Road Construction Technology

Mass haul diagram and its characteristics

A mass haul diagram is a curve of cumulative earthwork volume (cut positive, fill negative, after correcting for shrinkage or swell) plotted as ordinate against chainage as abscissa. It is used to plan the movement of earth along a road.

 Cum.
 volume
   ^            peak
   |          ,--*--.
   |  A -----'-------'--- B  balancing line
   |      ,'            '.
   | ____/                '.     valley
   |                        '-._*______
   +------------------------------------> chainage
      cut        fill         cut

Characteristics

1. A rising curve shows cut exceeding fill; a falling curve shows fill exceeding cut.
2. A peak marks a change from cut to fill and a valley a change from fill to cut.
3. A horizontal line (balancing line) cuts the curve at points where cut and fill are equal; the earth between these points is balanced.
4. The maximum or minimum ordinate between two such points gives the volume of earth moved from cut to fill in that section.
5. The length of the balancing line gives the haul distance for that balanced volume.
6. Curve above the balancing line means haul is forward (in the direction of chainage); below means backward.
7. The area between the curve and the balancing line is the haul moment, volume times distance (m3-m).
8. A steep slope shows heavy earthwork; a flat curve shows a balanced section.
9. The end ordinate gives the excess (to be wasted) or deficit (to be borrowed) for the whole length.

Definitions

• Free haul distance (FHD): the distance up to which excavated earth can be moved with no extra payment, because it is included in the unit rate of excavation and filling (commonly 100 m in Nepal/India specifications; as per contract).
• Overhaul: hauling beyond the free haul distance; paid as volume times extra distance (m3-station, 1 station = 20 m or 100 m as specified) $=V\times(L-L_f)$.
• Economic haul distance (limit of economic haul): the longest distance for which carrying cut to fill costs no more than excavating from a borrow pit and wasting the cut; beyond it, borrowing is cheaper. $L_e = L_f + \dfrac{\text{cost of borrow and waste}}{\text{overhaul rate per m}^3\text{-m}}$.
• Shrinkage: loose or in-situ soil, when compacted in an embankment, occupies less volume than the bank volume it came from (about 10 to 20 percent for soil); the fill volume is multiplied by a shrinkage factor to get the cut needed.
• Swelling: increase in volume of soil or rock after excavation (loose condition), for example 20-30 percent for common earth and 40-60 percent for rock, so more truck volume is needed.

Asphalt concrete (bituminous concrete, BC) is a dense-graded hot mix of aggregates, filler and bitumen (VG-30/60-70 penetration grade) laid as a wearing or binder course (IRC:29 / MoRTH Section 509, Nepal DoR specifications).

Materials

• Aggregate: crushed, clean, hard and cubical (LAA below 30 percent, AIV below 24 percent, flakiness index below 30 percent combined, stripping value below 5 percent, water absorption below 2 percent), graded to the BC Grading 1 or 2.
• Filler: stone dust, cement or hydrated lime passing 75 micron.
• Binder: bitumen 60/70 or VG-30; 5 to 6 percent by weight of mix as fixed by Marshall design (stability above 9 kN, flow 2-4 mm, air voids 3-5 percent, VMA 12-14 percent).

Plant and equipment: hot-mix plant (batch or drum mix) with dryer, mixing unit and temperature control, tippers with tarpaulin, bitumen distributor, paver finisher with screed, steel-wheel roller (8-10 t), pneumatic tyre roller (15-20 t), small plate compactor, thermometers.

Construction steps

1. Prepare the base: clean, repair defects, and apply a tack coat (0.20-0.25 kg/m2 of emulsion) to the old surface or a prime coat to a granular base.
2. Produce the mix at the plant: aggregate heated to about 160-175 C and bitumen to 150-165 C; the mixed material leaves the plant at not more than 170 C.
3. Transport in covered, insulated tippers; the mix must arrive hot enough to be laid at about 120-165 C depending on the binder grade.
4. Lay by paver with proper level sensors and uniform speed; hand-lay only at small patches.
5. Initial (breakdown) rolling by 8-10 t steel roller at 110-135 C, intermediate rolling with pneumatic tyre roller, final rolling with tandem roller at 70 C or more to remove marks. Roll longitudinally from edge towards centre, overlapping.
6. Joints: cut transverse joints vertical and apply tack coat; stagger longitudinal joints.
7. Traffic opens when the surface cools below 60 C.

