Essay: Highway construction

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  • Highway construction
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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.

3.Embankment materials.

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

Embankment Design

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.

1.Roadway Width

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.

2.Height

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.

3.Side Slopes

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

Most probable

I.S. classification

Possible I.S. classification

Visual Descrip -tion

Max. dry density gm/c.c

OMC

Anticipated embankment performance

A-1

GW, GP, GM, SW,SP,SM

Granular materials

1.84 – 2.28

7-15

Good to excellent

A-2

GM,GC, SM,SC

Granular materials with soil

1.76 – 2.16

9-18

Fair to excellent

A-3

SP

Sand

1.76 -1.84

9-15

Fair to good

A-4

ML, MH, OL,OH

CL, SM, SC

Sandy silts and silts

1.52 – 2.08

10-20

Poor to good

A-5

MH, OH

Elastic silts and silts

1.36-1.60

20-35

Unsatisfactory

A-6

CL, CI

MH, OH, SC

Silt clay

1.52-1.92

10-30

Poor to good

A-7

MH, CI, CH.OH

sc

Clay

1.36-1.84

15-35

Poor to fair

Drainage of Embankment

Special considerations have to be exercised for drainage of high embankment and protection of slopes from erosion due to surface flow.

1.Drainage of High Embankment

Embankments with height more than 8 m needs consideration.Such embankments have steep longitudinal gradients particularly at approaches to bridges.Serious damages can occur if the rain water is allowed to leave the carriage way in an uncontrolled manner.

It causes erosion of slopes and shoulders. This may undetermine the road pavement.

Typical drainage arrangement in high embankment

It is necessary to provide longitudinal drains from where the water is led down the slopes by means of stepped outfalls or lined chutes.Water from these outfalls finally discharge into the open drain at the bottom. Embankments slopes in between the stepped outfalls or lined chutes need to be protected from surface erosion.

2.Protection of Slope

Run-off from the pavement surface and the shoulders ultimately falls over the slopes of the embankments.Standing flood water against the road embankment in flood-prone zone causes erosion during draw-down.Erosion due to flows may cause the following harmful effects.

(i)Reduction in the effective width of roadway and shoulders due to the formation of rain cuts.

(ii)Stability of the slope is endangered due to increase in driving force due to saturation of the soil and reduction of shear strength.

(iii)Continued additional expenditure or maintenance.

Different factors contribute for erosion such as soil type, slope of embankment, height of embankment, intensity of rainfall, etc.

The following aspects are to be considered:

(i)High approaches to over-bridges.

(ii)Embankments in loose sandy soils.

(iii)Heavy rainfall zones.

Variety of treatment are available to protect the embankment and cut slopes depending on the severity of protection of the expected erosion. Some of the remedies are:

(i)Turfing

(ii)Pitching (with riprap, cement sand slabs, cement concrete slabs or bridges)

(iii)Raised curbs, gutters and flumes

(iv)Bitumen treatment

(i)Turfing

It is the most well-known and cheapest form of slope protection. Growth of turf depends on the type of soil.

Turf can be grown by seeding. Seeding is done in flatter slopes whereas seeds are preferred in steeper slopes.

(ii)Pitching

This is particularly done for high embankments in flood zones and heavy rainfall areas. In order to provide outlet for seepage of water in the embankment, open jointed pitching is preferred. A typical pitching is shown in figure.

Pitching embankment slope

(iii)Curbs

Curbs, gutters and flumes are provided where high approaches to over-bridges are needed in heavy rainfall zones.Dimensions, opening of flumes and slope depend on the demand.

Curbs,gutter and flume arrangement for drainage of highway embankment.

(iv)Bituminous Treatment

This treatment is given in different ways. The asphalt is sprayed as an emulsions which helps in a number of ways.It acts down the susceptibility to erosion.It helps to raise the soil temperature by absorbing light rays and promoting the emergence of tiny saplings.The other approach consists of laying and compacting asphalt mixes or spraying a low viscosity cut-back.

Embankments on Weak Soils

In locations on weak soils special design and construction features are adopted. If unsuitable soil conditions are encountered at the site of a proposed structure, one of the following five procedures are adopted:

1.Bypassing the unsuitable soil by means of deep foundation extending to a suitable bearing material.

2.The structure is redesigned with light weight material.

3.By removing the poor material and either treat it to improve and replace it by a suitable material.

4.By treating the soil in place to improve its properties.

5.By adopting suitable stage construction procedure with the use of sand drains.

EMBANKMENT CONSTRUCTION

Construction of embankment may be done either by rolling in relatively thin layers or by hydraulic fills. The former method is called rolled-earth method which is preferred in highway embankments. Embankment construction is performed in different stages as detailed below.

Borrow-pit

The required type of soil may not be available fully in quality and quantity but are to be processed in a pit called borrow-pit.This processed soil is only used for the construction of embankment.Following practice has been recommended by the Indian Road Congress (1961) for borrow- pits:

1.It should be located nearer to the road boundary as far as possible and should be rectangular in shape.

2.In order to provide due allowance for future widening of embankment, no borrow pit should be located within 5 m from the toe of the final embankment.

3.The depth of borrow-pit should be such that they do not cut an imaginary line having a slope of 1 in 4 projected from the edge of the bank. The maximum depth should be limited to 1.5 m.

4.Borrow-pits should not be dug continuously. Ridges should not be less than 8 m width and should be left at intervals not exceeding 300 m. In order to facilitate drainage small drains should be cut through the ridges.

5.In temporarily acquired land the depth of borrow pits should not be greater than 45 cm.

6.A collection of water in borrow-pit may give room for breeding of mosquitoes, they should not be dug within 0.8 km of towns and villages.

Cleaning of Natural Ground

Clearing and grubbing are the primary operation to be carried out in highway earthwork.It consists of removing trees, stumps, roots, shrubs, etc. from the area to be occupied by the embankment. Top soil is stripped in the areas where the embankment soil is not conducive to the growth of turf. Stripped material is then laid as a blanket on the embankment slopes.

Compaction of Original Ground

The dry density of the original ground should be 90% of the proctor’s density.The original ground is brought to the required density by loosening the soil, watering and rolling in layers of 25 cm. This treatment has to be extended to a depth of 0.5 m. If the soil 15 cm beneath this depth does not have a compaction of atleast 90%, it should be made up to at least 95%.

Compaction of Embankment

Embankment is constructed in layers by rolling.First the soil is dumped on the natural ground .The water content should be OMC ± 2%.

Required water is added at the borrow-pit or at the site. Soil clods should be broken to have a maximum size of 15 cm. In the top of the embankment, the maximum size should be less than 6 cm.

The soil is spread and depth of each layer shall be generally restricted to 25 cm. Mechanical equipment is needed for achieving the desired compaction.

Type of rollers used are sheep foot roller, pneumatic tyred roller, vibratory roller and power rammers.Compaction of each layer, called lift, should achieve 90 to 95% of proctor’s density.

Higher densities are desired in the top 0.5 m of the embankment immediately below the pavement than in the other portions.

Density requirements for embankments and subgrade materials, as per IRC (2001), are given in Table.

Density Requirements of Embankment and Subgrade (Source: IRC-2001)

Sl.No.

Type of Work

Maximum laboratory Dry unit weight kN/m3.

1.

Embankments up to 3 m height, not subjected to extensive flooding

Not less than 15.2

2.

Embankments exceeding 3m height or embankments of any height to long periods of inundation.

Not less than 16.0

3.

Subgrade and earthen shoulders verges/backfill

Not less than 17.5

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