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Feb 14 2013-Assoc Earth Sciences-Infil Response Letter.pdfAssodm:c,d Eart,,'h. . . .. . .. .... . a 'Albrtffivesl sb'vx I .S'ervir�q ff 'Pacift 98 1 W, 2013 Project N I o. 1<11.106064413 Burnme,.W C(.)j-)sauctj(..w 11980 NE 24"' Stj�eet, Suite 200 Bellevue, Washington 98005 Alterttion: MsTiffiny Brown sub.je& Response to City of Edmonds, Comments, Plat at'Woodway Eleaientai-y 23708 104" Avefitte Northwest Ednionds, Wasbington Dew- Ms., Browx-t: Associated Earth Sciences, Ine. (AESI) is pleased to 1wesent this coi-runctit-response le tel addressing CODIMCIIIS Whid)wex-e, provided in a sIveadsheet inauix dateld Jalfflary 25, 2013, by Jerry Shuster of the City of Ednionds. Comment I . 1. Full infiltration is proposed, see clarijications needed related to the proposedpil drains in item (L)Lbelcrw® it-5-SQQ�ry q Please see the response to Connnew 4(L) t, Additional inforination is required on how the proposed pit drains will affect the design iqfiltrafion rate, see itein, 4, ()J. below. .Re.§pqiiS.q AESI's February 12, 2013 "Case J.-.intofies Study" documents ii-nproved infiltration facility pe.t-formance with the installation of pit dj'ah)s, K i M I a n d, � I Ar C r c t It a c 0 ni ;,T� El Plat at Woodivay Elementary Edmonds, Washington Res ome to Comments Comment 4(L)I Provide more information on the effect of installing the pit drains and how the number and spacing will be determined. Response The effect of 'installing pit drains is addressed on page 2 in AESI's February 12, 2013 "Case Histories Study." As stated in the "Case Histories Study" letter, "The pit drains act to improve performance by penetrating low -permeability interbeds." The current proposal is to install one pit drain in the infiltration facility, Performance results described in Case #6 on page 5 document the effectiveness of a single pit drain. Comment 4(L)3 The Storm Drainage Report should state whether the infiltration facility needs to or does not need to be registered with the ,state Department of Ecology under the underground injection control (UIC) program. If not, the reason (s) for not registering should be stated. Response The proposed infiltration facility does not need to be registered with the Washington State Department of Ecology under the Underground Injection Control (UIC) program. According to the Washington State Department of Ecology's Guidance for UIC Wells that Manage Stormwater, dated December 2006, it does not meet the definition for the following reasons: (1) the proposed facility is not designed with perforated pipe, and (2) the facility, including the proposed pit drain, is not deeper than the largest surface dimension. Design Infiltration hate Discussion The project civil engineer, Blueline, is using a design rate of 7,0 inches per hour (in/hr) for the proposed infiltration vault. This design rate is based on infiltration testing and the application of the correction factors identified on page 5-59 of the 2005 King County Surface Water Design Manual, and described on pages 5 and 6 of AESI's October 5, 2012 "Subsurface Exploration, Infiltration Testing, and Design Infiltration Rate Determination" letter -report, As demonstrated in our February 12, 2013 "Case Histories Study," the addition of the proposed infiltration drain to the vault design will, in our opinion, enhance the infiltration performance of the vault. February 14, 2013 ASSOCIATED EARTH SCIENCES, INC. SSTIN - KHO606441313 - ProjeciA200606441KHMIP Page 2 Mal of Woodwa)j Elcnwnlmy onse to ("emunents ....... .... ("'losu're 'We then opportunily 11) be, of comi1jued servicle to yci�,a on this prqlecl. Should you havc, ;,my quest, I i , ons regarding this IeMm', pk",ase f"'afl us at Your carlit"'St Sincerely, I ASSOCIMIED EARTF1 Kirkhand, Washington Koger, LG., L—LG., 1-119 Senior Principal Geologist/ Hydrogeologist Rcferences: As,sociated Earth Sciences, IDC, (AESI), 2012, Subsutface exploration, infiltration testing, and desigri infiltration rate detenninatiom October 5, 2012. Associated Earth Scierices, Inc. (AESI), 2013, Case histories study -- infiltration pit drain: February 12, 2013, King County, 2005, St.irface water design manual. Washington State Department of Ecology, 2006, Guidance for UI C wells that manage stormwater: December 2006. Fel5rtany [4, 2013 SV/ld — K11060044B 0 Po qlert.s 00(O6064,M) 11141 AYSOC'1/177-,D IFARM ljvc. IP^agc 3 fn Assoc,30(�-ed Ewrth�. 41, tn,��'j t h Pf` Rfic "Wo rtthv es I S I n, e i�,',O F&WW-Y 12, 2013 Project 1'qo, KH.0606d,03 13urnstead Construc.11011 . 