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3106F.2 Site Characterization
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Each MOT shall be assigned at least one site class. Site Classes A, B, C, D, and E are defined in Table 31F-6-1, and Site Class F is defined by any of the following:
- Soils vulnerable to significant potential loss of stiffness, strength, and/or volume under seismic loading due to liquefiable soils, quick and highly sensitive clays, and/or collapsible weakly cemented soils.
- Peats and/or highly organic clays, where the thickness of peat or highly organic clay exceeds 10 feet.
- Very high plasticity clays with a plasticity index (PI) greater than 75, where the thickness of clay exceeds 25 feet.
- Very thick soft/medium stiff clays with undrained shear strength less than 1,000 psf, where the thickness of clay exceeds 120 feet.
- Site-specific investigations shall include adequate borings and/or cone penetration tests (CPTs) and other appropriate field methods, to enable the determination of geotechnical parameters.
- Adequate coverage of subsurface data, both horizontally and vertically, shall be obtained to develop geotechnical parameters.
- Exploration shall be deep enough to characterize subsurface materials that are affected by embankment behavior and shall extend to depth of at least 20 feet below the deepest foundation depth.
- During field exploration, particular attention shall be given to the presence of continuous low-strength layers or thin soil layers that could liquefy or weaken during the design earthquake shaking.
- CPTs provide continuous subsurface profile and shall be used to complement exploratory borings. When CPTs are performed, at least one boring shall be performed next to one of the CPT soundings to check that the CPT-soil behavior type interpretations are reasonable for the site. Any difference between CPT interpretation and subsurface condition obtained from borings shall be reconciled.
- Quantitative site soil stratigraphy is required to a depth of 100 feet for assigning a site class (see Table 31F-6-1).
- Laboratory tests may be necessary to supplement the borings and insitu field tests.