Introduction
CHAPTER 1 INTRODUCTION
1.1 Foundation
It is the interface between superstructure (or other load carrying component like machinery/tower/pipes tanks) and soil. The function of foundation is to transmit to, and into, the underlying soil or rock the loads supported by it and its self weight.
1.2 Foundation Engineer
The title foundation engineer is given to that person who by reason of training and experience is sufficiently versed in scientific principles and engineering judgment (often termed art) to design a foundation. The necessary scientific principles are acquired through formal educational courses in geotechnical (soil mechanics, geology, foundation engineering) and structural (analysis, design in reinforced concrete and steel, etc) engineering and continued self-study via short courses, professional conferences, journal reading, and the like.
1.3 Types of foundations
Foundations may be classified as Shallow foundations and Deep foundations. 1.3.1 Shallow foundations For shallow foundations, the depth to width ratio of footing is (D/B) 2.5 but may be somewhat more. Different types of shallow foundations are: Spread/Single/ Isolated footing: A footing carrying a single column is called spread footing, since its function is to spread the load laterally to the soil so that the stress intensity is reduced to a value that the soil can safely carry. Single footings may be of constant thickness (figure 1-1a) or either stepped (figure 1-1b) or sloped (figure 1-1c). Stepped or sloped footings are most commonly used to reduce the quantity of concrete away from the column where the bending moments are small and when the footing is not reinforced. Spread footings are most widely used because they are economical. Construction of footings requires a least amount of equipment and skill and no heavy or special equipment is necessary. Furthermore, the conditions of the footing and the supporting soil can be readily examined. Strip/wall footing: A wall footing is simply a strip of reinforced concrete or brick masonry wider than the wall (figure 11 d). The function of wall footing is also to distribute (spread) the load laterally as in isolated footings. A pedestal may be used to interface metal columns with spread or wall footings that are 2
located at the depth in the ground. This prevents possible corrosion of metal through direct contact with the soil (figure 1-1e)
Figure 1.1 Types of spread footings Combined footing: It may not be possible to place columns at the center of a spread footing if they are at the property line, near mechanical equipment locations, or irregularly spaced. Columns located off-center will usually result in a nonuniform soil pressure. To avoid the nonuniform soil pressure, an alternative is to enlarge the footing and place one or more of the adjacent columns in the same line on it (figure 1.2). These types of footings are called combined footing. A combined footing may have either rectangular or trapezoidal shape or be a series of pads connected by narrow rigid beams called a strap footing (figure 1.3). The footing can be rectangular if the column that is eccentric with respect to a spread footing carries a smaller load than the interior columns (figure 1.3a). The footing geometry is made such that the resultant of the several columns is in the center of the footing area. This footing and load geometry allows the designer to assume a uniform soil pressure distribution (figure 1.4). A combined footing will be trapezoid-shaped if the column that has too limited a space for a spread footing carries the larger load (figure 1.3b). In this case the resultant of the column loads (including moments) will be closer to the larger column load, and doubling the centroid distance as done for the rectangular footing will not provide sufficient length to reach the interior column. In most cases
Figure 1.2 (a) typical layout of combined footings for column loads as shown; more than two columns can be used
Figure 1.3 Types of combined footings (a) Rectangular (b) Trapezoidal (c) Strap
M
x
col1
P1
P2 S M 1 M 2 P 1 P 2
Col (1) w M1
P2
L w c x 2 2
Col (2) w
M2
L Uniform pressure distribution Figure 1.4 Rectangular footing when P1<P2 trapezoidal footing would be used with only two columns, however, more than two columns can also be supported on trapezoidal footing. The forming and reinforcing steel for a trapezoid footing is somewhat awkward to place. For these reason it may be preferable to use strap footing where possible, since essentially the same goal of producing a computed uniform soil pressure is obtained. A strap footing is used to connect an eccentrically loaded column footing to an interior column (figure 1.3c). The strap is used to transmit the moment caused from eccentricity to interior column footing so that a uniform soil pressure is computed beneath both footings. The strap serves the same purpose as the interior portion of a combined footing but is much narrower to save materials. A strap footing may be used in lieu of a combined rectangular/trapezoidal footing if the distance between columns is large and /or the allowable soil pressure is relatively large so that the additional footing area is not needed. A strap footing should be considered only after a careful analysis shows that spread footings-even if oversize-will not work. The extra labor and forming cost for this type of footing make it one to use as last resort. Mat/Raft foundation A mat/raft foundation is a large concrete slab used to interface one column, or more than one column in several lines, with the base soil (figure 1.5). It may encompass the entire foundation area or only a portion. A mat or raft foundation is used where 50% of the area is covered by conventional spread footings or in soils with extremely erratic characteristics. It is common to use mat foundations for 5
deep basements both to spread the column loads to a more uniform pressure distribution and to provide floor slab for the basement. A particular advantage for basements at or below the GWT is to provide water barrier. Depending on local costs and noting that a mat foundation requires both +ive and ive reinforcing steel, one may find it more economical to use spread footings-even if the entire area is covered. Spread footings avoid the use of ive reinforcing steel and can be accomplished as in figure 1.6 by pouring alternate footings, to avoid formwork, and using fiber spacer boards to separate the footings poured later.
