4. Clackamas Housing_Stormwater Management Report - COMPILED 2023-12-19.pdf

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Stormwater Management

Report

Clackamas County Emergency Housing

Clackamas County, OR

DCI Job Number 23032-0015

December 19, 2023

Table of Contents

Section I: Site Background Information ..................................................................... Page

1. Project Information

2. Stormwater Narrative....................................................................................... 2 - 3

Section II: Stormwater Design Information

1. Peak Detention Volumes

2. Rain Garden Section

3. Conveyance Calculation .................................................................................. 3 - 4

4. Exhibit: Site Drainage Area Map

5. Exhibit: Site Overflow Routes Map

Appendix

A. Soil Survey and Hydrologic Classification .................................................. A1 – A3

B. WES Stormwater Events ..................................................................................... B1

C. USDA SCS TR-55 SCS Curve Numbers ................................................................ C1

D. Rainfall I-D-R Curve Zone Map and Graph ............................................... D1 – D2

E. HydroCAD Routing Calculations ............................................................... E1 – E17

F. Geotechnical Report .................................................................................. F1 – F44

Section I: Site Background Information

1. Project Information

2. Stormwater Narrative....................................................................................... 2 - 3

Section I-1: Project Information

The Clackamas County Emergency Housing site is located in Clackamas County, Oregon and borders SE 115th Avenue to the west, Clackamas County Veterans Village across SE

115th Avenue, Precision Truss & Lumber to the north, and Pepsi Beverages south of the site. This report contains information for the private onsite stormwater quality and quantity control systems for the site.

The existing site conditions are primarily flat with gravel, dirt, and sparse vegetation with a few existing trees scattered. The east side of the site includes a mound, and the south half of the property features a hill sloped southerly. The site is proposing to construct thirteen (13) new buildings, eight (8) of which will be housing. The project proposes sidewalks, landscaping, a gravel common area, paved drop-off and parking areas, and stormwater facilities. The site is designed to treat and infiltrate stormwater runoff.

The proposed site is comprised of approximately 0.466 acres of impervious area. This is divided between the building roofs and decks, sidewalk areas, and paved areas.

Building stormwater runoff is proposed to be collected using roof drains and conveyed to the proposed stormwater rain gardens on the east and west ends of the site. The rain gardens will allow infiltration into the native soil and both have 16” of live storage.

The rain gardens for the site have been designed per WES Stormwater Standards. For detention and infiltration analysis, a HydroCAD model has been used.

Section I-2: Stormwater Narrative

The proposed site is designed to be emergency housing with additional administrative buildings. In addition to the proposed buildings, a vehicle drop off area and parking area are designed to be constructed on the northwest side of the site to allow access from SE 115th Ave. and for use by the County. The site has been designed for two drainage basins, one flowing westerly to the western rain garden, and the other flowing easterly to the eastern rain garden. The stormwater facilities used for this site are stormwater rain gardens and are designed to fully infiltrate the 10-year, 24-hour storm event for the area.

The stormwater facilities allow high rates of infiltration and do not typically allow standing water for extended periods of time. The facilities are designed according to

WES Standard Drawing SWM-04: 18” of growing medium for the top layer, 3” separation layer containing ¾” to ¼” open graded aggregate, 18” of 1 ½” to ¾” washed drain rock, followed by the existing uncompacted native soils.

The depth of the facility is designed to be 1.33’, minimum. Per the HydroCAD model used for this site, the maximum design height of a 10-year, 24-hour storm event for the new west facility, with all impervious and pervious areas contributing, is 0.78’ from bottom of rain garden. This means the west facility is designed to have a 0.55’ of freeboard above the 10-year, 24-hour storm water retention design surface. Similarly, the maximum design height of the same storm event for the new east facility is 0.80’ from bottom of rain garden. The east facility is designed to have 0.53’ of freeboard.

The rates used for infiltration was fifty inches per hour (50 in/hr), which is a conservative value per recommendations in the Geotechnical Report (see Appendix E).

Stormwater runoff is captured on site through various methods. For the buildings, the roof runoff is collected and conveyed through underground lines connected to roof downspouts. Paved areas are captured by having stormwater runoff sheet flow towards the west rain garden. Sidewalks and landscaped areas are graded to drain generally towards either the east or west rain garden.

As mentioned above, the new stormwater facilities have minimum 0.5’ of freeboard above the 10-year, 24-hour stormwater event stormwater level. If an overflow of the facilities were to occur, the runoff of the west rain garden would overflow towards the northwest into SE 115th Ave, and the runoff from the east garden would overflow towards the southeast down the hill (see Section II-5 for a map showing overflow routes). The buildings are designed to be at a higher elevation than the surrounding grades, so risk of flooding from the facilities is not present.

The stormwater pipes onsite have been designed to convey the 10-year peak storm event.

Section II: Onsite Stormwater Design Information

1. Peak Detention Volumes

2. Rain Garden Section

3. Conveyance Calculation .................................................................................. 3 - 4

4. Exhibit: Site Drainage Area Map

5. Exhibit: Site Overflow Routes Map

Section II-1

Peak Detention Volume Summary

For the West Rain Garden, the peak volume and elevation of the design storm events are listed below:

Annual Storm Event

(years)

Total Precipitation

Depth (in/24 hr)

Volume Required

(cf)

Peak Ponding Depth

(ft)

10 3.20 163 0.78

Bottom of West Rain Garden elevation = 118.13

Model reference elevation = 100.00

For the East Rain Garden, the peak volume and elevation of the design storm events are listed below:

Annual Storm Event

(years)

Total Precipitation

Depth (in/24 hr)

Volume Required

(cf)

Peak Ponding Depth

(ft)

10 3.20 152 0.80

Bottom of East Rain Garden elevation = Varies – 118.73 to 118.81

Model reference elevation = 100.00

Total precipitation depths are determined from Table 26 of the WES Stormwater

Standards. See Appendix B of these calculations for a copy of the referenced table.

Rain gardens have been designed with a minimum of 6” of freeboard above the 10-year, 24-hour storm event peak.

Appendix C: Stormwater Typical Drawings and Standard Details WES Stormwater Standards

April 2023 Page 198 of 269

SECTION II-2

Section II-3

Conveyance Calculations

Conveyance calculations for typical pipes on site are below, sized using the manning’s equation.

Mannings equation:

1.486

Where:

Q = Flow capacity of the pipe n = Mannings coefficient

A = Cross sectional area of a pipe (sf)

R = Hydraulic Radius of a pipe

S = Slope of the pipe (ft/ft)

Typical Pipes on Site:

1) 6” line @ 1.00% slope n = 0.013 for a PVC pipe

A = 0.196 sf for 6” pipe

R = 0.125 for a full 6” pipe

S = 0.0100 ft/ft

1.486

0.013 ∗ 0.196 �� ∗ 0.125

� ∗ 0.0100

� (6" @ 1.00%) = 0.56 ���

Determining the Maximum Design Flow Rate

To determine the maximum expected flow rate, the rational method was used for a 10-year storm event.

