Preliminary Geotechnical Memorandum – WRECO

8331 Sierra College Blvd., Suite 208 Roseville, CA 95661 Phone: 916.757.6150 Fax: 916.787.6191 www.wreco.com

Memorandum

Date: June 30, 2015 To: Keith D. Rhodes, PE – T.Y. Lin International From: Michael Wilson, PG, CEG – WRECO Subject: South County Corridor Project –Preliminary Geotechnical Memorandum

INTRODUCTION

Scope of Services This memorandum was prepared in support of the Feasibility Study for the South County Corridor (SCC) Project (Project). This memorandum evaluates the potential geotechnical/geologic impacts and mitigation measures for the Project.

Project Description The SCC is a planned east-west 4-lane divided expressway connecting State Route 99 (SR 99) to Interstate 5 (I-5) in the southern portion of Stanislaus County and bypassing the Cities of Patterson and Newman. The SCC Feasibility Study will analyze potential traditional and multi-modal corridor alignments that will enhance the east-west transportation link for all travel modes in southern Stanislaus County. Key goals of the Feasibility Study are as follows:

• Provide an efficient movement of goods and people for all modes of travel statewide • Improve safety through the roadway widening and improvements, limiting access to the expressway facility and divided traffic lanes • Enhance local, regional and statewide connectivity • Improve air quality and noise • Promote an increase of local and regional investments • Promote the support of General Plans applicable within the project limits • Assess the feasibility including planned land use, transportation and environmental issues • Develop project development and implementation strategies

Figure 1 is an aerial map showing the Project study area and Figure 2 is a map identifying the existing SCC and proposed SCC alternative alignments.

Project Need Stanislaus County is a vital hub for the movement of agricultural (farm to market) and other goods, both locally grown/produced and those that pass through the region, which links northern and southern California as well as the Bay Area. The lack of an efficient and direct travel route between

SR 99, SR 33, and I-5 in the southern part of Stanislaus County has become a pressing concern to the region.

Of primary concern is the amount of regional and interregional traffic generating congestion within the Patterson city limits and surrounding areas. This traffic congestion, noise, and related safety issues are of a larger concern to the region which depends on an efficient and safe transportation system to deliver manufactured and agricultural goods both regionally and inter-regionally. In addition, the centrally located nature of Stanislaus County has made it an ideal location for the distribution of goods through the Central Valley. The SCC will be key to the continued success of these industries.

The existing corridor is part of the 39.7 mile County Route J17 (CR J17) established in 1960 that runs through Stanislaus and Merced counties. The section of CR J17 (Sperry Road, E. Las Palmas Avenue, and W. Main Street) between SR 99 and I-5 functions as an agricultural trade corridor that extends 18 miles between the Cities of Turlock and Patterson. This section of CR J17 is generally a 2-lane highway through rural areas; although, the facility has 4-lane segments within the city limits of Turlock and 3-lane segments within the city limits of Patterson. East Las Palmas Avenue on the east side of town has 100 year old palm trees that prevent widening the road. Trucks experience approximately ten traffic signals along Sperry Road, East Las Palmas Avenue, and West Main Street to get from I-5 to SR 99. Since Patterson is becoming a west side hub for commerce distribution, the existing corridor route is heavily use and is often congested.

Figure 1, SCC Study Area Map

Figure 2, SCC Project Map

Source: TYLIN International, 2015. South County Corridor Conceptual Alignments, Overall Project Map, Level 2 Alignments, Exhibit 1 of 1, Scale 1” = 10,000’

EXISTING CONDITIONS

Physical Setting Climate The climate of Stanislaus County (within San Joaquin Valley) is classified as mild Mediterranean, with warm semi-arid to arid conditions. There is one wet season during the year, with 90-percent of the precipitation occurring October through April. Snow in the valley is infrequent, and occurs once in about every seven years. Precipitation ranges from 5- to 7-inches annually in the San Joaquin Valley and from 7- to 12-inches annually on the surrounding terraces, foothills, and mountains. In the San Joaquin Valley, the last frost in spring is generally in February or early March and the first frost in fall is late in November or early December. In the mountains, the last frost typically in spring is early in May and the first frost in fall for some areas is early in October. Summers are generally cloudless, hot, and dry with temperatures commonly exceeding 100-degrees F and rarely are less that 51-degrees F. Winters are typically mild and fairly humid. December and January are characterized by frequent fog or low clouds which occur mostly at night. Winter temperatures typically range from the low 30’s at night with daytime temperatures in the 50’s. A study of wind-speed records suggest that wind-speeds can attain velocities of 30- to 45-miles per hour about once in 2-years. As often as once in 50-years wind-speeds have reached 60- to 65-miles per hour, and at 100-year intervals they can reach 65- to 75-miles per hour.

Topography The Project consists of relatively flat topography with a slight rise in elevation in the west, near I-5. The elevation range is approximately 40 feet to 200 feet, with the San Joaquin River being the lowest and I-5 being the highest. Grassland and farmlands are the dominant habitats throughout the area (USGS, 20151-6).

Regional Geology The Project is located on the eastern edge of the Diablo Range Mountains. The Diablo Range is located within the California Coast Ranges subdivision of the Pacific Coast Ranges. The Diablo Range Mountains are nearly 200 miles long and start near the Carquinez Strait in Contra Costa County east of Oakland to the north and stretches south to Orchard Peak near where State Route 46 crosses over the Coast Ranges at Cholame in northwestern Kern County. The Diablo Range Mountains form the western wall of the Central Valley and are paralleled by the U.S. Route 101 to the west and by I-5 to the east. The Diablo Range Mountains are characterized by steep mountainous topography to the west that level out towards the valley on the east.

