4.6 geology and soils - butte county, california · 2014-10-02 · section 4.6 geology and soils...

Rio d’Oro Specific Plan EIR Section 4.6 Geology and Soils

County of Butte 4.6-1

4.6 GEOLOGY AND SOILS This section evaluates geologic and soil impacts from implementation of the proposed Specific Plan, including geologic hazards and soil erosion. The analysis is based in part on two reports produced in July 2008 by Neil O. Anderson & Associates: Preliminary Geotechnical Investigation Rio d’Oro Pacific Heights Road & Hwy 70, Oroville, California and Geohazard Investigation Rio d’Oro West of Highway 70, Oroville, California which are located in Appendix E to this EIR. 4.6.1 Setting

a. Regional and Local Geologic and Hydrogeologic Conditions. The Specific Plan area is located within the eastern foothills of the Great Valley geomorphic province and the western Sierra Nevada Mountains geomorphic province. The Great Valley province is an alluvial plain about 50 miles wide and 400 miles long in the central part of California. The northern portion is within the Sacramento Valley and drained by the Sacramento River. The southern portion is within the San Joaquin Valley and drained by the San Joaquin River. The Great Valley is a trough in which sediments have been deposited almost continuously since the Jurassic period (about 160 million years ago). Vast oil fields have been found in the southernmost San Joaquin Valley and along anticlinal uplifts on its southwestern margin. The Sierra Nevada geomorphic province is a tilted fault block nearly 400 miles long. Its east face is a high, rugged, multiple scarp, contrasting with the gentle western slope that disappears under the sediments of the Great Valley. Deep river canyons are cut into the western slope. Their upper courses, especially in massive granites of the higher Sierra, are modified by glacial sculpting, forming such scenic features as Yosemite Valley. The high crest culminates in Mt. Whitney with an elevation of 14,495 feet above sea level near the eastern scarp. The metamorphic bedrock contains gold bearing veins in the northwest trending Mother Lode. The northern Sierra boundary is marked where bedrock disappears under the Cenozoic volcanic cover of the Cascade Range. The Specific Plan area ranges from 120 feet above mean sea level in the southwest corner to 243 feet above mean sea level in the northeast corner and is underlain by the following geologic formations and material:

Red Bluff Formation: This formation is characterized by a thin veneer of highly weathered bright red gravels thought to have formed as a result of impeded drainages within the Sacramento Valley;

Turlock Lake Formation: The formation is characterized by weathered gravels and metamorphic rock fragments and quartz pebbles; silt and sand is present along the south and east side of the Sacramento Valley; and

Laguna Formation: This formation is characterized by a mixture of gravel, sand and silt. Pebbles and cobbles of quartz and metamorphic rock fragments are the predominant constituents of the gravels. In the Oroville area, volcanic rocks can comprise up to 20 percent of the gravel.

The closest groundwater basin to the Specific Plan area is the East Butte Subbasin of the Sacramento Valley Groundwater Basin, the eastern boundary of which is the Feather River,



located immediately to the west of the project site. The East Butte aquifer system is comprised of deposits of late Tertiary to Quaternary age. The Quaternary deposits include Holocene stream channel deposits and basin deposits, Pleistocene deposits of the Modesto and Riverbank formations, and Sutter Buttes alluvium. The Tertiary deposits include the Tuscan and Laguna formations (CDWR, February 2004). The only one of these deposits that occurs on the project site, as described above, is the Laguna formation. The following description of this formation is from the Department of Water Resource’s (DWR’s) California Groundwater Bulletin 118, Hydrologic Region South Coast California’s Groundwater Sacramento Valley Groundwater Basin, East Butte Subbasin, most recently updated in February 27, 2004.

Pliocene Laguna Formation. The Laguna Formation consists of interbedded alluvial sand, gravel, and silt deposits which are moderately consolidated and poorly-to-well cemented. The Laguna is compacted and generally has a low-to-moderate permeability, except in scattered gravels in the upper portion. The formation yields moderate quantities of water to wells along the eastern margin of the valley. Wells of higher capacity generally tap underlying Tuscan deposits. Surface exposures of the Laguna appear along the eastern margin of the subbasin in the vicinity of the Thermalito Afterbay and extend westerly in the subsurface. The lateral extent of the formation is unknown. The thickness of the formation is difficult to determine because the base of the unit is rarely exposed. Estimates of maximum thickness range from 180 feet (Helley and Harwood 1985) to 1,000 feet (Olmsted and Davis 1961). Geologic cross sections developed by California Department of Water Resources estimate the thickness to be approximately 500 feet (DWR 2000). Wells completed in the formation yield only moderate quantities of water.

