subsurface exploration and

a s s o c i a t e de a r t h s c i e n c e si n c o r p o r a t e d

Associated Earth Sciences, Inc. 911 5th AvenueKirkland, WA 98033P (425) 827 7701 F (425) 827 5424

Subsurface Exploration, Geologic Hazard, and Preliminary Geotechnical Engineering Report

9TH AND VIRGINIA Seattle, Washington

Prepared For: HFH SEATTLE TWO, LLC C/O HOWE FAMILY HOLDINGS, LLC

Project No. 180315E001 October 11, 2018

associated

earth sciences o r a t a d

October 11, 2018

Project No. 180315E001

HFH Seattle Two, LLC

do Howe Family Holdings, LLC

1521 2nd Avenue, Suite 605

Seattle, Washington 98101

Attention: Douglas Howe

Subject: Subsurface Exploration, Geologic Hazard, and

Preliminary Geotechnical Engineering Report

9th and Virginia (Tommie Tower)

1932 9th Avenue

Seattle, Washington

Dear Mr. Howe:

We are pleased to present our subsurface exploration, geologic hazard, and geotechnical

engineering report for the subject property. We understand you are considering construction

of a new high-rise building with three underground levels and 23 above-ground levels located

at the referenced address in Seattle, Washington. For preparation of this report, we were

provided with a preliminary plan set provided by Aedas Seattle, LLC, 7 sheet, dated August 21,

2018. Our conclusions and recommendations are presented herein.

We hope this report meets your current needs and look forward to assisting you with the

successful completion of your project. Should you have any questions, or require additional

information, please do not hesitate to call.

Sincerely,

ASSOCIATED EARTH SCIENCES, INC.

Kirkland, Washington

Kurt D. Merriman, P.E.

Senior Principal Geotechnical Engineer

KDM/ms - L80315E001-4 - Projects\20180315\KE\WP

Kirkland Office I 911 Fifth Avenue I Kirkland, WA 98033 P I 425.827.7701 Fl 425.827.5424 Everett Office I 2911 1/2 Hewitt Avenue, Suite 2 I Everett, WA 98201 P I 425.259.0522 F I 425.827.5424

Tacoma Office I 1552 Commerce Street, Suite 102 I Tacoma, WA 98402 P I 253.722.2992 F I 253.722.2993

www.aesgeo.com

SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND PRELIMINARY GEOTECHNICAL ENGINEERING REPORT

9TH AND VIRGINIA

Seattle, Washington

Prepared for: HFH Seattle Two, LLC

c/o Howe Family Holdings, LLC 1521 2nd Avenue, Suite 605 Seattle, Washington 98101

Prepared by Associated Earth Sciences, Inc.

911 5th Avenue Kirkland, Washington 98033

425-827-7701 Fax: 425-827-5424

October 11, 2018 Project No. 180315E001

Subsurface Exploration, Geologic Hazard, and 9th and Virginia Preliminary Geotechnical Engineering Report Seattle, Washington Project and Site Conditions

October 11, 2018 ASSOCIATED EARTH SCIENCES, INC. AWR/ms - 180315E001-4 - Projects\20180315\KE\WP Page 1

I. PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of our subsurface exploration, geologic hazard, and preliminary geotechnical engineering study for the proposed 9th and Virginia tower project located in downtown Seattle, Washington (Figure 1). If any changes in the nature, design, or location of the structure are planned, the conclusions and recommendations contained in this report should be reviewed and modified, or verified, as necessary. This scope of services was performed concurrently with environmental services performed under a separate scope of work, in which a “Limited Environmental Assessment,” report was prepared, dated October 10, 2018. 1.1 Purpose and Scope The purpose of this study was to provide geotechnical design recommendations to be used in the design of the above-mentioned project. Our study included reviewing available geologic literature, advancing two exploration borings and completing one as a groundwater monitoring well, reviewing available subsurface information for adjacent properties, and performing geologic studies to assess the type, thickness, distribution, and physical properties of the subsurface sediments and shallow groundwater conditions. Preliminary geotechnical engineering studies were also conducted to determine the type of suitable foundation, allowable foundation soil bearing pressures, anticipated settlements, temporary shoring recommendations, basement/retaining wall lateral earth pressures, floor support recommendations, and drainage considerations. This report summarizes our current fieldwork and offers development recommendations based on our present understanding of the project. 1.2 Authorization Written authorization to proceed with this study was granted by you. Our study was accomplished in general accordance with our scope of work letter dated July 2, 2018. This report has been prepared for the exclusive use of the Client and their agents, for specific application to this project. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner.



2.0 PROJECT AND SITE DESCRIPTION The approximate 0.17-acre site is bordered by Virginia Street to the northwest, 9th Avenue to the southwest, an adjacent apartment building to the southeast, and a paved driveway to the northeast. The existing site is occupied by a low-rise commercial building with a zero building setback from all property lines excluding the southeast property line where a driveway separates the existing building from the adjacent apartment building. Site topography across the parcel is relatively flat with overall vertical relief estimated at less than 5 feet. We understand that the currently proposed project involves the demolition of the existing building and the construction of a new mixed-use building that will contain a new hotel and residential apartments. We understand that the new building will have 23 above-grade floors and three below-grade floors. The below-grade floors will extend to a depth of 31 feet below the final grade. We anticipate that temporary shoring will be placed around the perimeter of the project area to support the excavation for below-grade construction. 3.0 SUBSURFACE EXPLORATION Our fieldwork to date included drilling two exploration borings, and completing one as a groundwater monitoring well. The location of Associated Earth Sciences, Inc.’s (AESI’s) explorations are depicted on Figure 2. The various types of sediments, as well as the depths where the characteristics of the sediments changed, are indicated on the exploration logs presented in the Appendix. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types. If changes occurred between sample intervals in our exploration borings, they were interpreted. Our explorations were approximately located in the field by measuring from existing features shown on Figure 2. The conclusions and recommendations presented in this report are based, in part, on the exploration borings completed for this study. The number, locations, and depths of the explorations were completed within current site and budgetary constraints. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that subsurface conditions differing from those indicated on the exploration logs may be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of any variations between the field explorations may not become fully evident until construction. If variations are observed at that time, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes.



