~ Geology, Soils and Seismicity ~

3.2.1-1 SETTING
This section describes geologic, soils, and seismic conditions in the Brooktrails Township Specific Plan area, and potential development constraints that may exist because of site-specific geotechnical characteristics.

Regional Geology
Mendocino County is in the northern portion of the Coast Ranges geomorphic province, a group of mountain ranges that extends about 600 miles from Santa Barbara County to the California/Oregon border. The Coast Ranges lie between the Central Valley and the Pacific Ocean, trend approximately north-northwest, and are about 65 miles wide at the latitude of Willits. The structure of the Coast Ranges is the result of global-scale crustal movements (plate tectonics), while the shape of the ranges is caused by local forces (landsliding and erosion). The Coast Ranges, on the North American tectonic plate, are separated from the Pacific plate to the west by the San Andreas fault zone, and from the Great Valley province to the east by a series of high-angle faults called the Coast Ranges.,

The Mendocino Range, near the center of which Brooktrails Township is located, is composed almost entirely by rocks of the Franciscan Assemblage, deposited about 140 million years ago and deformed by ongoing tectonic forces. In the vicinity of Brooktrails, the Franciscan Assemblage is represented by melange (a term used here to indicate a mixture of sandstone, mudstone, shale and chert, in which fossils are rare and rock types vary greatly over short distances) and greywacke (a term used here to indicate undifferentiated, weak to very strong, intensely fractured to massive, moderately to deeply weathered clastic sedimentary rocks, including arkosic sandstone, conglomerate, shale, with localized occurrences of greenstone). Two other types of geologic units occur in the area: continental basin deposits and Recent alluvium. The basin deposits are a heterogenous mixture of loosely cemented gravel, sand, silt and clay, deposited between 0.5 and 4 million years ago in the structural basin now occupied by Little Lake Valley. The Recent alluvium is unconsolidated gravel, sand, silt and clay deposited in the last 11,000 years, along most of the stream channels in the County.

Seismicity and Faulting
Because the northern Coast Ranges are adjacent to the boundary between two tectonic plates, they are subject to continuing seismic activity. Most seismic activity affecting Mendocino County occurs along four fault zones: the Coast Range Thrust zone on the eastern edge of the County, the offshore Mendocino fracture zone to the northwest, the Healdsburg-Rodgers Creek fault zone to the south , and the Maacama fault zone, which passes through the center of the County and across the northeast corner of Brooktrails Township. The fault zones trend north-northwest, approximately parallel to the general geologic structure of the Coast Ranges (Figure 3.2.1-1). Two major fault zones need to be considered in the Brooktrails area: the San Andreas and the Maacama. The San Andreas fault zone is widely known because it is historically active (during the last 200 years) and has produced numerous damaging earthquakes. It consists of a series of fault segments of various length, and passes through Mendocino County about 40 miles southwest of the Township. It is capable of generating a maximum credible earthquake (MCE) of Richter magnitude (M) 8.3. Despite its distance from the study area, it caused exceptionally intense groundshaking in Willits during the April 1906 earthquake. The earthquake epicenter was nearly 100 miles south of Brooktrails, but caused strong ground motion in the area because the deep alluvium in Little Lake Valley amplified the effects of the seismic waves.

The Maacama fault zone passes through the central part of Mendocino County and crosses Brooktrails Township along the valley of Upp Creek. Several structural basins, including Little Lake Valley, are associated with fault traces in this zone. The fault zone is active, as demonstrated by the M4.8 earthquake of November 1977, which caused moderate property damage in the Willits area. The epicenter of the quake was near Willits, and lead to speculation that an unknown trace of the fault passed through the town. However, there is no observed connection between this inferred fault trace and the Figure 3.2.1-1

Regional Fault Map known traces of the Maacama fault. The Maacama fault is capable of generating an MCE of M7.6. Although the MCE for the Maacama fault is lower than that of the San Andreas fault, it would produce intense groundshaking in the vicinity of Brooktrails because of the proximity of the earthquake epicenter to the Township, and because of the depth of alluvium in Little Lake Valley.

The state legislation dealing with large-scale construction near known active fault traces is the Alquist-Priolo Earthquake Fault Zoning Act. The purpose of the Act is to reduce the hazards posed by surface rupture of a fault. Maps prepared by the California Division of Mines and Geology delineate Earthquake Fault Zones of appropriate width to include known active traces of all major faults in California. Traces of the Maacama fault are in an Earthquake Fault Zone that passes through the Town of Willits and crosses the northeast corner of Brooktrails along the valley of Upp Creek. Structures for human occupancy, other than small single-family homes, are not permitted within the Earthquake Fault Zone until a geologic investigation demonstrates that the construction site is not crossed by an active fault trace.

A fault trace that extends beyond the Maacama Alquist-Priolo Earthquake Fault Zone has been mapped northwest of Willits, extending along Sherwood Road to Twin Lakes and continuing parallel to Dutch Henry Creek through the northeast corner of Brooktrails. Evaluation of this fault trace in 1981, by the California Division of Mines and Geology, determined that no movement had occurred along the trace during the last 11,000 years.

Site Topography
Most of Brooktrails is composed of hillsides that are moderately steep (20 to 40 percent slopes) to very steep (greater than 40 percent) slopes. Some gently sloping to flat-lying areas (0 to 20 percent slopes) exist on the ridge crests and in the valley bottoms. Overall topographic relief is approximately 1,200 feet, with elevations ranging from a high of about 2,680 feet above mean sea level (+2,680 feet msl) near the southwest corner of the Township to a low of about +1,480 feet msl at the southeastern corner.