As part of the geohazard investigation performed for the project (see Appendix E), several groundwater information sources were consulted to determine the depth and flow direction of groundwater below the site. According to Butte County water well information, groundwater was generally located at an elevation of approximately 70 to 131 feet above sea level in the Spring 2009 period in the north Yuba area. The closest monitored well to the site at this time was approximately 2,000 feet east of the site, and water elevation in the well was 122.8 feet in the Spring of 2008. If groundwater elevation is fairly consistent in the area, then groundwater may occur at approximately 120 feet below ground surface at the site (243 feet highest elevation on the site minus 122.8 feet groundwater elevation in the well located 2,000 feet to the east of the property). Based on existing information, it is assumed that groundwater follows the ground slope, flowing from the higher northeastern topography to the west or southwest below the site west toward the Feather River.

b. Seismic Setting. The Specific Plan area is located within a seismically active region, at the base of the foothills of the Sierra Nevada Mountains, and about 37 miles east of the Coast Range. No faults have been mapped directly beneath the site and the site does not lie within an Alquist-Priolo special studies zone. These are areas defined by the State of California where potential ground rupture may occur during an earthquake. However, there are several faults in the region capable of producing seismic ground shaking at the site, including the Foothills Fault System; the Coast Range-Central Valley (CRCV) geomorphic boundary, also known as the Great Valley Fault; and the San Andreas Fault Zone. Figure 4.6-1 shows these regional faults. The seismic and fault hazards relevant to the Specific Plan area are described below.

_̂

F o o t h i l l sF a u l t

S y s t e m

S a nA n d r e a s

F a u l tZ o n e

Cleveland Hills Fault

Regional Faults

Imagery provided by ESRI and its licensors © 2013.Fault data from US Geologic Survey, 2013.

Section 4.6 Geology and SoilsRio d'Oro Specific Plan EIR

County of ButteFigure 4.6-1

0 2814Miles ±

_̂ Project LocationSan Andreas Fault ZoneFoothills Fault SystemCleveland Hills FaultGreat Valley Fault Zone*

G r e a tV a l l e y

F a u l t Z o n e

*Location of Great Valley Fault Zone is Approximate



c. Seismic Hazards. Faults generally produce damage in two ways: ground shaking and surface rupture. Seismically induced ground shaking covers a wide area and is greatly influenced by the distance of a site to the seismic source, soil conditions, and depth to groundwater. Surface rupture is limited to very near the fault. Other hazards associated with seismically induced ground shaking include earthquake-triggered landslides, liquefaction, and settlement. The Uniform Building Code identifies the Specific Plan area as being in Seismic Zone 3, which is characterized as having the second-highest earthquake risk.

Faulting. The U.S. Geological Survey defines active faults as those that have had surface displacement within Holocene time (about the last 11,000 years). Holocene surface displacement can be recognized by the existence of cliffs in alluvium, terraces, offset stream courses, fault troughs and aligned saddles, sag ponds, and the existence of steep mountain fronts. Potentially active faults are those that have had surface displacement during Quaternary time, within the last 1.6 million years. Inactive faults have not had surface displacement within the last 1.6 million years. The most likely active faults to seismically affect the Specific Plan area are the Foothills Fault System; the Great Valley 1, Great Valley 2, and Great Valley 3 faults; and the San Andreas Fault Zone, as shown on Figure 4.6-1 and described below.

Foothills Fault System. This fault system, which roughly defines the boundary between the Central Valley and the Sierra Nevada, lies approximately 4 miles east of the site. The Foothills Fault System, between Folsom and Oroville, is bound on the east by the northward trending Melones fault zone and on the west by the northwestward trending Bear Mountain fault zone (Bulletin of the Seismological Society of America, February 1978). The magnitude 5.7 Oroville earthquake occurred on this system in August 1975 on the Cleveland Hill Fault near Lake Oroville Reservoir, approximately 4.3 miles to the southeast of the Specific Plan area, producing a rupture length of 3.4 miles.

The Coast Range-Central Valley (CRCV) geomorphic boundary (Great Valley Fault). This geomorphic boundary and 310 mile long seismically active fold and thrust belt, is located approximately 33 miles west of the site. Tectonic processes involved with the Coast Ranges are a significant source of seismicity, faulting, and folding, and have resulted in several significant earthquakes including the 1983 magnitude 6.5 Coalinga earthquake, which caused considerable damage in the Coalinga area.

San Andreas Fault Zone. The San Andreas Fault Zone is the dominant active fault in California. It is located about 145 miles to the west of the site. The San Andreas Fault Zone is considered the active boundary between the North American tectonic plate to the east, the Pacific plate to the west, and the Juan de Fuca plate to the north. Two of the biggest earthquakes in California history occurred along the San Andreas Fault, the 1857 Fort Tejon magnitude 7.92 earthquake and the 1906 San Francisco magnitude 7.68 earthquake.

Seismic Risk and Ground Acceleration. The proximity of active faults is such that the Specific Plan area has and will experience strong seismically induced ground motion. The Health and Safety Element of the Butte County General Plan 2030 states that “Conservatively, ground motions as strong as those observed during the 1975 Oroville earthquake (Modified Mercalli Intensity VIII) can be expected anywhere in Butte County.” The Modified Mercalli Intensity (MMI) scale is the most common intensity scale, and provides values ranging from I to XII as



shown in Table 4.6-1. A measurement of I indicates the seismic event was generally not felt. As the level of intensity increases, so does the rating. A measurement of XII indicates visible waves on the ground surface and destruction of buildings and related damage.

Table 4.6-1 Modified Mercalli Intensity Scale

I. Not felt except by a very few under especially favorable circumstances.

II. Felt only by a few persons, especially on upper floors of buildings. Delicately suspended objects may swing.

III. Felt noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing of truck. Duration estimated.

IV. During the day felt by many, felt outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows, etc. broken; a few instances of cracked plaster. Unstable objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.

