GEOTECHNICAL ENGINEERING REPORT PROPOSED RESIDENCE & DRIVEWAY IMPROVEMENTS 3835 WEST MERCER WAY MERCER ISLAND, WASHINGTON

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1 GEOTECHNICAL ENGINEERING REPORT PROPOSED RESIDENCE & DRIVEWAY IMPROVEMENTS 3835 WEST MERCER WAY MERCER ISLAND, WASHINGTON Project No Credit: Google Earth Prepared for: Bradford & Jill Jackson 3213 Eastlake Avenue East, Ste B Seattle, Washington Tel: Fax: Geotechnical & Earthquake Engineering Consultants

2 Geotechnical & Earthquake Engineering Consultants PanGEO Project No Bradford G. Jackson Jill A. Jackson 3835 West Mercer Way Mercer Island, Washington Subject: Geotechnical Engineering Report Proposed Residence & Driveway Improvements 3835 West Mercer Way Mercer Island, Washington Dear Jill & Brad: As requested, PanGEO Inc. completed a geotechnical engineering study to assist you and your designers with the design and construction of the proposed new residence and associated driveway improvements at 3835 West Mercer Way, on Mercer Island, Washington. In preparing this report, we performed a reconnaissance of the site, drilled six test borings at and adjacent to the site, and conducted engineering analyses. The results of our study and our design recommendations are presented in the attached report. In summary, in our opinion, the new residence may be supported by a conventional shallow foundation system. A soldier pile wall represents a feasible excavation support system to allow for the construction of the proposed basement. In addition, we recommend the use of permanent soldier pile walls to replace the existing driveway walls, and accommodate the proposed driveway widening. We appreciate the opportunity to be of service. Should you have any questions, please do not hesitate to call. Sincerely, Jon C. Rehkopf, P.E. Senior Project Geotechnical Engineer 3213 Eastlake Avenue East, Suite B Seattle, WA Tel: (206)

3 Section TABLE OF CONTENTS Page 1.0 GENERAL SITE AND PROJECT DESCRIPTION SUBSURFACE EXPLORATIONS TEST BORINGS LABORATORY TESTING SUBSURFACE CONDITIONS SITE GEOLOGY SOIL CONDITIONS GROUNDWATER CONDITIONS GEOLOGIC HAZARDS ASSESSMENT POTENTIAL LANDSLIDE HAZARDS SEISMIC HAZARDS EROSION HAZARDS GEOTECHNICAL RECOMMENDATIONS SEISMIC DESIGN PARAMETERS SPREAD FOOTINGS Allowable Bearing Pressure Lateral Resistance Perimeter Footing Drains Footing Subgrade Preparation FLOORS SLABS UNDER SLAB DRAINS BASEMENT WALL DESIGN PARAMETERS Lateral Earth Pressures Lateral Resistance Wall Backfill Wall Drainage & Damp Proofing TEMPORARY & PERMANENT SOLDIER PILE WALLS Soldier Pile Wall Tiebacks Lagging Drainage ON-SITE INFILTRATION CONSIDERATIONS STATEMENT OF RISK CONSTRUCTION CONSIDERATIONS DEMOLITION AND GRADING TEMPORARY UNSUPPORTED EXCAVATIONS TEMPORARY EXCAVATION SHORING GROUNDWATER CONTROL MATERIAL REUSE STRUCTURAL FILL WET WEATHER EARTHWORK SUBSURFACE DRAINAGE AND EROSION CONSIDERATIONS ADDITIONAL SERVICES CLOSURE REFERENCES...25

4 Table of Contents (Cont.) ATTACHMENTS: Figure 1 Vicinity Map Figure 2 Site and Exploration Plan Figure 3 Soldier Pile Wall Design Parameters Cantilever Wall/Single Tieback Lower Driveway Wall Figure 4 Soldier Pile Wall Design Parameters Cantilever Wall/Single Tieback Upper Driveway Wall Figure 5 Soldier Pile Wall Design Cantilever / Single Tieback Wall House Shoring Wall Figure 6 Soldier Pile Wall Design Multiple Row Tiebacks House Shoring Wall APPENDIX A TEST BORING LOGS Figure A-1 - Terms and Symbols for Boring and Test Pit Logs Figures A-2 through A-7 - Logs of Test Borings B-1 through B-6 APPENDIX B LABORATORY TEST RESULTS Figure B-1 Grain Size Distribution Figure B-2 Atterberg Limits _jackson_rpt.docx Page 2 PanGEO, Inc.

5 GEOTECHNICAL ENGINEERING REPORT PROPOSED RESIDENCE & DRIVEWAY IMPROVEMENTS 3835 WEST MERCER WAY MERCER ISLAND, WASHINGTON 1.0 GENERAL PanGEO, Inc. is pleased to present the following geotechnical engineering report to assist the project team with the design and permitting of the proposed residence and associated driveway improvements at 3835 West Mercer Way, in Mercer Island, Washington. This study was prepared in general accordance with our mutually agreed scope of services outlined in our original proposal dated January 21, 2013, and our subsequent proposal dated October 16, Our combined scope of services included reviewing readily available geologic and geotechnical data, conducting a site reconnaissance, advancing test borings at the site, conducting engineering analyses, and preparing the following geotechnical report. 2.0 SITE AND PROJECT DESCRIPTION The subject site is located at 3835 West Mercer Way, along the west shoreline of Mercer Island, Washington (see Figure 1). The site is accessed by means of Shoreclift Lane (private road) that switchbacks down the steep slope from West Mercer Way. The area of the site covers two lots and is approximately 1.1 acres in size. The site is irregularly shaped, and is bordered to the west by Lake Washington, to the north by a single-family residence, to the south by an undeveloped property, and to the east by a west facing steep slope. The ground surface within the approximately eastern two-thirds of the site slopes steeply down to the west, while the western portion of the site generally slopes gently down to the west. Based on our observations, as well as a review of the site topographic survey prepared by Encompass (dated 12/7/17), the steep slope on the site ranges in steepness from about 1½H:1V to 1H:1V. The steep slope is undeveloped and heavily vegetated with young to moderately sized trees and thick underbrush, consisting of ferns, ivy, and other native plants. A two-story single-family residence currently occupies the western portion of the site. Based on our observations of the exposed portion of the house foundation, the house is in good condition, with no significant cracks in the concrete walls or signs of settlement or distress noted. The gently sloping area of the site to the west of the existing house consists