3.1 Exploration Borings The exploration borings were generally completed by advancing hollow-stem auger tools with a truck-mounted drill rig. The upper approximate 2 feet of the explorations were completed with a vacuum truck (vactor) assisted excavation for the purpose of clearing utilities. During the hollow-stem auger drilling process, samples were obtained at generally 2½-foot and 5-foot depth intervals. The exploration borings were continuously observed and logged by a representative from our firm. The exploration logs presented in the Appendix are based on the field logs, drilling action, and observation of the samples secured. Disturbed, but representative samples were obtained by using the Standard Penetration Test (SPT) procedure in accordance with American Society for Testing and Materials (ASTM) D-1586. This test and sampling method consists of driving a standard 2-inch, outside-diameter, split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free-falling a distance of 30 inches. The number of blows for each 6-inch interval is recorded, and the number of blows required to drive the sampler the final 12 inches is known as the Standard Penetration Resistance (“N”) or blow count. If a total of 50 is recorded within one 6-inch interval, the blow count is recorded as the number of blows for the corresponding number of inches of penetration. The resistance, or N-value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils; these values are plotted on the attached exploration boring logs. The samples obtained from the split-barrel sampler were classified in the field and representative portions placed in watertight containers. The samples were then transported to our laboratory for further visual classification. A portion of the samples obtained were placed in a chilled cooler immediately following sampling, and subsequently transported to an analytical laboratory under standard chain-of-custody protocols for environmental testing, as described in our referenced “Limited Environmental Assessment” report. 3.2 Monitoring Well A groundwater monitoring well was installed at the project site in MW-1 to allow for documentation and monitoring of groundwater levels below the site. The well consists of 2-inch-diameter polyvinyl chloride (PVC) Schedule-40 well casing with threaded connections. The lower 10 feet of the well is a finely slotted (0.010-inch machine slot) well screen to permit water inflow. The annular space around the well screen was backfilled with silica sand, and the upper portion of annulus was sealed with bentonite chips and grout. A steel flush mount monument was placed over the top of the wellhead for protection. The as-built configuration of these wells is illustrated on the boring logs in the Appendix.



The monitoring well was completed to a depth of 65 feet below the surface with the bottom 10 feet screened in coarse-grained glaciomarine sediments. No groundwater was observed in the well at the time of drilling. 4.0 SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, visual reconnaissance of the site, review of applicable geologic literature and subsurface explorations completed by AESI in the project vicinity. The location of explorations completed for this study are shown on Figure 2. A copy of the exploration boring logs are included in the Appendix. As shown on the boring logs, the exploration borings generally encountered native sediments consisting of Possession-age glaciomarine drift sediments. The following section presents more detailed subsurface information. 4.1 Stratigraphy Possession-Age Glaciomarine Drift Both of our explorations generally encountered medium dense to very dense, some silt to very silty, fine to medium sand or stiff to hard silt and clay with trace to some sand and gravel (dropstones) interpreted as Possession-age glaciomarine sediments. Glaciomarine drift consists of sediment deposited when it melted out of floating glacial ice and then deposited on the sea floor. These glaciomarine sediments were deposited during the Possession Glaciation approximately 60,000 to 80,000 years ago, and have been glacially consolidated by at least one glacial ice sheet. Due to the high silt/clay and moisture content observed within the glaciomarine sediments in each of our explorations, we recommend that earthwork operations be limited to the dry season and that the contractor exercise extreme caution when working with this material to reduce erosion and disturbance of structural subgrades. The silty site soils would benefit from placement of a construction working surface as described in the “Site Preparation” section of this report. The moisture content observed within the glaciomarine sediments in each of our explorations was estimated to be above its optimum moisture content for compaction purposes. We recommend that excavated glaciomarine sediments not be reused in structural fill applications due to their high moisture content and fine-grained composition which makes them hard to work with. Treating excavated glaciomarine sediments with Portland cement might allow reuse in structural fill applications, and is discussed further in the “Site Preparation” and “Structural Fill” sections of this report.



4.2 Hydrology At the time of drilling, groundwater was not encountered in our explorations completed for this study. Exploration MW-2 was completed as a groundwater monitoring well to a depth of 65 feet below the surface. Based on the observations made at the time of drilling and the interpretation of the soil stratigraphy, in our opinion, groundwater will be limited to “perched” groundwater. Perched water occurs when surface water infiltrates down through relatively permeable soils and becomes trapped or “perched” atop a comparatively impermeable barrier, such as the clays or silts beneath the site. It should be noted that fluctuations in the level of the groundwater may occur due to the time of the year, variations in the amount of precipitation, and changes in site development. Based on the lack of seepage observed during our explorations, lack of groundwater within our well, and the site stratigraphy, in our opinion, the site will not require a major dewatering system to complete the planned excavation. Localized areas of groundwater accumulation or seepage may occur from the proposed cut walls, especially during the wet season.

