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HomeMy WebLinkAboutBLD2006-00361 - Geotechnical Report . JUL-11-2006 TUE 09:33 AM W LAUSEN FAX NO. 5116133 P. 01 04/24/2006 11:40 3600gerse NTI JLS GROUP PAGE 02 ,/�% NORTHWESTERN TERRITORIES, INC. 540.3 i f— A JLS GROUP COMPANY 717 SOUTH PGAMGOr STREET,PORT ANGORAS,WA u342 � P Engineers Land Sunnarar1 Ovobglsts NTI Conshuallon$napactlon 7E74fs T�sting ( O)452-3491 FAX 462.84A8 v wv.ntl4u.00m E•Malh ante2�I1U4u.com- J1.s GROUP, INC. April 19,2006 i. . Sue Zalokar and Ed Ansorg PO Box 1661 Forks, WA 98331 Subj: Geotechnical Observations for House Foundation for Tax Parcel#612042003, 195 Spike Lane in Forks,Washington Dear Sue&Ed: Background On April 6,2006;Bill Payton of NTI Engineering and Surveying, Inc. (NTI)met with you at the above referenced site to conduct a test pit Inspection to provide soil data to assist in the foundation design of a single-family residence. it is our understanding that the house will be elevated about 8'above the ground on columns and that you have retained W.B. Clausen Structural Engineers, Inc. (MC)to design the foundation for the house.You'indicated that you desired to construct as much of the house as possible yourself and preferred a foundation design that would allow this.The preliminary foundation design consists of piles imbedded about 8'-10'Into the soil and formed in Sono Tubes,which you could excavate,form and pour yourself. The property is located on Spike Lane off of Nansen Anderson Road off of Oil City Road south of Forks,WA(Figure 1).The property is bounded on all four sides by forest or grass lands. The east half of the property is wooded and the west half is grass.The house site is in the middle of the property at the edge of the clearing. Site Inspection • The inspection consisted of research of available geologic Information about the site,a site visit to observe the Soils from one test pit excavated just south of the proposed house location, analysis of the test pit Information, and this report. No laboratory testing was performed. The USDA Soll Survey of Jefferson County maps the soil in the area of the subject property as the Hob silt loam.This soil Is classified as silt from the surface down to 36",then as gravel from 36"to 60",the maximum depth that was attained. The Survey states that this soil is well drained and that permeability is moderate through the sift and rapid below it.The Survey also states that corrosivity to uncoated steel is high in the upper silt and moderate below It, and for concrete it is moderate to high in the silt and moderate below it. . .. JUL-11-2006 TUE 09:33 AM 1413iiiiAUSEN FAX NO. 51041133 P. 02 04/24/2006 11:40 3604531M8 NTT JLS GR0uP PAGE 03 The water well report for the on site well indicates black topsoil down to 4', gray gravel from 4'- 17'.grey sand from 17'-34', water bearing clean gravel from 30'-35'and gray soft sand from 35'- 95', with a static water level at 10'. During the site inspection, one test pit was dug to a maximum depth of 8'with a John Deere 310 backhoe.The test pit was dug south of the proposed house location.The surface was covered with grass, and the upper 2'o1soil was damp slit. From 2'to 3.5'the soil was damp fine to medium grained relatively clean sand. From 3.5'tog',the soil was damp rounded to sub- rounded gravel with a trace of sand and silt.The ppredominant gravel size was between 1"-2" with some in the 3"-4"range,and occasional larger material. From 5'to 8'the soil was interbedded sand and gravel. Water was encountered at 6.5'and the excavation was caving In. Due to the water and the caving soil,it was impossible to excavate deeper than e'_ Upon completion of the test pit observation,the hole was backfilled with the spoils but not compacted. Conclusions and Recommendations The subsurface exploration could only get to 8'below the ground surface due to the water table nr.d twee cnvinp in of the toot Fk. It is atnndord proo too to oxoloro ow'.•vral foot haloes two bottom of the foundation so that the soli profile is known,since it is the soil below the foundation that typically affects the foundation most.The only way to explore any deeper would be to bring in a geotechnicai drill rig,which is a completely different process than a water well drill rig. It would probably cost on the order of$3,000-00 to$5,000.00 to bring in this type of drill rig. The proposed foundation of plies formed In Sono Tubes in en excavated hole would be considered an end bearing pile because the soil around the pile would be disturbed and thus could not be relied upon to provide frictional resistance on the face of the pile, and we have no soil data below the proposed bottom of the pile. FL:rther, the proposed end depth of the pile is Just below the water table where the sail is loose and susceptible to liquefaction during an earthquake,which would significantly reduce the end bearing capacity of the pile. Finally, the excavation would likely cave In while attempting to install the pile and it would be difficult to achieve a firm undisturbed base for the pile,which would cause settlement concerns for the house. It Is therefore our opinion