Slope Stabilization Design Services
Site Stabilization and Excavation Project Yesler Terrace Site II-b Seattle, Washington for SMR Architects
February 4, 2015 Slope Stabilization Design Services
Site Stabilization and Excavation Project Yesler Terrace Site II-b Seattle, Washington for SMR Architects
February 4, 2015
8410 154th Avenue NE Redmond, Washington 98052 425.861.6000
Table of Contents
INTRODUCTION ...... 1 PROJECT DESCRIPTION ...... 1 SUBSURFACE CONDITIONS ...... 2 SLOPE STABALIZATION DESIGN ...... 4 Construction Sequence ...... 4 Soil Properties ...... 4 Groundwater Considerations ...... 4 Seismic Loading ...... 5 Surcharge Loading ...... 5 Design Safety Factors ...... 5 Design Methodology and Stability Analysis ...... 6 Soil Nails ...... 6 Wire Mesh Facing ...... 6 Slope Planting ...... 6 Slope Monitoring ...... 6 Design Drawings and Specifications ...... 7 GEOTECHNICAL SPECIAL INSPECTION ...... 7 LIMITATIONS ...... 7
APPENDICES Appendix A. Calculations
ATTACHMENTS Design Drawing Sheets Sheet G2.00 – General Notes and Sheet Index Sheet G2.10 – Site Plan Sheet G2.20 – Cross Sections Sheet G2.30 – Details Sheet G2.40 – Specifications
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INTRODUCTION
This report presents a summary of GeoEngineers’ slope stabilization design for the Site Stabilization and Excavation Project, which will be completed in advance of the planned 221 10th Avenue building development for Yesler Terrace Site II-b located in Seattle, Washington. The slope improvements also extend approximately 150 feet west of the 221 10th Avenue building to stabilize additional slope area for potential future development. GeoEngineers provided geotechnical engineering services to support design and construction of the planned 221 10th Avenue building and our recommendations are summarized in a report titled “Geotechnical Engineering Services, 221 10th Avenue, Yesler Terrace Redevelopment, Seattle, Washington” and dated February 3, 2015. Improvements to the slope are necessary to increase the static and seismic global stability safety factors to current design standards.
The project site is shown on Vicinity Map on Sheet G2.00 and the location and extent of the slope improvements are shown on Sheet G2.10, which are attached to this design report.
PROJECT DESCRIPTION
GeoEngineers’ understanding of the required slope stabilization is based on discussions with the project team, Seattle Housing Authority (SHA), and specialty contractors, City of Seattle codes and design standards, and our understanding of the site conditions. A geotechnical paid coaching session was also attended by GeoEngineers, SMR Architects, Coughlin Porter Lundeen (CPL), SHA, and the City of Seattle Department of Planning and Development (DPD) (Jim Mattoon) on December 11, 2014 to discuss our planned slope stabilization design.
The upper portion of the slope consists of the head scarp of historic landslides in the area. The slope flattens out below the head scarp and the surficial soils consist of slide debris from past landslide events. A detail description of the site history and historic landslides is provided in our February 3, 2015 geotechnical report. The slope is marginally stable under static loading conditions and is unstable under the design seismic event. The slope stabilization improvements are necessary to increase the slope stability safety factors to minimum values of 1.5 (static) and 1.1 (seismic) per the City of Seattle requirements.
The Hill Climb project is currently under construction and is located immediately south of the planned 221 10th Avenue building. The Hill Climb project includes construction of three rows of 3-foot-diameter drilled shafts at approximately 8-foot-on-center spacing to improve the stability of the landslide mass located in that area of the slope. The drilled shafts were installed in the fall of 2014, and the stair and ramp structures and associate walls will be constructed later this year. We previously prepared a geotechnical report to support design and construction of the Hill Climb project titled “Geotechnical Engineering Services, 10th Avenue Hillclimb to South Main Street Improvement Project, Yesler Terrace Redevelopment Project, Seattle, Washington” and dated January 18, 2013.
The slope stabilization improvements will extend approximately 150 feet west of the 221 10th Avenue building to the west boundary of Lot 19, and will tie into the west edge of the Hill Climb project. Improvements to the slope will cover an area approximately 300 feet long and about 25 to 30 feet high.
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The slope was evaluated for future grades and include an approximate 6-foot cut at the top of the slope near the planned 221 10th Avenue building. Excavation and grading above the slope will be completed prior to construction of the slope stabilization project. .