The west side of the Project is located near the western boundary of the northern portions of the San Joaquin Valley, near the Cities of Patterson and Newman, while the eastern side of the Project is located near Turlock near the center of the valley, in Stanislaus County. The physiographic location of the subject site is within the Great Valley Geomorphic Province. The province encompasses the San Joaquin and Sacramento valleys’ and is bounded by the Sierra Nevada Mountains to the east, the

Coast Ranges to the west, the Transverse Range (Tehachapi Mountains) to the south, and the Klamath Mountains to the north.

The San Joaquin Valley is a structural trough that covers 2,374 square miles, which makes up the southern two-thirds of the Great Valley Geomorphic Province. The San Joaquin Valley extends from the Stockton-Tracy area on the north to the Transverse Ranges on the south. The southern San Joaquin Valley basin is bounded by the Temblor Ranges on the west, the Sierra Nevada Mountains on the east, and the San Emigdio and Tehachapi mountains to the south. The west side of the basin consists of a tightly folded anticlinorium, which is sub-parallel to the San Andreas Fault. The east side of the valley is a broad homocline. The valley surface is relatively flat and is underlain by thousands of feet of alluvial (river), lacustrine (lake), and marine (ocean) deposits that have accumulated in an elongate, asymmetrical sedimentary basin to form the structural trough as the adjacent mountain ranges elevated. The main axis of the San Joaquin basin is north-northwestern trending along the valley’s main drainage axis.

During the late Mesozoic and to the early and middle Cenozoic eras (approximately 20 to 100 million years before present), deposition of thousands of feet of marine sediments occurred within the Great Valley. Continental deposits (generally alluvium) of late Tertiary and Quaternary age (approximately 20 million years ago to present) overlie these marine sediments. A total of 32,000 feet of continental deposits and underlying marine sediments were deposited into the San Joaquin Valley trough (or subbasin) during periodic inundation by the Pacific Ocean and by erosion of the surrounding mountains.

Project Site Geology Based on the Geologic Map of the San Francisco and San Jose Quadrangle (Wagner, et al, 1991), the Project site is underlain by soils belonging to the Fanglomerate, Valley Springs and Tesla formations’ on the western margin of the project (along I-5) and alluvial fan soils, Dos Palos alluvium (within the San Joaquin River floodplain) and Modesto formation trending eastward towards the center of the valley (Figure 3).

The Tesla formation consists of sedimentary rocks and gravel deposits. The Fanglomerate formation consists of mostly well consolidated sedimentary rocks, conglomerate, cobbles, and gravel deposits. The Valley Springs formation consists of mostly loosely consolidated sandstone, shale and gravel deposits. The alluvial fan soils are composed of unconsolidated and semi-consolidated lake, playa, and terrace deposits and surficial soils consisting of sand, silt and clay. The Dos Palos alluvium consists of unconsolidated, moderately to well sorted gravel, sand, silt and clay. The Modesto formation typically is composed of gravels, sands and silts and occurs east of the San Joaquin River.

Prelim i

Figure 3, SCC Geologic Map

Soil Profile The United States Department of Agriculture (USDA) Natural Resources Conservation Service’s Web Soil Survey (USDA, 2104) identifies a wide range of soils within the study area. The majority of these soils are classified as within hydrologic soil group (HSG) C, with HSGs B and D occurring the second most. Soils along the San Joaquin River are primarily HSG’s A and A/D, with smaller portions consisting of HSG C/D. HSG C soils have a slow infiltration rate when thoroughly wet and a slow rate of water transmission. These soils contain a layer which prevents the downward movement of water and soil (fine to moderately fine texture).

Groundwater Based on the State Water Resources Control Board’s (SWRCB) GeoTracker Groundwater Ambient Monitoring and Assessment (GAMA), monitoring wells in the study vicinity indicate an average depth to water of 23.88 feet below ground surface (bgs) in the western portion and 17.50 feet bgs in the eastern portion (SWRCB, 2015).

GEOLOGIC HAZARDS

Faulting The Project does not lie within or adjacent to an Alquist-Priolo Earthquake Fault Zone. One Quaternary age (active or potentially active) fault (the San Joaquin/Orestimba fault) has been mapped as crossing I-5 north of the I-5/Sperry Road interchange (William Lettis & Associates, Inc., 2007). This fault continues southward roughly paralleling the east side of I-5. This fault is coincident with the Great Valley 07 (Orestimba) fault that is recognized by Caltrans and the United States Geologic Survey (USGS). No other faults have been mapped as crossing the Project area. The southern end of the northwest trending Vernalis Thrust fault has been mapped as occurring approximately 8 miles northwest of the northern end of the Project study area. No other mapped faults appear to trend towards the Project study area.

Active faults in the project region capable of producing a maximum moment magnitude (Mmax) earthquake of 6.5 or greater include the San Joaquin – Orestimba fault (Great Valley 07), Ortigalita fault zone (Ortigalita – Cottonwood Arm section) and the San Andreas fault (Santa Cruz Mountains segment).

