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HomeMy WebLinkAboutBLD2001-00180 Geotechnical Report 7 oLDc /,_ IfE) /�� • c r T'` c fl 13256 Northeast 20th Street,Suite 16 V Li Li 1 1 Bellevue,Washington 98005 CONSULTANTS, INC. (425)747-5618 FAX(425)747-8561 /3i sic #__•) ?es 3 7 July 3, 2000 JN 00071 Marc Mauney 3821 Cascadia Avenue South Seattle, Washington 98188 Subject: Geotechnical Engineering Considerations Proposed Mauney Residence 2137 Oak Bay Road Jefferson County, Washington Dear Mr. Mauney: This letter presents our professional geotechnical opinions related to the stability of the subject property and the feasibility of constructing a new residence on the site. The scope of our work has consisted of the following: • Visiting the site once on February 16, 2000 to observe the existing site conditions and to monitor the excavation of four test pits on the property. • Discussing with you and John Simpson, your architect, the historical landslide information provided to you through discussions with Jefferson County personnel. • Developing this summary report. No test borings were completed for our assessment of the geology that underlies the site. Our work was authorized by your acceptance of our confirming proposal dated May 24, 2000. Based on our discussions with you and John Simpson, we anticipate that you intend to construct a single-family home on the property. Initially, this new house was to be located immediately west of the existing residence. However, following our preliminary assessment, and numerous discussions with both you and John Simpson, the house site has been shifted further upslope of the initial location. We were provided with a copy of the May 12, 2000 Conceptual Site Plan that illustrates the new house site. This plan contains a cross-section that indicates the house to consist of two floors over a daylight basement level. The lowest floor elevation is indicated to be 356 feet, which is up to approximately 5 feet below the existing grade. We anticipate that 3 to 4 feet of fill will be placed behind the west wall of the house for the driveway and sidewalks that will be located at the elevation of the main floor (366 feet). SITE CONDITIONS SURFACE The subject property is a large, trapezoidal-shaped lot situated along Oak Bay in unincorporated Jefferson County. The provided property dimensions indicate that the size covers a total area of Marc Mauney JN 00071 July 3, 2000 Page 2 10.2 acres. At the time of our site visit, the property was occupied by several structures. There is a primary residence and a smaller house located in the southeastern portion of the site. This area also contains several outbuildings (carport, pump house, and shed). Two sheds and a small trailer were located in the eastern, central, portion of the lot. An unpaved driveway curves through the western two-thirds of the property from Oak Bay Road. With the exception of the driveway and yard areas, much of the site is covered with medium to large-sized trees. The ground surface on the property generally slopes gently to moderately down toward the east. The eastern approximately two-thirds of the site exhibits hummocky topography, with several relatively-level bench areas separated by short, steep slopes. The proposed house site is located on one of these benches. We observed many of the trees growing on, and above, the steep slopes to have multiple curves in their trunks. This occurs in both deciduous and evergreen trees. Near the angled eastern property line is a very steep bluff that drops to the beach along Oak Bay. The height of this bluff increases from north to south. Based on our observations, the face of this bluff is subjected to undercutting from wave attack, and has undergone recent slope movement. The face of the taller, southern, portion of the bluff was bare of vegetation. This bare area extended onto the adjoining southern property. In the central portion of the site is a set of timber stairs that originally provided access down to the beach. These stairs have been wracked and distorted by slope movement within the last few years, with no indications of attempts to repair the damage. We also observed slide debris and trees accumulated on the beach from a landslide on the bluff further south of the site. In addition to the shallow soil movement that appears to have occurred in the recent past on the face of the bluff, we noted a wide tension crack between the primary residence and the top of the bluff. This tension crack was located 5 to 10 feet from the crest of the bluff, indicating that a relatively large mass of soil is experiencing failure. Similarly, a large tension crack was noted on the downslope edge of the driveway near the northern end of the property. The approximate locations of these tension cracks are shown on the attached reduced copy of the Conceptual Site Plan. Understanding of Landslide History Our understanding of the landslide history in the area is based on information you have gained from discussions with Jefferson County personnel, and local design professionals. We understand that slope movement frequently requires regrading or repaving of Oak Bay Road adjacent to the site. Immediately northeast of the site is a house that has been affected by slope movement. While we did not observe this house closely, it appears to be located on a bench area above the very steep bluff. We understand that Jefferson County has deemed this house as unsafe for