What Is WBM Road
The full form of WBM is Water Bound Macadam Road which is the most commonly used road construction procedure for more than 100 years. The Water Bound Macadam is named based on the Scottish engineer John Loudoun Macadam who first introduced and construct the WBM road.
Water Bound Macadam is a type of flexible pavement in which the base and surface layer contains crushed stone or broken rock pieces and materials are well interlocked with the help of a mechanical roller. Then the voids are filled with the help of screening material and binding materials (stone dust) along with water and compaction.
The thickness and the number of each compacted layer in WBM depend upon the design consideration and loading. However, they generally vary from 7.5 cm to 10 cm thick.
The Water Bound Macadam layers can easily disintegrate due to moving vehicle loads and surface water, thus the surface layer is usually prohibited to provide with the WBM layer. The road camber of 1 in 36 to 1 in 48 is usually preferable in WBM roads.
Materials Which Required For the Construction of WBM Road
There are mainly three types of construction material are required for the construction of WBM roads:
- Coarse Aggregate
- Binding Material
Coarse Aggregate is consists of a mixture of hard and durable crushed aggregate and broken stones. The aggregate used for every layer of the Water Bound Macadam Road construction should be well graded.
The below tables shows the gradation of the aggregates that can be adopted.
The coarse aggregates used in the construction of WBM road should possess the following properties:
- It should be hard and durable
- The coarse aggregate should be free from elongated and flaky particles.
- The coarse aggregates should be in acceptable shape and size as shown in the gradation table.
|Grading Number||Size Range in mm||Size of Sieve in mm||Percentage by Weight Passing the sieve|
|1.||90 to 40||100|
|2.||63 to 40||80 |
|3.||50 to 20||63|
The material which is used to fill up the excess void present in the compacted layer of coarse aggregate is called Screening. Screening material is nothing but aggregates of a smaller size than the coarse aggregates.
The following table shows the standard grading required for screening for the construction of WBM Road. To reduce the overall cost of WBM roads, IRC has recommended using non-plastic materials such as kankar, murram, or gravel instead of screening.
A Binding Material that is going to be utilized for the WBM road should be checked and approved by the engineer and it should have a plasticity index value of less than 6.
|Class of Grading||Size of Screening in mm||Size of sieve in mm||Percentage by Weight passing the sieve|
Usually, the Binding Material is not required if screening is utilized in the construction of WBM road or crushed rock as they have crushable property.
Cross Section of WBM Road
Following is WBM road cross section drawing,
Construction Procedure of WBM Road
The execution of WBM Road includes a series of sequential operations operated with a distinct interval of time. The followings are the steps to be followed during construction.
Steps of construction procedure of water-bound macadam road as below,
- Construction of the Foundation for Receiving the WBM Course.
- Providing of the Lateral Confinement.
- Coarse Aggregates Spreading
- Application of the Screenings.
- Sprinkling and Grouting.
- Application of the Binding Material.
- Setting and Drying.
- Preparation of Shoulder.
- Opening to Traffic.
1. Preparation of Subgrade or Foundation
- The subgrade layer contributes as a foundation layer of the road on which base, subbase, and wear course are rested. The required filling and cutting are done to bring the desire R.L. of ground.
- Preparation of this subgrade layer for the grade & camber and, dust & other loose materials is cleaned up.
- If necessary then modified subgrade materials are filled in the holes, depression, and uneven surface and then rolled and compacted.
- The level of the foundation must possess a high degree of uniformity, required cross-section, and elevation along with the well-drained condition.
2. Providing Lateral Confinement
- The lateral confinement is constructed before the laying of different layers of Water Bound Macadam materials.
- Before laying of aggregates the shoulders having a thickness as that of the compacted WBM layer should be constructed. It should be constructed with proper quality of murram or earth.
- But if the soil is unstable and the road is at a higher elevation or required to be filled, then the retaining wall can be constructed as lateral confinement.
- The main purpose of providing lateral confinement is that the road surface to be constructed is retain in between them and it becomes easy for laying of coarse aggregates.
