Hill Road Manual
1998 Edition

Recommended bridge structures for different span ranges in hilly terrains according to IRC SP 48 include:

Additional considerations for hilly settings:

• Prefer single-span bridges to avoid piers in swift, boulder-laden streams.
• Foundation type (open or well) depends on soil and site conditions.
• Portable steel bridges (e.g., Bailey bridges) are useful for emergency and temporary needs.
• Timber bridges are generally temporary and used where timber is locally abundant.
• Detailed site surveys and expert reviews are essential for safe designs.

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To prevent landslides through effective drainage design and upkeep as outlined in IRC SP 48:

Drainage Design Principles

• Horizontal Drains: Installed at slight outward inclinations to lower groundwater levels and reduce pore water pressure, thereby improving slope stability. These drains are suitable across varying soil types including weathered rock and are securely fixed to prevent displacement. They may be positioned behind retaining walls and discharge into side drains.
• Deep Trench Drains: Utilized to intercept subsurface water at depths less than 5-6 meters, these drains employ coarse gravel wrapped in geotextile fabric to avoid clogging. They are constructed in short sections with control shafts to facilitate monitoring.
• Surface and Catch Water Drains: Designed to intercept and safely divert surface runoff via lined channels to natural water bodies, thus preventing infiltration into unstable slopes.

Maintenance Measures

• Routine clearing of catch water and side drains to remove debris and vegetation, maintaining effective flow.
• Ensuring appropriate gradients and lining to prevent erosion within drainage channels.
• Temporary coverings may be used to reduce clogging risks.
• Properly maintained shoulders with adequate slope and hard surfaces prevent water ingress into the pavement subgrade.
• Regular inspections and timely repairs before monsoons reduce the risk of failures.

Conceptual Flow Diagram

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Summary: Properly designed and maintained surface and subsurface drainage systems are critical to lowering pore water pressure and increasing the factor of safety against landslides, especially when integrated with slope protection structures and vegetation.

According to IRC SP 48, suitable materials and construction methods for retaining walls in hilly road construction include:

Materials

• Dry Stone Masonry: Combined with Random Rubble (R.R.) masonry bands set in cement mortar to enhance strength.
• R.R. Masonry in Cement Mortar: Especially favored for slopes exposed to moisture.
• Stone Boulders and Large Gravel: Used for backfill and foundation layers.
• Reinforced Earth Walls: Granular backfill stabilized by tensile reinforcements such as geotextile strips or steel bands, with facing panels or turfing.
• Gabion or Sausage Walls: Wire mesh cages filled with stones, providing flexibility and drainage.
• Breast Walls: Stone masonry with numerous weep holes to relieve water pressure in wet slopes.

Construction Techniques

• Foundations should be placed on solid ground, protected from frost, scour, and water action, including toe projections to reduce pressure.
• Wall dimensions typically use base width formulas: for dry walls, base width ≈ 0.4 × height + 0.3 m; for wet conditions, base width ≈ 0.4 × height + 0.6 m.
• Wall thickness at the top is about 0.6 m; front face is battered (1 in 3 or 1 in 4), while back face is vertical or slightly inclined.
• Stones should be minimum size 20×15×10 cm, laid in courses perpendicular to the batter, with bond stones spaced approximately one per 0.5 m² overlapping at least 15 cm.
• Adequate drainage provisions include staggered weep holes (≥15×10 cm) spaced at 1 m intervals and loose stone drains behind the wall.
• Backfill is placed in layers of 150 mm, compacted and sloped away from the wall.

Design Considerations

• Maintain a factor of safety of at least 2 against sliding.
• Ensure foundation pressure does not exceed the soil bearing capacity.
• Prevent water pressure build-up through proper drainage.
• Avoid toe settlement by appropriate foundation depth and toe projection.

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This combination of materials and methods ensures stability and durability of retaining walls in challenging hill terrain.

IRC SP 48 provides specific guidance for managing expansive soils and salt contamination during subgrade preparation:

• Expansive Soils:

  • A non-expansive buffer layer is recommended over expansive soils such as black cotton soil.
  • If direct buffer placement is unfeasible, a blanket course at least 225 mm thick, composed of coarse or medium sand or non-plastic moorum (Plasticity Index below 5), should be provided to prevent soil movement.
  • This blanket should extend across the entire formation width.

• Salt Infestation:

  • When salt concentration exceeds 0.2%, a capillary cutoff layer of 225 mm thick coarse sand must be placed on the subgrade to prevent moisture migration upward.
  • Chemical stabilization may be enhanced by adding gypsum which converts soluble salts to less expansive calcium salts.
  • Refer to IRC: 34-1970 for detailed practices in salt-affected and waterlogged soils.

• General Subgrade Preparation:

  • Achieve the specified compaction density for the subgrade; if compaction is inadequate, increase the thickness of the sub-base accordingly.

Summary Table:

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These treatments ensure durability and stability of pavements over problematic soils.

IRC SP 48 recommends the following equipment selection and operational approaches to optimize earthmoving and construction efficiency in hilly terrain:

Equipment Deployment

• Dozers:
  • Use angle dozers with angled blades for side hill cutting.
  • Light dozers (D-50) are preferred for initial cutting on slopes exceeding 30°, followed by heavier dozers (D-80) for widening operations.
  • On slopes below 30°, light dozers can create access tracks without manual assistance.
• Drilling and Blasting: Necessary in rocky sections to facilitate dozer operations.
• Manual Labor: Approximately 25-30% of cutting is manual, with 70-75% mechanical.

Operational Strategies

• Single-stage construction is favored to expedite the process; however, a two-stage approach may be used for early light vehicle access.
• Work downhill with dozers to utilize gravity for pushing earth fill.
• Use dozer blades angled downhill to maximize fill placement.
• Side slopes are manually dressed to prevent rockfalls and ensure safety.
• Advance reconnaissance and clearing of rocky bottlenecks prevent machinery idling.

Summary Process Flow

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Key Considerations

• Manual cutting is essential for steep slopes to establish initial access.
• Mechanical cutting speeds up earthworks and reduces costs.
• Advance preparation and safety measures protect operators and equipment.

This approach balances safety, efficiency, and cost-effectiveness in challenging mountainous earthworks.