III. GEOLOGIC PROBLEMS

The project site is located in the Petaluma Valley approximately 1,000 feet east of the Petaluma River. The site is underlain by unconsolidated fluvial deposits of generally fine but variable grain size composed mainly of fine sand, silt, and silty clay. Previous test borings at the site encountered 2 to 2 _ feet of asphalt pavement, base rock and fill consisting of sandy clay, clayey sand, and silty sand with cobbles (PG&E, 1998). Below the fill, a layer of stiff silty clay with lenses of silty sand was encountered to a depth of 15 feet. This is underlain by a layer of medium dense to dense silty sand with clay lenses to a depth of 21 feet. A sand and gravel layer with 10% to 15% fines was encountered between 21 and 26 feet. The sand and gravel was underlain by silty clay and sandy clay to the depths of the borings. Groundwater was encountered at a depth of 5 to 7 feet.

  1. Surface rupture is a seismic hazard that can adversely affect structures or other improvements located on the surface traces of active faults. The California Department of Conservation Division of Mines and Geology (DMG) has evaluated active and potentially active faults in the State as required by the Alquist-Priolo Earthquake Fault Zoning Act. DMG has established hazard management zones, called Alquist-Priolo Earthquake Fault Zones (APEFZís), to regulate development within active fault zones. (Active faults are defined as those with surface displacement within Holocene time, or approximately within the last 11,000 years.) The Act requires that no structure for human occupancy be permitted on an active fault. Geologic investigations are required for most structures for human occupancy within APEFZís to evaluate surface rupture potential.

    There are two APEFZís within southern Sonoma County: the Rodgers Creek Fault, approximately 4.7 miles northeast of the site, and the San Andreas Fault, approximately 15 miles southwest of the site. The Tolay Fault was previously designated within an APEFZ, but was removed in 1983 (City of Petaluma, 1995). The site is not within an APEFZ. According to the DMG (1994), there are no known or suspected active faults nearer to the site or projecting toward the site. The potential for concealed bedrock faults (e.g., an undetected low angle thrust fault) that could result in surface fault rupture is considered to be low at the site, and thus the hazards to people and the proposed facilities caused by fault rupture are considered to be a less-than-significant impact.

  2. The site is within an area of high seismic activity. Several major faults extend through the region with the potential of generating damaging earthquakes of as much as 7.0 or more. Table 1 presents a list of active and potentially active faults within the region. There is a potential that the site will be subject to moderate to strong seismic ground shaking one or more times during the life of the project. The nearest significant earthquake epicenter to the site was the 1969 Santa Rosa earthquake series on the Healdsburg-Rodgers Creek fault (magnitude 5.6 - 5.7). Other large magnitude historic earthquakes have occurred on the San Andreas fault and the Hayward fault.

    Damage to substations was a primary cause for loss of power in recent earthquakes such as the Loma Prieta Earthquake and Northridge Earthquake. The site is within Seismic Zone 4 (highest hazard) of the 1994 Uniform Building Code (UBC). Damage to substation facilities from groundshaking may not be entirely mitigable, however, compliance with the UBC for design and construction of the substation would reduce ground shaking effects on the facility to levels of acceptable risk and, therefore, result in a less-than-significant impact from ground shaking.

    Groundshaking, and in some project areas liquefaction, could result in damage to power lines. The conductor wires are strung with sufficient length and catenary (sag) to accommodate vibratory motions and tensions set up by ground motions in earthquakes or high winds. In other words, it is considered a remote hazard that the power lines would "snap" because of earthquake groundshaking. On the other hand, earthquake induced vibratory motions in power lines have resulted in "wrapping" of the lines in which the separate conductor lines come into physical contact with each other. For example, wrapping was recorded as an effect of the 1989 Loma Prieta Earthquake. Wrapping is a potentially hazardous situation because the "hot wires" come into contact, although it would not likely cause the lines to break and fall. PG&Eís design and spacing requirements would be expected to be in conformance with requirements and industry standards for conductor separation.

    The primary potential cause of failure of power lines would result from the failure of one or more of the poles supporting the conductors. Tubular steel poles are structurally extremely strong and able to resist earthquake induced vibratory motions (or high winds) without failure, as evidenced by their performance in the Loma Prieta Earthquake, the 1994 Northridge Earthquake, and other earthquakes. Bending or breaking of the poles would be a remote hazard. The hazard would be greatest from the tipping of a pole caused by liquefaction or lateral movement of the ground (see following discussion). For this area, the ground dislocations from liquefaction would be minor and within the amount that could be well accommodated by the pole and its foundation design. Because PG&E would employ construction measures for installation of poles and stringing of conductors that meet accepted design standards, the hazards related to groundshaking, while not entirely avoidable, would not constitute an unacceptable level of risk; therefore, the impact is deemed to be less than significant.