Subsurface Exploration, Geologic Hazard, and 9th and Virginia Preliminary Geotechnical Engineering Report Seattle, Washington Geologic Hazards and Mitigations


II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic and groundwater conditions as observed and discussed herein. 5.0 SEISMIC HAZARDS AND MITIGATIONS Earthquakes occur in the Puget Lowland relatively frequently. The vast majority of these events are small and are usually not felt by people. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 1965, 6.5-magnitude event; and the 2001, 6.8-magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20- to 40-year period. 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. The potential for each of these hazards to adversely impact the proposed project is discussed below. 5.1 Surficial Ground Rupture Generally, the largest earthquakes that have occurred in the Puget Sound area are sub-crustal events with epicenters ranging from 50 to 70 kilometers in depth. Earthquakes that are generated at such depths usually do not result in fault rupture at the ground surface. Seattle Fault Zone The site is located north of the Seattle Fault Zone. Studies by the U.S. Geological Survey (USGS) and others have provided evidence of surficial ground rupture along splays of the Seattle Fault. The recognition of this fault is relatively new and data pertaining to it are limited, with the studies still ongoing. According to the USGS studies, the latest movement of this fault was about 1,100 years ago when about 20 feet of surficial displacement took place. This displacement can presently be seen in the form of raised, wave-cut beach terraces along Alki Point in West Seattle and Restoration Point at the south end of Bainbridge Island. Based on our review of the geologic map referenced above, the site is located about 1.5 miles north of the northern extent of the Seattle Fault Zone. Due to the suspected long recurrence interval, and the distance of the site to the fault trace, the potential for surficial ground rupture along the Seattle Fault Zone is considered low during the expected life of the proposed structure.

Subsurface Exploration, Geologic Hazard, and 9th and Virginia Preliminary Geotechnical Engineering Report Seattle, Washington Geologic Hazards and Mitigations


5.2 Seismically Induced Landslides Due to the lack of slopes and the strength of the site soils, there is little risk of seismically induced landslides occurring on the property. 5.3 Liquefaction The encountered stratigraphy has a low potential for liquefaction due to its dense state, high silt content, and lack of groundwaters. 5.4 Ground Motion Seismic site class for structural design of the building was determined using 2015 International Building Code (IBC) standards, which defines seismic site class by reference to American Society of Civil Engineers (ASCE) 7-10 – Minimum Design Loads for Buildings and Other Structures (ASCE Manual). Utilizing the SPT data from the two borings completed for this study, we determined average field standard penetration resistances for each boring per Section 20.4 of the ASCE Manual. This method uses SPT data for the upper 100 feet of the subsurface profile to calculate and average penetration resistance. Since our explorations extended to depths of 80.5 feet and 66.5 feet below the surface, SPT values were extrapolated beyond the depths of our borings to obtain a 100-foot profile. Average field standard penetration resistance values of 50.3 and 55.6 were calculated for explorations EB-1 and MW-1, respectively. Both values correlate to a seismic site class of “C” using Table 20.3-1 of the ASCE Manual.

Subsurface Exploration, Geologic Hazard, and 9th and Virginia Preliminary Geotechnical Engineering Report Seattle, Washington Preliminary Design Recommendations


III. PRELIMINARY DESIGN RECOMMENDATIONS 6.0 INTRODUCTION Our subsurface explorations indicates that, from a geotechnical standpoint, the parcel is suitable for the proposed development provided the recommendations contained herein are properly followed. The bearing stratum is relatively shallow and spread footing foundations may be utilized for foundation support. Foundations must bear completely on the undisturbed very dense or hard native soils to achieve the bearing capacity recommendations provided in this report. Below grade excavation for underground building floors will require the installation of a temporary excavation shoring system. Groundwater was not encountered in our explorations during drilling, and we anticipate that groundwater encountered during excavation will be limited to perched water and will not necessitate the need for a dewatering system. The contractor should be prepared to control limited groundwater seepage resulting from zones of perched groundwater. 7.0 SITE PREPARATION Old foundations presently on the site that are under building areas or not part of future plans should be removed. Any buried utilities should be removed or relocated if they are under building areas. Site preparation of planned building areas should include removal of all pavement, debris, and any other deleterious material. In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction. For estimating purposes, however, we anticipate that temporary, unsupported cut slopes, above the water table, in the very dense and hard natural soils can be made at a maximum slope of 1H:1V (Horizontal:Vertical). Flatter slopes should be provided adjacent to traffic lanes and/or utilities. As is typical with earthwork operations, some sloughing and raveling may occur and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA regulations should be followed at all times. The on-site soils contain a high percentage of fine-grained material that makes them moisture-sensitive and subject to disturbance when wet. The contractor must use care during site preparation and excavation operations so that the underlying soils are not softened. If disturbance occurs, the softened soils should be removed, and foundations extended down to competent natural soil. Once the base of the excavation is reached, consideration should be given to “armoring” the exposed subgrade with a layer of rock to provide a working surface during foundation construction. We recommend a 12-inch layer of washed or crushed rock for this purpose.



