W912P723B0002 Request for Information and Responses_JK Revisions.pdf

DRY CREEK ECOSYSTEM RESTORATION PROJECT – PHASE I

AS OF: 11 April 2023

QUESTIONS & RESPONSES

SOLICITATION NUMBER W912P723B0002

The intent of this section is to provide questions & comments received from interested parties on the above solicitation, and our responses to those questions and comments. It is possible that the information below does not reflect 100% of the questions and responses. The questions and comments do not necessarily represent questions received from any particular contractor.

Questions and responses are provided for information only and are not made part of the solicitation. Only a formal amendment (SF30) to the solicitation changes the solicitation.

Changes to any previous responses in this document will be struck-through and new, updated responses will be provided as appropriate

RFI # Spec Section RFI (Question) Responses

1 31 23 19

I am trying to get ahold of a copy of the geotechnical report for the project titled Geotechnical Investigation Report, Dry Creek Ecosystem Restoration, General Investigation Project Phase II, Sites 2A/2B and 4C/4D, Sonoma County, California.

a) Would you be able to provide a copy of this report or let me know where I can get one?

b) I did not see it on the SAM.GOV website listing for the project. Also, would you be able to confirm that the Site 2A Bank Repair will likely be the only portion of Phase II that will need dewatering?

a) Please see the Geotech report attached.

b) Water control and need for dewatering for all sites (2A, 2B, 4C) shall be done in according with Spec Section 31 23 19. Per the specification, the Contractor must dewater the subgrade of the Site 2A Bank Repair site at a minimum. It’s up to the Contractor to determine if that means complete dewatering.

2 00 21 00

Based on our heavily impacted bid schedule I am asking for the bid date to be extended by two weeks to allow for additional time to finish takeoffs, sequester vendor proposals and complete the cost estimate, thank you.

See Amendment 0002

Dry Creek Ecosystem Restoration General Investigation Project Phase II ESA / D130163.01

90% Design Report February 2020

Appendix B Geotechnical Investigation Report

Geotechnical Investigation Report Dry Creek Ecosystem Restoration General Investigation Project Phase II Sites 2A/2B and 4C/4D Sonoma County, California

SUBMITTED TO:

Ms. Ann Borgonovo, P.E.

Vice President ESA - Environmental Science Associates 550 Kearny Street, Suite 800 San Francisco, CA 94108 Aborgonovo@esassoc.com

February 12, 2020

A3GEO, Inc. • 821 Bancroft Way, Berkeley CA 94710 i

February 12, 2020

Ms. Ann Borgonovo, P.E.

Vice President ESA – Environmental Science Associates 550 Kearny Street, Suite 800 San Francisco, CA 94108 ABorgonovo@esassoc.com

RE: Geotechnical Investigation Report

Dry Creek Ecosystem Restoration General Investigation Project Phase II Sites 2A/2B and 4C/4D Sonoma County, California

Dear Ms. Borgonovo:

This report presents the results of A3GEO’s geotechnical investigation for the Dry Creek Ecosystem Restoration General Investigation Project Phase II (Sites 2A/2B and 4C/4D) in Sonoma County, California. The Dry Creek Ecosystem Restoration General Investigation Project Phase II (herein referred to as the “Project”) is part of the much larger, Dry Creek Habitat Enhancement project, managed by the Sonoma County Water Agency (SCWA). A3GEO has been providing geotechnical services as a subconsultant to ESA, one of the restoration design engineers for the Dry Creek Habitat Enhancement project, since 2014.

The overarching, Dry Creek Habitat Enhancement project was initiated in 2008 to enhance approximately six miles and dozens of Sites along Dry Creek with design and implementation occurring over a 12 year period.

This report specifically focuses on four of the Sites: Sites 2A, 2B, 4C and 4D.

A3GEO’s services performed in relation to Sites 2A, 2B, 4C and 4D were authorized under multiple agreements and amendments with ESA and are identified below:

Agreement for Subconsultant Services #D130163.00 dated February 24, 2014 in conjunction with the following amendments:

o Amendment No. 1 dated March 7, 2014, and o Amendment No. 2 dated March 22, 2016.

Agreement for Subconsultant Services #D130163.01 dated May 14, 2019 in conjunction with the following amendment:

o Amendment No. 1 dated July 26, 2019.

Since 2014, A3GEO has prepared numerous reports in association with the Dry Creek Habitat Enhancement project, some of which include data, evaluations and recommendations that pertain to Sites 2A, 2B, 4C and 4D.

This report incorporates the relevant information from our previous reports, provides updated design recommendations, and is intended to supersede all previous reports and serve as a stand-alone, Geotechnical Investigation Report for Sites 2A, 2B, 4C and 4D.

ii

The interpretations, conclusions and recommendations presented in this report were developed in accordance with generally accepted geotechnical principles and practices at the time that the report was prepared. Should you have any questions or comments concerning our findings, the concepts discussed, or our recommendations, please do not hesitate to call.

Yours very truly, A3GEO, Inc.

