Bridge foundation
-
Upload
9175764125 -
Category
Engineering
-
view
195 -
download
9
description
Transcript of Bridge foundation
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.
THANK YOU