Quality control: tests on aggregates, bitumen (penetration, softening point, ductility), mix design by Marshall method; at site: temperature of mix, binder content (extraction), gradation, Marshall stability and flow, density by core cutter (above 98 percent of laboratory Marshall density), thickness (+-5 mm) and surface regularity with 3 m straight edge (max 5 mm) and bump integrator.

Water Bound Macadam (WBM) is a base or surfacing course of clean crushed stone aggregate of graded sizes, bound by stone dust (screenings) and water, and compacted by rolling (IRC:19 and MoRTH Section 404; used in Nepal DoR specifications for base courses).

Materials

• Coarse aggregate: hard, tough crushed stone (Los Angeles abrasion value below 40 percent for base, aggregate impact value below 30-40 percent), flakiness index below 15 percent, in three grading sizes: Grading 1 (90-45 mm), Grading 2 (63-45 mm), Grading 3 (53-22 mm).
• Screenings: 13.2 mm size crushed stone dust to fill voids (Grading A/B); non-plastic with plasticity index below 6 percent.
• Binding material: a small quantity of clay or fine crusher dust (PI below 6 percent) only if the screenings lack fines.
• Water: clean and free from salts.

Equipment: three-wheel power roller (8-10 t) or vibratory roller (8-10 t), motor grader or spreader, water tanker, brooms, screening spreaders.

Construction procedure

1. Prepare the base: clean, trim to profile, fill gaps; provide lateral confinement by side shoulders, 20 cm wide.
2. Spread coarse aggregate in uniform thickness (75 mm compacted per layer, loose depth 1.25-1.30 times) by stone spreader or manually, with camber.
3. Dry rolling: roll from edges to centre with 8-10 t roller until aggregate is firmly keyed (no creep).
4. Apply screenings gradually in 3 or 4 stages with rolling and sweeping so that voids are filled.
5. Sprinkle water on the surface and roll with the roller; slurry formed binds the stone. Keep rolling until a wave of slurry ahead of the roller stops.
6. Apply binding material (if required), sprinkle, roll and let the surface dry overnight.
7. Next layer is placed after setting. Allow 1 to 2 days of curing, then give the surface treatment/premix carpet.

Quality control: aggregate tests (LAA, AIV, flakiness, water absorption, soundness), grading of aggregate and screenings, thickness and camber checks with a template, density by sand replacement, surface regularity by straight edge (max 12 mm undulation), and tests every 100-200 m.

Bituminous premix carpet (PMC) is a 20-25 mm thick open-graded wearing course in which pre-coated chippings are laid and compacted over a tack-coated base (IRC:14, MoRTH Section 508; used on rural and low-volume roads in Nepal under DoR/DoLIDAR specifications).

Materials

• Aggregate: 13.2 mm and 11.2 mm nominal size crushed stone/gravel; AIV below 30 percent, LAA below 35 percent, flakiness below 25 percent, water absorption below 1 percent, stripping value below 5 percent.
• Binder: bitumen 80/100 or 60/70 grade, or cutback / emulsion; quantities of binder and chippings per 10 m2 are taken from the IRC:14 table for the 20 mm carpet (about 14.6 kg of bitumen with 0.18 m3 of 13.2 mm chippings).
• Tack coat: 0.25-0.30 kg/m2 of bitumen emulsion.

Equipment: bitumen boiler with sprayer, mixer (drum mixer or mixing platform), 8-10 t smooth wheel roller, tipper, brooms, hand tools, thermometers.

Construction procedure

1. Prepare the base: clean, repair, apply tack coat (0.25-0.30 kg/m2) and allow to break.
2. Heat aggregates to 130-165 C and bitumen to 120-165 C, mix in a mixer so that each chipping is uniformly coated (a mixing platform for small jobs).
3. Spread the premix at 100-120 C immediately by paver or by hand rake, to uniform camber and thickness.
4. Roll with an 8-10 t roller from edges towards the centre, starting at about 100 C, until the chippings are firmly keyed and the surface is true.
5. Apply the seal coat (a bitumen film with 6.6 mm chips or sand) after the carpet has set; this closes the open texture and makes it waterproof.
6. Open to traffic after the surface cools.

Quality control: aggregate and bitumen tests; temperature checks of mix at laying; rate of spread of materials by measuring the area covered; thickness (+- 5 mm) and cross-section; surface evenness with a 3 m straight edge (max 8 mm); bitumen content by extraction on random samples.