Dona Mann, PE, GE Principal Engineer

(415) 425-0247

Dillon Braud, PE Project Engineer

(225) 439-0839 iii

TABLE OF CONTENTS

1. Introduction

1.01 Project Overview

1.02 Purpose and Scope of Services

2. Methods of Investigation

2.01 Review of Existing Information

2.02 Site Reconnaissance Visits

2.03 Geophysical Surveys

2.04 Borings

2.05 Geotechnical Laboratory Testing

3. Site Conditions

3.01 Local Geology

3.02 Surface Conditions

3.02.1 Site 2A

3.02.2 Site 2B

3.02.3 Site 4C

3.02.4 Site 4D

3.03 Subsurface Conditions

3.03.1 Overview

3.03.2 Sites 2A and 2B

3.03.3 Site 4C

3.03.4 Site 4D

4. Evaluations and Conclusions

4.01 General

4.02 Site 2A Bank Repair

4.02.1 Feasibility

4.02.2 Bank Repair Cross Section

4.02.3 Slope Stability Evaluation

4.03 Stability of Channel Cut Slopes

4.04 General Construction Considerations

4.04.1 Excavation

4.04.2 Shallow Bedrock

4.04.3 Groundwater / Dewatering

4.04.4 Temporary Cut Slopes / Shoring

4.04.5 Winter Construction

5. Recommendations

5.01 Site Preparation

5.02 Geotechnical Materials

5.03 Bank Repair Construction

5.04 Surface Drainage

5.05 Erosion Control

5.06 Future Services

5.06.1 Geotechnical Plan and Specification Review

5.06.2 Construction-Phase Geotechnical Services

6. Limitations

7. References iv

Figure 1 – Site Plan – Site 2A Figure 2 – Site Plan – Site 2B Figure 3 – Site Plan – Sites 4C & 4D Figure 4 – Interpretive Cross Section A-A’ – Site 2A Figure 5 – Interpretive Cross Section B-B’ Figure 6 – Interpretive Cross Section C-C’ – Site 2B Figure 7 – Interpretive Cross Section D-D’ – Sites 4C & 4D

Plate 1 – Vicinity Map Plate 2 – Site Location Map Plate 3 – Sites 2A/2B Geologic Map Plate 4 – Sites 4C/4D Geologic Map Plate 5 – Aerial Photographs – Site 2A Plate 6 – Site 2A/2B Photos Plate 7 – Site 2A Bank Photos Plate 8 – Site 4C/4D Photos

Appendix A – Exploratory Boring Logs Appendix B – Geotechnical Laboratory Testing Data Appendix C – NorCal Geophysical Report – Sites 2A, 2B, 4A, 4B, 4D, & 5A Appendix D – NorCal Geophysical Report – Site 4C Appendix E – Slope Stability Results

1. INTRODUCTION

1.01 Project Overview

This report presents the results of A3GEO’s geotechnical investigation for Sites 2A, 2B, 4C, and 4D of the Dry Creek Ecosystem Restoration General Investigation Project Phase II (Project). The Project’s objective is to create habitat enhancement graded features and design elements focused on improving summer rearing habitat for coho salmon. The primary graded features will include backwater alcoves with various types of embedded log structures and a 20-foot-high by 75-foot-long bank repair at Site 2A.

The general location of the Sites in relation to Warm Springs Dam (upstream) and Dry Creek’s confluence with the Russian River (downstream) is shown on the Vicinity Map (Plate 1). The specific location of each Site is shown on the Site Location Map (Plate 2). Individual Site Plans showing the proposed restoration designs are presented on Figures 1 through 3. Elevations (abbreviated El.) in this report are reported in feet and reference the North American Vertical Datum of 1988 (NAVD 88). Identifying information for each Site is summarized in the table below.

Table 1- Summary of Sites

Site

Approximate Upstream

River Station

Approximate Downstream

River Station

Approximate Length (feet)

Side of Creek1

Approximate Latitude and Longitude

Parcel Number(s)

2A 64+00 57+00 700 Left and Right

38.594722° N 122.873860° W

088-190-042 088-190-007 088-190-009

2B 69+00 64+00 500 Left 38.596380° N 122.874540° W 088-190-042

4C 216+00 207+00 900 Left 38.629039° N 122.899372° W 089-040-019

4D 213+00 204+00 900 Right 38.628033° N 122.899193° W 089-190-008

1.02 Purpose and Scope of Services

The primary purpose of our investigation was to: 1) investigate and characterize geotechnical conditions in the vicinity of Sites 2A, 2B, 4C, and 4D; and 2) provide geotechnical conclusions and recommendations in support of the proposed enhancement plans. The scope of our services generally included:

Reviewing available geotechnical, geologic and seismic information;

Conducting geotechnical site reconnaissance visits;

Obtaining drilling permits and exploring subsurface conditions with borings and geophysical surveys;

Performing geotechnical laboratory tests;

Characterizing geotechnical conditions and developing geologic cross sections;

Conducting geotechnical engineering analyses;

Developing geotechnical conclusions and recommendations for the project; and Preparing this geotechnical investigation report.

1 All references to “Right” and “Left” sides/banks in this report are assuming a vantage point of someone standing in the center of the creek looking downstream.

Due to the nature of the project, an evaluation of potential seismic hazards was not requested; accordingly, our scope of services did not include investigations or analyses relating to earthquake-induced ground shaking, liquefaction, lateral spreading, landsliding or fault rupture. Hydraulic and hydrologic assessments and related project design criteria were provided by ESA. There are no structures planned, so California Building Code seismic design criteria are not needed for the enhancement design. Our scope did not include environmental services relating to chemical constituents that could possibly be present at the site in soil, construction materials, groundwater or air.

This geotechnical report was prepared subject to the understanding that we will have the opportunity to: 1) provide geotechnical consultation when needed to check that our recommendations are appropriately implemented in planning and design documents; and 2) provide support during construction to facilitate design changes in the event that subsurface conditions differ from those anticipated.

2. METHODS OF INVESTIGATION

2.01 Review of Existing Information

A variety of published sources were reviewed to evaluate geotechnical data relevant to the subject sites. These sources included geotechnical and geologic literature, reports, and maps published by various public agencies including United States Geological Survey, California Geological Survey (formerly the California Division of Mines and Geology) and Sonoma County. Information obtained from our review of published information is discussed in Section 3, Site Conditions. A list of the references used in our investigation is presented in Section 7, References, at the end of this report.

2.02 Site Reconnaissance Visits

We conducted multiple site reconnaissance visits at each site at various times over the years (2014 through 2019). During these visits, we observed the surficial conditions at the site, took photographs, checked for obvious geotechnical/geologic concerns, assessed accessibility and identified locations for subsurface explorations. Observations made during our site reconnaissance visits are discussed in Section 3.02, Surface Conditions.

