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    City of Palm Desert/Adopted 3.15.04

    Comprehensive General Plan/Geotechnical Element

    Geotechnical ElementV-1

    GEOTECHNICAL ELEMENT

    PURPOSE

    The purpose of the Geotechnical Element is to provide information about the geologic and

    seismic conditions and hazards affecting the City of Palm Desert, its Sphere of Influence (SOI),

    and the expanded General Plan planning area. It sets forth a series of goals, policies, andprograms directed at protecting the general health and welfare of the community and reducing

    the potential for injuries, loss of life, and property damage resulting from seismic and other

    geologic hazards. The Element and its supporting documentation also serve as an informationdatabase on regional geotechnical hazards as a foundation upon which future land use policies

    and decisions will be based.

    BACKGROUND

    The Geotechnical Element focuses on the physical characteristics of the planning area and the

    safety of the community. It is directly related to a number of other General Plan elements,including Land Use, Circulation, Housing, Economic/Fiscal, Public Buildings and Facilities,

    Emergency Preparedness, and Water, Sewer and Utilities. Many of the issues addressed in the

    Flooding/Hydrology, and Police and Fire Protection Elements are also closely related to theGeotechnical Element.

    Both the California Government Code and Public Resources Code require the development of an

    element addressing seismic safety issues. Government Code Section 65302(g) requires that the

    General Plan address the need to protect the community from unreasonable risks associated withthe effects of seismically induced surface rupture, ground shaking, ground failure, seiching, dam

    failure, subsidence, and other known geologic hazards. The Geotechnical Element also satisfies,

    in part, Government Code Section 65303, which states that the General Plan may also address

    other subjects related to the physical development of the community. Government Code Section8876 establishes a program by which the City and all other jurisdictions located within the most

    severe seismic shaking zone, Zone 4 (as set forth in Chapter 2-23, Part 2, Title 24 of the

    Administrative Code), shall identify all potentially hazardous or substandard buildings andestablish a program for the mitigation of these structures.

    The most important piece of legislation directly related to this element, however, is the Alquist-Priolo Earthquake Fault Zoning Act, found in Public Resources Code Sections 2621 et. seq. The

    Act and its requirements are described in detail in subsequent sections of this Element.

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    City of Palm Desert/Adopted 3.15.04

    Comprehensive General Plan/Geotechnical Element

    Geotechnical ElementV-2

    GEOLOGIC CONDITIONS IN THE PLANNING AREA

    The Coachella Valley is the northern extension of a broad structural depression known as the

    Salton Trough. During about the last 10,000 years (Holocene Epoch), the Salton Trough has

    been inundated by more saline, mineral-rich water, which has formed lakes and inland seas.

    Among these bodies of water are Ancient Lake Cahuilla, which evaporated about 400 years agowhen the Colorado River changed its course to flow directly into the Gulf of California. The

    latest body of water to form in the Salton Trough is the present-day Salton Sea, which was

    formed in 1905 as a result of a break in a levee in the Colorado River.

    Four types of geologic deposits underlie the Palm Desert General Plan planning area, including:

    1) granitic and metamorphic basement rock associated with the mountains, 2) conglomerateunderlying the Indio Hills, as well as minor traces in the southern planning area, 3) stream

    deposits (alluvium) shed from the mountains, and 4) blowing sand deposits. The geologic

    composition of the valley has a great deal of influence over the geotechnical hazards affectingthe City and planning area. Among these hazards are windblown sand, wind erosion, and

    subsidence, which are discussed in more detail below.

    Granitic and Metamorphic Basement RockThe Santa Rosa Mountains in the southerly portion of the planning area are composed of hard

    crystalline granite and meta-sedimentary rock, which was emplaced more than 65 million years

    ago and forms the basement rock of the region. The Little San Bernardino Mountains in thenortherly planning area consist of Pre-Cambrian metamorphic rocks with similar engineering

    properties as the materials underlying the Santa Rosa Mountains. These rocks are typically non-

    water bearing, except where extensively jointed and fractured, and therefore have low tomoderately low permeabilities. They are typically rounded boulders that perch precariously on

    steep slopes. Boulders pose a rockfall hazard to areas adjacent to and downgradient from theseslopes.

