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HomeMy WebLinkAboutSWP272313(1) SUBSURFACE EXPLORATION, GEOLOGIC HAZARD AND GEOTECHNICAL ENGINEERING REPORT PRELLWITZ SHORTPLAT Renton, Washington p rin-1^,�re-d 1,0r MR. DOUG PRELLWITZ Project No. 960902 October 25, 1996 C) eo ENGINE RING, INC. ';`alG@t6ti+� LiCi73 Geo ,Source EIV G11VEE MI G, 11V C. GEOTECHNICAL ENGINEERING October 25, 1996 Project No. 960902 SUBSURFACE EXPLORATION , GEOLOGIC HAZARD AND GEOTECHNICAL ENGINEERING REPORT PRELLWITZ SHORTPLAT 11200 BLOCK OF NE 36th STREET RENTON, WASHINGTON I. PROJECT AND SITE CONDITIONS ■ INTRODUCTION This report presents the results of our subsurface exploration, geologic hazard and geotechnical engineering study for the abovementioned project. The proposed lot layouts and approximate locations of the explorations accomplished for this study are presented on the Site and Exploration Plan, Figure 1. In the event that any changes in the nature, design or lot locations of the houses are planned, the conclusions and recommendations contained in this report should be reviewed and modified, or verified, as necessary. Our study was to address potential geologic hazards from seismic and erosion considerations. We were also asked to address the concern of liquefaction. It was our opinion that slope stability was not considered an issue due to the low slope angles present on the site. Authorization Written authorization to proceed with this study was granted by Mr. Doug Prellwitz, owner. Our study was accomplished in general accordance with our scope of work/contract letter dated August 29, 1996. This report has been prepared for the exclusive use of Mr. 77 .. 2 Doug Prellwitz and his agents, for specific application to this site. Within the limitations of scope, schedule and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our report was prepared. No other warranty, expressed or implied is made. Our observations, findings, and opinions are a means to reduce the risks inherent to property development. ■ SITE DESCRIPTION The property was situated in the 11200 block of NE 36th Street in Renton, Washington. The 250 foot by 300 foot rectangular parcel gently sloped down toward the northwest at an approximate slope of 6H:1V (horizontal:vertical). Total elevation change across the property was on the order of 48 feet. Vegetation consisted of scattered deciduous and evergreen trees with moderate undergrowth. Nine lots are proposed with one extra area for stormwater control (Tract A). ■ SUBSURFACE EXPLORATION Our field study included excavating six exploration pits to gain information about the site. All of the pits were excavated with a tractor-mounted backhoe and were continuously logged by a geotechnical engineer/geologist from our firm. The various types of sediments as well as the depths where characteristics of the sediments changed are indicated on the exploration logs presented in the Appendix. The depths indicated on the logs where conditions changed' may represent gradational variations between sediment types in the field. Our explorations were approximately located on a topographic survey prepared by Touma Engineers of Kent, Washington. The conclusions and recommendations presented in this report are based on the exploration pits completed for this study. The number, location, and depth of the explorations were completed within site and budgetary constraints. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may sometimes be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of variation from the field explorations may not become fully evident until construction. If variations become known, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes. ■ SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, visual reconnaissance of the site, and past experience in the area. The overall geology of the site is an upper layer of relatively loose sand underlain by Vashon-age glacial sand. This is discussed in more detail below. Stratigraphy A thin layer of organic topsoil covered the site, ranging in thickness from 4 to 16 inches. Beneath the topsoil the native soils consisted of about 3 to 4 feet of loose to medium dense, dry to damp, tan, clayey, silty, fine sand. The roots from the trees penetrated into this layer. Natural soils underlying the loose sands consisted of glacially-compacted, very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. This material was overrun by several thousand feet of ice during the last glacial advance which resulted in a compact soil possessing high strength