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Home My WebLink About02776 - Technical Information Report - Geotechnical 2�� V SUBSURFACE EXPLORATION AND PRELIMINARY GEOTEC�CAL ENGINEERING REPORT �I�L�vi� ulvU� REI�TOi�, �'ASHINGTON � PREPARED FOR � �� White Star Development t-,, j .l . E� .r' ' "!V� . G PROJECT NO. G97177A JLTNE 1997 CORPORATE OFFICE 911 FiFth Avenue,s�ite �oo ASSOCIATE� Kirkland,Washington 98033 12061 ss�-no� EARTH FAX (206) 827-5424 SCIENCES, INC BAINBRIDGE ISLAND OFFICE 179 Madrone Lane North Bainbridge Island,WA 981 10 (206�780-9370 FAX(206►780-9438 SUBSURFACE EXPLORATION AND PRELIMINARY GEOTECHNICAL ENGINEERING REPORT TAI.BOT RIDGE 102� AVENUE SE RENTON, WASHINGTON June 20, 1997 ��! Project No. G97177A I I. PROJECT Al'�TD SITE CONDITIONS ' 1.OINTRODUCTION ` � This report presents the results of our subsurface exploration and preliminary geotechnical ' engineering feasibility study for the proposed storm water detention vault located one-fifth of a mile west of the end of 102`� Avenue SE (Figure 1). The approximate proposed vault location ! and approximate locations of the explorations accomplished for this study are presented on the Site and Exploration Plan, Figure 2. After site access is available for construction equipment, ' additional exploration pits are recommended to further evaluate subsurface conditions in the , vault area. In the event that any changes in the nature, design or location of the structure are planned, or if subsurface conditions are found to differ significantly from those encountered in the explorations for this study, the conclusions and recommendations contained in this report ' should be reviewed and modified, or verified, as necessary. 1.1 Purpose and Scope The purpose of this study was to provide subsurface data and preliminary geotechnical recommendations to be utilized in the design and development of the subject project. Our study included a review of available geologic literature, 3 hand borings, and perfornung geologic studies to assess the type, thickness, distribution and physical properties of the subsurface sediments and shallow ground water conditions. Preliminary geotechnical engineering studies were also completed to determine the type of suitable foundation, allowable foundation soil bearing pressures, anticipated foundation settlements and drainage considerations. This report summarizes our current field work and offers development recommendations based on our present understanding of the project and surrounding geologic conditions. 1.2 Authorization Verbal authorization to proceed with this study was granted by Mr. Ron Guest of Triad and Associates on behalf of the owner, White Star Development. Our work was completed in � accordance with our proposal dated June 10, 1997. This report has been prepared for the exclusive use of White Star Development and their agents, for specific application to this project. 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 wananty, expressed or implied is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. 2.0 PROJECT AND SITE DESCRIPTION This report was completed with and understanding of the project based on a May 28, 1997 grading and drainage plan completed by Triad and Associates that showed the site with elevation contours, existing structures, and the planned locations of lots, paved roadways, and the detention vault. We understand that the development will include fourteen (14) lots for single family residences. Access is planned from the east off 102'� Avenue SE. Present plans call for the construction of a 20 foot by 100 foot by 6 foot deep storm water detention vault located east of the proposed housing development. The north to south trending rectangular vault will be situated on a gently sloping ridge, also trending north to south, with well defined steep slopes on both the western and northern portion of the site. The steep, western slope is located about 100 feet west of the proposed detention vault, and extends down to a developing condominium site. Based on the previously referenced grading and drainage plan, the detention vault will be located in an area with an elevation change of about 40 feet from the end of 102`� Avenue SE. With the exception of a narrow mid-slope terrace, this slope has an approximate inclination of 2H:1V (Horizontal:Vertical). Located approximately 200 feet northeast of the proposed vault, the steep, northern slope extends down to a road on the valley floor, with an approximate slope of 2H:1V. The site was generally vegetated with fir and cedar trees, and a moderate to thick covering of brush, ferns, and blackberries. Other land use in the vicinity consisted of residences and undeveloped land. Our geologic reconnaissance did not reveal evidence of debris flow or landslide activity in the vicinity of the proposed vault or surrounding slopes. No evidence of adverse ground water/springs or surface water conditions were observed on the slopes either. 