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highway engineering 8th sem VTU Notes
Typology: Lecture notes
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General Objective
To understand the basic highway drainage system.
Specific Objectives
At the end of the unit you should be able to :-
Highway drainage may be defined as the process of interception and removal of water from over, under and the vicinity of the road surface. Road drainage is very important for safe and efficient design of the road way and hence is an essential part of highway design and construction.
A part of rainwater falling on road surface and adjoining area is lost by evaporation and percolation. The remaining water known as surface water , either remains on the surface of the road and adjoining area, or flows away from it, depending upon the topography and general slope of the area. Removal and diversion of this surface water from highway and adjoining land is known as surface drainage.
Due to percolation, if water table does not rise near of the road sub-grade, it does not create any problem as it does not affect the road sub-grade. If water table rises to the vicinity of road sub-grade, it requires to be lowered as it will definitely affect road sub-grade. Measures adopted to lower the subsoil water table are called sub surface drainage.
j. Excess moisture causes increase in weight and thus increase in stress and simultaneous reduction in strength of the soil mass. This main reason of failure of earth slopes and embankment foundation. k. Erosion of soil from the top of un-surfaced roads and embankment slopes in also due to surface water.
8.2 HIGHWAY DRAINAGE REQUIREMENTS
a. Surface water should not be allowed to remain standing on the road pavement and shoulders. Measures should be taken to drains off this water, immediately. b. The surface rain water from the adjoining area, should not be allowed to come towards the road surface. For this, general slope of the ground adjoining road, should be made slopping away from the road. This objective can be achieved by aligning road on ridge. c. Side drains should be of sufficient capacity and having sufficient longitudinal slope so that it may drain of all the collected surface water, efficiently. d. Surface water flowing across the road pavement should not develop cross ruts or erosions on road surface and shoulders. For this, high embankment slopes should be protected either by turfing or pitching. e. Seepage water and other capillary waters should be drained off by suitable underground drainage system. f. Maximum level of under ground water table should be maintained well below the sub-grade level of the road. Under ground water table should remain at least 1 m to 2 m below the road sub-grade.
g. In water logged areas, special measures should be taken to keep down the harmful salts.
The quantity of water to be carried by a side ditch along a highway is the run-off from the area contributing there to. Primarily this water comes from the portion of the road between the ditches and from shoulders flanking the roadway surface. As a broad principle, it may be accepted that for storms of short duration say of less than 15 minutes, about 75% of the water and that for storms exceeding 15 minutes duration, all the water that falls on this area would run to the ditches. Some more water might also reach the ditches from the adjoining lands sloping towards the roadway. Rate of run-off of water from these lands would depend upon the type of soil, type of vegetation and the slope of the land.
All these items should be carefully studied before arriving at the Design Discharge. For practical purposes, it may be assumed that for storms lasting 40 minutes or more a portion of water that falls on the adjoining lands also comes to the ditches. The design of cross section whether V-shaped or trapezoidal should be based upon these considerations.
Side-ditches should preferably be lined if financially feasible. The paving may consist of rubble masonry work, which is either laid dry or the joints filled with grout. The stones should be 15 to 20 cm deep and at least 5 cm wide for the purpose. This may be under-laid with a 10 cm layer of crushed rock or gravel, the particles of which may range in size from 10 to 20 mm. A typical section of such a ditch is shown in figure below.
Figure 6.1 : Section of a typical V-shaped side ditch
Intercepting drains are used on natural slopes to prevent erosion of the cut-slopes in the hilly areas. They are sometimes also used to relieve the side ditches of greater discharge than the one for which they are designed. The water from the intercepting drain is carried down to the side ditch at specified points along spillways. Typical intercepting drain are shown as figure below.
Figure 6.2 : Cross section of intercepting ditch
Shoulder Slope
to 0.45m
to 1.0m
45 to 60 cm
15 to 20 cm Stones laid dry or joint – grouted over 10 cm thick layer of crushed rock gravel
Intercepting Ditch
The kerb and gutter combination are fixed at the edge of the roadway and gullies are placed at suitable intervals. The gutters slope towards gully from both sides and a longitudinal gradient of 1 in 100 to 1 in 120 is given to these. The water from the gutter flows through the grating into the gully and is drained off through an outlet pipe to a water sewer, which takes the water away to a natural water channel.
The arrangement is of the type shown in figure 6.3. Sometimes V-shaped shallow concrete gutters are used for concrete or other high type of pavements. The shape is shown in figure 6.4. Such gutters do not create any traffic hazard. Sometimes, separate kerb and gutters are employed for draining off water from the city streets. Such as arrangement is shown in figure 6.5 below.
Figure 6.3 : The arrangement of side surface drainage
3 cm
Road Surface
8 cm 10 cm
cm 53 cm SECTION OF CHANNEL AT SUMMIT
8 cm
53 cm SECTION AT MID POINT BETWEEN SUMMIT AND GULYY
Road Surface
8 cm
Outlet Pipe
27 cm
14 cm
45 cm
Road Surface
SECTION AT GULLY
Figure 6.4 : Section of a shallow concrete gutter
Figure 6.5 : Section of a kerb and gutter as separate units.