The slope stabilization design consists of soil nail reinforcement connected to Geobrugg Tecco® System3 mesh facing. An erosion control blanket will be placed on the slope below the mesh facing. After the mesh facing is installed the slope will be hydroseeded per Seattle Standard Specifications 9-14.2 Seed Mix #1.
The slope stabilization project will be constructed entirely in SHA’s property and easements are not required to install the soil nails. Existing and planned future utilities were considered in the design; however, the contractor should field-verify that buried utilities do not interfere with soil nail locations. If conflicts between the soil nail system and existing utilities are identified, GeoEngineers should be contacted to provide modifications to the design.
SUBSURFACE CONDITIONS
Subsurface conditions in the project area were evaluated based on information obtained from borings completed as part of the 221 10th Avenue building design. The locations of borings are shown on Sheet G2.10 and the boring logs and our interpretation of the subsurface conditions are presented in our January 18, 2013 and February 3, 2015 geotechnical reports. The site is classified as an environmentally critical area (ECA) and includes (1) 40 percent steep slopes, (2) potential slide areas, and (3) known slide areas. A detailed description of the ECA at the site can be found in the above geotechnical reports.
The subsurface soils along the slope consist of fill, interbedded glacial soils, landslide debris, and fine-grained glacial soils as described below. ■ Fill: Approximately 5 to 10 feet of fill was encountered within the borings completed near the crest of the slope. The fill generally consists of loose to dense silty sand with gravel. Brick fragments, concrete, and wood debris were observed within the fill soils in several borings. ■ Landslide Deposits: The slope immediately south of the 221 10th Avenue Building site is underlain by extensive landslide deposits. The landslide deposits primarily consist of disturbed soils composed of silt and clay with sand. The landslide debris typically consists of highly fractured and disturbed deposits derived from native soils in the area, including the soils described below. The landslide debris is about 20 to 25 feet thick under the adjacent 10th Avenue Hill Climb project site. Relatively undisturbed fine- grained soils were observed below the landslide deposits. Our interpretation of the landslide area is described in our January 18, 2013 and February 3, 2015 geotechnical reports. ■ Interbedded Glacial Soils: A mixture of glacial till, outwash and transitional deposits was observed in each of the borings completed above the slope. Possible glacial outwash consisting of medium dense to dense silty sand and sandy silt was observed in borings LAI-3 and LAI-8 below the fill. This soil unit may also be a sandy glacial till deposit. Native glacial soils consisting of interbedded medium dense to dense/stiff to hard silty sand, sandy silt, and clay were observed below the fill or glacial till/outwash soils. For the purpose of this report the glacial till, outwash, and interbedded sand, silt, and clay soils are collectively referred to as interbedded glacial soils. ■ Fine-grained Glacial Soils: Fine-grained glacial deposits were observed below the interbedded glacial soils north of the slope and below the slide deposits on the slope face. The fine-grained glacial deposits
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are mapped as Pre-Olympia deposits, but other reports describe these soils as Lawton Clay deposits. The fine-grained soils generally consist of stiff to very stiff silt to fat clay (high plasticity) with variable sand and gravel content. These soils have been glacially consolidated with numerous slickensides and fractured surfaces. The fine-grained soils extended to the maximum depth explored (51½ feet) in the project site. The landslides on the southern slope initiated in these fine-grained glacial deposits.
Groundwater conditions at the site were assessed by observing the moisture content of the soil samples recovered during drilling, measuring the depth of water in the borehole prior to backfilling, and by measuring the depth to water in monitoring wells installed in some borings. In general, perched groundwater was encountered within the glacial soils’ more permeable layers and wet zones were commonly observed below a depth of about 9 feet. Perched groundwater levels are subject to fluctuations in the weather and vary with the seasons. The approximate groundwater elevations measured in the monitoring wells are summarized in Table 1. Perched groundwater observed at the time of drilling in the remaining borings are summarized in Table 2. Groundwater was observed discharging from the 10th Avenue South drainage tunnel outlet located on the south part of the slope within the 10th Avenue Hill Climb project at a rate of 1 to 2 gallons per minute (gpm). The observations were observed prior to starting construction of the Hill Climb project.
