Shallow Foundations E-Book

Tharwat M. Baban

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 941

Veröffentlichungsjahr: 2016

Beschreibung

Shallow Foundations: Discussions and Problem Solving is written for civil engineers and all civil engineering students taking courses in soil mechanics and geotechnical engineering. It covers the analysis, design and application of shallow foundations, with a primary focus on the interface between the structural elements and underlying soil. Topics such as site investigation, foundation contact pressure and settlement, vertical stresses in soils due to foundation loads, settlements, and bearing capacity are all fully covered, and a chapter is devoted to the structural design of different types of shallow foundations. It provides essential data for the design of shallow foundations under normal circumstances, considering both the American (ACI) and the European (EN) Standard Building Code Requirements, with each chapter being a concise discussion of critical and practical aspects. Applications are highlighted through solving a relatively large number of realistic problems. A total of 180 problems, all with full solutions, consolidate understanding of the fundamental principles and illustrate the design and application of shallow foundations.

Leseprobe

Shallow Foundations

Discussions and Problem Solving

Tharwat M. Baban

B.Sc. (Hons.); M.S. (Berkeley, Calif.)University Professor and Geotechnical Consultant(Retired)

This edition first published 2016© 2016 by John Wiley & Sons, Ltd

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Cover image: Courtesy of the Author

To The Baban Family

Preface

This book is intended primarily to introduce civil engineers, especially geotechnical engineers and all civil engineering students reading the specialist subjects of soil mechanics and geotechnical engineering, to the fundamental concepts and application of shallow foundation analysis and design. Also, the furnished material can be considered as an essential reference work for practising civil engineers, consulting engineers and government authorities. The primary focus of this book is on interfacing structural elements with the underlying soil, which is, in the author’s opinion, where the major focus of shallow foundation engineering lies.

The book is not intended to be a specific text book on soil mechanics or geotechnical engineering. Therefore, there is no part of the text alone that could be used as a core syllabus for a certain course. However, it is the author’s opinion that more than 70% of the book is core material at the advanced undergraduate levels. It is expected that civil engineering students will find the text helpful in better understanding the fundamental concepts and their implications for the analysis and design of shallow foundations. The author tried to present the material such that separable topics and subtopics are covered in separate sections, with clear and unambiguous titles and subtitles. Thus, it would not be difficult for a university lecturer to draw up a personalised reading schedule, appropriate to his or her own course. It is hoped that the book can establish itself as an effective reference and a useful text in most of the engineering colleges and technical institutions.

Generally, the given material is of an advanced level and, therefore, it is assumed that the reader has a good understanding of basic statics and the mechanics of materials and has studied the basic principles of soil mechanics, lateral earth pressures and reinforced concrete. SI units are used throughout all the chapters and, therefore, the reader also needs to have sufficient background knowledge regarding the use of these units.

The book would be very beneficial to the reader, since it provides essential data for the design of shallow foundations under ordinary circumstances. The necessary background concepts and theories are generally presented clearly in concise forms of formulas or charts, and their applications are highlighted through solving a relatively large number of realistic problems. Moreover, the worked problems are of the types usually faced by civil engineers in practice and, therefore, the obtained information will be most valuable.

Generally, the subject matter is introduced here by first discussing the particular topic and then solving a number of pertinent objective problems that highlight the relevant theories, concepts and analysis methods. A list of crucial references is given at the end of each chapter. Thus, each chapter consists of three parts: discussions, problem solving and references.

The “discussion” part is presented in a clear and concise but precise manner, keeping in view the avoidance of unnecessary details. In some chapters, where the topics are of special difficulty, full guiding explanations are given; where the subject of study is simpler, less detailed treatment is provided.

The “problem solving” part gives a relatively comprehensive range of worked out problems to consolidate an understanding of the principles and illustrate their applications in simple practical situations. A total of 180 worked problems have been provided. The author’s academic and professional career has proved to him that geotechnical engineers and civil engineering students need to be well acquainted with the correct and effective use of the theoretical and empirical principles and formulas they have learned. An effective way to lessen the deficiency may be through solving, as much as possible, a variety of problems of the type or nearly similar to those engineers face in practice. For these reasons the author considers the “problem solving” part, on which the book is partially based, as a vital portion of the text.

The “references” part that comes directly at the end of each chapter enriches the discussions part with valuable sources of information and increases its reliability. Moreover, the furnished references will be very beneficial to any ambitious fresh civil engineer or undergraduate student who may wish later to undertake higher studies in the subject.

The text comprises six chapters. The chapters are devoted mostly to the geotechnical and structural aspects of shallow foundation design. A brief overview of each chapter follows:

Chapter 1

deals with site investigation in relation to the analysis and design of shallow foundations. Unlike the other chapters, this chapter requires various topics, field tests in particular, to be discussed separately. Therefore, only a general and relatively brief overview of the overall subject matter, consistent with the chapter title, is given in the main “discussion” part. Discussion individual topics is given in the “problem solving” part directly below the relevant problem statement. Solutions of 27 problems have been provided. These solutions and those of the other five chapters have fully worked out calculations.