The San Joaquin/Orestimba fault marks the physiographic boundary between the Diablo Range and the Central Valley. This fault parallels the Diablo Range and is commonly divided into a northern and southern section, which splits at the Del Puerto Creek outlet (William Lettis & Associates, 2007). The northern section has been referred to by others as the San Joaquin Fault and other researchers have referred to the southern segment as the Orestimba fault. Both the northern and southern traces appear to have west-side-up thrust fault motion. The estimated slip rate is 0.4-0.6 mm/yr with roughly 60 meters of uplift in the past 300,000 to 200,000 years (Anderson & Piety, 2001 in William Lettis & Associates, Inc., 2007). Caltrans and the U.S. USGS recognize the San Joaquin/Orestimba fault as the Great Valley Fault Segment 07 (Orestimba). Current Caltrans and USGS databases indicate that this

fault is a shallow dipping reverse fault capable of producing a maximum moment magnitude (Mmax) earthquake of 6.7. Some researchers believe that, rather than blind thrust fault sources parallel to and underlying the range front, as assumed in the Working Group 1999 (WG99) model for seismic sources along the western San Joaquin Valley margin (i.e., Great Valley 07 and Great Valley 08), shortening appears to be accommodated by blind thrust fault sources that are oblique to the trend of the range front, and separated by right en echelon steps. This pattern of faulting generally implies smaller blind thrust fault sources and a lower potential for multi-segment ruptures than the fault geometry assumed by WG99. The restraining step-over model also implies that the slip rates on blind thrust faults are more directly related to slip rates on the bounding strike-slip faults, rather than regional plate-boundary-normal shortening rates (Unruh and Lettis, 1998). Further study will be required to establish the potential of a fault rupture hazard to structures in the near vicinity of the San Joaquin/Orestimba (Great Valley 07) fault.

The Ortigalita fault zone (Ortigalita – Cottonwood Arm section) is a strike-slip fault trending northwest with a vertical dip and is located approximately 14 miles to the southwest of the City of Newman. It is capable of producing a maximum moment magnitude (Mmax) earthquake of 7.0.

The San Andreas (Santa Cruz Mountains segment) is a right lateral strike-slip fault trending northwest, and is approximately 55 miles to the west-southwest of the study area. It is capable of producing a maximum moment magnitude (Mmax) earthquake of 8.0.

Figure 4, Quaternary Fault Map, shows the Project vicinity in relation to late Quaternary regional and local active faults. The nearest active faults, late Quaternary (within the past 700,000 years), to the Project site and capable of producing a Mmax earthquake of 6.5 or greater are listed in Table 1.

Figure 4, Quaternary Fault Map

Data Source: © 2007 William Lettis & Associates, Inc. Assessment and Documentation of Transpressional Structures, Northeastern Diablo Range, for the Quaternary Fault Map Database: Collaborative Research with William Lettis & Associates, Inc. and the U.S. Geological Survey; June 2007.

Table 1, Late Quaternary Faults1 Nearest To the Study Area with an Mmax of 6.5 or Greater Approximate Maximum Fault Name Distance to the Direction from Moment (Caltrans Fault Type Fault Rupture Site Magnitude Fault ID No.) Plane (RRUP) (Mmax) Great Valley fault Section 07 4.4 mi. Below Site2 Reverse 6.7 (Orestimba) (138) Great Valley fault Section 08 15 mi. Southeast Reverse 6.8 (Quinto) (160) Ortigalita fault zone (Ortigalita – Cottonwood 16 mi. Southwest Strike-Slip 7.0 Arm section) (159) Greenville (South) 19 mi. Northwest Strike-Slip 6.9 (144) Calaveras (Central) 31 mi. West Strike-Slip 6.9 (151) Hayward (Southern 35 mi. West Strike-Slip 6.7 Extension) (149) Silver Creek fault 36 mi. West Reverse 6.9 (148) Cascade fault 41 mi. Southwest Reverse 6.7 (153) San Andreas (Santa Cruz 55 mi. West Strike-Slip 8.0 Mountains) (158) 1 Caltrans Fault Database (V2a) for Caltrans ARS Online, October 23, 2012. 2 The fault trace is mapped crossing I-5 just north of the I-5/Sperry Road Interchange.

Ground Shaking The Project is located in a seismically active region of northern California and there is the potential for the site to experience strong ground shaking from local and regional earthquakes during the life of the project.

A preliminary seismic study was performed for the Project to develop preliminary seismic design parameters for the overall study area. Since the western margin of the Project is closer to potential seismic sources than the eastern side of the Project, the seismic parameters were derived from an area in the northwestern corner of the Project near where the San Joaquin fault (Great Valley 07) is mapped as crossing I-5. This will provide the most conservative values which is appropriate for this preliminary level of study.

Following the United States Geological Survey’s web-based U.S. Seismic Design Maps tool (http://earthquake.usgs.gov/designmaps/us/application.php) and utilizing the Uniform Building Code reference document (ASCE 7-10; with March 2013 errata), the preliminary seismic design coefficients presented in Table 2 were determined.

Table 2, Preliminary Seismic Design Coefficients

Latitude: 37.46352 Longitude: -121.12277 Site Class D – Stiff Soil

1 PGAM = 0.39g

Ss = 1.781g S1 = 0.588g

SMS = 1.781g SM1 = 0.882g

SDS = 1.188g SD1 = 0.588g 1 Peak Ground Acceleration for a Maximum Credible Earthquake where “g” is the acceleration due to gravity

An alternative preliminary seismic analysis was performed in accordance with the California Department of Transportation (Caltrans) Seismic Design Criteria (SDC) Version 1.7, (Caltrans, 2013), Memos to Designer (MTD) Section 20, and design tools outlined in the Caltrans Methodology for Developing Design Response Spectrum for Use in Seismic Design Recommendation, November 2012.

