June 23, 2020

MK West, Inc 2701 – 13th Street Place Southwest Puyallup, Washington 98371 (253) 222-0255

Attn: Mr. Michael Kerschner [email protected]

Geologic Hazard Assessment Addendum Proposed Residential Short Plat 1014 – 23rd Avenue Southwest Puyallup, Washington PN: 7065000020 Doc ID: MKWest.23rdAveSW.GHAa

This addendum to our Geologic Hazard Assessment addresses the hazard areas at the site and the slope stability of the proposed permanent slopes associated with the proposed development. We understand the City of Puyallup is requiring an update to our Geologic Hazard Assessment to address the presence of an erosion hazard area at the site. Additionally, we understand that permanent cut slopes are now proposed at the site, which will need to meet the performance standards per Puyallup Municipal Code (PMC) 21.06.1230. The proposed site and the locations of our previous subsurface explorations are shown on the Site & Exploration Plan, Figure 1. We previously prepared a Report dated April 24, 2019 and a Geologic Hazard Assessment dated November 14, 2019 for the proposed residential short plat at the site. Our soils report included the subsurface explorations and descriptive logs of 4 test pits excavated across the site. We also reviewed the Geotechnical Study Report Update by Pacific Geo Engineering dated December 8, 2016, which included 4 previous hand auger explorations at the site. We did not perform any additional subsurface explorations or laboratory testing a part of our original Geologic Hazard Assessment or this addendum. This addendum address comments from the Landau Associates, Inc, as the City of Puyallup third-party reviewer dated February 7, 2020. Landau’s only comment was that our report indicated that the site soils as mapped meet the criteria of an erosion hazard per title 21.06.1230 of the Puyallup Municipal Code.

ENGINEERING CONCLUSIONS AND RECOMMENDATIONS Based on our site observations and data review, subsurface explorations and our engineering analysis, it is our opinion the development of the site is feasible from a geotechnical standpoint, provided the recommendations included herein are incorporated into the project plans.

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Erosion Hazards per PMC 21.06.1210(3)(a) The PMC defines erosion hazard areas as those areas identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) or identified by a special study as having a “moderate to severe,” “severe,” or “very severe” erosion potential. The majority of the site is mapped by the NRCS soils map as underlain by Kitsap (20C and 20D). Where the Kitsap silt loam is on slopes of 15 to 30 percent (20D type), it is designated as having a “moderate to severe” erosion hazard when exposed. Based on the above, the portion of the site sloping at steeper than 15 percent does meet the definition of an erosion hazard area per the PMC.

Alteration of Erosion Hazard Area The proposed development includes regrading portions of the site that meet the definition of an erosion hazard area. Title 21.06.1230, lists the various criteria that needs to be satisfied in order to allow alteration of the portion of the site that are underlain by the Kitsap silt loam (20D) soils. As stated in our previous reports, the site does not have slopes steeper than 40 percent and the result of our demonstrates that proposed development will not have factors of safety below 1.5 and 1.2 for static and dynamic conditions, respectively. Additionally, the proposed alterations will not increase the geologic hazards to neighboring properties or increase the required buffers/setbacks. Between this report, our previous reports, and the civil drawings, short plat documents, and codes, covenants, and restrictions should satisfy the other remaining criteria for alteration of an erosion hazard area. We anticipate regrading is required, in part, to construct a common access drive with grades of 10 percent or less to meet emergency vehicle access requirements. The proposed site grades will still meet the existing erosion hazard area designation, which should be mitigated through permanent measures. As discussed below, the proposed site grading meets the required slope factors of safety per the PMC.

Slope Stability Analysis We analyzed the stability of the existing and proposed slope geometries using subsurface profiles A-A’ and B-B’, as shown on Figure 1. These subsurface profiles were selected as the most critical given the configuration of the proposed grading and development. The slope stability results for the existing and proposed configurations are included as Appendix A. Seismic loading of 0.254g was added to the stability models based on ½ of the mapped site modified peak ground acceleration with a 2% probability of exceedance within a 50-year period, retrieved from the ATC Hazards by Location website. We used the computer program Slide2, from RocScience, 2020, to perform the slope stability analyses. The computer program Slide2 uses a number of methods to estimate the factor of safety (FS) of the stability of a slope by analyzing the shear and normal forces acting on a series of vertical “slices” that comprise a failure surface. Each vertical slice is treated as a rigid body; therefore, the forces and/or moments acting on each slice are assumed to satisfy static equilibrium (i.e., a limit equilibrium analysis). The FS is defined as the ratio of the forces available to resist movement to the forces of the driving mass. A FS of 1.0 means that the driving and resisting forces are equal; an FS less than 1.0 indicates that the driving forces are greater than the resisting forces (indicating failure). We used the Generalized Limit Equilibrium method using the Morgenstern-Price analysis, which

