DR4344-284_Napa_Milliken_Geotechnical_Eval_Miller_Pacific_3426.001rpt_2023-05-31.pdf

135 Camino Dorado, Suite 3 Napa, California 94558 T (707) 265-7936 F (415) 382-3450

GEOTECHNICAL EVALUATION

CITY OF NAPA MILLIKEN WATER PIPELINE

HARDENING & PATHWAY IMPROVEMENTS

NAPA, CALIFORNIA

May 31, 2023

Job No. 3426.002

Prepared For:

Sandis 1700 S. Winchester Boulevard, Suite 200 Campbell, California 95008

CERTIFICATION

This document is an instrument of service, prepared by or under the direction of the undersigned professionals, in accordance with the current ordinary standard of care. The service specifically excludes the investigation of polychlorinated byphenols, radon, asbestos or any other hazardous materials. The document is for the sole use of the client and consultants on this project. No other use is authorized. If the project changes, or more than two years have passed since issuance of this report, the findings and recommendations must be updated.

MILLER PACIFIC ENGINEERING GROUP REVIEWED BY

(a California corporation)

Rusty Arend Mike Morisoli Geotechnical Engineer No. 3031 Geotechnical Engineer No. 3031 (Expires 6/30/23) (Expires 1/31/25)

HARDENING & PATHWAY IMPROVEMENTS

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 PROJECT DESCRIPTION

3.0 SITE CONDITIONS

3.1 Geology

3.2 Seismicity

Regional Active Faults

Historic Fault Activity

Probability of Future Earthquakes

3.3 Surface Conditions

3.4 Site Characterization and Laboratory Testing

3.5 Subsurface Conditions

4.0 GEOLOGIC HAZARDS

4.1 Seismic Shaking

4.2 Slope Instability

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1 Earthwork

Excavations

Fill Materials

Fill Placement and Compaction

5.2 Foundations

5.3 Retaining Walls

5.4 Safety Considerations

6.0 SUPPLEMENTAL GEOTECHNICAL SERVICES

7.0 LIMITATIONS

8.0 LIST OF REFERENCES

FIGURE 1 SITE LOCATION MAP

FIGURE 2 REGIONAL GEOLOGIC MAP

FIGURE 3 SITE-SPECIFIC GEOLOGIC MAP

FIGURE 4 ACTIVE FAULT MAP

FIGURE 5 HISTORIC EARTHQUAKE MAP

TABLE 1 DETERMINISTIC PEAK GROUND ACCELERATION FOR ACTIVE FAULTS

TABLE 2 PROBABILISTIC PEAK GROUND ACCELERATION FOR ACTIVE FAULTS

APPENDIX A: PRELIMINARY DESIGN DRAWINGS

APPENDIX B: LABORATORY TEST RESULTS

HARDENING & PATHWAY IMPROVEMENTS

1.0 INTRODUCTION

This report presents the results of our geotechnical evaluation for the City of Napa’s Milliken Water Pipeline Hardening and Pathway Improvements. As shown on the Site Location Map, Figure 1, the project area is located along the south side of Milliken Canyon northeast of the city limits. The upstream/northeastern terminus of the pipeline and access pathway are located at the Milliken Diversion Dam while the downstream/southwestern terminus are located at the Milliken Water Treatment Plant at 630 Chaparral Circle.

Our work was performed in accordance with our Subconsultant Agreement with Sandis authorized on December 12, 2022. The purpose of our evaluation was to assess geologic conditions throughout the proposed project area and to develop geotechnical recommendations and criteria for use in design and construction of the project. The scope of our services includes:

Reviewing published geologic and geotechnical background information.

Site reconnaissance to map surface geology, areas of potential slope instability and the delineation of other critical areas and surface features.

Laboratory testing to evaluate engineering properties related to rock hardness and excavation conditions.

Evaluation and discussion of relevant geologic hazards including seismic shaking, slope instability, and other hazards.

Engineering analyses to develop geotechnical recommendations and design criteria related to seismic design, earthwork, foundations, retaining structures, and other geotechnical-related items.

Preparation of this report which summarizes our site reconnaissance, anticipated subsurface conditions, laboratory testing, evaluation of relevant geologic hazards, and geotechnical recommendations and design criteria.

Issuance of this report completes our initial phase of services. Subsequent phases of work should include geotechnical plan review and observation and testing of geotechnical-related work items during construction.

2.0 PROJECT DESCRIPTION

The project includes replacing approximately 6,500 lineal feet of the Milliken raw water pipeline.

The existing pipeline consists of 16-inch welded steel pipe which conveys water from the diversion dam on Milliken Creek to the Milliken Water Treatment Plant. The pipeline is supported on wooden saddles above grade within a narrow access pathway that exists along the south side of the canyon. The pipeline and pathway were heavily damaged by the 2017 Atlas Fire. Additional damage has also occurred at numerous locations due to fallen trees, erosion, rockfall and instability of the steep slopes above and below the alignment. Therefore, a primary design consideration for the replacement will be to incorporate measures that better protect the pipeline from these hazards.

The portion of the alignment that extends from the treatment plant (near Station 0+00) to the vehicle turnaround area (near Station 31+00) will be replaced with 16-inch ductile iron pipe installed using open trench construction methods. The width of the pathway beyond the turnaround area becomes narrower, thereby limiting access for equipment and feasibility of a direct bury alternative. Therefore, the portion of the pipeline that extends from the turnaround to the diversion dam (Station 31+00 to 65+00) will consist of welded steel pipe installed at grade.

This portion of the pipeline will be located close to the inboard/uphill side. Measures to protect the pipeline include burying it within a containment wall from about Station 52+50 to 55+00 and installing a secondary steel casing over the pipeline in other areas.

