FS31_Interim_GeotechRpt_InfiltrationPavement_200414.pdf

April 14, 2020 Project No. T-8326

Ms. Kate Spitzer Sr. Project Manager, Capital Development Seattle Department of Finance and Administrative Services Seattle, Washington 94689

Subject: Geotechnical Engineering Evaluation FS 31 Interim Station N Northgate Way and Interlake Avenue North Seattle, Washington

Reference: Geotechnical Report, FS 31 Interim Station, N Northgate Way and Interlake Avenue N, Seattle, Washington, Project No. T-8326, prepared by Terra Associates, Inc., dated March 31, 2020

Dear Ms. Spitzer:

As requested, we have completed a geotechnical engineering evaluation for the FS 31 Interim Station project in Seattle, Washington. The purpose of our evaluation was to determine if the existing pavement along Interlake Avenue N is suitable for the project and to determine if the existing infiltration facility can be used for the new project.

Pavement

In order to determine if the existing pavement structure would be suitable to support the proposed project, we completed three asphalt cores within the drive apron adjacent Interlake Ave N. The approximate location of the cores is shown on attached Figure 1. The results of the cores indicate the pavement structure consists of two and one-half to three inches of hot mix asphalt (HMA) overlying two to five inches of crushed rock base (CRB). The soil subgrade consists of medium dense silty sand with gravel.

12220 113th Avenue NE, Ste. 130, Kirkland, Washington 98034 Phone (425) 821‐7777 • Fax (425) 821‐4334

Ms. Kate Spitzer April 14, 2020

Project No. T-8326 Page No. 2

To evaluate the existing pavement section, we used the American Association of State Highway and

Transportation Officials (AASHTO) procedures. For this procedure, we calculated the existing structural number for the pavement section. We used the thinnest section of 2.5 inches HMA over 2 inches CRB and the structural coefficients of 0.44 for HMA and 0.14 for CRB. The structural number for this section is 1.38. Additional design parameters required for the AASHTO procedure and selected for our analysis include the following:

Reliability – 50 percent

Standard deviation – 0.35

Present serviceability index – 4.0

Terminal serviceability index – 2.0

The supporting capability of the pavement subgrade is represented in the AASHTO procedure by the resilient modulus (Mr). Based on our exploration, the subgrade soils that support the roadway consist predominantly of medium dense silty sand with gravel material. Based on correlation with published data and our experience with similar soils, we have assigned the subgrade a Mr value of 7,000 psi.

Based on the above parameters, the current pavement can support a design ESAL of 14,601. This equates to approximately 15 fire trucks traversing the pavement every day.

The current pavement structure is in fair condition. Based on the results of our testing and our visual observations of the pavement structure, it is our opinion that the current pavement section is suitable for support of the proposed project. However, due to the current condition of the pavement, we recommend completing a two-inch grind and overlay of the existing pavement structure.

Infiltration

In accordance with the City of Seattle requirements, we have completed two small scale pilot infiltration tests near the existing infiltration facility to determine the design infiltration rate for the existing facility. The approximate location of the tests is shown on attached Figure 1. The results of the testing indicate the soils at the base of the facility have a design infiltration rate of 0.7 inches per hour. The simple infiltration checklists are attached to this letter.

We potholed the existing facility in two locations. The gravel near the surface of the facility has silt and soil mixed with the gravel. If the existing facility is to be utilized, we recommend removing the upper one-foot of gravel and replacing it with new clean gravel.

Environmental Earth Sciences

Terra

Associates, Inc.

Consultants in Geotechnical Engineering

Geology and

Figure 1

Date APR 2020

EXPOLRATION LOCATION PLAN

SEATTLE, WASHINGTON

FS 31 INTERIM STATION

Proj. No.T-8326

LEGEND:

APPROXIMATE PILOT INFILTRATION TEST LOCATION

REFERENCE:

SITE PLAN PROVIDED BY KING COUNTY IMAP AND CITY OF SEATTLE

REFERENCE ONLY AND SHOULD NOT BE USED FOR

DESIGN OR CONSTRUCTION PURPOSES.

