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HomeMy WebLinkAboutSoils Investigation Report - PLN General - 10/18/2000 1235 EAST 4TH AVE SUITE 101 _ OLYMPIA,WA 98506 (360)754-9339 FAX(360)352-2044 MC SQUARED E-mail:ang®mc2-inc.com I N C O R P O R A T E D August 1, 2000 David Strong Bradley-Noble Geotechnical Service PO Box 12267 Olympia, WA 98508-2267 RE: SOILS INVESTIGATION REPORT FOR THE LILLIWAUP POST OFFICE LILLIWAUP, WASHINGTON Dear David: We have reviewed the attached eight page soils report entitled "Soils Investigation Report for the New Lilliwaup Post Office" located in Lilliwaup, Washington. This report was prepared by you batted on field work performed by you and your firm. MC Squared, Incorporated has reviewed this report and the conclusions contained therein. It is our opinion that the report and the conclusions meet the standards of good geotechnical practice for this place and time. MC Squared's recommendations and opinion are based on our review of the field information and soils logs presented to us. MC Squared has made recommendations to you regarding allowing bearing stresses, lateral resistance and loads, site period, and other items of an engineering nature. These recommendations and conclusions have been included in the attached report. If you have any questions, or 1 may be of further help, please call me at (360) 754-9339. Sincerely yours, MC SQUARED, INC. � v QOF WAS y; Vince McClure, Ph.D., PE — o Principal z ,1 O Oc` �6GISTEP� ASS/ONALEta ('AMyFiles\Misc Files\Lilliwaup Post Oftice.08-0I-2000.wpd EXPtREs 081171 0 , i0 STRUCTURAL • FOUNDATION 0 CIVIL ENGINEERS TEST PIT LOGS Project number: 00 . 07-16 Lilliwaup Post Office Site Test pits excavated, logged, and backfilled on 26 July 2000 . Test Pit One: 0 to -3 . 5 feet Fill of gravelly sands with trash and debris, layers of organic soils with wood. -3 . 5 to -8 . 0 feet Reddish brown dense coarse slightly silty sandy gravels with cobbles and boulders . Test Pit Two: 0 to -2 . 2 feet Fill and topsoil comprised of a light brown silty sandy gravel with roots . -2 . 2 to -7 . 5 feet Reddish brown dense coarse slightly silty sandy gravels with cobbles and boulders . Test Pit Three: 0 to -1 . 0 feet Topsoil and roots, brown sandy gravel . -1 . 0 to -7 . 8 feet Reddish brown dense clean coarse sandy gravels with cobbles and boulders . Test Pit Four: 0 to -3 . 5 feet Fill of mixed sandy gravels with topsoil and organics . -3 . 5 to -8 . 0 feet Reddish brown pebbly coarse sand with scattered cobbles . Little fine sand fraction below a number 20 screen. No seepage or ground water was observed in the test pits . Below 4 to 5 feet from the surface, the soils became moist. Sloughing of the trench walls and undermining of the surface soils occurred in all the test pits . r \\ A br- �\ LLI O r7 �. \ J \\ INJ \ a- •� ,/ f .,. � l / �` c'..' j .. a \� � \ /\ `\ Q1 cl MOOYL,NI T \` O \ a 1p J 1 J J �9 SITE PLAN Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. 2401 Bristol Court SW, Olympia, WA 98502 Phone 360-357-7883 PO Box 12267, Olympia WA 98508-2267 FAX 360-754-4240 e-mail: bradleynoble@iuno.com SOILS INVESTIGATION REPORT FOR THE NEW LILLIWAUP POST OFFICE LILLIWAUP, WASHINGTON This report presents the results of our subsurface investigation for the proposed 1600-square foot modular post office and associated parking to be constructed on property shown on the enclosed site plan. Our purposes in exploring the subsurface soil conditions were to evaluate the bearing capacity of the site soils, to present recommendations for site development work, and to address other geotechnical considerations for this project. From the information provided to us, we understand that the new structure will be a double-wide modular building. The sanitary drainfield that serves the existing structure will be used for new structure. New asphalt parking and drive areas will be constructed. On-site infiltration of storm water will be included in the project. Work was authorized by the project design consultant, URS Greiner Woodward Clyde. SITE CONDITIONS Surface Conditions The area of proposed construction of the new facility is of low relief. It appears to have been used as a gravel borrow site in the past. In our field discussion with Mrs . Straatman, the property owner, we understand that this borrow area has not been used during the time of their ownership of the property. The site is overgrown with small trees, Scotch broom, and grasses . The area of Lots 5 and 6 are overgrown with a dense stand of Scotch broom. The underground locate service was not able to locate the telephone line that services the Straatman residence in this area. Underground telephone lines are found in the area of the driveway from Highway 101 to the Straatman residence and along the right-of-way of Highway 101 on the northeast property line. No other underground 00071601 Page 1 of 8 00071601 Page 2 of 8 utilities were marked, though we understand that a private water line to the existing post office does exist. A cut slope from the former gravel borrow operation has been constructed on the southwest side of the project. This slope appears to be stable with no major sloughing or erosion of soil from the slope observed. Above the cut slope, we found forest land vegetation, predominantly Douglas firs growing on the slope. Subsurface Conditions Subsurface soil conditions were explored by four test pits excavated under our supervision by a tractor-mounted backhoe. These test pits were logged, and the soils exposed were evaluated for their engineering properties . All test pits were loosely backfilled. During site preparation, these test pits should be re-excavated and re-backfilled as discussed in the Earthwork Criteria section of this report. This will prevent "soft" areas under structures or pavement. Because of the inability of the utility locate service to locate the telephone line in Lots 5 and 6, we did not conducted explorations there. We did find two shallow and partially filled test pits that were still open on Lot 6 . Soils in these test pits are similar to the native soils found in Test Pits 1 and 