The most prominent topographic feature is the relatively sharp, north to northwest-trending main ridge crest along the Township's western boundary, containing numerous southwest to southeast-trending spur ridges and intervening swales. Northwest-trending valleys occur along the southeast-flowing Willits and Dutch Henry Creeks. Willits Creek, flowing toward Little Lake Valley, has incised a steep valley approximately along the center line of the community. The Willits Creek valley, upstream from its confluence with Dutch Henry Creek at Lake Emily Reservoir, is fairly typical of the Brooktrails area. It is about 750 feet deep, lying between ridges of Franciscan rocks that rise to more than +2400 feet msl. The valley walls are forested slopes, varying in steepness from 2.5:1 (horizontal to vertical) to 1:1, with slightly flattened ridgetops. The valley floor contains the creek channel, and varies in width from about 10 feet to about 60 feet. It has a gradient between 1.5 percent and 5 percent.

In many places the Willits Creek valley follows the boundary between two rock units. The north side of the valley consists of massive, very hard, weathered graywacke, overlain by 10 to 20 feet of colluvium and residual soil. The south side of the valley consists of intensely fractured and highly weathered mudstone, shale and minor thinly bedded graywacke, overlain by 30 to 40 feet of residual soil.

Development that has caused topographic alteration in Brooktrails includes two reservoirs impounded by earth fill dams, approximately 1,280 single-family residences, a paved street network, community center, fire station, lodge and golf course. There is a network of unpaved logging roads throughout undeveloped areas of the Township. A former aggregate quarry in the northeastern portion of the Township is adjacent to the intersection of Sherwood Road and Lupine Way. Grading performed to construct these developments and for past logging activities have created some large cuts and fills on moderately steep to very steep hillsides. The logging road cuts generally are oversteepened and unstable. A massive earth slide occurred above the spillway (eastern dam abutment) on the Brooktrails 3N Dam in January 1971. Remedial grading was performed following the slide, and tiltmeters and survey monuments were installed to monitor slide movement.

Site Geology
Brooktrails Township is in the Mendocino Range west of Little Lake Valley. The area is underlain by graywacke and melange of the Franciscan Assemblage. Unconsolidated surficial deposits are generally thin to nonexistent along the ridge crests and upper hill slopes, but thicken toward the base of hill slopes, in swales, and in the creeks and other drainages in the Township.

The Franciscan greywacke consists of weak to very strong, intensely fractured to massive, moderately to deeply weathered clastic sedimentary rocks. Rock types included in this unit are arkosic sandstone, conglomerate, shale, with localized occurrences of greenstone (basalt, metagreywacke, schist and chert). The Franciscan melange consists of a similarly ancient sheared and altered sandstone and shale matrix, with localized inclusions of ultramafic rocks (serpentine, serpentinized ultramafics, and greenstone). The sheared matrix is often weak, highly erodible, and susceptible to landsliding even on relatively gentle hill slopes. The ultramafic rocks generally occur in masses too small to be shown separately on geologic maps of the Township.

Unconsolidated surficial deposits include colluvium, alluvium, and landslides. Colluvium consists of Recent hillside deposits which are a heterogeneous mixture of cobbles, gravel, sand, silt, and clay, with occasional boulders. Colluvium generally occupies swales and blankets lower hill slopes, locally forming colluvial fans. It also occurs on some Recent and dormant landslide deposits. Colluvium is generally massive, moderately to highly permeable, forms relatively unstable slopes in exposed cuts, and may be easily eroded and incised. Colluvial deposits grade down-slope into, and interfinger with, alluvium. Colluvium-filled swales are a potential source of debris flows on steep slopes.

Alluvium consists of crudely stratified Recent stream deposits of cobbles, gravel, sand, silt, and clay. It occurs in the relatively flat-lying valleys along Willits and Dutch Henry Creeks, and locally along tributary drainages in the Township.

Landslide deposits range in size from very small to very large, and may contain any of the materials previously discussed. They range in age from Quaternary (past 1.6 million years) to historic (past 200 years), and include predominately large bedrock and soil slumps, debris flows, debris slides, and soil slips.

Small, shallow (less than about 5 feet) to moderately deep (5 to 15 feet) debris flows and debris slides generally occur along cut slopes, creek banks, and in colluvium-filled swales. The large, deep (greater than 15 feet) bedrock and soil slumps occur on the steeper slopes.

Existing fill generally is associated with the earth fill dams, roadways, and building pads in Brooktrails. Most fills are too small to be shown on geologic maps of the Township, with the exception of the dams. The material usually is a combination of gravel, sand, silt and clay, placed in layers and compacted to provide a stable surface.

The most prominent structural geologic feature in Brooktrails is the active northwest-trending Maacama fault zone, which traverses the northeastern portion of the Township, and separates Franciscan melange and ultramafic rocks from greywacke. A generally northeast-trending bedrock thrust fault in the southern portion of the Township also separates melange from greywacke. There are several discontinuous, northwest-trending bedrock faults in Brooktrails, but they are not considered active by the State of California. The bedrock in the Township generally strikes northwest (but in some places to the northeast), with dips generally to the southwest and northeast ranging from gently dipping to vertical (and locally overturned) in the southern portion of the Township. There are several discontinuous northwest-trending bedrock folds in the Township.

Soils

Soils in the vicinity of Brooktrails Township consist of one major association, the Casabonne-Wohly loam, and several minor associations, of which the Zeni-Orboun loam is the most widespread (Figure 3.2.1-2). The soils are in many respects similar. They have developed on steep slopes underlain by sandstones or sandstone/shale mixtures, and they tend to be moderately deep to deep and are well-drained. Runoff is rapid and erosion hazard is high, particularly if the soils are stripped of vegetative cover. Both soil associations break down under heavy use of tracked vehicles, and roads in the Casabonne-Wohly loams may become impassable during the rainy season.

