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HomeMy WebLinkAboutRS_Geotech_Study_091016.pdfLIU & ASSOCIATES, INC. Geotechnical Engineering Engineering Geology February 20, 2009 Mr. Robert Wilson 720 South 55°i Street Renton, WA 98055 Dear Mr. Wilson: Subject: Geotechnical Engineering Study Wilson Plat 720 South 55`x' Street Renton, Washington L&A Job No. 9AO06 INTRODUCTION Earth Science qty of Renton We have completed a geotechnical engineering study for the site of the subject plat, located at the above address in Renton, Washington. The general location of the project site is shown on Plate 1 — Vicinity Map. We understand that the proposed development for the site is to plat it into 16 single-family residential lots, with supporting infrastructure. The purpose of this study is to characterize the subsurface conditions of the site and provide geotechnical recommendations for grading, slope stabilization, erosion mitigation, surface and ground water drainage control, foundation design and construction, etc., for the proposed development of the site. Presented in this report are our findings, conclusions and recommendations. PROJECT DESCRIPTION For our use in this study, you provided us with a set of topographic survey and plat plan of the proposed development for the site. According to this plan, the subject site is an rectangle -shaped land about 526 feet wide (east -west) by about 207 feet deep (north -south), lying to the north of South 55th Street. The platted building lots are to be accessed from 55th Avenue South to the south side of the site via a new roadway entering the site near its southwest corner and an existing gravel driveway at about 120 feet west of the southeast corner. These roadway/driveway 19213 Kenlake Place NE - Kenmore, Washington 98028 Phone (425) 483-9134 , Fax (425) 486-2746 February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 2 are to connect a paved road running mainly east -west down the middle of the site. Individual irafltrataori :acities X11 " be aged 'ispc s::;;stvrmwatex':rt'p residential _buildings wherever ; suitably .and a : concrete detention w;ult, designed ,tasuppletnent .the infiltration facilities, is to be located at the northwest corner of the site to store storm runoff collected over the impervious surfaces of the proposed development. We understand that grading for the proposed development of the site will ilnde'cutting .dpwn the .astern high -ground .area some Retaining walls and/or rockeryAtha# end litrig up',v_steM .1 AWeying .area.to tne i gree. g walls may be required to support or line the cut banks and/or fill embankments along the east, west and part of the south boundaries of the site. SCOPE OF SERVICES Our scope of services for this study comprises specifically the following: 1 Review the geologic and soil conditions at the site based on a published geologic map. 2. Explore the site for subsurface conditions with backhoe test pits to a firm bearing soil stratum or to the maximum depth (about 12 feet) capable by the backhoe used in excavating the test pits, whichever occurs first. 3. Perform necessary geotechnical analyses and provide geotechnical recommendations for grading, slope stabilization, erosion mitigation, surface and ground water drainage control, design and construction for building foundations and stormwater detention vault, etc., based on subsurface conditions encountered in the test pits and results of our geotechnical analyses. 4. Prepare a written report to present our findings, conclusions, and recommendations for the proposed development of the subject plat site. SURFACE CONDITIONS SITE CONDITIONS LIU & ASSOCIATES, INC. February 20, 2004 Wilson Plat L&A Job No. 9A006 Page 3 The subject site is situated on a broad, moderate to steep, westerly -declining hillside. It is backed into undeveloped wooded lands to the north and east, and adjoined by a single-family residence and undeveloped land to the south and a subdivision to the west. The site generally slopes down westerly at grades from 13 to 3.9 percent. The higher, grWnt portions of the site generally lie within the eastern 100 to 120 feet and the western 150 to 200 feet of the site, while the more moderate area lies in about the middle third of the site where the building pad and the yard of an existing residence are located. The existing residence is accessed from South 55` h Street via a long gravel driveway. The unpaved area around the existing house is covered with lawn grass. The 1. g6er:gradient eastern and western areas of the site is dotted with tall, mature evergreen trees with few mature deciduous trees mixed in between. The eastern steeper area is covered with thick brush, while the western steeper area is covered by sporadic brush and dense ivy. GEOLOGIC SETTING The Geologic Map of the Lake Stevens Quadrangle, Snohomish County, Washington, by James P. Minard, published by U. S. Geological Survey in 1985, was referenced for the geologic and soil conditions of the subject site. According to this publication, the surficial soil units at and in the vicinity of the site are mapped as Kame Terrace (Qit) deposits underlain by Ground Moraine Qgt)• The geology of the Puget Sound Lowland has been modified by the advance and retreat of several glaciers in the past and subsequent deposits and erosion. The latest glacier advanced to the Puget Sound Lowland is referred to as the Vashon Stade of the Fraser Glaciation, which has occurred during the later stages of the Pleistocene Epoch and retreated from the region some 14,500 years ago. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 4 The Kame terrace deposits were laid down by ice -marginal streams flowing between higher ground on one side and an ice margin on the other side during the last glaciation. They consist mostly of silty sand and gravel to cobble. The Kame terrace deposits also contain lenses and pods of till and beds of sand, silt and clay locally. The Kame terrace deposits are of moderately - high to high permeability and can provide good foundation support to structures in their native undisturbed state. The ground moraine soil unit, underlying the Kame deposits, is composed of a thin layer of ablation till over lodgmont till sediments, deposited by Puget glacial lobe of the Vashon Stade of the Fraser Glaciation. The lodgmont till is generally a compact mixture of unsorted clay, silt, sand, gravel and cobble, commonly referred to as "hard pan". The ablation till is similar to lodgmont till, but is much less compact and coherent. The thickness generally varies from 2 to 4 feet for the ablation till deposit, and 5 to 30 feet for the lodgmont till deposit. The lodgmont till deposit is practically impervious, except local lenses of sand and gravel. it has a compressive strength comparable to that of low-grade concrete and can stand in steep natural or cut slopes for a long period. The lodgmont till can provide excellent foundation support with little settlement expected to structures. The overlying ablation till is generally in a loose to medium -dense state, and is more compressible and permeable. SOIL CONDITIONS Subsurface conditions of the site were explored with seven test pits excavated within the site. These test pits were excavated on February 4, 2009, with a tract -mounted backhoe to depths from 6.5 to 9.5 feet. The approximate locations of the test pits are shown on Plate 2 - Site and Exploration Location Plan. The test pits were located with either a tape measure or by visual reference to existing topographic features in the field and on the topographic survey map, and their locations should be considered only accurate to the measuring method used. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 5 A geotechnical engineer from our office was present during subsurface exploration, who examined the soil and geologic conditions encountered and completed logs of test pits. Soil samples obtained from each soil unit in the test pits were visually classified in general accordance with United Soil Classification System, a copy of which is presented on Plate 3. Detailed descriptions of soils encountered during site exploration are presented in test pit logs on Plates 4 through 7. The test pits encountered a layer of loose, organic topsoil, from about 10 to 24 inches thick, mantling the site. In Test Pits 1 and 2, located in the eastern high ground area, the layer of topsoil was found underlain by a layer of ablation till of brown, medium -dense, silty, fine to medium sand with a trace of gravel, from 2.3 to 3.5 feet thick. This ablation till is underlain to the depths explored by a lodgmont till deposit of brown to brown -gray, dense, silty fine sand, with some gravel and occasional cobble. Test Pits 3 and 4, located at the downhill fringe of the moderately -sloped middle section of the site, encountered a Kame terrace deposit underlying the topsoil. The top 1.7 to 2.0 feet of this Kame terrace deposit is weathered to a loose to medium -dense state and is composed of brown, fine to medium sand, locally with some silt and/or gravel. The fresh Kane terrace deposit underneath is medium -dense to dense, and is composed of brown -gray to light -gray, fine to medium sand with a trace of gravel. This clean sandy deposit should be of high permeability. Test Pits 5, 6 and 7. Located near at the west end of the site, encountered a weathered and/or fresh Kame terrace deposit, up to about 9.1 feet thick, underlying the topsoil. The Kame terrace deposits were found underlain by a lodgmont till deposit in Test Pits 5 and 7 at a depth of 5.0 and 9.0 feet, respectively. GROUNDWATER CONDITION LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page b Groundwater seepage was not encountered by any of the seven test pits excavated on the subject plat site. The topsoil, ablation till, and weathered and fresh Kame terrace deposits are loose to medium -dense, and would allow some storm runoff to seep into the ground. The underlying dense to weakly -cemented lodgmont till deposit is of extremely low permeability and would perch stormwater infiltrating into the more permeable surficial soils. The amount of and the depth to this 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. The test pits excavated in the heart of winter did not encountered any groundwater. Therefore, it is our opinion that any groundwater encountered under the subject plat site during construction should be minimal and minor even in the winter months of the year. DISCUSSIONS AND RECOMMENDATIONS GENERAL Based on the soil conditions encountered by the test pits excavated on the site, it is our opinion that the site is suitable for the proposed development from the geotechnical engineering viewpoint, provided that the recommendations in this report are fully implemented and observed during construction. Due to moisture -sensitive fine-grained soils mantling the site and the local higher gradient areas within the site, we recommend that grading and foundation construction work for the proposed development be carried out and completed during the dryer period from April 1 through October 31. If grading work has to proceed beyond the above dryer period, the measures for slope stabilization, erosion mitigation, and surface and ground water drainage control recommended in this report should be in place and operational on a daily basis during construction. Surficial unsuitable soils, including topsoil and loose to medium -dense weathered soils mantling the site, should be stripped down to the medium -dense to dense, fresh Kame terrace and/or lodgmont tall soils within the building pads of the lots and the roadways. The underlying fresh LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 7 Kame terrace and/or lodgmont till soils are of high to moderately -high to high strength and are capable of providing adequate foundation support to the proposed residential buildings, roadways and stormwater detention vault. Conventional footing foundations constructed on or into the above competent basal soils may be used to support the proposed residential building and stormwater detention vault. Structural fill, if required for site grading, should be placed on proof -rolled, underlying undisturbed, competent basal soils following the stripping of the surficial unsuitable soils. GEOLOGIC HAZARDS AND REMEDIATIN Landslide Hazard The medium -dense to dense, fresh Kame terrace and lodgmont till soils underlying the site at shallow depth are of moderately -high to high shear strength and have good to excellent resistance against slope failures. Therefore, it is our opinion that the potential for deep-seated landslides to occur on the site should be minimal, provided the recommendations in this report are fully F implemented and complied with during construction. Erosion Hazard The topsoil and loose to medium -dense weathered soils are of low resistance against erosion. Erosion may occur in the weaker surficial soils over the higher gradient areas of the site if they are devoid of vegetation cover and overly saturated. Progressive erosion can lead to shallow, skin -type mudflows in the highex, gradient areas of the site. To mitigate such erosion hazard, vegetation outside of construction limits should be preserved and maintained. Unpaved exposed ground within the site resulted from construction activities should be re -seeded and re -vegetated as soon as possible. Concentrated stormwater should not be discharged uncontrolled onto the ground within the site. Stormwater over impervious surfaces, such as roofs and paved roadways and driveways, should be captured by underground drain line systems connected to roof downspouts or by catch basins installed in paved roadways and driveways. Water collected by these drain