Rational Method:

Where:

Q = Flow capacity of the pipe

C = Area coefficient i = Rainfall intensity of area

A = Land area contributing to flow (Acres)

C = 0.90 for impervious area i = See below.

A = Maximum contributing area = Approx. 3,500 sf of roof area = 0.080 acres

Intensity

= 0.64 in/hr (Refer to Appendix D)

Maximum Design Flow Rate

� = 0.90 ∗ 0.64 ℎ" ∗ 0.080 #�"$� = 0.046 ���

Conclusion

Based on the calculations above, we can determine that the pipes on site will have sufficient capacity to handle the stormwater events in the Clackamas County area. The calculations shown above are for a 10-year storm event.

SS

SS

Appendix

A. Soil Survey and Hydrologic Classification .................................................. A1 – A3

B. WES Stormwater Events ..................................................................................... B1

C. USDA SCS TR-55 SCS Curve Numbers ................................................................ C1

D. Rainfall I-D-R Curve Zone Map and Graph ............................................... D1 – D2

E. HydroCAD Routing Calculations ............................................................... E1 – E17

F. Geotechnical Report .................................................................................. F1 – F44

Soil Map—Clackamas County Area, Oregon

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

12/4/2023

535600 535620 535640 535660 535680 535700 535720 535740

535600 535620 535640 535660 535680 535700 535720 535740 535760

45° 24' 6'' N

2°

2'

2' ' W

45° 24' 6'' N

2°

2'

5'

' W

45° 23' 59'' N

2°

2'

2'

' W

45° 23' 59'' N

2°

2'

5'

' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 50 100 200 300

Feet 0 15 30 60 90

Meters Map Scale: 1:1,080 if printed on A portrait (8.5" x 11") sheet.

Soil Map may not be valid at this scale.

APPENDIX A1

MAP LEGEND MAP INFORMATION

Area of Interest (AOI) Area of Interest (AOI)

Soils Soil Map Unit Polygons

Soil Map Unit Lines

Soil Map Unit Points

Special Point Features Blowout

Borrow Pit

Clay Spot

Closed Depression

Gravel Pit

Gravelly Spot

Landfill

Lava Flow

Marsh or swamp

Mine or Quarry

Miscellaneous Water

Perennial Water

Rock Outcrop

Saline Spot

Sandy Spot

Severely Eroded Spot

Sinkhole

Slide or Slip

Sodic Spot

Spoil Area

Stony Spot

Very Stony Spot

Wet Spot

Other

Special Line Features

Water Features Streams and Canals

Transportation Rails

Interstate Highways

US Routes

Major Roads

Local Roads

Background Aerial Photography

The soil surveys that comprise your AOI were mapped at 1:20,000.

Warning: Soil Map may not be valid at this scale.

Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale.

Please rely on the bar scale on each map sheet for map measurements.

Source of Map: Natural Resources Conservation Service Web Soil Survey URL:

Coordinate System: Web Mercator (EPSG:3857)

Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required.

This product is generated from the USDA-NRCS certified data as of the version date(s) listed below.

Soil Survey Area: Clackamas County Area, Oregon Survey Area Data: Version 20, Sep 7, 2023

Soil map units are labeled (as space allows) for map scales 1:50,000 or larger.

Date(s) aerial images were photographed: Sep 26, 2022—Oct 11, 2022

The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident.

Soil Map—Clackamas County Area, Oregon

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

12/4/2023

APPENDIX A2

Map Unit Legend

Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI

18 Clackamas gravelly loam 2.3 47.3%

67 Newberg fine sandy loam 0.1 2.5%

76B Salem silt loam, 0 to 7 percent slopes

1.3 25.4%

77B Salem gravelly silt loam, 0 to 7 percent slopes

0.3 5.9%

92F Xerochrepts and Haploxerolls, very steep

0.9 18.8%

Totals for Area of Interest 5.0 100.0%

Soil Map—Clackamas County Area, Oregon

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

12/4/2023

APPENDIX A3

WES Stormwater Standards Appendix D: Facility Sizing Methodology and Resources

April 2023 Page 251 of 269

The curve numbers presented in Table 28 are for wet antecedent moisture conditions.

Wet conditions assume previous rainstorms have reduced the capacity of soil to absorb water. Given the frequency of storms in Clackamas County, wet conditions are most likely, and result in conservative hydrographic values.

Design Storm

The SBUH method also requires a design storm to perform the runoff calculations. For flow control calculations, the District uses an NRCS Type 1A 24-hour storm distribution.

The rainfall depths for 2-year through 100-year storm events are shown in Table 26.

Table 26. WES Design Storms

Design Storm/Recurrence Interval (years)

24-Hour Rainfall Depth (inches)

Water Quality 1.0

2-year 2.4

5-year 2.85

10-year 3.2

25-year 4.0

50-year 4.13

100-year 4.8

APPENDIX B1

April 2023 Page 255 of 269

Table 28. Runoff Curve Numbers2

Description

Curve Numbers for Hydrological Soil Groups

A B C D

Open space (lawns, parks, golf courses, cemeteries)

Poor condition (< 50% grass coverage) 68 79 86 89

Fair condition (50 to 75% grass coverage) 49 69 79 84

Good condition (>75% grass coverage) 39 61 74 80

Impervious Areas

Paved areas (parking lots, roofs, driveways) 98 98 98 98

Streets and roads

Paved with curbs 98 98 98 98

Paved with open ditches 83 89 92 93

Gravel 76 85 89 91

Dirt 72 82 87 89

Urban Districts

Commercial and business (85% impervious) 89 92 94 95

Industrial (72% impervious) 81 88 91 93

Residential districts by average lot size

1/8 acre or less (65% impervious) 77 85 90 92

1/4 acre (38% impervious) 61 75 83 87

1/3 acre (30% impervious) 57 72 81 86

1/2 acre (25% impervious) 54 70 80 85

Woods (Good Hydrologic Condition) 70*

* CN for Predeveloped Forest Condition is assumed to be equivalent to Woods condition with Hydrologic Soil Group C.

