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HomeMy WebLinkAboutBLD2005-00519 Geotechnical Report 4r. Gary A. Flowers, PLLC Geological & Geotechnical Consulting 19532 12th Avenue NE Shoreline,WA 98155-1106 August 7, 2005 Project No. GF05024 ( *-- • Dr. Peter& Deborah Ciani 111 831 Thorndyke Road AUG -- 9 2005 Port Ludlow, WA 98365-9291 7 � i Subject: Geological/Geotechnical Assessment Proposed New Ciani Residence 'j 1 ,6 Maxview Drive Port Ludlow, Jefferson County, Washington Dear Dr. and Mrs. Ciani: This report presents the rei' s of our geological/geotechnical evaluation of your proposed new home site to be located at T'36 Maxview Drive, near Port Ludlow, Jefferson County, Washington. It is our understanding that you plan on constructingtwo a new, story, approximate 1,800 square foot single family residence on the northern portion of the property. The purpose of our site evaluation was to document existing shallow soil and ground water conditions in the proposed building location and to provide geotechnical design recommendations for construction of the proposed improvements. A previous study was performed f 'ou by our office, titled Geological/Geotechnical Report, Proposed New Ciani Residence, Maxview Drive, Port Ludlow, Jefferson County, Washington, dated March 30, 2005. The purpose of the previous report was to address landslide and seismic hazard issues associated with the steep slope located to the west of your proposed building site. The results of the previous investigation resulted in a recommendation that the new house be located a minimum of 100 feet from the toe of the steep slope and that no further mitigation was required for these identified hazards. EXISTING SITE CONDITONS The subject site is a near trapezoidal shaped property encompassing approximately 3'/2 acres. The parcel contains both a level bench area (eastern portion) and a steep slope area (western portion). The steep slope area was evaluated in our previous report listed above. Therefore, this report will only focus on the bench area where the house will be constructed. Gary A. Flowers,PLLC. 19532 12th Avenue NE Shoreline,Washington 98155-1106 206-417-7640 1 Ciani Residence Geological/Geotechnical Report 156 Maxview Drive, Port Ludlow,Jefferson County, Washington The building site is comprised of two relatively level benches separated by an approximate 4 foot high step. Each bench is on the order of 70 feet wide. The proposed residence will span over the step area and will thus require a structural fill pad for the eastern portion of the house. The subject site has only field grass for vegetation. Subsurface Soil and Ground Water Conditions In order to characterize the shallow subsurface soil and ground water conditions on the property three exploration pits were excavated on the site on July 20, 2005. The results of these explorations indicated that suitable foundation bearing soils for the new residence are located at shallow depth. Logs of each of the exploration pits are as follows: EP-1 (near southeastern corner of house) 0-6" Sod/Topsoil 6"-4' Dense to very dense,moist to very moist with depth, brownish gray, silty, very fine sand BOH @ 4" No caving No ground water EP-2 (near northeastern corner of house) 0-6" Sod/Topsoil 6"-4' Dense to very dense,moist to very moist with depth, brownish gray, silty, very fine sand BOH @ 4" No caving No ground water(slight free water on soil at 3 3'� 1r EP-3 (near west central edge of house) AUG -� 9 2005 I 0-3" Sod/Topsoil 3"-4' Dense, moist, brownish gray, medium sand with some silt BOH @ 4" No caving No ground water(slight free water on soil at 3') In addition to the three exploration pits, an approximate 18 inch deep trench had previously been excavated along the lower bench area (east side of proposed house location) for purposes of placing an interceptor trench. An evaluation of the soils exposed within this trench indicated that they were the same dense to very dense silty, very fine sand soils encountered in EP-1 and EP-2. The encountered soils appear to be an advance outwash or transitional bed equivalent type soil. These sediments were deposited by meltwater emanating from the front of an advancing glacial ice sheet, or in a fluvial environment between advancing ice sheets, and were subsequently overrun and consolidated by the weight of the ice sheet. Gary A.Flowers,PLLC. 19532 12th Avenue NE Shoreline,Washington 98155-1106 206-417-7640 2 Ciani Residence Geological/Geotechnical Report 156 Maxview Drive, Port Ludlow,Jefferson County, Washington Hydrology No indication of standing or flowing water was present at the proposed house location at the time of our field work. A small stream was previously observed to the north of the house location. The water emanates from the steep slope area and flows offsite to the north. There was no evidence of erosion anywhere on the parcel. A water table was not encountered in any of our exploration pits. A small amount of interstitial water was observed in the soils at a depth of about 3 feet below existing ground surface in exploration pits EP-2 and EP-3. This would indicate that some amount of ground water is located at relatively shallow depths and that the ground water table is likely to be shallower during the winter months. Seismic Hazards Generally, there are four types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture; 2) seismically induced landslides; 3) liquefaction; and 4) ground motion. To the best of our knowledge at this time there are no fault zones located in the general area of the subject site so the risk of surficial ground rupture is considered to be low. Seismic hazards associated with the steep slope to the west of the proposed house location were addressed in our previous report. Based on the setback from the toe of the steep slope no further mitigation was recommended. Due to the dense to very