carbonel a#1 introduction to foundation engineering 2014-2015.pdf

8/10/2019 Carbonel a#1 Introduction to Foundation Engineering 2014-2015.PDF

http://slidepdf.com/reader/full/carbonel-a1-introduction-to-foundation-engineering-2014-2015pdf 1/19



FOUNDATION ENGINEERING, SOIL MECHANICS, AND

GEOTECHNICAL ENGINEERING

F O U N D A T I O N E N G I N E E R I N G

Foundation Engineering is the category of engineering concerned with

evaluating the ability of a locus to support a given structural load, and with

designing the substructure or transition member needed to support the

construction.

Source: http://www.dictionaryofconstruction.com/definition/foundation-

engineering.html

Foundation Engineering is the engineering field of study devoted to the design

of those structures which support other structure, 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.

Source: en.wikipidia.org/wiki/Foundation_engineering

That branch of engineering concerned with evaluating the earth's ability to

support a load and designing substructures to transmit the load of superstructures

to the earth.

Source: http://www.answers.com/topic/foundation-engineering

S O I L M E C H A N I C S

Soil Mechanics is the branch of science that deals with the study of the physical

properties of soil and the behaviour of soil masses subjected to various types of

forces.

Source: PRINCIPLES OF GEOTECHNICAL ENGINEERING, 7 th Edition by BMDas, page 1

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. Along with rock mechanics, soil

mechanics provides the theoretical basis for analysis in geotechnical engineering,



a subdiscipline of Civil engineering, and engineering geology, a subdiscipline 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. Example applications are building and bridge

foundations, retaining walls, dams, and buried pipeline systems. Principles of soilmechanics are also used in related disciplines such as engineering geology,

geophysical engineering, coastal engineering, agricultural engineering, hydrology,

and soil physics.

Source: http://en.wikipedia.org/wiki/Soil_mechanics

G E O T E C H N I C AL E N G I N E E R I N G

The use of engineering soils and rocks in construction is older than history and

no other materials, except timber, were used until about 200 years ago when an

iron bridge was built by Abraham Darby in Coalbrookdale. Soils and rocks are still

one of the most important construction materials used either in their natural state in

foundations or excavations or recompacted in dams and embankments.

Engineering soils are mostly just broken up rock, which is sometimes decomposed

into clay, so they are simply collections of particles. Dry sand will pour like water

but it will form a cone, and you can make a sandcastle and measure its

compressive strength as you would a concrete cylinder. Clay behaves more like

plasticine or butter. If the clay has a high water content it squashes like warmbutter, but if it has a low water content it is brittle like cold butter and it will fracture

and crack. The mechanics that govern the stability of a small excavation or a small

slope and the bearing capacity of boots in soft mud are exactly the same as for

large excavations and foundations.

Many engineers were first introduced to civil engineering as children building

structures with Meccano or Lego or with sticks and string. They also discovered

the behaviour of water and soil. They built sandcastles and they found it was

impossible to dig a hole in the beach below the water table. At home they played

with sand and plasticine. Many of these childhood experiences provide the

experimental evidence for theories and practices in structures, hydraulics and soil

mechanics. I have suggested some simple experiments which you can try at

home. These will illustrate the basic behaviour of soils and how foundations and

excavations work. As you work through the book I will explain your observations

and use these to illustrate some important geotechnical engineering theories and

analyses.



In the ground soils are usually saturated so the void spaces between the grains

are filled with water. Rocks are really strongly cemented soils but they are often

cracked and jointed so they are like soil in which the grains fit very closely

together. Natural soils and rocks appear in other disciplines such as agriculture

and mining, but in these cases their biological and chemical properties are more

important than their mechanical properties. Soils are granular materials andprinciples of soil mechanics are relevant to storage and transportation of other

granular materials such as mineral ores and grain.

Geotechnical engineering is simply the branch of engineering that deals with

structures built of, or in, natural soils and rocks. The subject requires knowledge of

strength and stiffness of soils and rocks, methods of analyses of structures and

hydraulics 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 thatmost engineers can select and specify the materials they use, but geotechnical

engineers must use the materials that exist in the ground and they have only very

limited possibilities for improving their properties. This means that an essential part

of geotechnical engineering is a ground investigation to determine what materials

are present and what their properties are. Since soils and rocks were formed by

natural geological processes, knowledge of geology is essential for geotechnical

engineering.

