INTRODUCTION TO BRIDGE FOUNDATION

BY:

DEBASREE GHOSH

JR. ENGINEER (CIVIL DEPT.)(Work of 2014, 8th Sem, NIT AGARTALA)

WHAT IS A BRIDGE? Bridge is a structure which covers any gap or any obstacle

to give a passage through it. Generally bridges carry a road or railway across a natural

or artificial obstacle such as, a river, canal or another railway or another road.

Approach slab

Deck slab

PARAPET

ABUTMENT

PIER

BEARING

PILE CAP

LONG GIRDERCROSS GIRDER

W H AT I S F O U N D AT I O N ?

A foundation is the part constructed under the pier/abutment and over 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 foundations of a bridge are particularly critical because they must support the entire weight of the bridge and the traffic loads that it will carry.

PURPOSE OF FOUNDATION

Purpose of foundation is to distribute the load of bridge over a large bearing area and provide stability of bridge against settlement and tilting.

It is provided sufficiently deep so that it is not affected by the scouring.

FIG2. FORCES ACTING ON A BRIDGE

HOW THE LOAD TRANSFER FROM BRIDGE TO THE FOUNDATION?

Beam

Pier

FIG3. WORKING OF FORCES

IN ORDER TO DESIGN THE FOUNDATION MUST DETERMINE THE FOLLOWING REASONABLY AND ACCURATELY:-

The maximum grip length required,The maximum likely scour depth,The soil pressures at the base, andThe stresses in the structure constituting the foundation.

The foundation should be taken to a depth which is safe from scour, and is adequate from considerations of bearing capacity, settlement stability of suitability of strata at the founding level.

FIG4. FORCES ON BRIDGE

Beam

Pier

SOME BASIC CONCEPTS

Scour depth: Scour denotes the washing away of the bed material in a stream due to the passage of a flood.

It depends on discharge, bed slope, bed material, direction of flow and alignment of pier.

Grip length: It can be defined as the extra length provided to ensure stability under heavy flood condition.

For road bridge: 1/3 (max scour depth) and

For railway bridge: ½ (max scour depth)

TYPES OF FOUNDATIONS

The type of foundation is designed when subsoil data of bridge site is obtained. The different types of foundations for bridges are given following;

Shallow Foundation (D<B) Spread Foundation and Raft foundation

Deep Foundation (D>B) Pile and Well foundation

B

D

FIG5. DEPTH AND WIDTH OF FOUNDATION

SHALLOW FOUNDATION

A shallow foundation is sometimes defined as one whose depth is smaller than its width. Shallow foundations can be laid using open excavation by allowing natural slopes on all sides. This is normally convenient above the water table and is practicable upto a depth of about 5m. Shallow foundation transfer the load to the ground by bearing at the bottom of the foundation.

FIG6. SHALLOW FOUNDATION

DEEP FOUNDATIONA deep foundation is defined as

one whose depth is larger than its width. Deep foundations can not be laid using open excavation.

Deep foundation transfer the load to the ground partly by end bearing at the bottom of the foundation and partly by skin friction with the soil around the foundation along its embedment in the soil.

Piles are preferred over wells as they are more economical than wells.

FIG7. DEEP FOUNDATION

WHICH TYPE OF FOUNDATION IS PREFERABLE FOR BRIDGES & WHY?

A bridge is a highly important structure comprising heavy structural loads constituting its own weight, overburdens around the supports, discharge loads, etc.

Due to this high intensity of loading deep foundations are generally preferred for the construction of bridges.

CONCLUSION:For small bridges or culverts shallow foundations may

be used whereas for long span bridges having very high loading conditions or loose soil strata then deep foundations are used.

REFERENCES

IRC Bridge Code – Section VII. Book of “Essential of bridge Engineering” sixth edition

reprinted 2009 by D. Johnson Victor. www.howstuffworks.com IRC: 78. for the depth of foundation for the

worst loading combination. Picture courtesy: Nat Geo www.slideshare.net

THANK YOU