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HomeMy WebLinkAboutRS_Geotech_Report_190415_v1April 15, 2019 Mr. Alex Castro 110 Aspen Lane South Pacific, WA 98047 SUBJECT: GEOTECHNICAL EVALUATION Proposed 4-Lot Residential Short Plat KCPN 073900-0085 12727 SE Petrovitsky Road Renton, Washington Project No. 19-102-01 Dear Alex, This report presents the results of our geotechnical evaluation performed at the site of the proposed 4-lot residential development to be located on the subject property in Renton. The purpose of our investigation was to explore the site and provide geotechnical engineering recommendations for design of foundations and site grading for the proposed structures plus evaluations with regard to storm water BMP's. Our work was performed in accordance with the scope of work and conditions of our proposal dated March 16, 2019. Based on discussions with Camille Washington of Beyler Consulting, LLC and review of the proposed short plat plan provided to us, we understand that the existing onsite structure will be demolished and the subject property will be subdivided into 4 new lots with a new structure to be constructed on each of the new lots. Based on our experience, the structures will likely be two to three -stories in height. Garage areas will likely be incorporated into each structure with access from a planned easement along the west side of the property. No foundation plans or foundation loadings were provided, however based on the expected two to three story structure height we estimate that the bearing wall loads could be in the range of about 2 to 3 klf. If actual structural loads exceed the above values by more than 50%, this office should be notified. Castro April 15, 2019 SCOPE OF WORK Our geotechnical evaluation included subsurface exploration, engineering evaluations, engineering analyses and the preparation of this report. The scope of work included the following specific tasks: o Reviewed published geologic mapping (see Figure 1) as well as iMap topographic mapping and USDA soil mapping of the site area. o Performed a site reconnaissance to observe the existing conditions and locate the exploratory test pits. o Observed the excavation of seven test pits to depths ranging from 6.5 to 10+ feet. Approximate locations of the test pits are shown on Figure 2 and logs of the test pits are included in Appendix A. o Performed laboratory testing of selected soil samples including moisture content, grain size analyses and soil classification. o Performed geotechnical engineering analyses and evaluations of options for foundation support for the proposed structures as well as and site grading and drainage plus evaluations of appropriate storm water BMP's for at the site based on our site explorations. o Prepared this geotechnical report summarizing our findings and geotechnical related recommendations for the proposed development. EXISTING SITE CONDITIONS The property is located in the city of Renton, at the SW corner of the intersection of SE Petrovitsky Road and 128 th Avenue SE (see Figures 1 and 2). At the time of our field investigation the lot was occupied by the existing house as well as various construction debris, construction materials, a travel trailer, construction equipment and hauling trailers. The property has small but steep cut/fill slopes along the east and west property lines and a rockery at the southeast corner but the interior is gently sloped down from northeast to southwest. The topographic mapping shown in Figure 2 indicates about 22+ feet of elevation drop from the northeast corner to the southwest corner of the property. Our supplemental measurements within the undeveloped neighboring property to the south indicated a natural slope of about 10 degrees down to the west/southwest. - Project No. 19-102-01 Page 2 Castro April 15, 2019 The topographic mapping of Figure 2 and our own observations and measurements indicate that the western slopes range in vertical height from about 3 to 6 feet in the northern half and increase to about 8+ feet at the southwest corner of the property. Gradients of the western slope range from about 33 percent up to about 100 percent with the 100 percent slopes observed only within the northwestern site area where slope heights range from only about 3 to 6 feet. We did not observe any evidence of instability of the western slope such as slide scars, ground cracks or offsets. We observed the eastern slopes and rockery to range in vertical height from about 5 to 6 feet in the north to 8+ feet in the area supported by the rockery within the proposed Lot 1 limits. We noted that the rockery included large rocks with dimensions up to about 5 feet in the lower 2/3 of the rockery. Gradients of the eastern slopes ranged from about 100 percent up to near vertical. We noted some sloughing and raveling of the of the steeper eastern slopes but the rockery appeared to be performing adequately. We could not observe the conditions above the slopes on the neighboring property due to an existing fence. Vegetation on the property includes grass areas, shrubs and deciduous trees ranging from about 12" to 24" and evergreen trees (typically fir) that ranged from about 14" to 50". We noted many small alder trees ranging from about 4" to 8" on Lot 4 and we noted an angled line of fir trees ranging from about 12" to 22" within the west half of Lot 3 as shown in Figure 2. We observed second growth fir trees on the undeveloped property immediately south of the site to range up to about 36" to 42" in diameter with a 42" fir tree observed near the gate at the southeast corner of the site. We observed that the site had some trees of similar size to the neighboring south natural area trees including a 50" fir at the northeast corner of Lot 3, a 30" fir along the west property line and a 28" fir at the southwest corner of Lot 1. Subsoils Our evaluation of the subsurface conditions was based on review of the geologic mapping (see Figure 1), and our subsurface explorations. Approximate locations of the test pits are shown on Figure 2 and logs of the test pits are included in Appendix A. Subsoils encountered at all of the test pits except TP-3 and TP-6 generally consisted of a surface layer of loose to medium dense debris fill consisting of silty fine sand with debris such as asphalt, brick, wood, metal, roots and organics that extended to depths ranging from of about 2 to 8 feet at the test pit locations. Please note that at TP-1 and TP-2 the asphalt debris was very concentrated and at the TP-2 location we encountered such large asphalt debris that the excavator could not penetrate below a depth of about 6.5 feet. Natural soils were encountered at the existing surface at TP-3 and TP-6 and below the debris fill encountered at the other test pit locations (except TP-2). Within the eastern half of the site (TP-1, TP-3, TP-6 and TP-7) the natural soils were classified as silty fine Project No. 19-102-01 Page 3 Castro April 15, 2019 sand or silty fine -medium sand (see grain size analyses of Figures A-5 and A-6). Within the western half of the site (TP-4 and TP-5) the natural soils contained more fines and were classified as sandy silt/silty fine sand. The natural soils were typically loose to medium dense, becoming more dense with depth and hard cemented soils were encountered at depths of about 8.5, 91 6, and 7.5 feet at TPs 1, 4, 6 and 7 respectively. Note that at TP-5 no dense/hard cemented soils were encountered to the 10 foot depth. Ground Water No ground water surface was encountered in any of the test pits to the maximum exploration depths. The subsoils were generally classified as moist to very moist with moisture contents ranging from about 6 to 25 percent and were generally higher along the western side of the site and red -brown mottling of the natural soils was