The construction steps of water bound macadam (WBM) roads and it's advantage and disadvantage.

The construction steps of WBM roads and it's advantage and disadvantage.

Water Bound Macadam (WBM) roads are a type of road construction in which broken stones (aggregates) are bound together using water and compacted to form a strong base or surface layer. The construction of WBM roads involves several systematic steps. Below is a detailed explanation of each step:

1. Preparation of Subgrade

• The subgrade is the natural soil layer on which the road is constructed.
• It should be properly leveled, graded, and compacted to achieve uniform strength.
• The subgrade must be well-drained to prevent water accumulation, which can weaken the road structure.
• Any soft spots should be replaced with better-quality soil and compacted.

2. Preparation of Sub-base and Base Course

• The sub-base and base course are layers made of compacted aggregates.
• The thickness of each layer typically ranges from 75 mm to 150 mm depending on the traffic load.
• The material used consists of well-graded broken stones, gravel, or crushed rock.

Steps Involved:

1. Spreading the Aggregates:

   • The aggregates are spread uniformly over the prepared subgrade using a motor grader or manually.
   • The material should meet the required specifications, including proper grading.

2. Watering the Surface:

   • Water is sprinkled over the aggregate layer to help with binding and compaction.
   • The quantity of water should be controlled to prevent excess moisture.

3. Rolling and Compaction:

   • A smooth-wheeled or vibratory roller (6-10 tons) is used to compact the aggregate layer.
   • Rolling is done until the aggregates are firmly locked in place and no further movement is observed.

3. Application of Screenings

• Screenings are fine aggregates (such as stone dust) used to fill voids between larger aggregates.
• They are applied in thin layers over the compacted base course.
• Water is sprinkled, and rolling is continued to ensure proper filling of gaps.

4. Application of Binding Material

• A binding material, such as stone dust, moorum (red clayey soil), or cement, is spread over the compacted layer.
• The purpose is to create a strong bond between aggregates.
• Water is added, and additional rolling is done to achieve a dense and stable layer.

5. Setting and Curing

• After compaction, the surface is left undisturbed for 24 hours to allow setting.
• Curing is done by sprinkling water at regular intervals to strengthen the bonding.

6. Surface Dressing or Wearing Course (if needed)

• Depending on the traffic load, an additional wearing course (such as a bituminous or cement concrete layer) may be applied.
• If WBM is the final surface layer, it may require periodic maintenance due to dust and displacement of aggregates.

7. Drainage Arrangements

• Proper drainage is essential to prevent water stagnation.
• Side drains, cross drains, and camber (sloped road surface) are constructed to facilitate water runoff.

Advantage and disadvantage of WBM roads.

Advantages of WBM Roads:

1. Cost-Effective – WBM roads are cheaper compared to other types like bituminous or concrete roads.
2. Locally Available Materials – The aggregates used are generally available locally, reducing transportation costs.
3. Good Load-Bearing Capacity – When properly compacted, WBM roads can bear moderate traffic loads.
4. Easy Construction & Maintenance – The construction process is straightforward, and repairs can be done quickly.
5. Good Drainage Properties – The layers allow water to drain quickly, preventing water accumulation.
6. Suitable for Rural Roads – It is ideal for low-traffic rural roads where heavy-duty pavement is not required.
7. Can be Upgraded – WBM roads can be easily converted into bituminous roads by adding an asphalt layer.
8. Less Environmental Impact – Since they don’t use harmful chemicals or petroleum-based products, they are more eco-friendly.

Disadvantages of WBM Roads:

1. High Maintenance – These roads require frequent maintenance due to loose aggregates and erosion.
2. Not Suitable for Heavy Traffic – They cannot withstand heavy loads and high-speed vehicles for long durations.
3. Dust Generation – Loose gravel and stones create dust, which can be a health hazard.
4. Erosion Due to Water – Heavy rains can wash away the surface layer, leading to potholes and damage.
5. Rough Surface – The surface is not as smooth as asphalt or concrete roads, leading to discomfort for vehicles and pedestrians.
6. Prone to Rutting & Potholes – Due to wear and tear, WBM roads can develop ruts and potholes quickly.
7. Slow Construction Process – Compared to bitumen or concrete roads, WBM roads take longer to construct due to water curing.
8. Limited Lifespan – These roads have a shorter lifespan compared to asphalt or concrete roads and require frequent resurfacing.

Materials are required for construction of WBM roads.

Water Bound Macadam (WBM) roads are constructed using compacted aggregates held together by water and binding material. The materials required for WBM road construction include:

1. Coarse Aggregates

• Types: Crushed stone, gravel, or crushed slag
• Size: Typically 90mm to 40mm for base course, 63mm to 40mm for intermediate, and 50mm to 20mm for the top
• Properties: Hard, durable, angular, and free from dust, clay, and organic matter
• Purpose: Provides structural stability and load distribution

2. Screening Material

• Types: Stone dust, crushed stone chips, or gravel
• Size: 12.5mm to 10mm for first layer, 10mm to 5mm for upper layers
• Purpose: Fills gaps between coarse aggregates for better interlocking

3. Binding Material

• Types: Murrum, gravel, kankar, moorum, or lime
• Properties: Should have good plasticity and binding capability
• Purpose: Helps in compacting and holding aggregates together

4. Water

• Source: Should be clean and free from organic impurities
• Purpose: Helps in the compaction process and activates the binding material

5. Sub-base or Base Course (If Required)

• Material: Granular sub-base (GSB) or compacted soil layer
• Purpose: Provides a stable foundation for WBM layers

6. Rolling Equipment

• Types:
  • 6 to 10-ton road rollers for compaction
  • Hand-operated or vibratory rollers for smaller sections
• Purpose: Compacts the aggregates and improves interlocking

7. Side Drain Materials (If Required)

• Materials: Bricks, stones, concrete pipes, or precast drains
• Purpose: Facilitates proper drainage to prevent waterlogging

Additional Considerations

• The WBM road should be built in layers (typically 75mm to 100mm thick)
• Proper curing is necessary for the binding material to hold the aggregates together
• In modern roads, WBM is often topped with a bituminous or concrete layer for durability