    Greater slope instability may occur along northeasterly and southwesterly facing slopes, giventhe general northwesterly trending shear and fracture zones contained in these granites, and

    perhaps to a greater degree in the meta-sedimentary rock of the Little San Bernardino Mountains.

    Mitigation of rockfall hazards is best accomplished by avoiding development on steep slopes and

    implementing structure setbacks at the toe of slopes. Detailed mapping of precarious rocks mayalso facilitate avoidance of rockfall hazards and help determine adequate setback distances.

    Quaternary ConglomerateQuaternary conglomerate underlies the Indio Hills, although minor traces of this unit also occur

    in the southern planning area, primarily along the margins of the valley. The conglomerate

    underlying the Indio Hills is referred to as Ocotillo Conglomerate and contains pebble to cobble-sized sub-rounded clasts of locally derived gneisses and granite, with a lesser amount of basic

    volcanic rocks, limestone and pegmatite. The distribution and thickness (greater than 2,000 feet)

    of this unit is evidence of rapid rates of deposition and subsidence in the Salton Trough duringthe last 2 million years. Because boulders exceeding 10 feet in diameter are known to occur in

    these units, a primary engineering concern is the generation and removal of oversized materials

    during grading.

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    City of Palm Desert/Adopted 3.15.04

    Comprehensive General Plan/Geotechnical Element

    Geotechnical ElementV-3

    Quaternary AlluviumAlluvial deposits, which are shed from the Santa Rosa and Little San Bernardino Mountains, as

    well as the Indio Hills occur within the mouths of drainages in the southern, northern and central

    portion of the General Plan planning area, respectively. Much of the developed portion of the

    City of Palm Desert is constructed on alluvium. These deposits have been laid down rapidly,without being saturated, and are therefore susceptible to collapse upon the introduction of

    irrigation water. Where alluvium consists of clay and/or silt layers, irreversible subsidence and

    compaction may occur as a result of ground water withdrawal.

    Sand Dune Deposits

    Aeolian, or wind-deposited, sediments occur both north and south of Interstate-10 and covermuch of the Coachella Valley floor. These deposits typically consist of reworked alluvium,

    which has been picked up by strong winds and redistributed as silty, fine to medium-grained

    sands that now form sand dunes and sand fields. A thick accumulation of these wind-blownsands, known as the Palm Springs Sand Ridge, has formed a broad, elongated and southeast

    trending ridge which rises as much as 120 feet above the surrounding desert floor. Engineeringconcerns associated with sand dune deposits include sand deposition, wind erosion, and

    collapsible soils.

    GEOLOGIC HAZARDS

    Slope Instability

    Slope stability is dependent upon several factors, including rock type, pore water pressure, and

    slope steepness. Although the granitic and metamorphic rock, which comprises the Santa Rosaand Little San Bernardino Mountains in the planning area, are generally considered grossly

    stable, the steepness of the slopes often results in locally precarious rocks that could fall as aresult of seismic ground shaking or intense rainfall. In addition, the mountainous terrain includes

    foliation, joints, fractures, and intrusive dikes which could act as potential failure surfaces and

    planes of weakness. Areas within the planning area, which are susceptible to slope instability, areillustrated in Exhibit V-1. Any proposed development in these areas would benefit from site-

    specific evaluation of potential slope instability hazards.

    Collapsible SoilsSoil collapse, or hydroconsolidation, occurs when soils undergo a rearrangement of their grains

    and experience a loss of cohesion or cementation, resulting in substantial and rapid settlement

    even under relatively low loads. This phenomenon typically occurs in recently deposited,Holocene soils deposited in an arid or semi-arid environment. Wind-deposited sands and silts,

    and alluvial fan and mudflow sediments deposited during flash floods are particularly susceptible

    to hydroconsolidation. Man-made fills, which are loose and unconsolidated, may also be subjectto collapse. When these soils are saturated from irrigation water or a rise in the groundwater

    table, pores and voids between the soil particles are removed, and the soils collapse.

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