The Casabonne-Wohly loams Figure 3.2.1-2 (missing)

Soils tend to have a higher clay content than the Zeni-Orboun loams, and are less permeable.

Hazards
Geologic and seismic conditions in Brooktrails Township are complex, but not unique. They are similar to conditions in other areas in the Coast Ranges of northern California. Geo-seismic constraints to development in the Township include hazards related to seismicity (groundshaking, landsliding and, possibly, liquefaction), to faulting (surface rupture along a trace of an active fault), to slope instability (landsliding), and to soil characteristics (settlement, erosion, impermeability).

The direct effects of seismically induced groundshaking result from a combination of the cyclic horizontal and vertical movements of the ground during an earthquake, the coherence of the geologic material (rock, soil, compacted fill) being shaken, and the quality of construction supported on that material. Seismic ground motions range from such low intensities that they cannot be detected, except by specialized equipment, to such high intensities that buildings are, literally, shaken apart and heavy objects are thrown into the air. One earthquake may create the entire range of effects, depending on the geologic conditions at a given site, the site's distance from the source of the earthquake, and the design of structures on the site. Bedrock formations, such as the Franciscan greywacke and melange that form the hills at Brooktrails, tend to be less affected by groundshaking than unconsolidated sediments, such as the mixtures of sand, silt, and clay that blanket the bases of the hills and fill the creek valleys in the Township.

As a general rule, the intensity of groundshaking increases with proximity to the source of the earthquake. However, given similar location and seismic energy output, the least amount of damaging vibration would occur on a site that was completely composed of bedrock. A site underlain by major thicknesses of sediments (such as the alluvium in the valleys) would experience more severe vibration because of the sediments' tendency to deform to a greater degree than the bedrock. For structures supported on sediments, the combination of ground deformation and susceptible building design appears to determine the extent of seismically caused damage, with well-constructed buildings founded on dense undisturbed native deposits preforming considerably better than moderately or poorly constructed buildings on loose soil or unengineered fill.

Earthquake-induced landsliding of steep slopes can occur in either bedrock or unconsolidated deposits. Firm bedrock usually can stand in steeper, more stable slopes than soils are able to maintain, but rock type, grain size, degree of weathering, and angle of the beds all contribute to the strength or weakness of a bedrock hillside. Some bedrock types, such as deeply weathered Franciscan melange, are more susceptible to slope failures than others, such as unfractured Franciscan greywacke.

Liquefaction is a response to severe groundshaking that can occur in loose, saturated soils. This transformation from a solid to a liquid ("quicksand") state can cause ground settling, landsliding, or lateral spreading. Earthquake-induced liquefaction does not affect bedrock; however, it does affect certain types of alluvium under conditions of saturation. The characteristics of a liquefaction-prone deposit include: (1) uniformly fine sand or sandy fill; (2) saturated conditions, usually caused by groundwater, but can be from other causes, such as reservoir inundation or water-supply system failure; (3) loose to moderately dense compaction; (4) little or no silt- and clay-sized particles to act as binders. If these conditions occur within about 30 to 40 feet below the ground surface, vibration sufficiently violent to increase pore pressure beyond the shear strength of the sand particles could cause such soils to liquefy. Any structures supported on these soils would be subject to tilting or settlement (sometimes very violent and rapid) as the supporting capability of the soil diminished.

Surface rupture of a trace of the Maacama Creek fault cannot be prevented, nor can its time of occurrence be predicted. Damage to road surfaces, foundations, and utilities crossed by the fault would be severe, but localized. Damage due to surface rupturing is limited to the actual location of the fault-line break. Various structural designs to resist the effects of seismic groundshaking are readily available, but there is a limited range of design elements capable of withstanding fault rupture. The safest approach is to avoid building across the fault trace, but where this is not possible, measures may be incorporated in designs to limit damage and to facilitate repair.

Static slope instability is the major cause of landslides in the Coast Ranges. Saturated slide-prone geologic material is the basis of most slope instability, but other natural processes and human activities may initiate landslides in otherwise stable areas. Geologic material, such as clay minerals, have a great capacity to absorb water, causing reduction of the strength of rocks containing the clays. The force of gravity, can cause landslides when saturated clays reduce the strength of a rock unit below its minimum stability threshold. The sheared and fractured matrix of the Franciscan melange contains clay minerals that tend to make this rock type unstable in very steep slopes. Another unstable situation exists where the bedding or planes of rock layers are parallel to the surface slope of the ground. In this case the potential exists for rock units to slip along the weak plane.

Several other conditions can cause, or contribute to, slope instability. Fault zones contain weakened rock, crushed by the repeated motion along the fault. Heavy rains can saturate a slope, reducing its strength. Stream cuts along the base of a slope can induce sliding by removing needed support from weak zones during high flood stages. Chemical and mechanical weathering can break down rock materials, and the seepage from high groundwater levels can increase water concentration, thus reducing strength.

The steepness of a slope is a major component of instability because of the unsupported weight of rock and soil that may bear on a weak zone. Such human activities as making road cuts, diverting surface runoff or impounding water can reduce the natural strength of slopes and generate landsliding in areas of normally low susceptibility.