The steep terrain and difficult site access represent a significant challenge for construction and operation of the pipeline. The City requires maintaining a minimum pathway width of five feet to provide access for small all-terrain vehicles (ATVs) and other maintenance equipment. New soldier pile retaining walls are planned at numerous locations on the outboard/downhill side of the pathway to achieve this minimum width. The pathway will also be supported on a steel-framed, elevated structure from about 62+90 to 65+50 where widening of the bench/pathway is not feasible. A similar steel-framed bridge will also be installed to replace an existing wood bridge near Station 52+00. The steel structures will be supported on concrete abutments with micropile foundations. The proposed pipeline, pathway and related improvements are shown on the design drawings in Appendix A.

3.0 SITE CONDITIONS

3.1 Geology

The project site lies within the Coast Ranges geomorphic province of California. Regional topography within the Coast Ranges province is characterized by northwest-southeast trending mountain ridges and intervening valleys that parallel the major geologic structures, including the San Andreas Fault System. The province is also generally characterized by abundant landsliding and erosion, owing in part to its typically high levels of precipitation and seismic activity.

The oldest rocks in the region are the sedimentary, igneous, and metamorphic rocks of the Jurassic- to Cretaceous-age (190- to 65-million years old) Franciscan Complex. Within Napa County, a variety of sedimentary and volcanic rocks of Tertiary (1.8- to 65-million years old) and Quaternary (less than 1.8-million years old) age locally overlie the basement rocks of the Franciscan Complex. Tectonic deformation and erosion during late Tertiary and Quaternary time

(the last several million years) formed the prominent coastal ridges and intervening valleys typical of the Coast Ranges province. The youngest geologic units in the region are Quaternary age (last

1.8 million years) sedimentary deposits, including alluvial deposits which partially fill most of the valleys and colluvial deposits which typically blanket the lower portions of surrounding slopes.

The project site is located along the south side of Milliken Canyon and above Milliken Creek.

Regional geologic mapping by the California Geological Survey shows the creek channel is underlain by stream and undivided alluvial deposits of late Pleistocene to Holocene age (CGS, 2004). The mapping further indicates the areas above the creek channel are underlain by Sonoma Volcanics of Tertiary age. The canyon sidewalls are mapped primarily as mafic flows and breccias while the areas further upslope are mapped as dacite and rhyolite ash flow tuff. A Regional Geologic Map and descriptions of the mapped geologic units are shown on Figure 2.

We completed a detailed site reconnaissance to observe existing conditions, map surface geology and evaluate areas of potential slope instability throughout the project area. The smaller scale mapping allowed for more detailed differentiation of the various subunits of the Sonoma Volcanics throughout the pipeline alignment. We also mapped several debris flow-type landslides that exist within the topographic chutes and ephemeral tributary streams that intersect the pathway and pipeline alignment. The results of our site-specific geologic mapping are presented on the Site Geologic Map, Figures 3-1 to 3-3.

3.2 Seismicity

The project site is located within the seismically active San Francisco Bay Area and will therefore experience the effects of future earthquakes. Earthquakes are the product of the build-up and sudden release of strain along a “fault” or zone of weakness in the earth's crust. Stored energy may be released as soon as it is generated or it may be accumulated and stored for long periods of time. Individual releases may be so small that they are detected only by sensitive instruments, or they may be violent enough to cause destruction over vast areas.

Faults are seldom single cracks in the earth's crust but are typically comprised of localized shear zones which link together to form larger fault zones. Within the Bay Area, faults are concentrated along the San Andreas Fault zone. The movement between rock formations along either side of a fault may be horizontal, vertical, or a combination and is radiated outward in the form of energy waves. The amplitude and frequency of earthquake ground motions partially depends on the material through which it is moving. The earthquake force is transmitted through hard rock in short, rapid vibrations, while this energy becomes a long, high-amplitude motion when moving through soft ground materials, such as Bay Mud.

Regional Active Faults

The California Geological Survey (previously known as the California Division of Mines and Geology), defines a “Holocene-active fault” as one that had surface displacement within Holocene time (the last 11,700 years). CGS mapped various faults in the region as part of their Fault Activity Map of California (CGS, 2010). Many of these faults are shown in relation to the project site on the attached Active Fault Map, Figure 4. The nearest known Holocene-active faults are the Green Valley, Cordelia and West Napa Faults which are located approximately 2.6 kilometers (1.6 miles) east, 8.4 kilometers (5.2 miles) east and 8.0 kilometers (5.0 miles) southwest of the site, respectively1. The Soda Springs Fault is also mapped approximately 3.4 kilometers (2.1 miles) west of the site. However, this fault is characterized by CGS as exhibiting late Quaternary fault displacement and is therefore not considered a Holocene-active fault.

Historic Fault Activity

Numerous earthquakes have occurred in the region within historic times. The results of our USGS earthquake search catalogue indicates that at least ten earthquakes with a Richter Magnitude of

5.0 or larger have occurred within 100 kilometers (62 miles) of the site between 1900 and 2023.

The approximate locations of these earthquakes that occurred before 2014 are shown on the Historic Earthquake Map, Figure 5.

Probability of Future Earthquakes

The site will likely experience moderate to strong ground shaking from future earthquakes originating on any of several active faults in the San Francisco Bay region. The historical records do not directly indicate either the maximum credible earthquake or the probability of such a future event. To evaluate earthquake probabilities in California, the USGS has assembled a group of researchers into the “Working Group on California Earthquake Probabilities” (USGS 2003, 2008, 2013) to estimate the probabilities of earthquakes on active faults. These studies have been published cooperatively by the USGS, CGS, and Southern California Earthquake Center (SCEC) as the Uniform California Earthquake Rupture Forecast, Versions 1, 2, and 3. In these studies, potential seismic sources were analyzed considering fault geometry, geologic slip rates, geodetic strain rates, historic activity, micro-seismicity, and other factors to arrive at estimates of earthquakes of various magnitudes on a variety of faults in California.