Tools, equipment and plants used in road construction

Tools, equipment and plants are grouped by the work they do.

Plants (hot-mix plant, crusher, batching plant) are fixed installations that produce materials; equipment is mobile machinery that places and compacts them. Choice depends on quantity of work, haul distance, site access and cost; for small hill roads in Nepal labour-based methods with light equipment are common.

Characteristics of the mass haul diagram

1. A rising curve shows cut exceeding fill; a falling curve shows fill exceeding cut.
2. A peak marks a change from cut to fill and a valley a change from fill to cut.
3. A horizontal line (balancing line) cuts the curve at points where cut and fill are equal; the earth between these points is balanced.
4. The maximum or minimum ordinate between two such points gives the volume of earth moved from cut to fill in that section.
5. The length of the balancing line gives the haul distance for that balanced volume.
6. Curve above the balancing line means haul is forward (in the direction of chainage); below means backward.
7. The area between the curve and the balancing line is the haul moment, volume times distance (m3-m).
8. A steep slope shows heavy earthwork; a flat curve shows a balanced section.
9. The end ordinate gives the excess (to be wasted) or deficit (to be borrowed) for the whole length.

Gravel road has a wearing course of naturally occurring gravel/aggregate-soil mix compacted on the subgrade; it is the common low-cost road in rural Nepal (Nepal Rural Road Standards / DoLIDAR, IRC:SP:20).

Materials: well-graded gravel with 50-70 percent gravel, 20-40 percent sand and 8-15 percent fines (plasticity index 4-12 percent, liquid limit below 35 percent), maximum size 40 mm (CBR above 30 percent for base, above 15 percent for sub-base).

Construction procedure

1. Clear the site, set out the centre line and width; construct subgrade to the grade and camber (4-5 percent), compact top 150 mm to 95-97 percent of proctor density; side drains and cross drains.
2. Haul gravel from the borrow/quarry and spread by grader or manually to the required depth, usually 100-150 mm loose in one or two layers.
3. Mix with water to near optimum moisture content, adding fines or sand if it is deficient in binder.
4. Shape with a grader to the camber and compact with a 8-10 t roller (or vibratory roller); repeat for the second layer.
5. Finish with the cross-fall, check the thickness and shoulders, and provide drainage structures.
6. Open to traffic after light curing. Regravelling of 25-30 mm is repeated every 3 to 5 years as the gravel wears away.

Methods of soil stabilisation and where they apply

Soil stabilisation is improving the engineering properties of a soil (strength, volume stability, durability, resistance to water) so that it can be used in a road layer. Methods and where each is used:

Lime stabilised road

Materials: soil (clay or silty clay, PI above 10); lime - quicklime or hydrated lime, 3 to 8 percent by dry weight of soil, with available lime above 60 percent (IS:712, IRC:SP:89); water.

Equipment: grader/pulveriser (rotavator), lime spreader, water tanker, sheep foot and smooth wheel/pneumatic rollers, cutting tools.

Construction procedure

1. Prepare the subgrade or soil layer: clear, grade and trim to line and level.
2. Pulverise the soil (all clods below 25 mm) with a rotavator or grader.
3. Spread the lime uniformly at the required percentage (by lorry spreader or in bags).
4. Mix lime with the soil by rotavator or grader; add water to get optimum moisture content; for first mixing leave the mix to cure (mellowing) for 24 to 48 hours (heavy clay).
5. Remix and compact using sheep foot then smooth-wheel or pneumatic tyre roller to 95-98 percent of maximum dry density.
6. Cure by keeping moist for 7 days (or cover with sand/ bitumen emulsion), and protect the surface from rain and traffic; give a surface course after curing.
7. Quality control: lime content test, pulverisation, moisture content, density and unconfined compressive strength (UCS above 0.7-1.0 MPa at 7 days).

Water Bound Macadam (WBM) is a base or surfacing course of clean crushed stone aggregate of graded sizes, bound by stone dust (screenings) and water, and compacted by rolling (IRC:19 and MoRTH Section 404; used in Nepal DoR specifications for base courses).

