brillante a#1 introduction to foundation engineering 2014-2015

8/10/2019 Brillante a#1 Introduction to Foundation Engineering 2014-2015

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Technological University of the Philippines

Ayala Boulevard, Ermita, Manila

College of Engineering

Department of Civil Engineering

CE 521-5A

Foundation Engineering, Lecture

Assignment No. 1

INTRODUCTION TO FOUNDATION ENGINEERING

Brillante, Ralph Kenneth V.

10205026

June 24, 2014

Engr. Jesus Ray M. Mansayon

Instructor


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I. INTRODUCTION

For engineering purposes, soil is defined as the uncemented aggregate ofmineral grains and decayed organic matter (solid particles) with liquid and gas in theempty spaces between the solid particles. Soil is used as a construction material invarious civil engineering projects, and it supports structural foundations. Thus, civilengineers must study the properties of soil, such as its origin, grain-size distribution,and ability to drain water, compressibility, shear strength, and load bearing capacity.Soil mechanics is the branch of science that deals with the study of the physicalproperties of soil and the behavior of soil masses subjected to various types of forces.

SOIL MECHANICS (Geotechnical Engineering: Principles and Practices of SoilMechanics and Foundation Engineering, by VNS Murthy, page 3)

Terzaghi defined Soil Mechanics as follows:

Soil Mechanics is the application of the laws of mechanics and hydraulics to

engineering problems dealing with sediments and other unconsolidated accumulationsof solid particles produced by the mechanical and chemical disintegration of rocksregardless of whether or not they contain an admixture of organic constituents.

GEOTECHNICAL ENGINEERING(The Mechanics of Soils and Foundation, 2nded. byJohn Atkinson, page 3)

Geotechnical engineering is simply the branch of engineering that deals withstructures built of, or in, natural soils and rocks. The subject requires knowledge ofstrength and stiffness of soils and rocks, methods of analyses of structures andhydraulics of groundwater flow. Use of natural soil and rock makes geotechnical

engineering different from many other branches of engineering and more interesting.

The distinction is that most engineers can select and specify the materials theyuse, but geotechnical engineers must use the materials that exist in the ground and theyhave only very limited possibilities for improving their properties. This means that anessential part of geotechnical engineering is a ground investigation to determine whatmaterials are present and what their properties are. Since soils and rocks were formedby natural geological processes, knowledge of geology is essential for geotechnicalengineering.

FOUNDATION ENGINEERING (Geotechnical Engineering: Principles and Practices of

Soil Mechanics and Foundation Engineering, by VNS Murthy, page 3)

The subject of Foundation Engineering deals with the design of various types ofsubstructures under different soil and environmental conditions.

During the design, the designer has to make use of the properties of soils, thetheories pertaining to the design and his own practical experience to adjust the designto suit field conditions. He has to deal with natural soil deposits which perform the


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engineering function of supporting the foundation and the superstructure above it. Soildeposits in nature exist in an extremely erratic manner producing thereby an infinitevariety of possible combinations which would affect the choice and design offoundations.

II. FOUNDATION ENGINEER

The foundation engineer must have the ability to interpret the principles of soilmechanics to suit the field conditions. The success or failure of his design dependsupon how much in tune he is with Nature.

Source :(Geotechnical Engineering: Principles and Practices of Soil Mechanics andFoundation Engineering, by VNS Murthy, page 3)

III. FOUR PERFORMANCE REQUIREMENTS

Strength Requirements

Once the design of the loads has been defined, we need to develop foundationdesigns that satisfy several performance requirements. The first category is strengthrequirements, which are intended to avoid catastrophic failures. There are two types:geotechnical strength requirements and structural strength requirements.

Geotechnical Strength Requirements

Geotechnical strength requirements are those that address the ability of the soilor rock to accept the loads imparted by the foundation without failing. The strength of

soil is governed by its capacity to sustain shear stress, so we satisfy geotechnicalstrength requirements by comparing shear stresses with shear strengths and designingaccordingly.

In the case of spread footing foundations, geotechnical strength is expressed asthe bearing capacity of the soil. If the load-bearing capacity of the soil is exceeded, theresulting shear failure is called bearing capacity failure


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Structural Strength Requirements

Structural strength requirements address the foundations structural integrity andits ability to safely carry the applied loads. For example, pile foundations made from A36steel are normally designed for a maximum allowable compressive stress of 12,600

lb/in2. Thus, the thickness of the steel must be chosen such that the stresses induce bythe design loads do not exceed this allowable value. Foundations that are loadedbeyond their structural capacity will, in principle, fail catastrophically.