VI. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.

VII. Everybody runs outdoor. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars.

VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amount. Changes in well water. Disturbs persons driving motor cars.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from riverbanks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII. Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.

Source: K.V. Steinbrugge 1982. Earthquakes, Volcanoes, and Tsunamis, and Anatomy of Hazards.

Earthquakes are characterized by magnitude, which is a quantitative measure of the strength of the earthquake based on strain energy released during a seismic event. The magnitude of an earthquake is constant for any given site and is independent of the site in question. The intensity of an earthquake at a given site; however, is not constant. The intensity is an indirect measurement of ground motion at a particular site and is affected by the earthquake magnitude, the distance between the site and the hypocenter (the location on the fault at depth where the energy is released), and the geologic conditions between the site and the hypocenter. Intensity, which is often measured by the Mercalli scale, generally increases with increasing magnitude and decreases with increasing distance from the hypocenter. Topography may also affect the intensity of an earthquake from one site to another. Topographic effects such as steep sided ridges or slopes may result in a higher intensity than sites located in relatively flat-lying areas.



Deterministic Seismic Analysis. The geohazard investigation (Appendix E) utilized the 2007 California Building Code (CBC) seismic design parameters and the 2007 California Building Code Soil Parameters to determine that the Specific Plan area has a peak design ground acceleration of 0.193g (one g equals one times the force of gravity). It also used the Probabilistic Seismic Hazards Mapping Ground Motion page from the California Geological Survey’s California fault and soil model, which estimated a ground motion for the site at 0.169g. This is the Design Basis Ground Motion, which has a 10 percent probability of being exceeded in a given 50-year exposure period. The degree of ground shaking that an area is subject to is primarily a function of the distance between the area and the seismic source, the type of material underlying a property and the motion of fault displacement. The Northridge (1994) earthquake showed how peculiarities in basin effects can play a substantial role in ground accelerations at particular locations. For instance, ground accelerations exceeding 1g were recorded at areas far from the epicenter of the Northridge earthquake. It is possible that accelerations near or over the upper bound earthquake ground motion could occur anywhere within the Specific Plan area. Ground shaking can also cause seismic settlement and subsidence, lurch cracking, and lateral spreading. Seismic settlement and subsidence is caused by the compaction of low density alluvium and soils. Lurch cracking is the development of ground fractures, cracks, and fissures produced by ground shaking, settlement, compaction, and sliding that can occur as a result of seismic ground acceleration. These features can occur if high ground accelerations affect an area. Lateral spreading is the horizontal movement or spreading of soil towards an open slope face, such as a stream bank. Lateral spreading is most likely to occur where inappropriately designed artificial fill slopes have been built. d. Other Geologic and Soil Hazards. Additional soil hazards potentially related to seismic activity are discussed below. Ground Rupture. Ground surface rupture results when the movement along a fault is sufficient to cause a gap or rupture along the upper edge of the fault zone on the surface. Since there are no known active faults that cross the Specific Plan area, the potential for ground rupture is considered remote. Landsliding. Landslides are slope failures that occur where the horizontal seismic forces act to induce soil and/or bedrock failures. The most common effect is reactivation or movement on a pre-existing landslide. Existing slides that are stable can become unstable and move during strong ground shaking. The site is located on a terrace with some rolling hills and drainages. According to the geohazard investigation findings (Appendix E), some minor slumping or stream bank erosion may occur during an earthquake in areas adjacent to the drainages, especially during periods of high runoff from rain or over irrigation. Areas along Pacific Heights Road and within the steep slopes of the drainages are most vulnerable to landsliding during an earthquake. If earthquake induced landsliding should occur, low topographic relief and the lack of existing landsliding observed on the site provides evidence that any impact would be relatively small and localized. However, geotechnical considerations should be made with respect to any construction adjacent to these slopes (Neil O. Anderson & Associates, 2008).



Liquefaction and Compaction. Liquefaction is defined as the sudden loss of soil strength due to a rapid increase water pressure within the soil resulting from seismic ground shaking. Liquefaction potential is dependent on such factors as soil type, depth to groundwater, degree of seismic shaking, and the relative density of the soil. When liquefaction of the soil occurs, buildings and other objects on the ground surface may tilt or sink, and lightweight buried structures (such as pipelines) may float toward the ground surface. Liquefied soil may be unable to support its own weight or that of structures, which could result in loss of foundation bearing or differential settlement. Liquefaction may also result in cracks in the ground surface followed by the emergence of a sand-water mixture. The potential for liquefaction is greatest in areas with loose, granular, low-density soil, where the water table is within the upper 40 to 50 feet of the ground surface. Liquefaction can result in slope and foundation failure. Other effects of liquefaction include lateral spread, flow failures, ground oscillations, and loss of bearing strength. Liquefaction is intrinsically linked with the depth of groundwater below the Specific Plan area and the types of sediments underlying an area. This phenomenon occurs only below the water table; however, after liquefaction has developed, it can propagate upward into overlying, non-saturated soil as excess pore water escapes. Liquefaction susceptibility is related to numerous factors, and the following conditions must exist for liquefaction to occur:

Sediments must be relatively young in age and must not have developed large amounts of cementation;

Sediments must consist mainly of cohesionless sands and silts; The sediment must not have a high relative density; Free groundwater must exist in the sediment; and The site must be exposed to seismic events of a magnitude large enough to induce

straining of soil particles. The geohazard investigation material (Appendix E) concluded that, based on estimated depth to groundwater, presence of fine grained soils, shallow depth to bedrock and the relatively low potential seismic acceleration rates at the site, the potential for seismically induced liquefaction is low.

