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4. Foundations and Geologic Site Hazards

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4. Foundations and Geologic Site Hazards 4. Foundations and Geologic Site Hazards 4.1 Scope 4.2.1 Foundation Information This chapter sets forth general requirements for Information on the foundation supporting the building consideration of foundation load-deformation to be rehabilitated, nearby foundation conditions, characteristics, seismic rehabilitation of foundations, design foundation loads, and load-deformation and mitigation of geologic site hazards in the characteristics of the foundation soils shall be obtained Systematic Rehabilitation of buildings. as specified in Sections 4.2.1.1 through 4.2.1.3. Section 4.2 specifies data collection for site 4.2.1.1 Foundation Conditions characterization and defines geologic site hazards. 4.2.1.1.1 Structural Foundation Information Section 4.3 outlines procedures for mitigation of The following structural information shall be obtained geologic site hazards. Section 4.4 provides soil strength for the foundation of the building to be rehabilitated: and stiffness parameters for consideration of foundation load-deformation characteristics. Section 4.5 specifies 1. Foundation type. seismic earth pressures on building walls. Section 4.6 specifies requirements for seismic rehabilitation of 2. Foundation configuration, including dimensions, foundations. locations, depth of embedment of shallow foundations, pile tip elevations, and variations in C4.1 Scope cross-section along the length of pile or belled caissons. This chapter provides geotechnical engineering provisions for building foundations and seismic- 3. Material composition and details of construction. geologic site hazards. Acceptability of the behavior of the foundation system and foundation soils for a given C4.2.1.1.1 Structural Foundation Information performance level cannot be determined apart from the context of the behavior of the superstructure. Foundation types may consist of shallow isolated or continuous spread footings, mat foundations, deep Geotechnical requirements for buildings that are foundations of driven piles, or cast-in-place concrete suitable for Simplified Rehabilitation are included in piers or drilled shafts of concrete. Chapter 10. Structural engineering issues of foundation systems are discussed in the chapters on Steel and Cast Foundation material types include concrete, steel, and Iron (Chapter 5), Concrete (Chapter 6), Masonry wood. Foundation installation methods include cast-in- (Chapter 7), and Wood and Light Metal Framing place and open/closed-end driving. (Chapter 8). With this minimum amount of information, presumptive or prescriptive procedures may be used to determine the ultimate bearing capacity of the 4.2 Site Characterization foundations. However, additional information is required for site-specific assessments of foundation Site characterization shall include collection of bearing capacity and stiffness. Acquiring this information on the building foundation as specified in additional information involves determining unit Section 4.2.1, and on seismic geologic site hazards as weights, shear strength, friction angle, compressibility specified in Section 4.2.2. characteristics, soil moduli, and Poisson’s ratio. C4.2 Site Characterization 4.2.1.1.2 Subsurface Soil Conditions The guidance of the State Historic Preservation Officer The following information on subsurface soil should be obtained if historic or archeological conditions shall be obtained as required by the selected resources are present at the site. performance level: FEMA 356 Seismic Rehabilitation Prestandard 4-1 Chapter 4: Foundations and Geologic Site Hazards 1. For Collapse Prevention and Life Safety 4.2.1.3 Load-Deformation Characteristics Performance Levels, the type, composition, Under Seismic Loading consistency, relative density, and layering of soils Load-deformation characteristics of foundations shall shall be determined to a depth at which the stress be obtained from geotechnical reports, or shall be imposed by the building is less than or equal to 10% determined in accordance with the requirements of of the building weight divided by the total Section 4.4. foundation area. For buildings with friction piles, the depth so calculated shall be increased by two-thirds of the pile length. For end bearing piles, the depth of C4.2.1.3 Load-Deformation Characteristics investigation shall be the pile length plus 10 feet. Under Seismic Loading Traditional geotechnical engineering treats load- 2. The location of the water table and its seasonal deformation characteristics for long-term dead loads fluctuations beneath the building shall be plus frequently applied live loads only. In most cases, determined. long-term settlement governs foundation design. Short-term (earthquake) load-deformation 