Materials

• Coarse aggregate: hard, tough crushed stone (Los Angeles abrasion value below 40 percent for base, aggregate impact value below 30-40 percent), flakiness index below 15 percent, in three grading sizes: Grading 1 (90-45 mm), Grading 2 (63-45 mm), Grading 3 (53-22 mm).
• Screenings: 13.2 mm size crushed stone dust to fill voids (Grading A/B); non-plastic with plasticity index below 6 percent.
• Binding material: a small quantity of clay or fine crusher dust (PI below 6 percent) only if the screenings lack fines.
• Water: clean and free from salts.

Equipment: three-wheel power roller (8-10 t) or vibratory roller (8-10 t), motor grader or spreader, water tanker, brooms, screening spreaders.

Construction procedure

1. Prepare the base: clean, trim to profile, fill gaps; provide lateral confinement by side shoulders, 20 cm wide.
2. Spread coarse aggregate in uniform thickness (75 mm compacted per layer, loose depth 1.25-1.30 times) by stone spreader or manually, with camber.
3. Dry rolling: roll from edges to centre with 8-10 t roller until aggregate is firmly keyed (no creep).
4. Apply screenings gradually in 3 or 4 stages with rolling and sweeping so that voids are filled.
5. Sprinkle water on the surface and roll with the roller; slurry formed binds the stone. Keep rolling until a wave of slurry ahead of the roller stops.
6. Apply binding material (if required), sprinkle, roll and let the surface dry overnight.
7. Next layer is placed after setting. Allow 1 to 2 days of curing, then give the surface treatment/premix carpet.

Quality control: aggregate tests (LAA, AIV, flakiness, water absorption, soundness), grading of aggregate and screenings, thickness and camber checks with a template, density by sand replacement, surface regularity by straight edge (max 12 mm undulation), and tests every 100-200 m.

Prime coat, tack coat and seal coat

• Prime coat: low-viscosity cutback bitumen (MC-30 / MC-70) or emulsion sprayed on an unbound granular (WBM/WMM) surface before bituminous layers are laid (rate 0.6-1.0 kg/m2). It binds loose dust, plugs the surface voids, hardens the top, and provides adhesion between the base and the bituminous layer (IRC:16).
• Tack coat: a thin film of bitumen emulsion (RS-1 / SS-1) or cutback (rate 0.20-0.30 kg/m2) sprayed on an existing bituminous or cement concrete surface before overlaying. It gives bond between the old and new layer, preventing slippage and delamination.
• Seal coat: a thin surface treatment of bitumen (0.6-0.9 kg/m2) with 6.6 mm chips or sand on a newly laid or old surface (IRC:16 / MoRTH Section 510). It seals voids, makes the surface waterproof and protects against weathering and skid.

Bituminous concrete pavement

Asphalt concrete (bituminous concrete, BC) is a dense-graded hot mix of aggregates, filler and bitumen (VG-30/60-70 penetration grade) laid as a wearing or binder course (IRC:29 / MoRTH Section 509, Nepal DoR specifications).

Materials

• Aggregate: crushed, clean, hard and cubical (LAA below 30 percent, AIV below 24 percent, flakiness index below 30 percent combined, stripping value below 5 percent, water absorption below 2 percent), graded to the BC Grading 1 or 2.
• Filler: stone dust, cement or hydrated lime passing 75 micron.
• Binder: bitumen 60/70 or VG-30; 5 to 6 percent by weight of mix as fixed by Marshall design (stability above 9 kN, flow 2-4 mm, air voids 3-5 percent, VMA 12-14 percent).

Plant and equipment: hot-mix plant (batch or drum mix) with dryer, mixing unit and temperature control, tippers with tarpaulin, bitumen distributor, paver finisher with screed, steel-wheel roller (8-10 t), pneumatic tyre roller (15-20 t), small plate compactor, thermometers.

Construction steps

1. Prepare the base: clean, repair defects, and apply a tack coat (0.20-0.25 kg/m2 of emulsion) to the old surface or a prime coat to a granular base.
2. Produce the mix at the plant: aggregate heated to about 160-175 C and bitumen to 150-165 C; the mixed material leaves the plant at not more than 170 C.
3. Transport in covered, insulated tippers; the mix must arrive hot enough to be laid at about 120-165 C depending on the binder grade.
4. Lay by paver with proper level sensors and uniform speed; hand-lay only at small patches.
5. Initial (breakdown) rolling by 8-10 t steel roller at 110-135 C, intermediate rolling with pneumatic tyre roller, final rolling with tandem roller at 70 C or more to remove marks. Roll longitudinally from edge towards centre, overlapping.
6. Joints: cut transverse joints vertical and apply tack coat; stagger longitudinal joints.
7. Traffic opens when the surface cools below 60 C.

