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23431 94TH AVE W.PDF111111111 9213 23431 94TH AVE W ;OVED BY ANNING A! V LIU & ASSOCIATES, INC. Geotechnical Engineedng Engineening Ms. Amy Trenner 23431 - 94h Avenue West Edmonds, WA 98020 Dear Ms. Trenner: ,1JOAAwgio tRk WWAC Ameft&rw-s 2WO06 1-.jition L8C wMIAC Arnend=�nrs E32006 Editim UPC w1WAC AmendnienIcs and Aqmdices k 8,- Q I - 02606 U,�uon IMC w/VJAC Amendments 16n-9'h',`200cj0 2006 E&Lion V) A Q WAC 5 1- 13 moob Edition wSEC WAC 5 1-11 -ROVED AS NOTED -,—;:Iy E& 91A lr�, — Subject: Stability Evaluation and Remedial Recommendations Precast Concrete Block Wall Amy Trenner Residence 23431 - 94h Avenue West Edmonds, Washington L&A Job No. 8AO88 INTRODUCTION Your. residence located at the above address in Edmonds, W RECEIVED JUL 11� 1009 BUILDING DEPARTMENT CITY OF EDMONDS ' cDp'l SUILDN-Gi DEPAMNENT WORK P,*--A-.- -" /1 APPROvED DATE BLDG OFFICIAL P 0 Dn, is s az-4ft— ide a soma, , —� I of 94h Avenue West, behind a neighboring residence fronting this street. The general cation of this residence is shown on Plate I - Vicinity Map, attached hereto. This residence is accessed from 94th Avenue West via a paved driveway fi7om 94h Avenue West, rising gently northward and entering the prop . erty at its southwest comer. Lining the south property boundary and about the northern half of the east side of the driveway is an L-shaped precast concrete block wall, varying from about 1.25 to 6.5 feet tall. The alignment of the block wall is shown on Plate 2 - Site Plan. We understand that this wall had been constructed a few years ago before you purchased the property. The above block wall appears to have been constructed to support fin placed over the southern portion of the property to create a nearly level yard around the south and east sides of the house. The eastern bulging and 1110"--Of Edmonds Building Department "A �Ath"c t concrete block wal h s expenenced outward c s t t I t your request, we have L 19213 Kenlake Place NE - Kenmore, Washington 98028 Phone (425) 483-9134 -. Fax (426) 486-2746 e- June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 2 REQUIRED SPECIAL INSPECTIONS FOR THIS PROJECT: 1 ) Excavation, grading, & site preparati on 2) Soil bearing verification 3) Placement of fill & compaction 4) Footing drain 5) Temporary Erosion Control 6) Final Erosion Control 7) Site retaining wall/ rockery construction. 8) General site monitoring 9) Final letter from geotechnical engineer of record completed a geotechnical and stability evaluation for this wall. Presented in this report are our findings of the conditions of and our remedial recommendations for this wall. DESCRIMON OF EXISTING BLOCK WALL The north leg of the precast concrete block wall, about 100 feet long, points to the east, and the south leg of the wall, about 45 feet long, points to the south, with a curved transition from one leg to the other. The block begins at its south end at about 1.25 feet tall, step -rising gradually northward to about 5 feet tall at the intersection of the two legs, rising again eastward to a maximum height of about 6.5 feet for a short distance, then decreasing gradually to about 2.1 feet tall at its east end (see Photos I and 2 under APPENDIX I attached hereto). The wan appears to be constructed of 8-inch high by 18-inch wide by 12-inch deep tall, precast concrete blocks. Geogrid reinforced embedded in the wall backfill can be seen between the rows of blocks. SUBSURFACE CONDITIONS Geologic Setting and Soil Conditions The Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles. Washingion, by James P. Minard, published by U. S. Geological Survey in 1983, was referenced for the geologic and soil conditions at the residence site. According to this publication, prior to the construction of the concrete block wall, the native surficial soil unit at and in the vicinity of the residence site is mapped as Vashon Till (Qvt). The Vashon till soil unit was plowed directly under glacial ice during the most recent glacial period as the glacier advanced over an eroded, irregular surface of older formations and sediments. It is composed of a mixture of unsorted clay, silt, s and, gravel, and scattered cobbles LIU & ASSOCIATES9 INC. