20 January 2017

CV Link and the Environment, part 2 geologysoils

Geology and soils

The CV Link is located in an area susceptible to landslides at one location, adjacent to Point Happy at the Indian Wells/La Quinta boundary. The Point Happy rock face will be evaluated and scaled of loose rock from the exposed slopes. Rock catchment devices, such as walls or steel mesh will be installed to mitigate the rockfall hazards.

Implementation of the CV Link could result in soil erosion or the loss of topsoil. Ground removal and recompaction of loose, near surface sandy soils will minimize dynamic settlement of dry soils. Deep dynamic compaction, additives (cement or fiber) to the soils and flooding of loose, granular soils will increase the density of the fill and remove or reduce the tendency to settle under dynamic shaking. Deep foundations will be considered at bypassing zones of loose sand.
Utility trench excavations in slope areas or near structures will be backfilled in conformance with governing agencies (water district, public works department, etc.). Backfill operations will be tested to monitor compliance.
Slope protection, cutoff walls, deep foundations below the maximum depth of scour and comparable measures will be applied.
Grading activities will cease during rainstorms or high wind events. Flow barriers and soil catchments (such as straw bales, silt fences, and temporary detention basins) will be installed during construction to control soil erosion.

Portions of the CV Link could be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property. CV Link design and engineering will conform to the prevailing California Building Code (CBC) for buildings and other structures, and Caltrans design standards for bridges to mitigate the effects of groundshaking and earthquake damage.
Foundation designs and subsurface soil improvements will follow the California Code of Regulations Volume 18, Title 14, Article 10, Section 3721[a]) to minimize liquefaction hazards. The design measures will avoid overexcavation and hydrocompaction, and include remedial grading, strengthening and deepening structural foundations.
All grading plans will include a soil erosion prevention/dust control plan. Blowing dust and sand during grading operations will be mitigated by adequate watering of soils prior to and during grading, and limiting the area of dry, exposed and disturbed materials and soils during these activities. After site development, measures (moist surface soils, planting stabilizing vegetation, establishing windbreaks with non-invasive vegetation or perimeter block walls, and using chemical soil stabilizers) will be installed.
Unprotected, permanent graded slopes will not be steeper than 3:1 (horizontal:vertical) to reduce wind and water erosion. Protected slopes with ground cover may be as steep as 2:1. Fill slopes will be overfilled and trimmed back to competent material. Fill slope surfaces shall be compacted to 90% maximum density either by over-filling and cutting back to expose a compacted core or by mechanical methods.
Positive site drainage shall be established during finish grading, for 2% away from structures for a minimum of 3 feet and a minimum gradient of 1% to the street, channel or other approved
drainage course.
Dust control measures will be implemented to control wind-blown sand during construction, in compliance with the SCAQMD and the Coachella Valley PM10 SIP.
Excavated soils may be used as fill material if they are free of organic or deleterious matter. Rocks or concrete larger than 6 inches will be removed from fill or backfill material. Prior to integrating reconditioned fill soil, the areas will be scarified, brought to optimum moisture conditions, and recompacted to at least 90%.
Any imported soils will be non-expansive, granular soils meeting the USCS classifications of SM, SP-SM, or SW-SM with a maximum rock size of 3 inches and 5 to 35 percent passing the No. 200 sieve. Imported fill shall be placed in maximum 8-inch lifts (loose) and compacted to at least 90 percent relative compaction (ASTM D 1557) near optimum moisture content.
Excavations of sandy soil will be kept moist, unsaturated, to reduce caving or sloughing. Where excavations over 4 feet deep are planned, lateral bracing or appropriate cut slopes of 1.5:1 (horizontal/vertical) will be provided. No surcharge loads from stockpiled soils or construction materials shall be allowed within a horizontal distance measured from the top of the excavation slope and equal to the depth of the excavation.
Removal and recompaction of susceptible soils, flooding and surcharging, and/or other ground densification techniques will mitigate hydro-collapse potential.
Standard geotechnical practices such as excavation of the expansive soils and replacement with non-expansive compacted fill (by using additional steel reinforcing in foundations, post-tensioned slabs, presoaking, and drainage control devices) will be used.

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