Last reviewed 16 Sept 2026 · 11 min read
Earthwork, embankment and subgrade
- Clearing and grubbing, removal of topsoil.
- Embankment construction in layers (commonly not more than about 200–250 mm compacted thickness) with suitable soil at optimum moisture content (OMC), compacted by rollers.
- Subgrade — the top portion (commonly 500 mm) compacted to a higher density.
Compaction requirements (MoRTH practice, modified Proctor): embankment about 95% of maximum dry density (MDD); subgrade and shoulders about 97% of MDD.
Field density checked by the sand replacement method or core cutter method (soft soils), or nuclear density gauges.
Compaction equipment: smooth-wheeled rollers (granular and bituminous layers), sheep-foot rollers (cohesive soils), pneumatic-tyred rollers (mixed soils, bituminous layers), vibratory rollers (granular soils), plate compactors and rammers (confined spaces).
Soil stabilisation
Improving soil properties (strength, stability, durability) for use as subgrade, sub-base or base.
| Method | Suitable for | Action |
|---|---|---|
| Mechanical stabilisation | Granular soils | Blending soils/aggregates to a proper gradation and compacting |
| Cement stabilisation | Sandy and low-plasticity soils | Cement hydration binds particles |
| Lime stabilisation | Clayey, plastic soils (black cotton soil) | Reduces plasticity and swelling; pozzolanic reactions increase strength |
| Bitumen stabilisation | Sandy soils | Waterproofing and cohesion |
| Chemical stabilisation | Various | Calcium chloride (dust control, moisture retention), sodium silicate, enzymes, polymers |
| Geosynthetics | Weak subgrades | Separation, reinforcement, drainage |
| Fly ash, slag | Various | Pozzolanic stabilisation, fill material |
Granular layers
Granular sub-base (GSB)
Well-graded or close-graded natural gravel, crushed stone or mixtures, laid at OMC and compacted; acts as a structural and drainage layer extending across the full formation width.
Water bound macadam (WBM)
Clean crushed coarse aggregates mechanically interlocked by rolling, with voids filled with screenings and binding material (e.g. fine soil) with the help of water.
Construction steps:
- Preparation of the base (shaping to camber, compaction), provision of lateral confinement (shoulders).
- Spreading of coarse aggregates in uniform layers (compacted thickness typically about 75 mm per layer).
- Dry rolling from the edges towards the centre (overlapping) until aggregates are firmly interlocked.
- Application of screenings to fill voids, with dry rolling and brooming.
- Sprinkling of water and grouting with continued rolling (wet rolling).
- Application of binding material with water to form a slurry filling remaining voids; rolling.
- Setting and drying — the layer is allowed to dry overnight before the next layer or surface treatment.
Wet mix macadam (WMM)
Graded crushed aggregates and granular material pre-mixed with water at OMC in a pugmill/mixing plant, laid with a paver and compacted with vibratory rollers. Faster, more uniform and denser than WBM; widely used as base course on highways.
Bituminous construction
Interface treatments
| Treatment | Purpose | Material |
|---|---|---|
| Prime coat | First application of low-viscosity binder on a porous granular surface (WBM/WMM) to plug voids, waterproof and promote bond with the bituminous layer | Medium-curing cutback or slow-setting bitumen emulsion |
| Tack coat | Very light application on an existing bituminous or concrete surface to bond it with the new bituminous layer | Rapid-setting bitumen emulsion or suitable binder |
| Seal coat | Thin surface treatment on open-textured surfaces (e.g. premix carpet) to seal voids against water, and to improve skid resistance | Liquid seal (binder + stone chips) or premixed sand–bitumen seal |
Bituminous surfacing and layers
| Layer | Description |
|---|---|
| Surface dressing (single/double) | Sprayed binder covered with stone chippings rolled in — waterproofing and skid resistance on low-volume roads or as renewal |
| Open-graded premix carpet (PC) | Thin (about 20 mm) premixed chips and binder, followed by a seal coat — rural and low-traffic roads |
| Mix seal surfacing | Closed-graded premix surfacing |
| Bituminous macadam (BM) | Open-graded premix used as base/binder course for lower traffic |
| Dense bituminous macadam (DBM) | Dense-graded premix binder/base course for heavy traffic |
| Semi-dense bituminous concrete (SDBC) | Wearing course for medium traffic |
| Bituminous concrete (BC) | Dense-graded, high-quality wearing course for heavily trafficked roads |
| Stone matrix asphalt (SMA) | Gap-graded, stone-on-stone skeleton with rich mastic and fibres; highly rut-resistant, durable |
| Mastic asphalt | Voidless mix of bitumen, fine aggregate and filler; for bridge decks, heavily stressed areas, waterproofing |
Hot mix construction
- Hot mix plant — batch plant (weighs and mixes each batch) or drum mix plant (continuous mixing in a drum); aggregates heated and dried, binder heated, mixed to specified temperatures.