TABLE 1. PERCHED GROUNDWATER: MONITORING WELL MEASUREMENTS
Approximate Well Screen Ground Groundwater Surface Interval Measurement Boring Elevation Depth Elevation Depth Elevation Date (feet) (feet) (feet) (feet) (feet) B-6 231.1 33.5 – 48.5 192.5 – 197.5 42.6 188.5 July 15, 2014 B-8 234.8 30 – 40 195 – 205 24.0 210.8 July 15, 2014 B-9 205.3 12 – 22 183 – 193 9.7 195.6 July 15, 2014 LAI-2 208 32 – 42 166 – 176 13 195 June 24, 2010 LAI-3 236 15 – 25 211 – 221 19 217 June 24, 2010
TABLE 2. PERCHED GROUNDWATER OBSERVATIONS DURING DRILLING*
Ground Surface At time of Drilling Boring Elevation (feet) Groundwater Depth (feet) Groundwater Elevation (feet) B-4 237.4 29 208 B-5 236.9 22 215 B-5A** 237.0 22 215 LAI-8 228.5 18 210.5 Note: * Perched groundwater conditions observed during drilling are approximate. ** Elevation estimated from nearby adjacent surveyed boring.
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SLOPE STABALIZATION DESIGN
Construction Sequence The construction sequence for the slope stabilization will consist of the following: 1. Excavate to grades shown on the drawings prior to nail installation; 2. Clear and grub the slope face; 3. Install soil nails; 4. Test the soil nails; 5. Install erosion control blanket; 6. Install wire mesh facing system; and 7. Hydroseed the slope face.
Soil Properties Selected engineering properties of the soil units were previously estimated as part of the design for the adjacent Hill Climb project and planned 221 10th Avenue building as presented in the geotechnical reports dated January 18, 2013 and February 3, 2015, respectively. Table 3 summarizes the selected soil properties used in the slope stabilization design.
The soil properties was previously estimated by completing a back-analysis of the existing slope as described in our January 18, 2013 geotechnical report.
TABLE 3. SELECTED ENGINEERING SOIL PROPERTIES
Total Unit Weight Friction Angle Cohesion Allowable Anchor Pullout Soil Unit (pcf) (degrees) (psf) (kips/foot) Fill 125 34 0 2.0 Slide Debris 120 18 75 N/A Slide Plane 120 16 0 N/A Varies Interbedded Glacial Soils 125 0 2.0 (34 – 36) Fine-Grained Glacial Soils 120 30 200 1.5 Notes: 1. Units of pounds per square foot shown as psf and pounds per cubic foot shown as pcf. 2. Allowable pullout values include a factor of safety of 2.0.
Groundwater Considerations The slope improvements were designed with a higher groundwater level than typically measured in the boring logs to account for seasonal variability. The design groundwater elevations are shown graphically on the slope stability results included in Appendix A.
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Seismic Loading Pseudostatic stability analyses for seismic loading conditions were performed for the site using the same slope profile as for the static analysis. Based on United States Geologic Survey (USGS) probabilistic seismic hazard maps, the peak horizontal ground acceleration (PGA) expected at the site from an earthquake with a 10 percent probability of exceedance in 50 years is approximately 0.38g. The return interval for this earthquake is 475 years. A pseudostatic horizontal acceleration coefficient of 0.19g was used in the seismic analyses, corresponding to 50 percent of the expected PGA at the ground surface. A pseudostatic slope stability analysis adds a constant horizontal force to each failure block that is analyzed. This force is equal to the weight of the failure block multiplied by the pseudostatic acceleration coefficient.
The justification for using 50 percent of the PGA is that during an earthquake, the majority of the acceleration time history is typically only a fraction of the PGA. Typically, only one or two cycles of an earthquake motion produce ground accelerations at or near the PGA. A pseudostatic acceleration coefficient of 50 percent of the PGA is the current standard of practice in the field of geotechnical engineering for pseudostatic slope stability analyses. Typical factor of safety values used in the pseudostatic analyses are 1.0 to 1.1 (Turner and Schuster, 1996). We understand that the City of Seattle will require a target seismic factor of safety of 1.1 for design of the slope stabilization project.