Structural strength analyses are conducted using their ASD or LRFD methods,depending on the types of foundation, the structural materials, and the governing code.

Serviceability Requirements

Foundations that satisfy strength requirements will not collapse, but they still may

not have adequate performance. For example, they may experience excessivesettlement. Therefore, we have the second category of performance requirements,which are known as serviceability requirements. These are intended to producefoundations that perform well when subjected to service loads. These requirementsinclude:

Settlement Most foundations experience some downward movement as aresult of the applied loads. This movement is called settlement. Keepingsettlements within tolerable limits is usually the most important foundationserviceability requirement.

HeaveSometimes foundations move upward instead of downward. We call this

upward movement heave. The most common source of heave is the swelling ofexpansive soils.

TiltWhen settlement or heave occurs only on one side of the structure, it maybegin to tilt. The Leaning Tower of Pisa is an extreme example of tilt.

Lateral movementSome foundations, such as those supporting certain kinds ofheavy machinery, are subjected to strong vibrations. Such foundations need toaccommodate these vibrations without experiencing resonance or otherproblems.

DurabilityFoundations must be resistant to the various physical, chemical, andbiological processes that cause deterioration. This is especially important inwaterfront structures, such as docks and piers.

Failure to satisfy these requirements generally results in increased maintenancecosts, aesthetic problems, diminished usefulness of the structure, and other similareffects.


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Settlement

The vertical downward load is usually the greatest load acting on foundations,and the resulting vertical downward movement is usually the largest and most importantmovement. We call this vertical downward movement settlement. Sometimes settlement

also occurs as a result of other causes unrelated to the presence of the foundation,such as consolidation due to the placement of a fill.

Although foundations with zero settlement would be ideal, this is not anattainable goal. Stress and strain always go together, so the imposition of load from thefoundation always causes some settlement in the underlying soils. Therefore, thequestion that faces the foundation engineer is not is the foundation will settle, but ratherdefining the amount of settlement that would be tolerable and designing the foundationto accommodate this requirement. This design process is analogous to that beamswhere the deflection must not exceed some maximum tolerable value.

Structural Response to Settlement

Structures can settle in many different ways. Sometimes the settlement isuniform, so the entire structure moves down as a unit. In this case, there is no damageto the structure itself, but there may be problems with its interface with the adjacentground or with other structures. Another possibility is settlement that varies linearlyacross the structure. This causes the structure to tilt.

The response of structures to foundation settlement is very complex, and acomplete analysis would require consideration of many factors. Such analyses would bevery time consuming, are thus not practical for the vast majority of structures. Therefore,we simplify the problem by describing settlement using only two parameters: totalsettlement and differential settlement.

Total Settlement

The total settlement, , is the change in foundation elevation from the originalunloaded position to the final loaded position.Structures that experience excessive total settlements might have some of the followingproblems:

a. Connections with existing structures Sometimes buildings must join existingstructures. In such cases, the floors in the new building must be at the same elevationas those in the existing building. However, if the new building settles excessively, thefloors will no longer match, causing serious serviceability problems.

b. Utility lines Buildings, tanks, and many other kinds of structures are connected tovarious utilities, many of which are located underground. If the structure settlesexcessively, the utility connections can be sheared or distorted. This is especiallytroublesome with gravity flow lines, such as sewers.


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c. Surface drainageThe ground floor of buildings must be at a slightly higher elevationthan the surrounding ground so rainwater does not enter. However, settlement mightdestroy these drainage patterns and cause rainwater to enter the structure.

d. AccessVehicles and pedestrians may need to access the structure, and excessivesettlement might impede them.

e. AestheticsExcessive settlement may cause aesthetic problems long before there isany threat to structural integrity or serviceability.

Some structures have sustained amazingly large total settlements, yet remain inservice. For example, many buildings have had little or no ill effects even after settlingas much as 250 mm (10 in.). Others have experienced some distress, but continue tobe used following even greater settlements. Some of the most dramatic examples arelocated in Mexico City, where buildings have settled more than 2 m (7 ft.) and are still in

use. However, these are extreme examples. Normally engineers have much stricterperformance requirements.