There are six soil conditions in Brooktrails Township that could affect development under the Specific Plan: impermeability, compressibility, shrink-and-swell potential, erosion, liquefaction and landsliding (Table 3.2.1-1). Soils with moderately slow percolation rates that are relatively impermeable during the wet season are the dominant soil types in the Township. Compressible and expansive layers are common in these soils; erosion hazards are high throughout the area. Soil strength throughout Brooktrails is fair at best. Combined with steep hillsides and shallow soil-depths to bedrock, this indicates generally low slope-stability even under static conditions. Liquefaction potential generally is low, but the variability of the soil conditions make generalization inappropriate when dealing with specific localities.

Impermeability, or a very slow rate of percolation, is an asset for the construction of storage reservoirs and dam cores. These structures would be intended to retain water, and not to act as recharge areas. However, impermeability can produce structural problems if water collects beneath or within the foundations of structures or roads. Positive drainage must be established to prevent supporting soils from becoming weakened by saturation.

Compressiveness, or the potential to collapse under loading, is another fairly common feature of soils containing any substantial amount of clay. This characteristic would not be an asset for the construction of building or roadway foundations because the soils do not provide adequate support unless they are specially treated. Sometimes they must be removed entirely and replaced with engineered backfill. If left untreated, these soils can cause unacceptable amounts of settlement. The effects can range from the nuisance level (sticking doors and windows) to the major structural damage level (shifted or collapsed foundations). Combined with seismic loads, the effect could be sufficient to make the difference between survival and destruction of a component of the foundation or building during a major earthquake.

Expansiveness, or the potential to swell and shrink with repeated cycles of wetting and drying, is another fairly common feature of clayey soils. This characteristic would be an asset for pond liners because the swollen soils would inhibit the absorption of water. However, like compressiveness, it would not be an asset for the construction of building or roadway foundations because such soils tend to be weak and compressible. Treatment to remediate this condition is similar to that described for compressive soils. If left in untreated, these soils can cause unacceptable amounts of settlement, as previously described.

Erosion potential is high throughout the area. This is especially true for the soils on the steep slopes.

Soil erosion can be a problem for the project components in much the same way as compressive soils. Basically, the loss of foundation support can result from excessive erosion. Development of steep hillsides could contribute to increased soil erosion in the areas of construction and thereby cause similar problems for existing structures in the Township. Additionally, erosion eventually results in transportation of soil particles removed from higher elevations and their deposition at lower elevations. The destination site often is a creek or pond. Excessive sedimentation in the waterways of Brooktrails would increase turbidity, thereby reducing the quality of the water captured in the Township's reservoirs.

Liquefaction is the transformation of a soil from a solid state to a liquid state as a response to seismically induced groundshaking. The transformation can be very rapid. The soil characteristics of a liquefaction-prone deposit are saturated conditions; loose, uniformly fine sand; little or no clay-sized particles to act as binders; sufficiently violent vibration to increase pore pressure beyond the shear strength of the sand particles. If these conditions occur within about 30 feet of the ground surface, any structures supported on the soils would be subject to tilting or settlement (sometimes very violent and rapid) as the supporting capabilities of the soil diminished. Liquefiable material at or near the ground surface would need to be replaced or recompacted before it could be used as structural support. The liquefaction potential of the soils in Brooktrails appears to be very low because of the relatively high clay component of the native loams. This may not be the case for Recent alluvial soils, which appear to be mostly sand. On-site testing would be necessary in alluvial areas to determine whether or not liquefaction was likely.

Landslides, earthslips, mudflows and soilcreeps are all expressions of soil conditions related to the instabilities created by steep slopes, shallow soil development, the presence of an excessive amount of water, or the lack of strength in the soil or at the soil/rock interface. Each of these conditions is observable in Brooktrails Township, but usually is reported simply as a "landslide." Earthquake activity does induce some landsliding, but most slides result from the weight of rain-saturated soil and rock exceeding the strength of the underlying material. Erosion of supporting material at the toe of a landslide or of a landslide-prone hillside further contributes to slope instability.

3.2.1-2 IMPACTS AND MITIGATION MEASURES

Brooktrails Township Specific Plan Policies.
Plan policies specifically related to erosion control, slope stability and seismic safety in the Plan area appear in the Environmental Resources chapter of the Plan as SOILS AND GEOLOGY GOAL ER-6.5-1 and ER-6.5-2. The Policies for implementing the Goals are so central to the treatment of grading, erosion control, slope stability and seismic safety in the Plan area that they are reiterated here to allow the reader easy reference to the actual language in the Plan.

SOILS AND GEOLOGY GOAL ER-6.5-1: Ensure that slopes, soils, geotechnical conditions and seismic constraints are adequately considered for all development within Brooktrails Township.

POLICY ER-6.5-1A: Discourage development or ensure adequate mitigation for development within areas characterized by steep slopes and soil limitations, including high erosion hazard, severe soil pressure variations, severe shrink-swell potential and septic system unsuitability.

POLICY ER-6.5-1B: Minimize the potential for soil erosion from all development, particularly in the vicinity of natural waterways and reservoirs.

POLICY ER-6.5-1C: Establish guidelines and criteria for development of areas with steep slopes and areas having soil limitations.

SOILS AND GEOLOGY GOAL ER-6.5-2: Avoid construction in areas that are not geotechnically or seismically suitable for development.

POLICY ER-6.5-2A: Within the Maacama Alquist-Priolo Earthquake Fault Zone, recommend geotechnical studies for those structures exempt from the provisions of the Alquist-Priolo Earthquake Fault Zoning Act to demonstrate feasibility of construction where buildings for human occupation are proposed.

POLICY ER-6.5-2B: Establish voluntary lot merger, conservation easement, and financial incentive programs to encourage the consolidation of lots that are characterized by steep slopes (40% or greater), or are not geotechnically or seismically suitable for development.