2 Urban Hydrology for Small Watersheds (TR-55), USDA Soil Conservation Service Engineering Division (1986).

APPENDIX C1

Appendix D: Facility Sizing Methodology and Resources WES Stormwater Standards

April 2023 Page 262 of 269

Figure 17. Rainfall Intensity Recurrence Curves (Zone 8)

Max. time = 60 minutes per WES Stormwater Standards Appenix D.3

- Rational Method

Intensity = 0.64"

APPENDIX D1

April 2023 Page 259 of 269

Figure 14. Rainfall I-D-R Curve Zone Map

Project site appears to lie within Zone 8

APPENDIX D2

1S

Predeveloped - WEST

BASIN

2S

Predeveloped - EAST

BASIN

3P

West Facility

3S

Developed - WEST

BASIN

4P

East Facility

4S

Developed - EAST

BASIN

Routing Diagram for 23032-0015_Storm Model_2023-12-04 Prepared by {enter your company name here}, Printed 12/19/2023 HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Subcat Reach Pond Link

APPENDIX E

23032-0015_Storm Model_2023-12-04 Printed 12/19/2023Prepared by {enter your company name here}

Page 2HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Area Listing (all nodes)

Area

(acres)

CN Description

(subcatchment-numbers)

0.922 79 <50% Grass cover, Poor, HSG B (3S, 4S)

0.318 89 <50% Grass cover, Poor, HSG D (4S)

0.025 85 Gravel roads, HSG B (3S)

0.018 98 Paved parking, HSG B (PVMT) (1S)

0.079 98 Roofs, HSG B (Back Bldgs, Pavilion) (4S)

0.026 98 Roofs, HSG B (Front 3 Bldgs) (3S)

0.006 98 Unconnected pavement, HSG B (CONC) (1S)

0.022 98 Unconnected pavement, HSG B (Conc swlk, stairs, ramps) (4S)

0.063 98 Unconnected pavement, HSG B (Conc swlks, ramps, stairs) (3S)

0.059 98 Unconnected pavement, HSG B (Decks) (3S, 4S)

0.193 98 Unconnected pavement, HSG B (Pvmt) (3S)

1.364 66 Woods, Poor, HSG B (1S, 2S)

0.318 83 Woods, Poor, HSG D (2S)

3.413 78 TOTAL AREA

Page 3HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Soil Listing (all nodes)

Area

(acres)

Soil

Group

Subcatchment

Numbers

0.000 HSG A

2.776 HSG B 1S, 2S, 3S, 4S

0.000 HSG C

0.637 HSG D 2S, 4S

0.000 Other

3.413 TOTAL AREA

Page 4HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Ground Covers (all nodes)

HSG-A

(acres)

HSG-B

(acres)

HSG-C

(acres)

HSG-D

(acres)

Other

(acres)

Total

(acres)

Ground

Cover

Subcatchment

Numbers

0.000 0.922 0.000 0.318 0.000 1.240 <50% Grass cover, Poor 3S, 4S

0.000 0.025 0.000 0.000 0.000 0.025 Gravel roads 3S

0.000 0.018 0.000 0.000 0.000 0.018 Paved parking 1S

0.000 0.105 0.000 0.000 0.000 0.105 Roofs 3S, 4S

0.000 0.343 0.000 0.000 0.000 0.343 Unconnected pavement 1S, 3S, 4S

0.000 1.364 0.000 0.318 0.000 1.683 Woods, Poor 1S, 2S

0.000 2.776 0.000 0.637 0.000 3.413 TOTAL AREA

Type IA 24-hr 10-Year Rainfall=3.20"23032-0015_Storm Model_2023-12-04

Page 5HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Time span=0.00-72.00 hrs, dt=0.03 hrs, 2401 points Runoff by SBUH method, Weighted-CN

Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method

Runoff Area=37,144 sf 2.78% Impervious Runoff Depth=0.69"Subcatchment 1S: Predeveloped - WEST Tc=0.0 min CN=67 Runoff=0.08 cfs 0.049 af

Runoff Area=37,185 sf 0.00% Impervious Runoff Depth=0.93"Subcatchment 2S: Predeveloped - EAST Tc=0.0 min CN=72 Runoff=0.15 cfs 0.066 af

Peak Elev=100.78' Storage=163 cf Inflow=0.39 cfs 0.130 afPond 3P: West Facility Outflow=0.33 cfs 0.130 af

Runoff Area=37,144 sf 34.64% Impervious Runoff Depth=1.84"Subcatchment 3S: Developed - WEST Tc=0.0 min CN=86 Runoff=0.39 cfs 0.130 af

Peak Elev=100.80' Storage=152 cf Inflow=0.37 cfs 0.125 afPond 4P: East Facility Outflow=0.32 cfs 0.125 af

Runoff Area=37,185 sf 17.12% Impervious Runoff Depth=1.76"Subcatchment 4S: Developed - EAST Tc=0.0 min UI Adjusted CN=85 Runoff=0.37 cfs 0.125 af

Total Runoff Area = 3.413 ac Runoff Volume = 0.370 af Average Runoff Depth = 1.30" 86.37% Pervious = 2.948 ac 13.63% Impervious = 0.465 ac

Page 6HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Summary for Subcatchment 1S: Predeveloped - WEST BASIN

[46] Hint: Tc=0 (Instant runoff peak depends on dt)

Runoff = 0.08 cfs @ 7.97 hrs, Volume= 0.049 af, Depth= 0.69"

Runoff by SBUH method, Weighted-CN, Time Span= 0.00-72.00 hrs, dt= 0.03 hrs Type IA 24-hr 10-Year Rainfall=3.20"

Area (sf) CN Description

* 765 98 Paved parking, HSG B (PVMT)

* 268 98 Unconnected pavement, HSG B (CONC)

36,111 66 Woods, Poor, HSG B

37,144 67 Weighted Average 36,111 97.22% Pervious Area 1,033 2.78% Impervious Area

268 25.94% Unconnected

Subcatchment 1S: Predeveloped - WEST BASIN

Runoff

Hydrograph

Time (hours) 727068666462605856545250484644424038363432302826242220181614121086420

F lo w

(c fs

0.085

0.08

0.075

0.07

0.065

0.06

0.055

0.05

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

Type IA 24-hr

10-Year Rainfall=3.20"

Runoff Area=37,144 sf

Runoff Volume=0.049 af

Runoff Depth=0.69"

Tc=0.0 min

CN=67

0.08 cfs

Page 7HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Summary for Subcatchment 2S: Predeveloped - EAST BASIN

[46] Hint: Tc=0 (Instant runoff peak depends on dt)

Runoff = 0.15 cfs @ 7.97 hrs, Volume= 0.066 af, Depth= 0.93"

Runoff by SBUH method, Weighted-CN, Time Span= 0.00-72.00 hrs, dt= 0.03 hrs Type IA 24-hr 10-Year Rainfall=3.20"

Area (sf) CN Description

23,316 66 Woods, Poor, HSG B 13,869 83 Woods, Poor, HSG D

37,185 72 Weighted Average 37,185 100.00% Pervious Area

Subcatchment 2S: Predeveloped - EAST BASIN

Runoff

Hydrograph

Time (hours) 727068666462605856545250484644424038363432302826242220181614121086420

F

0.16

0.15

0.14

0.13

0.12

0.11

0.1

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

Type IA 24-hr

10-Year Rainfall=3.20"

Runoff Area=37,185 sf

Runoff Volume=0.066 af

Runoff Depth=0.93"

