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HomeMy WebLinkAboutPettygrove Block 4, Lots 3 & 4 - Geotechnical Evaluation for 1206 Taylor Street - 2011.04.11Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 i i I I April 2011 E C E U Mr. Craig Owen, P.E.APR 14 2011 Owen Structural Engineering Inc. 220 E. First Street CITY Of PORT TOWNSEND Port Angeles, Washington 98362 1 DSD Subject: Geotechnical evaluation if in fact the slope at the Pat Kenna property, 1206 Taylor Street, Lots 3 &4 of the Pettygrove Addition, Block 4, Tax Parcel Number 984600402 in the City of Port Townsend is in fact a Geologically Hazardous Area, based on 19.05.100 Critical Area 4. Dear Mr. Owen: As requested, I met with you and the project contractor on 7 April 2011 at the above site in order to observe site soil profile and to develop an opinion if this is a potentially unstable slope. Our work has consisted of a site visit, review of geologic information, and preparation of this report. We have also reviewed the slope profile prepared by you as well as the information sheet provided for Geologically Hazardous Area in the Port Townsend Municipal Code. i At the time of our site visit, a trench had been excavated in the toe of the slope and extending north to behind the proposed retaining wall location. This trench has exposed the soil profile. You also provided us with a cross section of the slope prepared by you. On this cross section, the extent of the trench is indicated. Based on our field observation and discussion of the methods used to prepare this cross section, it is our opinion that the profile shown is accurate and does represent the actual slope profile which is typical along the back property line. The slope profile is relatively uniform from the toe to about 42 feet upslope towards the north property line. The lower slope has an average slope profile of 17 degrees 30% slope). At the top of slope, we find a short steep slope of about six feet of vertical relief. With this slope being steeper than 22 degrees (40% slope) but less than 10 feet in height it does not meet the criteria for identifying geologically hazardous areas. From our field discussion and review of the note on the profile, we understand that this is a fill section that was likely placed over the slope of this property during the construction of 11040201 Page I of 3 11040201 Page 2 of 3 the residence upslope, about 100 to 120 years ago. Based on our field observations, this slope appears to be stable with no indication of recent movement or instability. It is well vegetated and in our opinion presently stable. The test pit exposed a topsoil layer mantling the slope of about 14 to 18 inches thick. The topsoil overlies a gravelly silty fine sand that based on field observation is in the firm range of density. The sands will provide foundation support for the retaining wall structure as well as being the soils retained. No seepage or other indication of ground water was observed. Slope stability in granular soils, such as found at this site, are controlled by the angle of internal friction of the soils. Using Table 3-2 Empirical Values for the density and angle of internal friction in Bowles Foundation Analysis and Design 3rd edition for soils in the firm range, we can estimate that the firm native sands would have an angle of internal friction in the 27 to 32 degree range and a unit in the 105 to 115 p.c.f. range. We consider the angle of internal friction from this table to be conservative. The actual average unit weight of the soils will be influenced by the actual percentage of plus No. 4 material. With the angle of internal friction greater than the slope angle of the soils, slope stability can be inferred. With no ground water present, even after the recent heavy rains of the area, water than is not considered to have a destabilizing influence. Based on our work and our understanding of Section B of the Geologically Hazardous Area, this site does not meet the criterion nor is it in fact based on geotechnical evaluation a hazardous area. As part of our site meeting, we discussed the proposed development plans for the retaining wall. We understand that the retaining wall will be built in two section. We agree with this approach to limit removal of toe support. The plans indicate a retaining wall height of six feet. The contractor must be aware that once the forms or cast wall exceeds four feet in height, the void between the forms or wail and the slope is now considered to be a trench under the Department of Labor and Industries Standards. Sloping or shoring of the slope will be required as specified in Chapter 296-155 WAC, Part N, Excavation, Trenching and Shoring. We recommend a soil type C be used for control of sloping and shoring. To improve resistance of the retaining wall to sliding, you have indicated the use of coarse crushed rock placed and compacted between the bottom of footing and native soils. We agree with this approach. A coefficient of friction of 0.5 is permissible between the footing and coarse crushed rock. The plans include a perforated pipe at the inside base of the retaining wall to collect and dispose of any water attempting to pond against this wall. You have included another 11040201 Page 3 of 3 drain pipe at the outside base of the coarse crushed rock fill. We agree with this design approach to collect and control any water that may flow down the slope to the retaining wall. We do recommend that the drains be constructed using rigid plastic pipe rather than flexible ADS type of pipe. Rigid pipe is more resistant to crushing and deflection during placement of backfill. I We do recommend that you include weep holes in the design. The weep holes should have a minimum diameter of 3 inches and a maximum spacing of 10 feet. The bottom of the weep hole should be just above the crown of the wall drain pipe. If water is observed flowing from the weep holes, this indicates that the wall drain has failed and needs cleaning to restore its function. I G With gravelly silty fine sands being the soils retained, and if the owner is willing to accept the possibility of some minor lateral deflection of the wall, we recommend that the wall be designed to resist an active soil pressure of 35 p.c.f. With the slope extending above the wall, surcharge loading must be included in the design. We suggest that a uniformly distributed soil load equal to H2O highway loading be used for the surcharge value. If you have any additional geotechnical questions on this report, please contact us at our Olympia office. Cordially, 3 BRADLEY-NOBLE GEOTECHNICAL SERVICES i David C. Strong, L.E.G. E o a ,