Trancas Canyon Park EIR Section 4.5 Geology and Soils
4.5 GEOLOGY AND SOILS
The following analysis is based in part on a Geotechnical Report (July 2007) and a Grading Plan Review Report (June 2008), both performed by Geolabs-Westlake Village for the Trancas Park Project, as well as subsequent technical memorandums from Geolabs supplementing these reports. These reports and memorandums are included in their entirety as Appendix G to the EIR.
4.5.1 Setting
Trancas Canyon is located within the city of Malibu, which extends over 26 miles along the Pacific Ocean coastline within southwestern Los Angeles County. Malibu lies within the coastal and foothill areas on the south side of the Santa Monica Mountain Range. The City is within the Topanga, Malibu Beach, Point Dume, and Triunfo Pass Quadrangles which cover approximately 50 square miles of land comprising noncontiguous tracts of public lands, the westernmost portion of Malibu Creek State Park, Point Dume State Beach, Zuma Beach County Park, and Robert H. Meyer Memorial State Beach.
The project site is located approximately one-half mile north of the Pacific Ocean coastline in the Santa Monica Mountain Range, which is in the western part of the Transverse Ranges Geomorphic Province of California. The valleys and mountains of the Transverse Ranges are typically bounded by a series of east-west trending, generally north dipping reverse faults with left-lateral, oblique movement. Bedrock beneath this area consists of Miocene geologic-aged volcanic and marine sedimentary rocks. This province is considered seismically active.
The site is at an elevation that ranges between 100 and 300 feet above mean sea level within the Santa Monica Mountains. Although the project site is generally located in a moderate to steeply sloping canyon, the surrounding coastal area is characterized by broad, gently sloping, and relatively continuous terrace surfaces above narrow to moderately wide beaches. Trancas Canyon and the surrounding areas are composed of undifferentiated alluvial deposits (stream- deposited, unconsolidated) of gravels, sands, and silt, which extend down in the canyon.
a. Regional Geology. The faulting and seismicity of Southern California is dominated by the compressionary regime associated with the intersection of the San Andreas Fault Zone and the Garlock Fault. The San Andreas Fault Zone separates two tectonic plates. The western side of the fault is the Pacific Plate and the eastern side of the fault is the North American Plate. The Western Plate is moving in a northwesterly direction relative to the North American Plate. The San Andreas Fault generally trends northwest to southeast. However, north of the Transverse Ranges Province, the fault trends more in an east-west direction, causing the fault’s right-lateral strike-slip movement to produce north-south compression between the two plates. This compression has produced rapid uplift of many of the mountain ranges in Southern California. According to the Southern California Earthquake Center, north-south compression in southern California has been estimated at between 5 to 20 millimeters per year (SCEC, 1995).
b. Site Geology. The project site is located between Trancas Canyon Road and Paseo Canyon Drive, one half mile from the Pacific Coast Highway in the city of Malibu. The City of Malibu is bounded by the Santa Monica Mountains to the north, the Pacific Ocean to the south, the Santa Monica Fault to the east and Ventura County to the west. The project site is located at City of Malibu 4.5-1
Trancas Canyon Park EIR Section 4.5 Geology and Soils
approximately 6050 Trancas Canyon Road within the City of Malibu on generally east-facing manufactured and natural slopes (Geolabs, June 30, 2008) on the southern side of the Santa Monica Mountain Range. The major drainage in the vicinity of the site is Trancas Creek, which is located adjacent to the east of the site and drains south from the Santa Monica Mountains. According to the State of California Seismic Hazard Zone Report for the Point Dume Quadrangle (CDMG, 2001), the estimated historic highest groundwater depth in Trancas Canyon was 10 feet below grade.
According to the Geologic Map for the project area prepared by Geolabs (June 30, 2008, Plate 1.2), the site is underlain by artificial fill, topsoil, colluvium, landslide deposits, Zuma Volcanics, the Encinal Canyon Member of the Topanga Formation, and the Vaqueros Formation. According to Geolabs, several generations of artificial fill exist at the site, with a large canyon fill underlying the lower flat pad area. The large canyon fill was an engineered fill emplaced in 1964 according to the standards of care at that time. Based on Cross Section A-A’, shown on Plate 2.0 of Geolab’s June 30, 2008 report, the maximum depth of the large canyon fill is approximately 60 feet. Other areas of undocumented artificial fill are also present at the site.