Tc=0.0 min

CN=72

0.15 cfs

Page 8HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Summary for Pond 3P: West Facility

Inflow Area = 0.853 ac, 34.64% Impervious, Inflow Depth = 1.84" for 10-Year event Inflow = 0.39 cfs @ 7.88 hrs, Volume= 0.130 af Outflow = 0.33 cfs @ 8.01 hrs, Volume= 0.130 af, Atten= 14%, Lag= 7.7 min Discarded = 0.33 cfs @ 8.01 hrs, Volume= 0.130 af

Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.03 hrs Peak Elev= 100.78' @ 8.01 hrs Surf.Area= 279 sf Storage= 163 cf

Plug-Flow detention time= 1.2 min calculated for 0.130 af (100% of inflow) Center-of-Mass det. time= 1.3 min ( 774.9 - 773.7 )

Volume Invert Avail.Storage Storage Description

#1 100.00' 347 cf 12.00'W x 12.00'L x 1.33'H Prismatoid Z=3.0

Device Routing Invert Outlet Devices

#1 Discarded 100.00' 50.000 in/hr Exfiltration over Wetted area Conductivity to Groundwater Elevation = 1.00'

Discarded OutFlow Max=0.33 cfs @ 8.01 hrs HW=100.78' (Free Discharge) 1=Exfiltration ( Controls 0.33 cfs)

Pond 3P: West Facility

Inflow Discarded

Hydrograph

Time (hours) 727068666462605856545250484644424038363432302826242220181614121086420

F

0.42

0.4

0.38

0.36

0.34

0.32

0.3

0.28

0.26

0.24

0.22

0.2

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

Inflow Area=0.853 ac

Peak Elev=100.78'

Storage=163 cf

0.39 cfs

0.33 cfs

Page 9HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Pond 3P: West Facility

Discarded

Stage-Discharge

Discharge (cfs) 0.450.40.350.30.250.20.150.10.050

E le v a ti o n

(f e e t)

Exfiltration

Page 10HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Stage-Discharge for Pond 3P: West Facility

Elevation (feet)

Discarded (cfs)

100.00 0.00

100.01 0.17

100.02 0.17

100.03 0.17

100.04 0.17

100.05 0.18

100.06 0.18

100.07 0.18

100.08 0.18

100.09 0.18

100.10 0.18

100.11 0.19

100.12 0.19

100.13 0.19

100.14 0.19

100.15 0.19

100.16 0.20

100.17 0.20

100.18 0.20

100.19 0.20

100.20 0.20

100.21 0.21

100.22 0.21

100.23 0.21

100.24 0.21

100.25 0.21

100.26 0.22

100.27 0.22

100.28 0.22

100.29 0.22

100.30 0.22

100.31 0.23

100.32 0.23

100.33 0.23

100.34 0.23

100.35 0.23

100.36 0.24

100.37 0.24

100.38 0.24

100.39 0.24

100.40 0.24

100.41 0.25

100.42 0.25

100.43 0.25

100.44 0.25

100.45 0.26

100.46 0.26

100.47 0.26

100.48 0.26

100.49 0.26

100.50 0.27

100.51 0.27

Elevation (feet)

Discarded (cfs)

100.52 0.27

100.53 0.27

100.54 0.28

100.55 0.28

100.56 0.28

100.57 0.28

100.58 0.28

100.59 0.29

100.60 0.29

100.61 0.29

100.62 0.29

100.63 0.30

100.64 0.30

100.65 0.30

100.66 0.30

100.67 0.31

100.68 0.31

100.69 0.31

100.70 0.31

100.71 0.32

100.72 0.32

100.73 0.32

100.74 0.32

100.75 0.32

100.76 0.33

100.77 0.33

100.78 0.33

100.79 0.33

100.80 0.34

100.81 0.34

100.82 0.34

100.83 0.34

100.84 0.35

100.85 0.35

100.86 0.35

100.87 0.35

100.88 0.36

100.89 0.36

100.90 0.36

100.91 0.37

100.92 0.37

100.93 0.37

100.94 0.37

100.95 0.38

100.96 0.38

100.97 0.38

100.98 0.38

100.99 0.39

101.00 0.39

101.01 0.39

101.02 0.39

101.03 0.40

Elevation (feet)

Discarded (cfs)

101.04 0.40

101.05 0.40

101.06 0.41

101.07 0.41

101.08 0.41

101.09 0.41

101.10 0.42

101.11 0.42

101.12 0.42

101.13 0.42

101.14 0.43

101.15 0.43

101.16 0.43

101.17 0.44

101.18 0.44

101.19 0.44

101.20 0.44

101.21 0.45

101.22 0.45

101.23 0.45

101.24 0.46

101.25 0.46

101.26 0.46

101.27 0.46

101.28 0.47

101.29 0.47

101.30 0.47

101.31 0.48

101.32 0.48

101.33 0.48

Page 11HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Stage-Area-Storage for Pond 3P: West Facility

Elevation (feet)

Wetted (sq-ft)

Storage (cubic-feet)

100.00 144 0

100.02 147 3

100.04 150 6

100.06 153 9

100.08 156 12

100.10 160 15

100.12 163 18

100.14 166 22

100.16 169 25

100.18 173 28

100.20 176 32

100.22 179 35

100.24 183 39

100.26 186 43

100.28 189 46

100.30 193 50

100.32 196 54

100.34 200 58

100.36 204 62

100.38 207 66

100.40 211 70

100.42 214 74

100.44 218 78

100.46 222 83

100.48 226 87

100.50 229 92

100.52 233 96

100.54 237 101

100.56 241 105

100.58 245 110

100.60 249 115

100.62 253 120

100.64 257 125

100.66 261 130

100.68 265 135

100.70 269 140

100.72 273 145

100.74 277 151

100.76 281 156

100.78 285 162

100.80 290 167

100.82 294 173

100.84 298 179

100.86 303 185

100.88 307 191

100.90 311 197

100.92 316 203

100.94 320 209

100.96 325 215

100.98 329 222

101.00 334 228

101.02 338 235

Elevation (feet)

Wetted (sq-ft)

Storage (cubic-feet)

101.04 343 241

101.06 348 248

101.08 352 255

101.10 357 261

101.12 362 268

101.14 366 276

101.16 371 283

101.18 376 290

101.20 381 297

101.22 386 305

101.24 391 312

101.26 395 320

101.28 400 327

101.30 405 335

101.32 410 343

Page 12HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Summary for Subcatchment 3S: Developed - WEST BASIN

[46] Hint: Tc=0 (Instant runoff peak depends on dt)

Runoff = 0.39 cfs @ 7.88 hrs, Volume= 0.130 af, Depth= 1.84"

Runoff by SBUH method, Weighted-CN, Time Span= 0.00-72.00 hrs, dt= 0.03 hrs Type IA 24-hr 10-Year Rainfall=3.20"