DIMENSIONS ARE APPROXIMATE. IT IS INTENDED FOR

THIS SITE PLAN IS SCHEMATIC. ALL LOCATIONS AND

APPROXIMATE CORING LOCATION

APPROXIMATE POTHOLE LOCATION

PIT-2

PIT-1

CORE -1

CORE -2

CORE -3

City of Seattle Phone: 206-684-8850 Department of Construction and Inspections Applicant Services Center 700 Fifth Ave, Suite 2000, P.O. Box 34019 Seattle, WA 98124-4019 www.seattle.gov/sdci

City of Seattle Simple Infiltration Test Checklist

Call before you dig – Utility Locates 811

Project Address: _________________________________________ Date: ___________________________

Permit Number: _______________________

This Infiltration Test was performed by:

Company Name: ____________________________ Contact Name: _________________________________

Phone Number: _____________________________ Email Address: __________________________________

Include site map or drainage control plan, with test locations clearly marked.

The intent of this checklist is to provide a summary of stormwater BMP subsurface investigation and infiltration testing requirements associated with the Simple Subsurface Investigation. All projects and associated plans are also subject to the minimum requirements outlined in the City of Seattle Stormwater Manual and SMC Chapters 22.800 – 22.808, as well as the specific subsurface investigation and infiltration testing requirements outlined in Volume 3, Chapter 3 and Appendix D of the 2016 City of Seattle Stormwater Manual.

This checklist does not preclude the use of professional judgment to evaluate and manage risk associated with design, construction, and operation of infiltration BMPs.

See Appendix C for site constraints that may preclude infiltration facility feasibility for some BMPs. The Simple Infiltration Test is not allowed for projects with no off-site point of discharge (Section 4.3.2.1). These projects shall use a Small Pilot Infiltration Test (PIT).

Before you start call Utility Locates 811 to request locates of utilities at your site.

The Simple Subsurface Investigation involves an Infiltration Testing element and a Subsurface Investigation element.

Although the Infiltration Testing is listed first below, the Infiltration Testing and Subsurface Investigation can be done in any order.

INFILTRATION TESTING:

1. Is the infiltration test within the footprint of the proposed infiltration facility? Yes No

2. If “no,” is the test within 50 feet of the proposed infiltration facility? Yes No

Explain why: __________________________________________________________________

3. What is the total proposed new plus replaced impervious area (not including permeable pavement surfaces) infiltrated on the site? ___________________ ft2

Page 1 OF _____

4. Date and time of test(s): ______________________________________________________

• If performed November through March, one test is required.

• If performed April through October, two tests are required.

o Tests must be in the same hole within 2-days.

o The beginning of each test must be spaced 24-hours apart.

5. Dig an infiltration test hole at least 2-feet deep, measured from the proposed finished grade, and 2-feet across. It is recommended that the test hole depth be at the bottom of the facility to provide the best design information. (Note: this hole is separate from the hole in Step 11below)

6. Diameter of test hole (2-foot minimum): _____________ feet

7. Depth of test hole (2-foot minimum): _____________ feet

8. Describe soil type and texture (e.g., sand, clay, gravel.):_____________________________________________

9. Pre-soak period

a) Add water to the 12-inch mark. (Measure depth using a ruler, scale, or tape measure).

b) Stabilize water depth for a minimum of 30-minutes by adding water until the depth is maintained at a minimum of 12 inches, then move on to step c.

c) Stop adding water, then record the number of inches the water has fallen in 1 hour: ___________ inches

d) Record the number of inches the water has fallen from hour 1 to hour 2:_____________ inches

e) What is the smaller of the two numbers in row 9c and 9d above? (check only one box below)

> 3-inches (Use Table 1 below – 15-minute intervals.)