3. For enhanced rehabilitation objectives, the soil unit characteristics have not traditionally been used for weight, γ; soil cohesion, c; soil friction angle, φ; soil design; consequently, such relationships are not compressibility characteristics, soil shear modulus, generally found in the geotechnical reports for existing G; and Poisson’s ratio, ν, for each type, shall be buildings. determined. C4.2.1.1.2 Subsurface Soil Conditions 4.2.2 Seismic Geologic Site Hazards Specific foundation information developed for an Seismic rehabilitation shall include an assessment of adjacent or nearby building may be useful if subsurface earthquake-induced hazards at the site due to fault soils and ground water conditions in the site region are rupture, liquefaction, differential compaction, known to be uniform. However, less confidence will landsliding, and an assessment of earthquake-induced result if subsurface data are developed from anywhere flooding or inundation in accordance with but the site of the building being rehabilitated. Sections 4.2.2.1 through 4.2.2.5. Adjacent sites where construction has been done recently may provide a guide for evaluation of If the resulting ground movements cause unacceptable subsurface conditions at the site being considered. performance in the building for the selected performance level, then the hazards shall be mitigated Design drawings may indicate information regarding in accordance with Section 4.3. the allowable bearing capacity of the foundation elements. This information can be used directly in a 4.2.2.1 Fault Rupture presumptive or prescriptive evaluation of the A geologic fault shall be defined as a plane or zone foundation capacity. Construction records may also be along which earth materials on opposite sides have available indicating ultimate pile capacities if load tests moved differentially in response to tectonic forces. were performed. Information on the existing loads on the structure is relevant to determining the amount of Geologic site information shall be obtained to overload that the foundations may be capable of determine if an active geologic fault is present under the resisting during an earthquake. building foundation. If a fault is present, the following information shall be obtained: 4.2.1.2 Design Foundation Loads Information on the design foundation loads shall be obtained including separate information on dead loads and live loads. 4-2 Seismic Rehabilitation Prestandard FEMA 356 Chapter 4: Foundations and Geologic Site Hazards 1. The degree of activity based on the age of most A site shall be regarded as free from liquefaction hazard recent movement. if the site soils, or similar soils in the site vicinity, have not experienced historical liquefaction, and if any of the 2. The fault type; whether it is a strike-slip, normal- following criteria are met: slip, reverse-slip, or thrust fault. 1. The geologic materials underlying the site are either 3. The sense of slip with respect to building geometry. bedrock or have a very low liquefaction susceptibility according to the relative susceptibility 4. Magnitudes of vertical and/or horizontal ratings based upon the type of deposit and geologic displacements consistent with the selected age of the deposit, as shown in Table 4-1. earthquake hazard level. 2. The soils underlying the site are stiff clays or clayey 5. The width and distribution of the fault-rupture zone. silts. C4.2.2.1 Fault Rupture 3. The soils are not highly sensitive, based on local experience. Buildings found to straddle active faults should be assessed to determine if any rehabilitation is 4. The soils are cohesionless with a minimum warranted, possibly to reduce the collapse potential of normalized Standard Penetration Test (SPT) the structure given the likely amount and direction of resistance, N1 60, value of 30 blows/foot as defined fault displacement. in ASTM D1586-99, for depths below the ground water table, or with clay content greater than 20%. 4.2.2.2 Liquefaction 5. The ground water table is at least 35 feet below the Liquefaction shall be defined as an earthquake-induced deepest foundation depth, or 50 feet below the process in which saturated, loose, granular soils lose ground surface, whichever is shallower, including shear strength and liquefy as a result of an increase in considerations for seasonal and historic ground- pore-water pressure during earthquake shaking. water level rises, and any slopes or free-face conditions in the site vicinity do not extend below Subsurface soil and ground water information shall be the ground water elevation at the site. obtained to determine if liquefiable materials are present under the building foundation. If liquefiable If a liquefaction hazard is determined to exist at the site, soils are present, the following information shall be then a more
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