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Geotechnical Element City of Palm Desert/Adopted 3.15.04 Comprehensive General Plan/Geotechnical Element 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, and programs 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 information database 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 the Geotechnical 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 with the 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 Section 8876 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 and establish 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. Geotechnical Element V-1 City of Palm Desert/Adopted 3.15.04 Comprehensive General Plan/Geotechnical Element 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 ago when 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) conglomerate underlying 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 affecting the 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 Rock The 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 the northerly 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 to moderately 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 these slopes. Greater slope instability may occur along northeasterly and southwesterly facing slopes, given the 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 may also facilitate avoidance of rockfall hazards and help determine adequate setback distances. Quaternary Conglomerate Quaternary 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 during the 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. Geotechnical Element V-2 City of Palm Desert/Adopted 3.15.04 Comprehensive General Plan/Geotechnical Element Quaternary Alluvium Alluvial 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 cover much 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-blown sands, 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. Engineering concerns 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 Rosa and 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 a result 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, are illustrated in Exhibit V-1. Any proposed development in these areas would benefit from site- specific evaluation of potential slope instability hazards. Collapsible Soils Soil 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 subject to 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. Geotechnical Element V-3 City of Palm Desert/Adopted 3.15.04 Comprehensive General Plan/Geotechnical Element Soil collapse can result in the cracking of walls and foundations, tilting or sagging of floors, malfunctioning of doors and windows, and other structural damage. Alluvial fan deposits at the base of the Santa Rosa Mountains and wind-blown deposits in the northerly portion of the planning area are vulnerable to hydroconsolidation.
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