WBM road construction procedure | Features | Application of WBM


What is WBM Road?

Flexible pavement is the type of road pavement which have very less flexural strength and where load transfer is grain to grain to soil rather than flexural distribution. WBM road construction procedure is given below in detail.

The materials are compacted but don’t act as monolithic mass and deformation occur from top to bottom. The flexible pavement generally consists following basic layers: subgrade, base, sub-base, and surface layer.

Water bound Macadam is a type of flexible pavement un which the base and surface layer contains crushed stone or broken rock pieces as the material for construction which are well interlocked with help of mechanical rammer and roller first, and then the voids are filled with help of screening material and binding material (stone dust)  along with water and compaction.

The water-bound Macadam is named after the Scottish Engineer John Loudoun Macadam.  The thickness and number of layers in WBM depends upon the design consideration and load.

However, they generally range from 7.5 cm thick to 10 cm thick. The water-bound Macadam layers can easily disintegrate due to moving vehicle loads and the surface water over the road, thus the surface layer is generally prohibited to provide with WBM layer.

Instead, the surface layer of roads is preferred to be treated with asphalt or bituminous mix surface course, keeping remaining layers as WBM in order to solve this problem. The camber of 1 in 36 to 1 in 48 is preferred in WBM roads.

Features of WBM

  • Hard, crushed angular shaped crushed stones are used in WBM and the elongated and flaky rocks is extremely prohibited.
  • The size of coarse aggregate use for WBM is generally should be in the range of 90mm-40mm for base course and if thickness extends 10cm. And the size of 63mm – 40 mm or 50mm –  20mm are selected for the pavement thickness of 7.5 cm. (generally for the surface course)
  • The LAA value of coarse aggregates to be used in sub-base, base, and surface course are 60, 50, and 40 respectively
  • The abrasion value of coarse aggregates required for WBM as sub-base, base, and the surface course is 50,40 and 30 respectively
  • The maximum flakiness index of 15 is only allowed for base and surface course WBM coarse aggregates.
  • The size of screening material also called filler material is generally 10mm down or 12.5 mm down with the liquid limit below 20%, plasticity index below 6% and percentage passing through 0.075 mm sieve is below 10%. This filler material fills the void between interlocking spaces of coarse aggregates.
  • Limestone (or simply stone dust) is used as binding material which has a plasticity index of 4%-9% for the WBM surface course, and below 6% for the WBM base and sub-base course.  This binding material prevents the raveling and easy separation of WBM materials.
  • Camber of 2.5% –  3%  are desired for WBM roads.

Application of WBM

  • WBM is preferred over the traffic having above 100000 but less than 1000000
  • These kind of road are applicable to the village where the complete new road access is to be formed
  • Suitable where there abundantly availability of the stone i.e. near the quarry sides and crusher
  • The WBM can be applied as a base course and sub-base course for any kind of flexible road construction. And over the layer of WBM, the higher surface course of bituminous macadam, asphalt concrete or even rigid pavement can be done. So, very useful to be constructed when a low budget is available in a certain fiscal year and then can again be maintained or improvised with other superior quality

Procedure for construction of WBM road

WBM road construction procedure | Features | Application of WBM
WBM road construction procedure

The construction of WBM includes a series of sequential operations operated with distinct intervals of time. The above 100000 but less than 1000000 procedure is explained below

1) Preparation of subgrade

The subgrade acts as the foundation layer of the road over which base, subbase, and wear course are rested. The necessary cutting and filling are done in order to bring the required R.L of ground.

And if necessary then modified subgrade material are filled in the holes, depression, and uneven surface and then rolled and compacted. The loose particles and dust from the subgrade layer are completely removed.

The subgrade must be brought to a high degree of uniformity, required cross-section, and elevation along with the well-drained condition.

2) Provision of lateral confinement

The lateral confinement is constructed before the laying layers of Water bound Macadam material.

This confinement can simply be constructed by making trenches in stable soil confinement. But if the soil is unstable and the road is at high elevation or needed to be filled, then the retaining wall can be constructed as lateral confinement. For this reason, sometimes the shoulder is built-in advance too.

3) Spreading the course aggregate

For spreading the coarse aggregates over the surface of the subgrade, the lumped mass of coarse aggregates is stacked at the definite interval of the road throughout the required length of road to be constructed.

This is done so that the material is spread easily by the grader and there would be no problem of bringing material during the middle of work. Then, the material is spread with the help of types of machinery like backhoe or dozer.

Then the material is uniformly spread to obtain the required profile and thickness with help of a grader. All the surface irregularities are eliminated and the well level surface is checked and then subjected to rolling and compaction.

4) Rolling

For rolling WBM, the vibratory rollers or three-footed power rollers weighing 6 tons to 11 tons are used.

The rolling is done along the edges first and then moved to the center for the straight section of the road. And for super elevated curves, rolling is done from the lower end of superelevation and then moved to the high edge.

But the roller must pass parallel to the centerline of road alignment and should overlap half of the previous track while shifting and rolling to another pass. The sprinkle of water can be done if necessary. The process is repeated until the necessary compaction is achieved.

5) Spreading of screening (fines)  material

The screening material is generally small crushed stone girts, or kantars or broken brick that are spread till the voids of the coarse aggregates interlocking is filled. 

The number of layers from 3 to more is spread over the compacted coarse aggregates. The process of spreading, brooming, and rolling is carried out together for each layer till the surface of the coarse aggregate seems completely firm and well compacted without any voids.

6) Sprinkling of water and wet rolling

After the well Rolling of the screening material, the layer is sprinkled with water and then again rolled so that the water erodes the surface finer and seeps and deposits it to the inner voids.

For this purpose, sweeping is also dine along with water. The amount of water applied is dependent on a number of factors like the nature of the coarse aggregate, its size, shape,  type of screening material, and desired quality and compacted density of the course.

Thus Sprinkling and rolling are done till the desired density and surface of the course are obtained. The 50 percent

7) Application of binding material

The binding material is generally stone dust or lime dust depends on the suitability and requirements of WBM, which are applied in two or more consecutive thin layers.

After the spreading of the binder material, they are sprinkled with a lot of water in order to form a slurry. The slurry is then swept across the surface so that they seep and deposit to the voids. And the surface layer is rolled over by rollers and subjected to the next layer.

8) Setting and drying of WBM

For the WBM layer for the base and subbase course, the layer is allowed to dry thoroughly before subjected to the next course. And for surface (wear)  course, they are allowed to dry as well as well cured.

The WBM is left to cure for a single day and on the next day, the void or undulations are checked and again filled with the finer and binder material.

The WBM can be modified by using the top course as a bituminous mix as well so that the easy rotting and disintegration of the surface is prevented. The WBM is only allowed to traffic after its complete settings and drying.

I hope this article on “WBM road construction Procedure” remains helpful for you.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Rajan Shresth

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