20 cm (^) 16 cm
SLOPE: 1 IN 12
4 cm
15 cm 60 cm
30 cm
10 cm
8 cm
30 cm
Pavement Surface
Stability and strength of the road surface depends upon the strength of the sub grade. Sub-grade is the foundation layer of the road whose strength largely depends upon its moisture content. With increase in moisture content, the strength of the sub-grade decreases.
Variations in moisture content of sub-grade are caused by following:
a. See page of water from higher adjoining ground. b. Penetration of moisture through the pavement. c. Percolation of water from shoulders, pavement edges and soil formation slopes. d. Rise or fall of underground water table. e. Capillary rise of moisture in case of retentive type of soils. f. Transfer of moisture vapour through soil.
The first three methods of entrance of moisture are concerned with free water, and last three methods are connected with ground water. In the case of sub-surface drainage of the roads, every effort should be made to reduce the change or variation in moisture content to minimum. It is better if sub-grade is always kept dry, but in rainy season, some change in moisture content if sub- grade is inevitable. It should be remembered that by provision of sub-soil drainage, only gravitational water can be drained off, vapour water and cappillary water cannot de drained by this system.
If underground water table is more than 1.5 m below the sub-grade of the road, it does not require any sub soil drainage. But if it is closer than this, the best remedy is to raise the road formation to such a height that sub grade remains at least 1.2 m above the highest water table. But if due to other consideration, it is not possible to increase the height of formation, and sub- grade soil being drainable, deep side trenches should be constructed on both sides of the road to lower the water table. These trenches are provided with drain pipes and filled at the top by filter sand. The depth of trenches depends upon amount of lowering of water table, lateral distance between trenches and type of sub-grade soil.
If the sub-grade soil is of retentive type, only side trench drains may not prove adequate, as at the centre of the pavement lowering of water table may not take place up to the desired level. In that case, transverse drains may have to be provided at suitable intervals along with the side longitudinal drains. Transverse drains may be pipe drains or trapezoidal trench drains filled with stone or rubble. Stone or rubble filled transverse drains are also called french or blind drains. Transverse drains collect percolating water from the width of the pavement and discharge it into the side drains. Transverse pipe drains are given a little slope towards the side drains. The filter material surrounds the transverse trenches and the top of these trenches is filled with graded rubble, the bigger size rubble being nearer the pipe.
The diameter of the lateral pipe drain may be 10 cm and that of longitudinal pipe drain 20 cm or even more according to the requirements. The cross drains are placed staggered in plan, in the herringbone fashion.
If capillary rising water in very near to sub-grade of the road and is likely to affect its strength, steps should be taken to arrest the capillary rise of water. In the case of seepage, arresting of capillary rise is more useful than lowering the water table. In fact lowering of under ground water table is only economical method when sub-grade soil is of permeable type. In case of sub-grade having retentive type of soil, drainage is very difficult and costly and checking its rise by capillary cut-off’s proves more economical.
Capillary cut-off’s can be of three types as follows: a. Provision of a granular layer of suitable thickness or of sand blanket. b. Provision of impermeable layer of bituminous material. c. Heavy duty tar felt, polythene envelop, are the other measures which can be adopted.
During the construction of embankment, a granular layer of suitable thickness is inserted between the sub-grade and highest level of water table. Thickness of granular layer should be such that capillary rise of water remains within this layer. If this layer is too thin, it is likely that capillary rising water may rise above this layer and affect the road sub-grade. Refer to figure 6.7 and 6.8.
Granular Material
Capillary Rise
Highest Water Table
Figure 6.7 : Prevention of a capillary rise ( Using Granular Material )
Figure 6.8 : Prevention of a capillary rise ( Using Impermeable Method )
In second method, in place of granular layer, an impermeable layer generally of bituminous material is inserted to arrest the capillary rise. 50% straight rum bitumen 80/100 grade with 50% diesel oil at the rate 1kg/m^2 is used for this layer.
Impermeable Layer
Capillary Rise Highest Water Table
traffic load transmitted on pipe is of small intensity and also without vibrations.
Pipes may be made of stone ware, concrete, etc. Pipes should be laid on 15 cm cement concrete bedding. A protective rubble apron should be provided in sandy or clay beds.
Figure 6.9 : Pipe Culverts
b. Slab Culvert
These culverts have masonry abutments with stone slab over them. In localities where stone is easily available, these culvert are mostly used. These culverts are mostly used up to about 2 m span. Where stone patties are not available, Rein-concrete slabs are used. Rein-concrete slabs are designed as simply supported slabs. Span of rein-concrete slab may be about 3 m.
Road Surface
Figure 6.10 : Slab Culvert c. Box Culvert
These culverts are constructed where the nature of the soil below the foundation is not suitable for individual footing under piers and abutments. It is a monolithic rectangular drainage structure. The size of the rectangular passage should not be less than 60cm X 60 cm for easy cleaning of debris. Short span box cilverts can be precast.
Figure 6.11 : Box Culvert
Bed Pitching