MKWest.23rdAveSW.GHAa June 23, 2020 page | 3 satisfies both moment and force equilibrium, to search for the location of the most critical failure surfaces and their corresponding FS. The most critical surfaces are those with the lowest FS for a given loading condition, and are therefore the most likely to move. Soils at the site were modeled using Mohr-Coulomb strength properties. Table 1, below, summarizes the estimated soil parameters used in our stability analyses. In our opinion, based on the subsurface conditions encountered in our explorations, the assumed values below are appropriate.

TABLE 1: Mohr-Coulomb Strength Parameters

Unit Weight Angle Soil Type (pcf) (degrees) (psf)

Lacustrine silt (ML) 115 28 100 Weathered recessional outwash (SP-SM) 120 34 25

Based on our analyses, the current and proposed site configurations are stable and have the factors of safety shown in Table 2, below. The factors of safety meet the requirements of PMC 21.06.1230, of 1.5 for static conditions and 1.2 for dynamic conditions.

TABLE 2: Global Stability Analyses Results Cross Section Condition Loading Condition Factor of Safety Static 6.1 Existing Dynamic 1.9 A-A’ Static 2.0 Proposed Dynamic 1.2 Static 3.2 Existing Dynamic 1.4 B-B’ Static 2.1 Proposed Dynamic 1.2

Recommended Buffers and Setbacks Buffers and setbacks are typically used to protect critical areas from disturbance and also to protect the proposed development from damage due to the potential hazard. The following discussions regarding critical area buffers and structure setbacks are based on PMC 21.06 and International Building Code (IBC) 1808.7, respectively.

Vegetated Buffers Buffers typically consist of an undisturbed area of native vegetation, retained, or established, that extend from the edge of the critical area or hazard. The width of the buffer should be based on the potential hazard and associated risks. Buffer widths are generally measured from the edge of

MKWest.23rdAveSW.GHAa June 23, 2020 page | 4 the critical area being protected, in this case toe of slope. Per PMC 21.06.1240, the required or erosion hazard areas buffer width for slopes with a vertical elevation of more than 10 but less than 25 feet, shall be equal to the height of the slope divided by two. The proposed 2H:1V cut slopes at the site have heights of between about 10 to 16 feet. Therefore, the required vegetated buffer from the toe of slopes at the site will be between 5 and 8 feet, depending on the height of the adjacent cut slope.

Building Setbacks We recommend that building setbacks from slopes steeper than 33 percent be established in accordance with the 2015 IBC. The 2015 IBC Section 1808.7 requires a building setback from slopes that are steeper than 3H:1V (Horizontal: Vertical) unless evaluated and reduced and/or a structural setback is provided by a licensed geotechnical engineer. The setback distance is calculated based on the vertical height of the slope. The typical IBC setback from the toe of the slope equals one half the height of the slope, with a maximum setback of 15 feet from the toe of the slope. The IBC setback requirement is equal to the above requirement for vegetated buffer. Therefore, structures located outside the 5 to 8 feet buffer will also meet the required IBC setback. If these setbacks are not feasible, the IBC does allow the use of a “structural setback” where the foundations are deepened, or the daylight basement/stem wall are extended such that the setback distance is determined by measuring from the lowest/highest point of the . Where there is insufficient space the top of the daylight basement/crawl space wall or stem wall should be elevated such that the level backfill extending from the wall to the slope meets the required toe of slope

Erosion Control Weathering, erosion and the resulting surficial sloughing and shallow land sliding are natural processes. To manage and reduce the potential for these natural processes, we recommend erosion hazards be mitigated by applying Best Management Practices (BMPs), as outlined in the 2012 (with 2014 amendments) Stormwater Management Manual for Western Washington (SWMMWW). Temporary erosion control BMPs should be installed at the site prior to the beginning of clearing, grading, or other construction activities, and should be updated and maintained throughout construction until final site stabilization is established. Temporary erosion control BMPs may include, but are not limited to:

 Silt fencing and appropriate soil stockpiling techniques to prevent silty stormwater from leaving the site,  Jute matting, hydroseeding, or plastic covering to protect exposed soils,  Straw wattles, quarry spall armoring, check dams, or other energy attenuation BMPs to slow the flow of stormwater over slopes and within drainage channels, and,  Swales and berms to convey construction stormwater away from the slope.