habitation due to serious structural distress from the slope movement. SUBSURFACE The subsurface conditions were explored by excavating four test pits at the approximate locations shown on the attached reduced copy of the Conceptual Site Plan. Our exploration program was based on the proposed construction, subsurface conditions encountered during exploration, and the presence of buried utilities. The test pits were excavated during our February 16, 2000 site visit with a trackhoe. The undersigned principal engineer observed the excavation process, logged the test pits, and obtained representative samples of the soil encountered. "Grab" samples GEOTECH CONSULTANTS, INC. • Marc Mauney JN 00071 July 3, 2000 Page 3 of selected subsurface soil were collected from the backhoe bucket. No test pit logs are attached, but a detailed discussion of the conditions encountered in the explorations is provided in the following paragraphs. In addition to the test pits, we evaluated the subsurface conditions by examining the conditions exposed on the very steep, eastern bluff. Soil Conditions The four test pits were excavated to depths of 13 to 15 feet and encountered subsurface conditions that were generally similar. The soil revealed throughout the full depth of the test pits consisted of loose, gravelly, silty sand that contained small to large fragments of dense glacial till. Glacial till is a glacially-compressed mixture of gravel, silt and sand. Interspersed throughout this loose soil matrix were pockets of organics. Based on our observations, the soil encountered in the test pits has been disturbed and broken up by past slope movement. No indications of intact soil were noted to the maximum 15-foot depth of the explorations. Caving of the loose soils was observed in several of the test pits. The near-surface soils observed on the eastern bluff appear to consist of disturbed soils, similar to those found in the test pits. Near the base of the taller portions of the bluff, we observed what appears to be heavily-fractured, weathered sandstone. No groundwater seepage was observed in the test pits, or on the eastern bluff. CONCLUSIONS AND RECOMMENDATIONS GENERAL Based on the results of our explorations and observations, and the information provided to us regarding the landslide history of the site vicinity, it appears that the subject property lies within a large landslide mass that extends at least from Oak Bay to Oak Bay Road. No intact soils were encountered in the test pits, indicating that the slide plane lies beyond a depth of 15 feet. The presence of isolated benches separated by what appear to be slide scarps indicates that there are smaller slide blocks within the larger landslide mass. The presence of trees with curved trunks on the slide scarps is an indication that there has been at least some movement within the life of the trees. The neighboring affected house to the north appears to be situated on an individual slide block. In addition to the overall, larger landslide, we noted indications of incipient slope failures on the steep bluff and along the edge of the driveway. The landslide mass may be the result of ancient movement occurring several hundred or several thousand years ago. The more recent movement is likely the result of remobilization of all, or a part of, the landslide mass by excessive groundwater. Large earthquakes can also trigger movement in ancient landslide masses. Given the available information, it appears that movement of at least individual blocks occurs periodically within the larger landslide mass. This movement can be expected to continue in the future. It is impossible to accurately predict the frequency and magnitude of future slope movements that could occur. In recent years, we are familiar with several large, ancient landslides that have been reactivated following extended periods of wet weather. The size of the landslide GEOTECH CONSULTANTS,INC. • Marc Mauney JN 00071 July 3, 2000 Page 4 mass and the unknown depth to stable soil appears to make stabilization unfeasible for a single residence. As we have discussed, it is our opinion that the site could be developed with another residence, if the owners were willing to accept the likelihood that future soil movement will occur. Occupied structures would need to be constructed in such a manner as to limit the potential for catastrophic collapse in the event of continued movement of the landslide mass. Also, the development must not increase the potential for instability. We anticipate that Jefferson County will require a signed hold harmless before they will allow construction of a new home in a known landslide that will not be stabilized. The recommendations of this report are intended to provide safety for the occupants of the residence, by preventing rapid collapse of the foundation system due to any one episode of slope movement. Our recommendations are not intended to prevent damage to the residence and other improvements, or to ensure that these elements are usable after slope movement occurs. The proposed house location is in the center of a bench area, which may be smaller slide block. This will prevent the foundations from spanning over a known scarp in the overall slide mass. If large downsets or tension cracks form during future slope movement, they are most likely to occur at the old scarps around the bench areas. The house can be constructed on a conventional foundation system or a mat