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HomeMy WebLinkAboutPreliminary Geotechnical Engineering Evaluation04i sTRa-ra A PROFESSIONAL SERVICES Car.:.. Sl�,i'6gYl-iy /OM 4 -AG R`VYd Kd UP October 12, 2012 File: B012423A Kevin McCarthy, P.E. KM Engineering, LLP 9233 West State Street Boise, ID 83714 kevin@kmengllp.com RE: Preliminary Geotechnical Engineering Evaluation Tradewinds Subdivision Victory and Locust Grove Roads Meridian. Idaho Dear Mr. McCarthy Strata, A Pro fessional Services Corporation (STRATA) has performed our authorized preliminary geotechnical engineering evaluation for the proposed development located southeast of the intersection of Victory and Locust Grove Roads in Meridian, Idaho. Our geotechnical engineering evaluation's purpose was to explore the subsurface conditions in the proposed development area and provide geotechnical recommendations to assist project planning, design and construction. The attached report summarizes our field and laboratory test results and presents our geotechnical engineering opinions and recommendations. The northwestern corner of the property is covered by recently placed undocumented fill, up to approximately 4 feet. Below the recently placed fill and at the ground surface on the remainder of the project, native silt was encountered. Any future fill placement will be placed on-site as tested structural fill to the proposed project grading requirements. The following report provides specific geotechnical recommendations for preparing the site, over -excavating undocumented fill, earthwork recommendations, stormwater disposal and pavement design. It is our opinion that geotechnical continuity with the project team throughout construction will help identify undocumented fill during earthwork to allow its excavation and/or re -compaction as structural fill below the proposed infrastructure and building footprint. The project design and construction team must read, understand and implement this report in its entirety. Portions of the report cannot be relied upon individually without the supporting text of remaining sections, appendices and plates. Our opinion is the success of the proposed construction will depend on following the report recommendations, good construction practices, and providing the necessary construction monitoring, testing and consultation to verify that work has been constructed as recommended. We recommend STRATA be retained to provide construction observation, monitoring, testing and consultation services to verify our report recommendations are being followed. 8653 W. Hackamore Dr. Boise, Idaho 83709 P.208.376.8200 F.208.376.8201 www.stratageotech.com REPORT Limited Geotechnical Engineering Evaluation Tradewinds Subdivision Victory and Locust Grove Roads Meridian, Idaho Prepared For: Kevin McCarthy, P.E. KM Engineering, LLP 9233 W. State St. Boise, ID 83714 Prepared By: STRATA, Inc. 8653 W. Hackamore Dr. Boise, Idaho 83709 P. 208.376.8200 F. 208.376.8201 October 12, 2012 Limited Geotechnical Engineering Evaluation Tradewinds Subdivision Meridian, Idaho INTRODUCTION Our limited geotechnical engineering evaluation's purpose was to explore the subsurface soil conditions within the proposed development area located on the southeast corner of the intersection of Locust Grove and Victory Roads in Meridian, Idaho and provide recommendations for earthwork, stormwater disposal and pavement design. We accomplished our services referencing our authorized Proposal dated December 7, 2011. To accomplish our evaluation, STRATA performed the following services: 1. Coordinated exploration with the local Digline Utility Notification Center to help reduce the potential for damage to existing utilities. 2. Observed 4 exploratory test pits in the proposed development area extending 7 to 11 feet below existing grade. A professional engineer visually described, classified and logged soil encountered referencing the Unified Soil Classification System (USCS). Test pit locations were loosely backfilled with excavated soil after individual test pit exploration. 3. Performed laboratory tests with reference to ASTM International (ASTM) procedures including in-situ moisture, gradation (Minus No. 200 wash) and Atterberg Limit testing. We utilized these laboratory results to help characterize engineering parameters and to correlate soil engineering characteristics used in our design and recommendations. Lab testing results was not complete at the time of this report and will be issued as an addendum to this report. 