6 of a manicured yard area, that includes a concrete patio, low rock walls, and a small artificial pond. Current site conditions are depicted in Plate 1 below. Plate 1. Looking north and east at subject property from Lake Washington. We understand that the existing house will be removed entirely to allow for the construction of the new residence. We understand that the footprint of the new structure will be expanded to the east and west from the existing limits of the current house, and will have a long axis that extends along the shoreline as the existing residence does. The lowest floor elevation of the house will be approximately 24 feet. In addition, we understand that the existing driveway will be widened to the east, directly east of the house, into the steep slope, which will require a new retaining wall along the east side of the driveway. Lastly, additional driveway widening and new or replacement retaining walls may be needed along upper portions of the existing driveway, especially at the hairpin turn at the northernmost extent of the driveway. The conclusions and recommendations in this study are based on our understanding of the proposed development, which is in turn based on the project information provided to us. If the above project description is incorrect, or the project information changes, we should be consulted to review the recommendations contained in this study and make modifications, if needed _jackson_rpt.docx Page 2 PanGEO, Inc.

7 3.0 SUBSURFACE EXPLORATIONS 3.1 TEST BORINGS We advanced four borings (B-1 through B-4) to explore subsurface conditions at the site on February 25, We drilled an additional two borings (B-5 and B-6) on November 13, 2017, after the proposed driveway widening was added to the project scope. The approximate boring locations are shown in Figure 2. The borings were drilled to depths ranging from 8.5 to 26.5 feet below the existing ground surface using a portable drill rig owned and operated by CN Drilling (2013 drilling) and Boretec1, Inc. (2017 drilling). The drill rigs were equipped with 4-inch outside diameter hollow stem augers. Soil samples were obtained from the borings at 2.5- and 5-foot intervals in conjunction with Standard Penetration Test (SPT) sampling methods in general accordance with ASTM test method D-1586, in which the samples are obtained using a 2-inch outside diameter split-spoon sampler. The sampler was driven into the soil a distance of 18 inches using a 140-pound weight falling a distance of 30 inches. The number of blows required for each 6-inch increment of sampler penetration was recorded. The number of blows required to achieve the last 12 inches of sample penetration is defined as the SPT N-value. The N-value provides an empirical measure of the relative density of cohesionless soil, or the relative consistency of fine-grained soils. A representative from PanGEO was present during the field explorations to observe the drilling, to assist in sampling, and to describe and document the soil samples obtained from the borings. The summary boring logs are included in Appendix A, Figures A-2 and A-7. The soil samples were described using the system outlined on Figure A-1 in Appendix A. 3.2 LABORATORY TESTING Grain size analysis, Atterberg limits, and natural moisture content tests were performed on selected soil samples. The results from the moisture content tests are shown on the appropriate boring logs, and the grain size results and Atterberg Limits are summarized in Figures B-1 and B-2 in Appendix B of this report _jackson_rpt.docx Page 3 PanGEO, Inc.

8 4.0 SUBSURFACE CONDITIONS 4.1 SITE GEOLOGY According to the geologic map of Mercer Island (Troost and Wisher, 2006), the project site is underlain by older glacial and non-glacial deposits. The map indicates the underlying older glacial deposits (pre-olympia) in the area of the proposed development are finegrained deposits consisting of silt and clay with sandy interbeds. Pre-Olympia coarse-grain deposits are mapped above the fine-grained deposits approximately mid-slope. Recent lake deposits are also mapped along the shoreline of the site. Lake deposits generally consist of soft to medium stiff silt and clay with local loose to medium dense sand layers. Our test borings advanced at the site generally confirmed the mapped geology, and encountered hard silt and clay (pre-olympia deposits) below what we interpreted to be fill and colluvium on the upslope side of the existing house and below what we interpreted to be fill, colluvium and/or lake deposits along the downslope side of the house. The test boring located upslope along the driveway encountered sand, which we interpreted to be the pre-olympia coarse-grain deposits. 4.2 SOIL CONDITIONS Soil encountered in borings B-1 and B-2, which were advanced on the west side of the existing house, consisted of about 5 to 7 feet of what we interpreted to be fill or colluvium, underlain by hard glacially overridden deposits of clayey silt. The fill/colluvium unit is generally composed of medium dense silty and gravelly sand or stiff to very stiff sandy gravelly silt. Scattered organics and roots were observed at shallow depths near the ground surface. The layout of the existing front yard and property suggests that fill may have been previously placed during grading for the original development of the site. In B-1, an approximately 2-foot thick layer of medium dense silty gravelly sand and very stiff silt was encountered, which was interpreted to be lake deposits. The glacially overridden deposit generally consists of hard, silt and clay with gravel. Boring B-3, which was advanced to the east of the existing house, encountered about 5 feet of fill consisting of medium stiff, sandy silt with scattered charcoal fragments and ironoxide staining. It is likely that the previous construction of the existing driveway involved fill placement. The fill is underlain by about 18 feet of medium dense silty sand and stiff sandy silt, which we interpreted to be colluvium. Hard, glacially overridden silt and clay was encountered at a depth of about 23 feet below the ground surface. The hard silt and _jackson_rpt.docx Page 4 PanGEO, Inc.

