Lecture-1 Introduction to Foundations.pdf

7/27/2019 Lecture-1 Introduction to Foundations.pdf

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CE F313

FOUNDATION ENGINEERING

Department of Civil EngineeringBITS Pilani-Hyderabad Campus

INSTRUCTOR INCHARGEJ S KALYANA RAMA


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Objectives of the Course

The main objectives of the Course Foundation Engineering are

Provide an in-depth understanding regarding different types of foundations systems

for various structures and machines subjected to static and dynamic loads.

Analyze different types of foundation systems Spread, Combined, Raft, Ring, Pile,

Machine, Retaining Structures.

Understand various techniques adopted for ground improvement

Understand the suitable strengthening measures for existing and new foundations.

Computer Applications and case histories in the field of foundation engineering

Coverage of Indian Standard Codes of practice for various types of foundations and

retaining structures

CE F313-Foundation Engineering: Mr. J S Kalyana Rama 2


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TEXTBOOK

Murthy, V. N. S. Geotechnical

Engineering: Principles and

Practices of Soil Mechanics and

Foundation Engineering, CRCPress, Taylor & Francis Group, First

Indian Reprint, 2010



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REFERENCE BOOKS

Design Aids In Soil Mechanics

and Foundation EngineeringAuthor: Shenbaga Kaniraj

Tata Mc-Graw Hill Publications

Geotechnical Engineering

Author: Manoj Datta, S GulhatiTata Mc-Graw Hill Publications

CRC Press

Analysis and design of

foundations and retainingstructures subjected to

seismic loads

Author: Swami Saran

I K Publishers

Indian Standard Codes as

mentioned in the

Handout-2



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INTRODUCTION TO FOUNDATION ENGINEERING

Foundation is the art of selecting, designing, and constructing the elements that transfer the

weight (Weight may also include horizontal loads in addition to vertical loads) of a structure

to the underlying soil or rock. It also acts as an interface element between the superstructure

and the underlying soil or rock. The loads transmitted by the foundation to the underling soil

must not cause soil shear failure or damaging settlement of the superstructure.

The role of engineer is to select the type of foundation, its design and supervision of

construction. Before the engineer can design a foundation intelligently, he must have a reasonably accurate

conception of the physical properties and the arrangement of the underlying materials. This

requires detailed soil explorations.

The term "foundation engineering" is used to include the design of foundations for buildings

and other structures and also for such non foundation problems as designs of retaining walls,bulkheads, cofferdams, tunnels, and earth dams, as well as the design of natural slopes,

dewatering of soils, and stabilization of soils mechanically and chemically.



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construction site

Geo-Laboratory

~ for testing

Design Office

~ for design & analysissoil properties



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REQUIREMENTS OF FOUNDATIONS:

1. Economical

2. Adequate Safety Bearing Capacity, Sliding, Overturning

3. Settlements Total and Differential Settlements

4. Seasonal changes drying, frost and heave

5. Constructional Problems Stability of excavation, bottom heave,

ground water problems, vibrations and

noise.

6. Environmental Effects Permanent lowering of ground water

level



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General Requirements of Foundation

Location and Depth of Foundation

Bearing Capacity of Foundation

Settlement of Foundation



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Location of Foundation

The following considerations are necessary for deciding the location and depth of

foundationo As per IS:1904-1986, minimum depth of foundation shall be 0.50 m.

o Foundation shall be placed below the zone of

The frost heave

Excessive volume change due to moisture variation (usually exists within 1.5 to

3.5 m depth of soil from the top surface)

Topsoil or organic material

Peat and Muck

Unconsolidated material such as waste dump



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o Foundations adjacent to flowing water (flood water, rivers, etc.) shall be protected

against scouring. The following steps to be taken for design in such conditions

Determine foundation type

Estimate probable depth of scour, effects, etc. Estimate cost of foundation for normal and various scour conditions

Determine the scour versus risk, and revise the design accordingly



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Depth Of Foundation

The depth to which foundations should be carried depends upon the following principalfactors:

The securing of adequate allowable bearing capacity.

In the case of clayey soils, penetration below the zone where shrinkage and swelling due to

seasonal weather changes, and due to trees and shrubs are likely to cause appreciable

movements. In fine sands and silts, penetration below the zone in which trouble may be expected from

frost.

The maximum depth of scour, wherever relevant, should also be considered and the

foundation should be located sufficiently below this depth.

Other factors such as ground movements and heat transmitted from the building to the

supporting ground may be important.



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All foundations shall extend to a depth of at least 50 cm below natural ground level. On rock

or such other weather resisting natural ground, removal of the top soil may be all that is

required. In such cases, the surface shall be cleaned and, if necessary, stepped or otherwiseprepared so as to provide a suitable bearing and thus prevent slipping or other unwanted

movements.

Where there is excavation, ditch, pond, water course, filled up ground or similar condition

adjoining or adjacent to the subsoil on which the structure is to be erected and which is likely

to impair the stability of structure, either the foundation of such structure shall be carried

down to a depth beyond the detrimental influence of such conditions, or retaining walls orsimilar works shall be constructed for the purpose of shielding from their effects.