Figure 1.5 Common types of mat foundations. (a) Flat plate (b) plate thickened under columns (c) waffle slab (d) plate with pedestals (e) basement walls as part of ma
Possible fiberboard spacer boards between spread footings
Col.
Col.
Col.
Figure 1.6 Mat Versus possible use of spread footings to save labor, forming costs, and negative reinforcement 6
1.3.2 Deep foundations For deep foundations the length L/B ratio i.e. (length or depth of foundation to its width or diameter) 4. For types of deep foundations and uses please refer to chapter 5.
1.4 Requirement of foundation system
1.4.1 Safety requirement 1. Factor of safety against shear failure of the soil should be adequate (FOS 2.5-3)
2. Settlement (total or differential): The settlement should not cause any damage to the structure or interfere with the function of the structure 3. Factor of safety against structural failure of foundation should be adequate.
1.4.2 Depth requirement 1. Prevent movement due to soil volume changes by seasonal freezing and thawing of the ground. 2. Footing should be below zones of high volume changes due to moisture fluctuation. Many soils particularly with those of high plasticity shrink greatly on drying and swell upon the addition of moisture. 3. Prevent wind or water erosion. 4. By pass unsuitable soil layer such as peat, expensive clay, soft unconsolidated deposit, and old soil layer 5. Prevent footing movement or distortion by plant or tree root growth. 1.4.3 Spacing requirement The foundation must be spaced appropriately in order to prevent distress in adjacent foundation. 1.4.4 Economic and functional requirement The foundation should be economical and performs satisfactorily the intended function.
1.5 Steps for Designing a Foundation
Following minimum steps are required for designing a foundation: 1. Locate the site and the position of load. A rough estimate of the foundation load(s) is usually provided by the client or made in-house. Depending on the site or load system complexity, a literature survey may be started to see how others have approached similar problems.
2. Physically inspect the site for any geological or other evidence that may indicate a potential design problem that will have to be taken into account when making the design or giving a design recommendation supplement the inspection with any previously obtained soil data. 3. Establish the field exploration program and, on the basis of discovery (or what is found in the initial phase), set up the necessary supplemental field testing and any laboratory test program. 4. Determine the necessary soil design parameters based on integration of test data, scientific principles, and engineering judgment. Simple or complex computer analyses may be involved. 5. Design the foundation using the soil parameters from step 4. The foundation should be economical and be able to be built by the available construction personnel. Take into account practical construction tolerances and local construction practices. Interact closely with all concerned (client, engineers, architect, contractor) so that the substructure system is not excessively overdesigned and risk is kept within acceptable levels. A computer may be used extensively (or not at all) in this step.
