Navarro a#1 introduction to foundation engineering_2014-2015
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This document defines and discusses soil mechanics, geotechnical engineering, foundation engineering, the role of a foundation engineer, and classifications of foundations. It provides details on: 1) The differences between soil mechanics, geotechnical engineering, and foundation engineering. 2) The steps and responsibilities involved in designing a foundation as a foundation engineer. 3) The four main performance requirements for foundations: strength, serviceability, constructibility, and economic requirements. 4) The classifications of shallow foundations which include spread footings, strap footings, combined footings, raft/mat foundations, and deep footings. It also defines various deep foundation types such as pile foundations, pier foundations, caissons
Navarro a#1 introduction to foundation engineering_2014-2015
- 1. Technological University of the Philippines Ayala Blvd. Ermita, Manila College of Engineering Department of Civil Engineering CE 521-5A Foundation Engineering, Lec. Assignment No.1 Introduction to Foundation Engineering Navarro, Brylle Ephraiem Q. 10-205-053 June 24, 2014 Engr. Jesus Ray M. Mansayon Instructor
- 2. Define/Discuss/Enumerate/Differentiate the following: 1. Soil Mechanics, Geotechnical Engineering, and Foundation Engineering SOIL MECHANICS Soil mechanics is a branch of engineering mechanics that describes the behaviour of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and water) and particles (usually clay, silt, sand, and gravel) but soil may also contain organic solids, liquids, and gasses and other matter.1234 Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering,5 a sub discipline of Civil engineering, and engineering geology, a sub discipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils.6 Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as engineering geology, geophysical engineering, coastal engineering, agricultural engineering, hydrology and soil physics. GEOTECHNICAL ENGINEERING Geotechnical engineering is the branch of civil engineering concerned with the engineering behaviour of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground. Geotechnical engineering uses principles of soil mechanics and rock mechanics to investigate subsurface conditions and materials; determine the relevant physical/mechanical and chemical properties of these materials; evaluate stability of natural slopes and man-made soil deposits; assess 1 Mitchell, J.K., and Soga, K. (2005) Fundamentals of soil behavior, Third edition, John Wiley and Sons, Inc., ISBN 978-0-471-46302-7. 2 Santamarina, J.C., Klein, K.A., & Fam, M.A. (2001). Soils and Waves: Particulate Materials Behavior, Characterization and Process Monitoring. Wiley. ISBN 978-0-471-49058-6.. 3 Powrie, W., Spon Press, 2004, Soil Mechanics - 2nd ed ISBN 0-415-31156-X 4 A Guide to Soil Mechanics, Bolton, Malcolm,Macmillan Press, 1979. ISBN 0-333-18932-0 5 Fang, Y., Spon Press, 2006, Introductory Geotechnical Engineering 6 Lambe, T. William & Robert V. Whitman. Soil Mechanics. Wiley, 1991; p. 29. ISBN 978-0-471-51192-2
- 3. risks posed by site conditions; design earthworks and structure foundations; and monitor site conditions, earthwork and foundation construction.78 FOUNDATION ENGINEERING Foundation Engineering is the engineering field of study devoted to the design of those structures which support other structures, most typically buildings, bridges or transportation infrastructure. It is at the periphery of Civil, Structural and Geotechnical Engineering disciplines and has distinct focus on soil-structure interaction. 2. Foundation Engineer The title Foundation Engineer is given to the person who by reason of training and experience is sufficiently versed in scientific principles and engineering judgment to design a foundation. The necessary scientific principles are acquired through formal educational courses in geo-technical (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. Because of the heterogeneous nature of soil and rock masses, two foundations- even on adjacent construction sites- will seldom be the same except by coincidence. Since every foundation represents at least partly a venture into the unknown, it is of great value to have access to other’s solutions obtained from conference presentations, journal papers, and textbook condensations of appropriate literature. The amalgamation of experience, study of what others have done in somewhat similar situations, and the site-specific geotechnical information to produce an economical, practical, and safe substructure design is application of engineering judgment. The following steps are the minimum required for designing a foundation: 7 Terzaghi, K., Peck, R.B. and Mesri, G. (1996), Soil Mechanics in Engineering Practice 3rd Ed., John Wiley & Sons, Inc. ISBN 0-471-08658- 4 8 Holtz, R. and Kovacs, W. (1981), An Introduction to Geotechnical Engineering, Prentice-Hall, Inc. ISBN 0-13-484394-0
- 4. 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 this 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. For complex problems, compare the recommended data with published literature or engage another geotechnical consultant to give an outside perspective to the results. 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. The foundation engineer should be experienced in and have participation in all five of the preceding steps. In practice this often is not the case. An independent geotechnical firm specializing in sol exploration, soil testing, design of landfills, embankments, water pollution control, etc. often assigns one of its geotechnical designers to do steps 1 through 4.
