1 - Introduction - Foundation

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CE 152

GEOTECHNICAL ENGINEERING 2

INTRODUCTION

ENGR. ROGELIO FRETTEN C. DELA CRUZINSTRUCTOR


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FOUNDATION ENGINEERING

–Art and science of applying engineering judgment and the principles of soil mechanics.

Two parts consisting an engineering structure:

1. Superstructure –

upper part2. Substructure/Foundation – lower part

- part of the structure which is usually placedbelow the surface of the ground and which

transmits the load to the underlying soil or rock.


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Two principal kinds of failure for foundation:

1. Structural failure of foundation (bending,shear, bearing).

2. Bearing capacity failure of the supporting

soil.


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Three types of bearing capacity failures:

1. General shear failure

– It occurs in soils that are relatively incompressible and

reasonably strong, in rock, and in saturated, normally

consolidated clays that are loaded rapidly enough that

the undrained condition prevails. The failure surface is

well defined and failure occurs quite suddenly. Although

bulges may appear on both sides of the foundation,ultimate failure occurs on one side only, and is often

accompanied by rotations of the foundation.


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2. Local shear failure

– The shear surfaces are well defined under the

foundation, and then become vague near the ground

surface. A small bulge may occur, but considerable

settlement, perhaps on the other half of the foundation

width, is necessary before a clear shear surface forms

near the ground. Even then, a sudden failure does notoccur, as happens in the general shear case. The

foundation just continues to sink deeper into the ground.


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FOUNDATION CLASSIFICATIONS:

1. Shallow Foundation

- Satisfactory soil directly underlies the structure.

- Spread footings, combined footings

2. Deep Foundation

- Adequate soil is not found immediately below the

structure.

- Caisson foundation, pile foundation


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Typical Foundation Types

1. Foundation for buildings (controlled mainly byvertical loads)

2. Foundations for smoke stacks, radio & televisiontowers, bridge, piers, etc. (lateral loads)

3. Foundations for ports & marine structures(lateral loads & impact loads)

4. Foundations for machinery, turbines,generators, etc. (vibration & weight)

5. Foundation elements to support open cuts orretain earth masses or bridge abutments.


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ULTIMATE BEARING CAPACITY

THEORYReference: Fundamentals of Geotechnical Engineering 3rd Ed.

by Braja M. Das


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Later investigators, however, have suggested that foundations with

D f equal to 3 to 4 times the width of the foundation may also beconsidered as shallow foundations.


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The failure zone under the foundation can be separated into

three parts:


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Terzaghi expressed the ultimate bearing capacity in the form:

(12.3)


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P


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= 349.55 + 132.43 + 38.88

qu

= 520.86 kN/m2

4

86.520

2

all

kN/m22.130q


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Hence, the allowable load, P

LBP

APall

q

LBP all q

m5.1m5.1kN/m22.130 2

kN293P


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P


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Case I is applicable,

12.85 kN/m2


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Hence,

= 700.77

= (12.85)(23.18)(1.625)(1.23)

3

77.700

FS

u

all

qq

2

all kN/m59.233q

LBP all q

= 233.59 × (1.2)2

P = 336.37 kN


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Solution


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SEATWORK:

1. Determine the gross allowable bearing capacity for a strip

footing given the following:m2.1m,5.0,kN/m2.14',20',kN/m2.18

23 B Dc f

Use FS = 4, Eq. 12.3 and Table 12.1

2. A 2 m×

2 m square footing has the following conditions:0',34',kN/m9.15m,5.1

3 c D f

Determine the allowable gross vertical load that the column

could carry if FS = 3. Use Eq. 12.9


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END

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