing stable for a number of months, suggests that bearing capacity values will be moderately good. Therefore, !would class the soli as having a bearing capacity value of about 1.5 TSF to 2.0 TSF. Based on bearing capacity. values, I recommend that the footing width should be at least a 0.5 foot wider than standard and the depth of at least 1.5 feet or pipe piles be placed for foundations. Also no portion of the structure foundation and driveway wilt be placed on fill unless it is compacted to 95°~ to attain a bearing capacity of at least 2.0 TSF. However, because of sloping ground and limited surface area where any structure would be placed, 1 recommend that all foundations be founded on Pipe Piles. This would increase the factor of saafety plus and allow for greater bearing capacity values. As was suggested, since all of the possible building sees will be very limited in surface areas, this would allow for °cantilever° type extensions of any stn,rcture. This would also allow for reduced clearing adjacent to building site areas. As already suggested, upon plaang much of the surface and subsur€ace water into a pipe or pipe lines and covering those pipe lines at a depth 2.5 feet or more, I feel that the °creek" should not even be considered in regard to setback standards. As AAs. Blowers informed me, in January of 2006 at which time she inspected the site there was no water flow in the pipe crossing under Thomdyke Road. This suggests that flow down the pipeline will be very low. Therefore, when allowing fora 20 foot setback plus the "creek" zone from the property lines, that wilt then allow for about a 40 foot wide zone for building purposes. ff the county will not allow this, then the tat is not buildable. However, l must point out that !feel that with the proposed improvements that will improve the existing factor of safety, most of system would be better and have a better factor if safety than some of their existing. pipe lines in the general area. This will include the pipe that passes under Thomdyke Road adjacent to the project area. It is my understanding tha# that one pipe line crosses under Thomdyke Road and is adjacent or near the subject parcel. That pipe (culvert) is 105 feet tong and is about 16 feet deep under the Thomdyke Road. The pipe is 18 inches wide and is 90° or perpendicular to the county road at mile point 2.718. ! feel that the only condition that I would be concerned about would be a seismic. event and/or a condition generated by a Large underwater land movement which can then result in a Tsunami wave. 3 G i~"EII~ ~a~o ~' of t . ~ 6 We appreciate this opportunity to be of service to you. If you have any'questions regarding this report, please contact the undersigned Sincerely, James B. Scott, P.E. Attachment: Drainage Design System Lt~G ~T'EMi ~# 3 ~ ~ ' .-~ t June 3, 2002 James B. Scott, P.E. GEOTECl-1N1CAL CONSULTANT 360-293-6044 FAX 3b0-293-6044 Email: t~eascott26(c~comcttst.net J. B. SCOTT & ASSOCIATES An Engineering Corporation 3b01 West 5~~~ Street Anacortes, WA 98221 STANDARD DESIGN OF A TIGHTLINE DRAINAGE SYSTEM During the summer of 1992, while making a series of landslide investigations on Camano Island, it became obvious that many of the recent bluff slope failures were closely associated with surtace drainage. Then ifi was during another onsite evaluation of a parcel adjacent to a bluff slope on Camano Island that it was noticed that such a failure condition was in the making_ This condition was pointed out to the client with the recommendation that a surface water collection system along with a "tightline" be placed for disposal purposes. The client then requested #hat I provide him with instructions on how to install a "tightline" system in order tha# he could dispose of the collected surface water at a point "somewhere° lower down on the bluff slape. !t was at this point that I realized #hat many people had no concept of where to discharge the water and also the damage that can be caused by a relatively small amount of concentrated water flow being discharged on a steep slope. Therefore, it appeared that a standard design would be a major contribution to maintaining more stable shoreline bluff slopes. Based on this, a standard design was prepared and was first issued on August 11, 1992. Since that time hundreds of