foundation, provided they are heavily reinforced to span across tension cracks and downsets that may form during future movement of the landslide mass. The construction techniques should allow for future relevelling of the floors and walls in the event of small amounts of slope movement. If large movement occurs, it could be necessary to reconstruct the foundation system entirely. Ongoing slope movement could cause damage to on-grade elements, such as the driveway, decks, or slabs, or to utilities. Maintenance and repair of this damage should be anticipated. If the incipient slope failure along the east side of the driveway progresses, it may be necessary to reconstruct the driveway on the northern end of the property. Recession of the bluff face can be expected to continue. This process can be slowed, but not stopped, by constructing a bulkhead along the beach. Continued movement of the slope may push over the bulkhead, requiring possible periodic maintenance and repair. The proposed setback of over 200 feet from the face of the bluff should be more than adequate to protect the residence from damage due to continued recession of the bluff. Water from drains and impervious surfaces should not be discharged on, or close to, the eastern bluff. Alternatively, water could be discharged onto the ground near the house in a uniform fashion, or the water could be tigthlined to a discharge point located on the beach below the bluff. Only minor fill placement is planned on the upslope side of the residence. This should not adversely affect the stability of the landslide mass. We recommend that fill placed on the eastern, downslope, side of the house be limited to approximately 2 feet in thickness, and that the fill not reach further east than the existing driveway. No clearing or grading, other than possible removal of the existing residences, should occur within 50 feet of the crest of the bluff. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. GEOTECH CONSULTANTS, INC. Marc Mauney JN 00071 July 3, 2000 Page 5 We recommend including this report, in its entirety, in the project contract documents. Additionally, a copy of this report should be provided to future property owners so they may be aware of our findings and recommendations. CONVENTIONAL OR MAT FOUNDATION The house can be supported using an on-grade, shallow foundation system that has been heavily- reinforced to reduce the potential for catastrophic differential settlement. This can be accom- plished either using a series of interconnected continuous footings that are similar to grade beams, or a mat foundation. These foundations would bear on at least 12 inches of compacted crushed rock placed over firm soils underlying any topsoil or existing fill. We recommend that continuous footings have a minimum width of 16 inches. The footings or mat should be bottomed at least 18 inches below the lowest adjacent finish ground surface for frost protection. Continuous footings or a mat foundations should be sufficiently rigid to theoretically be able to span a minimum distance of 10 feet without soil support. This requirement will likely govern the reinforcement for a mat foundation, rather that utilizing a modulus of subgrade reaction of 100 kips per cubic foot (kcf) for the design. An allowable bearing pressure of 2,000 pounds per square foot (psf) is appropriate for footings or a mat supported as recommended above. A one-third increase in this design bearing pressure may be used when considering short-term wind or seismic loads. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level structural fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARA%IETER VALUE Coefficient of Friction 0.45 Passive Earth Pressure 300 pcf Where: (i) pcf is pounds per cubic foot,and (ii) passive earth pressure is computed using the equivalent fluid density. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate values. SEISMIC CONSIDERATIONS The site is located within Seismic Zone 3, as illustrated on Figure No. 16-2 of the 1997 Uniform Building Code (UBC). In accordance with Table 16-J of the 1997 UBC, the site soil profile within 100 feet of the ground surface is best represented by Soil Profile Type SD (Stiff Soil). The GENERAL section contains an additional discussion of seismic considerations related to slope stability and foundation design. GEOTECH CONSULTANTS, INC. Marc Mauney JN 00071 July 3, 2000 Page 6 PERMANENT FOUNDATION AND RETAINING WALLS Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures imposed by the soil they retain. The following recommended design parameters are for walls that restrain level backfill: PARAMETER DESIGN VALLE Active Earth Pressure* 40 pcf Passive Earth Pressure 300 pcf Coefficient of Friction 0.45 Soil Unit Weight 135 pcf Where: (I) pcf is pounds per cubic foot,and(if)active and passive earth pressures are computed using the equivalent fluid pressures. * For a restrained wall that cannot deflect at least 0.002 times its height, a uniform lateral pressure equal to 10 psf times the height of the wall should be added to the above active equivalent fluid pressure. The values given above are to be used to design permanent foundation and retaining walls only. The passive pressure given is appropriate for the depth of level structural fill placed in front of a retaining or foundation wall only. No safety factor is included in the passive and friction values. We recommend a safety factor of at least 1.5 for overturning and sliding, when using the above values to design the walls. Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from corners in the walls. The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid density. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. The wall design criteria assume that the backfill will be well-compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand-operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. Retaining Wall Backfill Backfill placed behind retaining or foundation walls should be coarse, free-draining structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. The on-site GEOTECH CONSULTANTS,INC. • Marc Mauney JN 00071 July 3, 2000 Page 7 soils should not be reused as wall backfill, due to their poor drainage characteristics and low compacted strength. The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the wall. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surface water to percolate into the backfill. Where necessary, surface drains should be provided to collect run-off and prevent ponding of water behind walls. The above recommendations are not intended to waterproof the below-grade walls. The performance of subsurface drainage systems will degrade over time. Therefore, waterproofing should be provided where moist conditions or some seepage through the walls are not acceptable. This typically includes limiting cold-joints and wall penetrations, and using bentonite panels or membranes on the outside of the walls. Applying a thin coat of asphalt emulsion is not considered waterproofing, but will only help to prevent moisture, generated from water vapor or capillary action, from seeping through the concrete. DRAINAGE CONSIDERATIONS Foundation drains should be provided around the perimeter of the structure, and at the base of all earth-retaining walls. These drains should be surrounded by at least 6 inches of 1-inch-minus, washed rock and then wrapped in non-woven, geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At its highest point, a perforated pipe invert should be at least 6 inches below the bottom of a slab floor or the level of a crawl space, and it should be sloped for drainage. All roof and surface water drains must be kept separate from the foundation drain system. A typical drain detail is attached to this report as Plate 1. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. Drainage should also be provided inside the footprint of a structure, where a crawl space will slope or be lower than the surrounding ground surface, or an excavation encounters significant seepage. We can provide recommendations for interior drains, should they become necessary, during excavation and foundation construction. The excavation and site should be graded so that surface water is directed off the site and away from the tops of slopes. Water should not be allowed to stand in any area where foundations, slabs, or pavements are to be constructed. Final site grading in areas adjacent to buildings should slope away at least 2 percent, except where the area is paved. Water from roof, storm water, and foundation drains should not be discharged on, or near, the eastern bluff. LIMITATIONS The analyses, conclusions, and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the test pits and observed in the soils exposures on the bluff are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those anticipated, we should be advised at once GEOTECH CONSULTANTS, INC. Marc Mauney JN 00071 July 3, 2000 Page 8 so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated soil conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking soil samples in test pits. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects. As discussed in the GENERAL section, the recommendations presented in this report are directed toward the protection of only the proposed structure from collapse due to slope movement. Predicting the future behavior of the identified landslide mass is an inexact and imperfect science that is currently based mostly on the past behavior of slopes with similar characteristics. The owner must ultimately accept the likelihood that some future slope movement could occur, possibly requiring repair or reconstruction of the house, driveway, utilities, or other on-grade elements. This report has been prepared for the exclusive use of Marc Mauney, and his representatives, for specific application to this project and site. Our recommendations and conclusions are based on observed site materials, and our understanding of the previous landslide history of the site. Our conclusions and recommendations are professional opinions derived in accordance with the scope of our services outlined in our proposal. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. ADDITIONAL SERVICES In addition to reviewing the final plans, Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. The following plates are attached to complete this report: Reduced Copy of Conceptual Site Plan Plate 1 Typical Footing Drain Detail GEOTECH CONSULTANTS,INC. Marc Mauney Page J N 00071 1 July 3, 2000 We appreciate the opportunity to be of service escate to contactisproject. us. If you have any questions, or if we may be of further service, please do not Respectfully submitted, GEOTECH CONSULTANTS, INC. Mc jA, 4G o wAsN co 27845 ck °.� � 'C/STE45 ��� SIONAL ECG 7J3/ l EXPIRES 10/25/21 Marc R. McGinnis, P.E. Principal cc: JWS Design Associate-John Simpson MRM/JHS: mrm GEOTECH CONSULTANTS,INC.