also only observed in the western test pits TP-4 and TP-5. Subsurface Variations Based on our experience, it is our opinion that some variation in the continuity and depth of subsoil deposits and ground water levels should be anticipated due to natural deposition variations and previous onsite grading. Due to seasonal moisture changes, ground water conditions should be expected to change with time. Care should be exercised when interpolating or extrapolating subsurface soils and ground water conditions between or beyond our test pits. Project No. 19-102-01 Page 4 Castro April 15, 2019 SITE EVALUATIONS General Review of published geologic mapping of the site vicinity (see Figure 1) indicates the site to be in an area mapped as ground moraine (glacial till) deposits (Qgt) which were deposited during the most recent glaciation (Vashon) which covered the Puget Sound area about 13,500 to 15,000 years ago. In addition the geologic map of Figure 1 shows areas of peat deposits (Qlp) mapped in the lowland areas to the west, south and east of the site. Based on the subsoils observed in our site explorations the natural site soils are most likely glacial till (Qgt) with thin surfical recessional outwash deposits. Based on our site observations and explorations it appears that the site has been modified by previous cut/fill grading. Natural soils are expected to be exposed at or near the existing ground surface within approximately the eastern half of Lots 1, 2 and 3 but the natural soils within approximately the western half of Lots 1, 2 and 3 and the most of Lot 4 are expected to be overlain by loose debris fill deposits that range in thickness up to 8+ feet. Structure Support Considerations As indicated above, the site is underlain by a variable thickness of loose debris fill ranges up to 8+ feet thick within the western half of Lots 1, 2 and 3 and most of the area of Lot 4. The existing debris fill and loose to medium dense weathered natural soils are not considered suitable for foundation support. Suitable natural bearing soils are expected to be at depths of about 2 to 5+ feet below the natural soil surface (below the existing fill deposits) which will range up to 10+ feet below the existing surface. Foundations must be extended through the existing fill and weathered soils to bear on dense/hard natural bearing soils or alternatively foundations could be supported on a zone of compacted structural fill over dense/hard natural soils. Based on our experience, optional means to mitigate the subsoil conditions at the site could include the following foundation systems: 1) Deep Spread Footings: This method requires existing fill and weathered soils within the foundation areas to be excavated (with proper excavation slopes for worker safety) to expose dense/hard natural bearing soils. Conventional spread footings would then be formed and supported directly on the exposed natural bearing soils and stem walls would be required to extend up from the deep footings to the above grade structure. 2) Lean Mix Concrete Monoliths: This method extends conventional spread footing support down to the deep bearing soils via lean concrete filled trenches. The method consists of excavating trenches along the foundation lines down through Project No. 19-102-01 Page 5 Castro April 15, 2019 the unsuitable fill and weathered natural soils to expose dense/hard natural bearing soils and immediately backfilling the trenches with lean concrete up to the minimum footing depth. Conventional spread footings can then be supported directly on the lean concrete monoliths. Considering the potential for caving and instability of trenches within the loose fill and weathered soils, in our experience this method is typically only practical for bearing soil depths of about 5 to 6+ feet. 3) Spread Footings on a Structural Fill Zone: This method includes the excavation (with proper excavation slopes) of a zone of existing unsuitable fill and weathered soils to expose dense bearing soils and backfilling the zone with properly compacted structural fill. Conventional shallow spread footings could then be supported directly on the zone of compacted structural fill. The zone of structural fill zone must extend down to bearing soils and laterally from the structure perimeter as recommended in this report. 4) Driven Pipe Piles: Pipe pile support is constructed by driving 2-inch to 3-inch diameter steel pipe to refusal into the bearing soils below the fill and unsuitable soils. Installation is normally accomplished with a tractor mounted hydraulic hammer system but can be accomplished using a hand-held pneumatic hammer for 2-inch piles. Piles larger than 2-inch should be load test. This method may not be suitable where the existing debris fill contains large amounts of concrete or asphalt debris such as was encountered in the area of Lot 1. 5) Helical Piers: Helical piers consist of one or more circular helixes welded to a support rod, which is "screwed" into the ground with a power torque head to refusal in the natural bearing soils below the fill and unsuitable soils. This method may not be suitable where the existing debris fill contains a large amount of concrete or asphalt debris such as was encountered in the area of Lot 1. Based on the conditions encountered in the area of Lots 1, 2 and 3 it appears that any of the 5 optional foundation systems described above could be considered for those lots. However, at Lot 4 options 1, 2 and 3 could be used but we would not recommend options 4 and 5 because the high concentration of asphalt debris observed within the fill at the TP-1 and TP-2 locations would likely prevent the piles/piers from penetrating through the fill to the bearing soils. The existing loose debris fill is also not considered suitable for direct slab -on -grade support. The best slab -on -grade support would be provided by full excavation of the unsuitable soils and replacement with structural fill, however as discussed above, full excavation could require 8+ feet of excavation and may not be practical. Alternatives could include the use of a structurally separate "floating" slab -on -grade supported on a limited zone of structural fill in subgrade areas. As a minimum we recommend that subgrade preparation for a structurally separate slab -on -grade or pavement include excavation of the existing fill and replacement with structural fill as required to provide a minimum of 2 feet of structural fill below the subgrade. Project No. 19-102-01 Page 6 Castro April 15, 2019 Recommendations for foundation design(s), retaining walls, site grading, site drainage and observations during construction are presented below in RECOMMENDATIONS. Slope Stability Evaluations The geologic map of Figure 1 shows no mapped landslides (QI) in the site vicinity. The property has small but steep cut/fill slopes and rockery along the east and west property lines. Based on our site explorations and observations, the eastern slope appears most likely to have been created by cut and fill grading while the western slope is most likely a fill slope. The topographic mapping of Figure 2 and our own observations and measurements indicate that the western slopes range in vertical height from about 3 to 6 feet in the northern half and increase to about 8+ feet at the southwest corner of the property. Gradients of the western slope range from about 33 percent up to about 100 percent with the 100 percent slopes observed only within the northwestern site area where slope heights range from only about 3 to 6 feet. We did not observe any evidence of instability of the western slope such as slide scars, ground cracks or offsets. We observed the eastern slopes and rockery to range in vertical height from about 5 to 6 feet in the north to 8+ feet in the area supported by the rockery within the