2.03 Geophysical Surveys

Seismic refraction (SR) surveys and multichannel analysis of surface waves (MASW) soundings were performed by A3GEO’s subconsultant, NORCAL Geophysical Consultants, Inc. (NorCal) from Cotati, California.

Overall, nine SR surveys and six MASW soundings were performed at the subject sites. The SR surveys and MASW soundings for Sites 2A, 2B, and 4D were performed between June 27 and July 5, 2016. The SR surveys and MASW soundings for Site 4C were performed on July 3, 2019. The locations of the SRs and MASWs performed are shown on the attached Site Plans, Figures 1 through 3.

For correlation purposes, each MASW sounding performed was also located at a SR survey location. The intent of the investigation was to survey locations where enhancement features (such as embedded or driven log structures) were planned. If the enhancement locations where inaccessible, then the surveys were performed in the nearest accessible area, usually at the top of the creek bank.

Seismic refraction and MASW are non-invasive, surficial, geophysical methods used to determine velocity characteristics of near-surface material. Seismic refraction measures primary waves (P-waves), while MASW soundings measure shear waves (S-waves). The velocities are then used to interpret the physical properties (e.g., density and hardness) of the materials. Unlike P-wave velocities, S-waves are not affected by saturation of the subsurface materials. The survey methods involve placing a continuous line of geophones on the ground surface and recording the arrival of P- or S-waves, which are induced into the ground by a hammer striking a steel plate.

NORCAL’s 20162 and 2019 Seismic Refraction/MASW Investigation Reports are presented in Appendices C and D, respectively. NORCAL’s reports include: 1) additional information about the survey methodology and procedures, 2) interpreted p-wave velocity profiles for the nine seismic refraction surveys, and 3) interpreted s-wave velocity profiles for the six MASW soundings.

2 NORCAL’s 2016 report includes Dry Creek Sites that are not part of this Project phase (i.e., Sites in addition to 2A/B and 4C/D).

The surveys performed at each site are summarized in the following table.

Table 2 –Geophysical Survey Summary

Site Survey ID Type of Seismic

Wave Measured

Approximate Elevation3 of the Existing Ground Surface at the Survey

Location (feet)

Approximate Depth of

Exploration (feet)

Year Performed

2A

SR 2A-1 p-wave 94 50 2016

SR 2A-2 p-wave 77 30 2016

MASW 2A-1 s-wave 94 60 2016

MASW 2A-2 s-wave 77 60 2016

2B

SR 2B-1 p-wave 95 50 2016

SR 2B-2 p-wave 76 25 2016

SR 2B-3 p-wave 77 - 80 25 2016

MASW 2B s-wave 77 60 2016

4C

SR-4C-1 p-wave 103 50 2019

SR-4C-2 p-wave 104 30 2019

MASW 4C-1 s-wave 103 100 2019

MASW 4C-2 s-wave 104 60 2019

4D

SR 4D-1 p-wave 118 50 2016

SR 4D-2 p-wave 120 50 2016

MASW 4D s-wave 120 60 2016

2.04 Borings

A total of eight exploratory borings were drilled for the Project. Three borings were drilled in 2015 (Site 2A) and five borings were drilled in 2019 (Sites 2B, 4C and 4D). The intent of the borings was to drill where design elements are planned (such as embedded/driven log structures and bank stabilization areas). If the enhancement locations were inaccessible, then the borings were drilled in the nearest accessible area, usually at the top of the creek bank.

Three borings at Site 2A (B-2A-1, B-2A-2, and B-2A-3) were drilled on October 12, 2015. The borings were drilled primarily to aid in the design of the geotechnical bank stabilization along the northern side of the tributary that flows east into the main Dry Creek channel at River Station 59+00. A3GEO subcontracted with Clear Heart Drilling of Santa Rosa, California, to advance the three exploratory borings at the approximate locations shown on the Site Plan (Figure 1). All three borings were drilled with a truck-mounted drill rig using hollow stem auger drilling equipment. Borings B-2A-1 and B-2A-3 were located near the top of the failed bank and were advanced to depths of 36.5 and 31.5 feet, respectively. Boring B-2A-2 was located 24 feet to the northwest of boring B- 2A-1 (away from the slope) and extended to a depth of 31.5 feet. The boring locations indicated on the Site Plan (Figure 1) were surveyed by ESA upon completion.

One boring was drilled at Site 2B (B-2B-1) on November 27, 2019. Two borings were drilled at Site 4C (B-4C-1 and B-4C-2) on September 20, 2019, and two borings were drilled at Site 4D (B-4D-1 and B-4D-2) on September 23, 2019. A3GEO subcontracted with Woodward Drilling of Rio Vista, California, to advance these borings at the approximate locations shown on the Site Plans (Figures 2 and 3). All five borings were drilled with a track-mounted drill rig using hollow stem auger drilling equipment. Latitude/Longitude coordinates of the boring locations were recorded in the field using a Google Earth application from a mobile smartphone and

3 Datum: NAVD 88 should be considered approximate. The coordinates of the locations were also cross-referenced by measuring from existing site features. A summary of the geotechnical borings drilled at the Project Sites is presented in the table below.