and low compressibility, and relatively low permeability characteristics. This material is very similar to the glacial Tills found in the Puget Sound region. Hydrology No surface water was encountered at the time of our field work. Ground water seepage was not encountered in our exploration holes at the time of our field study, however, perched water may be encountered during wet periods of the year atop the very dense, silty sand. Perched water occurs when surface water infiltrates down through surficial permeable soils and becomes trapped or "perched" atop the very dense, silty sands which have a comparatively low permeability. It should be noted that fluctuations in the level of the ground water may occur due to the time of the year and variations in rainfall. 4 October 25, 1996 Project No. 960902 II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards and mitigations are based on the geologic, slope, and potential ground water conditions anticipated on the site. The areas of concern to be addressed include seismic (including liquefaction), and erosion (including sediment transport). ■ SEISMIC HAZARDS AND MITIGATION Earthquakes occur in the Puget Lowland with great regularity. Fortunately, the vast majority of these events are small and are usually not felt by man. However, large earthquakes do occur as evidenced by the 1949, 7.2 magnitude event and the 1965, 6.5 magnitude event. The 1949 earthquake appears to have been the largest in this area during recorded history. Evaluation of earthquake return rates indicate what to expect within the life of the structure (50 to 100 years): an earthquake of magnitude between 5.5 and 6.0 will likely occur within the next 8 to 12 years; longer ranging, an earthquake of magnitude 6.6 to 7.2 will likely occur within the next 50 to 100 years. The City usually requires that engineering design be for a 100-year seismic event. On this site, there are 2 types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture and 2) the ground motion response. We were also asked to address the potential for seismically induced liquefaction. The potential for each of the hazards to adversely impact the site is discussed below. Surficial Ground Rupture Generally, the largest earthquakes which have occurred in the Puget Sound area are sub- crustal events with epicenters ranging from 50 to 70 kilometers in depth. No surficial faulting or earth rupture as a result of deep seismic activity has been documented, to date, in the tri-county Region. It is our opinion based on existing geologic data that the risk of surface rupture impacting the site is low. Ground Motion Response Based on the encountered site stratigraphy, local geology and visual reconnaissance of the site, it is our opinion that any earthquake damage to a proposed structure founded on the recommended bearing strata, and following our foundation and drainage recommendations, would be caused by the intensity and acceleration associated with the event and would not s be compounded by the site geology. Because of this fact, we recommend that seismic design of a house follow the minimum requirements of UBC standards. Liquefaction Potential Four conditions are required for a site to have a liquefaction potential; 1) The soils must consist of a uniform sand with a grain size distribution falling within a specific narrow range, 2) the sand must be in a loose condition, 3) the sand must be saturated (be below the water table), and the earthquake must have a duration of at least 20 seconds. The risk of liquefaction for this site is considered non-existent. The grain size distribution of the existing sand does not fall within the specified range, it is medium dense below 3 feet and the sloping, very dense layer would drain by gravity, thereby precluding any rise in a water table. ■ EROSION HAZARDS AND MITIGATION The surficial, loose, sand represents a moderate erosion hazard. The loose nature of the sand will allow it to be eroded by rain. The erodability of the underlying, very dense sand is considered low. To mitigate and reduce the erosion hazard and offsite sediment transport potential, we recommend the following: • Soils which are to be reused around the site should be stored in such a manner as to reduce erosion. Protective measures may include, but are not necessarily limited to, covering with plastic sheeting or the use of hay bales and/or silt fences. • In order to reduce the potential for erosion, we recommend that clearing not be done on the sloping areas unless they are replanted and stabilized. They