Standard of Significance
The CEQA Guidelines indicate that a project, such as the implementation of a Specific Plan, normally would have a significant geologic effect if it exposed people or structures to major geologic hazards. The potential geologic, soils and seismic effects of the proposed Brooktrails Township Specific Plan are considered from two points of view: construction impacts and hazards to people or structures. The basic criteria applied to the analysis of construction impacts are whether or not implementation of the Plan's goals as proposed would create substantial changes in the geologic environment at Brooktrails, would exacerbate erosion, or would create unstable slope conditions that would last beyond the short-term construction period. The analysis of hazards involves determining the degree to which implementation of the Plan's goals could endanger residents, visitors or structures through exposure to seismically induced groundshaking or other potentially hazardous geologic or soil conditions.

For the purposes of this EIR, significant geologic hazards are defined as rock, soil or seismic conditions so unfavorable that they could not be overcome by site-specific design, using reasonable construction and maintenance practices. While it is recognized that human activities may have other, nonhazardous, effects on geotechnical conditions, except as such changes relate to the protection of life or to the preservation of ecological values, they are not considered significant in the context of the proposed Specific Plan.

Impacts would be considered unavoidable significant effects of the proposed Specific Plan, if they could not be a) reduced to an acceptable level of risk, b) eliminated, or c) avoided by using existing techniques, generally recognized by geotechnical consultants working in northern California to be applicable and feasible.

Each impact identified in the following discussion is indicated as a significant (S), potentially significant (PS), or insignificant (I) impact of the Specific Plan. Mitigation measures to reduce each impact to an insignificant level are listed and described. One unavoidable hazard is identified: fault rupture along the Maacama fault zone. However, the effects of this hazard can be reduced through the application of mitigation measures.

Impact 3.2.1-1
Grading and excavation on, or adjacent to, existing steep slopes, whether underlain by bedrock or alluvial deposits, could create or exacerbate unstable slope conditions at the construction site. (PS)

The surface alteration necessary at individual lot sites to accommodate the construction permitted under the Specific Plan is not considered a significant geologic change in itself. However, the changes to topography for the addition of structures raises issues of slope stability at most sites in the Township. The creation of cuts in alluvium, and the placement of fill as road support or building pad terraces have the potential to create unstable slopes if the cuts and fills are not specifically engineered for stability. Substantial amounts of material could be needed to fill low areas, or deep cuts could be proposed if structural designs are not adapted to the specific sites, rather than attempting to adapt the sites to the designs.

An acceptable degree of cut-slope or fill-slope stability at these sites can be achieved only by adapting slope design (inclination, compaction, drainage control, etc.) to site-specific geologic conditions. Site-specific stability analysis is the basis of slope design in areas where instability is suspected. Such slope stability analyses contain recommendations for ground preparation, earthwork, foundations, etc., specific to the site, that become an integral part the construction design.

Before approving construction projects in the geologically constrained areas (Alquist-Priolo zone, landslides, slopes steeper than 40%), the County should have a completed report of soil and rock conditions at the project site, provided by the Project Sponsor, that evaluates potential slope instability conditions. The evaluations must be conducted by registered professionals, and measures to reduce or eliminate slope instability must be applied to the project site. The site-specific measures needed to achieve satisfactory slope performance cannot be determined until the soil and rock evaluations are complete and at least conceptual designs for the project have been prepared.

Detailed evaluation of geotechnical and seismic conditions at the sites of proposed structures and slope modifications within the Township are required to be prepared by California-licensed geologists and engineers, as part of the site-design for any proposed project. At a minimum, the investigations must provide information and recommendations for the following items:

1. The characteristics of the fill, soil and rock materials at the site.

2. The most appropriate type of foundations for the proposed structures, and support systems for the proposed slopes.

3. The design criteria for the recommended foundation type or support system, including necessary seismic considerations for the proximity of the Maacama fault.

4. The estimated ground settlement rate beneath the foundation or support system.

5. The necessary subgrade preparation for the foundations or support systems.

6. The lateral pressures for retaining walls.

7. The drainage conditions at the site.

8. The design slopes for cut and fill sections.

9. The suitability of on-site soils for use as backfill.

The recommendations of the foundation, slope and structural reports prepared for the construction of buildings or slopes at the specific project site are required to be incorporated in the Plans and Specifications for the design of the project. The minimum earthquake-resistant design that each project must include, must meet the current seismic engineering standards of the California Uniform Building Code for Seismic Zone 4. The appropriate standards to be met for near-field conditions (sites within one mile of a known active fault) are determined by the geotechnical and engineering professionals on the basis of the site-specific evaluations.

Mitigation Measure 3.2.1-1
In response to Soils and Geology Policies ER-6.5-1A, -1B and -2A, require site-specific minimal grading concepts, stability analysis and stabilization procedures, and design criteria for cut-slopes and fill-slopes, as recommended by a California Certified Engineering Geologist and Geotechnical Engineer during the design phase for each site inclusive of geologically constrained areas of the Alquist-Priolo zone, landslide areas and steep slopes.

Implementation of this mitigation measure, in a way similar to the following outline, would reduce this impact to an insignificant level. (I)

A. During the design phase for each site where construction is to occur or where substantial amounts of cutting or filling are to occur, the developer's registered geotechnical engineering consultant shall provide documentation that:

1. site-specific stability analyses has been conducted in the area proposed for grading to establish the design criteria for proposed cut or fill slopes, and
2. the recommended criteria have been incorporated in the design of cut and fill slopes.

B. During grading for these sites, the registered geotechnical professional shall be on the site:

1. to supervise the implementation of slope stability designs,
2. to observe areas of potential instability,
3. to supervise slope repairs, as necessary, and
4. to supervise compaction testing.