The results of this preliminary seismic evaluation indicate that the effects of seismic activity lessen to the east between the cities of Patterson (to the west) and Turlock (to the east). Both the deterministic and probabilistic (5% probability of exceedence in 50 years or 975 year average return period) methodologies were evaluated for use in developing the preliminary seismic design parameters. The Project corridor is not located in a deep sedimentary basin, and no basin amplification was necessary. For both the western and eastern ends of the Project area, the Probabilistic Acceleration Response

Spectrum controls. The preliminary peak ground acceleration (PGA) for the western end of the Project is estimated at 0.55g (“g” is the acceleration due to gravity). The associated preliminary peak spectral acceleration at the western end of the Project is estimated at about 1.4g at an approximately 0.2 second period. The preliminary PGA for the eastern end of the Project is estimated at 0.32g. The associated preliminary peak spectral acceleration at the western end of the Project is estimated at about 0.73g at an approximately 0.2 second period.

Liquefaction Potential A preliminary review of potential liquefaction of the Project site soils was performed to determine the possible extent of liquefaction within the Project corridor. Liquefaction is the process in which the seismic shear waves cause an increase in the pore water pressure in a cohesionless (sand and some non-cohesive silts) soil strata. This increase in pore water pressure reduces the effective stress confining the soil. The reduction in effective stress causes a reduction in the shear modulus of the soil, which in turn, results in increased soil deformation. Also associated with liquefaction is a loss in bearing strength. In the case of full liquefaction, when the increase in pore water pressure reduces the confining stress to zero, the soil experiences a full loss of strength and undergoes large viscous deformations. Lateral spreading (large lateral deformations) are possible when liquefaction occurs in ground having even minimal slope. We also reviewed the potential for dry dynamic settlement where cohesionless soils, which are in a loose to medium dense state, when subjected to seismic shear waves compact in place in a similar manner to being compacted with a vibratory roller. The energy of the seismic event reorganizes the grains to a more dense state and subsequently causes a reduction in the overall volume resulting in settlement of the soil.

There is a relatively moderate to high potential for significant ground motions in the Project area. The potential exists for loose to medium dense granular soils to be encountered throughout the study area within the San Joaquin Valley, particularly near the San Joaquin River and its tributaries. In addition, groundwater levels were found to range between 15 to 30 feet in depth within the Project area (based on the State of California Department of Water Resources data). Accordingly, liquefaction studies for this project will need to be performed within the Project area, where future subsurface investigations indicate the presence of potentially liquefiable materials.

Seismically Induced Settlement and Lateral Spreading Seismic settlement can occur as a result of compaction, densification, and/or reorientation of grains in embankment soils or the underlying native soil. Seismic settlement is not well understood; however, loose granular soil and elastic silts would be most susceptible to the phenomenon.

There is a relatively moderate to high potential for significant ground motions in the Project area. The granular soils and soft non-plastic silts and sandy silts encountered within the project limits may be susceptible to seismic settlement during a significant seismic event.

Liquefaction-induced lateral spreading is the lateral displacement of gently sloping ground as a result of pore pressure build-up or liquefaction in a shallow underlying deposit during an earthquake.

Liquefaction-induced lateral spreading generally occurs on mild slopes underlain by loose sands and high groundwater. If liquefaction occurs, the unsaturated overburden soil can slide as intact blocks, over the lower liquefied deposit. The geologic conditions conducive to lateral spreading (gentle surface slopes, high groundwater, and cohesionless soils) are frequently found along streams or other water courses, in recent alluvial or deltaic deposits and in loose, saturated sandy fills. Lateral spreading can cause slumping (settlement) of embankments and rotation of structures. The potential for liquefaction-induced lateral spreading of embankment fills over soft/loose alluvial soils within the Project limits are expected to be moderate during a significant seismic event.

Additional studies will need to be performed within the Project area, where future subsurface investigations indicate the presence of potentially liquefiable materials, which may lead to seismic settlement and/or lateral spreading.

Regional Subsidence Subsidence is a general lowering of the ground surface. The primary causes of subsidence are groundwater withdrawal, settlement and oxidation of peat deposits, and withdrawal of oil and gas. Due to extensive groundwater withdrawal and accompanying subsidence, the USGS has installed a land subsidence monitoring network, with particular attention being paid to the Delta-Mendota Canal. A recent study that analyzed the effects of subsidence along the Delta-Mendota Canal (Sneed, et al, 2013) indicated that land-surface deformation measurements along the northern reach of the Delta- Mendota Canal (including the study area) were fairly stable to minimally subsiding on an annual basis. The areas of greatest subsidence appear to occur south of the project study area between the towns of Los Banos and Huron near the west side of the valley. Accordingly, the potential for significant subsidence within the project area is considered to be low to moderate.

Expansive Soil Expansive soils tend to shrink and swell due to changes in moisture content. Repeated shrinking and swelling can cause damage to pavement and structures. The degree of shrink and swell will depend on the amount of clay in the soil and the type of clay (some clays tend to swell more than others). According to the USDA, Soil Survey of Stanislaus County, (Northern Part, 2007; Western Stanislaus County, 2014; and Eastern Stanislaus County, 2014) many of the soils in the project area consist of clayey soils which have a moderate to high shrink-swell potential. Additional studies will need to be performed within the Project area, where future subsurface investigations indicate the presence of potentially expansive soils.

Compressible Soil It is expected that the upper soils throughout the Project area may contain of soft/loose soils, which may be susceptible to settlement caused by compression from loading due to structures and/or embankments. Additional studies will need to be performed within the Project area, where future subsurface investigations indicate the presence of potentially compressible soils.