9 clay observed in the test boring are generally consistent to the glacially overridden deposits encountered in borings B-1 and B-2. Borings B-4 and B-5 were drilled just up from the toe of the steep slope, east of the existing driveway. The soil encountered in boring B-4 and B-5 consisted of about 4½ to 5 feet of medium dense, sandy silt that we interpreted to be colluvium. Below the colluvium test boring B-4 encountered the glacially overridden deposit consisting of a layer of very dense, silty sand underlain by hard slightly sandy silt and clay, and B-5 encountered hard clayey silt and silty clay. Test boring B-6, which was advanced upslope of the driveway just south of the hairpin turn, encountered about five feet of medium dense silty fine sand, which we interpreted to be colluvium, over medium dense becoming dense fine sand with some silt to the termination depth of the boring. We interpreted the medium dense to dense sand to be the mapped pre-olympia coarse-grained deposit. The soils encountered at each of the subsurface exploration locations are described in the boring logs presented in Appendix A of this report. 4.3 GROUNDWATER CONDITIONS Groundwater was encountered in borings B-1 through B-3 at the time of drilling. Groundwater was observed at about elevation 21 and 23 feet in borings B-1 and B-2, respectively. This elevation is generally consistent with the adjacent water elevation of Lake Washington. At boring location B-3, we observed groundwater at about elevation 25 feet. At boring location B-6, which was advanced upslope of the proposed house location, adjacent to the existing driveway, groundwater was encountered at a depth of about 20 feet, which corresponds to an elevation of about 70 feet. We infer that the groundwater observed in boring B-6 is perched on the underlying fine-grained deposits. It should be noted that groundwater levels may vary depending on the season, local subsurface conditions, and other factors, such as the level of Lake Washington. 5.0 GEOLOGIC HAZARDS ASSESSMENT 5.1 POTENTIAL LANDSLIDE HAZARDS The eastern approximately two-thirds of the subject site contains a west facing steep slope that has a maximum relief of about 100 feet on the subject property, based on the survey _jackson_rpt.docx Page 5 PanGEO, Inc.

10 information provided to us, and an approximate slope angle that ranges between about 1.5H:1V to 1H:1V. In general, the slope angle is fairly uniform from top to bottom, and the steepest slopes are located near the south end of the property. Directly east of the existing driveway, the slope angles appear to be over-steepened, likely due to the cuts made during the construction of the driveway. Based on our review of geologic mapping of the area and the results of our subsurface explorations, we infer that the slope is underlain by up to about 5 feet of colluvium over medium dense to dense silty sand in the middle to upper portion of the slope, and hard silt and clay along the lower portion of the slope. The subject site is mapped within a potential landslide hazard area according to the City of Mercer Island s Geologic Hazards Map. The map indicates that slopes of 15% or more and slopes between 40-79% are present at the site. The map also indicates that landslide or mass wasting deposits may exist near the northern portion of the site. The map depicts a landslide scarp near the middle of the slope above the project site, or about halfway up the slope between the shoreline and West Mercer Way. According to the map, the site does not contain a previously documented landslide location. Several documented slides have occurred several blocks north and south of the subject site. As part of our site reconnaissance we traversed the slope to look for evidence of past or on-going slope instability. During our site reconnaissance we did not observe evidence of past instability in the project area, such hummocky terrain, obvious slide scarps, uneven topography, or tension cracks. Although some leaning trees and bent trunks were noted, which is often an indication of soil creep, in general the majority of the tree trunks appeared straight, and the slopes angles were relatively uniform. Based on our reconnaissance of the slope and our understanding of subsurface conditions at the site, in our opinion a large deep-seated type of slope failure is relatively unlikely on the subject property. In our opinion, shallow surficial slides of various sizes (from large to small) are the likely type of failure that could occur on the steep slope at the site. However, due to the lack of observed evidence of recent shallow slides, and the thick vegetation cover which protects the surface of the slope from erosion, in our opinion the potential for a large shallow slide is relatively low. It is our opinion that the proposed development as currently planned is feasible from a geotechnical engineering standpoint, and in our opinion will not adversely affect the overall stability of the site or adjacent properties, provided the recommendations outlined herein _jackson_rpt.docx Page 6 PanGEO, Inc.

11 are followed and the proposed development is properly design and constructed. If mitigating measures such as a properly constructed and maintained catchment walls are incorporated into the design of the proposed driveway retaining walls, it is our opinion that adverse effects of potential shallow slides from the steep slope would be significantly reduced. Furthermore, because the proposed driveway wall east of the house would create more level area in between the steep slope and the proposed residence, in our opinion the proposed improvements would significantly decrease the potential of slope failures impacting the proposed residence. 5.2 SEISMIC HAZARDS Based on our review of the City of Mercer Island s Geologic Hazards Maps, the project site is mapped as a seismic hazard area. The City of Mercer Island Code defines seismic hazard areas as those areas subject to risk of damage as a result of earthquake-induced ground shaking, slope failure, soil liquefaction or surface faulting. Based on the very stiff to hard glacial soils underlying the proposed building site, in our opinion, the potential for soil liquefaction during an IBC-code level earthquake is considered minimal, and special design considerations associated with soil liquefaction are not required. It is also our opinion that the potential for significant seismic-induced landslidng is relatively low at the site due to the dense and hard glacial soils underlying the slope, and lack of steep slopes greater than 80%. However, if may be noted that shallow slides within over-steepened portions of the slope could have the potential to be triggered by a seismic event. Therefore, as noted below, we recommend that the new walls along the driveway include a catchment provision to limit the impact shallow slides could have on the proposed residence, or widened driveway. In addition, the permanent soldier pile walls along the driveway will be designed to consider the seismic loading. 5.3 EROSION HAZARDS The subject site is mapped within a potential erosion hazard area according to the City of Mercer Island s Geologic Hazards Map. Based on soil conditions encountered in the borings, the near-surface site soils are likely to exhibit moderate erosion potential. In our opinion, the erosion hazards at the site can be effectively mitigated with the best management practice during construction and with properly designed and implemented landscaping for permanent erosion control. During construction, the temporary erosion hazard can be effectively managed with an appropriate erosion and sediment control plan, _jackson_rpt.docx Page 7 PanGEO, Inc.

12 including but not limited to installing silt fence at the construction perimeter, limiting removal of vegetation to the construction area, placing gravel or hay bales at the disturbed/traffic areas, covering stockpile soil or cut slopes with plastic sheets, constructing a temporary drainage pond to control surface runoff and sediment trap, placing quarry spalls at the construction entrance, etc. Permanent erosion control measures should include establishing vegetation, landscape plants, and hardscape established at the end of project, and reducing surface runoff to the minimum extent possible. 6.0 GEOTECHNICAL RECOMMENDATIONS 6.1 SEISMIC DESIGN PARAMETERS The 2015 International Building Code (IBC) seismic design section provides a basis for seismic design of structures. Table 1 below provides seismic design parameters for the site that are in conformance with the 2015 IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1 Seismic Design Parameters Site Class Spectral Acceleration at 0.2 sec. [g] Spectral Acceleration at 1.0 sec. [g] Site Coefficients Design Spectral Response Parameters S S S 1 F a F v S DS S D1 D The spectral response accelerations were obtained from the USGS Earthquake Hazards Program website (2008 data) for the project latitude and longitude. Liquefaction Potential: Liquefaction is a process that can occur when soils lose shear strength for short periods of time during a seismic event. Ground shaking of sufficient strength and duration results in the loss of grain-to-grain contact and an increase in pore water pressure, causing the soil to behave as a fluid. Soils with a potential for liquefaction are typically cohesionless, predominately silt and sand sized, loose to medium dense, and must be saturated _jackson_rpt.docx Page 8 PanGEO, Inc.