3. Spreading the Coarse Aggregates
- To 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 through the required length of the road to be constructed.
- Then the coarse aggregates are evenly spread on the prepared base with the help of different types of machinery like backhoe or dozer.
- The total number of layers and thickness of aggregates depends on the details of pavement design. The thickness of every layer should be such that after compaction it gives a 7.5 cm thick layer.
- In general, for minor or ordinary roads, the thickness of a single compacted layer should 75 mm and is sufficient. For major or special roads, two layers of 150 mm each compacted thickness may be provided.
- Then, the road profile is checked by placing templates across the road every 6 m. the surface is checked from time to time to ensure the proper camber and grade of the road.
4. Rolling Process
- The process of rolling is carried out along the edges first and then moved to the center for the straight section of the road. For super elevated curves, rolling is done from the lower end of superelevation and then moved to the high edge.
- To carry out rolling in Water Bound Macadam Roads, the vibratory rollers or three-footed power rollers weighing 6 tons to 11 tons are used.
- Skilled operators must be used for driving the roller as the fault rolling operations lead to the formation of corrugations, the unequal finish of road surface, or wearing of the road in few months of construction.
- The main objective of this rolling is to fix the coarse aggregate thoroughly over one to another.
5. Application of Screening (Fine) Material
- The next step is the spreading of the screening material. After the process of rolling is completed, screening material is applied to fill all the voids that remained between the aggregates after the compaction.
- The quantity of screening is generally used to fill about 50% of the total voids and the remaining 50% voids are filled with filler materials.
- The process of spreading, booming, and rolling is carried out simultaneously for each layer till the surface of coarse aggregate seems firm and well compacted without any voids.
- The screening is applied in 3 layers or even more than that as per the sire conditions.
- After compaction, booming of the each layer should be done to remove the un-compacted screening material.
6. Sprinkling of Water and Grouting
- After the wet rolling of the screening, 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.
- The water sprinkling and process of rolling continued until all the voids are filled and a wave of grout flushed ahead of the roller.
- The quantity of water to be sprinkled is commonly dependent upon many factors such as size and nature of aggregate, type of surface desired, etc.
- If the voids are still viewable then an additional layer of screenings can also be applied and properly compacted.
7. Application of Binding Materials
- The same procedure is used for the application of binding materials as that of screenings. Hereafter each layer of water is sprinkled and a rolling operation is carried out.
- At the time of rolling operation, the wheel of the roller should be continuously watered to wash out the binding material that gets stuck to the wheels of the rollers.
- The binding material is generally stone dust or lime dust depends upon the suitability and requirements of WBM road, which are applied in 2 or more consecutive thin layers.
8. Setting and Drying of Surface
- After the final rolling operation, the WBM is left to cure for a single day and on the next day if any depressions or undulations are visible then again the adequate amount of screenings and binding materials can be spread and compacted.
- The WBM road can be changed by using the upper courses as a bituminous mix as well so that the easy rotting and disintegration of the surface are prevented.
9. Preparation of Shoulders
- While curing the road, shoulders are constructed alongside by filling earth to the specified cross slope. They are properly compacted.
10. Open for Traffic
- After proper drying and without any depressions, the road is then made upon for the traffic.
- For few days the traffic should be well disturbed over the full width of the road by placing obstacles longitudinally in the form of drums, barricades, etc.
Maintenance of WBM Road
- Within a short period, if any type of potholes or rugs shown on the newly constructed Water Bound Macadam Road, the concerned authority should repair it by filling it with the required material, and placing the material on potholes should be done properly.
- To prohibited aggregate from getting loose from the surface course of the road, a thin layer of moist soil should be spread over the surface periodically, particularly after the rainy season.
- The unevenness that occurred should be removed from the constructed road through dragging. It may cause the circumstance more badly for the constructed roads.
- Fresh materials should be used in place of any broken materials on the constructed road.