Subsidence and Settlement. Subsidence involves settlement resulting from the withdrawal of fluid (oil, natural gas, or water). When fluids are removed from the subsurface, the overburden weight, which the water had previously helped support through buoyant forces, is transferred to the soil structure. Subsidence typically occurs over a long period of time and results in a number of structural impacts. Facilities most affected by subsidence are long, surface infrastructure, such as canals, sewers, and pipelines. Seismically induced settlement occurs in loose to medium dense unconsolidated soil above groundwater. These soils compress (settle) when subject to seismic shaking. The settlement can be exacerbated by increased loading, such as from the construction of buildings. Settlement can also result solely from human activities, including improperly placed artificial fill, and structures built on soils or bedrock materials with differential settlement rates. In addition, settlement can occur in areas of alluvial deposits. The potential for seismically-induced subsidence (settlement) within the Specific Plan area was evaluated in the geohazard investigation, which found that the



Specific Plan area is not within any known subsidence zone. The geotechnical investigation (Appendix E) found that Specific Plan area soils are not highly subject to subsidence or settlement, but also made various soil engineering recommendations to help reduce the potential for soil problems after project construction. e. Soil Characteristics. Soil resources on the Specific Plan are depicted in Figure 4.6-2 and described below.

Soil Erosion. Soil erosion is the removal of soil by water and wind. The rate of erosion is estimated from four soil properties: texture, organic matter content, soil structure, and permeability. Other factors that influence erosion potential include the amount of rainfall and wind, the length and steepness of the slope, and the amount and type of vegetative cover.

Expansive Soils. Soils with moderate to high potential for shrink-swell exist in a large portion of the Study Area. During periods of water saturation, these soils tend to expand, and during dry periods, the soils tend to shrink. When volumes change with moisture content, cracking can occur within the structures built on these soils. Soils encountered during the field investigation for the preliminary geotechnical investigation (Appendix E) vary across the site. Generally, the upper soils consists of sandy clay and silt underlain by sandy cobbles and clayey sand that extend to depths of one to seven feet below the existing ground surface. The sandy cobbles and clayey sand is generally underlain by very dense sandy cobbles to the maximum depth explored (10 feet). Groundwater was not encountered during these excavations. Surface soils within the Specific Plan area include the following soil types, as described in the Geohazard Investigation (Appendix E), with classification based on the Butte County, California Soil Survey:

118 Xerorthents, Tailings, 0 to 50 Percent Slopes. This unit is composed of spoil piles on stream terraces and flood plains. The parent material is gravelly alluvium derived from igneous, metamorphic, and sedimentary rocks. The surface features consist of linear piles of gravel, cobbles, and stones 5 to 40 feet high. They include depressional areas that are often wet. The typical soil profile transitions from gravelly sandy loam at the surface to loamy sand between 8 and 81 inches below the surface.

331 Thompsonflat Loam, 15 to 30 Percent Slopes. This unit is composed of 85 percent

Thompsonflat loam and 15 percent minor components. This soil has a moderate potential shrink-swell. The typical soil profile transitions from loam in the first two inches beneath the surface, to gravelly loam from 2 to 19 inches below the surface and gravelly clay with varying degrees of loam and sand below 19 inches.

603 Oroville-Thermalito-Fernandez-Thompsonflat Complex, 0 to 9 Percent Slopes. This unit is composed of 30 percent Oroville, gravelly fine sandy loam, 25 percent Thermalito sandy loam, 15 percent Fernandez sandy loam, 15 percent Thompsonflat fine sandy loam, and 15 percent minor components (Gallaway Consulting, Inc., August 2007). This soil has high shrink-swell potential and shallow depth to a restrictive layer for drainage.

4

4

4

3

1

3

3 2

1

Project Area Soils

Imagery provided by ESRI and its licensors © 2013.

Section 4.6 Geology and SoilsRio d'Oro Specific Plan EIR

County of ButteFigure 4.6-2

0 1,600800Feet ±Soils

Project Site

1 - Xerorthents, tailings, 0 to 50 percent slopes2 - Xerorthents, tailings-Urban land complex, 0 to 2 percent slopes3 - Thompson Flat Loam, 15 To 30 percent slopes4 - Oroville-Thermalito-Fernandez-Thompsonflat complex, 0 to 9 percent slopes



f. Regulatory Setting.

Federal and State.

Alquist-Priolo Earthquake Fault Zoning Act. The Alquist-Priolo Earthquake Fault Zoning Act of 1972 (Sections 2621 through 2630 of the California Public Resources Code) is a state law that addresses hazards associated with construction in earthquake fault zones. The purpose of this law is to mitigate the hazard of surface fault rupture by prohibiting the construction of buildings intended for human occupancy along active fault lines. The state has identified Earthquake Fault Zones (known prior to 1994 as Special Studies Zones) along known active faults in California. The law provides policies and criteria to assist cities, counties and state agencies in the exercise of their responsibility to prohibit the location of developments and structures across the trace of active faults.

