It is constructed when the grade line of a highway is required to be raised above the existing ground level.Necessity for raising of ground level for construction of highway may arise due to the following reasons:
1.In order to make the subgrade level above the high ground water level.
2.In order to prevent the capillary water to enter the subgrade by capillary action or surface water through permeation.
3.In order to satisfy the vertical alignment of the designed highway.
Design and construction of embankment comprises of:
1.Deciding the height of embankment.
2.Finalizing side slopes and their short and long term stability.
4.Compaction requirement and compacting with suitable equipment.
5.Check for settlement and overall stability.
Figure shows the cross-section of a typical embankment.
Details of highway embankment
Important elements in embankment design are:
1.Selection of embankment dimension.
2.Stability of slopes.
3.Settlement analysis of embankment.
4.Selection of embankment materials.
5.Drainage of embankment.
6.Embankment on weak soil.
Selection of Embankment Dimension.
Roadway width are fixed as per Indian Standard recommendation for single lane and two lane roads.
In case of broader roads with more than two lanes, the road width has to be worked out to suit the number of lanes, medians and shoulders.
It is based on the desired grade line of the highway and the ground profile or topography.
The height of the fill is sometimes governed by stability of foundations, particularly when the foundation soil is weak.
In the view of erosion control, a flat slope is to be preferred.It involves, more earthwork but ensures safety of traffic.
2 horizontal and 1 vertical slope in un-inundated conditions and 3 horizontal and 1 vertical un-inundated conditions are satisfactory.
Selection of Embankment Materials
Granular soils are preferred for the embankment. Silts and clays are not desirable and organic soils are to be summarily rejected. Keeping the lead and lift to be low as the main consideration, the best of the soils available locally is often selected.
In case ,foundation soil is weak, light-weight fill material like cinder may be used. For a satisfactory performance the embankment material soils should possess the following properties:
1.A soil with large strength and of high stability.A well-graded soil gives high strength and stability should be preferred. Such a selection of soil will give an economical pavement design.
2.Compared to a fine-grained soil, course-grained are preferred.
3.Highly plastic soils should be avoided as it may exhibit large changes in volume which may lead to disruption of pavements.
4.Soil should have good drainage properties. In general well-graded coarse fractions exhibit good drainage properties and clayey soils should be avoided.
5.Soils which could be compacted to high densities should be preferred.
6.It is undesirable to use soils which are susceptible for capillary rise.
7.Frost-suceptible soils should be avoided.
8.Soils with high organic matter and with high presence of sodium sulphate should be avoided.
As per Indian Road Congress recommendations, the general guide to selection of soils on the basis of anticipated embankment performance is given in Table.
Table General guide to selection of soils on the basis of anticipated embankment performance (Source: IRC – 1970)
PRA classifi -cation
Possible I.S. classification
Visual Descrip -tion
Max. dry density gm/c.c
Anticipated embankment performance
GW, GP, GM, SW,SP,SM
1.84 – 2.28
Good to excellent
Granular materials with soil
1.76 – 2.16
Fair to excellent
Fair to good
ML, MH, OL,OH
CL, SM, SC
Sandy silts and silts
1.52 – 2.08
Poor to good
Elastic silts and silts
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