Conclusions from the most recent UCERF3 and USGS indicate the highest probability of an earthquake with a magnitude greater than 6.7 originating on any of the active faults in the San Francisco Bay region by 2043 is assigned to the Hayward/Rodgers Creek Fault system. The Rodgers Creek Fault is located approximately 26.5 kilometers (16.5 miles) southwest of the site and is assigned a probability of 33 percent. The San Andreas Fault, located approximately 58.5 kilometers (36.4 miles) southwest of the site, is assigned a 22 percent probability of an earthquake with a magnitude greater than 6.7 by 2043. Additional studies by the USGS regarding the probability of large earthquakes in the Bay Area are ongoing. These current evaluations include data from additional active faults and updated geological data.

3.3 Surface Conditions

Milliken Canyon comprises a deep, steep-walled drainage formed by downcutting of the Milliken Creek channel as it flows southwesterly through the site. Previous site grading appears to have consisted of excavation into the southern canyon wall to create a level bench for the existing

1 Distances to faults referenced herein were estimated using USGS Quaternary Faults and Folds in U.S.

KML file, accessed January 4, 2022.

pathway and pipeline alignment. The pathway width varies from about 10 to 35 feet within the direct bury portion of the alignment (from Station 0+00 to 31+00), and from about 5 to 15 feet throughout most of the above grade portion (from Station 31+00 to 62+00). The bench ends near Station 62+00 and the pathway transitions to a metal-framed structure which terminates at the diversion dam near Station 65+00.

The natural slopes extend several hundred feet above and below the pathway and are typically inclined at about 0.5:1 to 2:1 (horizontal:vertical). The previous grading has resulted in vertical cut slopes along portions of the inboard side of the pathway of up to about 30 feet in height and locally taller in some areas. The pathway is gravel surfaced and drainage is conveyed across the path by sheet flow and several culverts. An approximately 25-foot-long wooden bridge was constructed near Station 52+00 to provide access where the pathway is particularly narrow. The ground surface on either side of the pathway is partially covered by mature trees and other shrubbery with hard volcanic bedrock exposed in most locations.

Retaining walls have been constructed at numerous locations throughout the alignment. These structures are typically less than about three to four feet in height and consist of timber, metal or modular block construction. Many of the walls are leaning or exhibit other evidence of distress.

The existing 16-inch steel pipeline is generally situated near the toe of the cut slope along the inboard side of the path. Damage to the pipeline, retaining walls, trees and other vegetation from the 2017 Atlas Fire is evident throughout the site. Additional damage to the pipeline from impacts due to rock and debris fall is also apparent at numerous locations, as evidenced by the accumulation of soil and rock above or adjacent to portions of the pipe, localized crushing failures, rusting and scaling of the steel pipe surface, separation of joints and installation of pipe clamps as temporary repairs.

3.4 Site Characterization and Laboratory Testing

The narrow pathway and steeply sloping terrain present significant challenges with respect to site access required to explore subsurface conditions. The hard/strong volcanic bedrock underlying the site exacerbates these challenges in that relatively large, powerful equipment would be best suited to drill borings or excavate test pits. The use of portable drilling equipment for test borings was considered. However, the associated costs and risk of encountering slow production rates and/or practical refusal at shallow depths is relatively high using portable equipment due to hard rock conditions. Considering these challenges, and that the materials exposed in the existing cut slopes provide useful subsurface data throughout the majority of the alignment, we relied upon our geologic mapping of the cut slopes and adjacent areas to characterize subsurface conditions.

We also obtained several boulders of the various bedrock materials to perform laboratory testing aimed at evaluating engineering properties related to excavatability. The boulders were sent to Vermeer Corporation’s laboratory in Pella, Iowa where they were cored to produce samples for testing. The cores were tested to measure density, unconfined compressive strength, CERCHAR Abrasivity Index and Vermeer Energy Index. The results of the laboratory testing are summarized below and are presented in Appendix B.

3.5 Subsurface Conditions

Bedrock of the Sonoma Volcanics is exposed at or very near the ground surface throughout the project area. At its southwest end, the alignment parallels the north side of a knoll that extends from the treatment plant up to about Station 5+00. The knoll is composed of mafic lava flows at its base which are typically very hard/strong with flow surfaces dipping gently to the west and northwest. Densely to very densely welded ash flow tuff is visible within the pathway and adjacent slopes from about Station 5+00 to 32+00. The tuff is typically relatively low density, moderately hard and strong and forms natural slopes inclined at about 1:1 while standing vertically in the pathway cuts. The ash flow tuff is brecciated from about Station 10+00 to 17+00 and forms naturals slopes inclined between about 1.5:1 to 2:1.

The remainder of the alignment is primarily underlain by dacite which is typically hard and strong.

The dacite stands vertically in cliff faces and road cuts except where adverse joint orientations exist. An apparent fault zone extends from about station 32+00 to 40+00. Rhyolite does not underlie the pipeline but forms the uppermost portion of the knoll near the treatment plants and the upper portion of the cliff bands above the western half of the site. Debris flow-type landslides intersect the pathway and pipeline alignment at multiple locations. These debris flows are generally within incised channels either containing debris or exhibiting evidence of historic debris flow or avalanches. The debris typically consists of cobble- to boulder-size masses but, in some cases, range to more than about ten feet in maximum dimension.

Groundwater elevations are expected to fluctuate seasonally, and higher groundwater levels and seepage will likely be present during the winter and spring months when seasonal precipitation is generally higher. Zones of perched groundwater may also exist and are more common in areas where the permeability of the subsurface soils is higher (e.g. within shear or fault zones, where the bedrock is more fractured, and at contacts between surficial soils and underlying bedrock).

4.0 GEOLOGIC HAZARDS

This section summarizes our review of commonly considered geologic hazards and discusses their potential impacts on the pipeline. The primary hazards inherent to the site geologic conditions which could affect the proposed development include strong seismic ground shaking and slope instability. These hazards are discussed in further detail in the following sections.