Seismic Hazards Mapping Act. Whereas the Alquist-Priolo Earthquake Fault Zoning Act addresses the hazard of surface fault rupture only, the Seismic Hazards Mapping Act of 1990 (Sections 2690 through 2699 of the California Public Resources Code) was developed to protect the public from the effects of other subsurface seismic hazards, including strong ground shaking, liquefaction, landslides and other ground failure. The Seismic Hazards Mapping Act requires the State Geologist to prepare Seismic Hazard Zone Maps, also referred to as Zones of Required Investigation (ZORI), which identify areas prone to liquefaction and earthquake–induced landslides.

California Building Code. The California Building Code (CBC) is included in Title 24, Part 2 of the California Code of Regulations (CCR). Title 24 is administered by the California Building Standards Commission, which, by law, is responsible for coordinating all building standards. Under state law, all building standards must conform to Title 24 or they are not enforceable. The purpose of the CBC is to establish minimum standards to safeguard the public health, safety, and general welfare by regulating and controlling the design, construction, quality of materials, use and occupancy, location, and maintenance of all building and structures within its jurisdiction. The 2010 CBC is based on the 2009 International Building Code (IBC) published by the International Code Conference. In addition, the CBC contains necessary California amendments based on the American Society of Civil Engineers (ASCE) Minimum Design Standards 7-05. ASCE 7-05 provides requirements for general structural design and includes means for determining earthquake loads as well as other loads (flood, snow, wind, etc.) for inclusion in building codes. The provisions of the CBC apply to the construction, alteration, movement, replacement, and demolition of every building or structure or any appurtenances connected or attached to such buildings or structures throughout California.

Assembly Bill 6. In 1998 the State Legislature adopted AB 6, which expanded Civil Code Section 1102.6 regarding disclosure of earthquake hazards. Beginning in June 1998, the sellers of residential property must give prospective buyers a new “Natural Hazard Disclosure Statement” if the residential property lies within an earthquake fault zone or a seismic hazard zone. The new law is intended to warn prospective real estate buyers that local earthquake or seismic hazards may limit their ability to develop the property or obtain insurance and may affect their ability to obtain assistance after a disaster.



Surface Mining and Reclamation Act. The California Surface Mining and Reclamation Act of 1975 (SMARA) was enacted in response to land use conflicts between urban growth and essential mineral production. SMARA requires the State Geologist to classify land according to the presence or absence of significant mineral deposits. Local governments must consider this information before land with important mineral deposits is committed to land uses incompatible with mining. SMARA provides for the evaluation of an area’s mineral resources using a system of Mineral Resource Zone (MRZ) classifications that reflect the known or inferred presence and significance of a given mineral resource.

MRZ-1. Areas where adequate information indicates that no significant mineral deposits are present, or where it is judged that little likelihood exists for their presence.

MRZ-2. Areas where adequate information indicates that significant mineral deposits are present, or where it is judged that a high likelihood for their presence exists.

MRZ-3. Areas containing mineral deposits, the significance of which cannot be evaluated from available data.

MRZ-4. Areas where available information is inadequate for assignment into any other MRZ.

No MRZ-2 zones have been designed on or in proximity to the Specific Plan site (Butte County General Plan 2030 Draft EIR, 2010).

Butte County Air Quality Management District Rule 205. Butte County Air Quality Management District Rule 205 regarding fugitive dust emissions from construction activities. The purpose of Rule 205 is to recue ambient concentrations and limit fugitive emissions of fine particulate matter from construction activities, bulk material handling and storage, carryout and track out, and similar activities, weed abatement activities, unpaved parking lots, unpaved staging areas, unpaved roads, inactive disturbed land, disturbed open areas and windblown dust.

Butte County General Plan 2030. The Butte County General Plan 2030 contains several goals and policies related to geology and soils, listed below.

Goals

Goal HS-6. Reduce risks from earthquakes. Goal HS-7. Reduce risks from steep slopes and landslides. Goal HS-8. Reduce risks from erosion.

Goal HS-9. Reduce risks from expansive soils.

Goal HS-10. Avoid subsidence from groundwater withdrawal. Goal COS-12 Protect economically viable mineral resources and related industries while avoiding land use conflicts and environmental impacts from mining activities.



Policies Policy HS-P6.1. Appropriate detailed seismic investigations shall be completed for all public and private development projects in accordance with the Alquist-Priolo Earthquake Fault Zoning Act. Policy HS-P6. Geotechnical investigations shall be completed prior to approval of schools, hospitals, fire stations and sheriff stations, as a means to ensure that these critical facilities are constructed in a way that mitigates site-specific seismic hazards. Policy HS-P7.1. Site-specific geotechnical investigations shall be required to assess landslide potential for private development and public facilities projects in areas rated “Moderate to High” and “High” in Figure HS-4 or the most current available mapping.

Policy HS-P7. Site-specific geotechnical investigations shall be required to assess erosion potential for private development projects and public facilities in areas rated “Very High” in Figure HS-5 or the most current available mapping.

Policy HS-P9.1 Site-specific geotechnical investigations shall be required to assess risks from expansive soils for private development projects and public facilities in areas rated “High” in Figure HS-6 or the most current available mapping. Policy HS-P10.1 Continue to work with water providers and regulatory agencies to ensure that groundwater withdrawals do not lead to subsidence problems.