Area (sf) CN Description

* 8,400 98 Unconnected pavement, HSG B (Pvmt)

* 2,731 98 Unconnected pavement, HSG B (Conc swlks, ramps, stairs)

23,196 79 <50% Grass cover, Poor, HSG B

* 1,114 98 Roofs, HSG B (Front 3 Bldgs)

* 620 98 Unconnected pavement, HSG B (Decks)

1,083 85 Gravel roads, HSG B

37,144 86 Weighted Average 24,279 65.36% Pervious Area 12,865 34.64% Impervious Area 11,751 91.34% Unconnected

Subcatchment 3S: Developed - WEST BASIN

Runoff

Hydrograph

Time (hours) 727068666462605856545250484644424038363432302826242220181614121086420

F

0.42

0.4

0.38

0.36

0.34

0.32

0.3

0.28

0.26

0.24

0.22

0.2

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

Type IA 24-hr

10-Year Rainfall=3.20"

Runoff Area=37,144 sf

Runoff Volume=0.130 af

Runoff Depth=1.84"

Tc=0.0 min

CN=86

0.39 cfs

Page 13HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Summary for Pond 4P: East Facility

Building Roofs and Decks

Inflow Area = 0.854 ac, 17.12% Impervious, Inflow Depth = 1.76" for 10-Year event Inflow = 0.37 cfs @ 7.89 hrs, Volume= 0.125 af Outflow = 0.32 cfs @ 8.01 hrs, Volume= 0.125 af, Atten= 13%, Lag= 7.2 min Discarded = 0.32 cfs @ 8.01 hrs, Volume= 0.125 af

Routing by Stor-Ind method, Time Span= 0.00-72.00 hrs, dt= 0.03 hrs Peak Elev= 100.80' @ 8.01 hrs Surf.Area= 268 sf Storage= 152 cf

Plug-Flow detention time= 1.2 min calculated for 0.125 af (100% of inflow) Center-of-Mass det. time= 1.3 min ( 782.3 - 781.0 )

Volume Invert Avail.Storage Storage Description

#1 100.00' 326 cf 6.00'W x 20.00'L x 1.33'H Prismatoid Z=3.0

Device Routing Invert Outlet Devices

#1 Discarded 100.00' 50.000 in/hr Exfiltration over Wetted area Conductivity to Groundwater Elevation = 1.00'

Discarded OutFlow Max=0.32 cfs @ 8.01 hrs HW=100.80' (Free Discharge) 1=Exfiltration ( Controls 0.32 cfs)

Pond 4P: East Facility

Inflow Discarded

Hydrograph

Time (hours) 727068666462605856545250484644424038363432302826242220181614121086420

F

0.4

0.38

0.36

0.34

0.32

0.3

0.28

0.26

0.24

0.22

0.2

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

Inflow Area=0.854 ac

Peak Elev=100.80'

Storage=152 cf

0.37 cfs

0.32 cfs

Page 14HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Pond 4P: East Facility

Discarded

Stage-Discharge

Discharge (cfs) 0.450.40.350.30.250.20.150.10.050

E le v a ti o n

(f e e t)

Exfiltration

Page 15HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Stage-Discharge for Pond 4P: East Facility

Elevation (feet)

Discarded (cfs)

100.00 0.00

100.01 0.14

100.02 0.14

100.03 0.14

100.04 0.15

100.05 0.15

100.06 0.15

100.07 0.15

100.08 0.15

100.09 0.16

100.10 0.16

100.11 0.16

100.12 0.16

100.13 0.16

100.14 0.17

100.15 0.17

100.16 0.17

100.17 0.17

100.18 0.17

100.19 0.18

100.20 0.18

100.21 0.18

100.22 0.18

100.23 0.19

100.24 0.19

100.25 0.19

100.26 0.19

100.27 0.19

100.28 0.20

100.29 0.20

100.30 0.20

100.31 0.20

100.32 0.20

100.33 0.21

100.34 0.21

100.35 0.21

100.36 0.21

100.37 0.22

100.38 0.22

100.39 0.22

100.40 0.22

100.41 0.23

100.42 0.23

100.43 0.23

100.44 0.23

100.45 0.23

100.46 0.24

100.47 0.24

100.48 0.24

100.49 0.24

100.50 0.25

100.51 0.25

Elevation (feet)

Discarded (cfs)

100.52 0.25

100.53 0.25

100.54 0.26

100.55 0.26

100.56 0.26

100.57 0.26

100.58 0.27

100.59 0.27

100.60 0.27

100.61 0.27

100.62 0.28

100.63 0.28

100.64 0.28

100.65 0.28

100.66 0.29

100.67 0.29

100.68 0.29

100.69 0.29

100.70 0.30

100.71 0.30

100.72 0.30

100.73 0.30

100.74 0.31

100.75 0.31

100.76 0.31

100.77 0.31

100.78 0.32

100.79 0.32

100.80 0.32

100.81 0.32

100.82 0.33

100.83 0.33

100.84 0.33

100.85 0.33

100.86 0.34

100.87 0.34

100.88 0.34

100.89 0.35

100.90 0.35

100.91 0.35

100.92 0.35

100.93 0.36

100.94 0.36

100.95 0.36

100.96 0.36

100.97 0.37

100.98 0.37

100.99 0.37

101.00 0.38

101.01 0.38

101.02 0.38

101.03 0.38

Elevation (feet)

Discarded (cfs)

101.04 0.39

101.05 0.39

101.06 0.39

101.07 0.40

101.08 0.40

101.09 0.40

101.10 0.40

101.11 0.41

101.12 0.41

101.13 0.41

101.14 0.42

101.15 0.42

101.16 0.42

101.17 0.42

101.18 0.43

101.19 0.43

101.20 0.43

101.21 0.44

101.22 0.44

101.23 0.44

101.24 0.45

101.25 0.45

101.26 0.45

101.27 0.46

101.28 0.46

101.29 0.46

101.30 0.46

101.31 0.47

101.32 0.47

101.33 0.47

Page 16HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Stage-Area-Storage for Pond 4P: East Facility

Elevation (feet)

Wetted (sq-ft)

Storage (cubic-feet)

100.00 120 0

100.02 123 2

100.04 127 5

100.06 130 7

100.08 133 10

100.10 137 13

100.12 140 16

100.14 144 18

100.16 147 21

100.18 151 24

100.20 154 27

100.22 158 30

100.24 162 33

100.26 165 37

100.28 169 40

100.30 173 43

100.32 177 47

100.34 180 50

100.36 184 54

100.38 188 58

100.40 192 61

100.42 196 65

100.44 200 69

100.46 204 73

100.48 208 77

100.50 212 81

100.52 216 85

100.54 220 89

100.56 224 94

100.58 228 98

100.60 232 103

100.62 237 107

100.64 241 112

100.66 245 117

100.68 249 121

100.70 254 126

100.72 258 131

100.74 262 136

100.76 267 142

100.78 271 147

100.80 276 152

100.82 280 157

100.84 285 163

100.86 289 169

100.88 294 174

100.90 299 180

100.92 303 186

100.94 308 192

100.96 313 198

100.98 318 204

101.00 322 210

101.02 327 216

Elevation (feet)