A foundation in any type of soil shall be below the zone significantly weakened by root holes

or cavities produced by burrowing animals or works. The depth shall also be enough to

prevent the rainwater scouring below the footings.

Clay soils, like black cotton soils, are seasonally affected by drying, shrinkage and cracking in

dry and hot weather, and by swelling in the following wet weather to a depth which will varyaccording to the nature of the clay and the climatic condition of the region. It is necessary in

these soils, either to place the foundation bearing at such a depth where the effects of seasonal

changes are not important or to make the foundation capable of eliminating the undesirable

effects due to relative movement by providing flexible type of construction or rigid

foundations. Adequate load counteraction swelling pressures also provide satisfactory

foundations.



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FOUNDATION AT DIFFERENT LEVELS

Where footings are adjacent to sloping ground or where the bottoms of the footings of a structure

are at different levels or at levels different from those ,of the footings of adjoining structures, the

depth of the footings shall be such that the difference in footing elevations shall be subject to thefollowing limitations:

When the ground surface slopes downward adjacent to a footing, the sloping surface shall not

intersect a frustum of bearing material under the footing having sides which make an angle of

30 with the horizontal for soil and horizontal distance from the lower edge of the footing to

the sloping surface shall be at least 60 cm for rock and 90 cm for soil ( see Fig. 1 ).

Fig:1 Footing in Sloping ground


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In the case of footings in granular soil, a line drawn between the lower adjacent edges of

adjacent footings shall not have a steeper slope than one vertical to two horizontal (see Fig.

2a).

In case of footing of clayey soils a line drawn between the lower adjacent edge of the upper

footing and the upper adjacent edge of lower footing shall not have a steeper slope than onevertical to two horizontal ( see Fig. 2b ).

Fig:2a Footing on Granular Soil


Fig:2b Footing on Clayey Soil

CE F313 F d i E i i M J S K l R


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SPACING BETWEEN EXISTING AND NEW FOUNDATION

The deeper the new foundation and the nearer to the existing it is located, the greater

the damage is likely to be. The minimum horizontal spacing between existing and newfootings shall be equal to the width of the wider one. While the adoption of such

provision shall help minimizing damage to adjacent foundation, an analysis of bearing

capacity and settlement shall be carried out to have an appreciation of the effect on the

adjacent existing foundation.


CE F313 F d ti E i i M J S K l R 17


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CE F313-Foundation Engineering: Mr. J S Kalyana Rama

Foundations Systems

Foundation Engineering/Foundation Systems

Foundation

Structures

Retaining

Structures

Substructures

Shallow

Foundations

Deep

Foundations

1. Piles

2. Piers

3. Caissons

1. Footings

2. Rafts

1. Retaining Walls

2. Sheet Pile Walls

1. Coffer Dams

2. Cut Supports

1. Tunnels

2. Conduits

1. Diaphragm Walls

2. Pavements

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Shallow Foundations

Footings Rafts

For two or more

columns in one row

For Individual

Columns

For Load

Bearing Walls

For two or more

columns in several rows

Combined Footings

Shape:

a. Rectangular

b. Trapezoidal

c. Cantilever

Independent Footings

1. Isolated

2. Spread or Pad

Shape:

a. Rectangular

b. Square

c. Circular

Continuous or

Strip Footings

1. Grids

2. Rafts

Shape:

a. Rectangular

b. Squarec. Circular

d. Annular

e. Octagonal




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Deep Foundations

CaissonsPiersPiles

Box1. Drilled

2. Excavated

Shape:a. Circular

1. Replacement

2. Displacement

3. End Bearing

4. Frictional

5. Bored

6. Driven

7. Cast-in-situ

8. Precast

Shape:

a. Circular

b. Square

Open (Well)

Shape:a. Circular

b. Double-D

etc..

Pneumatic


CE F313 Fo ndation Engineering: Mr J S Kal ana Rama 20


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The foundations provided immediately beneath the lowest part of the structure, near to the ground

level are known as shallow foundations. The purpose of this type of foundations is to distribute

the structural loads over a considerable base area at the foundation bed. Since spread foundations

(shallow foundations) are constructed in open excavations, therefore, they are termed as open

foundationsShallow foundations are further classified into the following types: -

a. Spread or Isolated footings

b. Combined footing

c. Cantilever footing

d. Continuous or wall footing

e. Raft foundation

Shallow Foundations


CE F313 Foundation Engineering: Mr J S Kalyana Rama 21


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Spread Footing Shapes

Spread footing foundations: An enlargement at the bottom

of a column or bearing wall that spreads the applied

structural loads over a sufficiently large soil area.

i. Square spread footings : Supporting a single

centrally-supported column.

ii. Rectangular spread footings : In cases that

obstructions prevent construction of a square footing

with a sufficiently large base area and large moment

loads are present.

iii. Circular spread footings : Supporting a single

centrally-supported column, but less common than

square footing. (flagpoles).

iv. Continuous spread footings(Strip footings) : Used

to support bearing walls.

v. Combined footing : When columns are located tooclose together for each to give its own footing.

vi. Strap footing with a grade beam : Provides the

necessary moment resistance in the exterior footing

with eccentric load and a more rigid foundation

system.