C. The registered geotechnical engineering consultant should prepare an "as built" map, to be filed with the County, showing details of the site geology, the location of foundations, retaining walls, sub-drains and cleanouts, the results of stability analyses and compaction tests, and documenting the following requirements.

1. The CUBC Seismic Zone 4 standards shall be the minimum acceptable standards for stability of new or altered slopes.

2. Only the minimum amount of grading necessary for obtaining fill material, stabilizing slopes, and installing structures or access shall be performed in areas where slopes are steeper than 20 percent, to avoid the creation of potentially unstable slopes in borrow areas or at the construction sites.

3. Cut-slopes in alluvium, and fill-slopes shall be no steeper than 3:1 (horizontal to vertical) unless the design-level geotechnical investigation can demonstrate the satisfactory stability of a steeper configuration.

4. Cut-slopes in bedrock shall be no steeper than 2:1 (horizontal to vertical) unless the design-level geotechnical investigation can demonstrate the satisfactory stability of a steeper configuration.

5. Side-hill fills, if used, shall be keyed, provided with surface and subsurface drainage, and compacted according to the design specifications of the slope stability analyses for the site provided by the geotechnical professional.

Grading, excavation and construction activities would have the potential to increase erosion of soil from building sites, and to cause subsequent deposition of particles in drainage ways, creeks, or reservoirs. (PS)

During the grading and construction period, the potentially erosive effects of water leaving the construction areas would be of concern. Runoff during the grading period could carry particles of soil or fill from the grading or construction sites, or could erode soil down-gradient, if the flow were not controlled. In some cases the loss of the material by erosion may not be a significant impact in itself. However, the re-deposition of eroded material in water courses or lakes could create turbidity (endangering aquatic life), reduce wildlife habitat, and reduce the water-carrying capacity of streams and drainage ways, thereby potentially aggravating flood conditions (see Section 3.2.2, Hydrology). Erosive conditions uncontrolled during the grading period can persist into the occupation period of a building project.

The single most effective method to counteract the potential for water erosion, is to complete as much of the grading and construction as possible during the dry season. However, if portions of these phases extend into the wet season, sediment can be prevented from leaving the construction sites through the use of silt fences, straw bales, perimeter ditches, water bars, temporary culverts and swales, sediment traps, minimal grading concepts, and similar techniques appropriate for the site. These erosion and sediment transport control structures need to be in place prior to the onset of seasonal rains.

General grading activities, including those related to construction, are regulated by the California Uniform Building Code. Because Brooktrails contains a variety of slope gradients and of soil and rock materials, plans to control erosion and sediment transport must be suited to the sites where grading and construction is to occur. The concepts to be incorporated (as appropriate) in such plans have been published by the Association of Bay Area Governments and are reproduced in Mitigation Measure 3.2.1-2 below.

Mitigation Measure 3.2.1-2
In response to Soils and Geology Policies ER-6.5-1A and -1B, during design review, require an Erosion and Sediment Transport Control Plan, designed by an erosion control professional, or landscape architect or civil engineer specializing in erosion control, that would meet the following objectives for the grading and construction period of building projects in the Township, and throughout the lifetime of each project.

The implementation of this mitigation measure, in a way similar to the following outline, would reduce this impact to an insignificant level. (I)

A. The Erosion and Sediment Transport Control Plan shall be submitted, reviewed, implemented and inspected as part of the approval process for the grading plans for each project.

B. The Plan shall be designed by the developers' erosion control consultant, using concepts similar to those developed by the Association of Bay Area Governments, as appropriate, based on the specific erosion and sediment transport control needs of each area in which grading and construction is to occur. Those concepts include, but are not necessarily limited to the following items.

Confine grading and activities related to grading (demolition, construction, preparation and use of equipment and material storage areas, staging areas, preparation of access roads,) to the dry season, whenever possible.

If grading or activities related to grading need to be scheduled for the wet season, ensure that structural erosion and sediment transport control measures are ready for implementation prior to the onset of the first major storm of the season.

• Locate staging areas outside major streams and drainage ways.
• Keep the lengths and gradients of constructed slopes (cut or fill) as low as possible.
• Discharge grading and construction runoff into small drainages at frequent intervals to avoid buildup of large potentially erosive flows.
• Prevent runoff from flowing over unprotected slopes. Keep disturbed areas (areas of grading and related activities) to the minimum necessary for demolition or construction.
• Keep runoff away from disturbed areas during grading and related activities.
• Stabilize disturbed areas as quickly as possible, either by vegetative or mechanical methods.
• Direct runoff over vegetated areas prior to discharge into public storm drainage systems, whenever possible.
• Trap sediment before it leaves the site with such techniques as check dams, sediment ponds, or siltation fences.
• Make the contractor responsible for the removal and disposal of all sedimentation in retention ponds, that is generated by grading and related activities of the project.
• Use landscaping and grading methods that lower the potential for down-stream sedimentation. Modified drainage patterns, longer flow paths, encouraging infiltration into the ground, and slower storm-water conveyance velocities are examples of effective methods.
• Control landscaping activities carefully with regard to the application of fertilizers, herbicides, pesticides or other hazardous substances. Provide proper instruction to all landscaping personnel on the construction team.

C. During the installation of the erosion and sediment transport control structures, the erosion control professional shall be on the site to supervise the implementation of the designs, and the maintenance of the facilities throughout the demolition, grading and construction period.

D. The erosion control professional shall prepare an "as built" erosion and sediment control facility map, to be filed with the Township, showing details of the biological and structural elements of the plan, and providing an operating and maintenance schedule throughout the operational period of the project.