Wetted (sq-ft)

Storage (cubic-feet)

101.04 332 223

101.06 337 229

101.08 342 236

101.10 347 242

101.12 352 249

101.14 357 256

101.16 362 263

101.18 367 270

101.20 372 277

101.22 377 284

101.24 382 292

101.26 387 299

101.28 393 307

101.30 398 314

101.32 403 322

Page 17HydroCAD® 10.00-26 s/n 09306 © 2020 HydroCAD Software Solutions LLC

Summary for Subcatchment 4S: Developed - EAST BASIN

[46] Hint: Tc=0 (Instant runoff peak depends on dt)

Runoff = 0.37 cfs @ 7.89 hrs, Volume= 0.125 af, Depth= 1.76"

Runoff by SBUH method, Weighted-CN, Time Span= 0.00-72.00 hrs, dt= 0.03 hrs Type IA 24-hr 10-Year Rainfall=3.20"

Area (sf) CN Adj Description

* 3,456 98 Roofs, HSG B (Back Bldgs, Pavilion)

* 1,945 98 Unconnected pavement, HSG B (Decks)

* 965 98 Unconnected pavement, HSG B (Conc swlk, stairs, ramps)

16,950 79 <50% Grass cover, Poor, HSG B 13,869 89 <50% Grass cover, Poor, HSG D

37,185 86 85 Weighted Average, UI Adjusted 30,819 82.88% Pervious Area 6,366 17.12% Impervious Area 2,910 45.71% Unconnected

Subcatchment 4S: Developed - EAST BASIN

Runoff

Hydrograph

Time (hours) 727068666462605856545250484644424038363432302826242220181614121086420

F

0.4

0.38

0.36

0.34

0.32

0.3

0.28

0.26

0.24

0.22

0.2

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

Type IA 24-hr

10-Year Rainfall=3.20"

Runoff Area=37,185 sf

Runoff Volume=0.125 af

Runoff Depth=1.76"

Tc=0.0 min

UI Adjusted CN=85

0.37 cfs

Office: 18270 SW Boones Ferry Road, Suite 6, Durham, Oregon 97224

Mailing: P.O. Box 230997, Tigard, Oregon 97281

Carlson Geotechnical A Division of Carlson Testing, Inc.

Phone: (503) 601-8250 www.carlsontesting.com

Bend Office Eugene Office Salem Office Tigard Office

(541) 330-9155

(541) 345-0289

(503) 589-1252

(503) 684-3460

G2306020 Geotechnical Investigation

Report of

Geotechnical Investigation & Infiltration Testing

Clackamas Emergency Housing

16590 SE 114th Avenue

Clackamas County, Oregon

CGT Project Number G2306020

Prepared for

Mark Sirois

Clackamas County Community Development

2051 Kaen Rd # 245

Oregon City, OR 97045

December 1, 2023

APPENDIX F

http://www.carlsontesting.com/ http://www.carlsontesting.com/ mailto:atenold@carlsontesting.com mailto:bwilcox@carlsontesting.com

Clackamas Emergency Housing

Carlson Geotechnical Page 3 of 19

TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 Project Information

1.2 Scope of Services

2.0 SITE DESCRIPTION

2.1 Site Geology

2.2 Site Surface Conditions

2.3 Subsurface Conditions

3.0 SEISMIC CONSIDERATIONS

3.1 Seismic Design

3.2 Seismic Hazards

4.0 CONCLUSIONS

5.0 RECOMMENDATIONS

5.1 Site Preparation

5.2 Temporary Excavations

5.3 Wet Weather Considerations

5.4 Structural Fill

5.5 Permanent Slopes

5.6 Shallow Foundations

5.7 Floor Slabs

5.8 Pavements

5.9 Additional Considerations

6.0 RECOMMENDED ADDITIONAL SERVICES

6.1 Design Review

6.2 Observation of Construction

7.0 LIMITATIONS

ATTACHMENTS

Site Location ........................................................................................................................................... Figure 1

Site Plan ................................................................................................................................................. Figure 2

Site Photographs .................................................................................................................................... Figure 3

Subsurface Investigation and Laboratory Testing ............................................................................. Appendix A

Results of Infiltration Testing ............................................................................................................. Appendix B

Carlson Geotechnical Page 4 of 19

1.0 INTRODUCTION

Carlson Geotechnical (CGT), a division of Carlson Testing, Inc. (CTI), is pleased to submit this report summarizing the results of our geotechnical investigation and infiltration testing for the proposed Clackamas

Emergency Housing project. The site is located at 16590 SE 114th Avenue in Clackamas County, Oregon, as shown on the attached Site Location, Figure 1.

1.1 Project Information

CGT developed an understanding of the proposed project based on our correspondence with Base Design +

Architecture, LLC, and project documents provided to us on September 22 and 29, 2023. The documents provided included a Schematic Design Site Plan, prepared by BDA, dated July 28, 2023. Based on our review, we understand the project will include:

Construction of four new, multi dwelling structures as well as an office and facility buildings (bathrooms, kitchen, laundry) to serve the new dwellings. The buildings will be one story, wood-framed, with a slab on grade floor. The multi dwelling structures have an approximate footprint of 960 square feet with a deck.

For the purposes of this report, we have assumed maximum column, continuous wall, and uniform floor slab loads will be on the order of 25 kips, 2 kips per lineal foot (klf), and 150 pounds per square foot

(psf), respectively.

Construction of a parking lot and trash enclosure on the western portion of the site and a pathway to access the dwellings. We anticipate the parking lot will be surfaced with asphalt concrete (AC).

Although no stormwater management plans have been provided, we understand that, if conditions allow, stormwater collected from new impervious areas of the site will be disposed of, at least in part, via onsite infiltration. Design of on-site stormwater facilities will rest with others. Two infiltration tests were requested in the northeastern and northwestern portions of the site as part of this assignment.

Although no grading plans have been provided, we anticipate permanent grade changes at the site will be minimal, with maximum cuts and fills on the order of about 2 feet relative to existing grades.

1.2 Scope of Services

Our scope of work included the following:

Contact the Oregon Utilities Notification Center to mark the locations of public utilities within a 20-foot radius of our explorations at the site.

Explore subsurface conditions at the site by observing the excavation of thirteen test pits to depths of up to about 9½ feet below ground surface (bgs). Details of the subsurface investigation are presented in

Appendix A.

Conduct infiltration testing in two of the test pits. Results of the infiltration testing are presented in

Appendix B.