must also be protected during the interim by plastic sheeting or other means. Straw mulching over hydroseeding, fiber-reinforced hydroseeding, or other approved means should be used to re-establish ground cover. • All storm water from impermeable surfaces, including paved or concrete driveways and roofs, should be directed into a tightlined City-approved storm water system which discharges away from slopes. Uncontrolled discharge on sloping areas may promote erosion. b 6 October 25, 1996 Project No. 960902 III. DESIGN RECOMMENDATIONS ■ INTRODUCTION Our exploration indicates that, from a geotechnical standpoint, the parcel is suitable for single family residences provided the risks discussed are accepted and the recommendations contained herein are properly followed. The distribution of foundation loads of the wood- frame structures are expected to be typical; no concentrated loads are anticipated. Because our explorations indicate that the uniform, very dense sands (about 3 to 4 feet in depth) are capable of providing suitable building support, spread footing foundations may be utilized. ■ SITE PREPARATION Site preparation of planned building and road/parking areas should include removal of all trees, brush, debris and any other deleterious material. Additionally, the upper organic topsoil should be removed and the remaining roots grubbed. Areas where loose surficial soils exist due to grubbing operations should be considered as fill to the depth of disturbance and treated as subsequently recommended for structural fill placement. We recommend that road areas be proofrolled with a loaded dump truck to identify any soft spots; soft areas should be overexcavated and backfilled with structural fill. Loose sands should be stripped down to the underlying medium dense or very dense sands. Since the density of the soil is variable, random loose/soft pockets may exist and the depth and extent of stripping can best be determined in the field by the Geotechnical Engineer. Because of the many variables which can affect the required depth of stripping, it is our opinion that it is inappropriate to give exact stripping depths. It is important to understand that the quantity of soils to be stripped can increase dramatically due to rain-softening and equipment disturbance. In all actuality, the amount of stripping will probably be greater than estimated from the exploration logs. ■ STRUCTURAL FILL There is a possibility that structural fill will be necessary to establish desired grades. All references to structural fill in this report refer to subgrade preparation, fill type, placement and compaction of materials as discussed in this section. After overexcavation/stripping has been performed to the satisfaction of the Geotechnical Engineer, the upper 12 inches of exposed ground should be recompacted to 90 percent of the Modified Proctor Maximum Density using ASTM:D 1557 as the standard. If the subgrade contains too much moisture, adequate recompaction may be difficult or impossible to obtain and should probably not be attempted. In lieu of recompaction, the area to receive fill should be blanketed with washed rock or quarry spalls to act as a capillary break between the new fill and the wet subgrade. After recompaction of the exposed ground is tested and approved, or a free-draining rock course is laid, structural fill may be placed to attain desired grades. Structural fill is defined as non-organic soil, acceptable to the Geotechnical Engineer, placed in maximum 8 inch loose lifts with each lift being compacted to 95 percent of the Modified Proctor Maximum Density using ASTM:D 1557 as the standard. The top of any above-grade compacted fill upon which a building will be founded should extend horizontally outward a minimum distance of 5 feet beyond the outer edge of the perimeter footings before sloping down at an angle of 2H:1V (horizontal:vertical). The contractor should note that any proposed fill soils must be evaluated by GeoSource Engineering prior to their use in fills. This would require that we have a sample of the material 72 hours in advance to perform a Proctor test and determine its compaction curve which is required for field testing. Soils in which the amount of fine-grained material (smaller than No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture-sensitive. Use of moisture-sensitive soil in structural fills should be limited to favorable dry weather conditions. The onsite soils generally contained significant amounts of silt and are considered moisture-sensitive. In