Table 3 – Boring Summary

Site Location

ID

Surface Elevation4 (feet)

Boring Depth (feet)

Bottom of Boring Elevation (feet) Date Performed

2A B-2A-1 95.5 36.5 59.0 10-12-2015 B-2A-2 95.5 31.5 64.0 10-12-2015 B-2A-3 95.5 31.5 64.0 10-12-2015

2B B-2B-1 77 41.5 35.5 11-27-2019

4C B-4C-1 103 36.3 66.7 9-20-2019 B-4C-2 104 31.5 72.5 9-20-2019

4D B-4D-1 107 25.2 81.8 9-23-2019 B-4D-2 119 25.2 93.8 9-23-2018

Generally, soil and rock samples were collected from each boring at approximately five-foot intervals or less using a 2-inch outside diameter (O.D.) Standard Penetration Test (SPT) sampler without liners and a 3-inch O.D. Modified California samplers with liners. The samplers were driven using an automatic 140-pound hammer falling approximately 30 inches. The hammer blows required to drive the sampler the final 12 inches of each 18-inch drive are presented on the boring logs. Where a full 12-inch drive could not be achieved, the number of blows and amount of penetration achieved is shown. Sampler blow counts (in blows per foot) obtained using the SPT sampler correspond to SPT N-values. The Modified California sampler blow counts shown on the logs have been adjusted by a factor of 0.63 to account for the larger sampler end area (Adjusted N-Values).

During drilling, an A3GEO engineer visually/manually classified the soil in general accordance with ASTM D2488 classifications, which are based on the Unified Soil Classification System (USCS). Field classifications were subsequently checked and revised, where appropriate, based on laboratory test data. The logs of the borings are attached in Appendix A and are preceded by: 1) a Key to Exploratory Boring Logs that describes the USCS and the symbols used on the logs, and 2) a Key to Rock Descriptions. Upon completion of drilling, the boreholes were backfilled with grout, with the exception of the borings at Site 4D (B-4D-1 and B-4D-2), which were backfilled with bentonite chips due to the close proximity of the existing groundwater wells.

The attached boring logs represent our interpretation of the subsurface materials at the boring locations at the time of drilling. The passage of time may result in changes in the subsurface conditions. A summary of our findings from our subsurface exploration can be found in Section 3.03, Subsurface Conditions.

4 Datum: NAVD 88

2.05 Geotechnical Laboratory Testing

Samples from the borings were examined to check field classifications, assign laboratory tests and interpret geologic units. An A3GEO engineer examined the samples, edited the field versions of the logs and supervised the preparation of the final logs presented in Appendix A.

Our geotechnical laboratory testing program was directed toward a quantitative and qualitative evaluation of the physical properties of the soil that underlie the site. The following geotechnical laboratory tests were performed:

Atterberg limits per ASTM Test Designation D-4318;

Dry density per ASTM Test Designation D-2937;

Water content per ASTM Test Designation D-2216 Particle Size Distribution per ASTM Test Designation D-422; and Percent passing the #200 sieve per ASTM Test Designation D-1140.

The results of the tests are presented on the attached boring logs at the corresponding sample depths. The laboratory test data sheets are included in Appendix B.

3. SITE CONDITIONS

3.01 Local Geology

The USGS mapping generally shows lower Dry Creek near the west side of an alluvium-filled valley flanked on both sides by bedrock mountains/hills. The surficial alluvial deposits within the vicinity of the Project Sties are mapped by the USGS (Graymer and others, 2006) as Late Holocene, which signifies deposits interpreted as less than about 3,000 years old. The alluvium comprising the Dry Creek channel and banks are mapped by the USGS as Holocene (less than about 11,000 years old). Bedrock is present at or near the ground surface within the local hills/mountains and typically underlies alluvium on local valley floors.

USGS Geologic Maps of the areas surrounding Sites 2A/2B and 4C/4D are presented on Plates 3 and 4, respectively. As shown on Plate 3, Sites 2A/2B are located about 2,000 feet from the base of the bedrock hills mapped to the west. Sites 4C/4D are located at the base of the Franciscan Complex bedrock hills along the western edge of the valley floor (Plate 4). The Franciscan Complex bedrock at Site 4D is mapped approximately 50 to 150 feet southwest of the top of the creek banks.

3.02 Surface Conditions

3.02.1 Site 2A

Site 2A is approximately 700 feet long by 400 feet wide, spans both sides of the main channel of Dry Creek, and includes a bank repair on the unnamed tributary flowing east into the main channel at River Station 59+00.

Existing and proposed ground surface elevations, general topography and planned restoration areas at Site 2A are shown on the Site Plan (Figure 1).

Localized changes over time at Site 2A are shown in the sequences of historical aerial photographs presented on Plate 5. Comparisons between 1980, 1995, 2000 and 2003 photographs illustrate changes in the low-flow stream course, which has generally migrated towards the west. The 2008 aerial photographs show a nearly-full main stream channel under high-flow conditions. The 2013 photographs show dense vegetation present within the channel, which is in stark contrast to the mostly unvegetated channel shown in the 1980 photographs.

Top of bank elevations on the right and left sides of the creek are El. 95 and El. 94, respectively. The main channel thalweg is between El. 67 and El. 69. The tributary thalweg ranges in elevation from approximately El.

72 to El. 78 in the vicinity of the failed bank. The banks of the main channel are between 17 and 20-feet-high with inclinations between about 1.5H:1V and 2.2H:1V (horizontal to vertical). Generally, the banks are densely vegetated. On both sides of the main channel, wide terraces (100 or more feet wide) have formed between El.

75 and El. 80. As shown on the top photograph on Plate 6, large amounts of gravel deposits are visible on the terraces at low flow. The vantage point and direction the photo was taken is identified with a green arrow on the Site Plan, Figure 1. At high flows, the terraces are under water.

Site 2A includes a 75-foot-wide bank failure on the northern side of the tributary about 100 feet upstream of the tributary’s confluence with Dry Creek (Figure 1). The failed bank is approximately 20 feet high and is at an inclination of approximately 1.4H:1V (horizontal to vertical), with some portions near vertical. Two photographs on Plate 7, taken in March 2014 and February 2015, show the failed area from the same vantage point (top of bank looking southwest). The vantage point and direction the photo was taken is identified with a green arrow on the Site Plan, Figure 1. Sloughing and erosion are evident in the 11 months between the times that the two photographs were taken.