Source: THE MECHANICS OF SOILS AND FOUNDATION, 2 nd Edition by

Atkinson, page 1-3

Geotechnical Engineering is the subdiscipline of civil engineering that involves

natural materials found close to the surface of the earth. It includes the application

of the principles of soil mechanics and rock mechanics to the design of

foundations, retaining structures, and earth structures.

Source: PRINCIPLES OF GEOTECHNICAL ENGINEERING, 7 th Edition by BM

Das, page 1



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 isstrength requirements, which are intended to avoid catastrophic failures. Thereare two types: geotechnical strength requirements and structural strengthrequirements. The design of foundations of structures such as buildings, bridges,and dams generally requires a knowledge of such factors as

a) the load that will be transmitted by the superstructure to the foundationsystem,

b) the requirements of the local building code,c) the behaviour and stress-related deformability of soils that will support the

foundation system, andd) the geological conditions of the soil under consideration.To a foundation engineer, the last two factors are extremely important because

they concern soil mechanics.

Serviceability Requirements

Foundations that satisfy strength requirements will not collapse, but they still maynot 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 areintended to producefoundations that perform well when subjected to service loads. Theserequirementsinclude:

Settlement – Most foundations experience some downward movementas a result of the applied loads. This movement is called settlement.Keeping

settlements within tolerable limits is usually the most important foundationserviceability requirement.

Heave – Sometimes foundations move upward instead of downward. We

call this upward movement heave. The most common source of heave isthe swelling of expansive soils.

Tilt – When settlement or heave occurs only on one side of the structure, itmay begin to tilt. The Leaning Tower of Pisa is an extreme example of tilt.

Lateral movement – Some foundations, such as those supporting certainkinds of heavy machinery, are subjected to strong vibrations. Suchfoundations need to accommodate these vibrations without experiencingresonance or other problems.



Durability – Foundations must be resistant to the various physical,chemical, and biological processes that cause deterioration. This isespecially important in waterfront structures, such as docks and piers.

Constructability Requirements

The third category of performance requirements is constructability. The foundationmust be designed such that a contractor can build it without having to useextraordinary methods or equipment. There are many potential designs that mightbe quite satisfactory from a design perspective, but difficult or impossible to build.There are different types of deep foundations. One of these, a pile foundation,consists of a prefabricated pole that is driven into the ground using a modifiedcrane called a pile driver. The pile driver lifts the pile into the air, and thendrives it into the ground.Therefore, piles can be installed only at locationsthat have sufficient headroom. Fortunately, the vast majority of construction sites

have plenty of headroom.

Economic Requirements

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

a. Foundation designs rely on our assessments of the soil and rock conditions.These assessments 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 sidesof the excavation becomes the ―formwork for the concrete, compared toconcrete members in the superstructure that are carefully formed with plywood orother materials.

c. The structural materials may be damaged when they are installed. Forexample, 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 transferbetween foundations and the ground, so the stresses at any point in a foundation

are not always known with as much certainty as might be the case in much of thesuperstructure.

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

f. The additional weight brought on by the conservative design is of noconsequence, because the foundation is the lowest structural member andtherefore does not affect the dead load on any other member. Additional weight inthe foundation is actually beneficial in that it increases its uplift resistance.



Foundations are designed to have an adequate load capacity with limitedsettlement by a geotechnical engineer, and the footing itself may be designedstructurally by a structural engineer.

The primary design concerns are settlement and bearing capacity. Whenconsidering settlement, total settlement and differential settlement is normally

considered. Differential settlement is when one part of a foundation settles more

than another part. This can cause problems to the structure the foundation is

supporting.

Source: http://en.wikipedia.org/wiki/Foundation_(engineering)

The design of foundation requires the consideration of many essential factors

with regard to soil data, geology of the site, land use patterns, ground conditions

and the type of structure to be built. A detailed understanding of the field situation

is also very important apart from theoretical knowledge of the subject. This course

seeks to provide an overview of the essential features of foundation design. The

different aspects of foundation engineering ranging from soil exploration to the

design of different types of foundation, including the ground improvement

measures to be taken for poor soil conditions have been covered in this course.