Surcharge Loading The slope stabilization improvements have been designed for live load surcharges and the building surcharges from the planned 221 10th Avenue building. A uniform live load surcharge of 250 pounds per square foot (psf) was included in the design to account for potential traffic (pedestrian or maintenance vehicles) along the top of the slope. Each surcharge condition was included in both static and seismic conditions. The building surcharge included a uniform 6,000 psf surcharge load (North Wing of building) and 3,000 psf surcharge load (East Wing of building). The design sections were selected to model the nearest proximity of the building foundation to the crest of the slope. Design Section C included a 6,000 psf surcharge located approximately 36 feet from the crest of the slope. Design Section E included a 3,000 psf surcharge located approximately 35 feet from the crest of the slope. GeoEngineers should be contacted if additional surcharge loads are planned or if the location of the 221 10th Avenue building footprint changes significantly. Design Safety Factors The minimum design safety factors were selected to satisfy the City of Seattle minimum requirements and as recommended in the 2003 Federal Highway Administration (FHWA) Geotechnical Engineering Circular No. 7 – Soil Nail Walls. The minimum design safety factors used for the slope improvements is provided in Table 4.
TABLE 4. MINIMUM DESIGN SAFETY FACTORS
Design Component Minimum Static Factor of Safety Minimum Seismic Factor of Safety Global Stability 1.5 1.1 Anchor Pullout Resistance 2.0 1.5 Nail Bar Tensile Strength 1.8 1.35 Wire Mesh Facing and 1.5 1.1 Connections
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Design Methodology and Stability Analysis
Soil Nails The soil nail wall design for the project was completed using the procedures and recommendations outlined in the 1996 FHWA Manual for Design and Construction Monitoring of Soil Nail Walls and 2003 FHWA Geotechnical Engineering Circular No. 7 – Soil Nail Walls. The soil nail wall design was completed using the design software Slope/W V8.13 developed by GeoStudio. Spencer’s method of limit equilibrium analysis was used in the analysis and the soil nail reinforcement was modeled as an anchor. With this method a factor of safety is calculated as the ratio of the shear resistance of the soils along a failure plane to the shear forces applied on the failure plane due to the weight of the failure block. When the ratio of these forces (the factor of safety) is greater than 1, the slope is stable as modeled. For our analyses we assumed the critical failure surfaces to be the estimated failure plane from previous historic landslides below the site. Shear resistance of the anchors was neglected in the analysis. Slope/W also includes options for modeling surcharges, water levels, slopes above and below the reinforcement, and varying reinforcement parameters.
The nail length and spacing of the soil nail shoring system were varied to provide an overall factor of safety for soil strength of 1.5 (static) and 1.1 (seismic). Design calculations are presented in Appendix A.
The required nail lengths for the project range from 10 to 35 feet with nails at approximately 8-foot on-center spacing. The required thread bar size for the project consists of #9 Grade 75 bars. A row of vertical nails will be installed at the crest of the slope to support the wire mesh facing. The bars used for reinforcement will be epoxy coated for corrosion protection.
Wire Mesh Facing The facing design calculations are presented in Appendix A. The wire mesh facing for the slope consists of Geobrugg Tecco® System3 using the G65/3 mesh. The mesh selection and connection components were designed using the Ruvolum® software developed by Geobrugg. The wire mesh facing will be connected to the soil nails using P3 spike plates. The mesh facing and connection components will be galvanized for corrosion protection.
Slope Planting An erosion control blanket will be installed on the slope prior to constructing the wire mesh facing. The slope will be vegetated per Section 9-14.2 of the Seattle Standard Specifications using Seed Mix #1.
Slope Monitoring A slope monitoring program will be completed as described on Sheet G2.40. GeoEngineers will review the monitoring data as it is collected and submitted by the contractor.
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Design Drawings and Specifications The design plans for the project are attached to this document. The plans consist of 5 drawing sheets, as listed below. G2.00 General Notes and Sheet Index G2.10 Site Plan G2.20 Cross Sections G2.30 Details G2.40 Specifications
GEOTECHNICAL SPECIAL INSPECTION
GeoEngineers will complete geotechnical special inspection during construction, as required by the DPD. Special inspection will include observation of initial clearing and grubbing, soil nail installation and testing, facing installation, and temporary and permanent erosion control. The DPD may require additional geotechnical special inspections based on their review.
LIMITATIONS
We have prepared this report for the exclusive use of SMR Architects, SHA, and other members of the project team. The data and report should be provided to prospective contractors for their bidding or estimating purposes, but our report and interpretations should not be construed as a warranty of the subsurface conditions. This report is submitted as an addendum to our geotechnical report for the project titled “Geotechnical Engineering Services, 221 10th Avenue, Yesler Terrace Redevelopment, Seattle, Washington” and dated February 3, 2015, as is subject to the Limitations described therein.