4.1 Seismic Shaking

The site will likely experience seismic ground shaking similar to other areas in the seismically active Bay Area. The intensity of ground shaking will depend on the characteristics of the causative fault, distance from the fault, the earthquake magnitude and duration, and site specific geologic conditions. Estimates of peak ground accelerations are based on either deterministic or probabilistic methods.

Deterministic methods use empirical attenuation relations that provide approximate estimates of median peak ground accelerations. A summary of the active faults that could most significantly affect the planning area, their maximum credible magnitude, closest distance to the center of the planning area, and probable peak ground accelerations are summarized in Table 1. The calculated accelerations should only be considered as reasonable estimates. Many factors (e.g., soil conditions, orientation to the fault, etc.) can influence the actual ground surface accelerations.

Table 1 – Deterministic Peak Ground Accelerations for Active Faults

Fault

Moment Magnitude for Characteristic Earthquake

Closest Estimated

Distance (km)

Median Peak Ground

Acceleration (g)

Median PGA

+1 Std Dev (g)

Green Valley 6.30 2.6 0.39 0.72

West Napa 6.97 8.0 0.30 0.54

Rodgers Creek 7.30 26.5 0.19 0.35

Reference: Abrahamson & Silva, Boore & Atkinson, Campbell & Bozorgnia, and Chiou & Youngs 2008

NGA models using Vs30 = 760 m/s.

Probabilistic Seismic Hazard Analysis analyzes all possible earthquake scenarios while incorporating the probability of each individual event to occur. The probability is determined in the form of the recurrence interval, which is the average time for a specific earthquake acceleration to be exceeded. The design earthquake is not solely dependent on the fault with the closest distance to the site and/or the largest magnitude, but rather the probability of given seismic events occurring on both known and unknown faults.

We calculated the peak ground acceleration for two separate probabilistic conditions, including the two percent chance of exceedance in 50 years (2,475-year statistical return period) and the ten percent chance of exceedance in 50 years (475-year statistical return period). The peak ground acceleration values were calculated utilizing the USGS Unified Hazard Tool. The results of the probabilistic analyses are presented below in Table 2.

Table 2 – Probabilistic Peak Ground Accelerations for Active Faults

Probability of Exceedance

Statistical

Return Period Magnitude Peak Ground

Acceleration (g)

2% in 50 years 2,475 years 6.8 0.90

10% in 50 years 475 years 6.7 0.48

Reference: USGS Unified Hazard Tool accessed on January 16, 2023.

The potential for strong seismic shaking at the project site is high. Due to its proximity and historic rate of activity, the Green Valley Fault presents the highest potential for severe ground shaking.

The significant adverse impact associated with strong seismic shaking is potential damage to structures and improvements.

4.2 Slope Instability

The potential for instability of the steep slopes above and below the alignment represents a significant risk to the pipeline, as evidenced by previous damage that has occurred. Nearly the entire project area is exposed to rock fall and/or toppling hazards as a result of the steeply sloping conditions that are pervasive throughout the site. The debris flows identified during our site reconnaissance also present a risk of damage. While the risk of a large-scale rotational or translational landslide is generally low considering the relatively hard/strong bedrock that underlies the site, raveling/shallow failures may occur and impact the pipeline and pathway.

Fully eliminating risks associated with slope instability is not considered practical due to the significantly high costs that would be required. Therefore, project design efforts should focus on prioritizing measures that reduce these risks while maintaining costs that are compatible with project funding. Mitigation measures will include burying the pipeline below grade from about Station 0+00 to 31+00. In areas where the pipeline is above grade, it will be located as close as possible to the toe of the slope along the inboard side of the alignment. A steel casing or containment wall will also be installed to further protect the above grade portion of the pipeline.

5.0 CONCLUSIONS AND RECOMMENDATIONS

Based on evaluation, we judge that construction of the proposed pipeline is feasible from a geotechnical standpoint. Primary geotechnical considerations include the potential for instability due to the steeply sloping site conditions and potentially difficult excavation conditions due to the hard/strong bedrock. Additional discussion and recommendations addressing these and other considerations are presented in the following sections.

5.1 Earthwork

Earthwork will include excavations of up to about five feet in depth for the open trench portion of the alignment, retaining walls and bridge/pathway abutments. Earthwork should be performed in accordance with the recommendations and criteria outlined in the following sections.

Excavations

Excavations are expected to encounter bedrock consisting of various subunits of the Sonoma Volcanics at or very near the ground surface. Based on our site reconnaissance and laboratory testing, the bedrock is relatively hard/strong and variably weathered. For planning purposes, excavations within the welded ash flow tuff and ash flow tuff breccia should be considered Type B soils in accordance with OSHA Soil Classifications. Dacite and mafic lava flows should be considered OSHA Type A soils. If permanent cut slopes are incorporated into the project, we should provide supplemental recommendations for these cut slopes on a case-by-case basis.

Much of the volcanic bedrock exposed in the existing cut slopes is relatively hard/strong and will be difficult to excavate. Smaller and less powerful equipment will also likely be required due to the very difficult site access. We anticipate that much of the volcanic rock will require specialized techniques or equipment to excavate such as rock saws, jackhammers or hydraulic breakers.

Such conditions can significantly impact productions rates and overall costs for the project.

Therefore, we recommend inclusion of a line item and clear definition for “hard rock excavation” in the project bid documents. We also recommend that a Contractor that specializes in hard rock excavation in limited access conditions be consulted to aid in project planning and budgeting.

Minimizing the depth of the trench will also reduce challenges related to excavation.

Fill Materials

Unless otherwise recommended by the City or the pipe manufacturer, pipe bedding and embedment materials should consist of well-graded sand with 90 to 100 percent of particles passing the No. 4 sieve and no more than five percent finer than the No. 200 sieve. Provide the minimum bedding thickness beneath the pipe in accordance with the manufacturer’s recommendations (typically three to six inches).