Large slabs of reinforced concrete (approximately 2 to 7 feet long, approximately 8 to 12 inches thick), are present at the base of the failed bank and along the face of the slope immediately upstream. The concrete slabs can be seen in the photos on Plate 7. The slabs at the bottom of the slope appear to have once been on the slope face (presumably as armoring), and have since slid down to their current position. The failed slope had little to no vegetation at the time of our investigation. The exposed soil within the bank consists of very loose sandy silt with little to no cohesion. There is a large oak tree at the top of the bank with large roots exposed on the slope. It is our understanding that this tree will remain in place during and after construction.

3.02.2 Site 2B

Site 2B is approximately 500 feet long by 200 feet wide and is located on the left side of the main channel of Dry Creek from about Station 69+00 to 64+00. Existing and proposed ground surface elevations, general topography and planned restoration areas at Site 2B are shown on the Site Plan (Figure 2). The main channel thalweg is between El. 69 and El. 71. Top of bank is at El. 95. The banks at Site 2B are between 15 and 20-feet-high with inclinations between about 1.8H:1V and 3H:1V (horizontal to vertical). Generally, the banks are densely vegetated. A wide terrace (approximately 100 feet wide) has formed between about El. 75 and El. 80.

Large amounts of gravel deposits are visible on the terraces at low flow, which can be seen on the bottom photograph on Plate 6. The vantage point and direction the photo was taken is identified with a green arrow on the Site Plan, Figure 2. At high flows, the terraces are under water.

3.02.3 Site 4C

Site 4C [outlined in blue on the Site Plan (Figure 3)] is approximately 900 feet long by 250 feet wide and is located on the left side of the main channel of Dry Creek from about Station 216+00 to 207+00. Existing and proposed ground surface elevations, general topography and planned restoration areas are shown on the Site Plan. The main channel thalweg is between El. 95 and El. 97. Top of bank is at El. 115. The banks at Site 4C are between 9 and 13-feet-high with inclinations between about 1.6H:1V and 3H:1V (horizontal to vertical).

Generally, the banks are densely vegetated. A wide terrace (100 feet or more wide) between the main channel and the banks has formed between El. 102 and El. 105. A few open grassy meadows currently exist on the terrace, which can be seen in the top photograph on Plate 8. The vantage point and direction the photo was taken is identified with a green arrow on the Site Plan, Figure 3. The photograph on Plate 8 also shows the creek bank at Site 4C.

3.02.4 Site 4D

Site 4D [outlined in green on the Site Plan (Figure 3)] is approximately 900 feet long by 250 feet wide and is located on the right side of the main channel of Dry Creek from about Station 213+00 to 204+00. Existing and proposed ground surface elevations, general topography and planned restoration areas are shown on the Site Plan. The main channel thalweg is between El. 95 and El. 97. Top of bank is at El. 119. The banks at Site 4D are between 15 and 20-feet-high with inclinations between about 1.3H:1V and 3.5H:1V (horizontal to vertical).

Generally, the banks are densely vegetated. A wide terrace (100 feet or more wide) between the main channel and the banks has formed between El. 102 and El. 105. At the downstream end of Site 4D, the terrace narrows as the main channel of the creek meanders in a southerly direction. An existing side channel, approximately 25 feet wide, was visible during our investigation in September 2019 on the western side of the terrace adjacent to the bank. The channel appears to connect to the main Dry Creek channel at the southern end of the site, and has a length of approximately 500 feet. This side channel is visible in the bottom photograph on Plate 8. Also visible in this photo is a black corrugated pipe, likely used as an outfall for drainage from the vineyard. The vantage point and direction the photo was taken is identified with a green arrow on the Site Plan, Figure 3.

3.03 Subsurface Conditions

3.03.1 Overview

The materials encountered in the borings and interpreted from the geophysical surveys are consistent with those anticipated based on published geologic mapping. In general, the Sites are underlain by sandy/gravelly alluvial deposits over bedrock. At Sites 2A, 2B and 4C, bedrock is interpreted to be more than 40 feet below the top of the existing creek banks and about 20 to 40 feet below the existing channel corridor. At Site 4D, bedrock is shallower; bedrock at Site 4D is interpreted to be about 15 feet below the top of the existing creek banks and about 3 to 20 feet below the existing channel corridor.

Figures 4 through 7 present interpretive geologic cross sections5 based on the materials encountered in the borings and interpreted from the geophysical surveys. The locations of each cross section are identified on the Site Plans (Figures 1 through 3). The following table summarizes our interpretation of soil and bedrock consistency based on the s-wave velocity measurements obtained from the MASW geophysical surveys.

Table 4 – Summary of S-wave Velocity Measurements

Category S-wave Velocity (ft/sec) Interpreted Material Type

Low < 600 Soft / Loose Soil

Moderate to High 600 to 1,000 Stiff Soil, Saturated Sediments or Highly Weathered Bedrock

High > 1,000 Bedrock

General information on the subsurface materials and groundwater conditions encountered during our subsurface investigations are discussed in more detail in the following sections. Additional information is presented in Appendices A through D. Logs of the borings and detailed soil descriptions are included in Appendix A. Laboratory test results are included in Appendix B. The results of the geophysical investigations are presented in NORCAL’s reports (Appendices C and D).

3.03.2 Sites 2A and 2B

Interpretive Cross Sections A-A’, B-B’ and C-C’ (Figures 4 through 6) illustrate the generalized stratigraphy at Sites 2A and 2B. The locations of the cross sections are shown on the Site Plans, Figures 1 and 2. Based on the data collected for this investigation, both sites are interpreted to be underlain by about 25 to 50 feet of sand and gravel over bedrock. The sand and gravel has been divided into an upper and lower layer. The upper layer generally consists of loose to medium dense sand and gravel (SM, SP and GW) with trace to little fines, and the lower layer generally consists of medium dense to dense sand (SP, SW, SP-SM, SW-SM) with few fines. Grain size distributions and soil classifications [per the Unified Soil Classification System (USCS)] obtained from laboratory tests performed on samples collected from the borings, and one bulk sample collected on the terrace at Site 2B, are shown in Table 5 on the following page.