Policy HS-P10.2 Existing programs to monitor potential subsidence activity shall be supported.

Policy COS-P12.6 Discretionary development projects in the vicinity of permitted mining extraction sites or along existing haul routes shall record a notice of the right to mine against the property for which a discretionary permit is sought. The notice shall advise owners and subsequent interests in ownership that the existing mining operation has a permitted right to continued mining operations.

Actions

Action HS-A6.1 Continue to require applicants to seismically retrofit existing homes where required under existing building codes.

Butte County Code of Ordinances. The Butte County Code of Ordinances requires that every

building or structure be designed and constructed in conformance with the 2010 California Building Standards Code (Title 24), which sets procedures and limitations for design of structures based on seismic risk. The Butte County Code of Ordinances also contains regulations pertaining to grading; however, these are designed to protect air and water quality and are discussed within Section 4.3, Air Quality and Section 4.9, Hydrology and Water Quality of this EIR.



4.6.2 Impact Analysis

a. Methodology and Significance Thresholds. The assessment of geologic impacts is based on a review of Specific Plan area information and conditions, information contained in the geohazard investigation and geotechnical investigation performed for the project in 2008 (Appendix E), and goals, policies, and actions contained in applicable policy documents such as the Health and Safety Element of the Butte County General Plan 2030 and the Butte County Code of Ordinances, as discussed above. Project implementation would create a significant impact relative to geologic resources if it would result in any of the following conditions:

Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving rupture of a known earthquake fault, strong seismic ground shaking, seismic-related ground failure, including liquefaction, or landslides;

Result in substantial soil erosion or the loss of topsoil; Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of

the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction, or collapse; or

Be located on expansive soil, creating substantial risks to life or property. b. Project Impacts and Mitigation Measures. Impact GEO-1 Seismically-induced ground shaking could damage structures

in the proposed Specific Plan Area, resulting in loss of property and risk to human health. However, with implementation of State-mandated building standards, as well as mitigation measures contained in the geohazard and geotechnical reports, impacts would be Class II, significant but mitigable.

Although the Specific Plan area is not in an Alquist-Priolo special studies zone and the potential for ground rupture during an earthquake is remote, nearby active and potentially active faults could generate groundshaking that may adversely affect the Specific Plan area. The proximity of active faults is such that the area has experienced strong seismically induced ground motion and will likely experience strong seismically induced ground motion in the future. The Specific Plan area is located approximately 4 miles from the Cleveland Hills fault (part of the Foothills Fault System), approximately 33 miles from the Coast Range-Central Valley (CRCV) geomorphic boundary (the Great Valley Fault), and approximately 145 miles from the San Andreas fault zone. As part of the geohazards evaluation prepared for the proposed project, modeling performed by Neil O. Anderson & Associates indicates that peak horizontal ground acceleration of 0.169g has a 10% probability of occurring in the Specific Plan area in 50 years. It is possible that accelerations near or over the upper bound earthquake ground motion could occur anywhere within the Specific Plan area. Additionally, the Butte County General Plan 2030 states that ground motions as strong as those observed during the 1975 Oroville earthquake (Modified Mercalli Intensity VIII) can be expected anywhere in Butte County, which could produce slight damage in specially designed structures, considerable damage with partial collapse in ordinary substantial buildings, and great damage in poorly built structures. Besides the direct physical damage to structures



caused by ground shaking, slopes and inadequately compacted fill material could move and cause additional damage. Gas, water, and electrical lines would also be subject to groundshaking, which could rupture or break these lines and jeopardize public safety. Development of the Specific Plan area would be subject to the requirements of the 2010 California Building Standards Code (Title 24), which sets procedures and limitations for design of structures based on seismic risk, and which would ensure that the design and construction of new structures are engineered to withstand the expected ground acceleration that may occur in the Specific Plan area. The geotechnical and geohazard studies indicate that compliance with standard CBC construction methods and the recommendations in the technical reports would be sufficient to minimize potential groundshaking hazards. Thus, potential impacts related to seismic groundshaking would be significant but mitigable.

Mitigation Measures. Mitigation Measure GEO-1 shall be implemented to reduce the potential for adverse impacts from seismic shaking within the Specific Plan area. GEO-1 All project construction carried out as part of the proposed

Specific Plan shall comply with the California Building Code in effect at the time of construction, as well as the recommendations contained in report entitled Preliminary Geotechnical Investigation Rio d’Oro Pacific Heights Road & Hwy. 70 Oroville, California completed on July 31, 2008 by Neil O. Anderson & Associates, Inc. and any subsequent report addendums.

Significance After Mitigation. The risk of sustaining an earthquake with higher ground accelerations can’t be completely eliminated. Any structure built in California is susceptible to failure as a result of seismic activity. However, the potential for structural failure resulting from seismic ground shaking would be less than significant through implementation of the most recent industry standards for structural design, as required by the California Building Code and referenced in Mitigation Measure GEO-1.

Impact GEO-2 Construction of projects carried out under the proposed

Specific Plan could potentially expose soils in the Specific Plan area to erosion from the forces of wind and precipitation. However, compliance with existing regulations would minimize such exposure and erosion. Therefore, this impact would be Class III, less than significant.