Fill materials used for trench backfill should consist of non-expansive materials that are free of organic matter, have a Liquid Limit of less than 40 (ASTM D 4318), a Plasticity Index of less than 20 (ASTM D 4318), and have a minimum R-value of 20 (California Test 301). The fill material should contain no more than 50 percent of particles passing a No. 200 sieve and should have a maximum particle size of four inches. Some of the onsite soils may be suitable for re-use as trench backfill provided they meet the requirements above. Depending on the equipment used for excavation, smaller gravels will be produced if a rock saw is used as opposed to larger cobble-and boulder-sized material if jackhammers are used. The use of rock saws would be preferred so that excavated materials could be used as onsite fill.

Fill Placement and Compaction

Fill materials should be moisture conditioned to near the optimum moisture content prior to compaction. Properly moisture conditioned fill materials should subsequently be placed in loose, horizontal lifts of eight-inches-thick or less and uniformly compacted to at least 90 percent relative compaction. In pavement areas, the upper 12 inches of backfill should be compacted to at least 95 percent relative compaction. The maximum dry density and optimum moisture content of fill materials should be determined in accordance with ASTM D1557.

5.2 Foundations

The pathway will also be supported on a steel-framed, elevated structure from about 62+90 to 65+50 where widening is not feasible due to very steep and narrow conditions. A similar steel-framed bridge will also be installed to replace an existing wood bridge near Station 52+00. From discussions with the project team, we understand the structures will be designed to support relatively light vertical and lateral loads on the order of five to ten kips. While various foundations alternatives are feasible, we judge that drilled micropiles are well suited to support the anticipated foundation loads. Micropiles are also advantageous in that they can be constructed using relatively small equipment. Micropiles should be designed using a nominal skin friction of 2,000 pounds per square foot in firm rock which is expected within a few feet of the ground surface.

Passive resistance should be neglected since the piles are relatively slender, and any lateral resistance that is required should be obtained by battering.

5.3 Retaining Walls

New retaining walls will be constructed on the outboard side of the alignment as required to provide the minimum five-foot width for the pathway. The walls are expected to vary in length from about 20 to 100 feet with retained heights on the order of about five feet or less. While various wall alternatives were considered, soldier pile retaining walls have been selected primarily due to the ability to construct the walls with relatively small/portable equipment. With the exception of the containment wall, tiebacks will also be incorporated into the wall design to reduce the embedment depth and size of the steel beam for the soldier piles.

We recommend that the retaining walls be designed using an active earth pressure of 45 pounds per cubic foot (equivalent fluid pressure) and a uniform seismic pressure equal to ten times the wall height (in pounds per square foot). Passive resistance for the steeply sloping ground beneath the walls should be modeled as an equivalent fluid pressure of 200 pounds per cubic foot applied over a width of two drilled hole diameters. Tiebacks should be designed using a nominal skin friction of 2,000 pounds per square foot in firm rock. Wall drainage should be installed to prohibit the buildup of hydrostatic pressure behind the wall.

5.4 Safety Considerations

In addition to potential damage to the pipeline, slope instability also represents a hazard to workers during construction. Excavation of the hard bedrock is expected to generate significant vibrations which further increases the risk of inducing instability and the release of materials from areas above where personnel are working. This risk is generally higher below the cliffs and very steeply sloping terrain that exists along portions of the site. The Contractor’s planning and safety program should include measures for ensuring the safety of workers throughout construction.

Worker safety should be the sole responsibility of the Contractor.

6.0 SUPPLEMENTAL GEOTECHNICAL SERVICES

As project plans are nearing completion, we should review them to confirm that the intent of our geotechnical recommendations has been incorporated. We can also consult with project team to supplement or clarify geotechnical recommendations, if needed. During construction, we should be present intermittently to observe excavations, proper moisture conditioning of soils, fill placement and compaction, retaining wall construction and other geotechnical-related work items.

The purpose of our observation and testing is to confirm that site conditions are as anticipated, to adjust our recommendations and design criteria if needed, and to confirm that the Contractor’s work is performed in accordance with the project plans and specifications.

7.0 LIMITATIONS

We believe this report has been prepared in accordance with generally accepted geotechnical engineering practices in Napa County at the time the report was prepared. This report has been prepared for the exclusive use of Sandis and/or their assignees specifically for this project. No other warranty, expressed or implied, is made. Our evaluations and recommendations are based on the data obtained during our subsurface exploration program and our experience with soils in this geographic area. Our approved scope of work did not include a detailed environmental assessment of the site. We recommend that an environmental consultant be retained to evaluate environmental-related issues.

The evaluations and recommendations do not reflect variations in subsurface conditions that may exist between boring locations or in unexplored portions of the site. Should such variations become apparent during construction, the general recommendations contained within this report will not be considered valid unless Miller Pacific is given the opportunity to review such variations and revise or modify our recommendations accordingly. No changes may be made to the general recommendations contained herein without the written consent of Miller Pacific.

We recommend that this report, in its entirety, be made available to project team members, contractors, and subcontractors for informational purposes and discussion. We intend that the information presented within this report be interpreted only within the context of the report as a whole. No portion of this report should be separated from the rest of the information presented herein. No single portion of this report shall be considered valid unless it is presented with and as an integral part of the entire report.

8.0 LIST OF REFERENCES

American Concrete Institute, “318-14: Building Code Requirements for Structural Concrete and Commentary”.

American Society for Testing and Materials, (2009) “2009 Annual Book of ASTM Standards, Section 4, Construction, Volume 4.08, Soil and Rock; Dimension Stone; Geosynthetics,” ASTM, Philadelphia.

Association of Bay Area Governments (ABAG), Geographic Information System, http://quake.abag.ca.gov/mitigation/, 2022.

California Building Code, 2022 Edition, California Building Standards Commission/International Conference of Building Officials, Whittier, California.

California Department of Conservation, Division of Mines and Geology (1972), Special Publication 42, “Alquist-Priolo Special Studies Zone Act,” (Revised 1988).

California Department of Transportation (Caltrans) (2018), 2018 Standard Specifications.

California Geological Survey, Geologic Maps of the 7.5' Napa & Mt. George Quadrangles, 2004.