11980 NE 20' Street, Suite 200 Bellevue, Washington 98005 Attenrioar Ms. 'Fiffiny Brown SubjecIll: ("ase Histories Study Infiltration. Pit Drains Plat, at Woodway Elemeotaxy 23708 104"' Avenue Northwest Edmonds, Washington Dear Ms. Browrj.: Associated Eartb Sciences, fric, (AE'�Sl) is pleased to present this letter -rep ort providing the results of our case history study of projects where infiltration drains were used to improve the perforynance of storrnwciter infiltration facilities. AES) has proposed augmenting the proposed Tract C infiltration facility with one pit drain in order to improve infiltration facility performance, In response to this, the City of Edmonds has requested background infori-nation regarding pit drains and their effectiveness for irnproviiig infiltration in similar settings. Tris case history document provides six examples crf pare.jects where drilled or dug infiltration drains have been used to reniediate underperforining infiltratioii facilities, or where drains have been integral to the design of the facility to optimize infiltration pcYlorii-iarice- Site and Project Description The Plat at Woodway Elementary site is located at 23708 1,04"' Avenue Northwest, in Ednionds, Wmhington. It is out, understanding drat the miestern portion of the site is being developed, and the City of Ednionds has retained the eastern portiort for a park. The most recent site plan provided by The Blueline Group (Blueline) indicates that I.be site will consist of appro'Khriately 27 home sites, with associated roadway access, open spaces, and utilities,, Ttie ay. 11�1',V c, it (", r t , Fa (1, o M a 4f, 2, 5 - t,;2, 7., 7 7 0 1 is 21, 5 - 2 5 91 - 01 2, 5' 3 -7 �2 Z 2, 99 2 WVV W, R (,S 9 C 0 o CO �Vl Plat at Woodtiway Elementary Edmonds, Washh! ton Case Histories Stud - Infiltration Pit Drains current design concept is to infiltrate stormwater runoff in Tract C, as shown on the figure prepared by Blueline. Overview This case history study documents the effectiveness of infiltration drains at improving the performance of infiltration facilities. In four cases summarized in this letter -report, the drains were installed after constructed facilities yielded poor performance. Site investigations in these cases indicated that stratification in the sediments underlying the infiltration facility had prevented the downward migration of stormwater. Two case histories (Cases 5 and 6) are presented from sites where infiltration drains were installed as integral components of the drainage designs. Background — Stratification Considerations In situations similar to the proposed Plat at Woodway Elementary infiltration facility, AESI has previously used deep vertical pit drains to improve the flow characteristics of the receptor soils beneath infiltration ponds. The pit drains act to improve performance by penetrating low - permeability interbeds and preventing "micro -mounding" beneath the facility, and can result in a high degree of connectivity with more permeable receptor horizons. This allows for high rate horizontal flow to occur as early in the storm event as possible and results in improved facility performance. In addition, the pit drains provide an effective transfer of hydraulic head into the subsurface. This allows the head to drive water away from the facility as efficiently as possible in the subsurface. In summary, the pit drains allow for exposure to a large cross- sectional area of the most permeable intervals present in the receptor interval, and provide hydraulic head to optimize flow rates into the formation. It is AESI's experience that properly constructed pit drains can provide a significant improvement to infiltration pond performance. Pit drains are installed at the time of infiltration facility construction. A vertical pit is dug by an excavator to a prescribed depth beneath the bottom of the facility, and a piezometer (PVC pipe with slots at the bottom) is inserted into the pit for later performance testing. The pit is then backfilled with free -draining material as specified by AESI up to the subgrade elevation of the infiltration facility. Case History Examples The six project cases below illustrate the effectiveness of pit drains for improving infiltration facility performance. Februaty 12, 2013 ASSOCIATED EARTH, SCIENCES, INC.. MIN -- KH0606441111 -- Projects120006441KHMP Page 2 Plat at Woodway Elementary Edmonds, Washington Case histories Study -Infiltration Pat Drains Case #1 - Pond Remediation Subsurface exploration in a