The upper 6 to 10 feet of material adjacent to the failed bank at Site 2A is interpreted as artificial fill. Boring B- 2A-3 encountered a concrete obstruction at a depth of approximately 2 feet and some concrete fragments at a depth of approximately 5 feet. Because of the history of gravel mining in the area, artificial fill may be found in other areas as well.

5 Cross sections were developed using very limited data and actual conditions may vary significantly from what is shown.

Table 5 – Summary of Grain Size Distribution Tests - Sites 2A and 2B

The depth to bedrock at Sites 2A and 2B was interpreted from the MASW surveys; none of the borings encountered bedrock. The interpreted depths and elevations at which bedrock was encountered (i.e., s-wave velocity > 1,000 feet/sec) at each survey location within Site 2A and 2B is summarized in the table below.

Table 6 –Summary of Interpreted Depth to Bedrock at Surveyed Locations - Sites 2A and 2B

Interpreted Depth to Bedrock (feet below the ground surface)

Interpreted Elevation of Bedrock (feet)

2A

MASW 2A-1* 41 52

MASW 2A-2 31 46

2B MASW 2B 38 39

* Surveys performed at elevations above planned restoration areas due to site inaccessibility (i.e., dense vegetation).

Groundwater was encountered during drilling at each of the boring locations. A summary of groundwater levels observed at each boring location is provided below and is illustrated on the cross sections.

Table 7 – Observed Groundwater Levels - Sites 2A and 2B

Site Location Date

Groundwater Levels Observed During Drilling

Groundwater Levels Observed After Drilling

Depth (feet) Elevation (feet)

Time after Drilling

Depth (feet)

Elevation (feet)

2A

B-2A-1 10-12-2015 25.5 70.0 1 hour 25.5 70.0

B-2A -2 10-12-2015 25.5 70.0 0.5 hour 20.8 74.8

B-2A -3 10-12-2015 25.0 70.5 Immediately 25.0 70.5

2B B-2B-1 11-27-2019 4.5 72.5 Immediately 4.5 72.5

These groundwater levels do not represent stabilized groundwater conditions due to the effects of the drilling process and the limited time available for water level stabilization before borehole backfilling. Additionally, because the measurements were taken during the relatively dry, end-of-summer period, we anticipate that groundwater levels at the site would fluctuate due to rainfall, water levels in the creek, distance from the creek, and perhaps irrigation of the surrounding areas. We expect that groundwater levels could rise to near top of bank elevation during the winter and/or high-flow months.

3.03.3 Site 4C

Site 4C subsurface conditions are very similar to Sites 2A and 2B. Interpretive Cross Section D-D’ (Figure 7) illustrates the generalized stratigraphy at Site 4C. The location of the cross section is shown on the Site Plan, Figure 3. Based on the data collected for this investigation, the site is interpreted to be underlain by about 20 to 50 feet of sand and gravel over bedrock. The sand and gravel has been divided into three layers. The upper layer generally consists of loose to medium dense sand and gravel (SP, SM, GM, SW-SM, GW-GM) with few to little fines; the middle layer generally consists of medium dense to dense sand (SP, SW-SM) with trace fines, and the lower layer generally consists of dense to very dense sand (SP-SC) with few fines. Grain size distributions and soil classifications [per the Unified Soil Classification System (USCS)] obtained from laboratory tests performed on samples collected from the borings are shown in the table below.

Table 8 – Summary of Grain Size Distribution Tests – Site 4C

The depth to bedrock at Site 4C was interpreted from the MASW surveys and the borings. Boring B-4C-1 encountered bedrock at the bottom of the boring (at 36 feet). Boring B-4C-2 terminated at 31.5 feet below the ground surface and did not encounter bedrock. The interpreted depth to bedrock from the MASW survey (i.e., s-wave velocity > 1,000 feet/sec) and the depth to bedrock encountered in Boring B-4C-1 coincide exactly. The interpreted depths and elevations at which bedrock was encountered at each survey and boring location within Site 4C is summarized in the table below.

Table 9 –Summary of Interpreted Depth to Bedrock at Surveyed Locations - Site 4C

Interpreted Depth to Bedrock (feet below the ground surface)

Interpreted Elevation of Bedrock (feet)

4C

MASW 4C-1 36 67

MASW 4C-2 34 70

B-4C-1 36 67

Groundwater was encountered during drilling at each of the boring locations. A summary of groundwater levels observed at each boring location is provided below and is illustrated on the cross sections.

Table 10 –Observed Groundwater Levels - Site 4C

Groundwater Levels Observed During Drilling

Groundwater Levels Observed After Drilling

Depth (feet) Elevation (feet)

Time after Drilling

Depth (feet)

Elevation (feet)

4C B-4C-1 9/20/2019 4.0 99.0 Immediately 4.5 98.5

B-4C-2 9/20/2019 4.5 99.5 Immediately 4.5 99.5

These groundwater levels do not represent stabilized groundwater conditions due to the effects of the drilling process and the limited time available for water level stabilization before borehole backfilling. Additionally, because the measurements were taken during the relatively dry, end-of-summer period, we anticipate that groundwater levels at the site would fluctuate due to rainfall, water levels in the creek, distance from the creek, and perhaps irrigation of the surrounding areas. We expect that groundwater levels could rise to near top of bank elevation during the winter and/or high-flow months.

3.03.4 Site 4D

At Site 4D, bedrock was encountered at shallower depths than at Sites 2A, 2B, and 4C; bedrock at Site 4D is interpreted to be about 15 feet below the top of the existing creek banks and about 3 to 20 feet below the existing channel corridor. In addition, the creek banks at Site 4D contained a 9 foot thick layer of fine-grained material (i.e., clay and silt) which was not encountered at the other Sites (Boring B-4D-2). This material is likely colluvium from the adjacent hillside.

