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HomeMy WebLinkAboutRS_Geotech_Report_170515_v1GE®TECH CONSULTANTS, INC. Benchmark Development Company 5020 — 141St Avenue Southeast Bellevue, Washington 98006 Attention: Mark Sandler via email: mark sandier@comcast.net Subject: Transmittal Letter — Geotechnical Engineering Study Proposed Dental Arts Building 17816-108 th Avenue Southeast Renton, Washington Dear Mr. Sandler: 2401 10th Ave E Seattle, Washington 98102 (425) 747-5618 FAX (425) 747-8561 April 11, 2017 JN 17074 We are pleased to present this geotechnical engineering report for the proposed dental arts building to be constructed in Renton, Washington. The scope of our services consisted of exploring site surface and subsurface conditions, and then developing this report to provide recommendations for general earthwork, stormwater infiltration considerations, and design criteria for foundations and retaining walls. This work was authorized by your acceptance of our proposal, dated February 13, 2017. The attached report contains a discussion of the study and our recommendations. Please contact us if there are any questions regarding this report, or for further assistance during the design and construction phases of this project. ASM/JHS: mw Respectfully submitted, GEOTECH CONSULTANTS, INC. James H. Strange, P. . Associate GEOTECH CONSULTANTS, INC. GEOTECHNICAL ENGINEERING STUDY Proposed Dental Arts Building 17816 — 108th Avenue Southeast Renton, Washington This report presents the findings and recommendations of our geotechnical engineering study for the site of the proposed dental arts building to be located in Renton. We were provided with a preliminary site plan and a topographic map. KV Architecture Company developed the preliminary plans, which are dated October 24, 2016. The topographic map was developed by Barghausen Consulting Engineers, dated August 11, 1983. Based on these plans, we understand that the existing restaurant building will be demolished and a single -story dental arts building will be constructed in the northwest corner of the site; the proposed building will have the same general location as the existing building with a slightly larger footprint. The dental arts building will be offset 6 feet from the northern property line, 20 feet from the western property line, and will have a finished floor elevation of 402 feet. We understand there are no planned deep excavations for a basement floor. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. SITE CONDITIONS SURFACE The Vicinity Map, Plate 1, illustrates the general location of the site in the Benson Hill neighborhood of Renton. The rectangular -shaped subject site is located on the eastern side of 108th Avenue Southeast and has dimensions of 134 feet in the north -south direction and 292 feet in the east -west direction. A one-story restaurant building is located in the northwest corner of the property, which is offset 19 feet from the northern property line and 50 feet from the western property line. The existing building has a finished floor elevation of 402 feet, matching the surrounding site grade. A drive-through lane wraps around the northern and western sides of the building and an asphalt parking lot covers most of the remainder of the site. However, the eastern 60 feet of the site contains several mature evergreen trees and a grass lawn. The subject site slopes gently downwards from east to west with a total change in elevation of 12 feet across the site. No steep slopes are located on the property. There are several slopes across 108th Avenue Southeast to the east that are mapped as having an inclination between 15 to 40 percent on the City of Renton's online geographic information system (GIS) map. Commercial properties border the subject site to the north and south that contain restaurants surrounded by asphalt parking lots. The subject site is bordered by 108th Avenue Southeast and 109th Avenue Southeast to the west and east, respectively. At the time of our site visit, a sign was located on the adjacent northern property depicting the proposed development of a new tire retail store. The posted site plan indicated the new building would be located in the southwest corner of the adjacent northern property with an offset of 10 feet from the shared property line with subject site. GEOTECH CONSULTANTS, INC. Benchmark Development Company JN 17074 April 11, 2017 Page 2 SUBSURFACE The subsurface conditions were explored by drilling three borings at the approximate locations shown on the Site Exploration Plan, Plate 2. Our exploration program was based on the proposed construction, anticipated subsurface conditions and those encountered during exploration, and the scope of work outlined in our proposal. The borings were drilled on March 13, 2017 using a truck -mounted, hollow -stem auger drill. Samples were taken at approximate 5 -foot intervals with a standard penetration sampler. This split -spoon sampler, which has a 2 -inch outside diameter, is driven into the soil with a 140 -pound hammer falling 30 inches. The number of blows required to advance the sampler a given distance is an indication