We strongly recommend earthwork activities be completed during the dry season (May 1 through September 30), and that any exposed soil areas be stabilized during the wet season (October 1 through April 30). Where native vegetation is removed because of clearing and grading

MKWest.23rdAveSW.GHAa June 23, 2020 page | 5 activities, a dense vegetative groundcover, grass lawn, or native vegetation should be reestablished as soon as feasible. Permanent erosion control, such as mulched landscaping areas, groundcovers, hardscaping, or grass lawns, should be established as soon as feasible once final grades have been completed. All permanent erosion control methods should be maintained after construction activities have been completed. Because of the designation as an erosion hazard area, additional permanent erosion control products may be considered along the proposed 2H:1V cut slopes to provide greater mitigation than vegetation alone. Below is a discussion of several options.

Rolled Erosion Control Products (RECP) Rolled erosion control products typically consist of natural or geosynthetic material that is rolled out over the area to be protected. These erosion control products require a generally smooth, uniformly sloping surface for installation and are often anchored in place along the top of the slope and at regular intervals on the face of the slope. Most RECPs can provide adequate temporary erosion control when used as a standalone product. To function as permanent erosion control, natural fiber RECPs should be combined with seeding, mulching, or planting.

Hydraulic Erosion Control Products (HECP) Hydraulic erosion are spray-on products that can include various mixtures of mulches, bonded fiber matrix, and flexible growth medium along with seed mixtures. Common hydro- seeding type products perform in many applications as temporary erosion control. For steeper slopes, a mixture of seed and bonded fiber matrix or mulch is often required for additional erosion protection.

Cellular Confinement System A more robust method of permanent erosion control would be to install a cellular confinement system, such as GeoWeb. Cellular confinement system are geosynthetic cells, anchored to the slope which are then filled with a mixture and seeded. The system is tied to deadman anchors installed at the top of the slope and is held in place through an anchor and tendon system along the face of the slope. These systems require specialty design and installation.

There are a wide range of permanent erosion control solutions among these three categories. A specialty supplier should determine a specific product or combination of products based on the above recommendations. We can provide contacts for specialty suppliers in the area and additional consultation regarding a permanent erosion control solution for the proposed permanent slopes, as required.

LIMITATIONS We have prepared this report for use by MK West, Inc. and members of the design team. The data used in preparing this report and this report should be provided to prospective contractors for their bidding or estimating purposes only. Our report, conclusions and interpretations are based on data from others and limited site reconnaissance and should not be construed as a warranty of the subsurface conditions.

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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 our firm 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 earthwork and foundation installation activities comply with contract plans and specifications. The scope of our services does not include services related to environmental remediation and construction safety precautions. 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. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared. No other conditions, express or implied, should be understood.

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MKWest.23rdAveSW.GHAa June 23, 2020 page | 7

We trust this addendum is sufficient to meet your needs at this time. If you have any questions or comments, please do not hesitate to call at your earliest convenience.

Respectfully submitted, GeoResources, LLC

Neil Ferguson, PE Project Engineer

Keith S. Schembs, LEG Eric W. Heller, PE Principal Senior Project Engineer

NAF:KSS:naf Doc ID: MKWest.23rdAveSW.GHAa Attachments: Figure 1: Site & Exploration Plan Appendix A – Slope Stability Results Appendix B – Previous Subsurface Explorations

B’ B TP-1

TP-3

TP-2

A’

TP-4

A

Grading & Utility Plan prepared by C.E.S. NW Inc. Dated May 8, 2020. Site & Exploration Plan

Proposed Residential Short Plat Approximate Test Pit Location Global Stability Cross-section rd 1014 – 23 Avenue SW Puyallup, Washington Scale PN: 7065000020 1” = 60’ DocID: MKWest.23rdAveSW.F June 2020 Figure 1

Appendix A Slope Stability Results

400

Unit Material Strength Cohesion Phi Water Color Weight Ru Name Type (psf) (deg) Surface (lbs/ft3) Lacustrine Mohr- 115 100 28 None 0 silt (ML) Coulomb 350

6.1 300 250 200

-50 0 50 100 150 200 250 300 Project SLIDE - An Interactive Slope Stability Program

