1、Section 4 FOUNDATIONS 4.1 GENERAL Part A GENERAL REQUIREMENTS AND MATERIALS Foundations shall be designed to support all live and dead loads, and earth and water pressure loadings in ac- cordance with the general principles specified in this sec- tion. The design shall be made either with reference
2、to ser- vice loads and allowable stresses as provided in SERVICE LOAD DESIGN or, alternatively, with reference to load factors, and factored strength as provided in STRENGTH DESIGN. 4.2 FOUNDATION TYPE AND CAPACITY 4.2.1 Selection of Foundation Type Selection of foundation type shall be based on an
3、assessment of the magnitude and direction of loading, depth to suitable bearing materials, evidence of previous flooding, potential for liquefaction, undermining or scour, swelling potential, frost depth and ease and cost of construction. 4.2.2 Foundation Capacity Foundations shall be designed to pr
4、ovide adequate structural capacity, adequate foundation bearing capacity with acceptable settlements, and acceptable overall sta- bility of slopes adjacent to the foundations. The tolerable level of structural deformation is controlled by the type and span of the superstructure. 4.2.2.1 Bearing Capa
5、city The bearing capacity of foundations may be estimated using procedures described in Articles 4.4,4.5, or 4.6 for service load design and Articles 4.11, 4.12, or 4.13 for strength design, or other generally accepted theories. Such theories are based on soil and rock parameters measured by in situ
6、 and/or laboratory tests. The bearing capacity may also be determined using load tests. 4.2.2.2 Settlement The settlement of foundations may be determined using procedures described in Articles 4.4,4.5, or 4.6 for service load design and Articles 4.11, 4.12, or 4.13 for strength design, or other gen
7、erally accepted methodolo- gies. Such methods are based on soil and rock parameters measured directly or inferred from the results of in situ andor laboratory tests. 4.2.2.3 Overall Stability The overall stability of slopes in the vicinity of foundations shall be considered as part of the design of
8、foundations. 4.2.3 Soil, Rock, and Other Problem Conditions Geologic and environmental conditions can influence the performance of foundations and may require special consideration during design. To the extent possible, the presence and influence of such conditions shall be evalu- ated as part of th
9、e subsurface exploration program. A rep- resentative, but not exclusive, listing of problem condi- tions requiring special consideration is presented in Table 4.2.3A for general guidance. 4.3 SUBSURFACE EXPLORATION AND TESTING PROGRAMS The elements of the subsurface exploration and testing programs
10、shall be the responsibility of the designer based on the specific requirements of the project and his or her experience with local geologic conditions. 4.3.1 General Requirements As a minimum, the subsurface exploration and testing 0 Soil strata programs shall define the following, where applicable:
11、 -Depth, thickness, and variability 43 44 HIGHWAY BRIDGES 4.3.1 TABLE 4.2.3A Problem Conditions Requiring Special Consideration Problem Type Description Comments Organic soil; highly plastic clay Sensitive clay Micaceous soil Soil Expansive clay/silt; expansive slag Liquefiable soil Collapsible soil
12、 Pyritic soil Laminated rock Expansive shale Pyritic shale Rock Soluble rock Cretaceous shale Weak claystone (Red Beds) Gneissic and Schistose Rock Subsidence Sinkholes/solutioning Condition Negative skin friction/ expansion loading Corrosive environments Permafrost/frost Capillary water Low strengt
13、h and high compressibility Potentially large strength loss upon large straining Potentially high compressibility (often saprolitic) Potentially large expansion upon wetting Complete strength loss and high deformations due to earthquake Potentially large deformations upon wetting (Caliche; Loess) Pot
14、entially large expansion upon oxidation Low strength when loaded parallel to bedding Potentially large expansion upon wetting; degrades readily upon Expands upon exposure to aidwater Soluble in flowing and standing water (Limestone, Limerock, Indicator of potentially corrosive ground water Low stren
15、gth and readily degradable upon exposure to aidwater Highly distorted with irregular weathering profiles and steep Typical in areas of underground mining or high ground water Karst topography; typical of areas underlain by carbonate rock Additional compressive/uplift load on deep foundations due to
16、settlemenihplift of soil Acid mine drainage; degradation of certain soilhock types Typical in northern climates Rise of water level in silts and fine sands leading to strength loss loading exposure to aidwater Gypsum) discontinuities extraction strata -Identification and classification -Relevant eng
17、ineering properties (Le., shear strength, compressibility, stiffness, permeability, expansion or collapse potential, and frost suscep- tibility) 0 Rock strata -Depth to rock -Identification and classification -Quality (Le., soundness, hardness, jointing and presence of joint filling, resistance to w