Topsoil, defined as residual soil that caps the bedrock, was observed up to two feet thick by Geolabs (June 30, 2008), and reported up to five feet thick by previous geological consultants of record. Colluvium consisting of sandy clay and clayey sand with locally derived rock fragments was observed on the lower portions of existing natural slopes and upslope (and offsite) of the existing canyon fill (Geolabs, June 30, 2008).
Four landslides have been mapped in the northwest trending ravine north of the existing lower fill pad. According to Geolabs (June 30, 2008), three of the landslides are outside of the proposed park boundaries, but are elevated above the proposed park site. The fourth landslide extends onto the northwestern corner of the existing upper pad area onsite (Geolabs, June 30, 2008).
Zuma Volcanics are exposed in the southwestern and southern portions of the project area, south of the Malibu Coast Fault. The Zuma Volcanics generally consist of fractured, orange- brown andesite with interbeds of shale, siltstone and sandstone of the Trancas Formation (Geolabs, June 30, 2008). The Encinal Canyon Member of the Topanga Formation, consisting of dark brown to black fractured siltstone and claystone, was encountered in the northwestern portion of the project area, north of the Malibu Coast Fault. The Vaqueros Formation, a medium to coarse grained sandstone and conglomerate with interbedded platy to shaly siltstone and mudstone exists in the northeastern portion of the project area, north of the Malibu Coast Fault (Geolabs, June 30, 2008).
Perched groundwater was encountered by Geolabs during their exploratory excavations onsite (June 30, 2008). The perched groundwater was observed to be emanating from high-angle fractures in the bedrock, and was encountered between depths of 40 to 51 feet below ground surface. Geolabs indicated that the regional groundwater table would be expected to occur at or below the elevation of the thalweg of Trancas Canyon Creek (June 30, 2008).
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c. Seismic Hazards.
Seismic Potential. The Homeowners Guide to Earthquake Safety (California Seismic Safety Commission, 2005) briefly mentions and depicts seismic zones within California. As referenced in the Homeowners Guide, the International Building Code ranks the different regions according to their seismic hazard potential. Four regions have been established, Seismic Zones 1 through 4, with Zone 1 having the least seismic potential and Zone 4 having the highest seismic potential. As depicted in the Homeowners Guide to Earthquake Safety, the project site lies within Seismic Zone 4.
The proximity of active faults is such that the project area has experienced and will continue to experience strong seismically induced ground motion. The U.S. Geological Survey defines active faults as those that have had surface displacement within Holocene time (about the last 11,000 years). Surface displacement can be recognized by the existence of bluffs in alluvium, terraces, offset stream courses, fault troughs and saddles, the alignment of depressions, sag ponds, and the existence of steep mountain fronts. Potentially active faults are those that have had surface displacement during the last 1.6 million years. Inactive faults have not had surface displacement within the last 1.6 million years.
Several active and potentially active faults are located in the general site vicinity (see Figure 4.5- 1). These nearby faults include:
• Malibu Coast Fault (Active) • Santa Monica Fault (Potentially Active) • Palos Verdes Hills Fault (Active) • Newport-Inglewood Fault (Active)
According to the Geologic Map for the site by Geolabs (June 30, 2008), which is a compilation of their work onsite in addition to work performed by previous geologists, the Malibu Coast Fault trends northwest/southeast through the subject property, with two splays mapped (one of which is shown as concealed). Although the proposed park site is not located within a State- defined Alquist-Priolo Earthquake Fault Zone, the Malibu Coast Fault Zone is considered active by the City of Malibu and the Los Angeles County Seismic Safety Element unless proven otherwise (Geolabs, June 30, 2008). Other faults in the Southern California area that have the potential to seismically affect the site include the San Gabriel Fault, the San Andreas Fault, and the probable existence of large blind thrust faults currently undocumented.