Collapsible Soil Soils with a porous structure and low cohesion can have the potential to collapse upon saturation and/or loading. Based on review of available literature, no indication of the presence of collapsible soil in the Project area was found. The potential for collapsible soil in the Project area is expected to be low.

Slope Stability With the exception of the western margin of the Project area where the valley transitions to the Diablo Range Mountains to the west, the general topography of the Project area is essentially level to gently sloping. The only significant slopes within the Project are embankments created for the freeway interchanges, raised roadways, embankments of the California Aqueduct and Delta-Mendota Canal, and the banks of the various rivers and streams. A site-specific reconnaissance should be performed where the proposed alignments will encounter any of these features to assess the visually stability of the existing slopes and to determine if additional slope stability analysis is warranted.

Flooding The Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs) were researched for floodplain information within the SCC feasibility study area (Figure 5). The existing West Main Street/Las Palmas Avenue roadway extends through the San Joaquin River floodplain. This floodplain is identified as Zone A, which represents areas that are subject to inundation by the 100-year flood event where no base flood elevations have been determined. Del Puerto Creek, which extends through the northern end of the feasibility study area, and Orestimba Creek, which extends through the southern end of the feasibility study area, are identified as Zone AE. This zone represents areas that are subject to inundation by the 100-year flood event determined by detailed methods. Much of the city of Patterson is identified as Zone X (shaded), which designates areas within the 500-year floodplain. There are other areas throughout the southern portion of the feasibility study area that are designated as Zone AO and Zone X (shaded). These areas are located predominately adjacent to and surrounding the Southern Pacific Railroad; these floodplain areas are associated with named and unnamed channels, canals and laterals. Zone AO represents areas that are subject to inundation by the 100-year flood event with flood depths between 1 to 3 feet. All other areas not identified as Zone A, Zone AE, Zone AO, or Zone X (shaded), are classified as Zone X (unshaded). Zone X (unshaded) designates areas determined to be outside of the 500-year floodplain.

Widening, realignment, or modification of the existing roadway, or the construction of new segments of roadway, have the potential to result in additional fill and increases to floodplain elevations. Avoidance, minimization, and mitigation measures would need to be considered to reduce or maintain the existing floodplain characteristics. These efforts could include balancing the amount of proposed fill and cut, and designing retention or detention basins.

Figure 5, Floodplain Map Source: WRECO 2015

Earthquake Induced Flooding Due to the presence of earth embankments for the California Aqueduct and Delta-Mendota Canals, the potential for earthquake induced flooding is considered to be moderate to high on the west side of the Project area should a failure occur along these embankments during a significant seismic event. Inundation maps for Stanislaus County indicate that flooding from a failure of upstream dams in the Sierra Nevada foothills would likely only affect properties north of the Project.

Tsunami and Seiche Tsunamis are great sea waves typically generated by earthquake shaking or ground displacement. A Seiche is wave action created within restricted bodies of water typically in response to an earthquake. The potential hazard due to a tsunami is non-existent. Due to the lack of large bodies of water near the study area, the potential for a damaging Seiche is considered to be non-existent.

Volcanic Hazard Potential hazards associated with volcanic activity are estimated to be low (Miller, 1989).

Mining A review of the geologic literature did not reveal the presence of past mining activities within the Project.

Corrosion The California Department of Transportation (Caltrans) has the following definition of corrosive soils:

“For structural elements, the Department considers a site to be corrosive if one or more of the following conditions exists for the representative soil and/or water samples taken at the site:

o Chloride concentration is 500 ppm or greater, o Sulfate Concentration is 2000 ppm or greater, o pH is 5.5 or less.”

Future corrosion sampling and testing for the Project will be conducted following the Caltrans guidelines set forth in Section 6 - Requirements for Conducting Corrosion Investigations of Project Sites and Section 7 - Bridge Structures of the Corrosion Guidelines, Version 2.0, November, 2012.

The following mitigation measures should be employed as prudent engineering practice in the absence of site specific corrosive potential testing. For concrete, the use of mineral admixtures (such as fly ash, silica fume, metakolin, etc…), a reduced water content, and increased cementitious material content generally result in a high-density, durable concrete which is more resistant to corrosion. According to Table 854.1A in the Guide for the Protection of Reinforced and Unreinforced Concrete Against Acid and Sulfate Exposure Conditions, and Section 8.22 of the Bridge Design Specifications, the maximum water-to-cementitious material ratio shall not exceed 0.40 and a minimum of 3-inches clear cover shall be provided for all reinforcing bars where the concrete is cast against the surrounding soils. We also recommend the use of a minimum of 675 pounds per cubic yard of cementitious material and Type II Modified or Type V cement with 25-percent mineral admixtures be used on all locations where the concrete is to remain in permanent contact with the surrounding soils. Using Figure 854.3B, Minimum Thickness of Metal Pipe for 50 Years of Maintenance Free Service Life from the Caltrans Highway Design Manual, the minimum corrugated metal pipe thickness should be 16-gauge and should be constructed of galvanized steel. This minimum thickness is based upon corrosion assessment only and the pipe section should be checked structurally to determine the minimum thickness based upon the proposed loading requirements. To help mitigate the corrosion of the bridge foundations, Caltrans Standard Specification Section 50 should be followed for pre-stressing concrete.