Constructability Requirements

The third category of performance requirements is constructability. Thefoundation must be designed such that a contractor can build it without having to useextraordinary methods or equipment. There are many potential designs that might bequite satisfactory from a design perspective, but difficult or impossible to build.There are different types of deep foundations. One of these, a pile foundation, consistsof a prefabricated pole that is driven into the ground using a modified crane called a piledriver. The pile driver lifts the pile into the air, and then drives it into the ground.Therefore, piles can be installed only at locations that have sufficient headroom.Fortunately, the vast majority of construction sites have plenty of headroom.

Nevertheless, a design engineer who is not familiar with this constructionprocedure might ask for piles to be installed at a location with minimal headroom KarlTerzaghi expressed this concept very succinctly when he said: Do not design on paperwhat you have to wish into the ground.

This is why it is important for design engineers to have at least a rudimentaryunderstanding of construction.

Economic Requirements

Foundation designs are usually more conservative than those in thesuperstructure. This approach is justified for the following reasons:


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a. Foundation designs rely on our assessments of the soil and rock conditions. Theseassessments always include considerable uncertainty.

b. Foundations are not built with the same degree of precision as the superstructure.

For example, spread footings are typically excavated with a backhoe and the sides ofthe excavation becomes the formwork for the concrete, compared to concretemembers in the superstructure that are carefully formed with plywood or other materials.

c. The structural materials may be damaged when they are installed. For example,cracks and splits may develop in a timber pile during hard driving.

d. There is some uncertainty in the nature and distribution of the load transfer betweenfoundations and the ground, so the stresses at any point in a foundation are not alwaysknown with as much certainty as might be the case in much of the superstructure.

e. The consequences of a catastrophic failure are much greater.

f. The additional weight brought on by the conservative design is of no consequence,because the foundation is the lowest structural member and therefore does not affectthe dead load on any other member. Additional weight in the foundation is actuallybeneficial in that it increases its uplift resistance.

However, gross over conservatism is not warranted. An overly conservativedesign can be very expensive to build, especially with large structures where thefoundation is a greater portion of the total project cost. This also is a type of failure: thefailure to produce an economical design.

The nineteenth-century engineer Arthur Wellington once said that an engineersjob is that of doing well with one dollar which any bungler can do with two. We muststrive to produce designs that are both safe and cost-effective. Achieving the optimumbalance between reliability (safety) and cost is part of good engineering.

Designs that minimize the required quantity of construction materials do notnecessarily minimize the cost. In some cases, designs that use more materials may beeasier to build, and thus have a lower overall cost. For example, spread footingfoundations are usually made of low-strength concrete, even though it makes themthicker. In this case, the savings in materials and inspection costs are greater than thecost of buying more concrete.

Source:http://infohost.nmt.edu/~Mehrdad/foundation/hdout/PerformanceRequirements.pdf, page 24-43


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IV. GENERAL TYPES OF FOUNDATIONS

The various types of structural foundations may be grouped into two broadcategoriesshallow foundations and deep foundations. The classification indicates thedepth 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 lessthan its width and deep when it exceeds the width.

Classification of Shallow and Deep Foundation

The floating foundation, a special category, is not actually a different type, but itrepresents a special application of a soil mechanics principle to a combination of raft-caisson foundation.


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Spread foot ing s

Spread footing foundation is basically a pad used to spread out loads fromwalls or columns over a sufficiently large area of foundation soil. These are constructedas 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 forpermanent structures are rarely located directly on the ground surface. A spread footingneed not necessarily be at small depths; it may be located deep in the ground if the soilconditions or design criteria require. Spread footing required to support a wall is knownas a continuous, wall, or strip footing, while that required to support a column is knownas 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.If the footing supports more than one column or wall, it will be a strap footing, combinedfooting or a raft foundation. Two miscellaneous typesthe monolithic footing, used for

watertight basement (also for resisting uplift), and the grillage foundation, used forheavy loads are also shown.

Typees of Spread Footing

Strap foot ings

A strap footing comprises two or more footings connected by a beam calledstrap. This is alsocalled a cantilever footing orpump-handle foundation. This may be


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required when the footing of an exterior column cannot extend into an adjoining privateproperty.