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 3 SUBMIT ALL SPECIAL INSPECTION REPORTS TO CITY BUILDING INSPECTOR ON A WEEKLY BASIS and boulders. The Vashon till soils over the top two to three feet are normally weathered to a medium -dense state, and is moderately permeable and compressible. The underlying fresh till soils are very dense, and are commonly referred to as "hard pan". The fresh till soils are practically impervious to stormwater infiltration. The fresh till soils possess a compressive strength comparable to that of low-grade concrete and can stand in steep natural slopes or man - make cuts for a long period. If remained undisturbed and well -drained, the fresh till sods can provide excellent foundation support to structures with little settlement expected. Groundwater Conditions The fresh till sods underlying the site at shallow depth is practically impervious and would perch stormwater infiltrating into the more permeable surficial soils. This near -surface perched groundwater would flow laterally down -gradient along the surface of the underlying fresh till sods. The amount of and the depth to this near -surface perched groundwater would fluctuate seasonally, depending on precipitation, surface runoff, ground vegetation cover, site utilization, and other factors. The perched groundwater may dry up completely during the dryer summer months and accumulate and rise in the wet winter months. CONDITIONS OF EXISTING BLOCK WALL An eastern stretch of the north leg of the precast concrete block wall has experienced outward bulging and separation of rows of blocks (see Photos 3 and 4 — APPENDIX 1). The upper portion of this stretch of wall has leaned outward 2 to 3 inches relative to the base of the wall, and the separation between the rows of blocks is up to about 6 inches. Excessive moisture in blocks of some rows indicates inadequate drainage control of the wall. We believe the stability of this LIU & ASSOCIATES, INC. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8A088 Page 4 wall is compromised -and that there is a possibility that the wall could topple over and collapse in the future. We understand that the existing precast concrete block wall may not have been designed by a qualified engineer. The distress experienced by the wall may have been caused by one or all of the following: inadequately prepared foundation soils supporting the wall, a lack of drainage control of groundwater flowing towards the wall, insufficient. compaction of the wall backfill, inadequately reinforcement of geogrid mesh in the wall backfill. REMEDIAL RECOMMENDATIONS We believe the precast concrete block wall should be dismantled and reconstructed based on a proper design of the wall. The concrete blocks of the existing wall, if in good condition and free of cracks and chips, may be salvaged and reused in the reconstruction of the wall. DESIGN OF CONCRETE BLOCK WALL Design Soil Parameters Based on the soil conditions de scribed above, we recommend that the precast concrete block walls be designed for a fully drained condition in accordance with the following soil perimeters: . Reinforced Retained Foundation Leveling 'Soils Soils Soils Rock Base Unit Weight, r, pcf 130 125 130 130 Angle of Internal Friction, 36 34 36 40 degrees Cohesion, c, psf 0 0 0 0 LIU & ASSOCIATES, INC. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 5 Trafric Surcharge The portion of the to -be -reconstructed wall next to the paved driveway and paved parking area should be designed for a traffic surcharge. The traffic surcharge should be considered as to be equivalent to an uniform pressure of 200 psf Loading Conditions and Factors of Safety The to -be -reconstructed block wall should be designed for static and seismic loading conditions. For static loading conditions, the block wall should be designed to have a minimum factor of safety of 1.5 against sliding failure, 1.7 against over -turning failure, and 2.0 against foundation bearing failures. The Puget Sound region is in an active seismic zone and the block wall should be designed- for - a I 00-year seismic event under the seismic loading condition. The peak ground acceleration is about 0.3g (g = gravity force) for such an event in the Puget Sound region. The block wall, however, is to be built with interlocking concrete blocks with high flexibility, and the blocks will not mov e in unison during earthquakes. Therefore, for design of the block wall under the seismic loading condition, the peak ground acceleration may be reduced to 0.2g. The block wall should designed for a factor of safety of at least 1. 1 against sliding, 1. 2 against overturning and 1.5 against foundation bearing failure under the seismic loading condition. Stability Analyses and Designs The computer program, SRWall (Version 3.22 April 2002), was used in our stability analyses and design of the precast concrete block wall. The results of the block wall stability analyses are presented under APPENDIX 2 attached to this report. Our design of the precast concrete block wall is shown on the typical section of the wall shown on Plate 3. MU & ASSMATES, ffNC. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 6 CONSTRUCTION OF CONCRETE BLOCK WALL Reconstruction Of Wall Within Subject Property According to the topographic survey plan of the subject property provided to us, the existing block wall at the comer where the two legs of the wall intersect appears to have fallen within the adjoining neighboring property. In reconstruction, the block wall at this comer should be pulled in about 2.5 feet to the northwest such that the wall would be completely located within the subject property. Otherwise, a written consent from the neighboring property owner would be required if any part of the block wall falls within the neighboring property. Site Preparation and General Grading Existing block wall should be dismantled carefully with blocks in good shaped to be stored for reconstruction of the wall. Vegetation within the block walls and backfill construction' limits should be cleared and the roots thoroughly grubbed. Topsoil, disturbed soils, weak surficial soils and existing wall backfill should be stripped down to firm undisturbed native soils of an allowable bearing capacity of at least 3,000 psf. -The exposed sods within the block wall and backfill footprints should be compacted to a non -yielding state with a vibratory mechanical compac tor. During construction, storm runoff, if any, should be intercepted with ditches or interceptor trench drains, as required, and be conveyed into a storm sewer or suitable stormwater disposal facility. A layer of clean quarry spalls should be placed over areas of frequent traffic, such as the access to the area of the block wall, as required, to protect the subgrade soils from disturbance by construction traffic. LffU & ASSOCUTES9 ffNC. M June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 7 Keyway Trench The keyway trench of the block wall should be cut into undisturbed, finn, native. soils or into sufficiently compacted structural fill built on firm native soils, capable of rendering an allowable bearing pressure of at least 3,000 psf The soils exposed at bottom of the keyway trench should be compacted to a non -yielding state with a vibratory mechanical compactor. Over -excavation of unsuitable soils should be backfilled with sufficiently compacted structural fill. A minimum 4-inch layer of 7/8-inch-minus crushed rock leveling base, compacted to a non -yielding state, should be placed over bottom of the keyway trench. . The base -course blocks are to be placed on this crushed rock base with an embedment at least 8 inche's below the finish grade in front of -the block wall. The hollow cores of the precast concrete blocks should also be filled with 7/8-inch