13 Due to the presence of very stiff to hard clayey silt underlying the site, the potential for liquefaction at the site is considered to be low, in our opinion, and special design considerations associated with soil liquefaction are not necessary for this project. One exception was a thin, approximately 2-foot thick layer of medium dense sand which was encountered below the water level in boring B-1. While this soil deposit has the potential to liquefy during the code level event, because the proposed structure will not be located in this area of the site, and the anticipated impact of the liquefaction of this layer is minimal, in our opinion the impact of the liquefaction in this area of the site would have very little to no impact on the proposed development. If future development will be located in this area of the site, and the potential of relatively minor seismic induced settlements are not tolerable, we recommend that a deep foundation system, such as pin piles, be used to support future structures. 6.2 SPREAD FOOTINGS It is our opinion that conventional spread footings are appropriate to support the proposed residence. All footings should be founded on the native stiff to hard clayey silt, or properly compacted structural fill placed over competent native soil Allowable Bearing Pressure We recommend a maximum allowable soil bearing pressure of 2,500 pounds per square foot (psf) be used to size the footings. For allowable stress design, the recommended bearing pressure may be increased by one-third for transient loading, such as wind or seismic forces. Continuous and individual spread footings should have minimum widths of 18 and 24 inches, respectively. Total and differential settlements are anticipated to be within tolerable limits for footings designed and constructed as discussed above. Footing settlement under static loading conditions is estimated to be less than about 1-inch. We anticipate differential settlement across the footprint of the house should be less than about ½-inch. Most settlement will occur during construction as loads are applied Lateral Resistance Lateral forces from wind or seismic loading may be resisted by the combination of passive earth pressures acting against the embedded portions of the foundations and by friction acting on the base of the foundations. Passive resistance values may be determined using an equivalent fluid weight of 300 pounds per cubic foot (pcf). This value includes a factor _jackson_rpt.docx Page 9 PanGEO, Inc.

14 safety of at least 1.5 assuming that a properly compacted structural fill will be placed adjacent to the sides of the footings. A coefficient friction of 0.30 may be used to determine the frictional resistance at the base of the footings. This coefficient includes a factor safety of approximate 1.5. Unless covered by pavements or slabs, the passive resistance in the upper 12 inches of soil should be neglected Perimeter Footing Drains Footing drains should be installed around the perimeter of the residence, at or just below the invert of the footings. Under no circumstances should roof downspout drain lines be connected to the footing drain systems. Roof downspouts must be separately tightlined to appropriate discharge locations. Cleanouts should be installed at strategic locations to allow for periodic maintenance of the footing drain and downspout tightline systems Footing Subgrade Preparation The contractor should be aware that the site soils are highly sensitive to moisture due to their high silt and clay content, and will become disturbed and soft when exposed to inclement weather conditions or groundwater seepage. As a result, depending on the weather conditions at the time of footing construction, and the actual soil conditions encountered, it may be necessary to place 2 to 3 inches of crushed rock or lean-mix concrete on the exposed footing subgrade to protect it against moisture and disturbance. If groundwater seepage is encountered, the contractor should be prepared to dewater the footing excavations using sumps and pumps to allow for proper subgrade preparation. Footing subgrades should be in a firm, stable condition prior to setting forms and placing reinforcing steel. Any loose or softened soil should be removed from the footing excavations. The adequacy of the footing subgrade soils should be verified by a representative of PanGEO, prior to placing forms or rebar. If loose or disturbed soil is encountered at the footing elevation, the footing may be lowered to bear on the undisturbed soils, or the unsuitable soils should be removed and replaced with properly compacted structural fill, or lean-mix concrete. The structural fill should extend wider than the footing a distance equal to one-half of the over-excavation depth. 6.3 FLOORS SLABS Conventional slab on grade construction may be used for the floor slabs in the house and garage. The floor slab design may be accomplished using a modulus of subgrade reaction of 125 pci. We recommend that the slabs be constructed on a minimum 4-inch thick _jackson_rpt.docx Page 10 PanGEO, Inc.

15 capillary break placed on the undisturbed native soils. The capillary break should consist of free-draining, crushed rock or well-graded gravel compacted to a firm and unyielding condition. The capillary break material should have no more than 10 percent passing the No. 4 sieve and less than 5 percent by weight of the material passing the U.S. Standard No. 100 sieve. We also recommend that a 10-mil polyethylene vapor barrier be placed below the slab. 6.4 UNDER SLAB DRAINS We understand that the lowest level floor slab will be around elevation 24 feet. Because groundwater seepage was observed in our test borings around that same elevation, or slightly higher in B-3, we recommend that an under-slab drainage system be incorporated into the new house design to prevent accumulation of groundwater below the slab. In general, the under-slab drainage system should consist of 4-inch diameter perforated drainpipes placed in narrow (one foot or less), approximately 18-inch deep trenches (measured from the bottom of slab) spaced about 20 feet apart. The trenches should be backfilled with clean, free-draining 3/8 inch minus pea gravel or clean 5/8-inch crushed rock. Water collected in these drainpipes should be conveyed to an appropriate outlet. The design of the slab drain may be refined based on the final design of the structure, and the actual conditions observed during construction. 6.5 BASEMENT WALL DESIGN PARAMETERS Below-grade walls should be properly designed to resist the lateral earth pressures exerted by the soils behind the wall. Proper drainage provisions should also be provided behind the walls to intercept and remove groundwater from behind the wall. Our geotechnical recommendations for the design and construction of the below-grade walls are presented below Lateral Earth Pressures The below grade portions of walls with level back-slopes should be designed for an earth pressure based upon an equivalent fluid weight of 35 pcf for a wall that is allowed to yield, and 45 pcf for a wall that is restrained. For walls with backslopes up to 2H:1V, an earth pressure based upon an equivalent fluid weight of 50 pcf for yielding walls, and 60 pcf for restrained walls should be used. For walls cast against a cantilevered shoring wall, the _jackson_rpt.docx Page 11 PanGEO, Inc.