Chapter 2

presents introductory discussions and explanations of various topics pertaining to shallow foundations, their analysis and design. It discusses type and depth of shallow foundations, performance requirements, sulfate and organic acid attack on concrete, distribution of contact pressures and settlements and vertical stress increase in soils due to foundation loads. Solutions of 21 problems are presented.

Chapter 3

concerns settlements due to foundation loads. The chapter discusses various types of settlements of foundations on both coarse- and fine-grained soils, methods of settlement estimation, methods of estimating and accelerating consolidation settlement, settlement due to secondary compression (creep), estimation of settlements over the construction period and settlement of foundations on rock. Solutions of 56 problems are introduced.

Chapter 4

deals with the bearing capacity of shallow foundations. The chapter discusses most of the significant aspects of the subject matter, among them: bearing capacity failure mechanism, bearing capacity equations, bearing capacity of footings with concentric and eccentric vertical loads, bearing capacity of footings with inclined loads, effects of water table and other factors on bearing capacity, uplift capacity of shallow foundations, bearing capacity of foundations on layered soils and on slopes, and bearing capacity of rock. Solutions of 40 problems are provided.

Chapter 5

deals with the structural design of different types of shallow foundations. Structural design of plain concrete spread footings, pedestals, pile caps and the foundations of earth-retaining concrete walls are also included. The discussion part of the chapter covers most of the major aspects of the subject matter such as: selection of materials, design loads, structural action of isolated and continuous footings, eccentrically loaded footings, modulus of subgrade reaction and beams on elastic foundations, rigid and flexible design methods and so on. Design calculations of 28 typical problems are presented in a step by step order. All the structural designs conform to the Building Code Requirements for Structural Concrete (ACI 318) and Commentary, USA.

Chapter 6

deals with Eurocode Standards in relation to the design of spread foundations. The discussion part of the chapter covers certain important topics such as: Eurocode background and applications, basis of design and requirements, principles of limit states design, design approaches, partial factors and load combinations, geotechnical and structural designs of spread foundations and so on. The problem solving part of the chapter provides design calculations of eight typical problems, as an attempt to introduce the concerned engineer to application of the design rules stipulated by Eurocodes 2 and 7 (or EN 1992 and 1997) rather than the geotechnical and structural related issues.

It is well known that not all civil engineers are acquainted with all internationally recognised codes, such as the ACI Code and Eurocodes, at the same time. Therefore, this chapter is especially written for those civil engineers, specially geotechnical engineers, who are unfamiliar with the technical rules and requirements of the Eurocode Standards (Structural Eurocode). The implementation of the Eurocode is extended to all the European Union countries and there are firm steps toward their international adoption.

It must be clear that, despite every care taken to ensure correctness or accuracy, some errors might have crept in. The author will be grateful to the readers for bringing such errors, if any, to his notice. Also, suggestions for the improvement of the text will be gratefully acknowledged.

Tharwat M. Baban

Acknowledgements

The author wishes to express his gratitude to those writers and researchers whose findings are quoted and to acknowledge his dependence on those authors whose works provided sources of material; among them are:

Giuseppe Scarpelli, Technical University of Marche Region, Ancona, Italy

Trevor L.L. Orr, Trinity College, Dublin, Ireland

Andrew J. Bond, Geocentrix Ltd, Banstead, UK

Andrew Harris, Kingston University, London, UK

Jenny Burridge, The Concrete Center, London, UK

The author is truly grateful to Dr. Paul Sayer, who was kind enough to suggest the inclusion of Chapter 6.

The author wishes to thank all the members of his family, especially his wife Mrs. Sawsan Baban, for their encouragement, assistance and patience in preparing the manuscript of this book.

Finally, the author likes to thank John Wiley and Sons, Ltd and their production staff for their cooperation and assistance in the final development and production of this book.

CHAPTER 1Site Investigation in Relation to Analysis and Design of Foundations

1.1 General

The stability and safety of a structure depend upon the proper performance of its foundation. Hence, the first step in the successful design of any structure is that of achieving a proper foundation design.

Soil mechanics is the basis of foundation design since all engineered constructions rest on the earth. Before the established principles of soil mechanics can be properly applied, it is necessary to have a knowledge of the distribution, types and engineering properties of subsurface materials. Therefore, an adequate site investigation is an essential preliminary to enable a safe and economic design and to avoid any difficulties during construction. A careful site investigation can minimise the need for overdesign and reduce the risks of underdesign. A designer who is well equipped with the necessary reliable information can use a lower factor of safety, thereby achieving a more economical design. With enough information available, construction troubles can be decreased and, therefore, construction costs are decreased too.