Source:http://www.cdeep.iitb.ac.in/nptel/Civil%20Engineering/Foundation_Enginee

ring/Course%20Objective.html

Foundation design must support different kinds of loads, such as dead load, live

loads, rain and snow loads, wind loads, seismic loads etc. Foundation should be

so designed that it must satisfactorily meet building requirements.

The loads that a building foundation should support are:

Dead Load

Dead load is the combined weight of all the permanent components of the

building, including its own structural frame, floors, roofs, and walls, major

permanent electrical and mechanical equipment, and the foundation itself.

Live Loads

Live loads are non-permanent loads caused by the weights of the building’s

occupants, furnishings, and movable equipment.

Rain and snow loads

This load is one which acts primarily downward on building roofs.



Wind Loads

Wind loads acts laterally (sideways), downward or upward on a building. It is

based on local wind speed.

Seismic loads Seismic loads are horizontal or vertical forces caused by the motion of the

ground, relative to the building during an earthquake.

Other loads

Other loads include load caused by soil and hydrostatic pressure, including

lateral soil pressure loads, horizontal pressures of earth and groundwater against

basement walls, in some instances, buoyant uplift forces from the underground

water identical to the forces that cause a boat to float, and lateral force flood loads

that can occur in areas prone to flooding.

In some buildings, horizontal thrusts from long span structural systems such as

arches, rigid frames, domes, vaults, or tensile structures also acts on foundation



thicker for heavier structures.

Deep foundations A deep foundation is used to transfer the load of a structure down through the

upper weak layer of topsoil to the stronger layer of subsoil below. There aredifferent types of deep footings including impact driven piles, drillesshafts,cassions, helical piles, geo-piers and earth stabilized columns.The naming

conventions for different types of footings vary between different engineers.

Monopile foundation A monopile foundation is a type of deep foundation which uses a single,

generally large-diameter, structural element embedded into the earth to support allthe loads (weight, wind, etc.) of a large above-surface structure.

A large number of monopile foundations have been utilized in recent years foreconomically constructing fixed-bottom offshore wind farms in shallow-watersubsea locations. For example, a single wind farm off the coast of England wentonline in 2008 with over 100 turbines, each mounted on a 4.7-meter-diameter

monopile footing in ocean depths up to 18 metres of water.

Source: en.wikipedia.org/wiki/Foundation_(engineering)

When determining which foundation is the most economical, the engineer mustconsider the superstructure load, the subsoil conditions, and the desired tolerablesettlement. In general, foundations of buildings and bridges may be divided intotwo major categories:

1. shallow foundations and2. deep foundations.

Source: PRINCIPLES OF FOUNDATION ENGINEERING, 7 th

Edition by BM Das, page 1

TYPES OF SHALLOW FOUNDATION

1. Strip Footing: A strip footing is provided for a load-bearing wall. A strip footing is also provided

for a row of columns which are so closely spaced that their spread footings overlapor nearly touch each other. In such a case, it is more economical to provide a strip

footing than to provide a number of spread footings in one line. A strip footing isalso known as continuous footing.



2. Spread or Isolated Footing: A spread footing (or isolated or pad) footing is provided to support an individual

column. A spread footing is circular, square or rectangular slab of uniformthickness. Sometimes, it is stepped or haunched to spread the load over a largearea.



3. Combined Footing: A combined footing supports two columns. It is used when the two columns are

so close to each other that their individual footings would overlap. A combinedfooting is also provided when the property line is so close to one column that aspread footing would be eccentrically loaded when kept entirely within the property

line. By combining it with that of an interior column, the load is evenly distributed. Acombined footing may be rectangular or trapezoidal in plan.

4. Strap or Cantilever footing: A strap (or cantilever) footing consists of two isolated footings connected with a

structural strap or a lever. The strap connects the two footings such that they

behave as one unit. The strap is designed as a rigid beam. The individual footingsare so designed that their combined line of action passes through the resultant ofthe total load. a strap footing is more economical than a combined footing whenthe allowable soil pressure is relatively high and the distance between the columnsis large.



