III. GEOLOGIC PROBLEMS

Discussion

The project site is located in the flat to gently sloping portion of the Santa Clara Valley. The Nortech substation site is located approximately one-half mile south of the southern margin of the San Francisco Bay. Geologically, the Santa Clara Valley is characterized by a deep structural depression filled with sediment derived from the erosion of adjacent uplands and marine deposition. The Nortech substation site is essentially flat at an elevation of about 2.4 feet (National Geodetic Vertical Datum). The site is located on Holocene basin deposits, generally consisting of organic-rich clay to very fine silty clay. The Trimble-Nortech power line ranges in elevation from about 2 feet to 30 feet. The Kifer-Nortech power line ranges in elevation from about 2 feet to 40 feet. Both power lines cross areas with Holocene Basin deposits. The Trimble Substation is located in an area with Holocene flood plain deposits consisting of dense sandy to silty clay with locally present lenses of silt, sand and/or pebbles.

a) The active (surface displacement within the last 11,000 years) Hayward Fault is located approximately 4.5 miles east of the project site and the active San Andreas Fault lies approximately 13 miles west. The active Calaveras Fault lies approximately eight miles easterly of the proposed substation site. The active San Gregorio Fault is located approximately 25 miles to the west. Potentially active (surface displacement within the last 1.6 million years) traces of the Silver Creek and San Jose faults are located 0.5 mile and four miles, respectively, from the proposed substation site.

The Alquist-Priolo Earthquake Fault Zoning Act requires the delineation of zones along sufficiently active and well-defined faults. The purpose of the Act is to restrict construction of structures intended for human occupancy along traces of active faults, thus reducing the hazards associated with fault rupture. There is no evidence of the presence of an active fault crossing the site. The substation site is not located within an Alquist-Priolo Earthquake Fault Zone established for the active faults in this region. The substation and the power lines would operate unattended. The presence of a concealed fault, e.g., a low angle thrust fault, buried at great depth under the thick sediments of the area is a potential hazard that cannot be determined with available information. (An undiscovered concealed fault of this type was the source of the 1994 Northridge Earthquake in the Los Angeles area that damaged substation and transmission facilities.) While the possibility for a concealed fault cannot be entirely discounted, given the seismic history of the Santa Clara Valley, the potential hazard posed by a concealed undetected fault is considered speculative and a less than significant hazard.

  1. The main potential project-related hazard to structures and people in the project area would be from seismic activity. The project site is located in the Coast Range Geomorphic Province, which is an area of relatively high seismic activity. Several major northwest-trending fault zones are anticipated to generate major earthquakes that could induce significant ground shaking at the site, including the San Andreas Fault Zone (the dominant fault zone in California), and a number of fault zones are located within 60 miles of the project site. In addition to the San Andreas and Hayward faults, other major potentially active faults are listed in Table III-1. A major earthquake on any of the faults listed in Table III-1 could produce strong ground shaking at the site, affecting the proposed facilities (see discussion under [a], above). Shaking amplification is rated as "very high"(7 on a scale of 1 to 8, with 8 rating the highest amplification) and the modified Mercalli intensity is rated as high as IX-Heavy (9 on a scale of 1 to 10, with 10 rating as extreme) for both a major 7.0 earthquake on the southern segment of the Hayward Fault with a 7.0 magnitude event and a 7.3 magnitude earthquake on the entire Hayward Fault (ABAG, 1995). In an earthquake of that magnitude, damage to structures, roads and infrastructure would be heavy throughout the project area.

Similar to the existing Trimble Substation and Kifer Receiving Station, because the proposed Nortech Substation site would be fenced and locked, direct public access would be prevented. Therefore, unless workers were present onsite (which would occur only occasionally), no injuries to people on the site would occur during earthquakes. The earthquake hazards are potentially significant only for the substation facilities themselves. To the extent that these would be rendered inoperable by an earthquake, the result could be a loss of power in the service area. However, a major earthquake that could affect the site is also likely to affect a wide area in the South Bay. By providing better linkage of power transmission and distribution in the area, the project would likely result in a net improvement to system reliability during and following a major earthquake.