addition, they should not be used for backfilling directly against walls. Construction equipment traversing the site when the soils are wet can cause considerable disturbance. If fill is placed during wet weather or if proper compaction cannot be obtained. a select import material consisting of a clean, free-draining gravel and/or sand should be used. Free- draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve traction. Geotechnical Construction Monitoring A representative from our firm should inspect the stripped subgrade and be present during placement of structural fill to observe the work and perform a representative number of in- place density tests. In this way, the adequacy of the earthwork will be evaluated as filling progresses and any problem areas may be corrected at that time. It is important to understand that taking random compaction tests on a part-time basis will not confirm the uniformity or acceptable performance of a fill. As such, we are available to aid the owner in developing a suitable monitoring and testing program. 8 ■ FOUNDATIONS Spread footings may be used for building support when founded on the lower very dense natural sands. The upper sands are loose and variable for the most part and we recommend that footings bear on the lower, very dense sand layer. The following design strategy summary is discussed in more detail below: Design Strategy Summary • 16 inch wide continuous footings or 24"x24" for isolated pads up to 2 story high (including any daylight basement). • 2000 psf (pounds per square foot) allowable bearing pressure for footing design on very dense, lower sand stratum. 18 inches minimum depth below final grade to bottom of footings. • 4 inch diameter, rigid PVC (ASTM:D-2729) footing drains. We recommend that an allowable bearing pressure of 2,000 pounds per square foot (psf) be utilized for design purposes, including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. Perimeter footings should be buried at least 18 inches into the surrounding soil for frost protection; interior footings require only 12 inches burial. However, all footings must penetrate to the prescribed bearing stratum and no footing should be founded in or above loose or disturbed soils. To limit total settlements, all continuous footings should have a minimum width of 16 inches for 2-story structures (including daylight basements) and 24 inches for pad footings. Brick facing must be supported by an extension of the footings to reduce the potential of differential settlement between the brick and wood structure. It should be noted that the area bounded by lines extending downward at 1 H: l V (horizontal:vertical) from any footing must not intersect another footing or intersect a filled area which has not been compacted to at least 95 percent of ASTM:D 1557. In addition, a 1.5H:1V line extending down from any footing must not daylight because sloughing may eventually undermine the footing. Thus, footings should not be placed near the edge of steps or cuts in the bearing soils. Anticipated settlement of footings founded on the lower, very dense sand, with footing excavations inspected and approved by us, should be on the order of 1 inch. Differential settlements are expected to be less than 1/2 inch. However, disturbed soil not removed from footing excavations prior to footing placement, could result in increased settlements. All footing areas must be inspected by GeoSource Engineering prior to placing concrete, to verify 1) that the bearing soils have not been loosened during excavation, 2) that the design bearing capacity of the-soils has been attained, and 3) that construction conforms with the recommendations contained in this report. Such inspections may also be required by the governing municipality. Perimeter footing drains should be provided as discussed under the section on Drainage Considerations. 