Faults generally produce damage in two ways: surface rupture and seismically induced ground shaking. Surface rupture is limited to areas very near the fault and ground shaking covers a wide area.
Surface Rupture. Surface rupture along a fault is the surface expression of fault displacement. Fault displacement occurs when material on one side of a fault moves relative to the material on the other side of the fault. Surface displacement can range from a few inches to tens of feet during a rupture event. This can have disastrous consequences, including injury and loss of life, when buildings are located within the rupture zone. It is not practically feasible (structurally or economically) to design and build structures that can accommodate the rapid displacement involved with surface rupture.
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Alquist-Priolo Earthquake Fault Zones are areas within 500 feet from a known active fault trace. Pursuant to the Alquist-Priolo legislation, no structure for human occupancy is permitted on the trace of an active fault. The term “structure for human occupancy” is defined as any structure used or intended for supporting or sheltering any use or occupancy, which is expected to have a human occupancy rate of more than 2,000 person-hours per year. Any new development proposed within an Alquist-Priolo Zone must be preceded by a fault study. If the study indicates that an active fault is located within a proposed development site, then all structures associated with the development must be set back at least 50 feet on both sides of the fault trace. According to the California Division of Mines and Geology, the Alquist-Priolo legislation presumes that any area within 50 feet of an active fault is underlain by active branches of the fault unless proven otherwise (CDMG, revised 1997).
According to the 2000 California Division Mines and Geology Digital Images of Official Maps of Alquist-Priolo Earthquake Fault Zones of California, Southern Region, the nearest Alquist- Priolo Earthquake Fault Zone to the project site is located approximately 3.5 miles to the east- southeast of the site. This zone is associated with a segment of the active Malibu Coast Fault.
Seismically Induced Ground Shaking. Ground shaking covers a wide area and is greatly influenced by the distance of the site to the seismic source, soil conditions, and depth to groundwater. Ground shaking is a result of the seismic waves produced by a fault rupture event. Secondary hazards associated with seismically induced ground shaking include liquefaction, seismically induced settlement, earthquake-triggered landslides, tsunamis and seiches. Any of the faults listed above could generate substantial ground shaking at the project site.
As shown in the Peak Ground Acceleration Map of California by the California Geologic Survey (CGS Map, 2006), the site has a 10 percent probability of experiencing 0.4 to 0.5 gravity (g) peak horizontal ground acceleration within the next 50 years. A Probabilistic Seismic Hazard Assessment and Map produced by the California Geological Survey for the California/Nevada region (CGS, 2006) depicts peak ground acceleration (Pga), spectral acceleration (Sa) at short (0.2 second) and moderately long (1.0 second) periods. Ground movements (10% probability of being exceeded in 50 years) are expressed as a fraction of the acceleration due to gravity (g). Ground movement accelerations were calculated based on firm rock conditions, soft rock conditions, and alluvium site conditions and are discussed below (CGS, 2006).
The 10% exceedance in 50 years peak ground accelerations in units of gravity (g) expected at the site are 0.455 for firm rock conditions; 0.455 for soft rock conditions, and 0.475 for alluvium conditions.
The spectral acceleration (0.2 second) in units of g expected at the site are 1.055, 1.055, and 1.137 for firm rock, soft rock and alluvium conditions, respectively. The spectral acceleration (1.0 second) in units of g expected at the site are 0.387, 0.474, and 0.564 for firm rock, soft rock and alluvium conditions, respectively. The calculation of accelerations for firm rock, soft rock, and alluvial conditions assumes that the entire area is properly described as firm rock, soft rock, or alluvial material. Immediately beneath the project site, the earth materials consist of artificial fill and colluvial materials in the eastern portion of the site, clayey siltstone and sandstone in the northern and northwestern portions of the site, and Zuma Volcanics in the southern and southwestern portions of the site.