of the soil density or consistency. A geotechnical engineer from our staff observed the drilling process, logged the test borings, and obtained representative samples of the soil encountered. The Test Boring Logs are attached as Plates 3 through 5. Soil Conditions The borings conducted in the area of the proposed building encountered 2.5 to 4.5 feet of medium -dense, gravelly silty sand fill overlying native, medium -dense, silty sand. The underlying native soils became dense to very dense below at 5.5 to 6 feet below the existing ground surface. Very dense silty sand extended to the maximum -explored depth of 15.9 feet in our borings. No obstructions were revealed by our explorations. However, debris, buried utilities, and old foundation and slab elements are commonly encountered on sites that have had previous development. Although our explorations did not encounter cobbles or boulders, they are often found in soils that have been deposited by glaciers or fast-moving water. Groundwater Conditions Perched groundwater seepage was observed on top of the native soils at depths of 2.5 to 3.5 feet in Borings 2 and 3. Several perched groundwater layers were also observed in more permeable layers within the underlying silty sand at depths ranging from 8.5 to 13 feet. The borings were left open for only a short time period. Therefore, the seepage levels on the logs represent the location of transient water seepage and may not indicate the static groundwater level. Groundwater levels encountered during drilling can be deceptive, because seepage into the boring can be blocked or slowed by the auger itself. It should be noted that groundwater levels vary seasonally with rainfall and other factors. We anticipate that perched groundwater could be found in more permeable soil layers within the underlying silty sand and between the looser near -surface fill soil and the underlying denser silty sand as observed in our test borings. The stratification lines on the logs represent the approximate boundaries between soil types at the exploration locations. The actual transition between soil types may be gradual, and subsurface conditions can vary between exploration locations. The logs provide specific subsurface information only at the locations tested. If a transition in soil type occurred between samples in the borings, the depth of the transition was interpreted. The relative densities and moisture descriptions indicated on the boring logs are interpretive descriptions based on the conditions observed during drilling. GEOTECH CONSULTANTS, INC. Benchmark Development Company J N 17074 April 11, 2017 Page 3 CONCLUSIONS AND RECOMMENDATIONS GENERAL THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The borings conducted for this study encountered 2.5 to 4.5 feet of medium -dense, gravelly, silty sand fill soils overlying native, medium -dense silty sand. Conventional shallow foundations bearing on the underlying native silty sand or on structural fill placed on top of the native soils are suitable to support the proposed building. Some overexcavation will be necessary to extend through the upper fill soils to expose the competent native soils below. The onsite native and fill soils could be used as structural fill to backfill the required overexcavations provided they can be placed at or near their optimum moisture content. These fine-grained, silty soils are sensitive to moisture, which makes them impossible to adequately compact when they have moisture contents even 2 to 3 percent above their optimum moisture content. The reuse of these soils as structural fill to level the site will only be successful during hot, dry weather. It would also be prudent to cover the bearing surfaces with several inches of clean crushed rock during wet conditions to protect the soils from becoming disturbed and softened during the footing construction. An expanded discussion of structural fill can be found in the General Earthwork and Structural Fill section of this report. Based on the provided site plans and our observations onsite, it appears that sections of the new building may overly some of the existing underground utilities. Utility trench backfill is typically not adequately compacted to support the proposed building loads. It will be important that the foundation excavations extend through the trench backfill to reach competent native soils below or that the backfill be removed and replaced with competent backfill. If the depth of the trench backfill makes overexcavation unfeasible, continuous footings can be designed to span across the loose trench backfill (perpendicular to the trench). The section of footing crossing over a utility trench should be designed for no soil support, similar to a grade beam. This is typically accomplished with additional rebar reinforcing. The dense to very dense silty sand underlying the site is essentially impermeable. Groundwater seepage was observed perched on the native silty sand in our borings. Considering this, it is our opinion that concentrated stormwater infiltration will not be feasible on the subject site. It may be feasible to disperse low volumes of stormwater with the use of permeable paving; however additional testing will be needed once preliminary designs have been completed. The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. We anticipate that a silt fence will be needed around the downslope sides of any cleared areas. Existing pavements, ground cover, and landscaping should be left in place wherever possible to minimize the amount of exposed soil. Rocked staging areas and construction access roads should be provided to reduce the amount of soil or mud carried off the property by trucks and equipment. Wherever possible, the access roads should follow the alignment of planned pavements. Trucks should not be allowed to drive off of the rock -covered areas. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following clearing or rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. On most construction projects, it is necessary to periodically maintain or modify temporary erosion control measures to address specific site and weather conditions. GEOTECH CONSULTANTS, INC. Benchmark Development Company JN 17074 April 11, 2017 Page 4 The drainage and/or waterproofing recommendations presented in this report are intended only to prevent active seepage from flowing through concrete walls or slabs. Even in the absence of active seepage into and beneath structures, water vapor can migrate through walls, slabs, and floors from the surrounding soil, and can even be transmitted from slabs and foundation walls due to the concrete curing process. Water vapor also results from occupant uses, such as cooking and bathing. Excessive water vapor trapped within structures can result in a variety of undesirable conditions, including, but not limited to, moisture problems with flooring systems, excessively moist air within occupied areas, and the growth of molds, fungi, and other biological organisms that may be harmful to the health of the occupants. The designer or architect must consider the potential vapor sources and likely occupant uses, and provide sufficient ventilation, either passive or mechanical, to prevent a build up of excessive water vapor within the planned structure. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site soil profile within 100 feet of the ground surface is best represented by Site Class Type C (Very Dense Soil and Soft Rock). As noted in the USGS website, the mapped spectral acceleration value for a 0.2 second (Sr,) and 1.0 second period (Si) equals 1.39g and 0.52g, respectively. The site soils are not susceptible to seismic liquefaction because of their dense nature and the absence of near -surface groundwater. This statement regarding liquefaction includes the knowledge of the peak ground acceleration that is anticipated under a 1 -in -2,500 -year seismic event, which is significantly higher than the above -calculated ground acceleration. CONVENTIONAL FOUNDATIONS The proposed structure can be supported on conventional continuous and spread footings bearing on undisturbed, native, medium -dense, silty sand, or on structural fill placed above this competent native soil. See the section entitled General Earthwork and Structural Fill for comments regarding the existing utility trenches and associated backfill, as well as recommendations regarding the placement and compaction of structural fill beneath structures. Adequate compaction of structural fill should be verified with frequent density testing during fill placement. Prior to placing structural fill beneath foundations, the excavation should be observed by the geotechnical engineer to document that adequate bearing soils have been exposed. We recommend that continuous and individual spread footings have minimum widths of 12 and 16 inches, respectively. Exterior footings should also be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. GEOTECH CONSULTANTS, INC. Benchmark Development Company JN 17074 April 11, 2017 Page 5 Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints, this may require removing the disturbed soil by hand. Depending. on the final site grades, overexcavation may be required below the footings to expose competent native soil. Unless lean concrete is used to fill an overexcavated hole, the overexcavation must be at least as wide at the bottom as the sum of the depth of the overexcavation and the footing width. For example, an overexcavation extending 2 feet below the bottom of a 2 -foot -wide footing must be at least 4 feet wide at the base of the excavation. If lean concrete is used, the overexcavation need only extend 6 inches beyond the edges of the footing. An allowable bearing pressure of 2,500 pounds per square foot (psf) is appropriate for footings supported on competent native soil. A one-third increase in this design bearing pressure may be used when considering short-term wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction settlement of footings founded on competent native soil, or on structural fill up to 5 feet in thickness, will be about one inch, with differential settlements on the order of one half-inch in a distance of 50 feet along a continuous footing with a uniform load. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level, well -compacted fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARAMETER ULTIMATE VALUE Coefficient of Friction 0.50 Passive Earth Pressure 300 pcf Where: pcf is Pounds per Cubic Foot, and Passive Earth Pressure is computed using the Equivalent Fluid Density. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend maintaining a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate values. GEOTECH CONSULTANTS, INC. Benchmark Development Company April 11, 2017 FOUNDATION AND RETAINING WALLS JN 17074 Page 6 Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures imposed by the soil they retain. The following recommended parameters are for walls that restrain level backfill: PARAMETER VALUE Active Earth Pressure * 35 pcf Passive Earth Pressure 300 pcf Coefficient of Friction 0.50 Soil Unit Weight 130 pcf Where: pcf is Pounds per Cubic Foot, and Active and Passive Earth Pressures are computed using the Equivalent Fluid Pressures. * For a restrained wall that cannot deflect at least 0.002 times its height, a uniform lateral pressure equal to 10 psf times the height of the wall should be added to the above active equivalent fluid pressure. The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid density. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. The values given above are to be used to design only permanent foundation and retaining walls that are to be backfilled, such as conventional walls constructed of reinforced concrete or masonry. It is not appropriate to use the above earth pressures and soil unit weight to back -calculate soil strength parameters for design of other types of retaining walls, such as soldier pile, reinforced earth, modular or soil nail walls. We can assist with design of these types of walls, if desired. The passive pressure given is appropriate only for a shear key poured directly against undisturbed native soil, or for the depth of level, well -compacted fill placed in front of a retaining or foundation wall. The values for friction and passive resistance are ultimate values and do not include a safety factor. Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from corners or bends in the walls. This is intended to reduce the amount of cracking that can occur where a wall is restrained by a corner. Wall Pressures Due to Seismic Forces The surcharge wall loads that could be imposed by the design earthquake can be modeled by adding a uniform lateral pressure to the above -recommended active pressure. The recommended surcharge pressure is 7H pounds per square foot (psf), where H is the design retention height of the wall. Using this increased pressure, the safety factor against sliding and overturning can be reduced to 1.2 for the seismic analysis. GEOTECH CONSULTANTS, INC. Benchmark Development Company April 11, 2017 Retaining Wall Backfill and Waterproofing JN 17074 Page 7 Backfill placed behind retaining or foundation walls should be coarse, free -draining structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. A drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. Free -draining backfill or gravel should be used for the entire width of the backfill where seepage is encountered. For increased protection, drainage composites should be placed along cut slope faces, and the walls should be backfilled entirely with free -draining soil. The later section entitled Drainage Considerations should also be reviewed for recommendations related to subsurface drainage behind foundation and retaining walls. The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the wall. Also, subsurface drainage systems are not intended to handle large volumes of water from surface runoff. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surface water to percolate into the backfill. Water percolating through pervious surfaces (pavers, gravel, permeable pavement, etc.) must also be prevented from flowing toward walls or into the backfill zone. The compacted subgrade below pervious surfaces and any associated drainage layer should therefore be sloped away. Alternatively, a membrane and subsurface collection system could be provided below a pervious surface. It is critical that the wall backfill be placed in lifts and be properly compacted, in order for the above -recommended design earth pressures to be appropriate. The wall design criteria assume that the backfill will be well -compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand -operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. The section entitled General Earthwork and Structural Fill contains additional recommendations regarding the placement and compaction of structural fill behind retaining and foundation walls. The above recommendations are not intended to waterproof below -grade walls, or to prevent the formation of mold, mildew or fungi in interior spaces. Over time, the performance of subsurface drainage systems can degrade, subsurface groundwater flow patterns can change, and utilities can break or develop leaks. Therefore, waterproofing should be provided where future seepage through the walls is not acceptable. This typically includes limiting cold joints and