18、eathering, if exposed, and solutioning) -Compressive strength (e.g., uniaxial compres- sion, point load index) -Expansion potential Ground water elevation Ground surface elevation Local conditions requiring special consideration Exploration logs shall include soil and rock strata de- scriptions, pen
19、etration resistance for soils (e.g., SPT or qc), and sample recovery and RQD for rock strata. The drilling equipment and method, use of drilling mud, type of SPT hammer (i.e. safety, donut, hydraulic) or cone pen- etrometer (i.e., mechanical or electrical), and any unusual subsurface conditions such
20、 as artesian pressures, boulders or other obstructions, or voids shall also be noted on the exploration logs. 4.3.2 Minimum Depth Where substructure units will be supported on spread footings, the minimum depth of the subsurface explo- ration shall extend below the anticipated bearing level a minimu
21、m of two footing widths for isolated, individual footings where L 5 2B, and four footing widths for foot- ings where L 5B. For intermediate footing lengths, the minimum depth of exploration may be estimated by lin- ear interpolation as a function of L between depths of 2B and 5B below the bearing le
22、vel. Greater depths may be re- quired where warranted by local conditions. 4.3.2 DIVISION I-DESIGN 45 Where substructure units will be supported on deep foundations, the depth of the subsurface exploration shall xtend a minimum of 20 feet below the anticipated pile or shaft tip elevation. Where pile
23、 or shaft groups will be used, the subsurface exploration shall extend at least two times the maximum pile group dimension below the an- ticipated tip elevation, unless the foundations will be end bearing on or in rock. For piles bearing on rock, a mini- mum of 10 feet of rock core shall be obtained
24、 at each ex- ploration location to insure the exploration has not been terminated on a boulder. For shafts supported on or ex- tending into rock, a minimum of 10 feet of rock core, or a length of rock core equal to at least three times the shaft diameter for isolated shafts or two times the maximum
25、shaft group dimension for a shaft group, whichever is greater, shall be obtained to insure the exploration has not terminated in a boulder and to determine the physical characteristics of rock within the zone of foundation in- fluence for design. 4.3.3 Minimum Coverage A minimum of one soil boring s
26、hall be made for each substructure unit. (See Article 7.1.1 for definition of sub- structure unit.) For substructure units over 100 feet in width, a minimum of two borings shall be required. 4.3.4 Laboratory Testing Laboratory testing shall be performed as necessary to determine engineering properti
27、es including unit weight, shear strength, compressive strength and compressibility. In the absence of laboratory testing, engineering proper- ties may be estimated based on published test results or local experience. 4.3.5 Scour The probable depth of scour shall be determined by subsurface explorati
28、on and hydraulic studies. Refer to Article 1.3.2 and FHWA (1988) for general guidance regarding hydraulic studies and design. Part B SERVICE LOAD DESIGN METHOD ALLOWABLE STRESS DESIGN 4.4 SPREAD FOOTINGS 4.4.1 General 4.4.1.1 Applicability Provisions of this Article shall apply for design of iso- la
29、ted footings, and to combined footings and mats (foot- ings supporting more than one column, pier, or wall). 4.4.1.2 Footings Supporting Non-Rectangular Columns or Piers Footings supporting circular or regular polygon- shaped concrete columns or piers may be designed as- suming that the columns or p
30、iers act as square members with the same area for location of critical sections for mo- ment, shear, and development of reinforcement. 4.4.1.3 Footings in Fill Footings located in fill are subject to the same bearing capacity, settlement, and dynamic ground stability con- siderations as footings in
31、natural soil in accordance with Articles 4.4.7.1 through 4.4.7.3. The behavior of both the fill and underlying natural soil shall be considered. 4.4.1.4 Footings in Sloped Portions of Embankments The earth pressure against the back of footings and columns within the sloped portion of an embankment s
32、hall be equal to the at-rest earth pressure in accordance with Article 5.5.2. The resistance due to the passive earth pressure of the embankment in front of the footing shall be neglected to a depth equal to a minimum depth of 3 feet, the depth of anticipated scour, freeze thaw action, andor trench
33、excavation in front of the footing, whichever is greater. 4.4.1.5 Distribution of Bearing Pressure Footings shall be designed to keep the maximum soil and rock pressures within safe bearing values. To prevent unequal settlement, footings shall be designed to keep the bearing pressure as nearly unifo