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Sa n 33 An UV dr eas fa ult §¨¦5 UV138
Santa Clarita UV126
§¨¦5 1 UV Simi Valley §¨¦210 215 1 27 §¨¦ UVOxnard UV Thousand Oaks 101 Glendale 210 ¤£ §¨¦ 605 Rancho Cucamonga 101 210 San Bernardino £ lt N §¨¦ 66 ¤ u e §¨¦ UV fa w 215 a El Monte ic p 2 10 Pomona 10 §¨¦ Malibu Coast fault n o UV §¨¦ §¨¦ o r Ontario LOS AN GELS CO. M t f - 10 UV1 a a I 1 t n UV n u §¨¦ 710 ^ a 10 g _ l S t l §¨¦ §¨¦ e 71 Riverside w 72 UV o UV 1 42 P UV o UV 605 83 d UV a c 105 §¨¦ i §¨¦ 90 Moreno Valley f 19 i c UV FUVullerton 110 O 91 §¨¦ c UV Torrance e P a al os Orange n V Garden Grove er 710 de §¨¦ UV22 15 74 s §¨¦ 74 UV Hi UV lls 1 39 fa UV UV ul 215 t §¨¦405 Irvine §¨¦ Source: Bryant, W.A. (compiler), 2005, Digital Database of QHuuatnetringatroy na nBde aYcohungerUV F13a3ults from the Fault Activity Map of California, version 2.0: California Geological Survey Web Page,
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Thus, the appropriate horizontal ground acceleration expected would be modeled by the alluvial conditions (0.475 g), soft rock conditions (0.455 g), or firm rock conditions (0.455 g), depending on where structures are proposed onsite. The strength of ground shaking in an area is primarily a function of the distance between the area and the seismic source epicenter, the type of material underlying the property, and the motion of fault displacement. Because of the proximity to major active faults, such as the Malibu Coast Fault, and the San Andreas and Newport-Inglewood fault systems, it is possible that accelerations near or over 1.0g could occur anywhere within project area limits, including the project site.
d. Secondary Seismic Hazards and Soil Hazards.
Liquefaction. Liquefaction is a temporary, but substantial, loss of shear strength in granular solids, such as sand, silt, and gravel, usually occurring during or after a major earthquake. This occurs when the shock waves from an earthquake of sufficient magnitude and duration compact and decrease the volume of the soil; if drainage cannot occur, this reduction in soil volume will increase the pressure exerted on the water contained in the soil, forcing it upward to the ground surface. This process can transform stable granular material into a fluid- like state. The potential for liquefaction to occur 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.
Table 4.5-1 lists the relationship between liquefaction hazard and groundwater depth for the unconsolidated geologic units beneath the site.
Table 4.5-1 Liquefaction Zone Criteria
Geologic Unit Depth to Groundwater Greater than 40 feet Less than 40 feet Qal (alluvium) Low High Source: Department of Conservation Division of Mines and Geology, Seismic Hazard Zone Report, 7.5 Minute Point Dume Quadrangle, 2001.
According to the Geolabs Grading Plan Review Report (June 30, 2008), no alluvium exists beneath the project area. In addition, the depth to perched groundwater encountered during Geolab’s onsite investigation was greater than 40 feet. Therefore, Geolabs concluded that the potential for liquefaction to occur at the site was very low.
Subsidence, Settlement, and Hydroconsolidation. Subsidence is the withdrawal of fluid (oil, natural gas, or water) from compressible sediments. As water is withdrawn and the water table lowered, the effective pressure in the drained sediments is increased. Compressible layers then compact under the over-pressure burden that is no longer compensated by hydrostatic pressure. The resulting land subsidence is most pronounced in uncompacted sediments throughout Southern California. No withdrawal of fluid currently occurs from beneath the subject site and is not proposed as part of the park project.
Seismically induced settlement occurs in loose to medium dense unconsolidated soil. Loose to medium dense unconsolidated soil can compress (settle) when subject to seismic shaking. The
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settlement is exacerbated by increased loading, such as from the construction of structures onsite. This settlement can be mitigated prior to development through the removal and recompaction of loose soils. Geolabs has identified areas within the project area that are underlain by undocumented artificial fill, topsoil, and colluvium materials that may be subject to seismic compression.