3.0 SUBSURFACE EXPLORATION Our field study included advancing a series of 3 hand borings to gain information about the subsurface soil and shallow ground water conditions at the site. 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 2 conditions changed may represent gradational variations between sediment types in the field. Our borings were approximately located in the field by measuring from known site features shown on the topographic map. The conclusions and recommendations presented in this report are based on our geologic reconnaissance and the hand borings completed for this study. Because of the nature of exploratory work below ground and limited depth capability of hand exploration methods, extrapolation of subsurface conditions is necessary, and 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, or filling. The nature and extent of any variations in soil conditions may not become fully evident until equipment access is secured for esploration pits or until construction. If variations are observed at that time, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes. 3.1 Hand Borin�s � - The borings were completed on June 16, 1997 using a hand auger. Materials encountered from the borings were studied and classified by a geologist from our firm. All borings were backfilled after examination and logging. Selected samples were then transported to our laboratory for further visual classification as necessary. 4.0 SUBSURFACE CONDITIONS j Subsurface conditions on the parcel were inferred from the field explorations accomplished for I this study, visual reconnaissance of the site and review of published geologic literature. As shown on the field logs, the upper portion of the borings generally encountered about 1 to 2 ' feet of loose, moist, dark brown to brown silty sand (topsoil) with varying amounts of roots and other organic materials. !i Beneath the topsoil were sediments that were interpreted as undifferentiated glacial deposits. � These undifferentiated glacial deposits consisted of loose grading to medium dense, damp to moist, brown silty sands and soft grading to medium stiff, moist to wet, mottled gray and red sandy silts. Some organics, scattered cobbles and gravel lenses were also encountered. A lens of loose, brown, well-rounded, cobbly sand was encountered at a depth of approximately 4 feet in HB-2 and HB-3, but was impenetrable in HB-2 after 4 separate borings were attempted. However, the continuation of HB-3 at 4 feet was successful, and medium dense, moist, brown, silty fine sand was encountered below the upper sand and silt. All of the undifferentiated ' deposits are most likely the result of glaciofluvial sediment deposition. 4.1 Hvdrology Ground water seepage was not encountered in the borings at the time of our field study. I 3 ,� � A small drainage swale was present adjacent to 102`� Avenue SE, and sloped down to the northwest. Based on a USGS topographic map, this is a seasonal drainage that is only affected by heavy precipitation. ! ', � 4 June 20, 1997 Project No. G97177A II. PRELIMINARY DESIGN RECOMMENDATIONS 5.0 INTRODUCTION Our explorations, geologic reconnaissance and interpretations of published geologic literature indicate that from a geotechnical standpoint, the site is suitable for construction of the proposed detention vault. The bearing strata (medium dense sand) was relatively shallow in our exploration borings and should be encountered at the proposed 6 foot excavation depth for the vault foundation. No evidence of past landslide or debris flow activity were observed in the vicinity of the vault, nor were they mapped on geologic'literature. It is therefore our opinion that the vault may be constructed using a conventional, shallow foundation and retaining walls, without a significant adverse impact on slope stability. The construction area should be restored by grading to match the surrounding, natural contours and revegetating with a suitable seed/plant mixture. 6.0 SITE PREPARATION Site preparation for planned vault should consist of stripping and removing the upper, organic , topsoil horizon. This material may be set aside for later reuse as a mulch layer to promote gerniination and growth of natural plants. Once the vault footprint is stripped, the excavation for the vault chamber may proceed. It is anticipated that the depth of the excavation will be on the order of 6 feet, and based on our exploration borings, we anticipate loose, grading to medium dense silt, sand and gravel sediment will be encountered. We do not expect that significant ground water seepage will be encountered in the planned excavation, however, seepage zones may occur during the wetter periods of the year. In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction based on the encountered soil and ground water conditions. For