Occupational Safety and Health Administration (OSHA)(2005), Title 29 Code of Federal Regulations, Part 1926, 2005.

United States Geological Survey, “Database of Potential Sources for Earthquakes Larger than Magnitude 6 in Northern California,” The Working Group on Northern California Earthquake Potential, Open File Report 96-705, 1996.

United States Geological Survey (2003), “Summary of Earthquake Probabilities in the San Francisco Bay Region, 2002 to 2032,” The 2003 Working Group on California Earthquake Probabilities, 2003.

United States Geological Survey (2008), “The Uniform California Earthquake Rupture Forecast, Version 2,” The 2007 Working Group on California Earthquake Probabilities, Open File Report 2007-1437, 2008.

United States Geological Survey, Earthquake Hazards Program, Earthquake Circular Area Search http://neic.usgs.gov/neis/epic/epic_circ.html, accessed January 2023.

REFERENCE: United States Geological Survey, Topographic Maps of the 7.5' Napa & Mt. George Quadrangles, 2015

SITE LOCATION

(NO SCALE)

N O

R

TH

FIGURE

Drawn

Checked

Project No. Date: 1/10/2023

504 Redwood Blvd.

Suite 220

Novato, CA 94947

T 415 / 382-3444

F 415 / 382-3450 www.millerpac.comFILENAME: 3426.002 Figures.dwg

A CALIFORNIA CORPORATION, © 2022, ALL RIGHTS RESERVED

SITE LOCATION MAP

City of Napa Hardening of the Milliken Raw Water Pipeline

Napa, California 3426.002

RCA

SITE

REGIONAL GEOLOGIC MAP

(NO SCALE)

SITE

N O

R

TH

Qhc STREAM CHANNEL DEPOSITS (latest Holocene < 1,000 years) Deposits in active, natural stream channels, consists of loose alluvial sand, gravel and silt.

Qa ALLUVIUM, UNDIVIDED (latest Pleistocene to Holocene) Flat, relatively undissected fan, terrace and basin deposits.

Tsvm SONOMA VOLCANICS, MAFIC FLOWS AND BRECCIAS (late Miocene to Pliocene) Basalt, basaltic andesite and andesite flows and breccias, interbedded with volcanic agglomerate and tuff.

Tsvr SONOMA VOLCANICS, RHYOLITE ASH FLOW TUFF (late Miocene to Pliocene) Black to light grey vitrophyre with angular lithic clasts overlying welded tuff with flattened pumice lapilli and unwelded pumice lapilli tuff. This unit overlies the older rocks with angular unconformity.

Tsvdg SONOMA VOLCANICS, DACITE OF MT. GEORGE (late Miocene to Pliocene) Flows and domes of gray to tan porphyritic dacite. The dacite is typically strongly flow banded. The upper surfaces of flows are commonly perlitic.

Tsvdgi SONOMA VOLCANICS, DACITE INTRUSION (late Miocene to Pliocene) Gray porphyritic dacite intrusion in Milliken Canyon. The intrusion domed the overlying volcanic rocks.

LEGEND:

REFERENCE: California Geological Survey, Gelogic Maps of the 7.5' Napa & Mt. George Quadrangles, 2004.

FIGURE

Drawn

Checked

Project No. Date: 1/10/2023

504 Redwood Blvd.

Suite 220

Novato, CA 94947

T 415 / 382-3444

F 415 / 382-3450 www.millerpac.comFILENAME: 3426.002 Figures.dwg

A CALIFORNIA CORPORATION, © 2022, ALL RIGHTS RESERVED

REGIONAL GEOLOGIC MAP

City of Napa Hardening of the Milliken Raw Water Pipeline

Napa, California 3426.002

Tsvr

Tsvm

Tsvm

Tsvt

Tsvtb

Tsvt

Tsvr

Tsvt

MATCHLINE A

STA 5+00

STA 10+00

STA 15+00

STA 20+00

TREATMENT PLANT

Drawn

Checked

Designed

Project No. Date: 12/23/2022 FIGURE

504 Redwood Blvd.

Suite 220

Novato, CA 94947

T 415 / 382-3444

F 415 / 382-3450 www.millerpac.com

A CALIFORNIA CORPORATION, © 2022, ALL RIGHTS RESERVED

FILE: Topo Basemap.dwg

City of Napa Hardening of the Milliken Raw Water Pipeline

Napa, California 3426.002

SITE PLAN AND GEOLOGIC MAP - SOUTHWEST AREA

MFJ 3-1

Qdf DEBRIS FLOW (QUATERNARY) Incised channels either containing debris or exhibiting evidence of historic debris flow or debris avalanches. Debris typically consists of cobble- to boulder-size masses of volcanic rock ranging, in cases, to more than 10-feet in maximum dimension.

Tsvr RHYOLITE ASH FLOW TUFF (Late MIOCENE to PLIOCENE) Black vitrophyre with angular lithic clasts ovelrying welded tuff. Unconformably overlies older units. Forms significant cliff bands ranging to about a hundred feet high above the southeastern part of the pipeline. Typically very hard and very strong.

Tsvd DACITE (PLIOCENE) Light gray with plagioclase phenocrysts to +/-1-cm, typically hard and strong. Forms intrusive dome that gently warps older, overlying units. Forms very thin sandy to gravelly residual soils and stands in vertical cliff faces and roadcuts except where adverse joint orientations exist.

Tsvt WELDED ASH FLOW TUFF (PLIOCENE) Light brown to buff, densely- to very densely-welded vitric-lithic ash-lapilli tuff. Intermediate to mafic lithic fragments typically range from 2 to 6-inches. Includes interbeds of pumiceous tuff and clast-supported pebble agglomerate. Typically relatively low-density, moderately hard, and moderately strong. Forms natural slopes inclined near 1:1 (H:V) and stands in vertical roadcuts.