wall penetrations, and using bentonite panels or membranes on the outside of the walls. There are a variety of different waterproofing materials and systems, which should be installed by an experienced contractor familiar with the anticipated construction and subsurface conditions. Applying a thin coat of asphalt emulsion to the outside face of a wall is not considered waterproofing, and will only help to reduce moisture generated from water vapor or capillary action from seeping through the concrete. As with any project, adequate ventilation of basement and crawl space areas is important to prevent a build up of water vapor that is commonly transmitted through concrete walls from the surrounding soil, even when seepage is not present. This is appropriate even when waterproofing is applied to the outside of foundation and retaining walls. We recommend that you contact an experienced envelope consultant if detailed GEOTECH CONSULTANTS, INC. Benchmark Development Company April 11, 2017 JN 17074 Page 8 recommendations or specifications related to waterproofing design, or minimizing the potential for infestations of mold and mildew are desired. The General, Slabs -On -Grade, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of groundwater and excess water vapor for the anticipated construction. SLABS -ON -GRADE The building floors can be constructed as slabs -on -grade atop non-organic native soils, or on structural fill. The subgrade soil must be in a firm, non -yielding condition at the time of slab construction or underslab fill placement. Any soft areas encountered should be excavated and replaced with select, imported structural fill. Even where the exposed soils appear dry, water vapor will tend to naturally migrate upward through the soil to the new constructed space above it. This can affect moisture -sensitive flooring, cause imperfections or damage to the slab, or simply allow excessive water vapor into the space above the slab. All interior slabs -on -grade should be underlain by a capillary break drainage layer consisting of a minimum 4 -inch thickness of clean gravel or crushed rock that has a fines content (percent passing the No. 200 sieve) of less than 3 percent and a sand content (percent passing the No. 4 sieve) of no more than 10 percent. Pea gravel or crushed rock are typically used for this layer. As noted by the American Concrete Institute (ACI) in the Guides for Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below any on -grade slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture -sensitive equipment or products. ACI also notes that vapor retarders such as 6 -mil plastic sheeting have been used in the past, but are now recommending a minimum 10 -mil thickness for better durability and long term performance. A vapor retarder is defined as a material with a permeance of less than 0.3 perms, as determined by ASTM E 96. It is possible that concrete admixtures may meet this specification, although the manufacturers of the admixtures should be consulted. Where vapor retarders are used under slabs, their edges should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.01 perms when tested in accordance with ASTM E 96. Reinforced membranes having sealed overlaps can meet this requirement. In the recent past, ACI (Section 4.1.5) recommended that a minimum of 4 inches of well -graded compactable granular material, such as a 5/8 -inch -minus crushed rock pavement base, be placed over the vapor retarder or barrier for their protection, and as a "blotter" to aid in the curing of the concrete slab. Sand was not recommended by ACI for this purpose. However, the use of material over the vapor retarder is controversial as noted in current ACI literature because of the potential that the protection/blotter material can become wet between the time of its placement and the installation of the slab. If the material is wet prior to slab placement, which is always possible in the Puget Sound area, it could cause vapor transmission to occur up through the slab in the future, essentially destroying the purpose of the vapor barrier/retarder. Therefore, if there is a potential that the protection/blotter material will become wet before the slab is installed, ACI now recommends that no protection/blotter material be used. However, ACI then recommends that, because there is a potential for slab curl due to the loss of the blotter material, joint spacing in the GEOTECH CONSULTANTS, INC. Benchmark Development Company JN 17074 April 11, 2017 Page 9 slab be reduced, a low shrinkage concrete mixture be used, and "other measures" (steel reinforcing, etc.) be used. ASTM E-1643-98 "Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs" generally agrees with the recent ACI literature. We recommend that the contractor, the project materials engineer, and the owner discuss these issues and review recent ACI literature and ASTM E-1643 for installation guidelines and guidance on the use of the protection/blotter material. The General, Permanent Foundation and Retaining Walls, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of groundwater and excess water vapor for the anticipated