4. Performed engineering analyses in order to provide earthwork construction recommendations. We communicated with KM Engineers to coordinate necessary elements of design and construction into a specific discussion of the related soil and geologic conditions. Our engineering analyses provides geotechnical recommendations for: sir Earthwork • Site preparation • Excavation characteristics • Wet weather construction • Structural fill criteria • Geotextile applications 8653 W. Hackamore Dr. Boise, Idaho 83709 P.208.376.8200 F.208.376.8201 www.stratageotech.com Tradewinds Subdivision File: B012423A Page 3 utilizing a New Holland excavator with a 2 -foot -wide bucket. A professional engineer logged and visually classified soil encountered in each test pit location referencing the USCS. A brief explanation of the USCS is included in Appendix A and s hould be us ed to interpret terms presented on the test pit logs in this report. We obtained disturbed soil samples for classification and laboratory testing at select depths and locations. At the conclusion of our subsurface investigation, test pits were loosely backfilled with excavated soil level with the ground surface. A standpipe piezometer was installed in each test pit, for future groundwater monitoring. Test pit locations are labeled by the standpipe piezometer for future location during site survey and/or construction. SUBSURFACE CONDITIONS Generally, we encountered near surface recently placed undocumented fill in the northwestern corner of the site and anticipate fill near the southeastern property corner (reference Plate 1 for anticipated fill limits). Below fill (northwestern corner) and at the ground surface (majority of site), we encountered native silt, lean clay and silty sand underlain by gravel soil with depth. It does not appear that the contractor stripped the site of the upper vegetative layer prior to placing the fill near the northwest corner of the site. Significant vegetation and organics were observed at the fill to native interface in TP -3 and TP -4 for 1 to 1.5 feet. TP -1 and TP -2 exposed significant vegetation and organics at the ground surface to approximately 1 foot below existing grade. Underlying recently placed fill in TP -3 and TP -4 and at the ground surface in TP -1 and TP -2 approximately 3 to 4 feet of silt and lean clay was observed. Recently placed fill consisted of brown, medium dense silty gravel in TP -3 and TP -4. Brown, stiff and moist to wet silt and lean clay extended to tan silty sand at 3 to 8 feet below existing grade. Silty sand described as tan and wet to saturated extended to poorly -graded gravel with sand in each test pit. Poorly - graded gravel with sand and cobbles described as tan, dense and saturated extended to test pit termination depths of 7 to 11 feet. We provide specific soil descriptions and contacts on individual test pit logs in Appendix A. We observed groundwater in each test pit at depths ranging from 2.6 to 8.3 feet below existing grade. Groundwater in TP -3 and TP -4 is significantly deeper due to the 4 feet of fill placed in the northwest corner of the property. In the project vicinity, groundwater is a function of site development, precipitation and irrigation in the project vicinity. STRATA installed an www.stratageotech.com Tradewinds Subdivision File: B012423A Page 5 conditioning in order to achieve uniform moisture to reuse on-site silty and clayey soil. It is critical STRATA work in conjunction with the contractor to help verify the quality and consistency of the undocumented fill, over -excavation requirement, and/or anomalies in the fill during construction. Specific recommendations concerning existing fill are discussed below in the Earthwork section. GEOTECHNICAL OPINIONS AND RECOMMENDATIONS We present the following geotechnical recommendations to assist planning, design and construction of the proposed development to be located southeast of the intersection of Victory and Locust Grove Roads in Meridian, Idaho as illustrated on Plate 1, attached to this report. This report also provides specific geotechnical design criteria for the preliminary development, which the civil design and construction teams must review to verify the applicability to the planned structure. We base our recommendations on the results of our field evaluation, laboratory testing, our experience with similar soil conditions and our understanding of the preliminary proposed construction. If design plans change or if the subsurface conditions encountered during construction vary from those observed during our field evaluation, we must be notified to review the report recommendations and make necessary revisions. Earthwork Site Preparation Underlying undocumented fill and at the ground surface in the majority of the site significant vegetation and organics was observed to depths of 12 to 18 inches. Soil containing significant vegetation and organics is not suitable for use as structural fill or to bear structures over. As such, it must be excavated, removed and stockpiled for reuse as landscape fill, or removed from the site. As previously discussed, we observed recently placed undocumented fill up to 4 -feet - thick at the ground surface near the northwestern corner of the site. Undocumented fill is also anticipated near the southeastern corner of the site associated with the backfilling of a pond around 2006, see Plate 1 for estimated fill limits. Prior to placing structural fill for site grading or constructing residences or pavements, the following should be accomplished: www.stratageotech.com