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HomeMy WebLinkAboutGeotech Report 5-26-14GEOTECH CONSULTANTS, INC. RAD Holdings, LLC 1040 West Lake Sammamish Bellevue, Washington 98008 Parkway Southeast 13256 Northeast 20th Street, Suite 16 Bellevue, Washington 98005 (425) 747-5618 FAX (425) 747-8561 May 27, 2014 JN 14177 Attention: Rory Dees via email: rorydees@hotmail.com Subject: Transmittal Letter— Geotechnical Engineering Study Proposed Residential Development 3112 Talbot Road South Renton, Washington Dear Mr. Dees: We are pleased to present this geotechnical engineering report for the residential development to be constructed in Renton. The scope of our services consisted of exploring site surface and subsurface conditions, and then developing this report to provide recommendations for general earthwork and design criteria for foundations, 'retaining walls, and pavements. This work was authorized by your acceptance of our proposal, P-8823, dated September 6, 2013. 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. TRC/MRM: at Respectfully submitted, GEOTECH CONSULTANTS, INC. 1JoVr\C&hristen`Sen, P.E. Senior Engineer GEOTECH CONSULTANTS, INC. GEOTECHNICAL ENGINEERING STUDY Proposed Residential Development 3112 Talbot Road South Renton, Washington This report presents the findings and recommendations of our geotechnical engineering study for the site of the proposed residential development to be located in Renton. We were provided with a topographic survey of the site prepared by Axis Survey & Mapping dated August 28, 2013. We have also been provided with project plans by Land Development Advisors dated May 7, 2014. Based on these plans, we understand that the eastern of the two site residences will be removed and the western residence will remain. The development will consist of 8 residential lots and a stormwater detention pond. The lots will be accessed from the south with two driveways from South 32nd Place. Retaining walls up to 4 feet high will be constructed on the eastern side of the two proposed access driveways. Grading for the proposed lots will include cuts and fills of up to 4 feet. A stormwater detention pond will be located at the west side of the development, and a cut of up to 10 feet will be made for the pond. The pond slopes will have an inclination of 2:1 (H:V). 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 rectangular -shaped parcel. The site is surrounded by residences and is accessed from the west by a driveway from Talbot Road South. The site has dimensions of 100 feet in the north -south direction and 1,000 feet in the east -west direction. The property is developed with two residences; both of which are accessed from Talbot Road South by a driveway along the south edge of the site. The western residence has two stories and a basement, and the eastern residence has one story and a basement that daylights toward the west. The ground surface within the site slopes gently to moderately down toward the west, with a change in elevation of about 70 feet across a distance of 1,000 feet. There are no steep slopes on, or near, the site. Approximately the eastern 300 feet of the site is thickly vegetated with young to mature evergreen and deciduous trees and brush. Most of the remainder of the site is covered with grass lawn, with scattered mature trees and landscaping bushes. Blackberry vines grow in the western portion of the planned development area. SUBSURFACE The subsurface conditions were explored by excavating four test pits at the approximate locations shown on the Site Exploration Plan, Plate 2. Our exploration program was based on the proposed GEOTECH CONSULTANTS, INC. RAD Holdings, LLC JN 14177 May 27, 2014 Page 2 construction, anticipated subsurface conditions and those encountered during exploration, and the scope of work outlined in our proposal. The test pits were excavated on May 21, 2014 with a small excavator. A geotechnical engineer from our staff observed the excavation process, logged the test pits, and obtained representative samples of the soil encountered. "Grab" samples of selected subsurface soil were collected from the backhoe bucket. The Test Pit Logs are attached to this report as Plates 3 and 4. Soil Conditions The test pits found topsoil that had a thickness of about one foot. Below the topsoil, Test Pit 2 encountered loose to medium -dense silt with sand. Below this silt in Test Pit 2, and beneath the topsoil in the other explorations, we encountered loose to medium -dense silty sand with gravel. This material included pieces of dense silt in Test Pits 1 and 2. The silty sand with gravel became medium -dense at a depth of about 2 to 3 feet, and dense at a depth of about 4 to 7 feet. The dense silty sand with gravel extended to the maximum depth of the test pits, 6 to 8.8 feet below the surface. 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. Groundwater Conditions Perched groundwater seepage was observed at a depth of 3 feet in Test Pit 4. The test pits were left open for only a short time period, but were conducted following a very wet fall and winter. The seepage levels on the logs represent the location of transient water seepage and may not indicate the static groundwater level. It should be noted that groundwater levels vary seasonally with rainfall and other factors. We anticipate that groundwater could be found in more permeable soil layers and between the near -surface weathered soil and the underlying denser soil. 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. The relative densities and moisture descriptions indicated on the test pit logs are interpretive descriptions based on the conditions observed during excavation. The compaction of test pit backfill was not in the scope of our services. Loose soil will therefore be found in the area of the test pits. If this presents a problem, the backfill will need to be removed and replaced with structural fill during construction. 