8.0 STRUCTURAL FILL Should structural fill be necessary to establish desired grades beneath lightly loaded portions of the project, it should be placed and compacted according to the recommendations presented in this section. Due to the high foundation bearing loads, structural fill should not be placed beneath footings or columns. All references to structural fill in this report refer to subgrade preparation, fill type, placement, and compaction of materials as discussed in this section. If a percentage of compaction is specified under another section of this report, the value given in that section should be used. After the exposed ground to receive fill is tested and approved by the geotechnical engineer, structural fill may be placed to attain desired grades. Structural fill is defined as non-organic soil, acceptable to the geotechnical engineer, placed in maximum 8-inch loose lifts with each lift being compacted to at least 95 percent of the modified Proctor maximum dry density using ASTM D-1557 as the standard. In the case of roadway and utility trench filling, the backfill should be placed and compacted in accordance with City of Seattle standards. The top of the compacted fill should extend horizontally outward a minimum distance of 3 feet beyond the location of the perimeter footings or roadway edge before sloping down at an angle of 2H:1V. The contractor should note that any proposed fill soils must be evaluated by AESI prior to their use in fills. This would require that we have a sample of the material 48 hours in advance of filling activities to perform a Proctor test and determine its field compaction standard. Soils in which the amount of fine-grained material (smaller than No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture-sensitive. Use of moisture-sensitive soil in structural fills is not recommended. The on-site soils generally contained significant amounts of silt and are considered moisture-sensitive. For all fills, a select import material consisting of a clean, free-draining gravel and/or sand should be used, such as City of Seattle Type 2 or 17, crushed surfacing base course as specified in the 2018 edition of Standard Specifications for Road, Bridge, and Municipal Construction published by the Washington State Department of Transportation (WSDOT) Section 9-03.9(3). Free-draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction. A representative from our firm should inspect the stripped subgrade and be present during placement of structural fill to observe the work and perform a representative number of in-place density tests. In this way, the adequacy of the earthwork may be evaluated as filling progresses and any problem areas may be corrected at that time. It is important to understand that taking random compaction tests on a part-time basis will not assure uniformity or



acceptable performance of a fill. As such, we are available to aid the owner in developing a suitable monitoring and testing frequency. 9.0 FOUNDATIONS Conventional spread footings and column pads may be used for building support when founded on undisturbed, very dense to hard native soils. We recommend that an allowable bearing pressure of 10,000 pounds per square foot (psf) be utilized for design purposes, including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. All footings must penetrate to the prescribed bearing stratum and no footing should be founded in or above loose, organic, or fill soils. It should be noted that the area bounded by lines extending downward at 1H:1V from any footing must not intersect another footing or intersect a filled area that has not been compacted to at least 95 percent of ASTM D-1557. In addition, a 1.5H:1V line extending down from any footing must not daylight because sloughing or raveling may eventually undermine the footing. Thus, footings should not be placed near the edge of steps or cuts in the bearing soils. Anticipated settlement of footings founded on undisturbed, very dense or hard native soils should be on the order of 1 inch. However, disturbed soil not removed from footing excavations prior to footing placement could result in increased settlements. All footing areas should be inspected by AESI prior to placing concrete to verify that the design bearing capacity of the soils has been attained and that construction conforms to the recommendations contained in this report. Such inspections will be required by the Seattle Department of Construction and Inspections (SDCI) as part of special inspection requirements. Perimeter footing drains should be provided as discussed under the “Drainage Considerations” section of this report. 10.0 FLOOR SUPPORT A slab-on-grade floor may be used over structural fill or very dense to hard native ground. The floor should be cast atop a minimum of 6 inches of washed gravel to act as a capillary break. It should also be protected from dampness by an impervious moisture barrier or otherwise sealed.



11.0 DRAINAGE CONSIDERATIONS The underlying, glacially consolidated soils have a relatively high content of fine-grained particles and are considered moisture-sensitive; water will tend to perch atop this stratum. Additionally, traffic across these soils when they are damp or wet will result in disturbance of the otherwise firm stratum. Therefore, prior to site work and construction, the contractor should be prepared to provide drainage and subgrade protection as the excavation progresses. When permanent exterior walls are constructed, a drainage system should be incorporated to collect water seeping through the temporary shoring. Prior to casting the exterior walls, a proprietary prefabricated drainage mat should be placed in 4-foot-wide strips over the lagging from the top to the bottom of the wall. The strips should then be covered in a plastic sheeting prior to casting the new wall. The drainage mats should communicate with the permanent perimeter drainage system. This system could include a below-slab perimeter drain, either exterior or interior to the perimeter foundation drains consisting of rigid, perforated, PVC pipe surrounded by washed pea gravel, or a slab surface gutter with weep holes through the exterior wall. Weep holes 2 inches in diameter, 8 feet on-center are recommended. 12.0 TEMPORARY EXCAVATION SHORING SYSTEM A deep excavation is currently being planned for this project that may extend to roughly 40 feet below existing street grades based on the number of underground levels planned. If the proposed cuts for the underground parking will extend deeper, requiring soldier piles longer than about 50 feet, additional subsurface exploration and modifications to our shoring recommendations will be required. Temporary shoring will be required to support the sides of the excavation. In addition, shoring or limited underpinning may be required to protect adjacent buildings, streets, and utilities. This section of the report presents preliminary design considerations and criteria for use in the design of the excavation shoring. With this information and other pertinent data, it should be the responsibility of the shoring subcontractor(s) to determine the appropriate design details, construction methods and procedures for installation of the shoring system. 12.1 Soldier Pile Wall The most common method of shoring used in the Seattle area consists of a conventional soldier pile/waler shoring system utilizing steel soldier piles, often in conjunction with an internally braced or tieback system. Based on our present understanding of the project, we recommend soldier pile shoring walls for design and construction of the new high-rise tower.