9 ■ FLOOR SUPPORT Slab-on-grade floors may be used over structural fill or pre-rolled medium dense natural ground. Floors should be cast atop a minimum of 4 inches of washed granulithic material or pea gravel to act as a capillary break. They should also be protected from dampness by an impervious moisture barrier or otherwise sealed. We recommend bar reinforcement instead of wire mesh. ■ DRAINAGE CONSIDERATIONS Dense sands sometimes have ground water originating from rain which infiltrates into the upper looser sediments and flows above the denser sand at depth. Any excavations for basements may have flow in them at times. All retaining, basement and footing walls should be provided with drains at the footing elevations. Drains should consist of ASTM 2729 rigid, perforated PVC pipe surrounded by washed pea gravel and constructed with sufficient gradient to allow gravity discharge away from the houses. In addition, all basement walls should be backfilled with clean, free- draining sand. Roof and surface runoff should not dischargu into the footing drain system but should be handled by a separate, rigid tightline drain. In planning, exterior grades adjacent to walls should be sloped downward away from the structures to achieve surface drainage. We also recommend that the back side of any basement wall be waterproofed instead of dampproofed. 10 ■ PROJECT DESIGN AND CONSTRUCTION MONITORING At the time of this report, site grading, structural plans, and construction methods have not been finalized. We are available to provide additional geotechnical consultation as the project design develops and possibly changes from that upon which this report is based. We suggest that GeoSource Engineering perform a geotechnical review of the grading, drainage, and building plans prior to final design completion. In this way, our earthwork and foundation recommendations may be properly interpreted and implemented in the design. We are also available to provide geotechnical engineering and quality control monitoring services during construction. The integrity of the foundations depend on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Additional consultation, Plan Review and Construction Monitoring services are not part of this current scope of work. If these services are desired, please let us know and we will prepare a cost proposal. We have enjoyed working with you on this study and are confident that these recommendations will aid in the successful development of your site. If you should have any questions, or require further assistance, please do not hesitate to call. Sincerely, GeoSource :engineering, Inc. t 1 1086 w,ss,y,+c<< s s�lnN:ZL - £xPlncs 3 ¢ 97 Gary T. Lobdell, P.E., P.G. Principal 960902.DOC j� 9 / .\ 1 / --- ---- -- ---z --- ❑ EP-4 r--------•, I \ TS LOr. o 845 sq. ft. 10 aft es , �724 t; I` O.1 g cres `� \ / , � 0. acres) Z,� o ,o S ft. / _ OT9 1 2 res I / I ' 5,961 s, b. I ' ❑ EP-2 - - lT f I I r0 1 —� LDf•Jr / ' - 4, ft 45 s . n , .10 cres � • - in.f _ s to 7 h r, 0.15 a' s 30 1 r.3 sq.ft. I h I ' •,,,1 �� l' Ld _.... o ❑ EP-3: 25' 25' . 7 3 ; 6' 4 67 c e3 1 ❑ EP-5 2 65. v � 2 8 7 ' 2? \ � 30' 5 '2 — F� _7T 1/2' R"BAR & - NE 36th STREET - AP —�222224 _ c LEGAL DESCRIP77DN ` �� o �"0 ' EA LOT 9, BLOCK 4, HILLMAN'S C.D., LAKE WASHINGTON ;,ARDEN OF EDEN I SEc DI VISICiN Na 7, ACCORDING TO THE PLAT 7HEREOF, fc'CCOROED IN PRELL WITZ SHORTPLA T Renton, Washingtion Scale of Feet I I SITE AND EXPLORATION PLAN- FIG. 1 0 25 50 100 NORTH - LEGEND - Geo.Source ❑ EP-1 Exploration pit and ENGINEERING, INC. approximate location. Survey by Touma Engineers, Inc. 960902 OCT 1 996 11 APPENDIX EXPLORATION PIT LOGS Prellwitz Shortplat Renton, Washington Project No. 960902 EXPLORATION PIT NO. 1 Depth (ft) Soil Description 0.0-0.4 Loose, moist, black, topsoil. 0.4-3.8 Loose to medium dense, dry-damp, tan, clayey, silt, fine sand with roots to 3 feet. 3.8-6.0 Very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. No Seepage. No Caving. EXPLORATION PIT NO. 2 Depth (ft) Soil Description 0.0-0.9 Loose, moist, black, topsoil. 0.9-3.9 Loose to medium dense, dry, tan, clayey, silt, fine sand. 3.9-6.5 Very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. No Seepage. No Caving. EXPLORATION PIT NO. 3 Depth (ft) Soil Description 0.0-1.0 Loose, moist, black, topsoil. 1.0-2.8 Loose, dry, tan, clayey, silt, fine sand. 2.8-4.3 Very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. No Seepage. No Caving. 13 EXPLORATION PIT NO. 4 Depth (ft) Soil Description 0.0-1.3 Loose, moist, black, topsoil. 1.3-3.9 Loose to medium dense, dry-damp, tan, clayey, silt, fine sand. 3.9-5.7 Very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. No Seepage. No Caving. EXPLORATION PIT NO. 5 Depth (ft) Soil Description 0.0-1.0 Loose, moist, black, topsoil. 1.0-3.1 Loose to medium dense, dry-damp, tan, clayey, silt, fine sand. 3.1-5.0 Very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. No Seepage. No Caving. EXPLORATION PIT NO. 6 Depth (ft) Soil Description 0.0-1.0 Loose, moist, black, topsoil. 1.0-3.1 Loose to medium dense, dry-damp, tan, clayey, silt, fine sand. 3. 1-5.0 Very dense, damp, gray-tan, gravelly, silty, fine to medium sand with some cobbles. No Seepage. No Caving.