dense nature of the underlying sediments the risk of liquefaction on the site is low. Based on the encountered stratigraphy, it is our opinion that any earthquake damage to the proposed structure, when founded on suitable foundation bearing strata in accordance with the recommendations provided in this report would be caused by the intensity and acceleration associated with the event and not any of the above-discussed impacts. Design of the project should be consistent with 2003 International Building Code (IBC) guidelines. In accordance with the 2003 IBC, the following values should be used: -1 r Site Class D (Table 1615.1.1) l r I Ss = 125% (Figure 1516[1]) _ r �~ Si =44% (Figure 1516[2]) , 1 14 AUG - 95 CONCLUSIONS AND RECOMMENDATIONS On the basis of our geologic research and field explorations, the encountered soils will provide suitable support for a two story, single family residence. Due to the difference in elevation between the east and west sides of the proposed building location, an approximate 3 to 4 foot thick fill pad will be required to provide a level building pad. Gary A. Flowers, PLLC. 19532 12th Avenue NE Shoreline,Washington 98155-1106 206-417-7640 3 Ciani Residence eological/Geotechnical R part#I 156 Maxview Drive, Port Ludlow,Jefferson County, Washington AUG - 2005 J% Site Grading The site should be cleared of all vegetation and topsoil. These materials should be removed from the site or stored for future landscape use. Any non-organic soils that are to be reused on the site as structural backfill must be kept covered at all times and protected from the weather. A grading plan was not available at the time this study was completed. However, based on discussions with the owner we understand that 3 to 4 feet of structural fill will be placed to provide a level building pad for the new residence. We further understand that most of the structural fill will be imported and will consist of silty, gravelly sand, a sample of which was observed on the site at the time of our field study. The observed sample was very dry and will likely require moisture conditioning at the time of placement in order to obtain suitable compaction. Structural fill is defined as non-organic soil, placed in maximum 8-inch loose lifts with each lift being compacted to a dense, non-yielding condition prior to installation of the succeeding lift. Prior to installation of the structural fill pad, any disturbed, natural soils must first be recompacted to a dense, nonyielding condition. If the subgrade soils are wet and cannot be recompacted to a dense, nonyielding condition it may be necessary to place a layer of 4 to 6 inch quarry spalls prior to placement of structural fill. Should this occur, the quarry spalls should be placed in an 8 to 12 inch thick layer and properly consolidated with a compactor. The top of all compacted fill should extend horizontally outward a minimum distance of 3 feet beyond the location of perimeter footings or pavement edges before sloping down at a maximum angle of 2H:1 V. The planned structural fill pad will be placed over the top of the interceptor drain that was being constructed at the time of our site visit. As discussed at that time, the drain should be comprised of heavy duty, rigid, PVC pipe instead of the planned flexible ADS pipe. The drain should also be covered with a nonwoven geotextile filter fabric such as Mirafi 160N or equivalent. Foundation Recommendations A typical spread footing foundation will be suitable for the planned residential construction. Most of the foundation will be supported by a structural fill pad as described above in the Site Grading section. For the portion of the foundation that will be supported on the dense to very dense natural soils, the exposed soils should be scarified to a minimum depth of 12 inches and then recompacted to a dense, nonyielding condition. This will provide for more uniform settlement of the foundations between the different types of bearing soils. Provided the structural fill pad is correctly constructed and approved, and the natural soils are scarified and recompacted as described, an allowable soil bearing value of 2000 psf may be used in the design of these footings, including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. Perimeter footings for the proposed structures should be buried a Gary A. Flowers,PLLC. 19532 1211i Avenue NE Shoreline,Washington 98155-1106 206-417-7640 4 Ciani Residence Geological/Geotechnical Report 156 Maxview Drive, Port Ludlow,Jefferson County, Washington minimum of 18 inches into the surrounding soil for frost protection. Settlement of footings placed as detailed herein should be less than 1 inch total and %2 inch differential. However, foundations placed on disturbed or improperly compacted soil may result in increased settlement. All foundation excavations should be inspected by a representative of this firm, prior to concrete placement, to verify that the design bearing capacity of the soils has been attained and that construction conforms to the recommendations contained in this report. Lateral loads can be resisted by friction between the foundation and the supporting soils, and/or by passive earth pressure acting on the buried portions of the foundations. The foundations must be backfilled with structural fill compacted to a dense, nonyielding condition to achieve the passive resistance provided below. The structural fill must extend horizontally outward from the embedded portion of the foundation a distance equal to at least three times the embedment depth over which the passive resistance is applied. We recommend the following design parameters: • Passive equivalent fluid =250 pcf • Coefficient of friction= 0.35 The above values are allowable and include a factor of safety of at least 2.0. Retaining Walls At this time we are unaware of any retaining walls planned for