crushed rock, tamped to a non -yielding state. The blocks should be stacked tightly against one another. Each course of blocks should be interlocked with the blocks of the course below with inter - con necting lips or with Teflon pins. Drainage Control A minimum 4-inch diameter, perforated, rigid, PVC drain line wrapped in a non-woven'filter fabric sock should be laid behind the base blocks to collect and drain away groundwater flowing towards the walolWater dammed up behind the wall would exert hydrostatic pressure on the wall in addition to lateral soil pressur��.Hydrostatic pressure of inadequately drained wall may push .the wall outward and cause the wall to fa�.) The bottom of keyway trench and this drain line should be sloped down at sufficient grade (0. 5% minimum) to generate flow by gravit V. A minimum 12-inch-thick (horizontally) vertical drainage blanket, constructed of clean free-draiming 7/8-inch crushed rock, should be, placed against the back of the block wall. This vertical drainage blanket should be hydraulically connected to the drain line at the base of the wall. Structural fill, MU & ASSOCUTES9 EW. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 8 if required, should be placed behind the vertical drainage blanket. The vertical drainage blanket and structural fill should be constructed in lifts after each course of blocks is completed. We understand that there is no available nearby storm sewer line to which the ne w* block wall drain line can be tied to. --We recommend that groundwater collected in the wall drain line be discharged into and disposed through an infiltration trench. We estimated that the amount of groundwater flowing in the wall drain fine should be relatively minor c' omparing to roof runoff and that an infiltration trench installed under the driveway may be used to dispose the water in the wall drain fine. We estimate that We recommend the infiltration trench be at least 2.5 feet wide by 15 feet long by 3 feet deep. A typical section of the infiltration trench is shown on Plate 2. The side walls of the trench should lined by a layer of non -woven filter fabric. The trench should then be backfilled with clean, free -draining, 3/4 to 1- 1/2 inch crushed rock to within about 10 inches of the top of the trench. A 4-inch,.perforated, rigid, PVC dispersion PVC pipe should be set level in the crushed rock filled at about 15 inches below top of the trench, with the top of the crushed rock fill covered with filter fabric. The remaim*ng trench should then be backfilled with the same crushed rock fill to the top of the trench. The crushed rock fill should be placed in lifts no more than 8 inches in loose state, with each lift compacted to a non -yielding state with a vibratory mechanical compactor. Structural Fill Wall backfill should be constructed with structural fill. Structural fill is the fill that support structural or traffic load. Structural fill should consist of clean granular soils free of organic, debris and other deleterious substances and with particles not larger than three inches. Structural fill should have a moisture content within one percent of its optimum moisture content at the time of placement. Optimum moisture content is the water content in the soils that enables the soils to LIU & ASSOCIATES, INC. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 9 be compacted to the highest dry density for a given compaction effort. Onsite soils meeting the above requirements may be used as structural fill. Crushed rock fill and structural fill should be placed in lifts no more than 10 inches thick in loose state, with each lift com* acted to at least 92% p of the maximum dry density determined by ASTM D1557 (N 4odified Proctor method) with a vibratory mechanical compactor. Geogrid Mesh Placement The portions of the block wall taller than 3 feet should have the wall backfill reinforced with geogrid mesh. Each layer of geogrid mesh shall be anchored between rows of blocks and laid over level wall.backfill. The