Group A-A' Existing Scenario Master Scenario Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd 0.254 400

Unit Material Strength Cohesion Phi Water Color Weight Ru Name Type (psf) (deg) Surface (lbs/ft3) 1.9 Lacustrine Mohr- 115 100 28 None 0 silt (ML) Coulomb 350 300 250 200

-50 0 50 100 150 200 250 300 Project SLIDE - An Interactive Slope Stability Program

Group A-A' Existing Scenario Seismic Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd 400

Material Unit Weight Strength Cohesion Phi Water Color Ru Name (lbs/ft3) Type (psf) (deg) Surface Lacustrine Mohr- 115 100 28 None 0 silt (ML) Coulomb 350

300 2.0 250 200

-50 0 50 100 150 200 250 300 Project SLIDE - An Interactive Slope Stability Program

Group A-A' Proposed Scenario Master Scenario Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd 0.254 400

Material Unit Weight Strength Cohesion Phi Water Color Ru Name (lbs/ft3) Type (psf) (deg) Surface Lacustrine Mohr- 115 100 28 None 0 silt (ML) Coulomb 350

300 1.2 250 200

-50 0 50 100 150 200 250 300 350 Project SLIDE - An Interactive Slope Stability Program

Group A-A' Proposed Scenario Seismic Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd 500 Unit Weight Strength Cohesion Phi Water Material Name Color Ru (lbs/ft3) Type (psf) (deg) Surface Mohr- Lacustrine silt (ML) 115 100 28 None 0 Coulomb Weathered Mohr- 120 25 34 None 0 recessional (SP-SM) Coulomb 400

3.2 300 200

0 100 200 300 400 500 600 Project SLIDE - An Interactive Slope Stability Program

Group B-B' Existing Scenario Master Scenario Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd 0.254

500 Unit Weight (lbs/ Strength Cohesion Phi Water Material Name Color Ru ft3) Type (psf) (deg) Surface Mohr- Lacustrine silt (ML) 115 100 28 None 0 Coulomb Weathered recessional Mohr- 120 25 34 None 0 (SP-SM) Coulomb

1.4 400 300 200

0 100 200 300 400 500 Project SLIDE - An Interactive Slope Stability Program

Group B-B' Existing Scenario Seismic Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd 500

Unit Weight Strength Cohesion Phi Water Material Name Color Ru (lbs/ft3) Type (psf) (deg) Surface Mohr- Lacustrine silt (ML) 115 100 28 None 0 Coulomb Weathered Mohr- 120 25 34 None 0

400 recessional (SP-SM) Coulomb

300 2.1 200

-100 0 100 200 300 400 500 Project SLIDE - An Interactive Slope Stability Program

Group B-B' Proposed Scenario Master Scenario Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd Safety Factor 0.0 0.3 0.254 0.5

500 0.8 1.0 1.3 Unit Weight (lbs/ Strength Cohesion Phi Water 1.5 Material Name Color Ru 1.8 ft3) Type (psf) (deg) Surface 2.0 Mohr- Lacustrine silt (ML) 115 100 28 None 0 2.3 Coulomb 2.5 Weathered recessional Mohr- 2.8 120 25 34 None 0 (SP-SM) Coulomb 3.0 3.3 400 3.5 3.8 4.0 4.3 4.5 4.8 5.0 5.3 5.5 5.8 300 6.0+ 1.2 200

-100 0 100 200 300 400 500 Project SLIDE - An Interactive Slope Stability Program

Group B-B' Proposed Scenario Seismic Drawn By Company

Date File Name SLIDEINTERPRET 9.005 6/18/2020, 4:17:00 PM MKWest.23rdAve.slmd

Appendix A Previous Subsurface Explorations

SOIL CLASSIFICATION SYSTEM

MAJOR DIVISIONS GROUP GROUP NAME SYMBOL

GRAVEL CLEAN GW WELL-GRADED , FINE TO COARSE GRAVEL GRAVEL GP POORLY-GRADED GRAVEL COARSE GRAINED More than 50% GRAVEL GM SILTY GRAVEL SOILS Of Coarse Fraction WITH FINES Retained on GC CLAYEY GRAVEL No. 4 Sieve

SAND CLEAN SW WELL-GRADED SAND, FINE TO COARSE SAND

More than 50% SP POORLY-GRADED SAND Retained on No. 200 Sieve More than 50% SAND SM SILTY SAND Of Coarse Fraction WITH FINES Passes SC CLAYEY SAND No. 4 Sieve