16 wall pressures used for the shoring wall design should be used for design of the basement wall. A uniform pressure of 7H psf should be added to reflect the increase loading for seismic conditions, where H corresponds to the buried depth of the wall. The recommended lateral pressures assume that the backfill behind the wall consists of a free draining and properly compacted fill with adequate drainage provisions. If surcharge loads or building foundations will be located within a horizontal distance equal to the height of the wall, lateral earth pressures will need to be increased based upon the type and magnitude of surcharge. The surcharge diagram shown on Figure 5 may be used to calculate the surcharge pressure on the wall Lateral Resistance Lateral forces from wind or seismic loading may be resisted by the combination of passive earth pressures acting against the embedded portions of the foundations and by friction acting on the base of the foundations. Passive resistance values may be determined using an equivalent fluid weight of 300 pounds per cubic foot (pcf). This value includes a factor of safety of at least 1.5 assuming that a properly compacted structural fill will be placed adjacent to the sides of the footings. A coefficient friction of 0.30 may be used to determine the frictional resistance at the base of the footings. This coefficient includes a factor of safety of approximate Wall Backfill Based on the results of our test borings, the on-site soils consist of sandy silt and clayey silt, and would not be suitable to be re-used as wall backfill. We recommend that wall backfill consist of imported free draining granular soils such as Seattle Mineral Aggregate Type 17 or Gravel Borrow as defined in Section (1) of the WSDOT Standard Specifications for Road, Bridge, and Municipal Construction (WSDOT, 2016) In areas where the space is limited between the wall and the face of excavation, clean crushed 5/8- inch rock may be used as backfill without compaction. Wall backfill should be moisture conditioned to within about 3 percent of optimum moisture content, placed in loose, horizontal lifts less than 8 inches in thickness, and systematically compacted to a dense and relatively unyielding condition and to at least 95 percent of the maximum dry density, as determined using test method ASTM D Within 5 feet of the wall, the backfill should be compacted to 90 percent of the maximum dry density _jackson_rpt.docx Page 12 PanGEO, Inc.

17 6.5.4 Wall Drainage & Damp Proofing Provisions for permanent control of subsurface water should be incorporated into the design and construction of the below-grade walls. As a minimum, 4-inch diameter perforated drainpipes should be installed behind and at the base of the wall footings, embedded in 12 to 18 inches of pea or washed gravel. The gravel should be wrapped in a geotextile filter fabric to prevent the migration of fines into the drain system. The drainpipe should be graded to direct water to a suitable outlet. Where the below-grade wall will be constructed against a soldier pile wall, we recommend that prefabricated drainage mats, such as Mirafi 6000 or equivalent, be installed behind the walls (full face coverage) and the collected water should be directed through weep holes inside the building beneath the floor slab and tight-lined to an appropriate outlet. Please note that waterproofing considerations are beyond our scope of work. We recommend that a building envelope specialist be consulted to determine appropriate damp-proofing or water-proofing measures. 6.6 TEMPORARY & PERMANENT SOLDIER PILE WALLS We understand that retaining walls will be needed along portions of the new widened driveway. In particular, a lower wall will be required east of the new house to widen the driveway to the east, and additional walls may be needed along the upper portion of the driveway to straighten and widen the existing private Shoreclift Lane. In particular, we understand that a new wall will be needed behind the existing timber retaining wall at the hairpin curve. We anticipate that the driveway walls will be less than about 15 feet in height. In addition, we understand that temporary shoring walls will be utilized around the east side of the new house to facilitate construction of the basement. We anticipate that the temporary shoring walls may need to be up to 20 feet in height. In our opinion, based on our understanding of subsurface conditions at the site, cantilever and multiple tieback soldier pile walls are feasible retaining wall systems for the project. We offer the following geotechnical design recommendations for the proposed permanent and temporary soldier pile walls utilized for this project _jackson_rpt.docx Page 13 PanGEO, Inc.

18 6.6.1 Soldier Pile Wall A soldier pile wall consists of vertical steel beams, typically spaced from 6 to 8 feet apart along the proposed wall alignment, spanned by timber lagging. Prior to the start of excavation, the steel beams are installed into holes drilled to a design depth and then backfilled with lean mix or structural concrete. As the excavation proceeds downward and the steel piles are subsequently exposed, timber lagging is installed between the piles to further stabilize the walls of the excavation. A variety of facing schemes, including precast and cast in place concrete, can be applied to the face of the wall to give the wall a desired aesthetic appearance. Due to the height of the proposed walls, one or two levels of tie-backs will most likely be required to maintain stability of the soldier pile walls. In general, tiebacks are typically used for wall heights greater than about 12 feet to achieve a more economical design. However, considering the steep backslope behind the driveway walls, as well as the need for catchment, lower wall heights may also require one level of tiebacks. Design Lateral Pressures For a cantilevered soldier pile wall or a soldier pile wall with one level of tiebacks, the earth pressures depicted on Figures 3 and 4 should be used for design of the lower and upper permanent driveway walls, respectively. For the temporary basement excavations, Figure 5 may be used to design a cantilevered wall or wall with one level of tiebacks, and Figure 6 should be used to design the basement temporary shoring wall for two levels of tiebacks. Above the bottom of excavation, the recommended active earth, surcharge and seismic pressures should be applied over the full width of pile spacing. Below the bottom of excavation, the active and surcharge pressures should be applied over one pile diameter, and the passive resistance should be applied over two times the pile diameter. Catchment Wall Design Parameters Based on our subsurface explorations advanced on the slope, as well as our observations, we recommend that a catchment wall, with a minimum freeboard height of 3 feet, be incorporated into the permanent driveway retaining walls. Figures 3 and 4 present our recommended design pressures from potential debris that would accumulate behind the catchment wall. The design debris pressure should be applied over the full width of the pile spacing. It is important to note that periodic maintenance of the catchment wall is essential, as the functionality of the wall is directly related to the available catchment area behind the wall. Furthermore, permanent access to the back of the catchment wall should be incorporated _jackson_rpt.docx Page 14 PanGEO, Inc.