geogrid mesh should then be stretched taut and securely staked down before placing the next lift of backfill. Overlaps of geogrid mesh in the direction parallel to the wall alignment shall be at least 12 inches, while overlaps of geogrid mesh in the direction perpendicular to wall alignment should not be allowed. Friction force developed between the wall backfill and the geogrid mesh would provided lateral support to the block wall for required stability. Erosion Control Vegetation outside of construction limits should preserved and maintained. The wall backslopes should be thoroughly compacted to a non -yielding state with a vibratory compactor. Disturbed, unpaved, exposed ground should be re -seeded and re -vegetated as soon as possible. The seeded and vegetated areas should be covered with a layer of clear plastic tarp, as required, until the vegetation is fully established. The tarps should be securely weighted down with sandbags. MU & ASSOCUTES9 WC. June 30, 2009 Stability Evaluation and Reconstruction Precast Concrete Block Wall Amy Trenner Residence L&A Job No. 8AO88 Page 10 CLOSURE We are pleased to be of service to you on this project. Please feel free to call us if you have any questions regarding this report or the design of precast concrete block walls and rockery walls. r0114 IG S. Yours very truly, L11TJ SSOCIATES INC. 2758 J. S. (Julian) Liu, Ph.D., P.E. Consulting Geotechnical Engineer ,Emftu 7/17/ Attached: Plates. I through 3 Appendix I — Photos of Existing Concrete Block Wall Appendix 2 — Stability Analyses of New Precast Concrete Block Wall LIU & ASSOCIATES, INC. 3RDJ W =7 Cie 2RD 14TH �a ��V w Woo WrAvIlk Am OEM( 71cy I 9 2113THI!:3 23ZCC_-. a 211M io TH MJTR ST W_ 112T P, W rn S 1,7 (112TH AV W) C. 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P.L W u.), 87TH w CD m > < cp III Aj ".190ou V�; 'N' "M 7 AV coin < X 87 AV W i�3 c U) Z — -LINDEN ' - ! - " �z AV W A g Ln 1* 85n R 12 AV W m 0 I I �H! iHi --1 = AV 7 je AV W v 0 __j M MAN 84TH w :j�AV N QVALE� AV'-N'- 00 d m z Z 239uo (N TRUNK MEN a RD AV W 779) _0 M AV 8�� 8:ND AV W OP) 9-r-62ND AV W- (A N-4 81 PL w VE PL m ON- !I pl� le m am U; s 7f ON AV) lap 0 A fcS 7- -A v, MA > �pk ,LA' 'All 7— 7�A N a b-a a b (/ J11 T, >Z% �A ---A 1 t=A I I t=1 F-4 1 IT---4 [1 11 < -.;. 7-' V�% -6 - )q4 � 4,; ` , 7 T , , Z- t - �"" cv, � � -z_ I T- 'rFA t L i0a. - Sol Z- I . "I 11�1 . . - .1. - - ...- -I- - I- I ON 7 i1p v asm J. 1ww 1 "; �'da oj� 1 7) f;!; 7� 0EA> OD tv LIU& ASSOCIATES, Geotechnical Engineering Engineering Geology Earth Science I I JOB NO. 7 7 C) SITE PLAN AMY TRENNER RESIDENCE 23431 - 94TH AVENUE WEST EDMONDS, WASHINGTON TE, lZu Cr A Fl U- 4 14 04 V Wfe;R RVII E>0-rrom up) "HKI8 WX(2!C) 6.4 H L ... RIM] - a (1191 =4111 111 off.:== I—I's G LEA K - -7/i�-' MFP. PVC, P4ZA1W C�,KLJSH r-p RO&K= PIPS WgApfle:D .14 T LA -i�� e-Url ON YPI GEOGRID MESH SCHEDULE WaLL Height H, ft Geogrid MesK- (Length LN, ft / Type) Layer 1 1 Layer 2 _T Layer 3 Layer4 Layer5 3 or less Geogrid Mesh Not Re . quired 3 to 5 4.00 4.00 5.00 5.50 StrataGrid 150 StrataGrid 150 StrataGrid 150 StrataGrid 150 - 5 to 7 4.50 4.50 5.00 5.50 6.50 StrataGrid 150IStrataGrid 150IStrataGrid 150IStrataGrid 150IStrataGrid 150 5. INDEMNIFICATION 5.1 Contractor shall be solely responsible for construction safety and his methods, techniques, sequences or procedures used for the construction, and shall take necessary measure to maintain stability of cut banks during constrdction fo pr6v6fit odily injury or propetty damages. 