Shubham Dabhade Civil engg.

Bridge foundation

INTRODUCTION

Foundation is to provided to transmit the load from the piers or abutments and wings to and evenly distribute the load on the strata.

It is to be provided sufficiently deep so that it is not affected by the scour caused by the flow in river and does not undermined.

TYPES OF BRIDGE FOUNDATION

The choice of bridge foundation depends upon the importance of bridge, size, nature of soil and sub-soil in the bed and velocity of water flow.

foundations are grouped under

Shallow Foundation Open Foundation Block Foundation

Deep Foundation Pile Foundation Well Foundation

OPEN FOUNDATIONPipe and box culvert can be laid after

removing about 0.30D to 0.50D of top soil in bed and replacing it with well rammed moorum or laying a lean concrete bedding after the base is leveled and well consolidated by ramming or rolling.

OPEN FOUNDATION

Open Foundation in minor bridges will be in the footing form.

OPEN FOUNDATION

In designing the open foundation, the maximum pressure at base of foundation is limited to the values given in the table.

PILE FOUNDATION

Use of pile foundation has till recently not been a popular choice for bridges in India.

For road bridges, this had been used mostly as timber pile for temporary bridges and where subjected to lighter loads.

RCC piles both pre-cast and cast-in-situ as well as larger diameter bored pipes below bed level are becoming popular.

A minimum diameter of 1200mm for river bridge and 750mm for the others is preferred.

Use of piles for bridge in India

Cheap and quick in construction Timber piles have been used in railways and

roadways, particularly in the North-Eastern part in case of temporary bridge.

In the North-Eastern part of India, many of the rivers there are of shifting nature and there is always danger of bridges being out flanked by the rivers when the river shifts its course.

The recent development indicate that the pile foundation can be used quite economically, particularly, where the foundation have to be built very deep or taken through deep layers of soil subjected to a minimum of scour.

PILE FOUNDATION

Well foundation are constructed on dry ground in suitable form and then sunk into the ground to the desired level by grabbing the soil through dredge holes provided, surrounded by the solid masonry.

WELL FOUNDATION

Components of Well Foundation

Well Curb:The structure of well at the

bottom is tapered to end in a steel ‘cutting edge’ which facilitates cutting through soil for sinking of well. This tapering portion being thin, it has to be strengthened suitably to take up the heavy loading. This portion is called “Well Curb”.

Steining:The structure in the form of well

surrounding the dredge holes, is known as ‘steining’.

WELL FOUNDATION

Bottom Plug:

The concrete placed at the bottom to form the base of foundation for distributing the load to soil below, is known as “Bottom Plug”.

Top Plug:The concrete placed at the top, at

finishing level, to transmit the load of structure to the to the well steining, is known as “Top Plug”.

WELL FOUNDATION

Advantages

This provides a solid and massive foundation for heavy loads and high horizontal thrust transmitted by the moving loads.

This has a larger cross sectional area and hence the total foundation bearing capacity is much larger than piles.

Wells can be taken trough soil having boulders, logs of wood and such type of obstruction, without causing damage to the structure.

WELL FOUNDATION

The masonry in the steining of well is built in dry condition and hence quality of masonry or concrete can be assured.

The well provide good grip when taken sufficiently deep and hence most suited for river beds subjected to heavy scour.

Hence, the choice is mostly in favour of well foundation where large spans requiring deep foundations have to be provided.

WELL FOUNDATION

BLOCK FOUNDATION is a form of shallow foundation used for smaller bridges in locations where

there is a good founding strata at shallow depth

but is overlain by very soft layer of soil and high water table making open excavation difficult due to small quantum of work.

SITE SELECTIONIdeal characteristics of bridge site

1. Suitable, un-yielding, non-erodable

material for foundation should be present

at short depth for piers or abutments.

2. The bearing strata should be free from the

tendency to slip/slide/sink under load.

3. The stream at bridge site should be well

defined, as narrow as possible.

4. There should be straight reach of stream at

bridge site.

5. The site should have firm, permanent,

straight and high banks.

6. The flow of water at in stream at bridge

should be steady.

7. It should be feasible to have straight

approach road and square alignment.

8. There should be minimum obstruction of

water way so as to have minimum afflux.

9. There should be no need for costly river

training works in vicinity of bridge site.

SITE SELECTION

SITE SELECTION

10. At bridge site it should be possible to

provide secure and economical

approaches.

11. In order to have minimum foundation cost

the bridge site should be such that no

excessive work is carried out inside water.

12. In order to achieve economy, there should

be easy availability of labour, transport

facilities, construction material in the

vicinity of bridge site.

SITE SELECTION

13. There should be no confluence of large

tributaries in the vicinity of bridge site.

14. In case of curved alignment, the bridge

should not be on the curve but

preferably on the tangent, since

otherwise there is more probability of

accidents.

15. The bridge site should be such that

adequate vertical height and water way

is available underneath the bridge.

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