Tsvtb ASH FLOW TUFF BRECCIA (PLIOCENE) Light brown to buff, matrix-supported ash-lapilli flow breccia. Includes common bomb-size (10-feet or more in maximum dimension) inclusions/fragments of hard volcanic rock which appear to weather out of the matrix in-situ and form large surface "floaters". Typically weathers to erosion-prone silty residual soils and forms natural slopes inclined between 1.5 and 2:1 (H:V).

Tsvm MAFIC LAVA FLOW ROCKS (PLIOCENE) Dark gray lava flows, locally pumiceous, typically very hard and strong with gently northwest- to west-dipping flow surfaces.

GEOLOGIC CONTACT, DASHED WHERE APPROXIMATE

FAULT, DASHED WHERE APPROXIMATE

LEGEND AND KEY TO LOG SYMBOLS

APPROXIMATE ALIGNMENT OF 16-INCH TRANSMISSION PIPELINE

PARCEL BOUNDARY

TOPOGRAPHIC CONTOUR (5-FOOT INTERVALS)

SCALE

0 100 200 400 FEETN

O

R

TH

RISK ZONE 1 - TOPPLING/ROCKFALL

Areas where vertical or near-vertical cut slopes are susceptible to toppling or rockfall as a result of adversely-oritented internal joint structure and loss of lateral support created by road excavation/grading. Generally occurs where high vertical cuts exist in areas underlain by Dacite (Tsvd) and mafic flow rocks (Tsvm). Locally occurs in map unit Tst where secondary/tertiary joint sets exist and in areas of modest cut slope inclination in map unit Tsvtb where large boulders exposed in the cut slope weather out of weak, ashy matrix.

RISK ZONE 2 - DEBRIS AVALANCHES

Areas where steep to intermediate cut slopes lie below significant rockfall source areas. Primary rockfall sources include map units Tsvd and Tsvr. Locally, map unit Tsvtb may also be soiurce where large boulders weather out of ashy matrix., Individual blocks in all units locally range to more than 10-feet in greatest dimension. In general, entire alignment falls within Risk Zone 2 EXCEPT areas where vertical cut slopes are high enough that avalanche debris would likely clear roadway and not impact base of cut.

RISK ZONE 3 - DEBRIS FLOWS AND SLIDES

Areas where intermediate slopes are underlain by thicker colluvial soils, or where debris flows have developed in confined channels. Areas within map unit Qls typically involve the upper couple feet of residual or colluvial soils. Areas mapped as Qdf typically involve accumulated cobble- to boulder-size fragments washed down from upper slopes and cliff bands.

KEY TO RISK ZONE IDENTIFICATION

Tsvt

Tsvr

Tsvt

Tsvt

Tsvd

Tsvd

Tsvd

Tsvm

Tsvd

MATCHLINE A

MATCHLINE B

Tsvr

Qdf

Qdf

Qdf

STA 33+00

STA 25+00

STA 30+00

STA 35+00

STA 40+00

Drawn

Checked

Designed

Project No. Date: 12/23/2022 FIGURE

504 Redwood Blvd.

Suite 220

Novato, CA 94947

T 415 / 382-3444

F 415 / 382-3450 www.millerpac.com

A CALIFORNIA CORPORATION, © 2022, ALL RIGHTS RESERVED

FILE: Topo Basemap.dwg

City of Napa Hardening of the Milliken Raw Water Pipeline

Napa, California 3426.002

SITE PLAN AND GEOLOGIC MAP - CENTRAL AREA

MFJ 3-2

SCALE

0 100 200 400 FEETN

O

R

TH

Qdf DEBRIS FLOW (QUATERNARY) Incised channels either containing debris or exhibiting evidence of historic debris flow or debris avalanches. Debris typically consists of cobble- to boulder-size masses of volcanic rock ranging, in cases, to more than 10-feet in maximum dimension.

Tsvr RHYOLITE ASH FLOW TUFF (Late MIOCENE to PLIOCENE) Black vitrophyre with angular lithic clasts ovelrying welded tuff. Unconformably overlies older units. Forms significant cliff bands ranging to about a hundred feet high above the southeastern part of the pipeline. Typically very hard and very strong.

Tsvd DACITE (PLIOCENE) Light gray with plagioclase phenocrysts to +/-1-cm, typically hard and strong. Forms intrusive dome that gently warps older, overlying units. Forms very thin sandy to gravelly residual soils and stands in vertical cliff faces and roadcuts except where adverse joint orientations exist.

Tsvt WELDED ASH FLOW TUFF (PLIOCENE) Light brown to buff, densely- to very densely-welded vitric-lithic ash-lapilli tuff. Intermediate to mafic lithic fragments typically range from 2 to 6-inches. Includes interbeds of pumiceous tuff and clast-supported pebble agglomerate. Typically relatively low-density, moderately hard, and moderately strong. Forms natural slopes inclined near 1:1 (H:V) and stands in vertical roadcuts.

Tsvtb ASH FLOW TUFF BRECCIA (PLIOCENE) Light brown to buff, matrix-supported ash-lapilli flow breccia. Includes common bomb-size (10-feet or more in maximum dimension) inclusions/fragments of hard volcanic rock which appear to weather out of the matrix in-situ and form large surface "floaters". Typically weathers to erosion-prone silty residual soils and forms natural slopes inclined between 1.5 and 2:1 (H:V).

Tsvm MAFIC LAVA FLOW ROCKS (PLIOCENE) Dark gray lava flows, locally pumiceous, typically very hard and strong with gently northwest- to west-dipping flow surfaces.