  3. Strong earthquakes can cause secondary seismically-induced ground failures including liquefaction, lateral spreading, ground lurching, and densification settlement. Liquefaction is a phenomenon in which loose, saturated, granular soils suddenly lose shear strength due to earthquake-induced shaking and a rapid rise in pore water pressure. Liquefaction can result in bearing failures and settlement. Lateral spreading is horizontal displacement of weak soils or fill triggered by strong earthquake shaking. Lateral spreading most commonly occurs when weak, saturated soils are bordered by a steep embankment or slope. Ground lurching occurs as earthquake-triggered horizontal movements on relatively steep embankments or slopes result in the cracking of the ground surface.

    The site is underlain by a saturated sand and gravel layer at a depth of 21 to 26 feet that is potentially subject to liquefaction. This deposit is capped by stiff, cohesive soils, and it is likely that liquefaction-induced settlements would be small and within tolerable levels for the substation facilities. Project design using PG&E standard designs in accordance with CPUC General Order 95 would incorporate measures to consider potential liquefaction settlement and effects on structures (PG&E, 1998). Such measures would reduce the effects of potential liquefaction to levels of acceptable risk and result in a less than significant impact.

TABLE III-1

Known Active and Potentially Active Faults in Project Vicinity

Fault

Study Area

Distance from Fault

(km)

Maximum Credible

Earthquake (1)

Maximum Estimated

Bedrock

Acceleration

(g) (2)

Rodgers Creek

5

7.0

0.38

San Andreas

15

8.0

0.31

Hayward

22

7.5

0.18

Green Valley

25

7.0

0.11

Palo Colorado/
San Gregorio

26

7.7

0.18

Concord

32

6.7

0.07

Coast Range/
Sierran Block

35

8.0

0.17

Antioch

44

6.7

0.05

Calaveras

48

7.5

0.08

Greenville

50

7.3

0.07

Note: Distance shown is approximate, as measured from each end of the study area.

(1). Blake, 1995

(2). Average of Idriss (1994) and Campbell and Bozorgnia (1994)

 

The site is on a relatively flat slope (very gently sloping) and is approximately 1,000 feet away from the Petaluma River Channel, which is the nearest significant embankment to the site. The potential for lateral spreading and ground lurching is considered to be low due to the relatively large distance to the channel embankment. The test borings did not encounter loose, unsaturated sands subject to densification. The potential for lateral spreading, ground lurching, and densification is considered to be less than significant.

  1. Earthquakes can cause tsunami (tidal waves) and seiches (oscillating waves in enclosed bodies of water). The site is not near an ocean or a large enclosed body of water. The site is also not within a region subject to active volcanic activity. The site is not located within an area subject to inundation from an upstream dam failure. Therefore, there would be no impact on the project from tsunamis, seiches, dam failure or volcanic activity.

  2.  
  3. The site is nearly level and is not located near a slope subject to landslides or mudflows. The proposed construction would not affect slope stability on or adjacent to the site. The potential for landslides or mudflows is minimal; therefore, there would be no potential for an impact.

  4. The project will include minor grading to raise the grade from about 18 inches at the north end of the site to essentially no change in elevation at the south end. Settlement is anticipated to be minor at the site, provided that the grading be performed in accordance with common geotechnical engineering practice, including compaction of engineered fill and compliance with applicable building codes and ASTM standards. The proposed topographic changes and the potential for unstable conditions from proposed grading activities are considered to be less than significant.

    Grading and construction activities would disturb site soils. The soils may be subject to erosion from rainfall and stormwater runoff during periods of precipitation. Because the site is nearly level, construction activities are not likely to result in heavy erosion, gullying, or sedimentation, and are not considered to be a significant hazard.

  5. The proposed structures are relatively light, and the foundations will gain support from relatively stiff native soils or compacted fill. Test borings carried out for PG&E at the site did not encounter soft, compressible soils subject to consolidation and settlement at the site. The project will not include withdrawal of ground water. The potential for subsidence and settlement is considered to be low and would result in a less-than-significant impact.

  6.  
  7. Native soils at the site were judged to be moderately to moderately-highly expansive by PG&E. Expansive soils can damage foundations, damage underground facilities, cause power line poles to lean out of plumb, and damage pavement. The impact is potentially significant, but can be entirely mitigated. PG&E design of the foundations, in accordance with G.O. 95, would reduce the effect to less than significant.

  8.  
  9. The project area is essentially flat and has no unusual or unique geological features; therefore, there would be no impacts related to unique geologic or physical features.
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