19 into the layout of the proposed development to allow for periodic removal of accumulated debris from behind the wall to maintain the specified minimum freeboard. Vertical Capacity We recommend the vertical capacity of the soldier piles be determined using an allowable skin friction value of 1.0 ksf for the portion of the pile below the bottom of the excavation, and an allowable end bearing value of 15 ksf. Groundwater and Caving Soil Conditions - The drilling of soldier piles for the upper walls is anticipated to encountered medium dense to dense saturated sand. The drilling of the lower soldier piles for the driveway wall and house may also encounter loose soils (fill and colluvium) as well as the groundwater table. Caving in the colluvium and wet sand layers will likely occur during drilling. As a result, the drilling contractor should be prepared to use temporary casings and/or drilling fluids to stabilize the drill holes. Groundwater is likely to accumulate at the bottom of drill holes. We recommend that the lean concrete or structural concrete backfill be placed with tremie pipes if more than one foot of water is present at the bottom of the holes at the time of concrete placement Tiebacks If tiebacks will extend beyond the property boundaries, temporary or permanent easements will be needed from the neighboring property owners. Tieback Location Because excessive pile top deflections can occur before the first row of tiebacks is installed, it may be necessary to limit the first row of tiebacks to no more than 6 to 8 feet below the pile top unless steel beams of sufficient size will be used to limit the magnitude of the cantilever deflection. Corrosion Protection Because the proposed tieback soldier pile walls along the driveway will be a permanent structure, and the tiebacks are an integral component of wall support, tiebacks with double corrosion protection should be utilized. No-Load Zone - Tieback bond length should be located behind a no-load zone as indicated in Figures 3-6. The tiebacks should have a minimum bond length of 15 feet beyond the no-load zone in the load zone. Assumed Capacity The manner in which the tieback anchors carry load will depend on the type of anchor selected, the method of installation, and the soil conditions surrounding the anchor. Accordingly, we recommend use of a performance specification requiring the tieback contractor to install anchors capable of satisfactorily achieving the design structural loads, with a pullout resistance factor of safety of 2.0. For planning purposes, however, _jackson_rpt.docx Page 15 PanGEO, Inc.

20 the anchors may be sized for an allowable skin friction value of 2.5 kips per lineal foot of anchor bond length, assuming that small diameter (about 6 inches) pressure-grouted tiebacks will be used. Multiple post-grouting may be needed in order to achieve the design capacity, especially if initial pressure grouting is not utilized. We recommend that the allowable tieback loads be limited to about 120 kips per anchor. Anchors should have a minimum bond length of 15 feet. The actual capacity of the anchors should be checked with 200 percent verification tests. At least two 200-percent tests should be performed prior to installing production anchors. All production anchors should be proof tested to 130% of the design load. The anchor installations should be conducted in accordance with the latest edition of the Post Tensioning Institute (PTI) Recommendations for Prestressed Rock and Soil Anchors. Elements of the testing are as follows: Verification Tests (200% Tests) Prior to installing production anchors, perform a minimum of two tests each on each anchor type, installation method and soil type with the tested anchors constructed to the same dimensions as production anchors. Test locations to be determined in conjunction and approved by the geotechnical engineer. Test anchors, which will be loaded to 200% of the design load, may require additional prestressing steel (steel load not to exceed 80% of the ultimate tensile strength) or reinforcing of the soldier pile. Load test anchors to 200% load in 25% load increments, holding each incremental load for at least 5 minutes and recording deflection of the anchor head at various times within each hold to the nearest 0.01inch. At the 150% load, the holding period shall be at least 60 minutes. When silt and clay soils are encountered (i.e. lower driveway wall), at least one verification test should be held at the 150% load for 4 hours to test for creep. A successful test shall provide a measured creep rate of 0.04 inches or less at the 150% load between 1 and 10 minutes, and 0.08 inches or less between 6 and 60 minutes and 24 and 240 minutes, and all time increments shall have a creep rate _jackson_rpt.docx Page 16 PanGEO, Inc.

21 that is linear or decreasing with time. The applied load must remain constant during all holding periods (i.e. no more than 5% variation from the specified load). Proof Tests (130% load tests on all production anchors) Load test all production anchors to 130% of the design load in 25% load increments, holding each incremental load until a stable deflection is achieved (record deflection of the anchor head at various times within each hold to the nearest 0.01inch). At the 130% load, the holding period shall be at least 10 minutes A successful test shall provide a measured creep rate of 0.04 inches or less at the 130% load between 1 and 10 minutes with a creep rate that is linear or decreasing with time. The applied load must remain constant during the holding period (i.e. no more than 5% variation from the 130% load). Anchors failing this proof testing creep acceptance criteria may be held an additional 50 minutes for creep measurement. Acceptable performance would equate to a creep of 0.08 inches or less between 5 and 50 minutes with a linear or decreasing creep rate. Verification tested anchors or extended creep proof tested anchors not meeting the acceptance criteria will require a redesign by the contractor to achieve the acceptance criteria. In the tieback construction, a bond breaker shall be constructed in the no load zone when the installation procedures use single stage grouting. Groundwater and Caving Soil Conditions - The drilling for tiebacks may encounter wet sand where caving of the drilled holes are likely to occur. As result, we recommend the use of temporary casing during installation to keep the drilled holes open, and to minimize the risk of potential ground loss. Installation Considerations - The tiebacks for this project should be installed by experienced personnel. The use of compressed air to flush the drill cuttings must be properly controlled as the use of excessive amount of compressed air while drilling tiebacks could lead to reduction of soil strength and ground movements. Performance Monitoring The retaining wall should be designed to limit lateral and vertical deflection to about 1 inch. Ground settlements behind the wall are expected to be less than 1 inch _jackson_rpt.docx Page 17 PanGEO, Inc.