Tradewinds Subdivision File: B012423A Page 7 may be economically impractical or impossible at certain times of the year, especially during winter or spring months. Contractors should also anticipate isolated zones of wet soil during inclement weather that may require removal and replacement with granular structural fill in foundation or slab areas. Therefore, the contractor and developer should plan contingencies to remove and replace wet and disturbed soil with granular structural fill. If significant soft/wet soil conditions are encountered, the use of a woven geotextile fabric may be necessary. These material requirements are presented in the Geosynthetics report section. Allowing time for proper moisture -conditioning during dry weather is critical to reducing excessive over -excavations and importing granular structural fill. However, depending on the weather and moisture conditions during construction, drying of fine-grained soil may not be practical, and over -excavation in conjunction with the use of geosynthetics, may be necessary to help maintain project schedule. In short, using site soil may be impractical during certain weather or soil conditions and we recommend you consider necessary contingencies to remove and replace wet soil. During construction, intersect and divert surface runoff from rainfall or snowmelt to help reduce water ponding on the project site. Subgrades must always slope and be exposed to daylight to help direct water away from subgrades after the end of each construction day or before precipitation. Structural Fill Criteria All fill placed for the development must be placed as structural fill. The structural fill requirements described in Table 1 below, in general correlate to Idaho Standards for Public Works and Construction (ISPWC) material specifications. 14 www.stratageotech.com Tradewinds Subdivision File: B012423A Page 9 Geosynthetics Geosynthetic fabrics may be used to improve subgrade support when constructing on soft or wet soil and for soil improvement applications. Woven geosynthetics are required at all soil subgrades where moisture -conditioning and re -compaction, as recommended in the Site Preparation section of this report, is not practical due to wet soil conditions. A woven geotextile may only be required in localized areas, depending on time of construction. Where required, apply geosynthetics directly on approved subgrade, taut, free of wrinkles and over -lapped at least 12 inches. Woven geosynthetic fabrics for subgrade stabilization and soil improvements shall have the minimum properties of 700 pounds (CBR Puncture, ASTM D6241) and 200 pounds (Grab Tensile Strength ASTM 134632) such as a Contech C200. Non -woven separation geosynthetic fabrics are applicable for soil separation and for stormwater facilities and should have a maximum apparent opening size equivalent to the U.S. No. 70 sieve (ASTM D4751), a minimum weight of 3.5 ounces per square yard and minimum CBR puncture resistance of 200 pounds (ASTM D6241). Exterior Grading We recommend the ground surface outside of any structure be sloped a minimum of 5 percent away from the building for 10 feet to rapidly convey surface water or roof runoff away from foundations. Remaining landscapes should slope at least 2 percent away from structures. Roof downspouts must be provided and connected to a solid pipe placed away from structures and not allowed to infiltrate into the soil underlying the structure. Stormwater should be routed away from disturbed soil areas and should be disposed of in a suitable location as determined by the site civil engineer. Irrigation adjacent to or within 5 feet of the buildings is discouraged. Civil site grading and stormwater design must minimize water introduction into the site soil near structures. The site soil settlement potential will significantly increase upon changing moisture conditions. Therefore, selecting appropriate stormwater disposal techniques and positioning them down -gradient and away from structures is an important site design process. Additionally, careful consideration with respect to the landscape theme and irrigation locations must be considered. Stormwater Disposal We performed a percolation test within the native poorly -graded gravel with sand encountered at depth in each test pit. We measured a field infiltration rate of greater than 30 V www.stratageotech.com Tradewinds Subdivision File: B012423A Page 11 Local Roadways - Flexible Pavement (TI = 6.0) 2.5" - Asphalt concrete top course 16.0" -'/-inch-minus, crushed sand and gravel base course The above -recommended flexible pavement sections are based on a m aximum 20 - year design life and assumed traffic index (TI) of 6.0 for local subdivision roadway. The subgrade soil is anticipated to be either silt or lean clay. Based on our experience with lean clay in the Treasure Valley, we have utilized an R -value of 5 for design based on a lean clay subgrade. The design team should review these assumptions to ensure the assumed traffic index will meet the intended