Hydroconsolidation is the gradual reduction in soil mass resulting from an increase in compressive stress caused by the introduction of water. Geolabs indicated that they consider the existing undocumented artificial fills and mapped colluvial soils, other than those left in place beneath the 1964 compacted fill, as susceptible to hydroconsolidation (June 30, 2008). Geolabs performed consolidation testing on four samples of the 1964 large canyon fill, and determined that the potential for hydroconsolidation is low for the existing 1964 canyon fill and the underlying colluvium.
Landsliding. Landslides occur when slopes become unstable and masses of earth material move downslope. Landslides are generally considered to be rapid events, often triggered during periods of rainfall or by earthquakes. Areas that are more prone to earthquake-induced landslides are steep slopes in poorly-cemented or highly fractured rocks, areas underlain by loose, weak soil, and areas on or adjacent to existing landslide deposits.
The 2001 Seismic Report (CDMG, 2001), identifies areas throughout the Point Dume quadrangle as having a potential for landslides and slope instability. Due to the nature of numerous active faults in California, areas likely to be affected by strong earth-shaking events are the hillside areas that exist within the southern two-thirds of the Point Dume Quadrangle.
According to the Geologic Map of the project area by Geolabs (June 30,2008), one landslide extends into the northwestern corner of the existing upper pad area onsite. In addition, three other landslides have been mapped offsite in the northwest trending ravine north of the existing lower fill pad. The onsite landslide has landslide debris down to at least 17 feet below ground surface according to Geolabs (June 30, 2008). Geolabs indicates that this landslide appears to be a rotational slump deposit.
Other landslides exist to the north, northwest and northeast of the subject site. According to the Seismic Hazard Zones Map (CDMG, Point Dume Quadrangle, 2002), the eastern and northern portions of the subject property are depicted to be in a zone where previous occurrence of landslide movement or local topographic, geological, geotechnical, and subsurface water conditions indicate a potential for permanent ground displacement such that mitigation as defined in Public Resource Code Section 2693(c) would be required.
Geolabs evaluated the slope stability of the steep natural slopes flanking the northwest trending ravine on the subject property visually. The steep natural slopes flanking the northwest trending ravine exhibit signs of gross instability in the form of the three mapped offsite landslides. In addition to this visual evaluation, Geolabs calculated the stability of the landslide area that lies near the western edge of the proposed upper dog park pad. Geolabs concluded that the safety factors were below the City standards for new construction for the failure surfaces along the landslide slip surface for both the existing and the proposed conditions (June 30, 2008).
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Slope Instability. Mudslides, debris flows, and slumps are a more shallow type of slope failure compared to landslides. These typically affect the upper soil horizons, and are not commonly bedrock features. Historically, mudslides, debris flows, and slumps occur during or soon after periods of rainfall. Erosion can occur along manufactured slopes that are improperly designed or not adequately re-vegetated. In addition, slope instability can occur during inappropriate grading in areas of steep slopes.
Geolabs evaluated the slope stability of the steep natural slope descending from the upper northeastern level pad to an area where tennis courts have been constructed. The 170-foot-high, very steep, slope that descends to the tennis court area may be susceptible to rockfall and surficial instability. In addition to this visual evaluation, Geolabs calculated the stability of existing cut and fill slopes onsite, and calculated the stability of these slopes after proposed grading (June 30, 2008).
Geolabs concluded that the safety factor is adequate for the existing 120-foot-high fill slope that includes the proposed practice field area and descends to Trancas Canyon. In addition, the safety factor is adequate for the proposed grading on this fill slope that includes placing an additional 10 vertical feet of artificial fill on top of the pad area, above the artificial canyon fill placed in 1964.
Geolabs concluded that the safety factors are below the City standards for new construction, either currently or after proposed grading, for the proposed 35-foot-high, 2 horizontal to 1 vertical (2H:1V or 2:1) cut slope at the toe of the existing 95-foot-high 1 ½:1 to 2:1 cut slope that descends to the proposed future dog park pad. Geolabs indicated that mitigation of this condition should be considered (Geolabs, June 30, 2008).