2 . To confirm our interpretation of the site geology, we referred to the Geologic Map of North Central Mason County, Washington by R. J. Carson, 1976 . This area is mapped as Vrd, delta and alluvial fans of recessional outwash, in part, your alluvial fans of sand and gravel. At this site, we found three soil units . On the surface in the area of proposed construction, we found a fill section comprised of topsoil and debris . This fill section varied rapidly in thickness and composition. In our Test Pits 1, 2, and 3, and exposed in the existing test pits on Lot 6, we found silty, coarse sands and gravels with cobbles and boulders . In Test Pit 3, the coarse sands and gravels had a little silt adhering to the clasts . In Test Pit 4, under the fill section, we found rounded, 1 00071601 Page 3 of 8 coarse sands and small gravels with few fines and some cobbles . These are interpreted as being either beach or river bar deposits . No ground water or seepage were encountered in the test pits . We did not observe indication of high ground water levels at this site in the test pits . We do not expect that ground water levels would adversely affect the project as we understand the design concept. We do not expect that the site work will be adversely affected by rain except in the heaviest downpours, as infiltration of water should be rapid at this site. DISCUSSION AND RECOMMENDATIONS We expect that only minimal site grading and removal of existing fill material will be required to develop the site. Site clearing, grubbing and removal of unsuitable soils will reduce the present surface elevation from one to three feet. We do not expect that significant subsidence resulting from densification of the native soils will occur during the site development work as these are dense, coarse, granular materials . After removal of the topsoil and existing fill, the native soils are suitable for support of structural fill sections supporting paving sections and foundations . We do recommend that prior to placement of structural fill, the native soils be subjected to a heavy proof-roll as discussed in the Earthwork Criteria section of this report . In Test Pits 1 through 3, we found cobbles and boulders . The presence of oversized material and the soil being cohesionless limited depth of the test pits because of sloughing of the trench walls . During construction of underground utilities, the contractor should expect that excavation on neat trench walls will not be feasible. The coarse, pebbly sands in Test Pit 4 are cohesionless, and the depth of the test PY sloughing it was limited b the slou hin of the trench walls into the excavation faster than the backhoe could remove material . All underground utility trench work 00071601 Page 4 of 8 must conform to the requirements of the Department of Labor and Industries standards for a Type C soil . Even shallow trenches into these soils can fail suddenly and without warning. There are numerous trees growing at the top of the cut slope. We recommend that the services of a forester be provided to determine if any of these trees are prone to wind toss resulting from loss of root support by the cut slope. This will minimize the potential risk of damage to the structure from future wind or ice storm caused by falling trees . Footings Footings placed on or into the proof-rolled and densified native soils or structural fill sections meeting the recommendations of the Earthwork Criteria section may be designed to a bearing value of 3000 pounds per square foot. A one-third increase in this value is permissible for short-term wind or seismic loadings . For support of the modular building, we expect that strip footings with tie downs or continuous perimeter footings with a strip footing for support of the marriage line of the double wide will be used. We recommend that continuous or strip footings have a minimum width of 16 inches . Footings should be founded a minimum of 1 . 5 feet below planned finish grade for frost protection and confinement. Paving Sections Paving sections placed on the dense native soils may be designed to a CBR value of 50 based on soil classification. This design value requires that a uniform minimum density of 95% of ASTM D 698 be achieved on the subgrade. We recommend a minimum paving section of eight inches of ballast, two inches of 5/8-inch minus crushed rock for the leveling course, and two inches of Class B asphaltic concrete. All material used for the paving section should conform to the requirements of the current edition of the WSDOT/APWA specifications The project 's civil engineer should review the recommended minimum paving sections to ensure that the section meets the minimum 00071601 Page 5 of 8 design requirements based on the project's expected traffic loads and project pavement service life. Lateral Soil Pressure It is our understanding that no retaining walls or loading dock walls, or foundation walls over four feet high and restraining earth are to be incorporated in the design. If walls meeting these criteria are to be built, we should be consulted for design information Lateral loads may be resisted either by passive soil pressures or by friction between footings and native soil or structural fill sections . For passive soil pressures for footings cast and backfilled according to the Earthwork Criteria, a design value of 300 pounds per cubic foot may be used. A coefficient of friction of 0 . 