Impact 3.2.1-3
Use of weak soils for foundation support without prior treatment could create unstable soil conditions at the construction site. (PS)

The existence of impermeable, expansive, compressible and corrosive soils in Brooktrails make it necessary to ensure the soils used for foundation support are sound. The creation of cut or fill building pad terraces in unsuitable soils has the potential to create future problems of foundation settlement and utility line disruption if the soils are not specifically engineered for stability.

An acceptable degree of soil stability can be achieved by adopting soil treatment programs (grouting, compaction, drainage control, etc.) and foundation designs (grade beams, drilled piers, driven piles, etc.) that address site-specific soil conditions. Site-specific analysis is the basis of foundation design in areas where unsuitable conditions are suspected. Such analyses contain recommendations for ground preparation, earthwork, foundations, etc., specific to the site, that become an integral part the construction design.

Before approving a project in Brooktrails, the County should have a completed report of soil conditions at the project site, provided by the Project Sponsor, that identifies potentially unsuitable soil conditions. The evaluations must be conducted by registered soil professionals, and measures to eliminate inappropriate soil conditions must be applied to the project site. The site-specific measures needed to achieve satisfactory soil performance cannot be determined until the soil evaluations are complete and at least conceptual designs for the project have been prepared.

Mitigation Measure 3.2.1-3
In response to Soils and Geology Policies ER-6.5-1A and -1C, during design review, require site-specific soil suitability analysis and stabilization procedures, and design criteria for foundations, as recommended by a California-registered soil engineer during the design phase for each site where the existence of unsuitable soil conditions is known or suspected.

Implementation of this mitigation measure, in a way similar to the following outline, would reduce this impact to an insignificant level. (I)

A. During the design phase for each site where the existence of unsuitable soil conditions is known or suspected, the developer's registered soil engineering consultant shall provide documentation to the Township that:

1. site-specific soil suitability analyses has been conducted in the area of the proposed foundation to establish the design criteria for appropriate foundation type and support, and

2. the recommended criteria have been incorporated in the design of foundation.

B. During grading for these sites, the registered soils professional shall be on the site:

1. to observe areas of potential soil unsuitability,

2. to supervise the implementation of soil remediation programs, and

3. to verify final soil conditions prior to setting the foundations.

C. The registered soils engineering consultant shall prepare an "as built" map, to be filed with the County, showing details of the site soils, the location of foundations, sub-drains and cleanouts, the results of suitability analyses and compaction tests.

Impact 3.2.1-4
The northeast corner of the Plan Area is subject to the damaging effects of surface rupture along traces of the Maacama fault during the useful economic life of the Specific Plan. (S)

Surface rupturing along the trace of a fault affects all types of material; however, it does not always show clearly in unconsolidated soils or alluvium. Damage caused by surface rupturing is limited to the actual location of the fault-line break, unlike damage from groundshaking which can occur at great distances from the fault. Even a moderate earthquake can be accompanied by enough surface rupturing to damage foundations and buried utility lines that have not been adequately protected where they cross fault traces.

Under the Alquist-Priolo Earthquake Fault Zoning Act of 1972, the State is required to delineate "Earthquake Fault Zones" along known active faults, with the intent of regulating development near active faults in order to mitigate the hazard of surface fault-rupture. An Earthquake Fault Zone has been delineated in the Township along the Maacama fault zone.

Mitigation Measure 3.2.1-4
In response to Soils and Geology Policy ER-6.5-2A, secure the recommendations of a site-specific fault trace location and activity level investigation, performed by a California Certified Engineering Geologist, a California Registered Geologist or California Registered Geotechnical Engineer, to be incorporated in the design of all structures intended for human occupancy within the Earthquake Fault Zone that crosses the Township.

Implementation of this mitigation measure, in a way similar to the following outline, would not prevent rupture of the Maacama fault, but would reduce the effects of this hazard to an insignificant level for new structures in the Earthquake Fault Zone. (I)

A. The minimum setback from an active fault trace should be 50 feet, unless the site-specific fault investigation can demonstrate satisfactory safety conditions closer to the trace.

B. Additional seismic-resistant earthwork and construction design criteria shall be incorporated in the project as necessary, based design review and on the site-specific recommendations of a California Certified Engineering Geologist in cooperation with California Registered Geotechnical and structural engineering professionals.

C. During site preparation, the registered geotechnical professional shall be on the site to supervise implementation of the recommended criteria.

D. The geotechnical consultant shall prepare an "as built" map/report, to be filed with the County, showing details of the site geology, the location and activity level of fault traces, and the type and location of seismic-restraints used in the project facilities.

Impact 3.2.1-5
Brooktrails will be subject to damaging seismically induced groundshaking during the useful economic life of the Specific Plan. (S)

From the review of regional and local geo-seismic conditions, it is apparent that Brooktrails will be subjected to at least one major earthquake during the useful economic life of the Township Specific Plan. The design earthquake for the Brooktrails area is estimated to be about an M7 earthquake on
the Maacama fault, creating peak horizontal ground accelerations as high as 0.7g. The resulting vibration could cause damage to structural members of proposed commercial and residential facilities (primary effects), and could cause ground failures in alluvium and poorly compacted fill (secondary effects).

In Mendocino County, buildings constructed for human occupancy are required to reduce the exposure to potentially damaging seismic vibrations through seismic-resistant design, in conformance with the CUBC Seismic Zone 4 requirements. The CUBC Seismic Zones (1 through 4) roughly correspond to peak accelerations from the maximum credible earthquake expected within a given area (10 percent through 40 percent of the force of gravity). Because the parameters of the CUBC Seismic Zone 4 would be exceeded by the design earthquake for Brooktrails it is necessary to review the seismic protection requirements for all new or modified structures in more detail.