line systems should be tightlined to discharge into a storm sewer or a suitable LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 8 stormwater disposal facility. Areas devoid of vegetation cover should be re -seeded and re - vegetated as soon as possible, and should be covered with clear plastic sheets until the vegetation is fully established. Seismic Hazard and Design. Consideration The Puget Sound region is in an active seismic zone. The subject site is underlain at shallow depth by medium -dense to dense, fresh Kame terrace and lodgmont till soils of moderately -high to high shear strength. There is a lack of continuous, extensive, static groundwater table at shallow depth under the site. The combination of the above makes it rather unlikely for seismic hazards, such as landslides, liquefaction or soil lateral spreading, to occur on the site during strong earthquakes. Therefore, the seismic hazard of the site should be minimal. The residential buildings to be constructed on the site, however, should be designed to withstand seismic forces induced by strong earthquakes. Based on the soil conditions encountered by the test pits, it is our opinion that Seismic Use Group I and Site Class D should be used in the seismic design of the proposed residential buildings in accordance with the 2006 International Building Code (IBC). SITE PREPARATION AND GENERAL GRADING Vegetation within construction limits should be cleared and grubbed. Loose surficial soils, including topsoil and loose weathered soils, should be completely stripped down to the medium - dense to dense, fresh Kame terrace and/or lodgmont till soils within the building pads of the proposed residential buildings and roadways. The exposed soils should be compacted to a non - yielding state with a vibratory compactor and proof -rolled with a piece of heavy carthwork equipment. TEMPORARY EROSION CONTROL The onsite surficial soils contain a high percentage of fines, and are sensitive to moisture and can be disturbed easily by construction traffic when saturated. A layer of clean, 2 -to -4 -inch, quarry LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 9 spalls should be placed over excavated areas and areas of frequent traffic, such as the entrance to the site, as required, to protect the subgrade soils from disturbance by construction traffic. Silt fences should be installed along the downhill sides of construction areas to prevent sediment from being transported onto adjoining properties or streets by storm runoff. The bottom edge of silt fences should be folded inward and ballasted with onsite soils. Ditches or interceptor trench drains should be installed on the uphill sides of construction areas, as required, to intercept and drain away storm runoff and near -surface groundwater seepage. Water captured by such ditches or interceptor trench drains should be discharged into onsite detention/settling ponds or nearby storm inlets. The storm inlets, if into which stormwater is to be to be discharged, should be covered with a filter sock to prevent sediment from entering the storm sewer system. The filter socks should be cleaned frequently during construction to prevent clogging, and should be removed after completion of construction. EXCAVATION AND FILL SLOPES Under no circumstance should excavation slopes be steeper than the limits specified by local, state and federal safety regulations if workers have to perform construction work in excavated areas. Unsupported temporary cuts greater than 4 feet in height should be no steeper than 1 H:1 V in topsoil, loose to medium -dense weathered Kame terrace and ablation till soils, no steeper than 3/4H:1 V in medium -dense to dense Kame terrace deposits and may be vertical in lodgment till soils if the overall depth of excavation is no more than 15 feet. Otherwise, temporary cut in lodgment till should be no steeper than 1/2H:1V. Unsupported permanent cuts should be no steeper than 2H:1 V in topsoil, loose to medium -dense weathered Kame terrace and ablation till soils, no steeper than 1-3/4H:1 V in medium -dense to dense Kame terrace deposits, and no steeper than 1-1/2H:1V in lodgment till soils. A 5 -foot -wide level bench should be built into temporary or permanent cut slopes to keep the vertical rise between the benches no more than 15 feet. The soil units into which cut slopes and LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 10 the stability of the cut slopes are to be made should be verified by a geotechnical engineer during excavation. Permanent fill embankments required to support structural or traffic load should be constructed with compacted structural fill placed over undisturbed, proof -rolled, firm, native, fresh Kame terrace and/or lodgment till soils after the surficial unsuitable soils are completely stripped. Permanent fill to be placed over slopes steeper than 15 percent grade should be retained structurally. The exposed ground exceeding 15 percent grade should be benched with vertical steps not exceeding 4 feet tall after stripping of surficial unsuitable soils and prior to placing structural fill. The slope of permanent fill embankments should be no steeper than 2H:1 V. Upon completion, the sloping face of permanent fill embankments should be thoroughly compacted to a non -yielding state with a hoe -pack. The above recommended cut and fill slopes are under the assumption that groundwater seepage would not be encountered during construction. if groundwater is encountered, the cut and fill earthwork should be immediately halted and the slope stability re-evaluated. The slopes may have to be flattened and other measures taken to stabilize the slopes. Stormwater should not allowed to flow uncontrolled over cut and fill slopes. Permanent cut slopes or fill embankments should be seeded and vegetated as soon as possible for erosion protection and long-term stability, and should be covered with clear plastic sheets, as required, to protect them from erosion until the vegetation is fully established. STRUCTURAL FILL Structural fill is the fill that supports 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. The optimum moisture content is the water LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page I I content in the soils that enable the soils to be compacted to the highest dry density for a given compaction effort. Onsite clean silty sand to gravelly sand soils, meeting the above requirements, may be used as structural fill. Imported material to be used as structural fill should be clean, free -draining, granular soils containing no more than 5 percent by weight finer than the No. 200 sieve based on the fraction of the material passing No. 4 sieve, and should have individual particles not larger than three inches. The ground over which structural fill is to be placed should be prepared in accordance with recommendations in the SITE PREPARATION