22 Because ground deformations will occur due to the excavation (open cut or shored), we recommend that existing conditions on the adjacent private properties be photodocumented prior to the start of the project. We also recommend that survey points be installed on every other soldier pile and on adjacent structures. The survey points on the piles should be monitored at least weekly by the project surveyor until one week after the excavation has been completed to determine potential deformations. The monitoring program should include changes in both the horizontal (x and y directions) and vertical deformations to the nearest 0.01-foot, and the results be promptly submitted to PanGEO for review. After the initial baseline readings, which should be taken prior to the start of pile installations, the monitoring points on the adjacent structures only need to be shot if excessive soldier pile deflections are noted. The results of the monitoring will allow the design team to confirm design parameters, and for the contractor to make adjustments if necessary Lagging Lagging design recommendations for general conditions are presented on Figures 3 through 6. Because the proposed retaining wall will be a permanent structure, the lifespan of treated timber lagging should be considered in design. Typically, the useable life of timber lagging is on the order of 25 years before repair and/or replacement is necessary. To prolong the life of the lagging, other materials such as concrete (shotcrete) could be considered Drainage Adequate drainage provisions should be incorporated into the design of the permanent soldier pile retaining walls. If concrete lagging or a concrete facing over timber lagging is used, 3-inch diameter weep holes should be installed at the bottom of each soldier pile bay to allow drainage at the base of the wall. The discharged water from the weep holes, or seepage from the lagging, should be collected and discharged at an appropriate outlet, as allowing seepage to flow over the driveway could lead to slippery pavement conditions. 6.7 ON-SITE INFILTRATION CONSIDERATIONS Based on our review of the City of Mercer Island Low Impact Development (LID) infiltration feasibility map, the project site is located in an area were infiltrating LID is not permitted _jackson_rpt.docx Page 18 PanGEO, Inc.

23 7.0 STATEMENT OF RISK The site is mapped as a geologic hazard area by the City of Mercer Island. Per Mercer Island City Code, development within geologic hazard areas and critical slopes may occur if the geotechnical engineer provides a statement of risk with supporting documentation indicating that one of the following conditions can be met: a. The geologic hazard area will be modified, or the development has been designed so that the risk to the lot and adjacent property is eliminated or mitigated such that the site is determined to be safe; or b. An evaluation of site specific subsurface conditions demonstrates that the proposed development is not located in a geologic hazard area; or c. Development practices are proposed for the alteration that would render the development as safe as if it were not located in a geologic hazard area; or d. The alteration is so minor as not to pose a threat to the public health, safety, and welfare. It is our opinion that Criterion A and/or C can be met provided that the development is designed and constructed in accordance with the recommendations in this report. The proposed structures will be located at the toe of the steep slope, and will therefore not add a surcharge load to the existing slope. In addition, the design utilizes tieback soldier pile walls to support the cuts into the slope for driveway widening. Currently, the driveway cuts are supported with aging timber walls, low block walls, or rockeries, and the proposed soldier pile walls will offer increased stability over the existing condition. Furthermore, a catchment wall will be included in the design to reduce the potential that shallow slides from the upslope area will negatively impact the proposed structure and driveway. As such, in our opinion the development will not negatively affect the stability of the slope, or the surrounding properties, but will likely increase the stability of the site. In addition, in our opinion Criterion C can be met through best management practices during construction, including the proper use of a silt fence, minimize earthwork activities during periods heavy precipitation, minimize exposed areas in the wet season, etc. Permanent erosion control measures including landscape and hardscape installations will effectively mitigate the risk of erosion in the long term _jackson_rpt.docx Page 19 PanGEO, Inc.

24 8.0 CONSTRUCTION CONSIDERATIONS 8.1 DEMOLITION AND GRADING All footings and floor slabs of the existing house, as well as building debris and concrete rubble should be removed from the site prior to the start of excavations or grading. The temporary excavation shoring system will need to be installed prior to mass excavations to reduce the potential of site instability. 8.2 TEMPORARY UNSUPPORTED EXCAVATIONS Temporary excavations should be constructed in accordance with Part N of WAC (Washington Administrative Code) The contractor is responsible for maintaining safe excavation slopes and/or shoring. The proposed excavations are anticipated to encounter stiff to medium stiff sandy silt and medium dense silty sand. Where space is available, which is only anticipated to be along the south side of the house, as the north and east sides of the basement excavation will be adjacent to neighboring properties and the private driveway, an unsupported slope cut will be the most cost effective means of excavation support. If there is not enough room for an open cut, a temporary shoring system, such as a soldier pile wall, will be needed. Where space is available, an unsupported slope cut is feasible for the temporary excavations. For planning purposes, the temporary excavations may be sloped to as steep as 1H:1V (horizontal:vertical), but must be re-evaluated in the field during construction based on actual observed soil conditions and the presence of groundwater seepage. If a 1H:1V projection from the bottom of the excavation daylights outside the property line, either temporary shoring will be needed, or an easement is needed by the neighboring property owner. If zones of persistent seepage are encountered, the temporary slopes may need to be cut to shallower angles to maintain stability. During wet weather, runoff water should be prevented from entering excavations. If groundwater seepage is encountered, we anticipate that the use of drainage ditches and sump pumps will provide adequate construction dewatering. We also recommend that heavy construction equipment, building materials and excavated soil should not be allowed within a distance equal to 1/3 the slope height from the top of any excavation. 8.3 TEMPORARY EXCAVATION SHORING See Section 6.6 above for temporary soldier pile shoring wall recommendations _jackson_rpt.docx Page 20 PanGEO, Inc.