proposed Lot 1 limits. We noted that the rockery included large rocks with dimensions up to about 5 feet in the lower 2/3 of the rockery. Gradients of the eastern slopes ranged from about 100 percent up to near vertical. We noted some sloughing and raveling of the of the steeper eastern slopes but the rockery appeared to be performing adequately. We could not observe the conditions above the slopes on the neighboring property due to an existing fence. The nature of any future slope failures affecting your property is expected to be predominantly shallow "skin" failures of the weathered natural surficial slope materials or rockery failures. The failures are most likely to occur during very heavy rainfall periods and/or during strong ground shaking from moderate to large earthquake events. The risk of structure damage varies with the distance from the toe/top of the slope. As with all hillside development, the owner must be aware of and accept the risk that future slope failures may occur and may result in damage to his property and/or neighboring property. Considering the height and inclinations of the slopes on the property it is our opinion that the potential for future shallow failures of the onsite slopes is moderate to high. Project No. 19-102-01 Page 7 Castro April 15, 2019 Slope Classifications and Recommended Slope Buffers and Setbacks Considering that the onsite slopes along the eastern and western property lines are at most about 8+ feet in height, they do not meet the criteria for a protected slope per RMC 4-3-050-G-5-a.ii and would be classified as a sensitive slope per RMC 4-3-050-G- 5-a.i. Considering that the slopes have gradients exceeding 40 percent, the landslide hazard per RMC 4-3-050-G-5-b.iii is indicated to be high. Per the table of RMC 4-3- 050-G-2, no slope buffer or structure setback is required for sensitive slopes nor is any required for slopes with high landslide risk. However, considering the slope conditions on Lots 1 and 2 and IBC seismic criteria for the site (see Seismic Hazards Evaluations below), we recommend a minimum buffer on Lots 1 and 2 of at least 10 feet plus a structure setback of 15 feet from the buffer (total 25 feet) from the toe of the east slope/rockery and the top of the west fill slope. For the minor slopes on Lots 3 and 4 we do not consider a slope buffer to be warranted but do recommend a structure setback of at least 15 feet from the toe of the east cut/fill slope and the top of the west fill slope on Lots 3 and 4. Erosion Hazard Evaluations Per RMC 4-3-050-G-5-c, the general site is classified as low erosion hazard but the east and west slope areas would be classified as high erosion hazard based on slope gradients. Currently the property is well vegetated and we observed no indication of ongoing erosion areas on the site. Based on our site explorations the subsoils are generally silt -sand -gravel mixtures and are considered to have a moderate erosion potential when exposed in graded or disturbed areas. We have provided recommendations for drainage control and erosion control during and after construction and in our opinion, erosion risk should be low if our recommendations are followed. Seismic Hazard Evaluations Although the site is indicated to be within a low risk seismic hazard area per RMC 4-3- 050-G-5-d, the Puget Sound region is generally a seismically active area. About 17+ moderate to large earthquakes (M5 to M7+) have occurred in the Puget Sound and northwestern Cascades region since 1872 (147 years) including the 2/28/01 M6.8 Nisqually earthquake and it is our opinion that the site will very likely experience significant ground shaking during the life of the proposed development. Based on a published study the site lies about 6 miles south of the mapped location of the Seattle fault and about 21 miles southwest of the estimated trace of the South Whidbey -Lake Alice fault which both have postulated maximum credible magnitudes of 7.0 to 7-5. Another study of the Vashon-Tacoma area presents evidence for the east - west trending Tacoma Fault which is indicated to pass through the south end of Vashon and the middle of Maury Island about 10 miles southwest of the site. The study Project No. 19-102-01 Page 8 Castro April 15, 2019 suggests that the Tacoma Fault and the Seattle fault may be linked by a master thrust fault at depth. The Seattle fault has been documented to have moved at its west end (Bainbridge Island) about 1000 to 1100 years ago and evidence of movement at the east end has also been documented. Some experts feel that the recurrence interval between large events on the Seattle Fault may be on the order of several thousands of years but our calculations indicate it may be on the order of 1200 to 1400 years. The activity of the documented Tacoma fault is considered to be on the same order as the Seattle fault. The recurrence of a maximum credible event on the South Whidbey fault is not known but some experts have assigned a recurrence of about 3000 years, however smaller events will occur more frequently as evidenced by the 5.3 event on May 2, 1996 which was attributed to that fault. In addition to Puget Sound seismic sources, a great earthquake event (M8 to M9+) has been postulated for the Cascadia Subduction Zone (CSZ) along the northwest Pacific coast of Oregon, Washington and Canada. The risk of a future CSZ event is not precisely known at this time but the time of the last CSZ event has been documented to have been 319+ years ago (January 1700). Published studies have reported intervals between past CSZ events to range from as little as 106 years to as long as 754 years but the reported intervals between events has not exceed 300 years since about 327 B.C. and since that time CSZ event intervals have averaged only about 234 years. Considering this, in our opinion a CSZ event should be expected in the near future. Considering all of the above, it is our opinion that the proposed structures will very likely experience significant ground shaking during their useful life. The 2018 International Building Code (IBC) requires that a Maximum Considered Earthquake Geometric Mean (MCEG) ground motion peak horizontal ground acceleration (PGA) be used for site liquefaction evaluations but the 2018 IBC Design Earthquake which is defined as 2/3 of the MCEG ground motions in ASCE 7 may be used for consideration in other geotechnical seismic site evaluations for new construction. The MCEG PGA for the 2018 IBC per ASCE 7 is based on consideration of both probabilistic ground motions with a 2475-year recurrence interval and deterministic ground motions based on a model of known fault locations and characteristics adjusted for site specific soil conditions. Per section 1803.5.12(2) of the 2018 IBC, the MCEG PGA for this site is indicated to be about 0.585g based on USGS Seismic Design Web Service Documentation of Design Maps of ASCE 7-16. We estimate the IBC Design Earthquake ground motion PGA for this site to be 0.39g per the definition in Chapter 11 of ASCE 7. Please note that the Design Earthquake ground motion PGA is not intended for structural analyses. Spectral accelerations per the 2018 IBC should be considered in structural design. This site is considered to be a Site Class C per the 2018 IBC and the referenced definitions presented in Chapter 20 of ASCE 7-10. Project No. 19-102-01 Page 9 Castro April 15, 2019 Considering the lack of evidence of any shallow ground water and the dense/hard nature of the natural glacial till soils underlying the site which are recommended for structure support, it is our evaluation that the potential for structure damage due to liquefaction, lateral spreading and settlement is low. The onsite slopes and rockery are considered to have a moderate to high potential for induced shallow failures depending on the earthquake