Between 1-inch and 3-inches (Use Table 2 below – 30-minute intervals.)

< 1-inch (Use Table 3 below – 60-minute intervals.)

This is your “testing period”.

10. Testing period Based on the answer to 9e above, use either Table 1, 2 or 3 on the Results and Certification page to record your data and:

a) Refill the hole to the 12-inch mark.

b) Immediately record the time and depth of water in the appropriate table below.

c) Based on your time interval (answer to 9e above):

Record the time and depth of water in the hole at the specified intervals.

Complete the table by recording six measurements (in addition to the starting depth).

If the hole empties prior to the six measurements, refill to the 12-inch mark and continue recording until you have completed the table.

d) Using the depth of water recorded at each interval, calculate the infiltration rate and record the results:

• Table 1: Infiltration Rate = Change in depth between each interval x 4

• Table 2: Infiltration Rate = Change in depth between each interval x 2

• Table 3: Infiltration Rate = Change in depth between each interval x 1

e) If performed April through October, repeat steps 9 and 10 in the same hole 24 hours after the beginning of the first infiltration test and record the results in the Infiltration Test #2 Result tables.

SUBSURFACE INVESTIGATION:

11. Dig a hole to the depth required per Table 5 below (2-feet below proposed facility in the wet season and 3-feet below the proposed facility in the dry season) and approximately 5-feet from the proposed infiltration facility. (See the footnote at the end of Table 5 – depth is measured from the bottom of the proposed infiltration facility.)

12. Record total depth of hole from surrounding ground surface: ___________ feet

13. While digging the hole, did you:

No a) Hit hard pan? (i.e. hardened soil that is like concrete) Yes

b) Encounter standing water or seepage in the hole? Yes No

14. If you answered “yes” to either (13a) or (13b), infiltration is not feasible for this site. Test is finished.

Page 2 OF _____

INFILTRATION TEST RESULTS AND CERTIFICATION

Infiltration Test #1 Results Table 1 (15-min) Table 2 (30-min) Table 3 (60-min)

Time (15-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (30-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (60-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr) 12 --- 12 --- 12 ---

Infiltration Test #2 Results (Required if performed April through October – see step 4 above) Table 1 (15-min) Table 2 (30-min) Table 3 (60-min)

Time (15-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (30-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (60-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr) 12 --- 12 --- 12 ---

The lowest infiltration rate from the tables above = _______________in/hr (Measured infiltration rate)

• If the lowest measured infiltration rate is less than the minimum rate associated with an infiltration BMP

(see Table 4 below), that BMP cannot be used.

• If the measured infiltration rate is less than all minimum infiltration rates for infiltration BMPs, no further investigation is required.

Design infiltration rate = Measured infiltration rate x 0.5 = _________________in/hr

SIGNATURES ARE REQUIRED

I certify that I have followed the procedures outlined in this document to determine the infiltration BMP feasibility and infiltration rate.

Infiltration Test performed by:

Print Name ______________________________________

Signature _______________________________________ Date__________________________________

Subsurface Investigation performed by:

Print Name ______________________________________

Signature _______________________________________ Date__________________________________

Page 3 OF _____

REFERENCE TABLES

Table 4. Minimum Measured Infiltration Rates (Taken from the 2016 City of Seattle Stormwater Manual, Vol. 3, Section 3.2 – Table 3.3)

Infiltration BMP

Minimum Measured Infiltration Rate for On-site List Approach

(in/hr)

Minimum Allowed Measured Infiltration Rate for Meeting Flow Control, Water Quality Treatment, and On-site Performance Standards (in/hr)

Infiltration Trenches 5 5 Drywells 5 5 Infiltrating Bioretention without underdrain

0.6 0.6

Infiltrating Bioretention with underdrain 0.3 No minimum Rain Gardens 0.3 Not applicable (only for On-site List

Approach) Permeable Pavement Facility 0.3 0.3b Permeable Pavement Surface 0.3a No minimum Perforated Stub-out Connections 0.3 Not applicable (only for On-site List

Approach) Infiltration Basins Not applicable 0.6 Infiltration Chambers Not applicable 0.6 a Infiltration testing not required, only necessary to prove infeasibility.

b No minimum infiltration rate if underdrain is installed.