A site investigation usually costs a small percentage of total construction costs. According to Bowles (2001), elimination of the site exploration, which usually ranges from about 0.5 to 1.0% of total construction costs, only to find after construction has started that the foundation must be redesigned is certainly a false economy. However, a geotechnical engineer planning a subsurface exploration program for a specific job must keep in mind the relative costs of the exploration versus the total construction costs. It is understood that there is no hard and fast procedure for an economical planning a site investigation programme. Each condition must be weighed with good judgment and relative economy.

Nowadays, it is doubtful that any major structures are designed without site exploration being undertaken. Sometimes, for small jobs, it may be more economical to make the foundation design on conservative values rather than making elaborate borings and tests, especially, when the condition of the adjacent structures is an indication that the site is satisfactory. However, generally, design of structures without site investigation is not recommended by civil engineers.

The cheapest and most common method of subsurface exploration is by drilling boreholes. Test pits are too expensive for general exploration, but they may be used for more careful examination if found to be needed.

1.2 Site Investigation

A successful investigation of a site for an important structure will generally falls under the following five headings:

Reconnaissance

Subsurface exploration

Laboratory tests

Compiling information

Geotechnical report.

1.2.1 Reconnaissance

Office Reconnaissance: This phase of reconnaissance comprises the following duties:

Review of plans, boring logs and construction records of existing structures in the area.

Study of the preliminary plans and designs of the proposed structure, including the approximate magnitude of the loads to be transmitted to the supporting material.

Review of other backlogs of information already compiled on the same general area and similar structures.

Review of other information pertaining to the site area obtained from such sources as different types of maps (i.e., topographic, geologic and agricultural maps), photographs, records of adjacent bridge if exists, underground utility constructions and well drilling logs.

Formulation of a boring plan should be made during the latter phases of the office reconnaissance. This prepared boring plan should be reviewed during the field reconnaissance. The objective should be the development of a maximum of subsurface information through the use of a minimum number of boreholes. Spacing, number and depth of boreholes will be discussed later in conjunction with the Solution of Problem 1.4.

Field Reconnaissance: This phase of reconnaissance should commence with a visit to the site of the proposed structure. It should always be made by a Soils or Foundation Engineer who will complete the Geotechnical Report. Whenever possible, it is desirable that this engineer be accompanied by the driller or the driller foreman. Notes on items to be observed are as follows:

Surface Soils: Surface soils are easily revealed through the use of a shovel or post-hole diggers. These soils may sometimes be identified as belonging to some particular formation, and usually they indicate the underlying material.

Gullies, Excavations and Slopes: Any cut or hole near the proposed structure site is a subsurface window, and for its depth it will provide more information than borehole since it may be examined in detail.

Surface and Subsurface Water: The presence of either surface or subsurface water is an important factor in both preparation of boring plans and foundation design. All surface flows should be noted, and all opportunities should be taken to observe the groundwater level.

Study of Existing Structures: The existing structures within an area are valuable sources of information. A very close examination of them with regard to their performance, type of foundation, apparent settlement, load, location and age will yield a wealth of data.

Topography: To some extent, topography is indicative of subsurface conditions. In narrow, steep stream beds, rock is likely to be near the surface with little overlying stream-deposited soil. On the other hand, wide, flat valleys indicate deep soil deposits.

Information required by the Drill Crew. The drill crew needs to know how to get to the site, where to drill, what equipment to take, and what difficulties to expect. Generally, the following types of information are usually needed:

Information regarding verification of the boring plan which was already prepared during the office reconnaissance phase. The proposed locations of boreholes should be checked for accessibility. Desirable deletions, additions, and relocations should be made as are necessary to better suit the crew’s capabilities and to add completeness to the subsurface information.

Type of drilling and equipment needed. Notes should be made as to which type of drilling is best suited to the site (i.e., rotary, auger, etc.).

Reference points and bench marks. The reconnaissance should determine if reference points and bench marks are in place adjacent to the site and properly referenced on the plans.

Utilities. Underground and overhead utilities located at the site should be accurately shown on the plans or their locations should be staked on the ground.

Geophysical Survey: The field reconnaissance may require a geophysical survey of the site. The use of geophysical methods provides information on the depths to the

soil

and

rock layers

, the homogeneity of the layer and the type of soil or rock present. This information can be used to supplement the boring plan.

Field Reconnaissance Report: A concise and informative field reconnaissance report, in which all decisions concerning the boring plan and the drill crew are delineated, should be prepared. It can be facilitated by the use of a special check list or form.

1.2.2 Subsurface Exploration

General: After the reconnaissance has been completed, the results should be given to the foreman of the drill crew in order to carry out the foundation investigation at the site. Briefly, the subsurface or foundation exploration consists of making the borings and collecting the pertinent samples (i.e., drilling and sampling), performing the required field tests in conjunction with the drilling, coring, and identification of materials. Each job site must be studied and explored according to its subsurface conditions and the size and type of the proposed structure. The civil engineer or the experienced geologist in charge of the exploration task should endeavour to furnish complete data such that a reliable study of practical foundation types can be made.