The scope of our services does not include services related to construction safety precautions and our recommendations are not intended to direct the contractor’s methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design.
Variations in subsurface conditions are possible between the explorations and may also occur with time. A contingency for unanticipated conditions should be included in the budget and schedule. Sufficient monitoring, testing and consultation should be provided by GeoEngineers during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not slope stabilization installation activities comply with contract plans and specifications.
Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in the field of geotechnical engineering in this area at the time this report was prepared. No warranty or other conditions, express or implied, should be understood.
Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official document of record.
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APPENDIX A Calculations
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Upper 2 rows are 25' long Lower 2 rows are 30' long 220 Gravel Road 200 Name: Interbedded Glacial Soils Unit Weight: 125 pcf 180 Cohesion': 0 psf Phi': 34 ° Name: Slide Plane Unit Weight: 120 pcf Elevation Cohesion': 0 psf 160 Phi': 16 ° Name: Slide Debris Unit Weight: 120 pcf Cohesion': 75 psf Name: Glacially Consolidated Fine Grained Soils Phi': 18 ° Unit Weight: 120 pcf 140 Cohesion': 200 psf Phi': 30 ° 120 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Distance Page 25 6KDGHG$UHDV+DYH)6 *UDYHO5RDG 1DPH ,QWHUEHGGHG*ODFLDO6RLOV 8QLW:HLJKW SFI &RKHVLRQ SVI 3KL (OHYDWLRQ 1DPH 6OLGH'HEULV 1DPH *ODFLDOO\&RQVROLGDWHG)LQH*UDLQHG6RLOV 8QLW:HLJKW SFI 8QLW:HLJKW SFI &RKHVLRQ SVI &RKHVLRQ SVI 3KL 3KL 3LH]RPHWULF/LQH 'LVWDQFH Page 26 6KDGHG$UHDV+DYH)6 *UDYHO5RDG 1DPH ,QWHUEHGGHG*ODFLDO6RLOV 8QLW:HLJKW SFI &RKHVLRQ SVI 3KL (OHYDWLRQ 1DPH 6OLGH'HEULV 8QLW:HLJKW SFI 1DPH *ODFLDOO\&RQVROLGDWHG)LQH*UDLQHG6RLOV &RKHVLRQ SVI 8QLW:HLJKW SFI 3KL &RKHVLRQ SVI 3KL 'LVWDQFH Page 2 Yesler Terrace Slope Stabilization Seattle, WA 0241-029-02 Name: Fill Cross Section E-E' Unit Weight: 125 pcf kh = 0 g Cohesion': 0 psf Phi': 34 ° Edge of proposed 221 10th Ave Building 240 2.297 3000 psf Surcharge 250 psf Surcharge No. 9 Bars at 8' O.C. 230 Upper 2 rows are 25' long Lower 2 rows are 30' long 220 210 200 Gravel Road Name: Interbedded Glacial Soils 190 Unit Weight: 125 pcf Cohesion': 0 psf Phi': 34 ° 180 170 Elevation 160 Name: Slide Debris 150 Unit Weight: 120 pcf Name: Glacially Consolidated Fine Grained Soils Cohesion': 75 psf 140 Unit Weight: 120 pcf Phi': 18 ° Cohesion': 200 psf 130 Phi': 30 ° 120 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 Distance Page 2 Yesler Terrace Slope Stabilization Seattle, WA 0241-029-02 Name: Fill Cross Section E-E' Unit Weight: 125 pcf kh = 0.19 g Cohesion': 0 psf Phi': 34 ° Edge of proposed 221 10th Ave Building 240 1.310 3000 psf Surcharge 250 psf Surcharge No. 9 Bars at 8' O.C. 230 Upper 2 rows are 25' long Lower 2 rows are 30' long 220 210 200 Gravel Road Name: Interbedded Glacial Soils 190 Unit Weight: 125 pcf Cohesion': 0 psf Phi': 34 ° 180 170 Elevation 160 Name: Slide Debris 150 Unit Weight: 120 pcf Name: Glacially Consolidated Fine Grained Soils Cohesion': 75 psf 140 Unit Weight: 120 pcf Phi': 18 ° Cohesion': 200 psf 130 Phi': 30 ° 120 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 Distance Have we delivered World Class Client Service? 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