magnitude and shallow ground water conditions at, the time of the earthquake. Our recommended slope buffer and setbacks discussed above considered the IBC seismic criteria for the site. Construction Considerations The current moisture content of much of the onsite fill and weathered soils is considered to be too high for proper compaction. The shallow soils will likely need to be dried back before proper compaction can be achieved or alternatively they could be exported and replaced with granular material in structural fill areas such as under pavement and slabs or in the optional structural fill zone for support of spread footings. Storm Water Infiltration Feasibility Review of USDA soil survey mapping of the site vicinity (see Figure 1) indicates the site to expose Alderwood gravelly sandy loam soils (AgC). The Alderwood soils are described as gravelly sandy loam to a depth of 7 inches and very gravelly sandy loam to a depth of to 59 inches. However, based on the subsoils encountered in our site explorations, the upper natural subsoils at the site appear to have less gravel than indicated by the soil survey mapping with USDA classifications typically ranging from gravelly sandy loam to loamy sand with occasional gravel. Design of the storm water infiltration system should be in accordance with Appendix C of the 2017 Renton SWM. Our evaluation of storm water infiltration feasibility is based on our understanding of the minimum requirements for infiltration systems per Appendix C of the 2017 Renton SWM as follows: o The infiltration zone must be located entirely within permeable natural soils ranging from medium to coarse sand and cobbles for full infiltration to fine sand, loamy sand, sandy loam or loam for limited infiltration (no infiltration in fill soils or silt/clay soils or hardpan). If fill soils or silt/clay soils are present, the infiltration zone normally must be deepened as required to penetrate into permeable natural soils below the unsuitable soils (if feasible). o Minimum thickness of the permeable natural soil infiltration zone plus capping layer is 2 feet (18 inch infiltration zone plus 6 inch minimum cover for trench). o Minimum depth below the base of the infiltration zone to the maximum wet season water table or to the surface of a impermeable layer (silt/clay or very Project No. 19-102-01 Page 10 Castro April 15, 2019 dense/cemented hardpan soils) is 3 feet for full infiltration but only 1 foot for optional infiltration. Therefore the minimum thickness of permeable natural soils above the maximum, wet season water table or a impermeable layer for optional infiltration is 3 feet (2 feet of infiltration zone plus 1 foot above the impermeable layer. o Infiltration devices are not allowed on slopes steeper than 25 percent and infiltration devices on slopes steeper than 15 percent or within 200 feet of a steep slope hazard area must be approved by a geotechnical engineer. o Infiltration devices must be setback at least 5 feet from any property line and may not be placed within a steep slope buffer. Based on our site explorations and laboratory testing combined with review of soil mapping and topographic mapping, we conclude that the current site topography was created by cut/fill grading and that most of Lot 4 and roughly the western half of Lots 1, 2 and 3 are covered with a surface layer of debris fill soils. Natural soils below the fill layer and exposed within the eastern half of Lots 1, 2 and 3 were found to be predominately silt/sand/gravel mixtures and are generally consistent with the USDA mapped Alderwood soils. Based on the grain size analyses results of Figures A-5 and A-6, the onsite natural permeable soils contain about 70 to 86 percent sand and gravel and about 14 to 30 percent silt and are classified as gravelly sandy loam to loamy fine sand with occasional gravel per the USDA system. Although no ground water was encountered on our site explorations, all of the test pits (with the exception of TP-2) encountered impermeable layers (hard cemented soils or silt soils) and/or soil color mottling indicative of seasonal ground water levels. The thickness of natural permeable soils above the impermeable layers ranged from about 2+ feet at TP-1 and TP-4 up to 4 to 8+ feet at TP-3, TP-6 and TP-7. In our opinion, full stormwater infiltration is not feasible for the site per Appendix C due the loam (silt) content of the natural soils on the site, the presence of fill soils and inadeguate soil thickness. However in our opinion, limited stormwater infiltration is considered feasible within the eastern half of Lots 1, 2 and 3 per the conditions of Section C.2.3.2 of the 2017 Renton SWM . We recommend that the infiltration systems be located at least 10 feet from proposed structures and not directly upslope of the proposed structures and systems should also be located at least 10 feet from the toe of the eastern slopes and existing rockery and at least 30 feet from the top of the western fill slope. Design of limited storm water infiltration systems should be in accordance with Appendix C of the 2017 Renton SWM. We recommend that infiltration systems within the eastern half of Lot 1 be designed per Sections C.2.3.3 and C.2.3.4 for fine sand or loamy sand soils. Assuming rainfall region SeaTac 1.0 or less, for each 1000 square Project No. 19-102-01 Page 11 Castro April 15, 2019 feet of impervious surface on Lot 1 the minimum infiltration trench length is indicated to be 21 feet and the minimum dry well volume is indicated to be 315 cubic feet. We recommend that infiltration systems within the eastern half of Lots 2 and 3 be designed per Sections C.2.3.3 and C.2.3.4 for sandy loam soils. Assuming rainfall region SeaTac 1.0 or less, for each 1000 square feet of impervious surface on Lots 2 and 3 the minimum infiltration trench length is indicated to be 36 feet and the minimum dry well volume is indicated to be 360 cubic feet. All infiltration systems should be constructed within the natural permeable soils at least 1 foot above the surface of the underlying impervious soil layers. Impervious soil layers or soil mottling were encountered in our explorations at depths of 8.5, 8-5+7 6.51 97 67 and 7 feet in TP-1 , TP-3, TP-4, TP-5, TP-6 and TP-7 respectively. The storm water infiltration systems must include adequate de -silting to remove suspended fines from the water. Silt deposition from dirty storm water will reduce the infiltration capability of the system. Periodic cleaning of the de -silting systems should be performed to maintain the system capability. In our opinion neither full nor limited storm water infiltration is considered feasible within Lot 4 due to the apparent debris fill coverage of the entire lot and the inadequate thickness (2+ feet) of suitable natural permeable soils below the fill and above the impermeable cemented soils as indicated by our explorations. In our opinion a gravel filled trench Basic Dispersion system could be used for Lot 1 provided that the trench is located at least 10 feet west (downslope) of the proposed structure and is setback at least 30 feet from the top of the western fill slope. The dispersion trench should be designed per Sections C.2.4, C.2.4.1 and C.2.4.4 of the 2017 Renton SWM We do not recommend the use of splash blocks (C.2.4.2) for this site. Project No. 19-102-01 Page 12 Castro April 15, 2019 RECOMMENDATIONS The following subsections present our recommendations for the foundation options discussed above plus recommendations for retaining walls, general site grading, support of pavements and slabs, plan review and observations and testing during construction. Deep Spread Footing Conventional spread footings deepened as required to existing fill and weathered soils to found directly on the recommended undisturbed dense/hard bearing soils (by full