tightline systems have been installed using that standard design in many of the counties in northwest Washington. It provided a means by which collected surface and subsurface water could be collected and removed for disposal to or near high tide elevations without causing excessive erosion. In those ten years, not one failure, of the system has been reported. In the intervening years, the counties became mare and more aware of the necessity to better control the disposal of not only collected surface and subsurface water from a developed property and they also became more concerned about storm water runoff from the entire parcel or lot. It way during the major starm that occurred on December 31, 1996 when that storm runoff re:-afted in many hundreds of debr7s flows that did much damage in the Puget Sound area and even caused loss of life. Also as a result of that storm, for the next few years, additional failures developed on the already unstable or unbalanced slopes. As a result of those many failures, the counties are now requiring more engineering input into the design and construction of systems for drainage water removal This has resulted in the original design being updated twice and having more detailed input in regard to collection systems, pipe capac"ity, and energy dissipation. This "up date" of the standard design includes the original design format plus many new items, which address some of the recent requirements that, are now mandated by the counties. ~~~ ~~~IV ~~ t r , Tightline Design June 3, 2002 However, it must be understood that this is a general and standard design statement regarding placement of a "tightline system" by the property owner and therefore is to be considered only as a guide. This data was not prepared as "Plans and SpeciPa±ations" for bid purposes (a contract item). This will not replace a specific design for a specific location that should be designed by a qualified engineer. AR.o this does not take itrto account future increases in surface and subsurface water flows that might result from fuixrre development of adjacent properties. A drainage collection system, as shown in Figure 1, consists of three basic parts, which will be discussed, in some detail. The first part is the actual collection by which both surface and subsurface water is intercepted and directed into a conveyance system, which is usually a tightline, and then the disposal or discharge point where the flow energy must be dissipated to prevent erosion. The collection method can tonsis# of: {1 }gutters and downspout from the roof of a house or structure; {2) a collection ditch that intercepts both sheet flow and concentrated flows and then directs that flow toward a collection point; or (3) a curtain and/or French drain that intercepts subsurface water and by means of a pipe then directs that flow to a collection point. In some cases where water is being collected from many sources, the pipes will be directed into what is calved a drop structurep where the flows are mingled and directed into a Tightline System for disposal at a selected point. Thus a collection system can be quite simple or very complex. After the collected water enters the Tightiine System, which is the most critical part of the system, the tightline pipes are usually dire~ed down rather steep scopes to the point of discharge. Most people do not realize what happens during a period of excessive or full capacity water flow. The factors that have to be considered are: (1) weak points such as couplings which in the case of a f~xible polyethylene pipe is every 100 feet; (2) the effect of W on the pipe when exposed to the sun, which in time results in the pipe becoming brittle; {4) the strength or grade of the pipe; and (5) during a turbulent water flow period, the water surges result in vibration and pipe movement. Because of this last factor, it is good policy to always secure or anchor the tightline. The disposal point can be a very simple box like structure or depression filled with cobbles where the flow energy is dissipated or it can be a very complex discharge facility, such as a n3cent system being used and'desgned by Island County. The discharged water should be spread ~ ~ much as possible to reduce the risk of erosion. In some cases this done by having .a perforated pipe