estimating purposes, we anticipate that temporary, unsupported cut slopes in the silt, sand and gravel sediments may be made at a maximum slope of 1.SH:1V. As is typical with earthwork operations, some sloughing and raveling may occur and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA recommendations should he followed at all times Some of the on-site szciiments cuntaineci a nibti percenta�e �i tlnc;-�raineu matenai tivn��'�� makes them moisture sensitive and subject to disturbance when wet. The contractor must u� care during site pr� I�-- � � I softened. If disturbance occurs, softened soils should be removed and the area brought to grade with structural fill compacted to a minimum of 95 percent of ASTM:D 1557. Once the excavation is completed to the required depths and any softened soils removed to replace by structural fill, the subgrade should be recompacted to at least 95 percent of the above standard prior to foundation construction. After the subgrade is prepared and approved by the geotechnical engineer, foundation construction can begin. 7.0 FOUNDATIONS Conventional spread footings may be used for support of the vault when founded on the medium dense natural sands or structural fill placed as previously discussed. We recommend that an allowable soil bearing pressure of 2,500 pounds per square foot (ps� be utilized for design purposes, including 'clead and live loads. Footings should be buried at least 18 inches below surrounding grades for frost protection. All footings must penetrate to the prescribed bearing strata and no footings should be founded in or above loose/soft or organic soils. All footings should have a minimum width of 16 inches. The area bounded by lines extending downward at a 1H:1V inclination from any footing must not intersect another footing or intersect a filled area which is not being compacted to at least 95 percent of ASTM:D 1557. Anticipated settlement of footings founded on medium dense sand or approved structural fill I should be less than 1 inch. However, disturbed soil not removed from footing excavations prior to footing placement, could result in increased settlements. All footing areas should be 'I inspected by Associated Earth Sciences, Inc. (AESI) prior to placing concrete to verify that the ' design bearing capacity of the soils has been attained and that the construction conforms to the ', recommendations contained in this report. Such inspections may also be required by the �, governing municipality. 8.0 LATERAL WALL PRESSURES , Horizontally backfilled walls which are free to yield laterally at least 0.1 percent of their height may be design using an equivalent fluid equal to 35 pounds per cubic foot (pcfl. Fully I restrained, horizontally backfilled rigid walls which cannot yield should be designed for an equivalent fluid of 50 pcf. The upper, east wall will likely have a sloping backfill condition at I an approximate inclination of SH:1V and should be designed for an equivalent fluid of 40 pcf ' for yielding conditions and 60 pcf for restrained conditions. The lateral pressures presented above are based on the conditions of a uniform backf'ill consisting of a select on-site sand and gravel backfill compacted to 90 percent of the ASTM:D 1557. A higher degree of compaction is not recommended as this will increase the pressure acting on the walls. 6 Proper drainage must be provided so that hydrostatic pressures do not develop against the walls. This would involve installation of a minimum 1 foot wide blanket drain for the full height of the wall using imported, washed gravel. A perforated, rigid, PVC pipe should be placed at the wall base elevation and sloped to drain to a suitable outlet location. The drain pipe must be hydraulically connected to the free-draining wall backfill material. 9.0 SLOPE REMEDIATION Once the vault is constructed, the areas disturbed by the construction should be restored and planted to reduce the potential for erosion and slope instability. In general, we recommend that the following measures be used. 1. All excavations should be backfilled with granular, _natural soils derived from on-site, and compacted to at least 90 percent of ASTNi:D 1557. 