AND GENERAL GRADING and EXCAVATION AND FILL SLOPES sections of this report. Structural fill should be placed in lifts no more than 10 inches thick in its loose state, with each lift compacted to a minimum percentage of the maximum dry density determined by ASTM D 1557 (Modified Proctor Method) as follows: Application Within building pads and under foundations Roadway/driveway subgrade Retaining/foundation wall backfill Utility trench backfill of Maximum Dry Density 95% 95% for top 3 feet and 90% below 92% 95% for top 4 feet and 90% below STORMWATER DETENTION VAULT An underground concrete detention vault, designed to. supplement infiltration systems, is to be constructed at the northwest corner of the site to store stormwater collected over impervious surfaces of the proposed development of the site. Two test pits (Test Pits 5 and b) were excavated within the footprint of the proposed vault, and these test pits encountered dense LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 12 lodgment till soils and/or medium -dense to dense fresh Kame terrace deposit at depths about 4.0 to 5.0 feet. The vault may be supported on footings founded on these competent basal soils. An allowable soil bearing pressure not exceeding 3,500 psf may be used for the design of the vault footing foundations. A drain line consisting of perforated, rigid PVC, drain pipe or slotted, corrugated ADS, drain pipe, at least 6 inches in diameter, should be installed at a few inches below bottom of the perimeter footings of the vault walls to intercept and drain away groundwater which may flow towards the vault. The drain line should have sufficient slope (0.5% minimum) to generate flow by gravity, and water collected in the drain line should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. The vault footing drain line should be completely embedded in washed gravel wrapped in a layer of non -woven filter fabric, such as 140N by Mirafi Inc. or approved equal. A vertical drainage blanket at least 12 inches thick horizontally, consisting of clean 314 to 1 -1/2 -inch washed gravel or crushed rock, should be placed against the perimeter vault walls. The remaining backfill should be constructed of structural fill. Alternatively, a vertical drain mat, such as Miradrain 6000 by Mirafi Inc. or equivalent, may be placed against the perimeter vault walls as the vertical drainage blanket. The vertical drainage blanket or drain mat should be hydraulically connected to the drain line at the base of the vault perimeter walls. Sufficient number of cleanouts at strategic locations should be installed for periodical cleaning of the vault wall drain line to prevent clogging. The perimeter walls of the detention vault would also serve as retaining walls to support cut banks and backfill. The perimeter walls of the vault capped with a lid would be restrained at their top from horizontal movement and should be designed for at -rest lateral soil pressure. For the condition that groundwater behind the perimeter vault walls can be fully drained by the drain line provided at the base of the walls, we recominend an at -rest soil pressure of 50 pcf equivalent fluid density (EFD) be used for the design of vault perimeter walls. To counter the at -rest soil pressure, a passive lateral soil pressure of 375 pcf EFD may be used, except that the passive LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 13 pressure within the top 12 inches of the finish subgrade should be ignored. The above passive pressure assumes the backslope of the walls is level or ascending away from the walls. The at - rest soil pressure may also be resisted by the friction force between the footings and the subgrade soils based on a coefficient of friction of 0.55. If the site grades are such that it is not feasible to completely drain groundwater behind the vault walls with a gravity drain line system, the hydrostatic pressure on the perimeter vault walls should also be taken into consideration for the design of the vault perimeter walls. For the condition that a perimeter drain line has to be placed higher than the footing level, the perimeter vault walls should be designed for a lateral soils pressure of 50 pcf EFD above the drain line level and a combined lateral soil and hydrostatic pressure of 80 pcf EFD below the drain line level. The above lateral pressures on the walls may be countered by a passive soil pressure of 375 pcf EFD above the drain line and 210 pcf EFD below. The detention vault should also be designed for seismic loading based on a 100 -year seismic event. For seismic design of the detention vault walls, a pseudo static soil pressure diagram of inverted triangle from the finished ground level to the bottom of the foundations should be used. Based on the soil conditions in the detention vault area, we recommend the lateral soil pressure at the top of the triangle be 8H psf for a 100 -year seismic event, where H is the height from finish grade over top of the vault to bottom of footings in feet. A one-third increase in the above recommended allowable soil bearing pressure may be used when considering the seismic loading condition. The above design parameters are unfactored ultimate values. Proper factors of safety should be applied for the design of the vault walls against sliding and overturning failures. PAVED ROADWAYS AND DRIVEWAYS LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 14 Performance of roadway and driveway pavement is critically related to the conditions of the underlying subgrade soils. We recommend that the subgrade soils under the roadways/driveways be treated and prepared as described in the SITE PREPARATION AND GENERAL EARTHWORK section of this report. Prior to placing base material, the subgrade soils should be compacted to a non -yielding state with a vibratory roller compactor and proof. -rolled with a piece of heavy construction equipment, such as a fully -loaded dump truck. Any areas with excessive flexing or pumping should be over -excavated and re -compacted or replaced with structural fill or crushed rock placed and compacted in accordance with the recommendations provided in the STRUCTURAL FILL section of this report. We recommend that a layer of compacted, 7/8 -inch crushed rock base (CRB), be placed for the roadways/driveways. This crushed rock base should be at least 6 inches for the public roadways and 4 inches for the private driveways. The crushed rock base should be overlain with a 3 -inch asphalt treated base (ATB) topped by a 2 -inch -thick Class B asphalt concrete (AC) surficial course for public roadways and overlain by a 3 -inch -thick Class B asphalt concrete (AC) surficial course for the private driveways. BUILDING SETBACK The purpose of building setback from the top or toe or an overly steep portion of a slope is to establish a safe buffer such that if a slope failure should occur the stability