Soldier piles, which are usually wide-flange beams, are placed in predrilled holes that extend beyond the bottom of the excavation. The portion of each soldier pile extending below the bottom of the excavation is grouted in place with sufficient-strength concrete to transmit the vertical loads of the soldier beams to the soil below the excavation level. The upper portion of the soldier pile is then backfilled with a relatively weak grout so that it may be removed as necessary for placement of lagging. In our opinion, lagging would be required throughout the excavation. We also recommend that lagging be backfilled with clean sand or pea gravel during installation to minimize the potential for movement of the cut soil. The use of a weak mix (¼ to ½ sack) of flowable sand and cement can be used for lagging backfill. This will reduce the loss of backfill behind upper lagging boards due to excavation around lower lagging boards. Due to the depth of the cuts, the shoring system will require the installation of tiebacks to support the upper part of the wall. A tieback system consists of drilling behind the soldier pile wall at an angle to the horizontal and installing rods or cables with a grouted soil anchor. Walers at the soldier pile wall then transmit the restrained horizontal loads to the grouted anchor. Wall design can be performed based on an apparent earth pressure. We recommend the use of an apparent earth pressure of 25(H+2) psf presented as a trapezoidal distribution, where H is the height of the wall or excavation. This number reflects surcharging from “normal” construction equipment and activities, as well as the variability of the site soils. Any additional surcharge loads must be added and can be calculated from formulas shown on Figure 3. The use of an apparent earth pressure for the shoring system assumes sufficient deformation of the soil occurs to develop an active condition, typically on the order of 0.001 to 0.002 times the height of the excavation. Theoretically, an equal amount of settlement occurs behind the wall. The settlement is typically greatest immediately adjacent to the wall and decreases with distance away from the wall. The limits of settlement are typically within a distance of one wall height behind the top of wall. Therefore, a comprehensive survey of the surrounding streets, structures, and other critical reference points should be performed prior to construction activities. These points should then be accurately monitored as necessary, both horizontally and vertically, until the excavation and backfilling to surrounding site grade has been accomplished. Monitoring of adjacent streets and structures should be provided, regardless of the wall pressure design approach. Refer to Figure 4 for additional design details and a graphic representation of the recommended apparent earth pressures. From a soil standpoint, the grouted soldier piles must be designed for sufficient vertical capacity, and in the case where tiebacks are used, this should include the vertical component of the inclined tieback loads. It should be noted that settlement of the soldier piles under load could also cause a reduction in anchor pre-stress, allowing lateral tilting about the base. For design purposes, the vertical load capacity should be determined based on an allowable adhesion or side friction of 1 kip per square foot (ksf) and an allowable end bearing of 20 ksf, assuming a minimum embedment of 10 feet below the base of the excavation.



The soldier piles also need to be located a sufficient depth below the base of the excavation to provide adequate lateral or “kick-out” resistance to horizontal loads below the lowest brace or tieback level. In this regard, the lateral resistance may be computed based on passive pressure in the form of an allowable “apparent” earth pressure equivalent to 350(D-2) psf where D is the depth of embedment below the base of the excavation in feet. This pressure acts against twice the diameter of the grouted soldier pile section. The active or at-rest pressure distributions should be assumed to act over the tributary area of the piles above the excavation base and one concreted pile diameter below the base. For tiebacks used in the shoring system, the grouted anchors must be located far enough behind the soldier pile wall to develop anchorage within a stable soil mass to prevent a massive failure or excessive deformation. We recommend that this anchorage be obtained behind an assumed failure plane defined by a horizontal line extending a distance equal to H/3 behind the retained excavation at the base of the excavation, which then rotates 60 degrees from the horizontal and extends upward to the ground surface. The area between this assumed failure plane and the retained excavation is referred to as the “no-load zone.” These recommendations are presented on Figure 4. The anchor loads are transmitted to the surrounding soil by side friction or adhesion with the soil. The tieback anchors may be tentatively designed for an allowable shaft stress of 1,500 psf in the dense to very dense sands or hard silts/clays. Care must be exercised when installing tiebacks to avoid existing utilities and foundations. Demonstration of utility and foundation protection will be required to obtain a temporary tieback easement from the City of Seattle. All tiebacks will need to be de-stressed subsequent to wall and floor construction. We recommend for this site that each anchor be sized for a design or allowable load of not more than 50 percent of the ultimate load available through the anchor (as indicated by 200-percent verification tests). The test anchors should be capable of holding the ultimate load without excessive yield or creep so that a factor of safety of at least 2.0 is available for production anchors should further stressing occur. The rods or cables should transmit the anchor load to the soldier pile in such a manner to avoid eccentric loading. A series of anchor tests must be performed to verify the design and ultimate skin friction or adhesion of the tieback anchors. A common anchor testing program would consist of at least two, 200-percent verification tests of the design or allowable load in the soil within the slide zone and two such tests in the soil outside of this zone, plus proof-loading every production anchor to 130 percent of the design load. Because of the variation in the soil types and their densities, we recommend that AESI monitor the anchor test program. A common practice is to load an anchor in 10-kip increments held for