SUMMARY OF GEOLOGIC HAZARDS

The San Joaquin/Orestimba (Great Valley 07) fault is mapped as crossing I-5 just north of the Project and then is mapped as trending sub-parallel to I-5 (to the east) southeastward through the west side of the Project. Further study will be required to establish the potential of a fault rupture hazard to structures in the near vicinity of the San Joaquin/Orestimba (Great Valley 07) fault. Seismic shaking is expected to occur at the Project site with the degree of shaking lessening to the east further away from mapped faults near the west side of the Project. Our analyses indicate that potential for seismic shaking is expected to be moderate to high within the Project.

Since there is a relatively moderate to high potential for significant ground motions in the Project area combined with relatively high groundwater levels, the granular soils and soft non-plastic silts and sandy silts that may be encountered within the Project limits may be susceptible to liquefaction during a significant seismic event. Additional studies will need to be performed within the Project area, where future subsurface investigations indicate the presence of potentially liquefiable materials, to determine the potential extent of seismic settlement and/or lateral spreading.

Moderately expansive to expansive clay soils may be encountered within the Project. Potentially expansive soil of this type can be mitigated through standard grading practices and/or modification of structure foundation depths and loading, however at additional cost to the project. Mitigation measures may include excavation of expansive soils and replacement with approved borrow material; the use of geotextile materials; chemical treatment; and/or other approved methods. Areas that are expected to contain a greater percentage of clay (potentially expansive soils) are discussed further in the following section, “Preliminary Roadway Construction Assessment”.

A site-specific reconnaissance should be performed where the proposed alignments will encounter sloping ground to assess the visually stability of the existing slopes and to determine if additional slope stability analysis is warranted.

The potential flood risk associated with the implementation of the proposed Project includes but is not limited to: 1) change in land use, 2) change in impervious surface area, 3) fill inside the floodplain, or 4) change in the 100-year water surface elevation. Additional information related to flood risks to the Project are presented in the South County Corridor Project – Preliminary Hydrology, Floodplain, and Water Quality Technical Memo (WRECO, 2015).

Due to the presence of earth embankments for the California Aqueduct and Delta-Mendota Canals, the potential for earthquake induced flooding is considered to be moderate to high on the west side of the Project area should a failure occur along these embankments during a significant seismic event.

PRELIMINARY ROADWAY CONSTRUCTION ASSESSMENT

Based on the surface geologic conditions within the Project study area we have developed a preliminary assessment of the relative costs of roadway construction associated with the various soil types presented in the USDA Soil Surveys of Stanislaus County (USDA, 2007 and 2014).

USDA Soil Survey Soil Groups and Estimated Relative Roadway Construction Costs By overlying the USDA soil survey maps on the proposed Level 2 Alignments, the proposed alignments generally include five different soil groups: sandy loam, loamy sand, loam, gravelly clay loam, and clayey loam. According to the USDA, loam is defined as soil material that is 7 to 27 percent clay particles, and 28 to 50 percent silt particles, and less than 52 percent sand particles. Sandy loam is identified as predominately sand, likewise, clayey loam is identified as predominately clay and therefore referred to as clay soil.

Numerical ratings, ranging from 1 to 4, were assigned to the five different soil groups identified on the Level 2 Alignments and are presented in Table 3, below. These ratings are based on the soil group’s assumed strength and shrink-well potential properties presented in the USDA soil surveys that would affect their traffic supporting capacity. The soil group with the lowest number (Rating 1) is identified to be the relatively most favorable for local roads and streets use. Subgrade soil types with coarser material (sand/gravel) will typically have higher R-values, and thus will require a thinner pavement section which would be more economically favorable. The finer (clay/silt) material typically has lower strength (lower R-Values) than the granular material and thus would require relatively thicker (i.e., more costly) pavement sections to support the predicted traffic loads. Additionally, the alignments founded on clay soils may be prone to larger shrink-swell potential which may require mitigation (added cost). Refer to the next section for a discussion of subgrade enhancement geosynthetic (SEG) as a mitigation alternative.

Table 3, Level 2 Alignments Soil Rating Summary Rating Subgrade Relative General Soil Rating Segments/Alignments Description Soil Type1 Description2 • 6, 7, 8, 12 East of Jennings Least • 11, 14, 15, 16, 17, 18, 27, 28, Predicted Sandy Loam Moderately drained soils, 29, 30 (Appendix I, Figure 1. 1 Cost for and Loamy high relative soil strength, Part 3 of 7) Pavement Sand low swell-shrink potential • Construction 26 (Appendix I, Figure 1. Part 4 of 7) Poorly drained soils, • West of Sycamore: 3, 4, 12, medium relative soil 2 Loam 13 (Appendix I, Figure 1. strength, low swell-shrink Part 2 of 7) potential Poorly drained soils, • 1, 2 (Appendix I, Figure 1. Gravelly Clay medium relative soil 3 Part 2 of 7) Loam strength, high swell-shrink

potential • East Sycamore Ave: 3, 9, 13 • Between Sycamore and Ash Highest Ave only: 6, 7, 8, 12 Predicted Poorly drained soils, low Clay/Clay (Appendix I, Figure 1. Part 2 4 Cost for relative soil strength, high Loam of 7) Pavement swell-shrink potential • Construction 21, 22, 23, 25 (Appendix I, Figure 1. Part 5 of 7)

1 Generalized soil types per the USDA Web Soil Survey, Northern Part, 2007; Western and Eastern Stanislaus County, 2014. 2 Generalized soil descriptions per the USDA Web Soil Survey, Northern Part, 2007; Western and Eastern Stanislaus County, 2014. Figure 6 provides an illustrative summary of the soil types found on the Level 2 Alignments and Table 4 provides relative percentages of the proposed alignment segments per USDA soil classification. Appendix I contains a detailed illustrative breakdown of the USDA soil survey soil types of each section described above.