Table 5. Minimum Investigation Depth and Vertical Separation Requirements (Taken from the 2016 City of Seattle Stormwater Manual, Appendix D, Section D-2.3)

All BMPs

Season

Minimum Investigation

Depth (ft) a

Minimum Vertical Separation, ft a

Groundwater Hydraulically-

Restrictive Layer Wet Season (November – March) 2 1 1 Dry Season (April – October) 3 2 1 a The minimum investigation depth and vertical separation shall be measured from the bottom of the facility. The bottom of the facility is defined as the deepest portion of proposed facility where infiltrating water is expected to move into the underlying soil

Page 4 OF _____

City of Seattle Phone: 206-684-8850 Department of Construction and Inspections Applicant Services Center 700 Fifth Ave, Suite 2000, P.O. Box 34019 Seattle, WA 98124-4019 www.seattle.gov/sdci

City of Seattle Simple Infiltration Test Checklist

Call before you dig – Utility Locates 811

Project Address: _________________________________________ Date: ___________________________

Permit Number: _______________________

This Infiltration Test was performed by:

Company Name: ____________________________ Contact Name: _________________________________

Phone Number: _____________________________ Email Address: __________________________________

Include site map or drainage control plan, with test locations clearly marked.

The intent of this checklist is to provide a summary of stormwater BMP subsurface investigation and infiltration testing requirements associated with the Simple Subsurface Investigation. All projects and associated plans are also subject to the minimum requirements outlined in the City of Seattle Stormwater Manual and SMC Chapters 22.800 – 22.808, as well as the specific subsurface investigation and infiltration testing requirements outlined in Volume 3, Chapter 3 and Appendix D of the 2016 City of Seattle Stormwater Manual.

This checklist does not preclude the use of professional judgment to evaluate and manage risk associated with design, construction, and operation of infiltration BMPs.

See Appendix C for site constraints that may preclude infiltration facility feasibility for some BMPs. The Simple Infiltration Test is not allowed for projects with no off-site point of discharge (Section 4.3.2.1). These projects shall use a Small Pilot Infiltration Test (PIT).

Before you start call Utility Locates 811 to request locates of utilities at your site.

The Simple Subsurface Investigation involves an Infiltration Testing element and a Subsurface Investigation element.

Although the Infiltration Testing is listed first below, the Infiltration Testing and Subsurface Investigation can be done in any order.

INFILTRATION TESTING:

1. Is the infiltration test within the footprint of the proposed infiltration facility? Yes No

2. If “no,” is the test within 50 feet of the proposed infiltration facility? Yes No

Explain why: __________________________________________________________________

3. What is the total proposed new plus replaced impervious area (not including permeable pavement surfaces) infiltrated on the site? ___________________ ft2

Page 1 OF _____

4. Date and time of test(s): ______________________________________________________

• If performed November through March, one test is required.

• If performed April through October, two tests are required.

o Tests must be in the same hole within 2-days.

o The beginning of each test must be spaced 24-hours apart.

5. Dig an infiltration test hole at least 2-feet deep, measured from the proposed finished grade, and 2-feet across. It is recommended that the test hole depth be at the bottom of the facility to provide the best design information. (Note: this hole is separate from the hole in Step 11below)

6. Diameter of test hole (2-foot minimum): _____________ feet

7. Depth of test hole (2-foot minimum): _____________ feet

8. Describe soil type and texture (e.g., sand, clay, gravel.):_____________________________________________

9. Pre-soak period

a) Add water to the 12-inch mark. (Measure depth using a ruler, scale, or tape measure).

b) Stabilize water depth for a minimum of 30-minutes by adding water until the depth is maintained at a minimum of 12 inches, then move on to step c.

c) Stop adding water, then record the number of inches the water has fallen in 1 hour: ___________ inches

d) Record the number of inches the water has fallen from hour 1 to hour 2:_____________ inches

e) What is the smaller of the two numbers in row 9c and 9d above? (check only one box below)

> 3-inches (Use Table 1 below – 15-minute intervals.)