of the structure can be maintained and damages to the structure minimized. In general, the greater the setback, the lower the risk for the structure to sustain damages from a slope failure. To maintain stability, the residential buildings to be constructed on the site should be sufficiently setback from the top or toe of slopes of 40% gradient or more. We recommend the buildings be set back at least 15 feet from top or toe of slopes with grades 40% or more. If footing foundations are used to support the new residences of future development, the footing foundations should be embedded at least one foot into the underlying, medium -dense to dense, LILT & ASSOCIATES, INC. l,ebruary 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 15 fresh Kame terrace or lodgment till soils. If the buildings are to be constructed on the slopes with grades 40% or more, the footing foundations should be extended downward to such elevation that the horizontal distance from the edge of footings to the face of slopes should be at least 15 feet, and that a plane drawn from the edge of footings to the toe of slopes of 40% or more gradient should be no steeper than 2.5H: IV. BUILDING FOUNDATIONS Conventional footing foundations may be used to support the residential buildings to be constructed on the site. The footing foundations should be placed on or into the underlying, medium -dense to dense, fresh Kame terrace or lodgmont soils, or on structural fill placed over these undisturbed competent basal soils. Water should not be allowed to accumulate in excavated footing trenches. Disturbed soils in footing trenches should be completely removed down to undisturbed, competent basal soils and the basal soils should be thoroughly compacted to a non -yielding state with a vibratory mechanical compactor prior to pouring concrete for the footings. If the above recommendations are followed, our recommended design criteria for footing foundations are as follows: The allowable soil bearing pressure for design of footing foundations, including dead and live loads, should be no greater than 3,000 psf if constructed on or into native, undisturbed, competent basal soils, and no greater than 2,500 psf if constructed on structural fill placed over competent basal soils. The footing bearing soils should be verified by a geotechnical engineer after the footing trenches are excavated and before the footings poured. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L.&A Job No. 9AO06 Page 16 The minimum depth to bottom of perimeter footings below adjacent final exterior grade should be no less than 18 inches. The minimum depth to bottom of the interior footings below top of floor slab should be no less than 12 inches. The minimum width should be no less than 16 inches for continuous footings, and no less than 24 inches for individual footings, except those footings supporting light -weight decks or porches. A one-third increase in the above recommended allowable soil bearing pressure may be used when considering short-term, transitory, wind or seismic loads. For footing foundations designed and constructed per recommendations above, we estimate that the maximum total post - construction settlement of the buildings should be 314 inch or less and the differential settlement across building width should be 112 inch or less. Lateral loads on the proposed residential buildings may be resisted by the friction force between the foundations and the subgrade soils or the passive earth pressure acting on the below -grade portion of the foundations. For the latter, the foundations must be poured "neat" against undisturbed soils or backfilled with a clean, free -draining, compacted structural fill. We recommend that an equivalent fluid density (EFD) of 325 pef (pounds per cubic foot) for the passive earth pressure be used for lateral resistance. The above passive pressure assumes that the backfill is level or inclines upward away from the foundations for a horizontal distance at least 1.5 times the depth of the foundations below the final grade. A coefficient of friction of 0.55 between the foundations and the subgrade soils may be used. The above soil parameters are unfactored values, and a proper factor of safety should be used in calculating the resisting forces against lateral loads on the new garage. SLAB -ON -GRADE FLOORS Slab -on -grade floors, if used for the residential buildings to be constructed on the site, should be placed on firm subgrade soils prepared as outlined in the SITE PREPARATION AND LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 17 GENERAL EARTHWORK and the STRUCTURAL FILL sections of this report. Where moisture control is critical, the slab -on -grade floors should be placed on a capillary break which is in turn placed on the compacted subgrade. The capillary break should consist of a minimum four -inch -thick layer of clean, free -draining, 718 -inch crushed rock, containing no more than 5 percent by weight passing the No. 4 sieve. A vapor barrier, such as a 6 -mil plastic membrane, may be placed over the capillary break, as required, to keep moisture from migrating upwards. BASEMENT AND CIP CONCRETE RETAINING WALLS Building basement walls would be required to support backfill. Cast -in-place concrete walls may be used to retain fill embankments along the west and part of the south boundaries of the site. Basement walls restrained horizontally at the top are considered unyielding and should be designed for a lateral soil pressure under the at -rest condition; while cast -in-place concrete retaining walls free to move at the top should be designed for active lateral soil pressure. We recommend that a lateral soil pressure of 55 pcf EFD be used for the design of basement walls restrained at the top; and 40 pcf EFD for retaining walls unrestrained at the top. These lateral soils pressures are applicable to walls with level backslope. With a backslope rising away from the walls, an additional pressure of 0.75 pcf per degree of angle of the backslope above horizontal should be added to the above pressures. To counter the active soil or at -rest pressure, a passive lateral soil pressure of 300 pcf EFD may be used, except that the passive pressure within the top 12 inches of the finish subgrade should be ignored. The above passive pressure is applicable to walls with level backslope. The above lateral soil pressures are under the assumption that groundwater behind the walls is fully drained. To resist against sliding, the friction force between the footings and the subgrade soils may be calculated based on a coefficient of friction of 0.55. The above soil parameters are ultimate values, and proper factors of safety should be used in the design of the basement and retaining walls against sliding and overturning failures. Basement walls or retaining walls may be supported on footing foundations seated on or into the underlying, medium -dense to dense, fresh Kame terrace or lodgment till soils, with an allowable soil bearing pressure not to exceed 3,000 psf. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 18 A drain line consisting of perforated, rigid PVC, drain pipe or slotted, corrugated ADS, drain pipe, at least 4 inches in diameter, should be installed at a few inches below bottom of basement or retaining walls to intercept and drain away groundwater flowing towards the walls. The drain lines should have sufficient slope (0.5 percent minimum) to generate flow by gravity, and water collected in the drain line should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. The drain lines should be completely embedded in washed gravel wrapped in a layer of non -woven filter fabric, such as 140N by Mirafi Inc. or approved equal. A vertical drainage blanket at least 12 inches thick, consisting of clean, 314 to 1 -112 -inch, washed gravel or crushed rock, should be placed against the walls. Alternatively, a vertical drain mat, such as Miradrain 6000 by Mirafi Inc. or equivalent, may be placed against the walls as the vertical drainage blanket. The vertical drainage blanket or drain mat should be hydraulically connected to the drain lines at the base of the perimeter walls. ROCKERY WALLS General Rockery walls may be used to line the cut banks along the east boundary of the site. Rockery by nature is not an engineered retaining wall, such as a reinforced concrete wall. Although a rockery wall can provide some degree of retention capability, its main function is to serve as a protective facing to retard weathering and erosion process to the earth bank behind the rockery. To achieve a satisfactory rockery wall, the earth bank behind the wall must be stable by itself on a long-term basis. In addition, the rockery wall should be constructed in a proper way to assure long-term stability of the cut bank or fill embankment. The medium -dense to dense, fresh Kame terrace and/or lodgment till deposit underlying the site at shallow depth is of moderately -high to high shear strength. It is our opinion that cut banks in these soil deposits will be able to maintain long-term stability if lined by properly constructed rockery. Our design of rockery walls lining cut banks in these competent deposits is shown on Plate 8 attached hereto. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 19 Rockery Material and Construction Rock material used for rockery construction should be hard, well -cemented, sound, durable and free of cracks, fissures, joints, air holes and other defects. Construction of rockery walls should be in compliance with the Standard Rockery Construction Guidelines published by the Association of the Rockery Contractors. Subgrade and Keyway Trench Preparation Construction of the rockery walls should start immediately following completion of cut banks and keyway trenches. The keyway trenches should be cut into medium -dense to dense, fresh Kame terrace and/or lodgment till soils capable of rendering an allowable bearing capacity of at least 3,000 psf. The keyway trenches should be at least 12 inches deep below the finish grade in front of the rockery wall and should be wide enough such that the heel of the keyway trenches would be at least 12 inches from the back of the base -course facial stones. Keyway trenches should be free of loose disturbed soils or standing water and the exposed soils at bottom of keyway trenches should be compacted to a non -yielding state with a vibratory mechanical compactor prior to rockery installation. Groundwater Drainage Control A drain line should be placed in the keyway trenches behind the base -course facial stones to collect and drain away groundwater flowing toward the rockery walls. The drain lines should consist of minimum 6 -inch, perforated, rigid, PVC pipes, wrapped in a filter fabric sock. The bottom of the keyway trench and the drain line should have sufficient slope (0.5 percent minimum) to generate flow by gravity. Water collected in the drain lines should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. Facial Stones Facial stones of the rockery walls should be as nearly rectangular as possible with the long dimension of the stones placed perpendicular to the wall alignment. Facial stones should be LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 20 stacked tightly against one another to minimize voids between the stones. Excessive openings between the facial stones should be chinked with smaller rock from behind. The rockery walls should be constructed such that the facial stones of each successive course would be staggered over and firmly supported on stones of the previous course. Facial stones of the rockery walls should be tilted back at an inclination no steeper than 6V:1H. Drain Rock Course A drain rock course, consisting of 2 -to -4 -inch rock spalls, should be installed between the facial stones and the cut bank in lifts as each course of the facial stones is completed. The horizontal thickness of the drain rock course should be at least 12 inches. The purpose of the drain rock course is to retain soils in place while allowing groundwater to bleed out. PRECAST CONCRETE BLOCK WALLS General Precast concrete block walls may be used to support fill embankments to be constructed along the west and part of the south boundaries of the site. These walls may be constructed with commercially available precast concrete blocks, such as by Keystone, Lock Block or Redi Rock. Loose topsoil and loose to medium -dense weathered soils within the keyway trenches of the walls should be over -excavated down to medium -dense to dense, fresh Kame terrace and/or lodgment till soils. The block walls should be constructed with geogrid mesh reinforcement anchored to the block walls and embedded in the wall backfill. The wall should be founded on a minimum 4 -inch layer, 718 -inch crushed rock, leveling base, placed over firm undisturbed bearing soils capable of rendering an allowable bearing capacity of at least 3,000 psf. A vertical drainage blanket should be placed against the back of the concrete block facing and hydraulically connected to the drain lines at the base of the walls. The remaining wall backfill behind the vertical drainage blanket should consist of compacted structural fill consisting of clean granular soils. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 21 Design Soil Parameters We recommend that the precast concrete block walls to be constructed to support fill embankments be designed for a fully drained condition in accordance with the following soil perimeters: The block walls should be designed for a minimum factor of 1.5 against sliding failure and 1.7 against overturning failure under the static loading condition. The Puget Sound region is in an active seismic zone and the block walls should also be designed under the seismic loading condition for a 100 -year seismic event. The peak ground acceleration is about 0.3g (g = gravity force) for such an event in the Puget Sound region. The block walls, however, are built with interlocking concrete blocks with relatively high flexibility, and the blocks do not move in unison during earthquakes. Therefore, for design of the block wall under the seismic loading condition, the ground acceleration may be reduced to 0.29. The block walls should designed for a factor of safety of at least 1.15 against sliding and overturning failures under the seismic loading condition. Heavy equipment or material storage should not be allowed within 10 feet of the block walls; otherwise, the walls should be designed for 250 psf of uniform load. Construction of Precast Concrete Black Walls Vegetation within construction limits of the block walls and its backfill should be cleared and roots thoroughly grubbed. Unsuitable surficial soils, such as topsoil and loose to medium -dense weathered sand soils within the block walls and backfill footprint should be stripped down to the LIU & ASSOCIATES, INC. Reinforced Retained Foundation Leveling Soils Soils Soils Rock Base Unit Weight, r, pcf 130 120 135 135 Angle of Internal Friction, 36 33 36 40 0, degrees Cohesion, c, psf 0 0 0 0 The block walls should be designed for a minimum factor of 1.5 against sliding failure and 1.7 against overturning failure under the static loading condition. The Puget Sound region is in an active seismic zone and the block walls should also be designed under the seismic loading condition for a 100 -year seismic event. The peak ground acceleration is about 0.3g (g = gravity force) for such an event in the Puget Sound region. The block walls, however, are built with interlocking concrete blocks with relatively high flexibility, and the blocks do not move in unison during earthquakes. Therefore, for design of the block wall under the seismic loading condition, the ground acceleration may be reduced to 0.29. The block walls should designed for a factor of safety of at least 1.15 against sliding and overturning failures under the seismic loading condition. Heavy equipment or material storage should not be allowed within 10 feet of the block walls; otherwise, the walls should be designed for 250 psf of uniform load. Construction of Precast Concrete Black Walls Vegetation within construction limits of the block walls and its backfill should be cleared and roots thoroughly grubbed. Unsuitable surficial soils, such as topsoil and loose to medium -dense weathered sand soils within the block walls and backfill footprint should be stripped down to the LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 22 medium -dense to dense, fresh Kame terrace and/or lodgment till soils. Over -excavation down to these competent nasal soils should be backfilled with structural fill. The keyway trench for the block walls should be cut into native, undisturbed, medium -dense to dense, fresh Kame terrace and/or lodgment till 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 compactor. A minimum 4 -inch layer of 7/8 - inch -minus crushed rock leveling base, compacted to a non -yielding state, should be placed over firm subgrade soils supporting the block walls. The base -course blocks are to be placed on this crushed rock base with an embedment at least 10 inches below the adjacent finish grade in front of the block wall. The precast concrete blocks should be stacked tightly against one another. A minimum 6 -inch perforated, rigid, PVC drain line fitted in a non -woven filter fabric sock should be laid in the keyway trenches behind the base -course blocks. The bottom of keyway trenches and the drain line should have sufficient slope (0.5 percent minimum) to generate flow by gravity. The drain pipes should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. A minimum 10 -inch -thick (horizontally) vertical drainage blanket, constructed of clean % to 1 -112 -inch washed gravel crushed rock, should be placed against the back of the block wall facing. The vertical drainage blankets should be hydraulically connected to the drain lines at the base behind the block walls. The wall backfill behind the vertical drainage blankets should consist of structural fill. The vertical drainage blanket and structural fill should be constructed in lifts after each course of blocks is completed. Each layer of geogrid mesh should be laid on level backfill surface, with one end securely anchored between two rows of concrete blocks, stretched tight, and the other end staked down prior to the placement of the next lift of wail backfill. Overlaps of geogrid mesh in the direction of the wall alignments should be at least 12 inches, overlaps in the direction perpendicular to the wall alignments should not be allowed. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 23 DRAINAGE CONTROL Onsite Stormwater Disposal The moderately -sloped mid-section and the low-lying western portion of the subject plat site are underlain by fresh Kame terrace deposit of clean, gravelly, fine to medium sand deposit and infiltration of stormwater into this soil stratum is feasible for the residential buildings to be constructed in these areas. The impervious lodgmont till deposit, however, was found underlying Tract A", located at the northwest corner of the site, at a depth as less as 5 feet below the existing grade. Therefore, we do not recommend a concentrated infiltration facility in "Tract A" to dispose a large amount of stormwater into the ground. This is because the large amount of concentrated disposed stormwater in this tract may increase the potential of groundwater seepage downhill off the subject site where the lodgmont till may be exposed or near the ground surface. Increased groundwater seepage may elevate the possibility of erosion, sloughing and landlside. Appendix C Small Project Drainage Requirements, of the King County, Washington Surface Water Design Manual, January 24, 2005, allows stormwater collected on 2,000 or more but not more than 10,000 square feet of impervious surfaces of small residential projects to be disposed onsite with infiltration or dispersion method. The "full infiltration" method specified in Section C.2.2 — Full Infiltration of the above design manual can be used on sites underlain by medium sand or coarse sand or gravel and cobble. The moderately -sloped mid-section and the low-lying western portion of the subject plat site are underlain by fresh Kame terrace deposit of clean gravelly, fine to medium sand deposit and are qualified for using Full Infiltration method to dispose stormwater for the residential buildings in these areas. According to the above design manual, infiltration trenches installed into clean gravelly sand soils should be designed to be at least 30 feet long for every 1000 s.f, of impervious surface served (with a 2 -foot -wide trench). The length of infiltration trenches may be shortened proportionally with increased trench width. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 24 The infiltration trenches should be set back at least 10 feet from adjacent buildings and 5 feet from property lines. The schematic presentation of an infiltration trench with a single dispersion pipe to serve the residential buildings is shown on Plate 9. The infiltration trenches should be at least 24 inches wide. The bottom of infiltration trenches should be excavated at least 6 inches into the underlying clean, fresh, gravelly sand Kame terrace deposit. The side walls of the trenches should be lined with a layer of non -woven filter fabric. The trench is then filled with clean 3/4 to 1 -112 -inch washed gravel or crushed rock to within about 8 inches of the finish grade. The dispersion pipes should be constructed of 4 -inch rigid or 6 -inch flexible perforated PVC pipes and laid level in the gravel or crushed rock filled trenches at about 16 inches below the top of trenches. The top of the gravel or crushed rock fill should also be covered with the filter fabric liner. The remaining trench should then be filled with compacted on-site soils. Stormwater captured over impervious surfaces should be routed into a sediment control structure/oil-water separator structure before being released into the infiltration trenches. Building Footprint Excavation Building footprint excavation for the proposed residential buildings, if encountering groundwater seepage, should have the bottom of excavation sloped and ditches excavated along bases of the cut banks to direct collected groundwater into sump pits from which water can be pumped out. A layer of 2 -inch crushed rock should be placed over footing bearing subgrade soils, as required, to protect the soils from disturbance by construction traffic. This crushed rock base should be built to a few inches above groundwater level, but not less than 6 inches thick. The crush rock base should be compacted in 12 -inch lifts to a non -yielding state with a vibratory mechanical compactor. Runoff over Impervious Surfaces Storm runoff over impervious surfaces, such as roofs and paved roadways/driveways, should be collected by underground drain line systems connected to downspouts and by catch basins LIU & ASSOCIATES, INC. I , 1 February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 25 installed in paved roadways/driveways. Stormwater thus collected should be tightlined to discharge into a storm sewer or suitable stormwater disposal facilities. Building Footing Drains A subdrain should be installed, around the perimeter footings of each of the residential buildings. The subdrains should consist of a 4 -inch -minimum -diameter, perforated, rigid, drain pipe, laid a few inches below bottom of the perimeter footings of the buildings. The trenches and the drain lines should have a sufficient gradient (0.5% minimum) to generate flow by gravity. The drain lines should be wrapped in a non -woven filter fabric sock and completely enclosed in clean washed gravel. The remaining trenches may be backfilled with clean onsite soils. Water collected by the perimeter footing subdrain systems should be tightlined, separately from the roof and surface stormwater drain lines, to discharge into a storm sewer or suitable stormwater disposal facility. Surface Drainage Water should not be allowed to stand in any areas where footings, on -grade slabs, or pavement is to be constructed. Finish ground surface should be graded to direct surface runoff away from the residential buildings. We recommend the finish ground be sloped at a gradient of 3 percent minimum for a distance of at least 10 feet away from the buildings, except in the areas to be paved. Cleanouts Sufficient number of cleanouts at strategic locations should be provided for underground drain lines. The underground drain lines should be cleaned and maintained periodically to prevent clogging. LIU & ASSOCIATES, INC. I # February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 26 RISK EVALUATION STATEMENT The subject site is underlain at shallow depth by medium -dense to dense Kame terraec and/or lodgmont till soils. These soils are of moderately -high to high shear strength and groundwater was not eiucvunte ed by thetest pits,excavaiecl on the side 2herefaze the ,site should be quite stable. The key to maintain site stability during and after completion of construction is to have proper and adequate erosion and drainage controls. It is our judgment that provided the recommendations in this report are fully implemented and observed during construction, the areas disturbed by construction will be stabilized and will not increase the potential for soil movement. In our opinion, the risk of damage to the proposed development and from the development to adjacent properties due to soil instability should be minimal. LIMITATIONS This report has been prepared for the specific application to this project for the exclusive use by Mr. Robed Wilson `ad ;his associates, consultants and contractors. We recommend that this report, in its entirety, be included in the project contract documents for the information of the prospective contractors for their estimating and bidding purposes and for compliance with the recommendations in this report during construction. The conclusions and interpretations in this report, however, should not be construed as a warranty of the subsurface conditions. The scope of this study does not include services related to construction safety precautions and our recommendations are not intended to direct the contractor's methods, techniques, sequences or procedures, except as specifically described in this report for design considerations. Our recommendations and conclusions are based on the geologic and soil conditions encountered in the test pits, and our experience and engineering judgment. The conclusions and recommendations are professional opinions derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area. No warranty, expressed or implied, is made. LIU & ASSOCIATES, INC. t February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 27 The actual subsurface conditions of the site may vary from those encountered by the test pits excavated on the site. The nature and extent of such variations may not become evident until construction starts. If variations appear then, we should be retained to re-evaluate the recommendations of this report, and to verify or modify thern in writing prior to proceeding further with the construction of the proposed development of the site. 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 need further consultation. Nine plates attached Yours very truly, LIU & ASSOCIATES, INC. J. S. (Julian) Liu, MD., P.E. Consulting Geotechnical Engineer LIU & ASSOCIATES, INC.