construction. EXCAVATIONS AND SLOPES Excavation slopes should not exceed the limits specified in local, state, and national government safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil, if there are no indications of slope instability. However, vertical cuts should not be made near property boundaries, or existing utilities and structures. Based upon Washington Administrative Code (WAC) 296, Part N, the upper medium -dense native and fill soils at the subject site would generally be classified as Type B. Therefore, temporary cut slopes greater than 4 feet in height should not be excavated at an inclination steeper than 1:1 (Horizontal:Vertical), extending continuously between the top and the bottom of a cut. The above -recommended temporary slope inclination is based on the conditions exposed in our explorations, and on what has been successful at other sites with similar soil conditions. It is possible that variations in soil and groundwater conditions will require modifications to the inclination at which temporary slopes can stand. Temporary cuts are those that will remain unsupported for a relatively short duration to allow for the construction of foundations, retaining walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet weather. It is also important that surface runoff be directed away from the top of temporary slope cuts. Cut slopes should also be backfilled or retained as soon as possible to reduce the potential for instability. Please note that loose soil can cave suddenly and without warning. Excavation, foundation, and utility contractors should be made especially aware of this potential danger. These recommendations may need to be modified if the area near the potential cuts has been disturbed in the past by utility installation, or if settlement -sensitive utilities are located nearby. All permanent cuts into native soil should be inclined no steeper than 2:1 (H:V). To reduce the potential for shallow sloughing, fill must be compacted to the face of these slopes. This can be accomplished by overbuilding the compacted fill and then trimming it back to its final inclination. Adequate compaction of the slope face is important for long-term stability and is necessary to prevent excessive settlement of patios, slabs, foundations, or other improvements that may be placed near the edge of the slope. Water should not be allowed to flow uncontrolled over the top of any temporary or permanent slope. All permanently exposed slopes should be seeded with an appropriate species of vegetation to reduce erosion and improve the stability of the surficial layer of soil. GEOTECH CONSULTANTS, INC. Benchmark Development Company April 11, 2017 DRAINAGE CONSIDERATIONS JN 17074 Page 10 Footing drains should be used where: (1) Crawl spaces or basements will be below a structure; (2) A slab is below the outside grade; or, (3) The outside grade does not slope downward from a building. Drains should also be placed at the base of all earth -retaining walls. These drains should be surrounded by at least 6 inches of 1 -inch -minus, washed rock that is encircled with non -woven, geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At its highest point, a perforated pipe invert should be at least 6 inches below the bottom of a slab floor or the level of a crawl space. The discharge pipe for subsurface drains should be sloped for flow to the outlet point. Roof and surface water drains must not discharge into the foundation drain system. A typical drain detail is attached to this report as Plate 6. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. As a minimum, a vapor retarder, as defined in the Slabs -On -Grade section, should be provided in any crawl space area to limit the transmission of water vapor from the underlying soils. Crawl space grades are sometimes left near the elevation of the bottom of the footings. As a result, an outlet drain is recommended for all crawl spaces to prevent an accumulation of any water that may bypass the footing drains. Providing even a few inches of free draining gravel underneath the vapor retarder limits the potential for seepage to build up on top of the vapor retarder. Perched groundwater was observed during our field work. If seepage is encountered in an excavation, it should be drained from the site by directing it through drainage ditches, perforated pipe, or French drains, or by pumping it from sumps interconnected by shallow connector trenches at the bottom of the excavation. The excavation and site should be graded so that surface water is directed off the site and away from the tops of slopes. Water should not be allowed to stand in any area where foundations, slabs, or pavements are to be constructed. Final site grading in areas adjacent to a building should slope away at least 2 percent, except where the area is paved. Surface drains should be provided where necessary to prevent ponding of water behind foundation or retaining walls. A discussion of grading and drainage related to pervious surfaces near walls and structures is contained in the Foundation and Retaining Walls section. GENERAL EARTHWORK AND STRUCTURAL FILL