the project. If walls under 8 feet high are planned they should be designed for an equivalent fluid pressure of 35 pcf if free to rotate and 55 pcf if fixed, with level backfill. Passive resistance would be 250 pcf. Any retaining walls taller than 3 feet must be lined with a minimum of 12 inches of washed rock to within 1 foot of finish grade or an approved drainage mat such a MiraDrain or IncaDrain. The washed rock or drainage mat must tie into the footing drain. Floor Support Recommendations For slab-on-grade floors we recommend that the upper twelve (12) inches of natural soil beneath the slab be recompacted to a firm, unyielding condition. Any yielding areas should be overexcavated and filled with approved structural fill. A capillary break layer consisting of 4 inches of washed pea gravel and a heavy duty (minimum 10 mil), polyethylene plastic vapor barrier should be provided under any floor slabs where moisture intrusion is a concern. If the vapor barrier becomes compromised in any way during construction it should be replaced or an additional layer added. Penetrations through the vapor barrier should be wrapped and taped. ( _ _,F5) j`+ ; ii AUG 9 2005 I Gary A. Flowers,PLLC. 19532 l2'h Avenue NE Shoreline,Washington 98155-1 106 206-417-7640 5 Ciani Residence Geological/Geotechnical Report 156 Maxview Drive, Port Ludlow,Jefferson County, Washington Site Drainage All storm water runoff from impervious surfaces should be collected and piped into an approved storm water drainage system. We understand that storm water will be placed into the small existing drainage the flows off the site to the north. There is also a drainage ditch along the west side of Maxview Drive that could be used. Infiltration of storm water does not appear to be a suitable option for this site due to the very dense, fine grained nature of the underlying soils. All perimeter footing walls should be provided with a drain at the footing level. Drains should consist of rigid, perforated, PVC pipe surrounded by washed drain rock (maximum 5/8 inch) or pea gravel. Either the pipe or the drain rock should be wrapped with a nonwoven geotextile filter fabric. The level of the perforations in the pipe should be set approximately 2 inches below the bottom of the footing and the drains should be constructed with sufficient gradient to allow gravity discharge away from the buildings. Roof and surface runoff should not discharge into the footing drain system, but should be handled by a separate, rigid, tightline drain that discharges into an approved storm water conveyance system. In planning, exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. Care should be taken to avoid any grading or construction of any berms or other structures that may cause storm water to flow under any pavements. Water introduced under the pavement section could result in an unstable subgrade and future pavement failures. Erosion Protection The natural, on-site soils and proposed fill soils contain fines and are considered to be subject to erosion from both sheet flow and channelized flow. Due to the gentle grades on the site the erosion hazard can be handled with standard construction practices. An approved erosion control plan should be provided and implemented. The plan should include, at a minimum,placement of silt fences along the lower reaches of the property. The silt fences should be placed as per local municipal specifications and properly maintained. Soil stockpiles should be covered with plastic during periods of inclement weather. Areas stripped of vegetation during construction should be replanted, or otherwise protected, as soon as possible. Typically the construction entrance should be rocked to minimize off-site soil transport. However, since there is a long gravel road out to the first pavement this should not be necessary unless required by local regulations. SUMMARY Based on our site reconnaissance and subsurface explorations the site appears to be suitable for the proposed development provided the recommendations presented herein are properly implemented. SIP w !, r Gary A. Flowers,PLLC. ' 19532 12` Avenue NE Shoreline,Washington 98155-1106 -_,206-41L2.60 6 -.I, Ciani Residence Geological/Geotechnical Report 156 Maxview Drive, Port Ludlow,Jefferson County, Washington We recommend that we be retained to review those portions of the plans and specifications that pertain to grading or foundation installations to determine that they are consistent with the recommendations of this report. Construction monitoring and consultation services should also be provided to verify that subsurface conditions are as expected. Should conditions be revealed during construction that differs from the anticipated subsurface profile, we will evaluate those conditions and provide alternative recommendations where appropriate. Field construction monitoring and observation services should be considered an extension of this initial geotechnical evaluation and are essential to the determination of compliance with the project drawings and specifications. Such activities would include site clearing and grading, subsurface drainage, fill placement and compaction and bearing soils verification. Our findings and recommendations provided in this report were prepared in accordance with generally accepted principles of engineering geology and geotechnical engineering as practiced in the Puget Sound area at the time this report was submitted. We make no other warranty, either expressed or implied. Sincerely, *ILT Pkt FW o - ssti! n , 040 Ca ort/Y P-41*-\''''i r,l � +1 .. I:T ,,,Meilly. z iii , 7""' % 06 16271� , ': o‘-. ♦ , _ ioNAL 4 gios -. ,_ == Ge l EXPIRES '1—-yc�—o C l I GerY A- Flowers y Gary A. Flowers, P.G.,P.E.G. Robert M. Pride, P.E. Engineering Geologist Geotechnical Engineer AUG - 9 2005 ,_':./ Gary A. 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