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 GEOTECH CONSULTANTS, INC, RAD Holdings, LLC JN 14177 May 27, 2014 Page 3 CONTAINED IN THE REMAINDER OF THIS REPORT, ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The test pits conducted for this study encountered medium -dense silty sand with gravel that will provide adequate support to the proposed residences and pavements. The test pits found suitable bearing soils at a depth of 2 to 3 feet. The silty soils will be susceptible to disturbance and softening in wet conditions. As a result, it would be prudent to protect footing subgrades with a thin layer of crushed rock. If foundations are constructed within the footprint of the existing basements, it will be important to verify that suitable native bearing soils are first exposed. This usually requires removal of the foundations and slabs. We anticipate that perched water may be encountered in the sidewalls of the proposed stormwater detention pond excavation. This could cause erosion and instability near the seepage zone. We recommend that the portion of the pond more than 3 feet below the existing surface be armored with a one -foot -thickness of 2- to 4 -inch rock spalls to reduce the potential for erosion of the pond sides. The proposed excavations for the east sides of the two access driveways will be within 10 feet of adjacent residences. To avoid impacting those residences, no excavation should extend below a 1.5:1 (H:V) inclination extending outward from the base of the residence foundations. Shallow perched groundwater may result in seepage entering crawl spaces and/or basements under the planned houses. In addition to footing drains and free -draining wall backfill, drainage should be provided beneath the houses. This typically consists of a 6- to 9 -inch layer of free - draining gravel below the vapor retarder, with perforated pipes buried in the gravel on 15- to 20 -foot spacing. This underdrainage can be connected to the same outlet as the footing drains. 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. The on-site soil and groundwater conditions are not suitable for infiltration of runoff from impervious surfaces. This includes avoiding using drywells for downspout runoff. 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 GEOTECH CONSULTANTS, INC. RAD Holdings, LLC A 14177 May 27, 2014 Page 4 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, should also be provided to any future recommendations. SEISMIC CONSIDERATIONS in its entirety, in the project contract documents. This report property owners so they will be aware of our findings and In accordance with the International Building Code (IBC), the site class within 100 feet of the ground surface is best represented by Site Class Type C (Very Dense Soil and Soft Rock). The site soils are not susceptible to seismic liquefaction because of their dense nature. CONVENTIONAL FOUNDATIONS The proposed structure can be supported on conventional continuous and spread footings bearing on undisturbed, medium -dense, native soil, or on structural fill placed above this competent native soil. See the section entitled General Earthwork and Structural Fill for 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. 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. 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 30 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 GEOTECH CONSULTANTS, INC. RAD Holdings, LLC JN 14177 May 27, 2014 Page 5 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: ULTiMATE VALUE Coefficient of Friction 0.45 Passive Earth Pressure 350 pcf Where: (i) pcf is pounds per cubic foot, and (ii) 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. FOUNDATION AND RETAINING WALLS 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 Active Earth Pressure * 35 pcf Passive Earth Pressure 350 pcf Coefficient of Friction 0.45 Soil Unit Weight 130 pcf Where: (i) pcf is pounds per cubic foot, and (ii) 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 GEOTECH CONSULTANTS, INC. RAD Holdings, LLC May 27, 2014 JN 14177 Page 6 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. We recommend a safety factor of at least 1.5 for overturning and sliding, when using the above values to design the walls. 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, Retaining Wall Backfill and Waterproofing 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. The native soils are not free -draining. If they are used as compacted wall backfill, a minimum 12 -inch thickness of free -draining gravel should be placed against the wall. 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, ect.) 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 GEOTECH CONSULTANTS, INC. RAD Holdings, LLC May 27, 2014 JN 14177 Page 7 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 recommendations or specifications related to waterproofing design, or minimizing the potential for infestations of mold and mildew are desired. SLABS -ON -GRADE The building floors can be constructed as slabs -on -grade atop competent native soil 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. The General section should be reviewed for underdrainage recommendations. 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. GEOTECH CONSULTANTS, INC. RAD Holdings, LLC May 27, 2014 EXCAVATIONS AND SLOPES JN 14177 Page 8 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 soil 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 sand or 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). Permanent cut slopes encountering groundwater may require gravel armoring. Compacted fill slopes should not be constructed with an inclination greater 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. DRAINAGE CONSIDERATIONS 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 5. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. GEOTECH CONSULTANTS, INC. RAD Holdings, LLC A 14177 May 27, 2014 Page 9 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. 