excavation of the overlying unsuitable soils) should provide good support for the proposed structures. Square footings should be at least 24 inches wide and continuous wall footings should be at least 18 inches wide. Footings may be designed based on an allowable vertical bearing pressure of 2000 psf. Settlement of spread footing foundations supported directly on undisturbed dense/hard bearing soils is expected to be within tolerable limits. For example, the estimated settlement of continuous wall footings carrying loads up to 3 klf are expected to be on the order of '/4to Y2 inch. Maximum differential settlement within the proposed structure is expected to be on the order of % inch. Settlements are expected to occur primarily during construction. For lateral design of footings, resistance to lateral loads can be assumed to be provided by friction acting at the base of foundations and passive pressure from existing soil backfill. A coefficient of friction of 0.4 may be assumed with the dead load forces in contact with the monolith surface. An allowable passive pressure of 150 psf/ft may be assumed for existing soils or compacted backfill using onsite soils. Lean Concrete Monolith Supports As an alternative to deepened spread footings, foundation loads may be transferred through the unsuitable fill and weathered soils to the recommended dense/hard bearing soils by a monolith of lean concrete. Conventional spread footing foundations may be supported directly on top of the completed concrete monoliths and designed per the recommendations for deep spread footings given above. The lean concrete monolith should be constructed along the planned foundation lines and should be constructed using lean concrete with a minimum compressive strength of 1000 psi. The width of an un-reinforced lean concrete monolith should be at least as wide as the planned footing or at least one-third of the monolith height, whichever is greater. Reinforced concrete monoliths should be designed by a structural engineer. Project No. 19-102-01 Page 13 Castro April 15, 2019 A suitable width trench should be excavated with a smooth edged excavator bucket (no teeth) to expose the dense/hard bearing soils under observation by our office and backfilled as soon as possible with the lean concrete to the planned footing elevation. No personnel should enter the trench excavation without proper shoring or a trench shield and the trench should backfilled with concrete as soon as possible to the footing elevation the same day as excavated. Structural Fill Zone Construction of a structural fill zone for support of spread footings will require the excavation of the existing unsuitable fill and weathered soils to expose the underlying dense/hard bearing soils and placement of properly compacted structural fill to the planned foundation grade. The zone of compacted structural fill should extend vertically to the dense/hard bearing soils encountered in our test pits at depths ranging from about 6 to 10+ feet below the surface. At the surface of the bearing soils the structural fill zone should extend horizontally from the edge of footing at least 5 feet or at a 1: 1 projection down from the planned edge of footing, whichever is greater. The structural fill zone should be constructed using approved imported sand and gravel materials. The compacted structural fill must meet the following criteria: o Must be compacted in accordance with our recommendations for structural fill placement presented below under "Site Grading" o Must be compacted to at least 95 percent of the maximum dry density as determined by the ASTM D 1557 test method. Spread Footings on a Structural fill Zone Conventional spread footings founded on a properly compacted structural fill zone should provide good support for the proposed structures. All footings should be founded at least 18 inches below the lowest adjacent final grade. Square footings should be at least 24 inches wide and continuous wall footings should be at least 18 inches wide. Footings may be designed based on an allowable vertical bearing pressure of 2000 psf. Settlement of spread footings supported on a zone of structural fill is expected to be within tolerable limits for a wood -frame structure. For example, the estimated settlement of continuous wall footings carrying loads up to 3 kif are expected to be on the order of about 1/2 inch or less. Maximum differential settlement between adjacent footings is expected to be on the order of 1/4 inch. Settlements are expected to occur primarily during construction. Project No. 19-102-01 Page 14 Castro April 15, 2019 For lateral design, resistance to lateral loads can be assumed to be provided by friction acting at the base of foundations and by passive earth pressure. A coefficient of friction of 0.4 may be assumed with the dead load forces in contact with onsite soils. An allowable static passive earth pressure of 250 psf per foot of depth may be used for the sides of footings poured against properly compacted structural fill. The bearing values indicated above are for the total dead load plus frequently applied live loads. If normal code requirements are applied for design, the vertical bearing values may be increased by 50% for short durations of loading which will include the effects of wind or seismic forces. Allowable lateral passive pressures may be increased by 33% for wind and seismic forces. Driven Pipe Pile Foundations Properly constructed pipe piles are considered a feasible foundation system alternative to excavation of the loose soils overlying the bearing soils. This type of support is constructed by driving 2-inch or 3-inch diameter steel pipe to refusal into the bearing soils below the debris fill and weathered natural soils. Bearing soils are expected to be encountered at depths ranging up to 10+ feet across the site based on our explorations. It should be noted that in Lot 4 we encountered nested asphalt slabs which could potentially prevent pile penetration to bearing soils and therefore pile foundations are not recommended in Lot 4 or in other areas with a high concentration of asphalt or concrete debris. Based on our experience, piles typically penetrate about 5 to 15 feet into the bearing soils before encountering refusal. Based on our experience, an allowable vertical downward capacity of 6 kips can be assumed for properly installed 2-inch diameter piles and capacities of 15 kips can typically be achieved for 3-inch diameter piles installed as recommended below (Factor of Safety = 2+) but 3-inch pile allowable capacities should be confirmed based on a site specific load test of the proposed pile and installation criteria. We recommend that all pile load tests and installation be observed by our office to verify -the allowable capacity and refusal criteria for the production piles. No uplift capacity should be assumed for driven pipe piles. No lateral support should be assumed for the piles. Resistance to lateral loads can be provided by battered piles (compression only) and by passive earth pressure against the sides of grade beams. An allowable static passive earth pressure of 150 psf per foot of depth may be used for the sides of grade beams poured against existing soils. Pile installation should be accomplished with a tractor mounted hydraulic hammer system with a hammer weight in the range of at least about 850 to 1100 pounds for 3- inch piles but 2-inch piles may be driven with smaller hammers. Refusal penetration rates for piles will depend on the hammer size and the load testing results. We Project No. 19-102-01 Page 15 Castro April 15, 2019 recommend that all pile installation and load testing be observed by our office to verify the adequate penetration into the bearing soils and refusal criteria for the production piles. For 3-inch and larger piles, load