- Transport in covered tippers to limit heat loss.
- Preparation of the base — cleaning, correcting profile, tack coat.
- Laying with a sensor paver to line, level and camber, at the specified temperature.
- Compaction while hot:
- Breakdown rolling — smooth steel-wheeled/vibratory rollers.
- Intermediate rolling — pneumatic-tyred rollers (kneading action, sealing).
- Finish rolling — tandem smooth-wheeled rollers to remove marks. Rolling proceeds from the lower edge to the higher edge, with longitudinal joints compacted first.
- Quality control — temperature, binder content (extraction), gradation, density (cores), thickness, surface regularity (3 m straight edge), Marshall properties.
Warm mix asphalt technologies lower mixing temperatures, saving energy and reducing emissions.
Cement concrete pavement construction
- Preparation of subgrade, drainage layer and DLC sub-base; laying of separation membrane.
- Placing dowel bar assemblies (or automatic dowel inserters) and tie bars.
- Batching and mixing (central batching plant); transport in tippers/transit mixers.
- Paving with a slip-form paver (or fixed-form paving with vibrating screeds for smaller works); internal vibration.
- Finishing — floating, straight-edging; surface texturing (brush or tining) for skid resistance.
- Curing — application of curing compound immediately, followed by wet curing (e.g. wet hessian, ponding) for about 14 days.
- Joint cutting with saws at the correct time (before uncontrolled cracking), then joint sealing.
- Opening to traffic after the concrete gains required strength (typically about 28 days for full strength).
Failures of flexible pavements
| Distress | Description | Main causes |
|---|---|---|
| Alligator (fatigue) cracking | Interconnected cracks forming small polygons in wheel paths | Repeated loads exceeding structural capacity; weak base/subgrade; ageing binder |
| Longitudinal and transverse cracks | Cracks parallel/perpendicular to centre line | Poor joint construction, thermal shrinkage, reflection of underlying cracks, settlement |
| Block cracking | Rectangular blocks | Shrinkage and ageing of binder |
| Edge cracking and edge breaking | Near the pavement edge | Lack of lateral support (shoulders), drainage problems |
| Reflection cracking | Cracks in an overlay above cracks/joints in the old pavement | Movement of underlying cracks or concrete joints |
| Rutting | Longitudinal depressions in wheel paths | Mix rutting (excess binder, soft binder, rounded aggregates, high temperature) or structural rutting (subgrade/base deformation) |
| Corrugation and shoving | Transverse ripples and bulges | Unstable mixes, braking and acceleration zones, poor bond |
| Depressions and settlement | Localised low areas | Settlement of subgrade, poor compaction |
| Ravelling | Loss of aggregates from the surface | Insufficient binder, ageing, poor compaction, stripping |
| Stripping | Loss of bond between bitumen and aggregates | Moisture, hydrophilic aggregates |
| Potholes | Bowl-shaped holes | Water entry through cracks, ravelling progression, weak base |
| Bleeding (flushing) | Excess binder on the surface — shiny, sticky, slippery | Excessive binder, low air voids, soft binder in hot weather |
| Polishing | Smooth aggregates, low skid resistance | Polish-susceptible aggregates |