large infiltration pond located in Ring County, Washington indicated a high degree of stratification. The stratification was expected to limit the infiltration performance, and full-scale infiltration testing of the built facility was required to document the actual infiltration rate of the facility. Testing of the facility included approximately 11 million gallons of water and indicated an as -built infiltration rate of about 0.45 inches per hour (in/hr). Subsequent subsurface exploration determined that shallow ground water mounding within the highly stratified sediments resulted in the low infiltration rates observed during the full-scale pond testing. Pond remediation included the installation of drilled drains and deep pit drains. Following pond remediation, the full-scale pond test was repeated. The average infiltration rate over the entire duration of the pond test (52 hours) was 2.8 in/hr (in excess of a five -fold increase). The pond remediation minimized the adverse effects of ground water mounding. Long -tern infiltration performance in the pond has been monitored by a series of observation wells. Facility #1 has three wells (OBW-13, OBW-14, and OBW-15) installed in the footprint of the pond, one nearby upgradient well OBW-4R and one nearby downgradient well OBW-3R. All wells are equipped with data loggers. The attached hydrograph (Figure 1) shows water level response from two observation wells and a surface staff gauge over the period of record for Facility #1. The hydrograph demonstrates that from a ground water flow perspective, the pond is performing as intended. The hydrograph shows: 1) a direct hydraulic connection has been established between the infiltration pond and the top of the regional water table, 2) rapid dissipation of the ground water mound following an inflow event, and 3) no reduction in performance during the more than 12 -year period of record. Case #2 - Pond Remediation AESI performed forensic investigations and provided remediation design for a large infiltration pond located in Pierce County, Washington. Although the infiltration pond was designed and built to code, ground water mounding inhibited the infiltration capacity of the native sediments. Performance monitoring indicated that the original as -built infiltration rate was less than 0.2 in/hr. Pond remediation included the construction of pit drains across the pond bottom with a track - mounted excavator. Three pit drains were equipped with piezometers prior to backfilling with pea gravel to allow for water level monitoring during drain infiltration testing. February 12, 2013 ASSOCIATED EARTH SCIENCES, INC. CJKJId - KH060644811 - hojerls1200606441KHMP Page 3 Plat at Woodway Elementary Edmonds, Washin ton Case Histories Study - Infiltration Pit Drains The pit drains allow for exposure to a large cross-sectional area of the most permeable intervals present in the receptor interval, and provide hydraulic head to optimize flow rates into the formation. Pit drain installation improved the infiltration by a factor of 8 to 1.0 in/hr. Case #3 - Pond Remediation Exploration and test results indicated variable subsurface conditions ranging from highly permeable sand and gravel to relatively low -permeability silty sand beds underlying portions of the pond subgrade. In addition, relatively low -permeability silty interbeds were observed deeper in the subsurface in some of the exploration pits. Relatively low infiltration rates were obtained from tests conducted in the relatively silty sand soils exposed near the pond subgrade in the northern portion of the facility. Pond modifications included: 1) the removal of approximately 31/z feet of silty subgrade soils from the pond bottom, 2) excavation of a series of pit drains, 3) placement of clean 4x8 sand in the pit drains, and 4) placement of 3 feet of 4x8 sand on the pond subgrade with an overlay of 6 inches of C33 sand. The pond bottom was returned to the original design grade elevation by placement of the 4x8 and C33 sand. Following pond remediation, AESI observed the performance of the infiltration pond during a large storm event. We understand this storm event ranged from a 10- to 25 -year design event with over 4 inches of rain falling on Sea -Tac Airport. At the time of our visit, the pond contained only I foot of water. Based on information provided by the project civil engineer, it is estimated that at the time of our site visit, approximately 3.45 cubic feet per second (efs) was entering the infiltration pond and infiltrating. This equates