Table 4, Proposed Roadway Segments and USDA Soil Classification

Miles of Roadway per USDA Classification % Segment per USDA Classification

Length Segment of Gravelly Loamy Sand/ Gravelly Loamy Sand/ Clay Loam Clay Loam No. Segment Loam Clay Sandy Loam Clay Loam Sandy Loam Loam (Rating 2)1 (Rating 4)1 (mile) Loam (Rating 1)1 (Rating 3)1

02 18.6 11.7 2.6 2.2 2.1 63% 14% 12% 11% 1 3.1 0.0 0.0 3.1 0.0 0% 0% 100% 0% 2 3.4 0.0 0.0 3.4 0.0 0% 0% 100% 0% 3 5.2 0.0 0.0 1.7 3.5 0% 0% 33% 67%

4 1.9 0.0 1.9 0.0 0.0 0% 100% 0% 0%

6 6.3 4.3 0.0 0.0 2.0 68% 0% 0% 32%

7 5.4 3.3 0.0 0.0 2.1 61% 0% 0% 39%

8 5.7 3.6 0.0 0.0 2.1 63% 0% 0% 37%

9 3.0 0.0 0.0 0.0 3.0 0% 0% 0% 100%

11 3.0 2.7 0.0 0.0 0.3 90% 0% 0% 10%

12 7.3 3.3 1.8 0.0 2.2 45% 25% 0% 30%

13 5.1 0.0 0.0 1.7 3.4 0% 0% 33% 67%

14 2.0 2.0 0.0 0.0 0.0 100% 0% 0% 0%

15 1.0 1.0 0.0 0.0 0.0 100% 0% 0% 0%

16 5.8 5.8 0.0 0.0 0.0 100% 0% 0% 0%

17 2.0 2.0 0.0 0.0 0.0 100% 0% 0% 0% 18 1.0 1.0 0.0 0.0 0.0 100% 0% 0% 0% 21 9.0 0.2 0.0 0.0 8.8 2% 0% 0% 98% 22 1.8 0.0 0.0 0.0 1.8 0% 0% 0% 100% 23 1.8 0.0 0.0 0.0 1.8 0% 0% 0% 100% 25 7.0 0.1 0.0 0.0 6.9 1% 0% 0% 99% 26 1.8 1.8 0.0 0.0 0.0 100% 0% 0% 0% 27 2.4 2.4 0.0 0.0 0.0 100% 0% 0% 0% 28 3.3 3.3 0.0 0.0 0.0 100% 0% 0% 0% 29 1.0 1.0 0.0 0.0 0.0 100% 0% 0% 0% 30 1.0 1.0 0.0 0.0 0.0 100% 0% 0% 0% 1 Ratings refer to ratings described in Table 3, Level 2 Alignments Soil Rating Summary. 2 Segment 0 represents the existing SCC Alignment.

Legend: Soil Map Survey Summary

Rating 1: Sandy Loam & Loamy Sand

Rating 2: Loam

Rating 3: Gravelly Clay Loam

Rating 4: Clay & Clayey Loam

Figure 6, SCC Level 2 Alignment Soil Survey Summary

Source: TYLIN International, 2015. South County Corridor Conceptual Alignments, Overall Project Map, Level 2 Alignments, Exhibit 1 of 1, Scale 1” = 10,000’

Subgrade Mitigation Subgrade Enhancement Geosynthetic (SEG), which includes geotextile and geogrid, can be used to stabilize soft subgrade materials and increase the pavement subgrade bearing capacity. Geotextile is a sheet-like structure that can serve as a physical tensioned barrier between the subgrade and the pavement material to prevent the two from mixing. Geotextile also provides filtration and drainage, as well as an increase in the pavement’s lateral restraint and reinforcement. The friction created between the geotextile and pavement material provides the lateral restraint in the pavement section, while the stretching of the geotextile and deformation of the soft subgrade provides the reinforcement of the pavement section. On the other hand, geogrid is an apertured sheet-like structure that interlocks with the pavement aggregate material (subbase or base) to create a stiff aggregate platform that provides lateral restraint and reinforcement. Geogrid also serves to keep the pavement and soft subgrade material separate from one another, as well as providing vertical drainage (Caltrans, 2013). Use of SEG can reduce the pavement section thicknesses as well as reduce the long-term maintenance cost and premature failure in the alignments with poor soils. Figure 7 below provides a summary flowchart for selecting the type of geosynthetic.

Figure 7, SEG selection

Source: Caltrans, 2013

LIMITATIONS

This technical memorandum was performed in accordance with generally accepted geotechnical engineering principles and practices. No other warranty, expressed or implied, is made as to the conclusions and professional recommendations made in this report.

This technical memorandum is intended for use with the South County Corridor Project located in Stanislaus County, and any changes in the design or location of the proposed new improvements, however slight, should be brought to our attention so that we may determine how they may affect our conclusions and recommendations. The conclusions and recommendations contained in this technical memorandum are based upon the data relating only to this specific project and locations discussed herein.

This memorandum constitutes a preliminary study based on a review of readily available literature in support of the Feasibility Study for the South County Corridor Project and is not a comprehensive geotechnical investigation and should not be construed as such. Once the preferred alternative alignment(s) have been determined a comprehensive geotechnical investigation should be undertaken to provide detailed geotechnical analysis for the planned improvements.

Attachments: Appendix 1

REFERNCES

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Caltrans, 2009. Caltrans Deterministic PGA Map, by Merriam and Shantz, 2009.