5 minutes up to the final load of 200-percent design load, which is held for at least 30 minutes. Measurements of the rod or cable movement should be accurately recorded throughout the test. A linear or near-linear relationship between the unit anchor stresses and related movement would indicate a successful anchor test. Anchor creep should also be less than 0.08 inches per log cycle time for a successful test. The other component of the anchor tests for the project would be proof-loading each of the production anchors to 130 percent of the design load. Each anchor should withstand this load for at least 5 minutes. A total movement of the anchor of less than 6 inches would indicate a successful proof-load test. The anchor should then be locked-off at the design load. Subsequent to locking-off the tiebacks at the design load, all of the tieback holes should be backfilled to prevent possible collapse of the holes and any related consequences. Typically, sand is used as backfill material; however, most non-cohesive mixtures are suitable (subject to approval by the geotechnical engineer) provided there is no bonding to the tierods. 13.0 PROJECT DESIGN AND CONSTRUCTION MONITORING At the time of this report, site grading, structural plans, and construction methods have not been developed and the recommendations presented herein are preliminary. We are available to provide additional geotechnical consultation as the project design develops and possibly changes from that upon which this report is based. We recommend that AESI perform a geotechnical review of the plans prior to final design completion. In this way, our earthwork and foundation recommendations may be properly interpreted and implemented in the design. We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the foundation depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Construction monitoring services are not part of this current scope of work. If these services are desired, please let us know and we will prepare a proposal.

Ar(thonyW. 'o /anick, P.E.

Project Engineer


We have enjoyed working with you on this study and are confident these recommendations

will aid in the successful completion of your project. If you should have any questions, or

require further assistance, please do not hesitate to call.

Sincerely,

ASSOCIATED EARTH SCIENCES, INC.

Kirkland, Washington

Kurt D. Merriman, P.E.

Senior Principal Engineer

Attachments: Figure 1: Vicinity Map

Figure 2: Site and Exploration Aerial Photograph

Figure 3: Surcharge Pressures on Adjacent Walls

Figure 4: Temporary Soldier Pile Retaining Wall Design Criteria

Appendix: Exploration Logs

October 11, 2018 ASSOCIATED EARTH SCIENCES, INC.

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8\VM\

1803

15E0

01 F1

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DATA SOURCES / REFERENCES:USGS: 7.5' SERIES TOPOGRAPHIC MAPS, ESRI/I-CUBED/NGS 2013KING CO: STREETS, PARCELS, CITY LIMITS 1/18LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE

KitsapCounty

Snohomish County

Pierce County

King County

Stewart St

Virginia St

Boren Ave

9th Ave

Lenora St

Terry Ave

8th Ave

!(

¬«99

SITE

¥5

VICINITY MAP9TH & VIRGINIA

SEATTLE, WASHINGTON

!(

9TH AVE

VIRGINIA ST

EB-1

MW-1

0 4020

FEET

±NOTE: BLACK AND WHITEREPRODUCTION OF THIS COLORORIGINAL MAY REDUCE ITSEFFECTIVENESS AND LEAD TOINCORRECT INTERPRETATION

EXISTING SITE ANDEXPLORATION PLAN

9TH & VIRGINIASEATTLE, WASHINGTON

PROJ NO. DATE: FIGURE:180315E001 8/18 2Do

cume

nt Pa

th: G

:\GIS_

Proje

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Y201

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315 9

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15E0

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DATA SOURCES / REFERENCES:KING CO: STREETS, PARCELS 1/18AERIAL: KING CO, PICTOMETRY INT. 2015LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE

King County

SITE

LEGEND:SITE

!( EXPLORATION BORINGMONITORING WELL

9TH & VIRGINIA

SEATTLE, WASHINGTON

180315 9

th a

nd V

irgin

ia \ 1

80315E

001 F

3 S

UR

CH

AR

GE

PR

ES

SU

RE

2.c

dr

SURCHARGE PRESSURE

ON ADJACENT WALLS

PROJ NO. DATE: FIGURE:

180315E001 10/18 3

a s s o c i a t e d

e a r t h s c i e n c e s i n c o r p o r a t e d

LEGEND:

EXCAVATION DEPTH BELOW FOOTING IN FEET

LATERAL SOIL PRESSURE IN PSF

UNIT LOADING PRESSURE IN PSF

RADIANSb

sh

D

q

a

b

q

b/2

sh

D

GROUND SURFACE

BASE OF EXCAVATION

ISOLATED FOOTING

ab bsh = 0.64q ( - SIN COS2 )

sh

DGROUND SURFACE

BASE OF EXCAVATION

X = mDq

LINE LOADPRESSURE

z =

nD

CONTINUOUS FOOTINGPARALLEL TO EXCAVATION

(FOR m>0.4)

sh = 1.28qD

2m n2 2 2(m + n )

sh = qD

0.2 n2 2(0.16 + n )

(FOR m<0.4)

UNIFORM LOAD DISTRIBUTION

sh

q = VERTICAL PRESSURE IN PSF

= 0.4q

BASE OF EXCAVATION

shUNIFORM LOAD

q

BASE OFEXCAVATION

2’

GROUND SURFACE

SO

LD

IER

PIL

E

H/4

D =

10’ M

IN IN

TO

25 (H+2) PSFACTIVE

40 (H+2) PSFAT-REST

HTIE BACK

15 MIN

0.2

H

350 (D-2) PSF

PASSIVE PRESSURE ACTSOVER TWICE PILE DIAMETER

PASSIVE PRESSURE TRUNCATED 2 FEET BELOW BASE OF EXCAVATION

Dense or HardGlaciomarineSediments

ACTIVE OR AT-REST EARTH PRESSUREACTS OVER SOLDIER PILE TRIBUTARY AREA

NOTES:1. SOLDIER PILE EMBEDMENT DEPTH “D” SHOULD CONSIDER NECESSARY VERTICAL CAPACITY, KICKOUT, AND OVERTURNING RESISTANCE.