- 5. The output of step 4 is given to the client- often a foundation engineer who specializes in the design of the structural elements making up the substructure system. The principal deficiency in this approach is the tendency to treat the design soil parameters- obtained from soil tests of variable quality, heavily supplemented with engineering judgment- as precise numbers whose magnitude is totally inviolable. Thus, the foundation engineer and geotechnical consultant must work closely together, or at least have frequent conferences as the design progresses. It should be evident that both parties need to appreciate the problems of each other and, particularly, that the foundation design engineer must be aware of the approximate methods used to obtain the soil parameters being used. This understanding can be obtained by each having training in the other’s specialty. To this end, the primary focus of this text will be on analysis and design of the interfacing elements for buildings, machines, and retaining structures and on those soil mechanics principles used to obtain the necessary soil parameters required to accomplish the design. Specific foundation elements to be considered include shallow elements such as footings and mats and deep elements such as piles and drilled piers. Geotechnical considerations will primarily be on strength and deformation and those soil-water phenomena that affect strength and deformation with the current trend to using sites with marginal soil parameters for major projects, methods to improve the strength and deformation characteristics through soil improvement methods.9 3. Four Performance Requirements STRENGTH REQUIREMENTS Strength requirements are intended to avoid catastrophic failures. There are two types: geotechnical strength requirements and structural strength requirements. Geotechnical strength requirements are those that address the ability of the soil or rock to accept the loads imparted by the foundation without failing. The strength of soil is governed by its capacity to sustain shear stresses, so we satisfy geotechnical 9 Bowles, Joseph. 1995. Foundation Analysis and Design., 5th Edition., USA. P24-26
- 6. strength requirements by comparing shear stresses with shear strengths and designing accordingly. In the case of spread footing foundations, geotechnical strength is expressed as the bearing capacity of the soil. If the load-bearing capacity of the soil is exceeded, the resulting shear failure is called a bearing capacity failure as shown in the Figure 1. Structural strength requirements address the foundation’s structural integrity and its ability to safely carry the applied loads. Foundations that are loaded beyond structural capacity will, in principle, fail catastrophically. Structural strength analyses are conducted using the ASD or LRFD methods, depending on the types of foundation, the structural materials, and the governing code. SERVICEABILITY REQUIREMENTS Serviceability requirements are intended to produce foundations that perform well when subjected to the service loads. These requirements include: Settlement – Most foundations experience some downward movement as a result of the applied loads. This movement is called settlement. Keeping settlements within tolerable limits is usually the most important foundation serviceability requirement. Fig.1. A bearing capacity failure beneath a spread footing foundation. The soil has failed in shear, causing the foundation to collapse
- 7. Heave – Sometimes foundations move upward instead of downward. We call this upward movement heave. The most common source of heave is the swelling of expansive soils. Tilt – When settlement or heave occurs only on one side of the structure, it may begin to tilt. The Leaning Tower of Pisa is an extreme example of tilt. Lateral movement – Foundations subjected to lateral loads (shear or moment) deform horizontally. This lateral movement also must remain within acceptable limits to avoid structural distress. Vibration – Some foundations, such as those supporting certain kinds of heavy machinery, are subjected to strong vibrations. Such foundations need to accommodate these vibrations without experiencing resonance or other problems. Durability – Foundations must be resistant to the various physical, chemical, and biological processes that cause deterioration. This is especially important in waterfront structures, such as docks and piers. Failure to satisfy these requirements generally results in increased maintenance costs, aesthetic problems, diminished usefulness of the structure, and other similar effects. Fig.2. Modes of settlement: (a) uniform, (b) tilting with no distortion, (c) distortion
- 8. CONSTRUCTIBILITY REQUIREMENTS Constructibility requirements meaning the foundation must be designed such that the contractor can build it without having to use extraordinary method or equipment. ECONOMIC REQUIREMENTS Economic requirements are intended to produce designs that minimize the required quantity of construction materials do not necessarily minimize the cost. In some cases, designs that use more materials may be easier to build, and thus have a lower overall cost.10 4. Classification of Foundation The various types of structural foundations may be grouped into two broad categories — shallow foundations and deep foundations. The classification indicates the depth of the foundation relative to its size and the depth of the soil providing most of the support. According to Terzaghi, a foundation is shallow if its depth is equal to or less than its width and deep when it exceeds the width. Further classification of shallow foundations and deep foundations is as follows: 10 http://infohost.nmt.edu/~Mehrdad/foundation/hdout/PerformanceRequirements.pdf
- 9. The ‘floating foundation’, a special category, is not actually a different type, but it represents a special application of a soil mechanics principle to a combination of raft- caisson foundation, explained later. A short description of these with pictorial representation will now be given. Spread footings Spread footing foundation is basically a pad used to ‘‘spread out’’ loads from walls or columns over a sufficiently large area of foundation soil. These are constructed as close to the ground surface as possible consistent with the design requirements, and with factors such as frost penetration depth and possibility of soil erosion. Footings for permanent structures are rarely located directly on the ground surface. A spread footing need not necessarily be at small depths; it may be located deep in the ground if the soil conditions or design criteria require. Spread footing required to support a wall is known as a continuous, wall, or strip footing, while that required to support a column is known as an individual or an isolated footing. An isolated footing may be square, circular, or rectangular in shape in plan, depending upon factors such as the plan shape of the column and constraints of space.