GEOLOGIC CONTACT, DASHED WHERE APPROXIMATE

FAULT, DASHED WHERE APPROXIMATE

LEGEND AND KEY TO LOG SYMBOLS

APPROXIMATE ALIGNMENT OF 16-INCH TRANSMISSION PIPELINE

PARCEL BOUNDARY

TOPOGRAPHIC CONTOUR (5-FOOT INTERVALS)

RISK ZONE 1 - TOPPLING/ROCKFALL

Areas where vertical or near-vertical cut slopes are susceptible to toppling or rockfall as a result of adversely-oritented internal joint structure and loss of lateral support created by road excavation/grading. Generally occurs where high vertical cuts exist in areas underlain by Dacite (Tsvd) and mafic flow rocks (Tsvm). Locally occurs in map unit Tst where secondary/tertiary joint sets exist and in areas of modest cut slope inclination in map unit Tsvtb where large boulders exposed in the cut slope weather out of weak, ashy matrix.

RISK ZONE 2 - DEBRIS AVALANCHES

Areas where steep to intermediate cut slopes lie below significant rockfall source areas. Primary rockfall sources include map units Tsvd and Tsvr. Locally, map unit Tsvtb may also be soiurce where large boulders weather out of ashy matrix., Individual blocks in all units locally range to more than 10-feet in greatest dimension. In general, entire alignment falls within Risk Zone 2 EXCEPT areas where vertical cut slopes are high enough that avalanche debris would likely clear roadway and not impact base of cut.

RISK ZONE 3 - DEBRIS FLOWS AND SLIDES

Areas where intermediate slopes are underlain by thicker colluvial soils, or where debris flows have developed in confined channels. Areas within map unit Qls typically involve the upper couple feet of residual or colluvial soils. Areas mapped as Qdf typically involve accumulated cobble- to boulder-size fragments washed down from upper slopes and cliff bands.

Tsvdg

Tsvdg

Tsvd

Tsvm

Tsvd

MATCHLINE B

Tsvdg

Qdf

Qdf

Qdf

Qdf

Qdf

Qdf

STA 40+00

STA 45+00

STA 50+00

STA 55+00

STA 60+00

STA 65+00

DIVERSION DAM

Drawn

Checked

Designed

Project No. Date: 12/23/2022 FIGURE

504 Redwood Blvd.

Suite 220

Novato, CA 94947

T 415 / 382-3444

F 415 / 382-3450 www.millerpac.com

A CALIFORNIA CORPORATION, © 2022, ALL RIGHTS RESERVED

FILE: Topo Basemap.dwg

City of Napa Hardening of the Milliken Raw Water Pipeline

Napa, California 3426.002

SITE PLAN AND GEOLOGIC MAP - NORTHEAST AREA

MFJ 3-3

SCALE

0 100 200 400 FEETN

O

R

TH

Qdf DEBRIS FLOW (QUATERNARY) Incised channels either containing debris or exhibiting evidence of historic debris flow or debris avalanches. Debris typically consists of cobble- to boulder-size masses of volcanic rock ranging, in cases, to more than 10-feet in maximum dimension.

Tsvr RHYOLITE ASH FLOW TUFF (Late MIOCENE to PLIOCENE) Black vitrophyre with angular lithic clasts ovelrying welded tuff. Unconformably overlies older units. Forms significant cliff bands ranging to about a hundred feet high above the southeastern part of the pipeline. Typically very hard and very strong.

Tsvd DACITE (PLIOCENE) Light gray with plagioclase phenocrysts to +/-1-cm, typically hard and strong. Forms intrusive dome that gently warps older, overlying units. Forms very thin sandy to gravelly residual soils and stands in vertical cliff faces and roadcuts except where adverse joint orientations exist.

Tsvt WELDED ASH FLOW TUFF (PLIOCENE) Light brown to buff, densely- to very densely-welded vitric-lithic ash-lapilli tuff. Intermediate to mafic lithic fragments typically range from 2 to 6-inches. Includes interbeds of pumiceous tuff and clast-supported pebble agglomerate. Typically relatively low-density, moderately hard, and moderately strong. Forms natural slopes inclined near 1:1 (H:V) and stands in vertical roadcuts.

Tsvtb ASH FLOW TUFF BRECCIA (PLIOCENE) Light brown to buff, matrix-supported ash-lapilli flow breccia. Includes common bomb-size (10-feet or more in maximum dimension) inclusions/fragments of hard volcanic rock which appear to weather out of the matrix in-situ and form large surface "floaters". Typically weathers to erosion-prone silty residual soils and forms natural slopes inclined between 1.5 and 2:1 (H:V).

Tsvm MAFIC LAVA FLOW ROCKS (PLIOCENE) Dark gray lava flows, locally pumiceous, typically very hard and strong with gently northwest- to west-dipping flow surfaces.

GEOLOGIC CONTACT, DASHED WHERE APPROXIMATE

FAULT, DASHED WHERE APPROXIMATE

LEGEND AND KEY TO LOG SYMBOLS

APPROXIMATE ALIGNMENT OF 16-INCH TRANSMISSION PIPELINE

PARCEL BOUNDARY

TOPOGRAPHIC CONTOUR (5-FOOT INTERVALS)

RISK ZONE 1 - TOPPLING/ROCKFALL

Areas where vertical or near-vertical cut slopes are susceptible to toppling or rockfall as a result of adversely-oritented internal joint structure and loss of lateral support created by road excavation/grading. Generally occurs where high vertical cuts exist in areas underlain by Dacite (Tsvd) and mafic flow rocks (Tsvm). Locally occurs in map unit Tst where secondary/tertiary joint sets exist and in areas of modest cut slope inclination in map unit Tsvtb where large boulders exposed in the cut slope weather out of weak, ashy matrix.

RISK ZONE 2 - DEBRIS AVALANCHES

Areas where steep to intermediate cut slopes lie below significant rockfall source areas. Primary rockfall sources include map units Tsvd and Tsvr. Locally, map unit Tsvtb may also be soiurce where large boulders weather out of ashy matrix., Individual blocks in all units locally range to more than 10-feet in greatest dimension. In general, entire alignment falls within Risk Zone 2 EXCEPT areas where vertical cut slopes are high enough that avalanche debris would likely clear roadway and not impact base of cut.

RISK ZONE 3 - DEBRIS FLOWS AND SLIDES

Areas where intermediate…