Classify the soils encountered in the explorations in general accordance with ASTM D2488 (Visual-

Manual Procedure).

Provide a technical narrative describing surface and subsurface deposits, and local geology of the site, based on the results of our explorations and published geologic mapping.

Provide recommendations for the Seismic Site Class, mapped maximum considered earthquake spectral response accelerations, and site seismic coefficients.

Carlson Geotechnical Page 5 of 19

Provide a qualitative evaluation of seismic hazards at the site, including earthquake-induced liquefaction, landsliding, and surface rupture due to faulting or lateral spread.

Provide geotechnical recommendations for site preparation and earthwork.

Provide geotechnical engineering recommendations for use in design and construction of shallow foundations, floor slabs, and pavements.

Provide this written report summarizing the results of our geotechnical investigation and recommendations for the project.

2.0 SITE DESCRIPTION

2.1 Site Geology

Based on available geologic mapping 1,2 of the area, the site is underlain by younger terrace deposits of the

Clackamas River. The terrace deposits were produced by the periodic buildup of fluvial deposits about

10,000 years ago, which flank the current channel and floodplain of the Clackamas River. The terrace deposits typically consist of unconsolidated sand, gravel and cobbles. Based on nearby well logs, this unit extends to depths in excess of 100 feet below ground surface in the vicinity of the site.

2.2 Site Surface Conditions

During our field investigation, the approximate 3¾-acre site was bordered by commercial properties to the north, east and south, and an emergency housing pods facility to the west. Lithgow Creek was observed to cut across the southern portion of the site. The site was terraced; the upper portion (area of proposed site development) was relatively flat and then descended to the south at gradients of up to 1¾ horizontal to 1 vertical (1¾H:1V), to the southern portion which was also relatively flat. Partially buried structures (e.g.

foundations and retaining walls) were observed near the crest of the upper terrace. A relatively large soil stockpile was observed in the eastern portion of the upper terrace. Site layout and surface conditions at the time of our field investigation are shown on the attached Site Plan (Figure 2) and Site Photographs (Figure

3).

2.3 Subsurface Conditions

2.3.1 Subsurface Investigation & Laboratory Testing

Our subsurface investigation consisted of thirteen test pits (TP-1 through TP-13) completed on November

10, 2023. While onsite we also observed and logged an additional test pit (TP-14) which had been excavated by others prior to our arrival onsite. The approximate exploration locations are shown on the Site Plan, attached as Figure 2. In summary, the test pits were excavated to depths ranging from about 3 to 9½ feet bgs. Details regarding the subsurface investigation, logs of the explorations, and results of laboratory testing are presented in Appendix A. Subsurface conditions encountered during our investigation are summarized below.

Wells, R.E., Haugerud, R.A., Niem, A.R., Niem, W.A., Ma, Lina, Evarts, R.C., O’Connor, J.E., Madin, I.P., Sherrod, D.R., Beeson, M.H., Tolan, T.L., Wheeler, K.L., Hanson, W.B., and Sawlan, M.G., 2020. Geologic map of the greater Portland metropolitan area and surrounding region, Oregon and Washington: U.S. Geological Survey, Scientific Investigations Map SIM-3443. scale 1:63,360.

Ma, L., Madin, I.P., Duplantis, S., and Williams, K.J., 2012. Lidar-based surficial geologic map and database of the greater

Portland, Oregon, area, Clackamas, Columbia, Marion, Multnomah, Washington, and Yamhill Counties, Oregon, and Clark County, Washington: Oregon Department of Geology and Mineral Industries, Open-File Report 0-2012-02, scale 1:8,000.

Carlson Geotechnical Page 6 of 19

2.3.2 Subsurface Materials

The following describes each of the subsurface materials encountered at the site.

2.3.2.1 Site Soils

Undocumented Poorly Graded Gravel with Sand and Cobbles Fill (GP Fill)

Undocumented poorly graded gravelly fill was encountered at the surface of TP-1 through TP-12.

Undocumented fill refers to materials placed without (available) records of subgrade conditions or evaluation of compaction. The poorly graded gravel fill was typically brown to gray, moist, subrounded to subangular, and contained varying amounts of cobbles up to 12-inches in diameter, and varying amounts of sand and silt fines. Varying amounts of debris was also observed within the fill soil, the debris consisted of glass, metal, plastic, concrete, brick and asphalt pieces up to 4 inches in diameter. The fill soil extended to depths of about

1 to 2½ feet bgs in TP-1 through TP-12.

Poorly Graded Gravel with Sand, Cobbles and Boulders (GP-GM)

Underlying the undocumented fill soils in TP-1 through TP-12 was native, poorly graded gravelly soil. This soil was typically dense, dark brown, moist, contained subangular gravel and cobbles up to 12-inches in diameter, a varying amount of boulders up to 18-inches in diameter, and a varying amount of silt and sand.

Some caving was observed in this material at depths of about 3 to 9½ feet bgs. This soil extended to the full depth explored in TP-1 through TP-12, about 3 to 9½ feet bgs.

The native soils encountered during our subsurface investigation were consistent with the lower Clackamas

River terrace deposits described in Section 2.1.

2.3.2.2 Stockpiled Soils

Exploration TP-13 and TP-14 were excavated into the existing soil stockpile that was observed on the eastern portion of the site. The log for TP-14 was based on the observations made of the sidewalls of the excavation.

Undocumented Poorly Graded Silty Gravel to Gravel with Sand Fill (GM Fill, GP Fill)

Undocumented gravelly fill was observed at the surface of TP-13 and TP-14. This soil was gray/brown to brown, moist to wet, angular to subrounded, up to 2-inches in diameter, and contained varying amounts of sand and silt. Varying amounts of debris consisting of plastic, brick, concrete and asphalt chunks up to 2 feet long. The stockpiled soil extended to a depth of about 2 feet bgs in TP-14. In TP-13, the silty gravel fill extended to the full depth explored, about 6 feet bgs.

Undocumented Silt with Sand Fill (ML Fill)

Underlying the undocumented silty gravel fill in TP-14, we observed undocumented silt with sand fill. This soil was typically gray/brown, wet, and exhibited low plasticity. This soil extended the full depth of the excavation, about 6 feet bgs.

2.3.3 Groundwater

We did not encounter groundwater within the depths explored at the site on November 10, 2023. To determine approximate regional groundwater levels in the area, we researched well logs available on the

Carlson Geotechnical Page 7 of 19

Oregon Water Resources Department (OWRD) website for wells located within Section 15, Township 2

South, Range 2 East, Willamette Meridian. Our review indicated that groundwater levels in the area generally ranged from about 45 to 65 feet bgs. It should be noted groundwater levels vary with local topography. In addition, the groundwater levels reported on the OWRD logs often reflect the purpose of the well, so water well logs may only report deeper, confined groundwater, while geotechnical or environmental borings will often report any groundwater encountered, including shallow, unconfined groundwater. Therefore, the levels reported on the OWRD well logs referenced above are considered generally indicative of local water levels and may not reflect actual groundwater levels at the project site. We anticipate that groundwater levels will fluctuate due to seasonal and annual variations in precipitation, changes in site utilization, or other factors.