Tsunamis and Seiches. Tsunamis are large ocean surges that are created as a result of a subsea earthquake or landslide. The waves created by the subsea earthquake or landslide travel across the ocean at high speeds (several hundreds of miles per hour). As the waves reach shore, their amplitudes increase. Once the waves reach land, they can cause widespread flooding. The areas susceptible to tsunamis are those near to the ocean and along low-lying river channels.
A seiche is a wave or series of waves that are produced within an enclosed or partially enclosed body of water (such as a lake or bay). Most seiches are created as landslides fall into the body of water and displace the water. The water then sloshes out of the bay or lake, creating the seiche. If a seiche overtops a dam, the water can erode the dam face to the point where the dam can fail.
The subject property is located approximately one-half mile north from the Pacific Ocean at elevations ranging from 100 to 300 feet above mean sea level. As such, the risk of damage from a tsunami is considered low. The site does not lie in an area near any large bodies of water or bays that could be affected by a seiche. Therefore, the risk from seiches is also considered to be low.
5.2 Environmental Impact Analysis
a. Methodology and Significance Thresholds. This evaluation is based on review of existing information that has been developed for the project site and other available regional sources. An impact is considered potentially significant if it would expose people or structures to major geologic hazards. Therefore, impacts are considered significant if the proposed
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development would be exposed to high potential for such seismic hazards as ground surface rupture, ground shaking, liquefaction, subsidence, settlement, hydroconsolidation, landslides, slope instability, or tsunamis and seiches.
b. Project Impacts and Mitigation Measures.
Impact GEO-1 Seismically induced ground surface rupture could occur in the project area due to the onsite Malibu Coast Fault as depicted on the Geologic Map by Geolabs (June 30, 2008). This is considered a Class II, significant but mitigable, impact.
The Malibu Coast Fault is mapped as two splays that transverse the subject property in a northwest/southeast direction, in the central portion of the property. The northern splay of the fault is mapped as concealed. Other segments of the Malibu Coast Fault have been documented to be active within the last 11,000 years as defined by the Alquist-Priolo Act, and the Malibu Coast Fault on the subject property is considered active by the City of Malibu and the Los Angeles County Seismic Safety Element unless proven otherwise.
Mitigation Measures. The following measures would reduce impacts associated with seismically induced ground surface rupture to a less than significant level:
GEO-1 Fault Setbacks. Any proposed habitable structures that meet the definition per the Alquist-Priolo legislation (January 1, 1994), shall be setback a minimum of 50 feet from the traces of the Malibu Coast Fault as mapped by Geolabs (June 30, 2008, Plate 1.2).
Significance After Mitigation. Any recommendations for building setbacks shall be implemented in conjunction with all city, state, and federal regulations, thus ensuring impacts would be less than significant.
Impact GEO-2 Seismically induced ground shaking could destroy or damage structures and infrastructure developed for the proposed project, resulting in loss of property or risk to human health. Provided that the design and construction of any proposed structures complies with all applicable provisions of the most recent International Building Code and the California Building Code, impacts are considered Class III, less than significant.
The strongest ground-shaking event at the site is calculated to occur from a rupture along the Malibu Coast or Santa Monica faults. Deterministic ground acceleration estimates are provided in Table 4.5-2. According to the probabilistic model (CGS, 2006) for the project site, there is a 10% chance of peak ground accelerations to exceed 0.455 g over a 50 year time span. The faults listed in Table 4.5-2 are not the only faults in the area that can produce earthquakes, but they are the most probable to affect the project site according to the latest data. Earthquakes along these faults could produce potentially significant impacts to structures onsite. Although nothing can ensure that structures do not fail under seismic stress, proper engineering can minimize the risk to life and property.