The geologic conditions at the construction sites, the criteria for determining the design earthquake for the specific project site, and the seismic-restraint criteria for areas in which new structures will be located need to be reviewed by a California Registered Geologist or Certified Engineering Geologist in consultation with the geotechnical and structural engineers for the project. The review would produce recommendations regarding seismic restraints that would be incorporated in the design of projects in Brooktrails to increase the chances of survival for residents and visitors to the Township during a major earthquake.

Mitigation Measure 3.2.1-5
In response to Soils and Geology Goal ER-6.5-2 and Policy ER-6.5-2B, require site-specific seismic-restraint criteria, as recommended by a California-registered geotechnical or structural engineer, to be incorporated in the design of slopes, foundations and structures for projects in the Township.

Implementation of this mitigation measure, in a way similar to the following outline, would reduce this impact to an insignificant level. (I)

A. The minimum seismic-resistant design standards for all proposed facilities shall conform to the CUBC Seismic Zone 4 Standards.

B. Additional seismic-resistant earthwork and construction design criteria shall be incorporated in the project as necessary, based on design review and the site-specific recommendations of a California Certified Engineering Geologist in cooperation with California-registered geotechnical and structural engineering professionals.

C. During site preparation, the registered geotechnical professional shall be on the site to supervise implementation of the recommended criteria.

D. The California Certified Engineering Geologist consultant shall prepare an "as built" map/report, to be filed with the County, showing details of the site geology, the location and type of seismic-restraint facilities, and documenting the following requirements, as appropriate.

1. Engineering analyses shall demonstrate satisfactory performance of bedrock, alluvium and fill where they form part or all of the support for structures.

2. Analysis of soil expansion potential and appropriate remediation (compaction, removal, etc.) shall be completed prior to using expansive soils for foundation support.

3. Roads, foundations and underground utilities in fill or alluvium shall be designed to accommodate settlement or compaction estimated by the site-specific investigations of the geotechnical consultant.

Impact 3.2.1-6
The Specific Plan would increase the number of dwelling units in an area subject to seismic groundshaking with its attendant secondary effects of ground failure. (S)

If the Specific Plan is implemented, the establishment of a total of approximately 3800 dwelling units in an area subject to geological hazards could not be avoided. These hazards, previously discussed in this section, can be reduced to a great extent through properly applied engineering design, and management procedures, but they cannot be entirely eliminated. To offset the effects of these hazards, the Specific Plan proposed the implementation of the above listed Soils and Geology Goals and Policies, in addition to the County's requirement of the use of the current CUBC standards for all new structures. Appropriate site-specific investigation of geologic conditions for new structures intended for human occupancy are necessary to the safety of development projects in the Township.

Mitigation Measure 3.2.1-6
Implement Mitigation Measures 3.2.1-1 through 5. In addition, this EIR should remain available at the Township for potential builders and/or lot purchasers. This would reduce this impact to an insignificant level. (I)

Impact 3.2.1-7
The dam for the proposed water-supply reservoir could be vulnerable to damage in an earthquake. (PS)

Damage to the proposed water-supply reservoir dam in a very severe earthquake is unlikely because the dam would be designed to meet the stringent requirements of the California Department of Water Resources, Division of Safety of Dams (DSOD). The dam is required to be built to withstand the forces produced by the maximum credible earthquake (M8.3) likely to occur on the San Andreas fault.

The dam itself would not lie on an active or potentially active fault. The nearest fault trace in the vicinity of the proposed dam is about 2 miles to the northeast, with no evidence of Holocene (last 11,000 years) displacement. As such, it is not considered to pose a greater threat to the proposed dam than the design earthquake on the San Andreas fault.

No dams in California have failed as a result of earthquakes; however, at least two dams of inadequate structural design have been so damaged by seismically induced secondary ground failures that they had to be abandoned. In 1971, the Lower Van Norman Dam was damaged by a severe landslide on the reservoir side of the dam during the San Fernando earthquake. Prior to World War II, the Scheffield Dam near Santa Barbara was damaged by earthquake-generated liquefaction. According to the DSOD, there was no substantial water loss in either case. Residents in the potential inundation zone were evacuated successfully and the dams eventually were abandoned as structurally unsafe. Under increasingly stringent DSOD regulations, the risk of earthquake-induced dam failures is reduced to a reasonable level.

The dam would be designed and constructed to meet the current static load and seismic load standards of the DSOD. The structure would be inspected periodically by DSOD for static and dynamic stability.

Mitigation Measures 3.2.1-7
In response to Soils and Geology Goal ER-6.5-1 and -2, require a detailed Earthquake Preparedness Plan to be prepared by the dam operations personnel and submitted for review and approval by the County.

Implementation of this mitigation measure, in a way similar to the following outline, would reduce this impact to an insignificant level. (I)

A. The specific language of earthquake preparedness plans varies, but should include the following items:

1. Ensuring existing and proposed seismic designs meet current County, State and Federal standards, where applicable.

2. Making structural and non-structural elements secure from the effects of expected levels of groundshaking.

3. Assigning specific personnel primary and backup responsibilities to be carried out during a seismic emergency.

4. Providing supplies of emergency water, food and shelter for project personnel to remain on-site for at least three days.

5. Providing training for personnel in First Aid, CPR and other emergency response procedures.

6. Carrying out practice drills of emergency response procedures.

7. Preparation of an inundation zone map and evacuation plan.

B. Township public safety and maintenance facilities should be located out of the dam failure inundation zone.