to an infiltration rate of roughly 38.5 in/hr across the pond bottom. The as -built performance, following pit drain installation, exceeded the design infiltration rate by a factor of 2 1/4 . Case #4 - Pond Remediation Full-scale pond testing indicated a relationship of increasing infiltration rate with increasing stage. However, the maximum rate measured during testing was less than 2.0 in/hr. Several restrictive layers were subsequently identified during detailed subsurface exploration. The restrictive layers, combined with dirty construction water inflow, caused shallow ground water mounding, which resulted in very low infiltration rates during full-scale testing. Following remediation, which consisted of drilled drains, full-scale pond testing was repeated, and rates exceeding 17.0 in/hr were recorded at higher stage elevations, The remediated rate exceeded the initial infiltration rates by a factor of 81/2. Figure 2 shows stage -rate relationships for the pre-remediated and remediated pond during full-scale pond testing. February 12, 2013 ASSOCIATED EARTH SCIENCES, INC, CJKAd - KH060644B]l - Prnjec1s120060644WMWP Page 4 Plat at Woodway Elementary Edmonds, Washington Case Histories ,Study - .infiltration Pit Drains Case ##5 -- Pit Drains During Initial Construction Prior to construction of this large infiltration pond, a design infiltration rate of 2.9 in/hr was determined based on subsurface exploration and in-situ infiltration testing. Infiltration drains were installed in this pond as an integral part of construction to account for stratification observed during the field exploration phase. Full-scale pond testing yielded an infiltration rate in excess of 4.3 in/hr at higher pond stages, which is about 11/a times greater than the design rate of 2.9 in/hr. Stage -rate relationships (Figure 3) document a trend of increasing infiltration rate with increasing stage height indicating maximum infiltration rates would exceed 4.3 in/hr. Case #6 - Pit Drain During Initial Construction This infiltration vault, located in Kirkland, Washington, is founded in recessional outwash which overlies advance outwash at depth. During the design phase of the project, a large -ring infiltration test was performed within stratified recessional outwash, at a depth equivalent to I to 2 feet below the proposed vault bottom. During the infiltration test, the rate continued to drop, due to an underlying low -permeability silty sand layer. In order to maximize infiltration capacity while minimizing the footprint of the facility, AESI recommended installation of a pit drain system. The design infiltration rate was 85 gallons per minute (gpm) for the entire facility (pit drain plus vault bottom area). The drain bottom area was 312 square feet, and extended 19 feet below the bottom of the vault. The vault bottom area was 1,760 square feet. A full-scale drain test was conducted at a flow rate of approximately 93 gpm into the pit drain. The maximum head rise in the pit drain during the test period was 10.3 feet. Since the total depth of the pit drain was approximately 19 feet, the as -built flow capacity of the pit drain significantly exceeded the design rate requirements. Following the conclusion of the test, AESI was asked to repeat the flow test at a 150 gpm rate, in order to account for additional runoff from an adjacent parcel. The 150 gpm flow test was completed with a total head rise of approximately 15 feet above the bottom of the infiltration drain, or about 4.5 feet below the bottom of the infiltration vault. Based on this testing, the as -built flow capacity of the pit drain alone still exceeded the combined pit drain/vault design requirements, even with the addition of the adjacent parcel, The installation of the pit drain in this infiltration vault allowed the design flow rate to infiltrate into the subsurface efficiently by effectively minimizing the mounding effects of stratification, and by allowing the infiltrated water to access a greater thickness of the receptor sediments. February 12, 2013 ASSOCIATED EARTH SCIENCES, INC. UK& - Ki106064011 - Projects1200606441KHMP Page 5 111m, ca Woodway Elementew)p E --d= m--,o-ndsW8_up )I .C.... ... ....e... .. . I.. . . . .s....l..... Sm ._ 1T) fir.o....d...on.. Pit Drcdns� C I losurc" We are (,,,onfident thas this let( er r% will fid in the successful compk"tion ofyour- plo,ject. [I., you should have any questims, please do not hesitate to ca].L Silicerely, ASSOCINTIE'D EARTH. SCIENCES, IINC. 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