tests should be performed on representative piles prior to production pile installation to verify the appropriate allowable vertical capacity and refusal criteria. At least 3% of the piles should be load tested to confirm capacity. Testing should be accomplished according to the ASTM quick test procedure described in the ASTM D 1143-81 test method for piles under static axial compressive load. Allowable capacity should be based on test pile settlement of 1/4 inch or less for the design load and a Factor of Safety of 2.0+ of ultimate capacity. Pile capacity may be limited by the structural capacity of the pipe and connections which should be determined by the structural engineer. The piRe and couplers which form the pile, must be of structural quality (schedule 40+) and must be provided with a corrosion resistant coating (qalvanize . The pipe pile supports should be capped with a grade beam to transfer structural loads to the piles. The pile/grade beam system should be designed by a qualified structural engineer. If the pipe pile supports are designed and installed in accordance with the recommendations given above, the settlement of a single isolated pile is estimated to be on the order of 1/4 inch. Helical Pier Supports Helical piers are considered a feasible alternative foundation system to transfer loads through the upper soils into the underlying bearing soils. This system consists of one or more circular helixes welded to a support rod, which is "screwed" into the ground with a power torque head. We recommend the use of SS150 or SS175 type helical piers as per the A.B. Chance Co. or equivalent, using a double helix lead section with a 6-inch lead helix to help penetrate into the dense bearing soils. Both the lead section and extensions should be properly galvanized. Piers should penetrate through the weathered surface soils and into the dense/hard natural bearing soils. It should be noted that in Lot 4 we encountered nested asphalt slabs which could potentially prevent helical pier penetration to bearing soils and therefore pier foundations are not recommended in Lot 4 or in other areas with a high concentration of asphalt or concrete debris. Based on our experience, piers typically penetrate about 5 to 15 feet into the bearing soils before encountering refusal. The depth to bearing soils is expected to range up to 10+ feet across the site and generally the deeper bearing soils will be in the western portion of the site. Minimum spacing between pier centers should be at least 3 diameters or 3 feet whichever is greater. Minimum penetration into the bearing soils should be at least 3 feet for downward loaded anchors and 5 feet for uplift anchors (see below). Adequate Project No. 19-102-01 Page 16 Castro April 15, 2019 penetration into the bearing soils and installation torque should be verified by a representative of this office during the production pier installations. Recommended allowable single pier static downward capacity should be taken as 3.33 times the installation torque in ft-lbs (3000 ft-lbs torque fora 10 kip allowable capacity). Based on static analyses and our experience, we expect that capacities of 10 kips are achievable and capacities as high as 20 kips could be developed with larger installation equipment. The allowable capacity may be used for dead loads plus frequently occurring live loads. The recommended pier capacity may be increased by 33% to resist total vertical downward loads which include a transient component such as loads due to wind forces or seismic shaking. No lateral support should be assumed for helical piers. Resistance to lateral loads can be provided by battered piers (compression only) and by passive earth pressure against the sides of grade beams. An allowable static passive earth pressure of 150 psf per foot of depth may be used for the sides of grade beams poured against existing soils. The helical pier system should be designed by a qualified structural engineer. Pier capacities may be limited by the structural capacity of the extension rods and connections which should be determined by the structural engineer. The helical pier supports should be capped with a grade beam or other properly designed connection to transfer structural loads to the piers. If pier foundations are designed in accordance with the recommendations given above, the settlement of a single isolated pier is estimated to be less than 1/4 inch. Calibration of any pressure operated torque indicators equipment should be verified by correlation to torque shear pin torque indicator during the first pier installation. Retaining Wall Design for the production installation measured via an electronic or Cantilevered retaining walls as referred to in this report are walls which yield or move outward during and after backfilling. Actual wall movements will depend on the wall design and method of backfilling and can range from 0.1% to 0.3% of the wall height. Design pressures for cantilevered walls given below assume that the top of the wall will deflect at least 0.15% of the wall height. Design of wall foundations should be in accordance with the recommendations presented in this report. Static design of permanent cantilevered retaining walls which support a horizontal surface of properly compacted clean free -draining granular material may be based on an equivalent fluid density of 40 pcf. These pressures assume that there is no water pressure with the wall backfill. An additional uniform lateral pressure due to backfill surcharge should be computed using a coefficient of 0.27 times the uniform vertical surcharge load. Project No. 19-102-01 Page 17 Castro April 15, 2019 Static design of walls supporting horizontal backfill and structurally braced against movement should be based on an equivalent fluid density of 60 pcf. This pressure assumes that the wall supports a horizontal backfill of properly compacted free -draining granular material and that there is no water pressure behind the wall. Uniform lateral pressure due to a uniform vertical surcharge behind a braced wall should be computed using a coefficient of 0.43 times the uniform vertical surcharge load. Seismic design of retaining walls should include a dynamic soil loading. Dynamic soil pressure should be assumed to have an inverted triangular distribution. Based on a 0-39g IBC ground motion level the dynamic soil pressure at the top of the wall should be at least 26H (psfl where H is the height of the wall above the footing base. The dynamic soil pressure should diminish linearly to zero at the base of the wall. Combined static plus dynamic soil pressure should be used for seismic design of the walls. Care should be exercised in compacting backfill against retaining walls. Heavy equipment should not approach retaining walls close enough to intrude within a 1:1 line drawn upward from the bottom of the wall. Backfill close to walls should be placed and compacted with hand -operated equipment. Recommendations for placement and compaction of structural fill are presented under "Site Grading". Design wall pressures given above assume no water pressure behind the wall. We recommend that a drainage zone be provided behind all walls and a adequate drain system be provided at the base of the walls. Wall drains should consist of a four -inch diameter perforated PVC drain pipe placed in at least one cubic foot of drain gravel per lineal foot along the base of the wall. Drain gravel should be washed material with particle sizes in the range of 3/4 to 1-1/2 inches. As a minimum, the drainage zone within the upper wall should consist of a Miradrain drainage mat or equivalent attached to the wall surface for the full height and embedded into the drain gravel surrounding the perforated PVC drain pipe at the base of the wall. As an alternative a sand drainage zone