Interpretive Cross Section D-D’ (Figure 7) illustrates the generalized stratigraphy at Site 4D. The location of the cross section is shown on the Site Plan, Figure 3. Based on the data collected for this investigation, the site is interpreted to be underlain by about 3 to 25 feet of soil over bedrock. As shown on Figure 7, the soil profile at Site 4D has varied layers on either side of the proposed backwater alcove. The channel corridor generally consists of loose to medium dense sand (SP, SC, SW) with trace fines over medium dense to dense sand (SP, SW-SM) with trace fines over bedrock; the creek banks generally consist of medium dense clayey sand (SC) over stiff/medium dense sandy lean clay to clayey sand (CL/SC) over bedrock. Fines contents on the clayey material within the banks ranged from 51 to 68 percent; an Atterberg Limits test on this material resulted in a liquid limit (LL) of 33 and plasticity index (PI) of 13. Grain size distributions and soil classifications [per the Unified Soil Classification System (USCS)] obtained from laboratory tests performed on samples collected from the borings are shown in the table below.

Table 11 – Summary of Grain Size Distribution Tests – Site 4D

The depth to bedrock at Site 4D was interpreted from the MASW surveys and the borings. The interpreted depths and elevations at which bedrock was encountered (i.e., s-wave velocity > 1,000 feet/sec) at each survey and boring location within Site 4D is summarized in the table below.

Table 12 –Summary of Interpreted Depth to Bedrock at Surveyed Locations - Site 4D

Interpreted Depth to Bedrock (feet below the ground surface)

Interpreted Elevation of Bedrock (feet)

4D

SR 4D-1* 18 – 19 99 - 100

SR 4D-2* 9 – 19 101 - 111

MASW 4D* 12 108

B-4D-1 15 92

B-4D-2* 15.5 103.5

* Surveys performed at elevations above planned restoration areas due to site inaccessibility (i.e., dense vegetation).

The bedrock encountered at Site 4D is interpreted as Franciscan Complex including both sedimentary and volcanic rocks, which is mapped approximately 50 to 150 feet southwest of the top of the creek banks (Plate 4).

The bedrock typically slopes downwards toward the creek and away from the hills, which can be seen from the Site 4D and 4C exploration locations (Figure 7). The bedrock encountered in the borings was logged as moderately weathered, moderately hard, moderately strong and crushed to intensely fractured. The Site 4D MASW survey indicates maximum s-wave velocities approaching 1,800 ft/sec within the upper portion of the bedrock.

Groundwater was encountered during drilling at one of the boring locations within Site 4D. A summary of groundwater levels observed at each boring location is provided below and is illustrated on the cross sections.

Table 13 –Observed Groundwater Levels - Site 4D

Groundwater Levels Observed During Drilling

Groundwater Levels Observed After Drilling

Depth (feet) Elevation (feet)

Time after Drilling

Depth (feet)

Elevation (feet)

4D B-4D-1 9/23/2019 8.0 99.0 Immediately 8.0 99.0

B-4D-2 9/23/2019 Not Encountered --- Not Encountered --- ---

At Boring B-4D-2, located on top of the bank, groundwater was not encountered. These groundwater levels do not represent stabilized groundwater conditions due to the effects of the drilling process and the limited time available for water level stabilization before borehole backfilling. Additionally, because the measurements were taken during the relatively dry, end-of-summer period, we anticipate that groundwater levels at the site would fluctuate due to rainfall, water levels in the creek, distance from the creek, and perhaps irrigation of the surrounding areas. We expect that groundwater levels could rise to near top of bank elevation during the winter and/or high-flow months.

4. EVALUATIONS AND CONCLUSIONS

4.01 General

We understand that the objectives of this project include enhancing the stability, erosional resistance and aquatic habitat within the immediate vicinity of the bank failure at Site 2A and the proposed restoration areas at Sites 2A, 2B, 4C and 4D. We conclude these objectives can reasonably be achieved under normal conditions, with the understanding that lesser performance may result under rarer events such as extreme flooding or a large earthquake. Seismic slope stability investigations, conclusions and recommendations pertaining to earthquake shaking, soil liquefaction, lateral spreading and other types of geologic hazards were outside our authorized scope of services.

4.02 Site 2A Bank Repair

4.02.1 Feasibility

Based on our review of available information and our onsite observations, we conclude that the repair and stabilization of the failed bank at Site 2A is generally feasible from a geotechnical standpoint. We presently interpret that the slope failure we observed is relatively shallow. We did not observe any direct obvious evidence of deep-seated global instabilities that would preclude the construction of conventional earthwork-type repairs. In this report, we present recommendations for the design and construction of the bank repair judged appropriate for the site conditions, as presently interpreted. Our recommendations include the provision that we provide geotechnical engineering services during the construction of the stabilization project, in order to check geotechnical design assumptions and verify that the construction is appropriate for the site conditions encountered.

4.02.2 Bank Repair Cross Section

Multiple bank stabilization alternatives were developed jointly between ESA and A3GEO. The slope restoration alternatives considered various restoration techniques, including use of a geogrid reinforced slope, vegetated soil lifts (VSLs) and rock slope protection. These options were presented to the Sonoma County Water Agency (SCWA) and various agencies for evaluation, and ultimately a preferred restoration scheme was selected, the general concept of which is shown below in the cross section prepared by ESA.

The restored section features front-slope inclinations between 1.7H:1V and 1.3H:1V (horizontal to vertical) and back-slope inclinations between 1.7H:1V and 1.5H:1V. As shown in the preceding section, the bank repair involves several different geotechnical elements serving various purposes:

Rock Slope Protection (RSP) – Below El. 87, the outboard face of the bank repair is comprised of two layers of rock sized material (total of 3-foot minimum thickness) to resist erosion under frequent low-flow events. The elevation at the top of the RSP is the Q100 water surface elevation determined by ESA. The RSP will be placed using Caltrans “Method A”, which utilizes individual placement by equipment to achieve “3-point bearing” (no wobbling) on adjacent rocks. The RSP is anchored (keyed) into the channel while utilizing the existing concrete slabs as a buttress to ensure adequate bearing and resist sliding. The intent is to keep all concrete in place and not move or remove any of the slabs. We recommend a 4 foot deep, 4 foot wide keyway. Excavation for the keyway is discussed in greater detail in Section 5.03, Bank Repair Construction.

