Guidelines for Causeway construction

 Guidelines for Construction of Causeways

yogendra borse

1. General information.

A road network is never complete till all C.D., works, Minor bridges and Major bridges on it are completed. Due to several constraints on the allocated funds For roads, many a time the C.D. works and bridges on the allocated funds for roads, may a time the C.D. works and Bridges on the road network are taken up later on, depending on the availability of funds. However, till these bridges and C.D. works are completed to make these crossings fordable during monsoon and during floods, the Causeways or such type of lower level passages have to be constructed at least to make them fordable during the post monsoon period. These causeways or hard passages are no substitute for high level bridges with permissible interruptions to traffic.such causeways are essentially low cost solutions  designed for floods of frequency much higher than those for bridges. Apart from floods, these causeways are not designed for all such forces which are considered for design of bridges. So some damages during floods have to be accepted on such causeways. However, if proper precautions are taken in planning and designing of the causeways, some of the damages though they can not be avoided entirely, could atleast be minimised. These guidelines can help in planning, designing and construction of causeways to minimise major ave dable damages.

2. Definition.

2.1 A causeway is a small submersible structure with or without openings which will allow floods to pass over it.

2.2 Depending on the type of construction and the road level above the bed of the water course, these structure can be classified as follows:

Flush Causeway: is a paved dip or road dam on the road crossing a shallow water course and road top level almost at the same level as that of the bed of the water course.

Low level or a vented causeway: is a structure provided with a few openings comprising of pipes, small span slabs or small arches etc. and a raised road top level to a moderate height upto 1 to 1.20 mtrs. In exceptional cases it may be 1.50 mtrs.

 high level causeway: is a submersible structure Provided with larger openings comprising of slabs, arch or boxes and a raised road top level to a reasonable height upto 2.50 to 3.00 mtrs.

3. Causes leading to Damages.

3.1 Study of some damages to causeways and malfunctioning thereof revealed that they have one of the following characteristics :

i) Outflanking of the Causeway.

ii) Collapse of downstream headwall.

iii) Damages to road pavement on the causeways.

3.2 The probable causes of the above damages could be

i) Inadequate or blocked openings.

ii) Scour on the down-stream side.

iv) Inadequate keying of the head-wall in the natural banks. 

iv) Silting on the upstream side

v) High velocity and inadequate pavement protection.

4. Precautions to be taken

4.1 It is generally observed that the road top level selected is abnormally high compared to the vent way provided. This condition causes heading up of water upstream of structure and results into high velocities leading to structure failure and also out flanking. Road top level of the causeway should be kept as low as possible and should be restricted to the heights mentioned in above definitions.

4.2 Area of the Vents. 

Waterway provided in the causeway should not be less than 30% of the area of the stream, measured between the stream bed level and the proposed road top level. However, higher vent area will be desirable. In cases where the causeway is located on the existing natural crossings, these areas should be calculated at the defined cross section of the stream. In scanty rainfall area i.e. Annual rainfall less than 750 mm., the vent area can be brought down to 15%. 

4.3 The upstream and downstream face walls of the causeway should cover the whole bed width of the water course in case of vented and high level causeways. A wing face wall should be provided upto the top level of the protected bed. 

This face wall serves two major Purposes -

i) It coverages the flow and avoids outflanking. 

ii) It protects the pavement and makes the shape hydraulically efficient.

4.4 Foundations of the head walls should be taken sufficiently deep to avoid exposure due to scouring. Extreme care should be taken about the depth of foundation of the down-stream head walls in erodible strata like soil, soft murum and sand etc. In erodible strata, downstream protection by way of apron should invariably be provided.

4.5 it is observed that in causeway having openings with hume pipes, a general tendency is to provide 600 mm earth cushioning above the pipes. Even Though this cushioning is essential for the load distribution and safety of the pipes in generel road use, this provision increase the R.T.L. undesirably high and makes the submersible structure hydraulically inefficient. This cushioning should be reduced to the barest minimum by providing C.C., stone paving or penetration macadam etc. above the pipe openings.

4.6 As the causeway are expected to allow the flood water to pass over it and the traffic expects to use them immediately, the road surface on the causeway should be such that it does not get damaged due to frequent over topping. Sufficiently stable pavement should therefore, be provided for the full width of the causeway. It should also cover the protected bed portion of the causeway. In case of large submergence where submersible approaches are provided, similar suitable precautions should be taken while providing the pavement and downstream protection.

4.7 It has been noticed in many washed out and Pavement failure causeways that the filling between the head walls is done with clayey material. As it is very difficult to ensure proper consolidation around pipes, use of clayey material should be avoided in causeway construction. Filling between the head walls should be done with granular material like sand, hard murum, pebbles, gravels, boulders etc. As the river, nalla bed generally consists of such materials, best use of the same should be made. If required, such material should be brought from outside but in no case clayey material be used.

4.8 To increase hydraulic efficiency and structural stability -

i) it is preferable that the downstream head wall should have batter on the outside i.e. on down-stream side;

ii) it is also preferable to provide rounding at outer corners to both the headwalls. 

iii) it is preferable to provide slight camber to Pavement on one side only i.e. sloping to downstream side only;

iv) road to level and top of coping above head walls should be same.

4.9 In case of stream carrying heavy silt and floating debris, it is desirable to provide a catch pit at the upstream side opening to the vents.

In case of streams carrying large floating debris, it is desirable to provide large openings comprising of slabs, or arches rather than smaller openings of pipes. 

In case streams carrying large floating material such as small bushes, trees, debris, arrestors should be fixed.

Some modifications may help in reducing this problem as explained in video below 

Widen area of pipe at Upstream and downstream by cutting in conical shape.

4.10 The work of causeway proper, its approaches, paving and the protective works like apron, paved drains in approaches, pitching for the approaches etc. should be so planned and undertaken that all the works are completed before outset of the monsoon or else the function of the causeway structure gets impaired and the work may be damaged.

5. Hydraulic calculations.

5.1 For hydraulic calculations for causeways are explained with simple procedure with worked out examples, in article Hydraulics for Causeway 

6. General.

6.1 It should noted that while adopting the option of causeway and effecting economy, basic principles of sound engineering practice and minimum technical requirements are not overlooked. Some mistakes on the part of Engineers in understanding the hydraulic behaviour of the structure and endangering the natural regims of the water course lead to failures and involve extra expenditure. Such failures create doubts about the suitability of the structure.

6.2 Specific attention should be paid to the following:

i) Road top level above bed as low as possible as per guidelines for different types of openings. 

ii) Vent opening below road top level. 

a. Lower limit 30% of the area below R.T.L.

b. Lower limit 15% for scanty rainfall ares However, higher vent area is desirable.

iii) Head wall shape in elevation-

trapezoidal, blending smoothly with the natural cross section of the water course with desirable grade of 1 in 30. 

iv) Apron for downstream protection in erodible strata.