All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and other deleterious material. It is important that existing foundations be removed before site development. The stripped or removed materials should not be mixed with any materials to be used as structural fill, but they could be used in non-structural areas, such as landscape beds. Structural fill is defined as any fill, including utility backfill, placed under, or close to, a building, behind permanent retaining or foundation walls, or in other areas where the underlying soil needs to support loads. All structural fill should be placed in horizontal lifts with a moisture content at, or near, the optimum moisture content. The optimum moisture content is that moisture content that results in the greatest compacted dry density. The moisture content of fill is very important and must be closely controlled during the filling and compaction process. The allowable thickness of the fill lift will depend on the material type selected, the compaction equipment used, and the number of passes made to compact the lift. The loose lift thickness should not exceed 12 inches. We recommend testing the fill as it is placed. If the fill is not GEOTECH CONSULTANTS, INC. Benchmark Development Company April 11, 2017 sufficiently compacted, it can be recompacted before need to remove the fill to achieve the required recommended relative compactions for structural fill: JN 17074 Page 11 another lift is placed. This eliminates the compaction. The following table presents LOCATION OF FILL MINIMUM RELATIVE PLACEMENT COMPACTION Beneath footings, slabs 95% orwalkways Filled slopes and behind 90% retaining walls 95% for upper 12 inches of Beneath pavements subgrade; 90% below that level Where: Minimum Relative Compaction is the ratio, expressed in percentages, of the compacted dry density to the maximum dry density, as determined in accordance with ASTM Test Designation D 1557-91 (Modified Proctor). Use of On -Site Soil If grading activities take place during wet weather, or when the silty, on-site soil is wet, site preparation costs may be higher because of delays due to rain and the potential need to import granular fill. The on-site soil is generally silty and therefore moisture sensitive. Grading operations will be difficult during wet weather, or when the moisture content of this soil exceeds the optimum moisture content. The moisture content of the silty, on-site soil must be at, or near, the optimum moisture content, as the soil cannot be consistently compacted to the required density when the moisture content is significantly greater than optimum. The moisture content of the on-site soil was generally above the estimated optimum moisture content at the time of our explorations. The on-site native silty sand underlying the topsoil could be used as structural fill, if grading operations are conducted during hot, dry weather, when drying the wetter soil by aeration is possible. During excessively dry weather, however, it may be necessary to add water to achieve the optimum moisture content. Moisture -sensitive soil may also be susceptible to excessive softening and "pumping" from construction equipment, or even foot traffic, when the moisture content is greater than the optimum moisture content. It may be beneficial to protect subgrades with a layer of imported sand or crushed rock to limit disturbance from traffic. The General section should be reviewed for considerations related to the reuse of on-site soils. Structural fill that will be placed in wet weather should consist of a coarse, granular soil with a silt or clay content of no more than 5 percent. The percentage of particles passing the No. 200 sieve should be measured from that portion of soil passing the three -quarter -inch sieve. LIMITATIONS The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions GEOTECH CONSULTANTS, INC. Benchmark Development Company JN 17074 April 11, 2017 Page 12 encountered in the borings are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those observed in our explorations, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking samples in borings. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects. This report has been prepared for the exclusive use of Benchmark Development Company and its representatives, for specific application to this project and site. Our conclusions and recommendations are professional opinions derived in accordance with our understanding of current local standards of practice, and within the scope of our services. No warranty is expressed or implied. The scope of our services 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 our report for consideration in design. Our services also do not include assessing or minimizing the potential for biological hazards, such as mold, bacteria, mildew and fungi in either the existing or proposed site development. ADDITIONAL SERVICES In addition to reviewing the final plans, Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work we actually observe. It is still the responsibility of your contractor or on-site construction team to verify that our recommendations are being followed, whether we are present at the