Quality control: tests on aggregates, bitumen (penetration, softening point, ductility), mix design by Marshall method; at site: temperature of mix, binder content (extraction), gradation, Marshall stability and flow, density by core cutter (above 98 percent of laboratory Marshall density), thickness (+-5 mm) and surface regularity with 3 m straight edge (max 5 mm) and bump integrator.

Rolled concrete and normal concrete

Construction of cement concrete pavement

Cement concrete pavement (Pavement Quality Concrete, PQC) is built as a slab on a sub-base following IRC:15 (construction of concrete roads), IRC:58 and MoRTH Section 600; Nepal DoR specifications follow the same steps.

Materials: ordinary Portland cement (OPC 43/53, IS:269) or PPC; fine aggregate (zone II sand), coarse aggregate 20-40 mm crushed stone (abrasion below 35 percent, flakiness below 25 percent); potable water; admixtures if needed; dowel bars (mild steel, plain), tie bars (deformed), joint filler and sealant (hot or cold poured), curing compound. Concrete of flexural strength about 4.5 MPa (M40, w/c ratio below 0.45).

Construction steps

1. Sub-grade and sub-base: prepare and compact the subgrade; lay a Dry Lean Concrete (DLC) or granular sub-base with separation layer of polythene sheet (125 micron) on top.
2. Setting out and forms: fix steel side forms (for fixed-form paving) rigidly to line and level, or set string lines for slip-form paver.
3. Batching and mixing: batch aggregates and cement by weight in a batching plant; mix in a transit mixer or central plant, slump 25-50 mm (workability for paver).
4. Placing: place concrete in a uniform layer, slightly above final level; place dowel bars on cradles/baskets at transverse joints and tie bars at the longitudinal joint.
5. Compaction: use needle vibrators along edges and joints, and a screed or surface vibrator; slip-form paver compacts, shapes and finishes in one pass.
6. Finishing: float, straight-edge, texture (broom or tining) for skid resistance; surface tolerance 3 mm under a 3 m straight edge.
7. Curing: spray a curing compound immediately and cover with wet hessian or polythene; keep moist for at least 14 days (7 days minimum), protect from rain, sun and wind.
8. Joints: saw-cut contraction joints (3 mm wide, 1/4 to 1/3 slab depth) within 6-18 hours of laying; form expansion and construction joints; seal with sealant after cleaning.
9. Opening: allow traffic after concrete attains the specified strength, generally 28 days (or 75 percent of strength for light vehicles).

Quality control: cement, aggregate and water tests; slump (every batch/at truck); cube strength (3 cubes per 30 m3 at 7 and 28 days) and flexural beam tests; temperature of concrete (between 5 and 30 C); thickness by levels or cores (+- 5 mm); level and evenness with a straight edge; checking of dowel alignment, joint depth and curing record.

Mass haul diagram

A mass haul diagram is a curve of cumulative earthwork volume (cut positive, fill negative, after correcting for shrinkage or swell) plotted as ordinate against chainage as abscissa. It is used to plan the movement of earth along a road.

 Cum.
 volume
   ^            peak
   |          ,--*--.
   |  A -----'-------'--- B  balancing line
   |      ,'            '.
   | ____/                '.     valley
   |                        '-._*______
   +------------------------------------> chainage
      cut        fill         cut

Characteristics

1. A rising curve shows cut exceeding fill; a falling curve shows fill exceeding cut.
2. A peak marks a change from cut to fill and a valley a change from fill to cut.
3. A horizontal line (balancing line) cuts the curve at points where cut and fill are equal; the earth between these points is balanced.
4. The maximum or minimum ordinate between two such points gives the volume of earth moved from cut to fill in that section.
5. The length of the balancing line gives the haul distance for that balanced volume.
6. Curve above the balancing line means haul is forward (in the direction of chainage); below means backward.
7. The area between the curve and the balancing line is the haul moment, volume times distance (m3-m).
8. A steep slope shows heavy earthwork; a flat curve shows a balanced section.
9. The end ordinate gives the excess (to be wasted) or deficit (to be borrowed) for the whole length.

Soil cement stabilised base course

Soil-cement (cement-stabilised) base is a mix of soil, cement and water, compacted to give a strong, durable layer (IRC:SP:89, IS:4332).