34、rm as practical. For foot- ings supported on piles or drilled shafts, the spacing be- tween piles and drilled shafts shall be designed to ensure nearly equal loads on deep foundation elements as may be practical. When footings support more than one column, pier, or wall, distribution of soil pressur
35、e shall be consistent with properties of the foundation materials and the structure, and with the principles of geotechnical engineering. 4.4.2 Notations The following notations shall apply for the design of spread footings on soil and rock: A A = Contact area of footing (ft) = Effective footing are
36、a for computation of bearing capacity of a footing subjected to eccentric load (ft); (SeeArticle4.4.7.1.1.1) 46 HIGHWAY BRIDGES 4.4.2 b, b, b, B B C CI C* c2 CC, cc, CO D Eo Em = Base inclination factors (dim); (See Article = Width of footing (ft); (Minimum plan di- mension of footing unless otherwi
37、se noted) = Effective width for load eccentric in direc- tion of short side, L unchanged (ft) = Soil cohesion (ksf) = Effective stress soil cohesion (ksf) = Reduced effective stress soil cohesion for punching shear (ksf); (See Article 4.4.7.1) = Adhesion between footing and foundation soil or rock (
38、ksf); (See Article 4.4.7.1.1.3) = Coefficient of consolidation (ft2/yr); (See Article 4.4.7.2.3) = Shear strength of upper cohesive soil layer below footing (ksf); (See Article 4.4.7.1.1.7) = Shear strength of lower cohesive soil layer below footing (ksf); (See Article 4.4.7.1.1.7) = Compression ind
39、ex (dim); (See Article 4.4.7.2.3) = Recompression index (dim); (See Article 4.4.7.2.3) = Compression ratio (dim); (See Article 4.4.7.2.3) = Uniaxial compressive strength of intact rock (ksf) = Recompression ratio (dim); (See Article 4.4.7.2.3) = Coefficient of secondary compression de- fined as chan
40、ge in height per log cycle of time (dim); (See Article 4.4.7.2.4) = Influence depth for water below footing (ft); (See Article 4.4.7.1.1.6) = Depth to base of footing (ft) = Void ratio (dim); (See Article 4.4.7.2.3) = Void ratio at final vertical effective stress (dim); (See Article 4.4.7.2.3) = Voi
41、d ratio at initial vertical effective stress (dim); (See Article 4.4.7.2.3) = Void ratio at maximum past vertical effec- tive stress (dim); (See Article 4.4.7.2.3) = Eccentricity of load in the B direction mea- sured from centroid of footing (ft); (See Ar- ticle 4.4.7.1.1.1) = Eccentricity of load i
42、n the L direction mea- sured from centroid of footing (ft); (See Article 4.4.7.1.1.1) . 4.4.7.1.1.8) = Modulus of intact rock (ksf) = Rock mass modulus (ksf); (See Article 4.4.8.2.2) Es F FS H e L L Li n N Ni Nnl Nms NS = Soil modulus (ksf) = Total force on footing subjected to an in- clined load (k
43、); (See Article 4.4.7.1.1.1) = Unconfined compressive strength of con- crete (ksf) = Factor of safety against bearing capacity, overturning or sliding shear failure (dim) = Depth from footing base to top of second cohesive soil layer for two-layer cohesive soil profile below footing (ft); (See Artic
44、le 4.4.7.1.1.7) = Height of compressible soil layer (ft) = Critical thickness of the upper layer of a two-layer system beyond which the under- lying layer will have little effect on the bear- ing capacity of footings bearing in the upper layer (ft); (SeeMicle 4.4.7.1.1.7) = Height of longest drainag
45、e path in com- pressible soil layer (ft) = Height of slope (ft); (See Article 4.4.7.1.1.4) = Slope angle from horizontal of ground sur- face below footing (deg) = Load inclination factors (dim); (See Article 4.4.7.1.1.3) = Influence coefficient to account for rigidity and dimensions of footing (dim)
46、; (See Arti- cle 4.4.8.2.2) = Center-to-center spacing between adjacent footings (ft) = Length of footing (ft) = Effective footing length for load eccentric in direction of long side, B unchanged (ft) = Length (or width) of footing having positive contact pressure (compression) for footing loaded ec
47、centrically about one axis (ft) = Exponential factor relating BL or L/B ra- tios for inclined loading (dim); (See Article 4.4.7.1.1.3) = Standard penetration resistance (blowdft) = Standard penetration resistance corrected for effects of overburden pressure (blows/ ft); (See Article 4.4.7.2.2) = Bea
48、ring capacity factors based on the value of internal friction of the foundation soil (dim); (See Article 4.4.7.1) = Modified bearing capacity factor to account for layered cohesive soils below footing (dim); (See Article 4.4.7.1.1.7) = Coefficient factor to estimate quit for rock (dim); (See Article
49、 4.4.8.1.2) = Stability number (dim); (See Article 4.4.7.1.1.4) 4.4.2 DIVISION I-DESIGN 47 = Modified bearing capacity factors for ef- fects of footing on or adjacent sloping ground (dim); (See Article 4.4.7.1.1.4) = Tangential component of force on footing. (k) = Maximum resisting force between footing base and foundation soil or rock for sliding failure (k) = Effective overburden pressure at base of footing (ksf) = Normal component of force on footing (k) = Allowable uniform bearing pressure or con- = Cone penetration resistance (ksf) = Maximum footing contact pressure (ksf) = Maximu