2. ALL TIEBACK ANCHORS SHALL BE PRESTRESSED TO 130 PERCENT OF DESIGN LOAD AND LOCKED OFF AT 100 PERCENT OF DESIGN LOAD. AT LEAST TWO ANCHORS PER SIDE AT THE EXCAVATION SHALL BE PRESTRESSED TO 200 PERCENT AND MONITORED FOR CREEP. TIEBACK ANCHOR ZONE IS TO BE LOCATED BEHIND THE NO-LOAD ZONE.

3. PRESUMPTIVE ALLOWABLE TIEBACK - SOIL ADHESION 1.0 KSF TO BE CONFIRMED BY 200 PERCENT VERIFICATION TEST.

4. PASSIVE PRESSURES INCLUDE A FACTOR OF SAFETY OF 1.5.

5. ALLOWABLE SKIN FRICTION OF SOLDIER PILE - 1.0 KSF OVER DEPTH “D-2”. ALLOWABLE END BEARING = 20 KSF.

6. DIAGRAM DOES NOT INCLUDE HYDROSTATIC PRESSURES OR SLOPE SURCHARGES AND ASSUMES WALLS ARE SUITABLY DRAINED TO PREVENT BUILDUP OF HYDROSTATIC PRESSURE WITH NO SLOPE AT TOP OF WALL.

7. DIAGRAM IS ILLUSTRATIVE AND NOT REFERENCED TO A PARTICULAR LOCATION.

8. DIAGRAM DOES NOT INCLUDE PRESSURES DUE TO SURFACE SURCHARGES FROM ANY ADJACENT STRUCTURES. THESE PRESSURES MUST BE PROVIDED BY THE STRUCTURAL ENGINEER.

9. BASE OF EXCAVATION SHALL BE DEFINED AS THE FOUNDATION SUBGRADE ELEVATION.

60

NO

-LOA

D Z

ON

E LIM

IT

9TH & VIRGINIA

SEATTLE, WASHINGTON

180315 9

th a

nd V

irgin

ia \ 1

80315E

001 F

4 S

old

ier.

cdr

TEMPORARY SOLDIER PILE

RETAINING WALL DESIGN CRITERIA

PROJ NO. DATE: FIGURE:

180315E001 10/18 4

a s s o c i a t e d

e a r t h s c i e n c e s i n c o r p o r a t e d

APPENDIX

Asphalt - 1.5 inchesConcrete - 4 inches

Possession Glacial Marine Drift

Slightly moist, dark gray, very silty, fine SAND, trace to some gravel, tracemedium sand; unsorted (SM).

As above.

As above.

As above.

As above, poor recovery.

As above.

As above.

Moist, dark gray, clayey, SILT, trace to some fine sand; massive; mild reactionwith hydrochloric acid (ML).

As above.

3915

92031

122027

50/1"

162631

6922

4714

4615

S-1

S-2

S-3

S-4

S-5

S-6

S-7

S-8

S-9

1 of 2

NAVD 88

Sheet

Dep

th (

ft)

Exploration Number180315E001

M - Moisture

8 inches

40

Datum

ST G

raph

ic

10

Oth

er T

ests

Hole Diameter (in)

DESCRIPTION

Location

Water Level () Approved by:

30

Blows/Foot

Driller/Equipment

Blo

ws/

6"

EDI / Truck

Wel

l

5

10

15

20

25

30

35

40

Wat

er L

evel

Project Name

EB-1

Sym

bol

DV2" OD Split Spoon Sampler (SPT)

3" OD Split Spoon Sampler (D & M) CJK

Com

plet

ion

Sam

ples

Ground Surface Elevation (ft)

Grab Sample

8/20/18,8/20/18

Logged by:

Shelby Tube Sample

140# / 30"

Ring Sample

No Recovery

Water Level at time of drilling (ATD)

9th & Virginia

Project Number

20

Seattle, WADate Start/Finish

Hammer Weight/Drop

Sampler Type (ST):

Exploration LogA

ES

IBO

R 1

8031

5.G

PJ

Oct

ober

3, 2

018

2424

51

4747

5050/1"

57

3131

2121

2121

Slightly moist, grayish brown, silty, fine SAND, trace gravel; trace very silty, finesand interbeds (SM).Moist, dark gray, silty, fine to medium SAND, some gravel; unsorted (SM).

Moist, gray, silty, fine to medium SAND, some gravel; unsorted (SM).

Turbidite Interbed ?Moist, grayish brown with trace zones of oxidation, very silty, fine SAND, tracegravel; unsorted (SM).

Very moist, brownish gray, very silty, fine SAND; stratified (SM).

Moist, brownish gray with some zones of oxidation, medium SAND, some silt(SP-SM).

Very moist, brownish gray, gravelly, fine to coarse SAND, some silt (SP-SM).

Very moist, brownish gray, silty, fine to medium SAND, some gravel (SM).

Moist, dark gray, clayey, SILT, some fine to coarse sand, some gravel; unsorted(ML).

Possession Glacial Marine DriftVery moist, dark gray, very silty, fine to medium SAND, some gravel; unsorted(SM).