Materials: granular or sandy soil (PI below 20, fines below 50 percent, maximum size 40 mm; clay and organic soil unsuitable); ordinary Portland cement 4-8 percent by dry weight (decided by laboratory trial: 7-day UCS 1.7-3 MPa); clean water.

Equipment: pulveriser (rotavator) or grader, cement spreader (or bags), water tanker, rollers (vibratory 8-10 t, pneumatic tyre), motor grader, curing sprayer.

Construction procedure

1. Prepare the subgrade, trim and compact; set line and levels.
2. Spread the soil in a uniform layer and pulverise to break clods.
3. Spread the cement in a uniform layer at the specified rate (by bags at fixed spacing or by mechanical spreader).
4. Mix dry cement and soil with rotavator or grader until colour is uniform; add water to the optimum moisture content and mix again.
5. Compact immediately (within 2 hours of adding water) to 98 percent of modified Proctor density, using a vibratory roller and then a pneumatic roller; shape with the grader.
6. Cure for 7 days with moist cover or a bituminous prime coat (bitumen emulsion) to retain moisture; keep traffic off.
7. Provide the bituminous surface or the next layer after curing.
8. Control: cement content (titration), moisture and density, UCS cubes, thickness and level.

Types of bituminous pavements

Bituminous pavements are classified by the layer, method of mixing and surface treatment.

Field quality control

Quality of bituminous construction is controlled in three stages.

1. Materials - test aggregate (LAA, AIV, flakiness, water absorption, stripping value, soundness) and bitumen (penetration, softening point, ductility, viscosity, flash point) before use and at fixed frequency (e.g. every 100 m3 of aggregate and every consignment of bitumen).
2. Mix design and plant control - Marshall mix design (stability, flow, air voids, VMA), job-mix formula; temperature of aggregate and bitumen; accuracy of proportioning; check of grading from hot bins; mix temperature at plant.
3. Field control - temperature of mix at delivery and rolling (not less than 120-130 C at start, 70 C at end), rate of spread of binder (tray test), thickness (level survey/cores), compaction (core density not less than 98 percent of laboratory density), surface regularity (3 m straight edge, bump integrator), camber and joints; extraction test for binder content and gradation on random samples; no laying in rain or temperatures below 10 C.

Records are kept lot-wise; defective lots are rejected or reworked as per the contract.

Equipment and plants for road construction activities

Tools, equipment and plants are grouped by the work they do.

Plants (hot-mix plant, crusher, batching plant) are fixed installations that produce materials; equipment is mobile machinery that places and compacts them. Choice depends on quantity of work, haul distance, site access and cost; for small hill roads in Nepal labour-based methods with light equipment are common.

Double bituminous surface dressing (DBSD)

Double bituminous surface dressing (DBSD) is a surface treatment of two successive applications of bitumen and stone chips on a prepared WBM or granular base, forming a 20-25 mm waterproof wearing surface (IRC:17; Nepal DoR "Standard specification for road and bridge works", used for low-cost roads).

Materials

• Binder: bitumen 80/100 or 60/70, cutback or emulsion; typical rates are about 1.2 kg/m2 for the first coat (with 13.2 mm chips) and about 1.0 kg/m2 for the second coat (with 11.2 mm chips), as fixed from the IRC:17 table.
• Aggregate: crushed stone or gravel, AIV below 30 percent, flakiness below 25 percent, water absorption below 1 percent, stripping value below 5 percent: first coat 13.2 mm (about 0.15 m3 per 10 m2); second coat 11.2 mm or 6.6 mm (0.09 m3 per 10 m2).
• Prime coat over WBM base if required: 0.6-1.0 kg/m2 of cutback or emulsion.

Construction procedure

1. Prepare the base: sweep and clean; fill all depressions, patch potholes and ensure correct camber. Apply tack/prime coat and let it cure.
2. Spray the first coat of binder at the specified rate and temperature (150-170 C) with a pressure distributor, in a uniform sheet, within one lane width at a time.
3. Spread the first coat chippings (13.2 mm) immediately by chip spreader or manually, to cover 100 percent without piling.
4. Roll with an 8-10 t roller, moving from edge to centre; brush off surplus chips.
5. Apply the second coat of binder with smaller chips (11.2 mm) in the same way, spread them, and roll.
6. Final rolling with a pneumatic roller; sweep off loose chips; open to traffic after 24 hours at low speed (20 km/h) for a few days.
7. Quality control: bitumen temperature and rate of spread (tray test), chip spread rate, grading, cleanness and surface regularity.