2. Finished grades should be contoured to blend into the surrounding natural slopes, and to avoid concentrated flows of runoff water. Concentrated runoff must not be directed onto or above more steeply sloping areas of the site. 3. Disturbed areas should be revegetated with a suitable hydroseed mixture. To promote plant germination and growth, mulching the slopes with the on-site topsoil sediments is recommended prior to hydroseeding. The mulch/topsoil thickness should not exceed 1 foot. 10.0 CLOSURE At time of this report, site grading, structural plans, and construction methods had not been finalized. The recommendations presented in this report are therefore preliminary and should be reviewed and/or modified as necessary as new design information becomes available. We are available to provide additional geotechnical engineering exploration or consultations as the project design develops and possibly changes from that upon which this report is based. We are also available to provide geotechnical engineering and monitoring services durina construction. The integrity of the foundation depends 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. � We have enjoyed working with you on this study and are confident that our recommendations will aid in the successful completion of your project. If you have any questions or require additional information, please do not hesitate to call. Sincerely, ASSOCIATED EARTH SCIENCES, �1C. Kirkland, Washington J�� BLYT ��+S � o� yi,�, O � �ti � c„ �,. ;�z �` c "• �_ i �(��f`�7 i Ly� c ''I . �� I �P��iS'S_�hn �,w .^ ss�0,'�AL.F1�. ,,/ Exo��ES SI�77 � � �i �l �� I Andrea S. Lonner Bruce L. Blyton, P.E. '� Staff Geologist Associate Engineer BLBi Id G97177A 1 6/l0/97 ld-W W g ' ,\ I � � I �1J I I � �� :�Y ;L�'Jl �v+ �� i � i� 1 I :s �y �:sc� � - ., �� -� J\7 .',l �� . \ I . :vrcJ =�,�H � I c _ . � i � � .�C�� _� _._ n I �:� u c �� _ � . . �'- a _-C. a � I _. • _ � •� <J - � '- ! ' � X< :� I:� — � I - - =tiv ��L� -� n � I y ' � - c - -„ W� �i� � � � i 7]C .� . . . __ . . _ W - —� /''��t I l ;\ �/ y I ; - _ _ _- `��\, j r _ - + •.> ; v - � -�� _;; , �; � _ v J � ` i� ��_ ,_ �; _ . . � � � , ;�= I �� : � �I _E C�'Yl. � � I • . /� L� 'S- `j /� �� '�� __" �� ,��, ; ; �, _ ��,� � ' � � 1� \ . I Cc ;2i—r/ ., 1� Z . �, � i =I �, =� � � � �; ; �� •'�i.pj � I nl � �� ":w Jf _::na.. �;�� S ��r ::NLS � _� � \ ^ I � ( �; � � S i1r H1=8 O�, n i i J)� _ �� f Y�� ' =I n .�� J�y �i_3 -I ol �� ^� � �ej = = m� ��I � a f N I / � .J�7 y1.1'e � w 1 � ._� � i17 ti179 �/� al I a�... I� �i� Associated Earth Sciences, Inc. 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' ' ` � ` ` ` f ' JUNE 1997 , /' /, ' ' . ! _ � f � `` ; . ,' . ,.�� - .. � ' }� � � ' � i� .l�� , < < f � � i i : . , , ��. . � "'=, ,` ' .. � f : r; r . �� f� ' �� / �' ��• �/ � 1 � .' � i q�, � �' �: I ' " • "IS'`' ,' '� ;' ,' ' ; ; t .�� ' 3� ASSOCIATED I, , � � � � �� ' -� �' - � ' r' � ' }�69 �. NORrf, . . -;��g ,� � ' � �- � - � � 6 EARTH � � `• '6� '� ` ' � �� ',��}9 � ' � SCIENCES, INC I . / ' ; " . 0 4p $O � Y I . '� �685 / � � — i � SCA�E IN F�ET HAND EXPL4RATION HOLE �� Number HB-1 0 _,____ _ .__.._ .___ _. .� Forest duff/topsoil. Soft to medium dense, moist, tan, silty, fine SAND; increases in silt content and red and gray mottling with depth; occasional organics and medium to coarse sand. 2.5 I Medium dense, moist, mottled red and gray, silty, fine SAND; occasional rootlets. @ 3'-8" �i appearance of inedium to coarse sand lenses. �i 5 Medium stiff, moist, mottled gray and red, fine, sandy SILT; @ 6' a 3" x 2"well rounded rock in shoe. II _ Medium stiff, moist, mottled gray and red, fine, sandy SILT with occasional coarse sand and small gravel. I�I 7.5 ' BOH @ 7'-G" . � Number HB-2 I, Forest duff/topsoil. i, Loose, moist, brown, medium SAND, some silt; occasional coarse sand and organics. 2.5 Medium dense moist li ht brown medium to coarse SAND� increasin in coarseness with de th. BOH @ 3'-7" Note: Terminated due to difficult digging. 5 7.5 � SubsurFace conditions depicted represent our observation at the time and location of this exploratory hole,modified by geologic interpretation,engineering analysis,and judgment. They are not necessarily representative of other times and location. We will not � � accept responsibility for the use or interpretation by others of information presented on this log. Reviewed By � Associated Earth Sciences, Inc. Talbot Ridge Detention Vault 911 Fifth Avenue, Suite 100 Renton/King County, Washington - Kirkland, Washington 98033 Pro ect No. G97117A _ Phone: 425-827-7701 �u e 1997 Fax: 425-827-5424 HAND EXPLORATI � N HOLE Number HB-3 0 Forest duff/topsoil. Loose, moist, reddish-tan, silty, fine SAND with organics. Loose moist reddish-tan sil fine SAND some coarse sand� occasional cobbles. 2.5 Loose, moist, brown SAND with cobbles. 5 Loose to medium dense, moist, mottled red and gray, silty, fine SAND; lenses of inedium sand. Medium dense, moist, reddish-brown, silty SAND with red and gray mottling. BOH @ 6'-6" 7.5 � Number 2.5 5 7.5 Subsurface conditions depicted represent our observation at the time and location of this exploratory hole,modified by geologic interpretation,engineering anaysis,and judgment. They are not necessarily representative of other times and location. We will not accept responsibility for the use or interpretation by others of information presented on this log. Reviewed By Associated Earth Sciences, Inc. Talbot Ridge Detention Vault 911 Fifth Avenue, Suite 100 Renton/King County, Washington Kirkland, Washington 98033 Pro ect No. G97117A Phone: 425-827-7701 1 Fax: 425-827-5424 J u ne 1997 ,