SILT AND INORGANIC ML SILT

FINE CL CLAY GRAINED SOILS Liquid Limit ORGANIC OL ORGANIC SILT, ORGANIC CLAY Less than 50

SILT AND CLAY INORGANIC MH SILT OF HIGH PLASTICITY, ELASTIC SILT

More than 50% CH CLAY OF HIGH PLASTICITY, FAT CLAY Passes No. 200 Sieve Liquid Limit ORGANIC OH ORGANIC CLAY, ORGANIC SILT 50 or more

HIGHLY ORGANIC SOILS PT

NOTES: SOIL MOISTURE MODIFIERS:

1. Field classification is based on visual examination of soil Dry- Absence of moisture, dry to the touch in general accordance with ASTM D2488-90. Moist- Damp, but no visible water 2. using laboratory tests is based on ASTM D2487-90. Wet- Visible free water or saturated, usually soil is obtained from below water table 3. Description of soil density or consistency are based on interpretation of blow count data, visual appearance of soils, and or test data.

Unified Soils Classification System Proposed Residential Short Plat 1014 – 23rd Avenue SW Puyallup, Washington PN: 7065000020

DocID: MKWest.23rdAveSW.F April 2019 Figure A-1

Test Pit TP-1 Location: North central portion of site, proposed Lot 3 Approximate Elevation: 226’

Depth (ft) Soil Type Soil Description 0 - 1 - Dark brown sandy SILT (loose, moist) (topsoil/rootzone) 1 - 8 ML Tan to gray SILT with fine sand (medium stiff to stiff, moist to wet) (recessional lacustrine deposits)

Terminated at 8 feet below ground surface. No caving observed at the time of excavation. Slow groundwater seepage observed at 2 feet below ground surface. Mottling observed at 2 feet below ground surface.

Test Pit TP-2 Location: Central portion of site, proposed Lot 1 Approximate Elevation: 262’

Depth (ft) Soil Type Soil Description 0 - 1 - Dark brown silty SAND (loose, moist) (topsoil/rootzone) 1 - 3½ SP-SM Brown SAND with silt (loose, moist) (weathered recessional lacustrine) 3½ - 8 ML Tan to gray SILT with fine sand (medium stiff to stiff, moist) (recessional lacustrine deposits)

Terminated at 8 feet below ground surface. Caving observed 1 to 3½ feet below ground surface at the time of excavation. Slow groundwater seepage observed at 3½ feet below ground surface. Mottling observed at 3 feet below ground surface.

Test Pit TP-3 Location: South central portion of site, proposed Lot 1 Approximate Elevation: 273’

Depth (ft) Soil Type Soil Description 0 - 1 - Dark brown silty SAND (loose, moist) (topsoil/rootzone) 1 - 5 SP-SM Brown SAND with silt (loose, moist) (weathered recessional lacustrine) 5 - 7 SP Gray stratified poorly-graded SAND with some gravel (medium dense, moist) 7 - 9 ML Tan to gray SILT with fine sand (medium stiff to stiff, moist) (recessional lacustrine deposits)

Terminated at 9 feet below ground surface. No caving observed at the time of excavation. No groundwater seepage observed. Mottling observed at 2 feet below ground surface.

Logged by: ND Excavated on: March 15, 2019 Test Pit Logs Proposed Residential Short Plat 1014 – 23rd Avenue SW Puyallup, Washington PN: 7065000020

DocID: MKWest.23rdAveSW.F April 2019 Figure A-2

Test Pit TP-4 Location: Northwest portion of site, proposed Lot 4 Approximate Elevation: 228’

Depth (ft) Soil Type Soil Description 0 - 1 - Dark brown sandy SILT (loose, moist) (topsoil/rootzone) 1 - 8 ML Tan to gray SILT with fine sand (medium stiff to stiff, moist) (recessional lacustrine deposits)

Terminated at 8 feet below ground surface. No caving observed at the time of excavation. Slow to moderate groundwater seepage observed from 0.5 to 1 feet below ground surface. Mottling observed at 1 foot below ground surface.

Logged by: ND Excavated on: March 15, 2019 Test Pit Logs Proposed Residential Short Plat 1014 – 23rd Avenue SW Puyallup, Washington PN: 7065000020

DocID: MKWest.23rdAveSW.F April 2019 Figure A-3