Soil stabilisation

Soil stabilisation is the process of improving the engineering properties of a soil, such as strength, bearing capacity, durability, volume stability and resistance to water, by mechanical means or by adding admixtures (cement, lime, bitumen, chemicals) so that it can be used in a road subgrade, sub-base or base.

Equipment used for road construction

• Earth work: bulldozer, excavator, loader, scraper, motor grader, tippers.
• Compaction: smooth-wheel, vibratory, sheep-foot and pneumatic tyre rollers, plate compactors.
• Granular layers and stabilisation: grader, water tanker, pulveriser (rotavator), cement/lime spreader.
• Bituminous work: bitumen heater, pressure distributor, hot-mix plant, paver-finisher, chip spreader.
• Concrete work: batching plant, transit mixer, paver, vibrators, joint cutter.
• Aggregate: stone crusher and screens.

Characteristics of the mass haul diagram

1. A rising curve shows cut exceeding fill; a falling curve shows fill exceeding cut.
2. A peak marks a change from cut to fill and a valley a change from fill to cut.
3. A horizontal line (balancing line) cuts the curve at points where cut and fill are equal; the earth between these points is balanced.
4. The maximum or minimum ordinate between two such points gives the volume of earth moved from cut to fill in that section.
5. The length of the balancing line gives the haul distance for that balanced volume.
6. Curve above the balancing line means haul is forward (in the direction of chainage); below means backward.
7. The area between the curve and the balancing line is the haul moment, volume times distance (m3-m).
8. A steep slope shows heavy earthwork; a flat curve shows a balanced section.
9. The end ordinate gives the excess (to be wasted) or deficit (to be borrowed) for the whole length.

Methods of soil stabilisation

Soil stabilisation is improving the engineering properties of a soil (strength, volume stability, durability, resistance to water) so that it can be used in a road layer. Methods and where each is used:

Construction of soil cement stabilised road

Soil-cement (cement-stabilised) base is a mix of soil, cement and water, compacted to give a strong, durable layer (IRC:SP:89, IS:4332).

Materials: granular or sandy soil (PI below 20, fines below 50 percent, maximum size 40 mm; clay and organic soil unsuitable); ordinary Portland cement 4-8 percent by dry weight (decided by laboratory trial: 7-day UCS 1.7-3 MPa); clean water.

Equipment: pulveriser (rotavator) or grader, cement spreader (or bags), water tanker, rollers (vibratory 8-10 t, pneumatic tyre), motor grader, curing sprayer.

Construction procedure

1. Prepare the subgrade, trim and compact; set line and levels.
2. Spread the soil in a uniform layer and pulverise to break clods.
3. Spread the cement in a uniform layer at the specified rate (by bags at fixed spacing or by mechanical spreader).
4. Mix dry cement and soil with rotavator or grader until colour is uniform; add water to the optimum moisture content and mix again.
5. Compact immediately (within 2 hours of adding water) to 98 percent of modified Proctor density, using a vibratory roller and then a pneumatic roller; shape with the grader.
6. Cure for 7 days with moist cover or a bituminous prime coat (bitumen emulsion) to retain moisture; keep traffic off.
7. Provide the bituminous surface or the next layer after curing.
8. Control: cement content (titration), moisture and density, UCS cubes, thickness and level.

Mass haul diagram

A mass haul diagram is a curve of cumulative earthwork volume (cut positive, fill negative, after correcting for shrinkage or swell) plotted as ordinate against chainage as abscissa. It is used to plan the movement of earth along a road.

 Cum.
 volume
   ^            peak
   |          ,--*--.
   |  A -----'-------'--- B  balancing line
   |      ,'            '.
   | ____/                '.     valley
   |                        '-._*______
   +------------------------------------> chainage
      cut        fill         cut

It shows the earthwork balance, the direction and length of haul, and the quantities to be borrowed or wasted, so the cheapest plan of earth movement can be made.

Construction of WBM road

Water Bound Macadam (WBM) is a base or surfacing course of clean crushed stone aggregate of graded sizes, bound by stone dust (screenings) and water, and compacted by rolling (IRC:19 and MoRTH Section 404; used in Nepal DoR specifications for base courses).