Between 1-inch and 3-inches (Use Table 2 below – 30-minute intervals.)

< 1-inch (Use Table 3 below – 60-minute intervals.)

This is your “testing period”.

10. Testing period Based on the answer to 9e above, use either Table 1, 2 or 3 on the Results and Certification page to record your data and:

a) Refill the hole to the 12-inch mark.

b) Immediately record the time and depth of water in the appropriate table below.

c) Based on your time interval (answer to 9e above):

Record the time and depth of water in the hole at the specified intervals.

Complete the table by recording six measurements (in addition to the starting depth).

If the hole empties prior to the six measurements, refill to the 12-inch mark and continue recording until you have completed the table.

d) Using the depth of water recorded at each interval, calculate the infiltration rate and record the results:

• Table 1: Infiltration Rate = Change in depth between each interval x 4

• Table 2: Infiltration Rate = Change in depth between each interval x 2

• Table 3: Infiltration Rate = Change in depth between each interval x 1

e) If performed April through October, repeat steps 9 and 10 in the same hole 24 hours after the beginning of the first infiltration test and record the results in the Infiltration Test #2 Result tables.

SUBSURFACE INVESTIGATION:

11. Dig a hole to the depth required per Table 5 below (2-feet below proposed facility in the wet season and 3-feet below the proposed facility in the dry season) and approximately 5-feet from the proposed infiltration facility. (See the footnote at the end of Table 5 – depth is measured from the bottom of the proposed infiltration facility.)

12. Record total depth of hole from surrounding ground surface: ___________ feet

13. While digging the hole, did you:

No a) Hit hard pan? (i.e. hardened soil that is like concrete) Yes

b) Encounter standing water or seepage in the hole? Yes No

14. If you answered “yes” to either (13a) or (13b), infiltration is not feasible for this site. Test is finished.

Page 2 OF _____

INFILTRATION TEST RESULTS AND CERTIFICATION

Infiltration Test #1 Results Table 1 (15-min) Table 2 (30-min) Table 3 (60-min)

Time (15-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (30-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (60-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr) 12 --- 12 --- 12 ---

Infiltration Test #2 Results (Required if performed April through October – see step 4 above) Table 1 (15-min) Table 2 (30-min) Table 3 (60-min)

Time (15-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (30-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr)

Time (60-min)

Depth of Water

(inches)

Infiltration Rate

(in/hr) 12 --- 12 --- 12 ---

The lowest infiltration rate from the tables above = _______________in/hr (Measured infiltration rate)

• If the lowest measured infiltration rate is less than the minimum rate associated with an infiltration BMP

(see Table 4 below), that BMP cannot be used.

• If the measured infiltration rate is less than all minimum infiltration rates for infiltration BMPs, no further investigation is required.

Design infiltration rate = Measured infiltration rate x 0.5 = _________________in/hr

SIGNATURES ARE REQUIRED

I certify that I have followed the procedures outlined in this document to determine the infiltration BMP feasibility and infiltration rate.