PG&E, in conjunction with other utilities and equipment vendors throughout the country, have revised IEEE 693, "Recommended Practices for Seismic Design of Substations," to address equipment and voltage-specific seismic qualification requirements. These requirements are generally more stringent than the Uniform Building Code (PG&E, 1998). New equipment at the existing substations and for the proposed Nortech Substation will be procured using the seismic qualification requirements of IEEE 693. Following these requirements, it is anticipated that no structural damage would occur if the substation were subjected to peak ground accelerations levels approaching 1 g (gravitational acceleration). The mean peak horizontal ground acceleration is estimated at 0.5 g; therefore, the project would be expected to perform adequately if designed and constructed to established standards required by the CPUC. Compliance with the IEEE 693 and, where applicable, the Uniform Building Code, would reduce ground shaking effects to levels of acceptable risk and result in a less than significant impact from seismic hazard.

Ground shaking, and liquefaction in some limited project areas where poles would be constructed, 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 ground shaking. 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&Es 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 failure of poles is more likely potentially related to a failure of the foundation support as a result of liquefaction (or landsliding, which is not a hazard present in the project area). See the discussion and mitigation under item III.c, below.

Table III-1

Selected Faults in the Project Vicinity, Their Maximum Credible Earthquake Magnitude, Fault Activity Classification, and Distance from the Nortech Substation Site


Fault


Activity3

Distance (miles)


MCE1

Peak Ground Acceleration 2

Hayward (southern segment)

Holocene (Active)
(1836, 1868)

4.5

7.0

0.41

San Andreas (Peninsula segment)

Holocene (Active)

13

7.9

0.21

Calaveras (southern segment)

Holocene (Active)

13.7

6.5

0.22

San Gregorio

Holocene (Active)

25

7.3

0.13

San Jose

Quaternary (Potentially Active)

4

NA

NA

Silver Creek

Quaternary
(Potentially Active)

0.5

NA

NA

_________________________

N/A = Accurate Estimates Not Available

  1. MCE is the Maximum Credible Earthquake, Richter Magnitude, an estimate of the largest earthquake that is judged by geologic studies to be capable of occurring on a fault or segment of a fault.
  2. The peak ground acceleration expressed in gravitational acceleration.
  3. Age is the period of recorded or most recent geologic evidence of earthquake displacement on a fault.

_________________

Source: PG&E, PEA

c) Earthquakes or aftershocks may cause secondary ground failures. Ground failures are caused by soil losing its structural integrity. Examples of seismically induced ground failures are liquefaction, lateral spreading, ground lurching, and subsidence. Liquefaction (the rapid transformation of soil to a fluid-like state) affects loose saturated sands. Earthquake ground shaking induces a rapid rise in excess pore pressure and the soil loses its bearing strength, and it may spread laterally, undergo settlement and form fissures and sand boils (upwellings of sand at the surface). Lateral spreading is the horizontal movement of loose, unconfined sediment and fill deposits during seismic activity. Ground lurching is the horizontal movement of soil, sediments, or fills located on relatively steep embankments or scarps as a result of seismic activity, forming irregular ground surface cracks. The potential for lateral spreading or lurching is highest in areas underlain by soft, saturated materials, especially where bordered by steep banks of a river or adjacent hard ground. Subsidence is vertical downward movement of the ground surface as the soil densifies.

The Nortech Substation site is located in an area considered to have a low to moderate liquefaction potential. Soils in the vicinity of Coyote Creek (Trimble-Nortech power line) may have a high potential for liquefaction (rated low to high, indicating that the hazard is site specific) (PG&E, 1998). A low to high liquefaction rating is also indicated for the Kifer-Nortech power line. Lateral spreading, lurching and ground settlement (subsidence) are rated low to high hazards and localized in their effect. Lateral spreading or lurching could occur along the banks of the Guadalupe River, threatening the integrity of the proposed transmission poles. A loss of foundation support for the poles could cause them to tip, bringing down the conductors. If the wires were energized at the moment of tipping or collapse, the "hot" wire would pose a potential hazard to people in the area and could ignite fires. The project includes high-speed relays that would de-energize the line within about one-tenth of a second after detecting a broken line. See Section IX, Hazards. While the potential for earthquake induced hazards are unavoidable, conformance with industry design standards and CPUC design requirements for the poles and their foundations would reduce the hazard to an acceptable level of risk. Therefore, the impact, with proposed mitigation, is considered less than significant.

d) Earthquakes can cause tsunami ("tidal waves"), seiches (oscillating waves in enclosed water bodies), and landslide splash waves in enclosed water bodies such as lakes and reservoirs. The project site is not located near a tsunami run-up area or near an enclosed body of water such as a reservoir or lake. Therefore, this is considered a less than significant impact.