- Every 2 to 5 years after the constriction, the surface of the constructed road required to be renewed, keeping in view the volume of traffic.
- After the road opens for traffic, the loose mixture should be removed that comes on the top of the road surface. Along with the fresh binding material, the leveled road surface should be added and it should be properly compacted and watered.
- The dust problem can be effectively removed by providing a surface dressing of bituminous materials.
Advantages of WBM Road
Followings are the advantages of WBM road,
- It is cost-effective as the cost of construction of Water Bound Macadam road is comparatively low.
- In the construction of the WBM road, no skilled laborers are required.
- Water Bound Macadam roads are constructed with the help of locally and easily available materials.
- If WBM roads are maintained perfectly from time to time then they could resist heavy traffic and approximately about 900 tons per day.
Disadvantages of WBM Road
Followings are some disadvantages of WBM road,
- The maintenance cost of Water Bound Macadam road is high as compared to the bitumen base course.
- The overall life span of Water Bound Macadam road is less than the bitumen road.
- A high probability of danger to traffic and inconvenience is there if these roads do not receive proper maintenance periodically.
- These roads lead to yielding and softening of the subsoil as these roads are absorbent to rainwater.
- When a fast-moving vehicle passes over a WBM road, the stone piece gets disturbed and finally, the road surface disintegrates
What is WBM road?
WBM in the term ‘WBM Road’ stands for Water Bound Macadam. It is a type of flexible pavement in which the base and surface layer contains crushed stone or broken rock pieces and materials are well interlocked with the help of a mechanical roller.
What maintenance is required for WBM roads?
MAINTAINANCE FOR WBM ROADS
1. The concerned authority should repair it by filling it with the required material, and placing the material on potholes should be done properly.
2. Fresh materials should be used in place of any broken materials on the constructed road.
3. Every 2 to 5 years after the constriction, the surface of the constructed road required to be renewed.
4. The dust problem can be effectively removed by providing a surface dressing of bituminous materials.
What is WBM Road Specification?
WBM ROAD SPECIFICATION
In WBM Road the binding material consists of fine-grained material passing through a 0.425 mm sieve is used, in order to prevent raveling of the stones. Locally available kankar nodules or limestone dust may also be utilized. The binding material having a plasticity index value of 4 to 8 is used in construction.
State the Advantages of Macadam Roads?
Advantages of Macadam Roads
1. It is cost-effective as the cost of construction of Water Bound Macadam road is comparatively low.
2. In the construction of the WBM road, no skilled laborers are required.
3. Water Bound Macadam roads are constructed with the help of locally and easily available materials.
4. If WBM roads are maintained perfectly from time to time then they could resist heavy traffic and approximately about 900 tons per day.
What are the Disadvantages of Macadam Roads?
Disadvantages of Macadam Roads
1. The maintenance cost of Water Bound Macadam road is high as compared to the bitumen base course.
2. The overall life span of Water Bound Macadam road is less than the bitumen road.
3. A high probability of danger to traffic and inconvenience is there if these roads do not receive proper maintenance periodically.
4. These roads lead to yielding and softening of the subsoil as these roads are absorbent to rainwater.
The full form of WBM is Water Bound Macadam. It is the layer of road constructed from broken stone aggregates bound together by stone dust or screening material and water applied during construction and compacted by heavy smoothed wheel roller.
What Does WBM Mean?
WBM means “Wanna-Be Moderator”.
WBM Road Specification
The thickness and number of layers in WBM road depend upon the details of design pavement. In general, for ordinary roads, 75 mm thickness of layers is sufficient for a single layer. For special roads, 150 mm thickness for 2 layers pavement can be selected.
WBM Road Construction Procedure
Construction Procedure of WBM Road as follows,
Construction of the Foundation for Receiving the WBM Course.
Providing of the Lateral Confinement.
Coarse Aggregates Spreading
Application of the Screenings.
Sprinkling and Grouting.
Application of the Binding Material.
Setting and Drying.
Preparation of Shoulder.
Opening to Traffic.
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