could be placed the full height of the wall with a horizontal width equal to at least 1 foot. Backfill within the drainage zone should be a clean sand/gravel mixture with less than 5 percent fines based on the sand fraction. A membrane of Mirafi 140 filter fabric or equivalent should be provided between the drainage zone material and onsite silty soil backfill. The drainage zone backfill should be capped with 12 inches of silty soils to reduce surface water infiltration. Site Grading Site grading is expected to consist primarily of subgrade preparation for construction of foundations, slabs and pavements. Recommendations for site preparation, temporary excavations, structural fill and subgrade preparation are presented below. Project No. 19-102-01 Page 18 Castro April 15, 2019 Site Preparation: Existing fill soils and loose/medium dense natural soils should be stripped from planned structural fill areas. Debris and trash, plus rocks and rubble over 6 inches in size, should be removed from the subgrade. Subsoil conditions on the site may vary from those encountered in the test pits. Therefore, the soils engineer should observe the prepared areas prior to placement of any new fills. Temporary Excavations: Sloped temporary construction excavations may be used where planned excavation limits will not undermine the adjacent existing structures or interfere with other construction. Based on the conditions observed at the site it is our opinion that temporary excavations which will require workers to enter them can be made vertically to 4 feet but deeper excavations in un-saturated soils above the water table should be sloped no steeper than 1:1 (horizontal: vertical). If ground water is encountered, dewatering and flatter slopes will be required. Where there is not enough room for sloped excavations, shoring should be provided. It should be noted that the contractor is responsible for maintaining safe construction excavations. Structural Fill: On site soils may be used for general structural fill (subject to final approval during construction) provided that the soil moisture content is suitable for compaction and they do not contain any organics. All imported fill should be clean, sand and gravel materials free of organic debris and other deleterious material. Structural fill should be placed in horizontal lifts not exceeding 8 inches in loose depth and compacted to the required density. General structural fill should be compacted to at least 90 percent of the maximum dry density as determined by the ASTM D1557 test method unless otherwise specified. Structural fill within the optional structural fill zone for foundation support should be compacted to at least 95 percent of the maximum dry density as determined by the ASTM D 1557 test method. Pavement and Slab Subgrade Preparation: Concrete slabs -on -grade should be supported on a subgrade consisting of at least 24 inches of general structural fill. Asphalt pavement sections (AC and base course) should be supported on a subgrade consisting of at least 6 inches of crushed gravel over 24 inches of general structural fill In driveway areas a minimum 8-inch depth of crushed gravel should be provided above the general structural fill. All topsoil and organic soils in the subgrade areas should be excavated and replaced with compacted structural fill. The imported crushed gravel fill should be compacted to at least 95 percent of the maximum dry density as determined by the ASTM D 1557 test method. Risk of slab cracking can be reduced by placing 2-way reinforcement steel, and greater excavation and replacement of the existing soils with new structural fill. If interior concrete slabs are constructed they should be underlain by a polyethylene vapor barrier of at least 6 mil thickness. Project No. 19-102-01 Page 19 Castro April 15, 2019 Drainage Control Adequate positive drainage should be provided away from the structures and on the site in general to prevent water from ponding and to reduce percolation of water into subsoils adjacent to foundations. Granular backfill in non-structural areas should be capped with 6 inches of onsite soils. A desirable slope for surface drainage is 2% in landscaped areas and 1 % in paved areas. Roof drains should be tightlined into the storm drain system (no discharge by splash blocks). Perimeter drains consisting of a four -inch diameter perforated PVC drain pipe placed in at least one cubic foot of washed gravel zone per lineal foot that is fully encapsulated with Mirafi 140 filter fabric or equivalent along the perimeter foundations. Erosion Control Onsite materials are expected to be erodible when exposed to concentrated water flow. Siltation fences or other suitable detention devices should be provided around soil stockpiles and around the exposed soil areas during construction to control the transport of eroded material. The lower edge of the silt fence fabric should have "J" shaped embedment in a trench extending at least 12 inches below the ground surface. Exposed final graded soil areas should be covered with anon -erosive surface covering or planted immediately with grass and deep rooted plants to provided permanent erosion control. In addition we recommend that the exposed soil surfaces of the site be temporarily covered with straw mulch or other suitable erosion resistant material during the wet season (11/1 through 3/31) if final erosion control measures are not completed. Plan Review This report has been prepared to aid in the evaluation of this site and to assist the owners and their consultants in the design and construction of the project. It is recommended that this office be provided the opportunity to review the final design drawings and specifications to determine if the recommendations of this report have been properly implemented and to make any supplemental design recommendations which may be required. Observations and Testing During Construction Foundation recommendations given in this report are based on the assumption that foundation construction will be observed by our office to see that foundations penetrate existing unsuitable soils and are supported on the recommended bearing soils. All structural fill and subgrade areas should be observed by a representative of this office after stripping and prior to placing fill. Proper fill placement and compaction should be verified with field and laboratory density testing by a qualified testing laboratory. Infiltration systems installation should be observed, by our office to confirm that our recommendations were followed. Project No. 19-102-01 Page 20 Castro April 15, 2019 CLOSURE This report was prepared for specific application to the subject site and for the exclusive use of Mr. Alex Castro and his representatives. The findings and conclusions of this report were based on the data obtained from our subsurface explorations. Our evaluations based on that data were prepared with the skill and care ordinarily exercised by local members of the geotechnical profession practicing under similar conditions in the same locality. We make no other warranty, either express or implied. Variations may exist in site conditions between those described in this report and actual conditions encountered during construction. Unanticipated subsurface conditions commonly occur and cannot be prevented even by making site specific explorations and performing reconnaissance. Such unexpected conditions frequently require additional expenditures to achieve a properly constructed project. If conditions encountered during construction appear to be different from those indicated in this report, our office should be notified. Respectfully submitted, GEOSPECTRUM CONSULTANTS, INC. ames A. Doolittle Principal Engineer Encl: Figures 1 and 2 Appendix A Dist: 1/Addressee via email 1/ Beyler Consulting, LLC via email I HO(P71REES 3119/ Project No. 19-102-01 Page 21 I ref 1 ogi c map of the Renton wr). , uadrangle by D. R. Mullineaux., USGS Map GC,)-405,, 1965 Enlarged Scale: --1 "=1 000 SITE VICINITY GEOLOGIC MAP GEt�SPECTRUM CONSULTANTS, INC. Proposed 4-Lot Residential Short Plat 12727 SE Petrovitsky Road Sl Renton Washington Proj. No.19-1021 Date 4/,119 1 Figure 1 fAf F- ROW DED 2 770:1 SF 0.018:E ACREZ 4. COMA, 002 FOUND Re5AR & CAP LS #30450 EASEMENT FOR SLOPES PER—' ZEC, NO. 85-,008C.1,09 & REC.