The Specific Plan area is shown on Figure HS-5 of the Butte County General Plan 2030 Health and Safety Element as having moderate erosion potential. Erosion potential is highly correlated to slope characteristics. Loose soils and/or steep slopes create conditions favorable to erosion. The potential for erosion generally increases as a result of human activity, primarily through the removal of vegetative cover and development of structures and impervious surfaces. These conditions increase runoff during storm events and also expose soils to wind-based erosion. While the proposed project would be developed within generally flat topography and would remain in this condition after its development, areas of greater topographic relief occur along the arroyos and the steep slopes adjacent to Pacific Heights Road and the main drainages within the



site. These areas could be particularly susceptible to erosion. However, all areas of the site would be susceptible to erosion during grading and related site preparation activities. To avoid or minimize erosion, projects carried out under the proposed Specific Plan would be subject to the state Construction General Permit (2009-0009-DWQ). The California State Water Board adopted the most recent Construction General Permit (2009-0009-DWQ) on September 2, 2009. This permit became effective on July 1, 2010. Under the current permit, potential water quality impacts are assessed based on what is referred to as the Risk Level. The three risk levels are defined by the project’s combined “Sediment Risk” and the “Receiving Water Risk”, with Risk Level 1 being the lowest risk category and Risk Level 3 being the highest. A higher “Risk Level” requires more stringent pollution control. A project’s “Sediment Risk Factor” is based on the amount of rainfall, soil characteristics and topography. A project’s “Receiving Water Risk Factor” is based on watershed characteristics and whether the project discharges to a 303(d) water body impaired by sediment or if the receiving water provides certain beneficial uses. There may be multiple Notices of Intent (NOI) submitted throughout the life of the proposed project; therefore, the quantity and nature of construction activities included in each NOI would determine the risk category for each construction effort associated with the proposed project. All risk categories are required to submit a Permit Registration Document (PRD). The PRD includes the NOI, Risk Assessment, site map, Storm Water Pollution Prevention Plan (SWPPP), fee and signed certification statement. The State Water Resources Control Board is currently considering adoption of amendments to the state Construction General Permit, but these have not yet been adopted (California State Water Resources Control Board, August 2013). Projects carried out under the proposed Specific Plan would be subject to whatever Construction General Permit is in effect at the time of application for grading permits. The Feather River is not a 303 (d) listed water body impaired for sediment; and therefore, construction activities would likely be categorized either as either a Risk Level 1 or Risk Level 2 project depending on the construction schedule relative to the likelihood of rain events. Prior to construction of the proposed Project, a SWPPP would be prepared to provide standard Best Management Practices (BMPs) that would be implemented as part of the SWPPP to minimize soil erosion during construction. These measures may include the following:

Slope/Surface Protection Systems, such as re-vegetated slopes with minimal cut/fill, or, in some locations, retaining walls to reduce the steepness of slopes.

Concentrated Flow Conveyance Systems such as flared culvert inlets/outlets, and rock slope protection/velocity dissipation devices at pipe outlets or ditches.

Maximize protection of desirable existing vegetation to provide erosion and sediment control benefits.

Silt fences at bottom of slopes and around stock piles to intercept flow of sediments. Fiber rolls on face of slope to slow down runoff and remove sediments. Stabilize construction entrances to reduce tracking of dirt onto roadways. Concrete washouts to avoid cement flowing into drainage systems. Gravel bags as additional protection to intercept sediments. Standard DOT inlet protection and energy dissipaters at drainage systems.

The above list represents measures that could be implemented. Specific measures would be incorporated into a site-specific SWPPP prior to construction. No measures other than



compliance with provisions of the Construction General Permit 2009-0009-DWQ (or subsequently adopted Construction General Permit) would be required to minimize soil erosion or loss of topsoil associated with stormwater runoff. Construction of projects carried out under the proposed Specific Plan would also be subject to Butte County Air Quality Management District Rule 205 regarding fugitive dust emissions from construction activities. The purpose of Rule 205 is to reduce ambient concentrations and limit fugitive emissions of fine particulate matter from construction activities, bulk material handling and storage, carryout and track out, and similar activities, weed abatement activities, unpaved parking lots, unpaved staging areas, unpaved roads, inactive disturbed land, disturbed open areas and windblown dust. Rule 205 provides a list of Best Available Control Measures (BACM) to reduce fugitive dust emissions associated with the above referenced activities. These BACM are available through ARB’s website (California Air Resources Board, August 2013). The Specific Plan area would either be developed with impervious surfaces surrounded by landscaping or left in its current condition as open space or a conservation easement. Both pre-and post-construction soil erosion would be minimized. Mitigation Measures. This impact would be less than significant without mitigation. Significance after Mitigation. This impact would be less than significant without mitigation.

Impact GEO-3 While the Specific Plan area is not located on highly unstable

geologic unit or unstable soils, including expansive soils, proper soils engineering practices would be required to ensure that soil conditions would not significantly impact the proposed project. With required implementation of standard engineering practices, impacts associated with unstable or expansive soils would be a Class II, significant but mitigable impact.