5. Mat or Raft Foundations: A mat or raft foundation is a large slab supporting a number of columns and

walls under the entire structure or a large part of the structure. A mat is requiredwhen the allowable soil pressure is low or where the columns and walls are soclose that individual footings would overlap or nearly touch each other.

Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is a large variation in the loads on individualcolumns.

TYPES OF DEEP FOUNDATION

Deep foundations are required to carry loads from a structure through weakcompressible soils or fills on to stronger and less compressible soils or rocks atdepth, or for functional reasons. These foundations are those founding too deeply

below the finished ground surface for their base bearing capacity to be affected bysurface conditions, this is usually at depths >3 m below finished ground level.Deep foundations can be used to transfer the loading to a deeper, more competentstrata at depth if unsuitable soils are present near the surface.

The types of deep foundations in general use are as follows:1. Basements2. Buoyancy rafts (hollow box foundations)3. Caissons4. Cylinders5. Shaft foundations

6. Piles

Basement foundations: These are hollow substructures designed to provide working or storage space

below ground level. The structural design is governed by their functionalrequirements rather than from considerations of the most efficient method ofresisting external earth and hydrostatic pressures. They are constructed in place inopen excavations.

Buoyancy rafts (hollow box foundations) Buoyancy rafts are hollow substructures designed to provide a buoyant or semi-

buoyant substructure beneath which the net loading on the soil is reduced to thedesired low intensity. Buoyancy rafts can be designed to be sunk as caissons, theycan also be constructed in place in open excavations.

Caissons foundations: Caissons are hollow substructures designed to be constructed on or near the

surface and then sunk as a single unit to their required level.



Cylinders: Cylinders are small single-cell caissons.

Shaft foundations: Shaft foundations are constructed within deep excavations supported by liningconstructed in place and subsequently filled with concrete or other pre-fabricatedload-bearing units



Pile foundations:

Pile foundations are relatively long and slender members constructed by drivingpreformed units to the desired founding level, or by driving or drilling-in tubes to therequired depth – the tubes being filled with concrete before or during withdrawal orby drilling unlined or wholly or partly lined boreholes which are then filled withconcrete.

Source: http://theconstructor.org/geotechnical/



Soil properties

Some of the important properties of soils that are used by geotechnical engineers

to analyze site conditions and design earthworks, retaining structures, and

foundations are: Unit Weight

Total unit weight: Cumulative weight of the solid particles, water and air inthe material per unit volume. Note that the air phase is often assumed to beweightless.

PorosityRatio of the volume of voids (containing air, water, or other fluids) in a soil tothe total volume of the soil. A porosity of 0 implies that there are no voids inthe soil.

Void ratiois the ratio of the volume of voids to the volume of solid particles in a soil.Void ratio is mathematically related to the porosity.

Permeability A measure of the ability of water to flow through the soil, expressed in unitsof velocity.

CompressibilityThe rate of change of volume with effective stress. If the pores are filled withwater, then the water must be squeezed out of the pores to allow volumetriccompression of the soil; this process is called consolidation.

Shear strengthThe shear stress that will cause shear failure.

Atterberg LimitsLiquid limit, plastic limit, and shrinkage limit. These indices are used forestimation of other engineering properties and for soil classification.

Source: https://en.wikipedia.org/wiki/Geotechnical_engineering



IGNIF ICANCE OF FOUNDATION ENGINEERING

Foundation Engineering is an important component of any construction project.

The structural loads of buildings, bridges, towers, and other civil engineering works

must be transmitted to the underlying natural soil or rock material using a

foundation system that is safe, stable, and economical. The course provides

participants with the necessary geotechnical engineering skills to analyze shallow

and deep foundation systems under different loading conditions.

Source:http://www.mcgill.ca/continuingstudies/programs-and-courses/engineering-

0/engineeringce/foundation

• • • • • •EN D • • • • • •

carbonel a#1 introduction to foundation engineering 2014-2015.pdf

Documents