Vegetated Soil Lifts (VSLs) – Above El. 87, the outboard face of the bank repair is comprised of VSLs designed to resist erosion during infrequent high-flow events. The VSLs consist of compacted soil surrounded on the bottom, outer face, and top by a double layer coir fabric. The VSLs include a planted slope face that enhances erosion resistance. Live cuttings will be placed between the VSLs.

Backing Material – Behind the layer of RSP, the bank repair contains a minimum 8-inch thick layer of Backing Material. The purpose of the Backing Class Material is to serve as a filter blanket, which is a layer of material placed between the RSP and the Class 2 Permeable Material to prevent migration of materials through the RSP.

Class 2 Permeable Material – A vertical drainage layer comprised of Caltrans Class 2 Permeable Material fills the space between the back of the VSLs, the Backing Material, and the adjacent natural ground. Class 2 Permeable Material is engineered to resist the migration of fines (silt and clay particles) and thereby remain relatively free-draining. In this application, most of the groundwater flowing from the natural ground will be intercepted by the vertical drainage layer and drain (by gravity) out of the bank repair through the Backing Material and the RSP.

4.02.3 Slope Stability Evaluation

We analyzed the static stability of the proposed bank repair cross section using commercially-available slope stability analysis software (SLIDE, version 6.039 by Rocscience, Inc.); key inputs to the program include soil shear strength, soil unit weight and groundwater surface. A simplified analytical cross section was developed to model the planned bank repair using the following parameters.

Material Properties

Material Unit Weight (lbs/ft3)

Cohesion (lbs/ft2)

Friction Angle (degrees)

RSP 165 0 45

VSLs 120 50 30

Backing Material & Class 2 Permeable Material 140 0 40

Upper Bank Deposits 120 50 30 Lower Bank Deposits 125 0 35

Concrete Slabs 150 0 45

The properties of native materials within the natural (unfailed) bank were estimated based on: 1) the subsurface data obtained from the borings and laboratory tests performed at Site 2A (Appendix A and B); and 2) back-calculation performed using the SLIDE software based on the existing slope geometry which is known to have a factor of safety of at least 1.0. The SLIDE output plots of the back-calculation of material properties are included in Appendix E. The properties of VSLs were assigned the same parameters as the Upper Bank Deposits;

corresponding recommendations for VSL fill materials are presented in a subsequent section of this report.

Both the temporary and the restored slope conditions were analyzed with various slope inclinations and water levels. The temporary conditions were analyzed with a “low” groundwater level and a maximum slope inclination of 1.5H:1V. The restored conditions were analyzed with a “low” and a “high” groundwater level and a maximum slope inclination of 1.3H:1V. The “low” groundwater case was modeled with a uniform (horizontal) water surface coincident with the bottom of the keyway (assuming the keyway would be dewatered during construction if water was present). The “high” groundwater case was modeled with a uniform (horizontal) water surface at El.

87 within the channel and slope (El. 87 corresponds to the Q100 water surface elevation).

For each analysis case, the slope’s minimum static factor of safety (FS) against sliding was calculated. The FS is commonly defined as the sum of the forces resisting movement divided by the sum of the forces driving movement. A slope with a FS greater than unity (1.0) is theoretically stable. The results of our analyses are tabulated in the tables below, and graphical results are presented in Appendix E.

Temporary Slope Conditions (“Low” Groundwater Level)

Inclination FS (min)

1.7H:1V 1.25 1.5H:1V 1.13

Restored Slope Conditions

Inclination GW Level

FS

(min)

1.7H:1V Low 1.55 High 1.43

1.3H:1V Low 1.40 High 1.30

The results of our slope stability analyses generally show the slopes to be stable under static (i.e. non-earthquake) conditions for both the temporary and restored conditions. Typical minimum design static factors of safety for permanent slopes range from about 1.25 to 1.5 for situations where the cost of slope repair is comparable to the cost of construction and the risk to human life and property is small (Duncan et al., 1987). As summarized in the table above, our analysis indicates factors of safety of 1.3 or greater are achievable for the restored conditions and the factors of safety are judged to be adequate for the project conditions (e.g., risk to human life and property is small). Factors of safety of 1.13 and 1.25 were calculated for the temporary conditions which indicate that temporary cut slopes in native materials should be kept at 1.5H:1V or flatter unless engineered shoring is installed (see Section 4.04.4 for additional information regarding temporary cut slopes and shoring).

4.03 Stability of Channel Cut Slopes

We analyzed the static stability of the proposed 2H:1V backwater alcove channel slopes using commercially-available slope stability analysis software (SLIDE, version 6.039 by Rocscience, Inc.). A simplified analytical cross section was developed to model the slopes using the following parameters.

Channel Material Properties

Material Unit Weight (lbs/ft3)

Cohesion (lbs/ft2)

Friction Angle (degrees)

Loose to medium dense sands 120 0 35

The properties of materials within the slopes were estimated based on the subsurface data obtained from the borings and laboratory tests performed for the project (Appendices A and B). The slopes were analyzed with a “low” and a “high” groundwater level. The “low” groundwater case was modeled with a uniform (horizontal) water surface with 2 feet of water in the channel. The “high” groundwater case was modeled with a uniform (horizontal) water surface with the channel full.

For each analysis case, the slope’s minimum static factor of safety (FS) against sliding was calculated. The FS is commonly defined as the sum of the forces resisting movement divided by the sum of the forces driving…