in a ditch or having the discharge point in the bottom of a box., #ype structure filled with cobbles. In `the case of storm drainage from large areas, this can result in large volumes of water that cannot be handled by a simple energy dissipation system. In some of those cases, the Island County design, which. is available at the County Engineers Office, may be required. The bluff and or slope conditions along with the volume of water to discharge will dictate the actual design of the system. The basic Standard Design in this discussion will probably be mare than sufficient for the average sized house and lot with an average bluff slope angle of about 30°. it should be pointed out that the system shown in this Standard Design can be considered as being over designedA. 3 ,.~.,~ Z ~_ . ~ ,' ` Tightline Design June 3, 2002 However it is know that many "tightline" systems have failed because of poor anchorage, the. use of under strength pipe, or under capacity pipe. Therefore it is better to err in favor of over design than to have a system fail. Failure of a tightline system can and does result in very rapid erosion, which can then result in associated debris flow conditions and the unbalancing of the slope. This then results in slope instability and landslides. The typical system, as shown and allowing for annual inspections can have a project life of up to 20 years for the exposed portion of the pipe that will ultimately deteriorate due to solar UV. Polyethylene pipe becomes brittle after extended exposure to the sun. However, buried pipe is reported to have a project life estimated to exceed a 100 years. Therefore, covering the pipe with burlap or vegetation will prolong the project life of the system. Pesearch was done to select a product that would give a reasonable project life yet. allow for "ease of installation". Based on data from a manufacturer (see Figure 2), it appears .that the best pipe in regard to both resistance to UV and strength for steep slopes and allowing for the estimated flow of a 100 year storm generated from 3000 square foot roof, will be 4 inch Heavy Duty - AASHTO or equivalent, flexible pipe as produced by Rancor, Inc. The product code is HY PL5 (Specification AASHTO M252). The Washington State distributorfor Rancor pipe is H.D. Fowler Company of Bellevue. !n the case of a high or steep. bluff slope, during a period of heavy flow, the collected water will discharge water much like ahigh-pressure fire hose. This will result in vibration and surges as a result of the turbulent flow. Thus anchoring the pipe to reduce or dampen movement is required. Two types of anchoring (encasing the pipe) or placement of pins (rebar) will be discussed. In addition to anchoring the pipe this will also give some support to the pipe in regard to weight loading and/or elongation: The function of an anchor at the up-hill portion of the slope is to provide just that.. An anchor or anchors are needed since the weight of the total length of pipe will tend to pull and elongate the pipe. On very steep slopes it may be necessary to have an anchor every 20 feet. Atypical section showing anchorage is shown in Figure 1. You will note that my examples show only flexible pipe, which is much easier to install and less expensive than ridged pipe. Also, in the event of minor slumps a flexible pipe can adjust to the ground surtace while a ridge pipe will tend to span such zones. If the span is too long and other material rolls or slide down the shpe, it could break the pipe which then would be worse than nat. having any drainage coNection system. If you provide your own labor, the cost of placing the anchor points and pipe should be quite .economics! with regard to materials costs. However, the labor effort to dig trenches in order to encase a pipe or place a drop structure might be rather difficult, especially if "hard panty or cemented till is encountered. After the system has put into service, an annual inspection of the pipeline is mandatory. Also, if a drop structure has been installed, it should able to be opened and inspected. Then if debris is present it can and should be removed. At the first sign of an exposed pipe becoming brittle; it should be replaced. As already stated this is a standard design for average conditions. Should the project site be