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Table 4.5-2. Significant Faults and Deterministic Peak Ground Acceleration in Firm Rock Approximate Distance from Age of Most Moment Peak Ground Fault Name A B C Site Recent Offset Magnitude Acceleration (g) (kilometers) Malibu Coast* Onsite Holocene 6.7 0.455 Santa Monica 26 Holocene 6.6 0.463 San Andreas (Near 80 Holocene 7.1 0.750 Palmdale) A Los Angeles Region Fault Map by the California Geological Survey was created using public-domain fault data (Also, the 1994 Fault Activity Map of California and Adjacent Areas by Charles W. Jennings was used as a guide for this map) B California Department of Conservation Division of Mines and Geology Open Report, 1996 C California Geological Survey-Probabilistic Seismic Hazards Assessment Mapping (10% probability of being exceeded in 50 years), 2006 * Dibblee, 1993
The most recent International Building Code and California Building Code ensure that the design and construction of new structures are engineered to withstand the expected ground acceleration that may occur. The calculated design base ground motion for the site has been performed by Geolabs (June 30, 2008), and takes into consideration the soil type, potential for liquefaction, and the most current and applicable seismic attenuation methods that are available.
Mitigation Measures. Provided that the design and construction of any buildings or structures complies with all applicable provisions of the most recent International Building Code and California Building Code, no further mitigation is necessary.
Significance After Mitigation. The probability of the occurrence of a larger-than- expected earthquake with higher ground accelerations is never zero. Any structure built in California is susceptible to failure due to seismic activity. However, the potential for structural failure due to seismic ground shaking would be considered less than significant through implementation of the most recent industry standards for structural designs.
Impact GEO-3 Portions of the project site underlain by undocumented artificial fill, topsoil, and colluvium materials may be subject to seismic compression and hydroconsolidation. This is considered a Class II, significant but mitigable impact.
According to Geolabs, areas of undocumented artificial fill, topsoil, and colluvium are present beneath the project area. The Geologic Map shows the location of these materials (June 30, 2008). The presence of this material results in the potential for geologic hazards to the park site and facilities as well as park users.
Mitigation Measures. The following measure would reduce impacts associated with settlement and hydroconsolidation hazards to a less than significant level:
GEO-3 Stripping of Undocumented Fill. Areas scheduled to receive engineered fill during the proposed grading onsite shall first be stripped of any
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undocumented fill, topsoil, or colluvium. Undocumented fill here does not include the large canyon fill placed under controlled conditions in 1964, or the colluvium left in place below this canyon fill.
Significance After Mitigation. Provided that the recommendations contained in the Geolabs Grading Plan Review Report (June 30, 2008) for mitigation to prevent or reduce the risk of settlement and hydroconsolidation are implemented in conjunction with all city, state, and federal regulations, the impacts related to settlement would be reduced to less than significant.
Impact GEO-4 The project site is located in an area underlain by geologic units that are prone to landslides. However, because the landslide that exists on the subject property projects beneath the proposed dog park pad, where no habitable structures are proposed, these impacts are considered Class III, less than significant.
According to the 2001 Seismic Report (CDMG, 2001), areas have been identified throughout the Point Dume Quadrangle as having a potential for landslides and slope instability. Landslides are mapped in the Encinal Member of the Topanga Formation offsite and onsite, and in the Zuma Volcanics offsite. Bedding in the Topanga Formation on the subject property is dipping to the south and southwest, potentially down slope or daylighting, providing a higher risk for landslide. Grading in hillside areas with adverse (daylighted or down slope) formational geologic bedding conditions can create potential landslide conditions, particularly if the toe of slopes are cut.
Geolab’s factor of safety calculations for the existing landslide onsite near the western edge of the proposed dog park pad indicate that the safety factors are below City standards for new construction of habitable structures for both existing and proposed conditions. However, Geolabs does indicate that post-grading safety factors increase by approximately 0.2 compared to the safety factors for the landslide as it currently exists. Geolab stated that based upon the tests the conducted, and if the project is constructed in accordance with their recommendation and properly maintained, it is their opinion “that (1) the park site will be safe for its intended use, although portions of it might be impacted by landslide, settlement or slippage, and that (2) the proposed building or proposed grading construction will have no adverse effect on the geologic stability of property outside the building site, with the exception of the dog park cut slope…minor increase in the magnitude of the estimated seismic deformation (34cm to 41 cm) affecting an undeveloped offsite area could result.” In addition, because no habitable structures are proposed on the proposed dog park pad, typical safety factors that meet or exceed 1.5 for static safety and 1.1 for pseudostatic safety are not required. Therefore the project would not increase the potential for landslides either on or off the site.