that this type of foundation should not be considered_ A similar option to the above pile foundation would be driven piles.The piles,which could be steel or concrete,are driven into the soil with a large hammer suspended from a crane to refusal or until sufficient resistance Is achieved. This technique was done on a house on the Fiwha River just south of Highway 101 west of Port Angeles using 12"steel piles filled with concrete and a rebar cage. A local contractor,Wilson Construction, installed the piles. The benefit of this system is that there is no excavation necessary; so caving is not an issue-Also,the soli around the pile gets densified as the pile is driven. and thus frictional resistance can be developed.The down side to this option is that the depth of the pile will be unknown because of the lack of soil data at depth; the piles need to be driven to refusal or until sufficient resistance is achieved. Thus the cost of pile installation will not be known precisely.Also,given the corrosion potential of the soil mentioned in the Soil Survey, the pile may need to be coated for protection.Finally, you would loose the ability to perform this work yourself,as it requires special equipment and experience, not to mention the added cost of hiring a specialty contractor and the high price of steel if steel piles are used.A third option would be shallow concrete spread footings with grade beams if needed or a mat/raft type foundation.This option would keep the foundation above the water table where the chance of cave in of the excavation would be reduced.We would have information about the soil below the foundation. You could do the work yourself and not have 2 . . JUL-11-2006 TUE 09:34 AM W LAUSEN FAX NO. 510 1133 P. 03 04/24/2000 11:40 3604 zM 50 NTI JLS GROUP PAGE 54 the expense of a specialty contractor.And concrete is probably cheaper than steel piles. Given our understanding of the project,we would recommend this option. We recommend that for shallow foundations,the foundation be below the±2'silt layer,and founded in the underlying sand or gravel above the water table. In the foundation design,we recommend a system at,the connection between the columns and the house that will allow for re-leveling the house if settlement occurs.The amount of potential settlement Is an unknown, but some settlement should be allowed for in the design of the foundation. Because of the potential for liquefaction of the soils below-6.5'during an earthquake,the foundation should be designed to handle differential settlement without failure of structural elements. The connection between the foundation and the house should be designed with provisions for sufficient adjustment to correct the effects of differential settlement following an earthquake event. Following are the empirically derived soil index values to be used in the design of the foundation: Silt Layer(0'-21 • Unit Weight(y)(pcf): 85 • Allowable Foundation Pressure(psf): 1500 • Allowable Lateral Bearing Pressure(psf/f below natural grade): 100 a Lateral Sliding Resistance(psf): 130 • Friction Angle(4)' 26° Sand Layer(2'-3.5') • Unit Weight(y)(pcf): 100 • Allowable Foundation Pressure (psf): 2000 • Allowable Lateral Bearing Pressure(psf/f bo3low natural grade): 150 • Lateral sliding Coefficient of Friction: 0.25 • Friction Angle($): 30° Gravel Layer(3.5'-5') • Unit Weight(y)(pcf): 120 • Allowable Foundation Pressure(pal): 3000 • Allowable Lateral Hearing Pressure (psf/f below natural grade):200 • Lateral Sliding Coefficient of Friction: 0.35 • Friction Angle(e): 32` If you have any questions regarding this report or need further Information,please give us a call at(360)452-8491. Limitations • This report has been prepared exclusively for your use In conjunction with the above referenced project.The report has not been prepared for use by others or for other locations. Others may use ft only with the express written permission of the Engineer. Within the limits of scope,schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared.No other warranty,expressed or implied, is made as to the conclusions and professional advice included In this report. 3 , JUL-11-2006 TUE 09:34 AM WBliAUSEN FAX N0, 5106, 1133 P. 04 • 04/24/2006 11:40 3604SIWB NTI JLS GROUP PAGE 05 The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes endlor surface outcrop& If there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report,we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, a'[ A, /4. cr. *r4 NORTHWESTERN TERRITORIES, INC. �• y� o* ti� i, ywa 41, Robert A. Leach, P.E., MBA �ANA� Principal Engineer �ll DARES 17/7012006 Peet7t , Bill Payton,L.E.G. Engineering Geologist fir, � � Vie; T I1�1 • --.001.1111111 4:1O.r\8131'ReportslZALSO601,test pits fcr foundation.4(26.12)01 City Rd.doc -1 William C, Pa ton Jr. lExpires 11/06/05• 4 • • 5-24 CIVIL ENGINEERING REFERENCE MANUAL 12f f rtq Appendix B: Manning Nomograph 5\pk.P- ft,anD t.ete-k?- Z"ob -e,' 1-f /4 1110 c f%. ' Ac. 0-1?c7 .3 .2 -50 2 =40 .3 -30 4 -.01 .10 - - .09 . .08 .5 c 120 .07 0, . .06 .6 .05 \5_, ' - .04 6' ,, .8 \ c H- -.02 9 \ © 6k4C 50 . 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