Combined foot ings

A combined footing supports two or more columns in a row when the areasrequired for individual footings are such that they come very near each other. They arealso preferred in situations of limited space on one side owing to the existence of theboundary line of private property. The plan shape of the footing may be rectangular ortrapezoidal; the footing will then be called rectangular combined footingor trapezoidalcombined footing, as the case may be.

Raft fou ndations (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 footingswould tend to be too close or tend to overlap; further, this is considered suitable whendifferential settlements arising out of footings on weak soils are to be minimized.

Common Arrangement of Strap Beams to Strap Footings


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Combined Footings

Deep foot ing s

According to Terzaghi, if the depth of a footing is less than or equal to the width,it may be considered a shallow foundation. However, if the depth is more, the footingsare considered as deep footing; Meyerhof (1951) developed the theory of bearingcapacity for such footings.

Pi le found at ions

Pile foundations are intended to transmit structural loads through zones of poorsoil to a depth where the soil has the desired capacity to transmit the loads. They aresomewhat similar to columns in that loads developed at one level are transmitted to alower level; but piles obtain lateral support from the soil in which they are embedded sothat there is no concern with regard to buckling and, it is in this respect that they differfrom columns. Piles are slender foundation units which are usually driven into place.They may also be cast-in-place. 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 abatter.

Pier found at ions

Pier foundations are somewhat similar to pile foundations but are typically largerin area than piles. An opening is drilled to the desired depth and concrete is poured tomake a pier foundation. Much distinction is now being lost between the pile foundationand pier foundation, adjectives such as driven, bored, or drilled, and precast andcast-in-situ, being used to indicate the method of installation and construction. Usually,pier foundations are used for bridges.


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Caisson s (Wells)

A caisson is a structural box or chamber that is sunk into place or built in placeby systematic excavation below the bottom. Caissons are classified as opencaissons,pneumaticcaissons, and boxor floatingcaissons. Open caissons may be box-type

of pile-type. The top and bottom are open during installation for open caissons. Thebottom 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 fromentering the working chamber, the top of the caisson is closed. Excavation andconcreting is facilitated to be carried out in the dry. The caisson is sunk deeper as theexcavation proceeds and on reaching the final position, the working chamber is filledwith concrete.

Box or floating caisson is one in which the bottom is closed. It is cast on land andtowed 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 ortemporary bases of timber are sometimes used for floating the caisson to the site.

Types of Caissons

Float ing foun dat ion

The floating foundation is a special type of foundation construction useful inlocations where deep deposits of compressible cohesive soils exist and the use of pilesis impractical. The concept of a floating foundation requires that the substructure beassembled as a combination of a raft and caisson to create a rigid box

This foundation is installed at such a depth that the total weight of the soilexcavated for the rigid box equals the total weight of the planned structure. Theoretically


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speaking, therefore, the soil below the structure is not subjected to any increase instress; consequently, no settlement is to be expected. However, some settlement doesoccur usually because the soil at the bottom of the excavation expands after excavationand gets recompressed during andafter construction.

Rigid Foundation Caisson using Floating Foundation Concept

Source: Geotechnical Engineering by C. Venkatramaiah, pages 607-613

Deep Foundation

Shallow foundations are normally used where the soil close to the ground surfaceand up to the zone of significant stress possesses sufficient bearing strength to carrythe superstructure load without causing distress to the superstructure due to settlement.However, where the top soil is either loose or soft or of a swelling type the load from thestructure has to be transferred to deeper firm strata.

The structural loads may be transferred to deeper firm strata by means of piles.Piles are long slender columns driven, bored or cast-in-situ. Driven piles are made of avariety of materials such as concrete, steel, timber etc., whereas cast-in-situ piles areconcrete piles. They may be subjected to vertical or lateral loads or a combination ofvertical and lateral loads. If the diameter of a bored-cast-in-situ pile is greater than about0.75 m, it is sometimes called a drilled pier, drilled caisson or drilled shaft. The


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distinction made between a small diameter bored cast-in-situ pile (less than 0.75 m) anda larger one is just for the sake of design considerations. This chapter is concerned withdriven piles and small diameter bored cast-in-situ piles only.

Source: Geotechnical Engineering: Principles and Practices of Soil Mechanics andFoundation Engineering, by VNS Murthy, page 605

brillante a#1 introduction to foundation engineering 2014-2015

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