5.2 Contractor shall indemnify and save harmless the Owner and Engineer from and against any damage, cost and liability for injury or death to persons and from damage to property caused ..by negligence of Contractor, his agents, employees or subcontractors. GENERAL NOTES SCOPE OF WORK Contractor shall provide material, equipment, labor and incidental tools to complete the precast concrete block wall reconstruction to match the lines and grades of the existing block wall and shown on this drawing. I 2. SITE CONDITIONS Contractor shall visit site and familiarize himself with site conditions, and shall verify existing grades, dimensions and elevations in the field. Contractor shall report any discrepancy with the drawing to Engineer (Liu & Associates, Inc.) before proceedin1g with wall construction. Contractor shall exercise extreme care not to damage any underbround utilities, existing structures, driveways, houses, landscape, etc. Damage to the above shall be repaired and restored to their original conditions or better at Contractor's expense. 3. MATERIAL 3.1 Precast Concrete Blocks of the existing wall shall be 'dismantled and salvaged. Blocks in good conditions without cracks and chips shall be use'd in the reconstruction of the wall. New blocks, if req u-ired shall match the existing blocks. 3.2 Levelinq Base and Drainage Blanke shall consist of clean, free -draining 7/8-inch- crushed rock. 3.3 Drain Line shall be consist of min. 4-inch diameter, rigid', perforated, PVC pipes, with glued watertight joints and fitted with a non -woven filter fabric S`ock. 3.4 Structural Fill shall be clean granular soils free of organics, debris and other deleterious substances and with particles no larger than 3 inches. . Imported material used as structural fill shall contain no more than 5% by weight finer than the No. 200 sieve based on the fraction of the material passing No. 4 sieve. 3.5 Geogrid Mesh shall be as specified on the drawing or eq i ual. 4. INSTALLATION 4.1 Removed existing block wall and wall backfill down to fi6 undisturbed native soils. Keyway trench for the new block wall shall be into native soils capable of rendering an allowable bearing pressure of 3,000 psf. Over -excavation to 'remove loosetweak soils shall be backfilled with structural fill. Exposed soils on bottom 'of keyway shall be compacted to a non -yielding state with a vibratory compactor. 4.2 Bottom of keyway trench and drain line shall have sufficient slope (0.5% minimum) to generate flow by gravity. Drain line shall be tightlined 'to discharge into the existing catch basin near south end of driveway. 1 4.3 The blocks of the wall should be tightly stacked and interlocked against one another. The hollow cores of. the blocks shall be filled with 7/8-inch cr�shed rock tamped to a non -yielding state. 4.4 Structural fill shall be placed in lifts no more than 10 inches in loose state, with each lift compacted to at least 92% of the maximum dry density determined by ASTM D1557 (Modified Proctor method). 4.5 Each layer of geogrid mesh shall be laid on level fill surface, anchored with interconnecting lips or with Teflon pins, stretched light, and staked down, prior to placing the next lift of structural fill. Overlaps for geogrid mesh inbe directi6n6f wall alignment shall be at least 12 inches, overlaps in the direction perpendicula: r to Wall alignment shall not be allowed. i 4.6 Wall backslope and disturbed areas shall be te-seeded and re -vegetated as soon as possible. Re -seeded and re -vegetated areas shall be covered with clear plastic sheets, its required, until the vegetation is fully established. 4.7 Debris and construction waste shall be removed hauled off the site and the site thoroughly cleaned.,afte'r completion of block wall reconstruction. Z 0 P :5 F- 0 a- 0 CO Z 0 0 LLJ LLi (-) F- CD Z LLJ 0 00 cc::,' 64 C�4 Z LLJ 3: 1,- _J < 0 — U) LLJ 0 Z =) LLJ uj W Z LLJ (_5 of LLJ > < < C, 0 Z Z� LLJ = W 0 UJI,Z — LLJ f— LLJ IX I.— >_ 1 0 , Ce) 0 < 0 C11) LLJ cc) 00 5 Z 0 < 00 0 F_ C/) < d Z 0 LLJ M 0 CL --.% 5-d V 05 U r_ R 0 Z a) 0 CD C W CD S LU 0 4) 0 APPENDIX I Photos Existing Precast Concrete Block Wall Amy Trenner Residence Nit C51 . X Photo I — West Leg and West Part of North Leg of Wall �q- 7: �7 t Photo 2 — East Part of North Leg of Wall Note wall bulges out where fence posts tilt backward Photo 3 — North Leg of Wall Showing bulge of wall and separation between rows of blocks Photo 4 — Close-up Look of Protrusion and Separation of Rows of Blocks I -7�06W 3q3) 7 q4 Alz- lu a"Mtl FIVE