1.6 Foundation Selection Process
The rational selection of a safe foundation involves a systematic process of evaluation of many factors, including structural design load, environmental effects, subsurface condition, performance requirement, construction methods and economics. A suggested sequence of steps in this process is outlined in figure 1.7 and discussed briefly below. Additional discussion of the various phases of the process is presented in subsequent of this synthesis. The foundation selection must be based on information about the proposed structure and the site conditions. Ideally, a preliminary evaluation of the subsurface conditions and potential foundation problems should be included in the preliminary site location studies. However, foundation conditions frequently are overlooked in site selection. Similarly, the type of structure usually is established prior to the foundation investigations. Thus, the type and site of structure, the foundation design loads and the required performance criteria often are specified by the structural engineers with little or no geotechnical input. The field and laboratory geotechnical investigations should be planned by a geotechnical engineer or engineering geologist who understands the type of information that will be needed in the foundation selection studies. Thus, this individual must recognize the requirements of various types of structures, the foundation alternatives that may be considered and the types of analyses that will be required to make a rational selection among these alternatives. The analyses of behavior of various potential foundation systems in reponse to design loads and environmental factors are the responsibility of the geotechnical or foundation engineer. The predicted behavior of each alternative then is compared with the performance requirements established by the structural engineer. For foundations that appear to provide satisfactory performance, potential construction problems and cost are considered. Maintenance costs also should be considered. Finally, the foundation system that will provide satisfactory performance at least cost is recommended. As noted in figure 1.7 normally shallow foundation should be evaluated first. 8
If shallow foundations will perform satisfactorily, they usually will be the most economical alternative. If the response of a shallow foundation appears to be satisfactory or marginal, other alternatives must be considered. Various types of deep foundation and/or ground modification techniques may be evaluated. Ideally, modification of the primary structure to reduce performance criteria also should be considered. However, this option is seldom used in current practice. The foundation investigation and recommendations are presented in a foundation report, which is prepared by the geotechnical engineer. The report should include, Site description Boring logs and subsurface profile Results of laboratory and field tests for identification, classification and relevant engineering properties of strata Review of design loads Analyses of behavior of each foundation alternative Evaluation of predicted performance in relation to performance requirements Discussion of potential construction problems (excavation, dewatering, etc.) Discussion of relative costs Recommendations Foundation type Foundation design criteria (allowable loads, depth, etc.) Special construction methods Construction monitoring where required
The geotechnical engineers recommendations are submitted to the structure designers, who ultimately must approve the design recommendations and prepare the detailed structural design and the construction plans and specification for the foundation. Finally, the geotechnical engineer must be prepared to respond to problems that may develop during construction. Because of the inherent variability of subsurface conditions, it is not uncommon to encounter unanticipated conditions which may significantly affect the foundation design. Minor and occasionally major design revisions may be necessary to accommodate the unforeseen conditions. In other instance, the geotechnical report may have recommendation monitoring of field behavior during and/or after construction. In summary, the selection, design and construction of an adequate cost affective foundation require coordination among geotechnical engineers. It is desirable for the agency to be organized in a manner that permits the geotechnical engineer to be directly involved in all phases of the foundation work from preliminary planning through construction.
OBTAIN SITE INFORMATION Surface -Topography -Hydrology -Climate Subsurface -Soil/Rock Strata -Soil/Rock properties -Ground water table
OBTAIN STRUCTURE DATA Type Performance criteria Foundation loads
EVALUATION FOUNDATION ALTERANTIVE 1. Shallow foundations 2. Deep foundations 3. Ground modification shallow foundation for
PREDICT BEHAVIOR -Settlement -Bearing capacity -Lateral stability -Environmental factor
Select Another Foundation Alternative
DETERMING FEASIBILITY -Predicted vs. Required performance -Potential construction problems -Cost estimate Acceptable RECOMMENDATION -Foundation type -Design data -Construction procedures
Not Acceptable
After selection Process completed Prepare Detailed Design plans and Specification, Monitor Construction
Figure 1.7 Flow chart of foundation selection process
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1.7 Type of loads
A structure may be subjected to a combination of some or all of the following loads and forces. 1.7.1 Dead loads Dead loads are those that are constant in magnitude and fixed in location through out the lifetime of the structure. Usually the major part of the dead load is the weight of the structure itself. 1.7.2 Live loads Live loads consist chiefly of occupancy loads in buildings and traffic loads on bridges. They may be either fully or partially in place or not present at all and may also change in location. Their magnitude and distribution at any given time are uncertain, and even their maximum intensities thorugho0ut the lifetime of the structure are not known with precision. 1.7.3 Environmental loads These mainly consists of snow loads, wind pressure and suction, earthquake loads (i.e. inertia forces caused by earthquake motions), soil pressures on subsurface portions of structures, water pressure acting laterally against basement walls and vertically against base slabs, loads from possible ponding of rainwater on flat surfaces, and forces caused by temperature differentials. Like live loads, environmental loads at any given time are uncertain both in magnitude and distribution.
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