site or not. The scope of our work did not include an environmental assessment, but we can provide this service, if requested. The following plates are attached to complete this report: Plate 1 Vicinity Map Plate 2 Site Exploration Plan Plates 3 - 5 Boring Logs Plate 6 Typical Footing Drain Detail GEOTECH CONSULTANTS, INC. Benchmark Development Company April 11, 2017 JN 17074 Page 13 We appreciate the opportunity to be of service on this project. Please contact us if you have any questions, or if we can be of further assistance. ASM/JHS:mw Respectfully submitted, GEOTECH CONSULTANTS, INC. -J�PIN- Adam S. Moyer Geotechnical Engineer James H. Strange, Jr., P.E. Associate GEOTECH CONSULTANTS, INC. 7—OV' tet tis, (q ,r <,F s .., z.. .. p r ry CO��y £' ` I µ{ 3 1 CO _'£� :S, .<Q t# t .y,.hu. ''�s �µ'-'�a z? '} ':. Oxy T£ £fd4�.. lhy m _ i, £: 1.J bE�."-.....,...... -«„ i w4hJ Lm..:: •i—.:...,.. v'.;:"' 3a'aa.. «iA'/k'n}".n.. i h #� �tC *'.. ., ...< iii .. #.^. ...... .. e 9 s �.--. YLJ � � � #� s ........... s, c*- t { ,x...E 135 : ;SITE �` �£$. s„ ti .......... GEOTECH CONSULTANTS, INC. (Source: Microsoft MapPoint, 2013) VICINITY MAP 17816 - 108th Avenue Southeast Renton, Washington Job No: I Date:Plate: 17074 1 Apr. 2017 1 1 a"z' t5Vpr'FR'tF MW'Mnw r4. Z,%E, Af 10 416 4114 4 ft1?1?1kVV 111 1 W tl_p, N et'114 A C /A. w 'V' aj , �4Riv'k r"z ,* ✓ 0 Vip C'am V w1r a4im oscb $An FM QW. WA N. V . . . . . . . ..... 4w, N AW Urn Ap 396, 1a°7%Re#fitir�r^Tir� E117 R4 safE. r Am N. B-3 AO� 4* TW W, Lim E00 Gd L �ml MAO plenta t2l, mqm 14J7' 1 00 I -STORY BRICK a WOOD LD RAX RESTAURANT BGlow . lf,romt too "'. 03" f R1W MR. MR. EL. A 402.04 x of rk 60, Box B-2 % s G�' Ctaa�nks€ ! AX No. $301304"' 4 Rim 431 141 QQ AD 14, Ao 404 1*4 $41191 Rim L A Legend, Q Test Boring Location L f&!Box P. Plate: I 17074 I Apr. 2017 No Scale I 2 apt,% 413✓ -I'A VAX -Vz Power N. 4W., u. un, I -411 sit.jeftsre GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 17816 - 108th Avenue Southeast Renton, Washington Job No: Date: Plate: I 17074 I Apr. 2017 No Scale I 2 5 15 20i BORING 1 Description GEOTECH CONSULTANTS, INC. TEST BORING LOG 17816 - 108th Avenue Southeast Renton, Washington Job Date: Logged by: Plate: 17074 Apr. 2017 ASM 1 3 M Grass and topsoil over: FILL Dark -gray -brown gravelly, silty SAND, fine to medium -grained, moist, *42 1 medium -dense (FILL) Gray -brown mottled orange, silty SAND with gravel, fine to medium -grain 37 2 €F very moist, medium -dense -becomes gray -brown, moist, dense 84 _ 3 € - n with occasional pieces of charcoal, becomes very dense 9.. 50 4 sM -with rust mottling € 6" -becomes wet 50 —, 5 5 €1 € t:::,: ::: €€ -no charcoal l -becomes 2ray, moist * Test boring was terminated at 15.9 feet on March 13, 2017. * Perched groundwater was encountered at 10.5 feet during drilling. GEOTECH CONSULTANTS, INC. TEST BORING LOG 17816 - 108th Avenue Southeast Renton, Washington Job Date: Logged by: Plate: 17074 Apr. 2017 ASM 1 3 M 10 15 20 BORING 2 Description GEOTECH CONSULTANTS, INC. TEST BORING LOG 17816 - 108th Avenue Southeast Renton, Washington Job Date: Logged by: Plate: 17074 2 -inches of asphalt over; ASM 1 4 FILL Dark -gray -brown gravelly, silty SAND, fine to coarse-grained, moist, medium -dense 21 1 Rust -brown slightly silty SAND with gravel, fine to medium -grained, SM SP very moist to wet, medium -dense .. . . a. 54 2 Brown silty SAND, fine-grained, moist, very dense — 54 3 -becomes wet, increased silt content SM -becomes moist, increased silt content �► — 63 4 -becomes wet, increased sand content 50 -becomes gray and gray -brown, moist 6 * Test boring was terminated at 15.5 feet on March 13, 2017. * Perched groundwater was encountered at 3.7 to 5 feet, 8.5 to 10.2 feel and 11.2 to 13 feet during drilling. * Groundwater was measured at 7.5 feet after drilling. GEOTECH CONSULTANTS, INC. TEST BORING LOG 17816 - 108th Avenue Southeast Renton, Washington Job Date: Logged by: Plate: 17074 Apr. 2017 ASM 1 4 10 15 K11 BORING 3 Description GEOTECH CONSULTANTS, INC. TEST BORING LOG 17816 - 108th Avenue Southeast Renton, Washington Job Date: Logged by. Plate: 17074 Apr. 2017 ASM 1 5 IL Asphalt over; FILL V Dark -brown gravelly, silty SAND, fine to coarse-grained, moist, medium -dense (F 31 1 € 1t� Brown slightly silty SAND with gravel, fine to medium -grained, very moist to wet, dense -increased silt content, becomes fine-grained, moist, very dense 65 2 70 3 -increased sand content I SM V SP � :::a::::: — 50 4 -becomes moist, increased silt content 5" 50 5 -becomes gray 3" * Test boring was terminated at 15.8 feet on March 13, 2017. * Perched groundwater was encountered at 2.7 to 3.7 feet and 9.5 to 10.5 f during drilling. GEOTECH CONSULTANTS, INC. TEST BORING LOG 17816 - 108th Avenue Southeast Renton, Washington Job Date: Logged by. Plate: 17074 Apr. 2017 ASM 1 5 IL Slope backfill away from foundation. Provide surface drains where necessary. Tightline Roof Drain (Do not connect to footing drain) Backfill (See text for requirements) 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. FOOTING DRAIN DETAIL 17816 - 108th Avenue Southeast Renton, Washington Job No: Date: Plate: 17074 1 Apr. 2017 6