Caltrans, 2012. Caltrans Geotechnical Services Design Manual, Version 1.0, June, 2012.

Caltrans, 2012. Caltrans Corrosion Guidelines, Version 2.0, Caltrans Division of Engineering Services, Materials Engineering and Testing Services, Corrosion and Structural Concrete Field Investigation Branch, November 2012.

Caltrans, 2012. Methodology for Developing Design Response Spectrum for Use in Seismic Design Recommendations, Caltrans Division of Engineering Services, Geotechnical Services, November 2012.

Caltrans, 2013. Caltrans Seismic Design Criteria, Version 1.7, April, 2013.

Caltrans, 2013. Caltrans ARS Online Version 2.2.0.6, Division of Research and Innovation, Caltrans GeoResearch Group, Updated: April 18, 2013. http://dap3.dot.ca.gov/ARS_Online/index.php

Caltrans, 2013. California Department of Transportation Pavement Program, Office of Concrete Pavements and Pavement Foundations. Subgrade Enhancement Geosynthetic Design and Construction Guide.

California Geological Survey, 2003. Probabilistic Seismic Shaking Hazards in California, USGS/CGS Probabilistic Seismic Hazards Assessment (PSHA) Model, 10% probability of being exceeded in 50 years. http://www.consrv.ca.gov/CGS/rghm/pshamap/pshamain.html.

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Appendix I: USDA Soil Maps

Legend: Soil Map Survey Summary

Rating 1: Sandy Loam & Loamy Sand Figure 1 (Part 1 of 7) Rating 2: Loam Soil Map Rating 3: Gravelly Clay Loam Segment Key South County Corridor Rating 4: Clay & Clayey Loam Stanislaus County, CA WRECO Project No. P14104

Legend

Segment No. ## Segment Alignment 6

Node

8 12

3 12 2 9

Zacharias Rd. 1 4

Note: See Figure 1, Part 6 of 7 for map unit description.

Figure 1 (Part 2 of 7) Soil Map Segment 1, 2, 3, 4, 6, 7, 8, 9, 12, 13 Reference: USDA Natural Resources Conservation Service, Web Soil Survey National Cooperative Soil Survey, 2014. Soil Map – Western South County Corridor Stanislaus County, California, Map Scale 1:24,000, Map projection: Web Mercator, Corner Coordinates: WGS84, Edge tics: UTM Zone 10N WGS84. Stanislaus County, CA WRECO Project No. P14104

6 Legend

## Segment No.

12 17 18 Segment Alignment

7 Node

29 30

13 W. Main Avenue 16 3 11

15 14

Jennings Rd

27 Rd.

4 Carpenter

Note: See Figure 1, Part 7 of 7 for map unit description.

Figure 1 (Part 3 of 7) Soil Map Segment 6, 7, 8, 11, 12, 14, 15, 16, 17, 18, 27, 28, 29, 30 Reference: USDA Natural Resources Conservation Service, Web Soil Survey National Cooperative Soil Survey, 2014. Soil Map – Eastern Stanislaus County, California, Map Scale 1:24,000, Map projection: Web Mercator, Corner Coordinates: WGS84, Edge tics: UTM Zone 10N South County Corridor WGS84. Stanislaus County, CA WRECO Project No. P14104

14 15 W. Main Avenue 16

27 28

W. Hardening Rd

3 28

4 Legend ## ## Segment No.

S. Morgan S. Rd. Segment Alignment 21 25 Node

Note: See Figure 1, Part 7 of 7 for map unit description. Figure 1 (Part 4 of 7) Soil Map Segment 15, 16, 26, 27, 28 Reference: USDA Natural Resources Conservation Service, Web Soil Survey National Cooperative Soil Survey, 2014. Soil Map – Eastern South County Corridor Stanislaus County, California, Map Scale 1:24,000, Map projection: Web Mercator, Corner Coordinates: WGS84, Edge tics: UTM Zone 10N Stanislaus County, CA WGS84. WRECO Project No. P14104

E. Marshall Rd.

Legend

## Segment No.

Segment Alignment

Node

Fink Rd. 23 25

Note: See Figure 1, Part 6 of 7 for map unit description. Figure 1 (Part 5 of 7) Soil Map Segment 21, 22, 23, 25 Reference: USDA Natural Resources Conservation Service, Web Soil Survey National Cooperative Soil Survey, 2014. Soil Map – Western South County Corridor Stanislaus County, California, Map Scale 1:24,000, Map projection: Web Mercator, Corner Coordinates: WGS84, Edge tics: UTM Zone 10N Stanislaus County, CA WGS84. WRECO Project No. P14104

Figure 1 (Part 6 of 7) Soil Map Map Unit Symbols Reference: USDA Natural Resources Conservation Service, Web Soil Survey National Cooperative Soil Survey, 2014. Soil Map – Western South County Corridor Stanislaus County, California, Map Scale 1:24,000, Map projection: Web Mercator, Corner Coordinates: WGS84, Edge tics: UTM Zone 10N Stanislaus County, CA WGS84. WRECO Project No. P14104

Figure 1 (Part 7 of 7) Soil Map Map Unit Symbols South County Corridor Reference: USDA Natural Resources Conservation Service, Web Soil Survey National Cooperative Soil Survey, 2014. Soil Map – Eastern Stanislaus County, CA Stanislaus County, California, Map Scale 1:24,000, Map projection: Web Mercator, Corner Coordinates: WGS84, Edge tics: UTM Zone 10N WGS84. WRECO Project No. P14104