5 is permissible between footings and native soils or structural fill sections . Infiltration We understand that the existing sanitary drainfield will be used for this new structure. Infiltration of storm water on site is practical . Coarse sands and gravels or coarse sands with pebbles are found along the property line where it fronts onto Highway 101 . We recommend that the native soils found in Test Pit 4 be used for storm water infiltration. These soils infiltrated water rapidly, and a stabilized rate in free draining soils could not be determined. We estimate the rate of infiltration using the EPA standard test method is in excess of 10 seconds per inch. For design of infiltration systems, we recommend that a conservative rate of infiltration of six inches per hour be used. Seismic Site Period The project area is in Seismic Zone 3 as shown by Figure 18-2 , Seismic Zone Map of the United States, in the 1997 edition of the Uniform Building Code. Actual ground motion during a seismic event is influenced by the distance from the epicenter, the depth to bedrock under the site, and the condition (degree of consolidation and saturation) of the soils supporting the structure. We do not believe that the underlying 00071601 Page 6 of 8 soils are prone to spontaneous liquefaction under seismic loadings . Available geologic information, as g g g presented in Thickness of Unconsolidated Sediments, Puget Lowland, Washington by Hall and Othberg, 1974, shows a sediment thickness of about 400 feet under the project area. We believe that these soils are granular and are well consolidated as a result of loading by the Vashon glacial ice mass and overburden pressure. We recommend that a soil profile type SC be used for seismic design. Earthwork Criteria In areas under structures, paving sections, and sidewalks , strip all topsoil and organic material and existing fill material. For structural fill in areas under footings and slabs-on-grade, we recommend that all soils be compacted to a minimum density of 90 percent of ASTM D-1557 . This includes proofrolling native soils exposed in the bottom of the excavation before placing fill . Materials under the paving section should also be compacted to the minimum density specified in the Paving Section by proofrolling before placing the paving section. This includes proofrolling in-place soils, soils that have been disturbed during construction, and all structural fill materials . For imported structural fill, we recommend that a clean, six-inch minus, well graded gravel or gravelly sand (classifying as GW or SW as determined by ANSI/ASTM test method D-2487 ) , conforming to APWA specification 9-03 . 14 for gravel borrow, be used. We also recommend that no more than 7% by weight pass the number 200 screen as tested by ANSI/ASTM D-1140 test procedure. Other material may be used after the review and written approval by the soils engineer or engineering geologist. All fill should be placed in uniform horizontal lifts of six- to eight- inch loose thickness . Each lift should be conditioned to the optimum moisture content and compacted to the specified minimum density before placing the next lift. We further recommend that all utility trench backfill be compacted as specified above. Earthwork should be performed by an approved testing laboratory under the supervision of Bradley-Noble Geotechnical Services to ensure compliance with the compaction requirements . A r 00071601 Page 7 of 8 �I Placement of fill sections on slopes greater that 4: 1 (horizontal to vertical) will be benched as directed into the native soils. Height and width of the bench will be determined in the field by the soils engineer or engineering geologist. Unrestricted slopes shall not exceed 2 :1 (horizontal to vertical) for fill embankments and cuts that expose native soils . All fill slopes will be rolled. The project's civil engineer is responsible for the protection of the constructed fill slopes from uncollected runoff. We recommend that all cut-and-fill slopes be seeded as soon as possible after construction, so that vegetation can protect the slopes from sheet washing. No fill is to be placed during periods of unfavorable weather or while the fill is frozen or thawing. When work is stopped by rain, placement of fill will not resume until the soils engineer or engineering geologist determines that the moisture content is suitable for compactive effort and that the previously placed fill has not been loosened. The contractor will take appropriate measures during unfavorable weather to protect the fill already placed. Measures that may be required include limiting wheeled traffic and grading to provide temporary drainage of the fill . At the direction of the soils engineer or engineering geologist, the contractor will be responsible for the removal and reworking of fill that has softened or has less than the required compaction. LIMITS OF LIABILITY BRADLEY-NOBLE GEOTECHNICAL SERVICES is responsible for the opinions and conclusions contained in this report. These are based on the data relating only to the specific project and locations discussed herein. This report was prepared within the standard and accepted practices of our industry. In the event conclusions and recommendations based on these data are made by others, such conclusions and recommendations are not the responsibility of the soils engineer or engineering geologist unless he has been given an opportunity to review them and concurs in such conclusions or recommendations in writing. 00071601 Page 8 of 8 The analysis and recommendations submitted in this report are based upon the data obtained in the explorations at the locations indicated on the attached plan. This report does not reflect any variations that may occur between these explorations . The nature and extent of variations between explorations may not become evident until construction is underway. Bradley-Noble is to be given the opportunity to review the final plans and specifications for soils work. This is to verify that our geotechnical engineering recommendations have been correctly interpreted and implemented in the final design and specifications . We also recommend that we be retained to provide geotechnical services during the foundation construction and trenching. These services would include review of backfill operations, excavations, and other geotechnical considerations that may arise during construction. We would observe compliance with the design concept and project specifications . If the subsurface conditions differ from those anticipated in our explorations, we would also evaluate changes in construction specifications . BRADLEY-NOBLE GEOTECHNICAL SERVICES Report prepared by: l David Strong Engineering Geologist Please see attached soil engineer's review letter. 2 August 2000