The Specific Plan area is not located on an unstable geologic unit or unstable soils. The Specific Plan area has minimal natural or man-made slopes. To minimize impacts to slopes associated with the arroyos and along Pacific Heights Road, the Specific Plan would preserve these areas as open space or within an environmental conservation area. Further, the Specific Plan requires a 75 foot setback from these areas to minimize erosion. The Specific Plan area is in an area designated as having a landslide potential of “low to none” in the Butte County General Plan 2030. Lateral spreading is lateral ground movement that takes place when liquefaction occurs adjacent to a slope or open face. The loss of strength in the liquefied material near the base of a slope can result in a slope failure. The Specific Plan area has a low potential for liquefaction; thus, the potential for lateral spreading is low given that liquefaction is required for lateral spreading to occur. Furthermore, because the Specific Plan area is relatively flat, no steep slopes or open slope faces would be created during grading. Thus, lateral spreading is not expected to occur.



Subsidence or ground settlement may occur with fault movement, slope instability, liquefaction and soil compaction. Settlement is not necessarily destructive. It is usually differential (i.e., uneven) settlement that damages structures. Differential settlement occurs when the subsoil is non-uniform in depth, density, or character, and/or when the severity of shaking varies from one place to another. The Specific Plan area has low potential for liquefaction and contains few slopes that would be susceptible to landslides. It is also not located within an active fault line; therefore, the potential for subsidence to occur is low. As shown in Figure 17-7 of the Butte County General Plan 2030 Setting and Trends Report (Butte County, August 2007), part of the Specific Plan area may be located within an area of potential subsidence. Subsidence/settlement can also occur in areas where structures have been placed on improperly compacted artificial fill. The Preliminary Geotechnical Investigation (Appendix E) concluded that the Specific Plan area is suitable for construction of the proposed development as long as all of the conclusions and recommendations of the report are incorporated into the design and construction of the project. Also, consistent with General Plan 2030 Policies HS-P6.2 and HS-P9.1, site-specific geotechnical reports would be required by the County as part of the design phase of the proposed project to determine localized soil suitability and remediation measures, if needed. The proposed project would also be constructed consistent with the CBC and standard practices of the Association of Structural Engineers of California, including design standards, grading, and construction practices, to avoid or reduce geologic hazards to bridges, roadways and all related structures. Thus, it is unlikely that the proposed project would create or be subject to landsliding, lateral spreading, subsidence, liquefaction or collapse, either during or post construction. As explained in Section 4.6.1e, near-surface sandy clay soil within the Specific Plan area has a low expansion potential, but the native clay component of on-site soil is expansive. Groundwater was not encountered during excavations. As stated above in Section 4.6.1a, groundwater is expected to be located approximately 120 feet beneath the site. To minimize the risk of soil expansion, the Preliminary Geotechnical Investigation recommends mixing the clay soils with the cobbles, sand and silt during grading. Furthermore, foundation design criteria are recommended to minimize potential risk from expansive soils. To address site-specific issues regarding expansive soils, a geotechnical report is recommended as part of the design process and required per Butte County General Plan 2030 Policy HS-P9.1. The Preliminary Geotechnical Investigation also contains grading recommendations relating to proper site clearance, soil compaction, and requirements for properly engineered fill. By incorporating recommendations within the Preliminary Geotechnical Investigation and site-specific geotechnical reports as mitigation herein, potential impacts associated with expansive soils would be less than significant.

Mitigation Measures. The following mitigation measures are required to reduce potential soil instability impacts.

GEO-3(a) Adherence to Geotechnical Report. All recommendations

contained in the Specific Plan geotechnical report (Neil O. Anderson & Associates, 2008) shall be agreed to by the developer of individual projects carried out under the Specific Plan prior to issuance of grading permits, and subsequently carried out by the developer and monitored by a qualified geotechnical engineer.



GEO-3(b) Further Geotechnical Studies. In compliance with Butte County General Plan 2030 Policy HS-P9.1, further geotechnical study for specific sites or phases to be developed under the Specific Plan shall be performed and their recommendations carried out by project developers if determined to be necessary by Butte County.

Significance After Mitigation. Proper treatment of soils with expansion potential and properly designed and constructed structures, foundations, roads, and utilities, as well as preparation of site-specific geotechnical investigations for specific sites or phases to be developed under the proposed Specific Plan, as required by Butte County General Plan 2030 Policy HS-P9.1, would adequately mitigate the potential for problems caused by unstable or expansive soils, thereby reducing impacts to less than significant.

c. Cumulative Impacts. Development of the proposed Specific Plan, as well as cumulative projects listed in Tables 3-1 and 3-2 in Section 3.0, Environmental Setting of this EIR, would expose new residents and property to geology and soils hazards in the area. However, the proposed project, with implementation of applicable design requirements and mitigation measures, would not have significant direct or indirect permanent or temporary impacts related to geology and soils. Geology and soils issues for other projects would be addressed on a project-by-project basis through preparation of required soils and geotechnical engineering studies and adherence to the recommendations therein, as well as existing County and State regulations and the California Building Code. Because potential impacts associated with Specific Plan buildout can be mitigated to a less than significant level, and impacts from future projects would be addressed on a case-by-case basis, the Specific Plan’s contribution to cumulative impacts would be less than significant.

4.6 geology and soils - butte county, california · 2014-10-02 · section 4.6 geology and soils...

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