�..,,,,,�,4'ROWDED 9662" .51150. SEE N.O. 87051 25' :,E.s-.RjCTJ0N 2 & 3. SHEET i ;TERSECITION OF & 4290-9 ON THE 3KY POI 2 4:,.A.ID, PL SE. of "-K) NUMEN7175 -7- __1 JIfU.U.I_ f r_ GEOSPECTRUM CONSULTANTS, INC. .... . .... . .... ........ . .. ............ ........ ..... 'A 1, FOUPjD 1/2" PEBAR & t::AFG;�4.�Fv_ 0. I,ijii x o. IS, OF CA S75002'2 I "E 5. i 27',W) FOUND 2" 3,z�45S 001"IF CASE Wl'PUNC.-H DOWN 0.85 tHEL D) FOUND 112" REBAP & CAP G4 INC-#304.50 LU %n co 101 EASEPIEPff 70 I-P MAINTAIN SLOPES Pr - NO, 20000--)13001594 CA VAIN -w/pUN!-H 03 CL T\ P-3 N TR 3 co a to 32 31 )LOUNf) 2!' BRASS Damtc CASE WIPUNCH DOWN 0 (DIST FROM CA LI:6j SE 176TH ST rioe &JPJLJLJL:A Jt:JA'XJC- IJ%JXNt& L JL %ill Jr JUIALI Proposed 4-Lot Residential Short Plat 12727 SE Petrovitsky Road Renton, Washington Proj. Nol 9-102 Date 4/19 Figure 2 Castro April 15, 2019 APPENDIX A FIELD EXPLORATION Our field exploration included a site reconnaissance and subsurface exploration program. During the site reconnaissance, the surface site conditions were noted, and the locations of the test pits were approximately determined. Elevations of the test pits were based on the topographic mapping provided to us (see Figure 2). The test pits were excavated with a track -mounted excavator. Soils were continuously logged and classified in the field by visual examination, in accordance with the ASTM Soil Classification system. Logs of the test pits are presented on the test pit summary sheets A-1 and A-4. The test pits summaries include descriptions of the soils and pertinent field data. Soil consistency and moisture conditions indicated on the logs are interpretations based on the conditions observed in the field. Boundaries, between soil strata indicated on the logs are approximate and actual transitions between strata may be gradual. Grain size analyses testing were performed on selected samples from the test pit explorations. Results of the grain size analyses are presented in Figures A-5 and A-6. Project No. 19-102-01 Page 22 TEST PIT NO. 1 Logged by JAD Date: 4/2/19 Elevation:408' Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments 0 SM Silty fine Sand with gravel loose moist brown FILL &SMwSt�ihso�me�al�,� 3b1' to 2' nested bl ck 2 silty saWd T brown 4 & boulder to 2' 6— 1 ................... — loose/ moist light 11.1 SM Silty f-m Sand with occ gravel m.dense to brown moi8— t------------------------------ ------------------- ----------------- very cementhard moist 7.4 10— Maximum depth 9.5 feet. — No ground water encountered. 12- 14— TEST PIT NO. 2 Logged by JAD Date: 4/2/19 Elevation: 404' Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments 0 M SM Silty fine Sand w/occ. gravel 1Lose moist ki J!y h t FILL 2 & AC pavement fragmentc.. own medium dense 4 5.8 AC slabs 3".y 1) to 2'+ nested black x�Yih some silty sand & occ. grave 9 6 brown on 10 12 Maximum depth 6.5 ft.. due to ref sal on large slab. 14] No ground water encountered. GEOSPECTRUM CONSULTANTS., INC. Proposed 4-Lot Residential Short Plat ...... -------- m.gM 01 01111U! 1:1 & 1: 0 , 11 0 12727 SE Petrovitsky Road nn ....... 1 11 .. W: RNW: NOUN 10 Geotechnical Engineering and Earth Sciences Renton, Was hin gton Proi. No. 19-102 Date 4/19 Figure A-1 TEST PIT NO. 3 Logged by JAD Date: 4/2/19 Depth Blows Class. Soil Description 0 SM Silty fine -medium Sand 21 4 6 8 10 12 14 Consistency Moisture Color m.dense .................... moist dk brn dense ra �ro�vn ................... dense ........................................ with occ gravel Maximum depth 8.5 feet. No ground water encountered. ------------------- moist light to brown verv. TEST PIT NO. 4 Elevation:408' W(%) Comments I W** M*O Logged by JAD Date: 4/2/19 Elevation: 404' Depth Blows Class. Soil Description Consistency Moisture Color W(%) Comments 0 2 SM &AC ISilty fin - me iand w/occgravel c- ��cramment loose moist�r��vn ck 8.0 FILL pave ertiragment 4— 6 -------------------------------------- moist to I_ ....... V ........... N Sandy lilt/Silty ve fine sand occ. grav/%l very moist e- d i'u" m' Te ___9�oflrn MY & Pe r5 ........................................... ... .0 - se� ------ ------------------- 8— some cementation dense very5 "-h-'td --------- MQ1 -1 --------- --- dte!V'bmj 10- 12— Maximum depth 9.5 ft. No ground water encountered. 14 GEOSPECTRUM CONSULTANTS., INC. Proposed 4-Lot Residential Short Plat 12727 SE Petrovitsky Road Ing jgg! US: ill: kGeotechnical Engineering and Earth Sciences Renton, Washington Proj. No.19-102 Date 4/19 Figure A-2 TEST PIT NO. 5 Logged by JAD Date: 4/2/19 Depth Blows Class. Soil Description 0 SM Silty fine and wi h gravel s I f raci t I Sri W008 2 s�e�eence�nP9st 4 6 8 10 12 14 . t Toot's --------------------------- .... organics---------------------- Elevation:401' Consistency Moisture Color W(%) Comments loose moist brown FILL to 10.5 very gray - moist rown .................... & black very 21.8 most light brown ... brn-g"hl ..SM.... ... .. $ilty. Fine S--and -with sod",o'ss' . ..................... ­ ------- --- 11 - ------ 11 --- -- --&'m- --- ­' ML Sandy Silt, mottled m.dense �rr��ci=�r 22.8 Maximum depth 10 feet. No ground water encountered. TEST PIT NO. 6 Logged by JAD Date: 4/2/19 Depth Blows Class. Soil Description Consistency Moisture Color 0 ��I/ Silty fine Snd/fine Sand dense moist �rownd- 2weaKly cemented to 4 no cementation ------------------- edium Tense very ................. kiy h t moist own 6 'Vety -cemented ------ .............. ------------------- n ___geg�. a . ..... ... gray-:brn 10 12 Maximum depth 7 feet.. 14 No ground water encountered. Elevation: 406' W(%) Comments 13.6 11.9 GEOSPECTRUM CONSULTANTS, INC. Proposed 4-Lot Residential Short Plat 12727 SE Petrovitsky Road Geotechnical En-gineenng and Earth Sciences Renton, Washington Proj. No. 19-102 Date 4/19 Figure A-3 TEST PIT NO. 7 Logged by JAD Date: 4/2/19 Depth Blows Class. Soil Description 0 SM Silty fine Sand with gravel 2 tnnqnil & ront-q 41 1 RV/ I Silty fine Sand/fine Sand Consistency Moisture loose moist to . ................... Tedium As very e e moist ................... 6 dense ......................................... I ................... cemented 81 �eWse/ 10 12 Maximum depth 8.5 ft. 1 No ground water encountered. 14 Elevation: 404' Color W(%) Comments kiowyht n Ft --- - ------- ... dkbrowr �ed- 15.4 rown brown ................. �i�ht own ----------------- 9 rm 19.2 ro n GEOSPECTRUM CONSULTANTS, INC. Proposed 4-Lot Residential Short Plat 12727 SE Petrovitsky Road Geotechnical Engineering and Earth Sciences Renton, Washington Proj. No.19-102 Date4/19 Figure A-4 SIZE OF OPERIAG W--INCAES U.S. STANDARD SIEVE SIZE HYDROMETER 100 100 s0 so so so 70 �. 70 w 60 60 m w s0 so z U. z 40 U 40 w o. 30 30 20 20 10 1a 1000 100 10 1.0 0.1 0.01 0.001 GRAIN SIZE IN MILLIMETERS COBBLES Coarse I Fine Coarsel Medium I Fine Silt clay GRAVEL SAND FINE GRAINED SYmbol _ Location USGS USDA Sample # Depth % Moisture % Passip3 #200 TP-6 1 ' - 2' 13.6 18.1 18.1 TP-6 3' - 4' 11.9 17.8 18.7 TP-7 2.51- 3.5' 15.4 14.0 16.0 GRAIN SIZE ANALYSES RESULTS GEOSPECTRUM CONSULTANTS, INC.INC12727 roposed 4-Lot Residential Short Plat ................:sue::......: SE P etrovitsky Road �.:::.:: �. :: •:.:::::ti::•:.::::.;. v. • Renton Washion ngton Go©t®chnlca/ EngrJn�►��JnB► crnc� Ec�rrrt�/� Sc/s►nces Proj. No.19-102 Date 4 / 19 Figure A-6