- 10. If the footing supports more than one column or wall, it will be a strap footing, combined footing or a raft foundation. The common types of spread footings referred to above are shown in Fig. 15.2. Two miscellaneous types—the monolithic footing, used for watertight basement (also for resisting uplift), and the grillage foundation, used for heavy loads are also shown. Strap footings A ‘strap footing’ comprises two or more footings connected by a beam called ‘strap’. This is also called a ‘cantilever footing’ or ‘pump-handle foundation’. This may be required when the footing of an exterior column cannot extend into an adjoining private property. Common types of strap beam arrangements are shown in Fig. 15.3. Combined footings A combined footing supports two or more columns in a row when the areas required for individual footings are such that they come very near each other. They are also preferred in situations of limited space on one side owing to the existence of the boundary line of private property.
- 11. The plan shape of the footing may be rectangular or trapezoidal; the footing will then be called ‘rectangular combined footing’ or ‘trapezoidal combined footing’, as the case may be. These are shown in Fig. 15.4. Raft foundations (Mats) A raft or mat foundation is a large footing, usually supporting walls as well as several columns in two or more rows. This is adopted when individual column footings would tend to be too close or tend to overlap; further, this is considered suitable when differential settlements arising out of footings on weak soils are to be minimised. A typical mat or raft is shown in Fig. 15.5.
- 12. Deep footings According to Terzaghi, if the depth of a footing is less than or equal to the width, it may be considered a shallow foundation. Theories of bearing capacity have been considered
- 13. for these in Chapter 14. However, if the depth is more, the footings are considered as deep footings (Fig.15.6); Meyerhof (1951) developed the theory of bearing capacity for such footings. Pile foundations Pile foundations are intended to transmit structural loads through zones of poor soil to a depth where the soil has the desired capacity to transmit the loads. They are somewhat similar to columns in that loads developed at one level are transmitted to a lower level; but piles obtain lateral support from the soil in which they are embedded so that there is no concern with regard to buckling and, it is in this respect that they differ from columns. Piles are slender foundation units which are usually driven into place. They may also be cast-in-place (Fig. 15.7). A pile foundation usually consists of a number of piles, which together support a structure. The piles may be driven or placed vertically or with a batter. Pier foundations Pier foundations are somewhat similar to pile foundations but are typically larger in area than piles. An opening is drilled to the desired depth and concrete is poured to make a pier foundation (Fig. 15.8). Much distinction is now being lost between the pile foundation and pier foundation, adjectives such as ‘driven’, ‘bored’, or ‘drilled’, and ‘precast’ and ‘cast-in-situ’, being used to indicate the method of installation and construction. Usually, pier foundations are used for bridges. Caissons (Wells) A caisson is a structural box or chamber that is sunk into place or built in place by systematic excavation below the bottom. Caissons are classified as ‘open’ caissons,
- 14. ‘pneumatic’ caissons, and ‘box’ or ‘floating’ caissons. Open caissons may be box-type of pile-type. The top and bottom are open during installation for open caissons. The bottom may be finally sealed with concrete or may be anchored into rock. Pneumatic caisson is one in which compressed air is used to keep water from entering the working chamber, the top of the caisson is closed. Excavation and concreting is facilitated to be carried out in the dry. The caisson is sunk deeper as the excavation proceeds and on reaching the final position, the working chamber is filled with concrete. Box or floating caisson is one in which the bottom is closed. It is cast on land and towed to the site and launched in water, after the concrete has got cured. It is sunk into position by filling the inside with sand, gravel, concrete or water. False bottoms or temporary bases of timber are sometimes used for floating the caisson to the site. The various types of caissons are shown in Fig. 15.9
- 15. Floating foundation The floating foundation is a special type of foundation construction useful in locations where deep deposits of compressible cohesive soils exist and the use of piles is impractical. The concept of a floating foundation requires that the substructure be assembled as a combination of a raft and caisson to create a rigid box as shown in Fig. 15.10. This foundation is installed at such a depth that the total weight of the soil excavated for the rigid box equals the total weight of the planned structure. Theoretically speaking, therefore, the soil below the structure is not subjected to any increase in stress; consequently, no settlement is to be expected. However, some settlement does occur usually because the soil at the bottom of the excavation expands after excavation and gets recompressed during and after construction.11 11 Venkatramaiah, C. 2006. Geotechnical Engineering, Revised 3rd Edition. New Age International (P) Limited Publishers. P607-613.