Mitigation Measures. As impacts would be less than significant, no mitigation is required.
Significance after Mitigation. Since the proposed grading (i.e. cut slope above the dog park pad) in the area of the existing landslide will increase the factors of safety by about 0.2, and since no habitable structures are proposed in the dog park pad area, grading in the area of the existing landslide will be less than significant without mitigation.
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Impact GEO-5 The project site is located in an area underlain by geologic units that are prone to slope instability, and slope instability could be created through inappropriate grading. This is considered a Class II, significant but mitigable impact.
Geolabs (June 30, 2008) calculated the stability of existing cut and fill slopes onsite, and calculated the stability of these slopes after proposed grading. Geolabs concluded that the safety factors are adequate for the existing 120-foot-high fill slope that includes the proposed practice field area and descends to Trancas Creek (both pre- and post grading).
Geolabs evaluated the slope stability of the 170-foot-high, mostly off-site steep natural slope descending from the existing upper northeastern level pad to an area where tennis courts have been constructed adjacent to Trancas Creek. This slope may be susceptible to rockfall and surficial instability under existing conditions. The potential for rockfall and surficial instability on the upper portions of the slope, comprised of Vaqueros sandstone, could be exacerbated by grading of the slope above the northeastern end of the proposed dog park pad.
Geolabs concluded that the safety factors do not meet the City standards for new construction of habitable structures, either currently or after proposed grading, for the proposed 35-foot-high cut slope at the toe of the existing 95-foot-high cut slope that descends to the proposed future dog park pad. In fact, the safety factors are reduced post grading. However, since no habitable structures are proposed in the dog park pad area, the post-grading safety factors are considered adequate as long as the integrity of the slope is monitored.
Mitigation Measures. The following measures would reduce impacts associated with slope instability to a less than significant level:
GEO-5(a) Tennis Court Area. During grading of the slope above the northeastern end of the proposed dog park pad, the tennis courts shall be fenced off and temporarily closed due to the potential for rockfall. This measure shall be printed on all grading plans.
GEO-5(b) Structures Prohibited on Dog Park Pad. Structures shall be prohibited on the dog park pad. This measure shall be printed on all grading plans.
GEO-5(c) Slope Monitoring. The proposed cut slope at the toe of the existing cut slope descending down to the proposed future dog park shall be checked annually by a certified engineering geologist. The Geologist shall report findings in writing to the City’s Building Official. If any surficial failure or evidence of slope instability is detected, the dog park shall be closed until the condition is remedied to the satisfaction of the Building Official.
Significance After Mitigation. Provided the tennis court areas are closed during grading in the northeastern end of the proposed dog park pad area, and provided the City regularly monitors the cut slope proposed above the dog park area and no structures are proposed in that area, impacts related to slope instability would be reduced to less than significant.
City of Malibu 4.5-12
Trancas Canyon Park EIR Section 4.5 Geology and Soils
c. Cumulative Impacts. The proposed project plus cumulative projects would increase development in the City of Malibu. Such development exposes current residents and property to seismic hazards such as seismically induced ground rupture and ground shaking, liquefaction, settlement, hydroconsolidation, and landslide hazards that exist in the area. The proposed project would incrementally contribute to these cumulative impacts. However, seismic hazards, liquefaction, settlement, hydroconsolidation and landslides issues are addressed on a case-by-case basis to mitigate impacts resulting from individual projects. It should be noted that proposed structural development is limited to four shade structures, a restroom building and small structure, and that although much of the park area would be improved with play fields, landscaping and other improvements, more than half of the site would be left in open space. Cumulative impacts are less than significant.
City of Malibu 4.5-13
Trancas Canyon Park EIR Section 4.5 Geology and Soils
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City of Malibu 4.5-14