located in an area of much higher than normal rainfall or the system will co{lest both sheet flow and storm flows from areas having a large watershed, then a more #3 3 ~' r ' Tightline Design June 3, 2002 elaborate system wilt be required. in that case a speccrFFc design for the specific site will be required. There are other sources of information regarding collection and dispasai systems of surface water runoff. Many of the counties and cities are constantly updating their requirements and some of these requirements have been put into pamphlets or bulletins for public distribution. The state has issued the following publication regarding the collection and disposal of surface and groundwater. "Surface Water and Groundwater on Coastal Bluffs, Publication 95-107, Washington Department of Ecology, 1995. James B. Scott, PE ~~~ ~~~ ~~ r ~~rn~~ s _' r Backfill ~~~ ~ ncrete ball ,+-'~''~ Co N®a~s' St ~, ~ Drop Structure r Rebar Backfitl -~ ~~ `'~, ~~~,, ~ Coupling ,:"~~ Rebar ,~"" TOP OF SLOPE AREA TYPICAL ANCFIOR --~''t~ ,.~''~ i~° ®ISCI-FARCE AREA HIGH TIDE LINE Drain Rock 1rfa~' '; L, r, ~~ cJc`IIllP_S B. Scott, P.~. E" GEDTECHNICAL.GOIVSIILTAAtT 3b0-293-fiOBd Fnx 350-293-fipd4 ~~ s. B. sco~.~r ~ ~ssocr~zrs 3601' W. 5TH ST. ~~ AIQACOR7HS, WA $$221 -- .~~ s w^~ ,-- _~ Mofes Note ~ 1 _ Compact trenc#i wilttt cit3}ley backfi{I and armor surface to protect against erosion and seepage (plpin9) along pipe. It clayey ~ is not available. then ii>ix (volume) 9 parts dry sand and 1 part tieritonife and plaxse as backfill then vibrate. Note #2 - Drop stnar~uri3 is concrete utifiiy box-with intake and outlet holes #or pipes. The outlet pipe must be secured to the box. At a point just beyond the Duffer: anchor the pipe as shown fn Note #3. Top should be removal fior inspection purposes ', Note it -Secure the pipe by means of t" rebar driven to refusal. Bend the bar aver the'pipe but allow room for expansion of the pipe. Tie to anchors vath vinyl coated cable (c{othes line). Dip ends of v'snyl Coated cable in "rope dip" to prnteck against corrosion. Make ties looms to allow for heat expansion of pipe Note #4 - ff coupling is made, then make it just back from top of slope Double vamp each side. Place anchor rebar so stickup will be adjacent to coupling and pour concrete ball. Note #5 -Discharge point can be barrel filled with gravel, box with gravel or cobbles. If volume of bvater is large then place a trench and place horizontal pei#orated rigid pipe capped on both ends in bed of gravel. 7o atlaw for volume to exceed capacity of perforations, then place slits in the top of pipe for water .discharge purposes s I ~ r gn~;t~Vr it%lij3tx fS i ,t~~ Y)' p~~ t0„ t2" iw" tsd'~ ~~ HEAVY DUTY-AA5HT0 pipe is a flexible high density pot}ethylene pipe, corrugated on the inside and outside. HI-AVY DUTY AASI-iT0 pipe is heavier than HEAVY [DUTY pipe. Thi ~ Fxtra weight pr<~vides additior7al stiffness desired in some installations II is often used for highway-related drainage projec;is or for applications where heavy loads will be placed on the pipe. HEAVY DUTY-AASNTQ pipe also works well for general drainage applications. i~z$si#tt °ri;+:czt~tftt' ~>c_t'ir4_C7lti~itl til~lc t^rotluct ~tt~C . ~~ n:~SEi"I'tl At«~2 E'luln HY PL! Ycrf i-IY PFI tD3 IertiVti'rap IiY PWI 03 ~~ i~" _ Af~SId"IY) rii2S2 Ylaln HY Pi.,504 Pert HY PF3 tDs I'ert/W'rap HY PW5 04 ~ SUGGESTED ~PPLfCATiQNS l a" Ar4SH'TO 14t252 I'tutn HY PLi=f16 Teri HY PF1 06 nertlWrap - HY PWY ~ ''•"~" >1A;;F-i'Tt? 14!252 I'I~tn HY PL1 08 PcrC Ei1( PFl (1$ ~ - PerflWrap HX PWf 08 I'•`~" iAASH'Y'(7-~i2SZ I'luln - HY PG! 10 Pert HY PFi 1{t I~erClWrs~p HY PW1 lU ~ I' o.D. '' AASI•iTU tN29d I'[aln HY PG1 1~ 0 I'e-1 HY t'F1 12 Pecf (AASI#TO HY'D36 12 M3b Pattern) -"- I'irC(Wrap NY PW1 12 s].7" ArOtifiTdi rv129~1 Irt:etn F[Y PL1 I5 I'crt HY PFi 15 I'crt (AASHTO HY D36 15 Ji136 Pattern) P'crt/Wrap HY PWl 1S i.5~' AASI~'I'di tki2')4 I'I~rln t44' I'LI 1$ PerC tPY'PFl 18 GHEGKUST QF tTEt11$ TO GOrvsiiDER i'crt (AASFI'E'O NY D36 18 WHEN PURGFtAStNG TM1S PR®DtlCT t4t3d Patters) ` . ~ i'erf/V4'rap Egy pW1 7S / Fittings (see page 4) ~~'.`~" AAStiTO t~iZy4 I'tuin HY ['EJT 24 ~ Extra couplers for fittings t'erf ~y pF~ 24 / Gaotextile wrap I'erf (AASEITQ HY p36 Zq +~ The need tar perfotattans 14134 Pattern) ETCrfJW rap HY PW 1' 2jt ,~. = _ 6 •. ~.