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. Final site grading in areas adjacent to buildings 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. Drainage measures on multi -lot developments sometimes have to be modified or upgraded to address post -grading conditions. A discussion of grading and drainage related to pervious surfaces near walls and structures is contained in the Foundation and Retaining Walls section. PAVEMENT AREAS The pavement section may be supported on competent, native soil or on structural fill compacted to a 95 percent density. The pavement subgrade must be in a stable, non -yielding condition at the time of paving. Granular structural fill or geotextile fabric may be needed to stabilize soft, wet, or unstable areas. To evaluate pavement subgrade strength, we recommend that a proof roll be completed with a loaded dump truck immediately before paving. In most instances where unstable subgrade conditions are encountered, an additional 12 inches of granular structural fill will stabilize the subgrade, except for very soft areas where additional fill could be required. The subgrade should be evaluated by Geotech Consultants, Inc., after the site is stripped and out to grade. Recommendations for the compaction of structural fill beneath pavements are given in the section entitled General Earthwork and Structural Fill. The performance of site pavements is directly related to the strength and stability of the underlying subgrade. The pavement for lightly loaded traffic and parking areas should consist of 2 inches of asphalt concrete (AC) over 4 inches of crushed rock base (CRB) or 3 inches of asphalt -treated base (ATB). We recommend providing heavily loaded areas with 3 inches of AC over 6 inches of CRB or 4 inches of ATB. Heavily loaded areas are typically main driveways, dumpster sites, or areas with truck traffic. Increased maintenance and more frequent repairs should be expected if thinner pavement sections are used. The pavement section recommendations and guidelines presented in this report are based on our experience in the area and on what has been successful in similar situations. As with any pavements, some maintenance and repair of limited areas can be expected as the pavement ages. Cracks in the pavement should be sealed as soon as possible after they become evident, in order to reduce the potential for degradation of the subgrade from infiltration of surface water. For the same reason, it is also prudent to seal the surface of the pavement after it has been in use for several years. To provide for a design without the need for any maintenance or repair would be uneconomical. GEOTECH CONSULTANTS, INC, RAD Holdings, LLC May 27, 2014 GENERAL EARTHWORK AND STRUCTURAL FILL JN 14177 Page 10 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 sufficiently compacted, it can be recompacted before another lift is placed. This eliminates the need to remove the fill to achieve the required compaction. The following table presents recommended relative compactions for structural fill: Beneath footings, slabs 95% or walkways Filled slopes and behind 90% retainina 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. IVloisture-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. GEOTECH CONSULTANTS, INC. RAD Holdings, LLC JN 14177 May 27, 2014 Page 11 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 encountered in the test pits 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 test pits. 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 RAD Holdings, LLC 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 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 following plates are attached to complete this report: Plate 1 Vicinity Map GEOTECH CONSULTANTS, INC. RAD Holdings, LLC May 27, 2014 Plate 2 Site Exploration Plan Plates 3 - 4 Test Pit Logs Plate 5 Typical Footing Drain Detail A 14177 Page 12 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. TRC/MRM: at Respectfully submitted, GEOTECH CONSULTANTS, INC. %�\ Ltn T"or Christensen, P.E. �nior Engineer irc R. McGinnis, P.E. ncipal GEOTECH CONSULTANTS, INC. GEOTECH CONSIJLTANTS, INC. (Source: Microsoft Streets and Trlps, 2004) VICINITY MAP 3112 Talbot Road South Renton, Washington Job No: Date: Plate: 14177 May 2014 1 Legend: Lj Test pit location GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 3112 Talbot Road South Renton, Washington Job No: Date: I Plate: 14177 May 2014 No Scale 2 61 10 5 10 'Polo������e JSG� TEST PIT 1 PSOIL Description Dark -brown silty SAND with occasional gravel, roots, and organics, fine to coarse-grained, moist, loose to medium -dense -becomes brown and medium -dense, with pieces of dense silt -decreased gravel content -becomes dense * Test Pit terminated at 8.8 feet on May 21, 2014. * No groundwater seepage was observed during excavation. * No caving observed during excavation. J�oS TEST PIT 2 Description TOPSOIL Rust -brown mottled gray SILT with sand, fine to medium -grained, non -plastic, moist, loose to medium -dense Brown silty SAND with gravel and pieces of dense silt, fine to coarse-grained, moist, medium -dense -becomes dense * Test Pit terminated at 6.0 feet on May 21, 2014. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GEOTECH CONSULTANTS, INC. TEST PIT LOG 3112 Talbot Road South Renton, Washington Job Date: Logged by: Plate: 14177 1 May 2014 1 TRC I 3 R, 10 5 10 �Go��v���a��e JSG� TEST PIT 3 TOPSOIL Rust -brown mottled gray silty SAND loose -becomes brown and medium -dense -becomes dense Description , fine to * Test Pit terminated at 6.0 feet on May 21, 2014. * Slight groundwater seepage was observed at 3.0 feet during excavation. * No caving observed during excavation. Qo� Go � J5 TEST PIT 4 TOPSOIL Description Rust -brown mottled gray silty SAND with gravel, fine to coarse-grained, moist, loose -becomes brown and medium -dense -becomes dense * Test Pit terminated at 6.0 feet on May 21, 2014. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GEOTECH CONSULTANTS, INC. TEST PIT LOG 3112 Talbot Road South Renton, Washington Job Date: Logged by: Plate: 14177 1 May 2014 1 TRC 1 4 Slope backfill away from foundation. Provide surface drains where necessary. Backfill (See text for requirements) Nonwoven Geotextile Tightline Roof Drain (Do not connect to footing drain) Possible Slab �'o OP °D�'ODO o0�'Ooo °��ODO °��'O00 °Do.opO 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 bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, GEOTECH CONSULTANTS, INC. Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) prevent buildup of water that waterproofing, and slab considerations. FOOTING DRAIN DETAIL 3112 Talbot Road South Renton, Washington Job No: Date: Plate: 14177 May 2014 1 1 5