The depth to groundwater map for the Portland area indicates groundwater is present at depths of 30 feet bgs in the vicinity of the site. It should be noted that the levels reported by the referenced map are average values for a given location and incorporate a degree of uncertainty.

3.0 SEISMIC CONSIDERATIONS

3.1 Seismic Design

Section 1613.2.2 of the 2022 Oregon Structural Specialty Code (2022 OSSC) requires that the determination of the seismic site class be in accordance with Chapter 20 of the American Society of Civil Engineers

Minimum Design Loads for Buildings and Other Structures (ASCE 7-16). We have assigned the site as Site

Class D (“Stiff Soil”) based on geologic mapping and subsurface conditions encountered during our investigation. Earthquake ground motion parameters for the site were obtained in accordance with the 2022

OSSC using the Seismic Hazards by Location calculator on the ATC website . The site Latitude 45.401263°

North and Longitude 122.54433° West were input as the site location. The following table shows the recommended seismic design parameters for the site.

Table 1 Seismic Ground Motion Values

Parameter Value

Mapped Acceleration Parameters Spectral Acceleration, 0.2 second (Ss) 0.833g

Spectral Acceleration, 1.0 second (S1) 0.368g

Coefficients

(Site Class D)

Site Coefficient, 0.2 second (FA) 1.167

Site Coefficient, 1.0 second (FV) 1.932

Adjusted MCE Spectral

Response Parameters

MCE Spectral Acceleration, 0.2 second (SMS ) 0.972g

MCE Spectral Acceleration, 1.0 second (SM1 ) 0.711g

Design Spectral Response Accelerations Design Spectral Acceleration, 0.2 second (SDS ) 0.648g

Design Spectral Acceleration, 1.0 second (SD1 ) 0.474g

Seismic Design Category (Risk Category II) D

Value determined from 2022 OSSC Table 1613.2.3(2).

Oregon Water Resources Department, 2023. Well Log Records, accessed November 2023, from OWRD web site:

http://apps.wrd.state.or.us/apps/gw/well_log/.

Snyder, D.T., 2008, Estimated depth to ground water and configuration of the water table in the Portland, Oregon area: U.S.

Geological Survey, Scientific Investigations Report SIR-2008-5059, scale 1:60,000.

Applied Technology Council (ATC), 2023. USGS seismic design parameters determined using “Seismic Hazards by Location,” accessed November 2023, from the ATC website https://hazards.atcouncil.org/.

http://apps.wrd.state.or.us/apps/gw/well_log/ https://hazards.atcouncil.org/

Carlson Geotechnical Page 8 of 19

3.2 Seismic Hazards

3.2.1 Liquefaction

In general, liquefaction occurs when deposits of loose/soft, saturated, cohesionless soils, generally sands and silts, are subjected to strong earthquake shaking. If these deposits cannot drain quickly enough, pore water pressures can increase, approaching the value of the overburden pressure. The shear strength of a cohesionless soil is directly proportional to the effective stress, which is equal to the difference between the overburden pressure and the pore water pressure. When the pore water pressure increases to the value of the overburden pressure, the shear strength of the soil approaches zero, and the soil can liquefy. The liquefied soils can undergo rapid consolidation or, if unconfined, can flow as a liquid. Structures supported by the liquefied soils can experience rapid, excessive settlement, shearing, or even catastrophic failure.

For fine-grained soils, susceptibility to liquefaction is evaluated based on penetration resistance and plasticity, among other characteristics. Criteria for identifying non-liquefiable, fine-grained soils are constantly evolving. Current practice to identify non-liquefiable, fine-grained soils is based on moisture content and plasticity characteristics of the soils 6,7,8

. The susceptibility of sands, gravels, and sand-gravel mixtures to liquefaction is typically assessed based on penetration resistance, as measured using SPTs, CPTs, or

Becker Hammer Penetration tests (BPTs).

Based on their dense relative density, lack of saturated conditions, static groundwater, etc., the coarse-grained soils encountered within our explorations are considered non-liquefiable. Based on review of geologic mapping and our previous experience in the area, we do not anticipate liquefiable conditions are present at depths below those explored as part of this assignment.

3.2.2 Slope Instability

We did not observe any obvious signs of past or on-going slope instability at the site. Review of the

Statewide Landslide Information Database for Oregon (SLIDO), available at the DOGAMI website , shows no historic or prehistoric landslides at or in the immediate vicinity of the site. HazVu shows a moderate to high hazard for landslides at the site, however, we anticipate those hazard levels were assigned based on the on-site slope gradients. Given the lack of evidence of previous landslides in the vicinity, and our observations while onsite, the risk of seismically-induced slope instability occurring at the site is considered low. Provided our recommendations for grading and stormwater management, as described below, are followed, the proposed development is not anticipated to increase this risk.

Seed, R.B. et al., 2003. Recent Advances in Soil Liquefaction Engineering: A Unified and Consistent Framework. Earthquake

Engineering Research Center Report No. EERC 2003-06.

Bray, Jonathan D., Sancio, Rodolfo B., et al., 2006. Liquefaction Susceptibility of Fine-Grained Soils, Journal of Geotechnical and

Geoenvironmental Engineering, Volume 132, Issue 9, September 2006.

Idriss, I.M., Boulanger, R.W., 2008. Soil Liquefaction During Earthquakes, Earthquakes Engineering Research Institute Monograph

MNO-12.

Oregon Department of Geology and Mineral Industries, 2023. Statewide Landslide Information Database for Oregon (SLIDO), accessed November 2023, from DOGAMI web site: https://gis.dogami.oregon.gov/maps/slido/.

https://gis.dogami.oregon.gov/maps/slido/

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3.2.3 Surface Rupture

3.2.3.1 Faulting

Although the site is situated in a region of the country with known active faults and historic seismic activity, no known faults exist on or immediately adjacent to the site. Therefore, the risk of surface rupture at the site due to faulting is considered low.

3.2.3.2 Lateral Spread

Surface rupture due to lateral spread can occur on sites underlain by liquefiable soils that are located on or immediately adjacent to slopes steeper than about 3 degrees (20H:1V), and/or adjacent to a free face, such as a stream bank or the shore of an open body of water. During lateral spread, the materials overlying the liquefied soils are subject to lateral movement downslope or toward the free face. Based on the non-liquefiable nature of the soils at the site, the risk of damage associated with lateral spread is negligible.

4.0 CONCLUSIONS

Based on the results of our field explorations and analyses, the site may be developed as described in

Section 1.1 of this report, provided the recommendations presented in this report are incorporated into the design and development.