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Strap Footing

Use of strap footing to support exterior

Columns when construction cannot

Occur beyond the wall

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Spread or Isolated Footings:- They are used to support individual column. Isolated footings are

stepped type, simple type or slope type, having projections in the base concrete. To support heavy

loads, reinforcement is also provided at the base. The reinforcement provided is in the form of

steel bars and is placed in both directions

Single spread footing Stepped spread footing Sloped spread footing


CE F313-Foundation Engineering: Mr J S Kalyana Rama 25


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Combined Footing:- A combined footing supports two or sometimes three column in a row.

Combined footing is used when property lines, equipment locations, column spacing or other

considerations limit the footing clearance at the column locations. The combined footing can be

rectangular in shape if both the columns carry equal loads, or can be trapezoidal if there is a space

limitation and they carry unequal loads. Generally they are constructed of reinforced concrete.




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Cantilever or Strap Footing: - Cantilever footing consists of two individual footings connected

by a beam called a strap. It is also sometimes called as strap footing. Cantilever footing may be

used where the distance between the columns is so great that a trapezoidal combined footing

becomes quite narrow, with resulting high bending moments. The strap beam does not remain in

contact with soil so a strap doesnt transfer any pressure to the soil.




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Continuous or Wall Footing:- In this type of footing a single continuous reinforced concrete

slab is provided as foundation of wall and three or more columns in a row. This type of footing is

suitable at locations liable to earthquake activities. This also prevents differential settlement in

the structure.

CE F313 Foundation Engineering: Mr. J S Kalyana Rama 27



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Raft Foundation:- A raft or mat is a combined footing that covers the entire area beneath a

structure and supports all the columns. When the allowable soil pressure is low or the structure

loads are heavy, the use of spread footings would cover more than one-half of the area, and it

may prove more economical to use raft foundation. It is also used where the soil mass contains

compressible layers so that the differential settlement would be difficult to control the raft tendsto bridge over the erratic deposits and eliminates the differential settlement.




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GENERAL PRINCIPLES OF FOUNDATION DESIGN

The usual approach to a normal foundation-engineering problem is

1. To prepare a plan of the base of the structure showing the various columns, load bearing walls

with estimated loads, including dead load, live load, moments and torques coming into the

foundation units.

2. To study the tentative allowable bearing pressures allocated for the various strata below the

ground level, as given by the soil investigation report.

3. To determine the required foundation depth. This may be the minimum depth based on soil

strength or structural requirement considerations.

4. To compute the dimensions of the foundation based on the given loading and allowable bearing

pressure.5. To estimate the total and differential settlements of the structure. If these are excessive the

bearing pressure will have to be reduced or the foundation taken to a deeper and less

compressible stratum or the structure will have to be founded on piles or other special measures

taken




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Loads on Foundation

An accurate estimation of all loads acting on the foundation should be made before it can be

properly designed. A foundation may be subjected to two or more of the following loads.

a) Dead load: - - Weight of structure

- All material permanently attached to structure

- Static earth pressure acting permanently against the structure below ground surface.

- Water pressures acting laterally against basement walls and vertically against slab.

b) Live load: - temporary loads expected to superimpose on the structure during its useful life.

c) Wind load: - lateral load coming from the action of wind. Local building codes provide

magnitude of design wind pressure.

d) Earth-quake load: - lateral load coming from earthquake motion. The total lateral force (base

shear) at the base of a structure is evaluated in accordance with local building code.

e) Dynamic load: - load coming from a vibrating object (machinery). In such case, separate

foundation should be provided. The impact effect of such loads should be considered in design.




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Selection of Foundation Type

In selecting the foundation type the following points must be considered

a. Function of the structure

b. Loads it must carry

c. Subsurface conditions

d. Cost of foundation in comparison with the cost of the superstructure.

Having these points in mind one should apply the following steps in order to arrive at a

decision.

i. Obtain at least approximate information concerning the nature of the superstructure

and the loads to be transmitted to the foundation

ii. Determine the subsurface condition in a general way.iii. Consider each of the usual types of foundations in order to judge whether or not

a. They could be constructed under existing conditions.

b. They are capable of carrying the required load.

c. They experience serious differential settlements.

C 3 3 ou d o g ee g: . J S y 31



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The types that are found to be unsuitable should then be eliminated.

iv. Undertake a detailed study of the most promising types. Such a study may require

additional information on loads and subsurface conditions.Determine the approximate size of footing or the approximate length and number of piles

required

v. Prepare an estimate for the cost of each promising type of foundation.

vi. Select the type that represents the most acceptable compromise between

performance and cost.

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Table 1. Soil Types and Foundation Consideration

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Table 2. Possible Solutions to some Problems in Foundations

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Documents

Lecture-1 Introduction to Foundations.pdf