As above.

3832

50/5"

50/3"

101934

41642

622

50/6"

1130

50/6"

50/3"

50/6"

S-10

S-11

S-12

S-13

S-14

S-15

S-16

S-17

Bottom of exploration boring at 80.5 feetNo groundwater encountered.

2 of 2

NAVD 88

Sheet

Dep

th (

ft)

Exploration Number180315E001

M - Moisture

8 inches

40

Datum

ST G

raph

ic

10

Oth

er T

ests

Hole Diameter (in)

DESCRIPTION

Location

Water Level () Approved by:

30

Blows/Foot

Driller/Equipment

Blo

ws/

6"

EDI / Truck

Wel

l

50

55

60

65

70

75

80

85

Wat

er L

evel

Project Name

EB-1

Sym

bol

DV2" OD Split Spoon Sampler (SPT)

3" OD Split Spoon Sampler (D & M) CJK

Com

plet

ion

Sam

ples

Ground Surface Elevation (ft)

Grab Sample

8/20/18,8/20/18

Logged by:

Shelby Tube Sample

140# / 30"

Ring Sample

No Recovery


9th & Virginia

Project Number

20

Seattle, WADate Start/Finish

Hammer Weight/Drop

Sampler Type (ST):

Exploration LogA

ES

IBO

R 1

8031

5.G

PJ

Oct

ober

3, 2

018

5050/5"

5050/3"

53

58

5050/6"

5050/6"

5050/3"

5050/6"

Asphalt - 1 inchConcrete - 4 inches

Possession Glacial Marine Drift

Moist, dark gray, very silty, fine SAND, some gravel; unsorted (SM).

As above.

Contains trace organics (SM).

Poor recovery. Moist, dark gray, very silty, fine to medium SAND, tracegravel; unsorted (SM).

Very silty, fine sand.

As above.

Sand becomes fine to medium.

Moist, dark gray, clayey, SILT, some fine sand, trace gravel; poorlystratified (ML).

As above.

Becomes moist, tan, silty, CLAY, trace fine sand; stratified (CL).

3715

71621

6916

50/5"

81222

262940

1738

50/6"

4618

61834

Flush mount monumentJ-plug well capGrout 0 to 1.5 feetBentonite chips 1.5 to 52 feet

2-inch I.D. PVC well casingwith threaded connections 0.5to 55 feet.

Well Number

140# / 30"

Project NameElevation (Top of Well Casing)

N/AW

ater

Lev

el

CJKApproved by:

Grab Sample

115.00

3" OD Split Spoon Sampler (D & M)

MW-2Location

1 of 2

8/21/18,8/21/18

Seattle, WA

Sampler Type (ST):

WELL CONSTRUCTION

Gra

phic

Sym

bol

Blo

ws/

6"

Sheet

5

10

15

20

25

30

35

40

EDI / Truck

115

Ring Sample

Logged by:

9th & Virginia

Shelby Tube Sample

Drilling/Equipment

Water Level ()


180315E001

Water Level Elevation

M - Moisture

ST

Surface Elevation (ft)

Project Number

Date Start/Finish

Hammer Weight/Drop8 inches

Dep

th(f

t)

DESCRIPTION

2" OD Split Spoon Sampler (SPT) No Recovery

Geologic & Monitoring Well Construction Log

DV

Hole Diameter (in)

NW

WE

LL-

B 1

8031

5.G

PJ

BO

RIN

G.G

DT

10/

3/1

8

Moist, dark gray, very silty, fine to medium SAND, some gravel; unsorted(SM).

As above, contains trace organics.

Contains trace medium sand seams, some silt (SP-SM).

Turbidite Interbed ?Moist, brownish gray with trace zones of oxidation, fine to mediumSAND, some silt, trace gravel; some very silty, fine sand interbeds;stratified (SP-SM).As above.

Very moist, brownish gray, fine to medium SAND, some silt, trace gravel;unsorted (SP-SM).

3850/2"

50/5"

313442

143240

2435

50/6"

62840

Silica sand 10/20 52 to 66.5feet

2-inch I.D. PVC well screen0.010-inch slot width 55 to 65feet

Threaded end cap

Well tag # BKH-547

Boring terminated at 66.5 feet.Well completed at 65 feet on 8/21/18.No groundwater encountered.

Well Number

140# / 30"

Project NameElevation (Top of Well Casing)

N/AW

ater

Lev

el

CJKApproved by:

Grab Sample

115.00

3" OD Split Spoon Sampler (D & M)

MW-2Location

2 of 2

8/21/18,8/21/18

Seattle, WA

Sampler Type (ST):

WELL CONSTRUCTION

Gra

phic

Sym

bol

Blo

ws/

6"

Sheet

50

55

60

65

70

75

80

85

EDI / Truck

115

Ring Sample

Logged by:

9th & Virginia

Shelby Tube Sample

Drilling/Equipment

Water Level ()


180315E001

Water Level Elevation

M - Moisture

ST

Surface Elevation (ft)

Project Number

Date Start/Finish

Hammer Weight/Drop8 inches

Dep

th(f

t)

DESCRIPTION

2" OD Split Spoon Sampler (SPT) No Recovery

Geologic & Monitoring Well Construction Log

DV

Hole Diameter (in)

NW

WE

LL-

B 1

8031

5.G

PJ

BO

RIN

G.G

DT

10/

3/1

8

subsurface exploration and

Documents