Road or rail line projects involves construction of culverts and minor bridges. Siting considerations and investigation of catchment area for construction of culverts and minor bridges is discussed.
If detailed investigation is carried for every project, more time will be consumed resulting in an expensive construction.
Approximation during the construction can be carried out if required. This is because the repairs or any kind of replacement due to inadequacy can be done with minimum dislocation and cost.
Construction of Culverts and Minor Bridges
The two main investigations that is carried out during construction of culvert and minor bridge, i.e. the siting and the catchment area study.
A crossing with a total length not greater than 6m, that is measured between the faces of the abutments or the extreme vent way boundaries is called as the culvert. This measurement is carried out in the direction perpendicular to the axis of the vent away.
Pipes, arches, slabs of reinforced concrete on piers or abutments, reinforced concrete boxes are used to make these minor drainage works. In the areas where the bank is appreciably high or where the cushion is sufficiently available pipes and arch culverts are provided.
The pipe culverts can be provided for low discharges up to 10 cubic meters per second (cumecs). This is found to be cheap and quickest form to construct in the before-mentioned discharge range.
Box culverts are also constructed in individual spans for length from 1 to 4m. These are provided in single or multiple units. For span ranging from 2 to 6m, arches and slab culverts are the best choices.
While constructing box and slab culverts, a wearing course is directly placed on the top of the slab without the need for any earth cushion. The figure-1 below shows the section of culverts.
Fig.1: Typical Sections of Different Types of Culverts
Siting Considerations and Investigation of Catchment Area for Construction of Culverts and Minor Bridges
For the investigation for any major road or rail line, the survey is conducted through the center line of the alignment that is proposed as the initial survey. The details of the same are obtained by undergoing plane table survey for about 100m, done on either side of the alignment.
To plot the longitudinal section, levels are taken along the alignment at certain intervals. These intervals must not be less than 25m in plains.
In the case of an undulating country, the intervals must be close when chosen. Hence, the interval is selected based on the terrain characteristics.
Clear and efficient contours can be obtained for the area that is surveyed by taking a sufficient number of the cross section. The intervals that is considered for vertical contour levels should be 1 to 2m.
The rivers, canals and natural streams that are crossed while surveying is marked and noted down with respect to that location. These are points called the obligatory points that will be considered for drainage crossing provision.
Openings are provided in the embankments wherever necessary, to consider any form of low level points that will result in the collection of water to the upstream during the embankment construction or result in the formation of water impound.
The valley points can be discerned by plotting and studying the longitudinal section. Once the valley points are noted, the contours are examined properly to decide whether there is any need for draining the major local pockets.
In areas where a continuous form of ups and down undulations are prominent, say with a difference of successive points not more than 0.3m, a diversion at the suitable point will enable the local pocket to get drained.
The Proper examination must be made to undergo any kind of diversion. Based on this assessment the catchment is drained by means of a culvert at any location.
Other investigations that must be carried out are on the soil characteristics, the catchment area that must be drained and on the flood level that is anticipated to determine the clearances.
Provision of Diversions during Construction of Culverts and Minor Bridges
The investigating engineer will see the possibility of diversion in the case of the culvert and minor bridge construction.
The following details are noted during investigations for the same:
- The catchment area
- Soil characteristics
- The nature of the stream – It can be either perennial or seasonal
- The highest flood level
- The lower water level slope
- The requirements for navigation
- Other construction problems that may arise
No need for alternative sites in the case of the man-made or another type of culverts. The culverts and the minor bridges can be provided without altering the road or rail alignment.
To run the flow normal to the road or the alignment of the rail, special modifications or training can be carried out. The bridge span will have shorter length if a normal crossing is carried out. It also reduces the length of the pier and the abutments.
This arrangement will enable smooth flow and facilitates construction of segmental wing wall and return walls. This will bring structures that have a minimum sharp angle, which would facilitate the reduction of eddies and cross currents.
If normal crossing cannot be provided due to the space constraint problems, skew crossing can be carried for the water course training works. The skew is preferred to be less than 30 degrees.
Catchment Area Consideration for Culvert Construction
A catchment area is the area from where a river mostly gets its water or pour out, that varies with rivers to lakes be in meters or in km.
A direct measurement from a scaled map with essential contours can give the catchment area for any bridge or culvert crossing. In the case of small crossings, the area can be prepared from the survey plan that covers details up to 100m on either side.
In the case where the catchment area exceeds the surveyed boundaries, a traverse must be run at the site along the extreme ridge line that envelope the area which has to be drained. After this, the area to be drained is computed.
In the case of small streams, the topographic sheets help in direct calculations from the contour details given in the sheet. It is good if the topographic sheet has scale 1” = 1 mile or 1:50,000 or 1: 25,000.
If this is not available, a traverse must be run along the ridges on either side which separate the stream under consideration from the neighboring streams. This traverse can be carried out with the help of a chain, prismatic compass, and an Abney Level.