  1. The project site is essentially flat, and is not located in the vicinity of uplands characterized by unstable slopes; therefore, hazards associated with landsliding are not considered a hazard on the project site.

f) Unstable soil conditions include settlement and failure from low strength. Substation site soils are not of the types characterized by low strength. Settlement can occur either uniformly or differentially. Uniform settlement of a structure can cause poor drainage. Differential settlement can damage foundations and cause mechanical and structural problems within a structure. The magnitude of settlement of a fill or native clay material will depend on their properties, the manner in which the fills are placed, the thickness of the material, the type of underlying subsurface soil, and the load placed on the material. Differential settlement at the Nortech Substation site is rated as low to moderate. Settlement beneath the proposed transformer bank foundations is expected to occur due to compressibility of native, near-surface clay. Total settlement is expected to be low to moderate, on the order of a few inches. This could be accommodated within the project design. Settlement is generally a gradual hazard. As standard engineering, design, and construction practices are proposed in conformance with PG&E construction guidelines and CPUC required standards, impacts resulting from settlement, would be minor and the hazard would be less than significant. Differential settlement hazard is rated as low to high for both power lines. The slow action of settlement and the expected small amount of settlement is not expected to impair the operation of the power lines. Poles that settle differentially could create a requirement for corrective action as part of long-term maintenance requirements; this would constitute a less than significant impact.

The project site would require minimal additional grading of the flat site to construct the proposed Nortech Substation and would not result in any substantial changes in topography. Two to three feet of engineered fill would be placed at the substation site. No fills or changes in topography would be needed at the Trimble Substation and Kifer Receiving Station or for construction of the power lines. Construction of the substation would disturb site soils: temporarily exposed site soils may be subject to erosion by rain splash and overland flow of storm water for the duration of the construction activities. Site preparation would entail minor regrading, fill placement, resurfacing, and paving of portions of the site, eliminating any long-term hazard. Because the site is flat and the soils have a high clay content, soil erosion from construction activities would not result in significant hazards of gully formation. Runoff from the site could entrain loose soil and discharge it into storm drains. While the hazard is deemed less than significant, the impacts from erosion and sediment discharges could be eliminated by implementation of standard best construction management practices, as contained in Mitigation Measure IV.c.1, below.

g) Historic land subsidence due to extraction of groundwater from the underlying Santa Clara Formation has been recorded in this portion of the Santa Clara Valley. However, subsidence was virtually halted by 1971 due groundwater recharge and importation of water. The project would not require the removal of groundwater or any change in groundwater use; therefore, there would be no impact related to ground subsidence.

h) Expansivity, or shrink-swell, is the cyclic change in volume that occurs in fine-grained sediments because of expansion and contraction of clay caused by wetting and drying. Soils that are expansive (have shrink-swell potential) can damage foundations and other structures. This problem can be overcome with proper foundation engineering (Helley, 1979). Soils on the project site were observed to be clay mixtures with varying degrees of expansive potential. The hazard is rated as moderate to high for the Nortech Substation site and low to high for the proposed power lines. Foundation designs would be based on assumptions of high groundwater depth. A rise in groundwater following construction at the facility could cause the lean clays to swell. Proposed placement of engineered fill would reduce the hazard to the foundation of the Nortech Substation. Soils with high shrink-swell hazard potentially could affect some of the proposed poles. The slow action of expansive clay soils on the poles could cause them to lean out of plumb but is not expected to impair the operation of the power lines. Poles affected by expansive soils could create a requirement for corrective action as part of long-term maintenance requirements; this would constitute a less than significant impact.

i) 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. There is no evidence of potentially significant paleontological resources present in the area of the project. There are no significant mineral resources present in the project area. Soils in project area at one time were involved in extensive use for agriculture. Some of the soils along the power line routes potentially would be considered prime agricultural soils. However, as the area in general has been largely converted to urban uses, and because the project itself would not affect agricultural uses in the area, the use of these soils for the project would be a less than significant impact.

TOP
Forward to Next Section
(Water)
Back to Previous Section
(Population and Housing)
Back to North San Jose Capacity Project Main Page PG&E Substation Projects Main Page CPUC Home Page