25 8.4 GROUNDWATER CONTROL Groundwater seepage may be encountered within the foundation excavations, and should be anticipated. Groundwater seepage, although expected to be relatively minor, may be controlled by sloping the base of the excavation to a low point and removing the water using a sump and pump. 8.5 MATERIAL REUSE Based on the results of our test borings at the site, the majority of the site is underlain by sandy silt and clayey silt. The contractor should be aware that these soils are very moisture sensitive, and will become disturbed and soft when exposed to inclement weather conditions. In addition, these soils will be difficult or impossible to adequately compact. As a result, the excavated on-site material will not be suitable for use as structural backfill, but may be used as backfill in non-structural areas. If use of the existing soils is planned, any excavated soil should be stockpiled and protected with plastic sheeting to prevent softening from rainfall. 8.6 STRUCTURAL FILL Based on the proposed site development, we anticipate that structural fill may be needed below slab-on-grades, and to backfill basement walls of the house. Imported structural fill should consist of free-draining granular soils, such as City of Seattle Type 17. The structural fill should be moisture conditioned to within about 3 percent of optimum moisture content, placed in loose, horizontal lifts less than 8 inches in thickness, and systematically compacted to a dense and relatively unyielding condition and to at least 95 percent of the maximum dry density, as determined using test method ASTM D WET WEATHER EARTHWORK In our opinion, the proposed site construction may be accomplished during the winter without adversely affecting the site stability. However, earthwork construction performed during the drier summer months likely will be more economical. Winter construction will require the implementation of best management erosion and sedimentation control practice to reduce the chances of off-site sediment transport. The site soils contain a high percentage of fines and are moisture sensitive. Any subgrade soils that become softened either by disturbance or rainfall should be removed and replaced with structural fill or lean _jackson_rpt.docx Page 21 PanGEO, Inc.

26 mix concrete. General recommendations relative to earthwork performed in wet conditions are presented below: Site stripping, excavation and subgrade preparation should be followed promptly by the placement and compaction of clean structural fill or lean-mix concrete; The size and type of construction equipment used may have to be limited to prevent soil disturbance; The ground surface within the construction area should be graded to promote run-off of surface water and to prevent the ponding of water; and Bales of straw and/or geotextile silt fences should be strategically located to control erosion and the movement of soil. 8.8 SUBSURFACE DRAINAGE AND EROSION CONSIDERATIONS Surface runoff can be controlled during construction by careful grading practices. Typically, this includes the construction of shallow, upgrade perimeter ditches or low earthen berms in conjunction with silt fences to collect runoff and prevent water from entering excavations or to prevent runoff from the construction area from leaving the immediate work site. Temporary erosion control may require the use of hay bales on the downhill side of the project to prevent water from leaving the site and potential storm water detention to trap sand and silt before the water is discharged to a suitable outlet. All collected water should be directed under control to a positive and permanent discharge system. Special consideration should be placed on the erosion control measures along the shoreline of Lake Washington. Potential problems associated with erosion around the development may be reduced by establishing vegetation within disturbed areas immediately following grading operations. 9.0 ADDITIONAL SERVICES To confirm that our recommendations are properly incorporated into the design and construction of the proposed project, PanGEO should be retained to conduct a review of the final project plans and specifications, and to monitor the construction of geotechnical elements. The City of Mercer Island, as part of the permitting process, will also require geotechnical construction inspection services. PanGEO can provide you a cost estimate _jackson_rpt.docx Page 22 PanGEO, Inc.

27 for construction monitoring services at a later date. Specifically, we anticipate that the following construction support services may be needed: Review final project plans and specifications; Verify implementation of erosion control measures; Observe installation of excavation shoring system; Observe installation of permanent soldier pile and tieback walls; Observe the stability of open cut slopes; Evaluate optical survey data provided by others to evaluate the performance of the shoring system; Verify adequacy of footing and slab subgrades; Confirm the adequacy of the compaction of structural backfill; Observe installation of subsurface drainage provisions, and; Other consultation as may be required during construction. Modifications to our recommendations presented in this report may be necessary, based on the actual conditions encountered during construction CLOSURE We have prepared this report for Bradford & Jill Jackson, and the project design team. Recommendations contained in this report are based on a site reconnaissance, a subsurface exploration program, review of pertinent subsurface information, and our understanding of the project. The study was performed using a mutually agreed-upon scope of services. Variations in soil conditions may exist between the locations of the explorations and the actual conditions underlying the site. The nature and extent of soil variations may not be evident until construction occurs. If any soil conditions are encountered at the site that are different from those described in this report, we should be notified immediately to review the applicability of our recommendations. Additionally, we should also be notified to review the applicability of our recommendations if there are any changes in the project scope. The scope of our work does not include services related to construction safety precautions. Our recommendations are not intended to direct the contractors methods, techniques, _jackson_rpt.docx Page 23 PanGEO, Inc.

28 sequences or procedures, except as specifically described in our report for consideration in design. Additionally, the scope of our services specifically excludes the assessment of environmental characteristics, particularly those involving hazardous substances. We are not mold consultants nor are our recommendations to be interpreted as being preventative of mold development. A mold specialist should be consulted for all mold-related issues. This report has been prepared for planning and design purposes for specific application to the proposed project in accordance with the generally accepted standards of local practice at the time this report was written. No warranty, express or implied, is made. This report may be used only by the client and for the purposes stated, within a reasonable time from its issuance. Land use, site conditions (both off and on-site), or other factors including advances in our understanding of applied science, may change over time and could materially affect our findings. Therefore, this report should not be relied upon after 24 months from its issuance. PanGEO should be notified if the project is delayed by more than 24 months from the date of this report so that we may review the applicability of our conclusions considering the time lapse. It is the client s responsibility to see that all parties to this project, including the designer, contractor, subcontractors, etc., are made aware of this report in its entirety. The use of information contained in this report for bidding purposes should be done at the contractor s option and risk. Any party other than the client who wishes to use this report shall notify PanGEO of such intended use and for permission to copy this report. Based on the intended use of the report, PanGEO may require that additional work be performed and that an updated report be reissued. Noncompliance with any of these requirements will release PanGEO from any liability resulting from the use this report. Sincerely, PanGEO, Inc. Jon C. Rehkopf, P.E. Senior Project Geotechnical Engineer Siew L. Tan, P.E. Principal Geotechnical Engineer _jackson_rpt.docx Page 24 PanGEO, Inc.

29 11.0 REFERENCES International Code Council, 2015, International Building Code (IBC), Troost, K.G., and Wisher, A. P, Geologic Map of Mercer Island, Washington, scale 1:24,000. United States Geological Survey, Earthquake Hazards Program, 2008 Data, accessed via: WSDOT, 2016, Standard Specifications for Road, Bridge and Municipal Construction, M _jackson_rpt.docx Page 25 PanGEO, Inc.

30 Fig1_vicinity.ppt 3/26/2013(10:12 AM) JCR Lake Washington Site Location Image Credit: Google Maps N No Scale Proposed Residence 3835 West Mercer Way Mercer Island, Washington Project No. VICINITY MAP Figure No