Infiltration Test performed by:

Print Name ______________________________________

Signature _______________________________________ Date__________________________________

Subsurface Investigation performed by:

Print Name ______________________________________

Signature _______________________________________ Date__________________________________

Page 3 OF _____

REFERENCE TABLES

Table 4. Minimum Measured Infiltration Rates (Taken from the 2016 City of Seattle Stormwater Manual, Vol. 3, Section 3.2 – Table 3.3)

Infiltration BMP

Minimum Measured Infiltration Rate for On-site List Approach

(in/hr)

Minimum Allowed Measured Infiltration Rate for Meeting Flow Control, Water Quality Treatment, and On-site Performance Standards (in/hr)

Infiltration Trenches 5 5 Drywells 5 5 Infiltrating Bioretention without underdrain

0.6 0.6

Infiltrating Bioretention with underdrain 0.3 No minimum Rain Gardens 0.3 Not applicable (only for On-site List

Approach) Permeable Pavement Facility 0.3 0.3b Permeable Pavement Surface 0.3a No minimum Perforated Stub-out Connections 0.3 Not applicable (only for On-site List

Approach) Infiltration Basins Not applicable 0.6 Infiltration Chambers Not applicable 0.6 a Infiltration testing not required, only necessary to prove infeasibility.

b No minimum infiltration rate if underdrain is installed.

Table 5. Minimum Investigation Depth and Vertical Separation Requirements (Taken from the 2016 City of Seattle Stormwater Manual, Appendix D, Section D-2.3)

All BMPs

Season

Minimum Investigation

Depth (ft) a

Minimum Vertical Separation, ft a

Groundwater Hydraulically-

Restrictive Layer Wet Season (November – March) 2 1 1 Dry Season (April – October) 3 2 1 a The minimum investigation depth and vertical separation shall be measured from the bottom of the facility. The bottom of the facility is defined as the deepest portion of proposed facility where infiltrating water is expected to move into the underlying soil

Page 4 OF _____

S am pl e

N o.

D ep th ft)

PROJECT NAME: PROJ. NO: LOGGED BY:

LOCATION:

DATE LOGGED:

APPROX. ELEV:

DEPTH TO CAVING:

FIGURE

DEPTH TO GROUNDWATER:

SURFACE CONDITIONS:

Description Consistency/

Relative Density W interpreted as being indicative of other locations at the site.

NOTE: This subsurface information pertains only to this test pit location and should not be

A-2

T-8326 MJX

Seattle, Washington Grass

April 6 & 7, 2020

FS 31 Interim Station

LOG OF TEST PIT NO. PIT-1

N/A

6.5 Feet N/A

Medium Dense

Dense

(2 inches TOPSOIL)

FILL: Brown silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, trace brick deposits, trace 2-inch layer of 5/8" clean crushed rock at approximately 1-foot.

Reddish-brown silty SAND, fine to medium sand, moist, scattered gravel. (SM)

Gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, trace cobbles. (SM)

Test pit terminated at approximately 8.5 feet.

Small-scale PIT performed at approximately 5.5 feet.

Minor perched groundwater seepage observed at approximately 6.5 feet following infiltration testing - seepage out the sides of the excavation.

No caving observed.

S am pl e

N o.

D ep th ft)

PROJECT NAME: PROJ. NO: LOGGED BY:

LOCATION:

DATE LOGGED:

APPROX. ELEV:

DEPTH TO CAVING:

FIGURE

DEPTH TO GROUNDWATER:

SURFACE CONDITIONS:

Description Consistency/

Relative Density W interpreted as being indicative of other locations at the site.

NOTE: This subsurface information pertains only to this test pit location and should not be

A-3

T-8326 MJX

Seattle, Washington Mixed Gravel

April 6 & 7, 2020

FS 31 Interim Station

LOG OF TEST PIT NO. PIT-2

N/A

5 to 7 Feet N/A

Medium Dense

Dense

(1-inch MIXED GRAVEL)

FILL: Brown to grayish-brown silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, trace brick debris, trace cobbles, occasional organic. (SM)

Reddish-brown silty SAND, fine to medium sand, moist, scattered gravel. (SM)

Gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, mottled, scattered sand seams. (SM)

Test pit terminated at approximately 8 feet.

Small-scale PIT performed at approximately 5 feet.

Minor groundwater seepage was observed between approximately 5 to 7 feet at localized points following infiltration testing - seepage out of the sides of the excavation.

No caving observed.