use of the pavement areas. If the subgrade is wet at the time of construction, we recommend a woven geotextile such as Contech C200 be considered over subgrade prior to placing the subbase course. The subbase and base course should be consistent with the Structural Fill criteria above and should be compacted in accordance with the Structural Fill section of this report. The asphalt concrete for the flexible pavement area should have material properties as specified in ASTM D-3515 and have a mix design with a maximum aggregate size between 3/4 -inch and 3/8 -inch. The asphalt concrete should be compacted as required by ISPWC Sections 809 and 810. Timely maintenance will help reduce the potential for surface water infiltration through the pavement section and into the underlying subgrade. Poor maintenance and absence of crack repair of the new pavement may allow saturated conditions to occur in the section and in the underlying subgrade. The native soil may soften if saturated and experience a reduction of load bearing capacity. In addition to a reduction in subgrade strength, saturation of the subgrade soil may also create conditions where frost expansion or heaving can occur. E ither or both of these occurrences would result in higher maintenance requirements and a pot entially shortened service life of the pavement. Therefore, we strongly recommend crack maintenance and slurry sealing be accomplished in all pavement areas as needed and at least once every 3 to 5 years. ADDITIONAL RECOMMENDED SERVICES Groundwater Monitoring The groundwater level should be monitored during upcoming irrigation season to verify the seasonal high groundwater level beneath the site. A piezometer was installed in each test www.stratageotech.com I Tradewinds Subdivision File: B012423A Page 13 lots and are not assuming the role of geotechnical engineer of record for residential structures. We recommend individual builders and owners retain a geotechnical engineer to evaluate foundation subgrade requirements, and observe foundation subgrade excavation. Our geotechnical findings and opinions have been developed based on the authorized subsurface exploration and laboratory testing, as well as our understanding of the project at this time. Our geotechnical design recommendations are specific to the planned design and infrastructure construction and should not be extrapolated to other future site developments without allowing adequate geotechnical consultation by STRATA. Our services consist of professional opinions and f indings made in accordance with generally accepted geotechnical engineering principles and practices in southwest Idaho at the time of this report. The geotechnical recommendations provided herein are based on the premise that appropriate geotechnical consultation during subsequent design phases is implemented and an adequate program of tests and observations will be conducted by STRATA during construction to verify compliance with our recommendations and to confirm conditions between exploration locations. This acknowledgment is in lieu of all warranties either express or implied. The following plates accompany this report: Plate 1: Test Pit Location Plan Appendix A: Unified Soil Classification System (USCS) & Exploration Logs www.stratageotec h, corn APPENDIX A LEAN CLAY, (CL) brown, stiff; moist SILTY SAND, (SM) tan, dense, moist to wet SAND, (GP) tan, dense, saturated BG 2.5 CL SM a �p �l• P fl' QQ 5.0 0 6'' GP O.n, 3 Client: KM Engineering Test Pit Number: TP -1 EXPLORATORY t: Project: B012423A Date Excavated: 10-03-2012 Y FR a-r,Y, TEST PIT LOG m F Backhoe: Bucket Width: 2' Depth to Groundwater: 2.9' Logged By: BN Sheet 1 Of 1 X vi y J J O m o -o N 12 REMARKS REMARKS USCS Description a= cig E ,a, ¢T'= @� d o a H d �� Ground Lu C6 M 012 2 ' o' � v Note: BGSSurfaceflow y Z o o 0 7 U D U SILT WITH SAND, (native), (ML) ' brown, stiff, moist Significant vegetation and organics observed to 6" BGS ML LEAN CLAY, (CL) brown, stiff; moist SILTY SAND, (SM) tan, dense, moist to wet SAND, (GP) tan, dense, saturated BG 2.5 CL SM a �p �l• P fl' QQ 5.0 0 6'' GP O.n, 3 Client: KM Engineering Test Pit Number: TP -1 EXPLORATORY t: Project: B012423A Date Excavated: 10-03-2012 Y FR a-r,Y, TEST PIT LOG m F Backhoe: Bucket Width: 2' Depth to Groundwater: 2.9' Logged By: BN Sheet 1 Of 1 = Vi rqm J O m co m m° ' m REMARKS USCS Description a x g E, a x m o a H d m w❑ —' j m F a Z 2 0 o �- Note: BGSSurface Ground C N rp 7 ?.1 ❑ U a SILTY GRAVEL, (fill), (GM) brown, ' medium dense, moist GM 2.5 SILT, With Organics (native), (ML) black, firm, moist to wet ML SILT WITH SAND, (ML) brown, 5.0 BC' stiff, moist MIL LEAN CLAY, (CL) brown, stiff, moist CL 7.5 SILTY SAND, (SM) tan, medium dense, saturated SM ' POORLY GRADED GRAVEL WITH 10.0 SAND, (GP) tan, dense, saturated GP PQ Test Pit Terminated at 11.0 Feet. i i Client: KM Engineering Test Pit Number: TP -3 T - EXPLORATORY TEST PIT LOG Project: B012423A Date Excavated: 10-03-2012 i Backhoe: Bucket Width: 2' ': Depth to Groundwater: 8.2' Logged By: BN Sheet 1 Of 1