Materials

• Coarse aggregate: hard, tough crushed stone (Los Angeles abrasion value below 40 percent for base, aggregate impact value below 30-40 percent), flakiness index below 15 percent, in three grading sizes: Grading 1 (90-45 mm), Grading 2 (63-45 mm), Grading 3 (53-22 mm).
• Screenings: 13.2 mm size crushed stone dust to fill voids (Grading A/B); non-plastic with plasticity index below 6 percent.
• Binding material: a small quantity of clay or fine crusher dust (PI below 6 percent) only if the screenings lack fines.
• Water: clean and free from salts.

Equipment: three-wheel power roller (8-10 t) or vibratory roller (8-10 t), motor grader or spreader, water tanker, brooms, screening spreaders.

Construction procedure

1. Prepare the base: clean, trim to profile, fill gaps; provide lateral confinement by side shoulders, 20 cm wide.
2. Spread coarse aggregate in uniform thickness (75 mm compacted per layer, loose depth 1.25-1.30 times) by stone spreader or manually, with camber.
3. Dry rolling: roll from edges to centre with 8-10 t roller until aggregate is firmly keyed (no creep).
4. Apply screenings gradually in 3 or 4 stages with rolling and sweeping so that voids are filled.
5. Sprinkle water on the surface and roll with the roller; slurry formed binds the stone. Keep rolling until a wave of slurry ahead of the roller stops.
6. Apply binding material (if required), sprinkle, roll and let the surface dry overnight.
7. Next layer is placed after setting. Allow 1 to 2 days of curing, then give the surface treatment/premix carpet.

Quality control: aggregate tests (LAA, AIV, flakiness, water absorption, soundness), grading of aggregate and screenings, thickness and camber checks with a template, density by sand replacement, surface regularity by straight edge (max 12 mm undulation), and tests every 100-200 m.

Factors affecting soil stabilisation

The success of stabilisation depends on:

1. Type of soil - its grain size, plasticity index, mineralogy and organic content decide the suitable method (cement for sands and gravel, lime for plastic clays).
2. Type and amount of stabiliser - cement 4-8 percent, lime 3-8 percent, bitumen 4-7 percent; too little gives no strength gain and too much is costly or causes cracking.
3. Mixing efficiency - uniform pulverisation and mixing of soil and stabiliser; clods must be broken (below 25 mm).
4. Moisture content and compaction - optimum moisture and high density are needed; delay between mixing and compaction (more than 2 hours for cement) reduces strength.
5. Curing and time - adequate moist curing for 7 days or more; temperature (cement and lime reactions are slow below 10 C).
6. Presence of harmful materials - sulphates, organic matter and salts retard cement and lime reactions.
7. Climate and drainage - freezing, wetting-drying and water access affect durability.
8. Cost and availability of the stabiliser and equipment.

Mechanical method of soil stabilisation

Mechanical stabilisation improves the soil by changing its gradation and density only, without a chemical binder.

• Two or more soils are blended (for example, a coarse gravel lacking fines with a clayey soil lacking coarse particles) to get a well-graded mix with the right proportions of gravel, sand and fines (plasticity index 4-10 percent).
• The mix is spread at optimum moisture content and compacted with rollers to high density (above 98 percent of modified Proctor).
• Used for gravel roads, sub-base and base of low-volume roads, and as the first step before chemical stabilisation. It is cheap but depends on a good proportion of materials.

Construction of cement soil stabilised road base

Soil-cement (cement-stabilised) base is a mix of soil, cement and water, compacted to give a strong, durable layer (IRC:SP:89, IS:4332).

Materials: granular or sandy soil (PI below 20, fines below 50 percent, maximum size 40 mm; clay and organic soil unsuitable); ordinary Portland cement 4-8 percent by dry weight (decided by laboratory trial: 7-day UCS 1.7-3 MPa); clean water.

Equipment: pulveriser (rotavator) or grader, cement spreader (or bags), water tanker, rollers (vibratory 8-10 t, pneumatic tyre), motor grader, curing sprayer.

Construction procedure

1. Prepare the subgrade, trim and compact; set line and levels.
2. Spread the soil in a uniform layer and pulverise to break clods.
3. Spread the cement in a uniform layer at the specified rate (by bags at fixed spacing or by mechanical spreader).
4. Mix dry cement and soil with rotavator or grader until colour is uniform; add water to the optimum moisture content and mix again.
5. Compact immediately (within 2 hours of adding water) to 98 percent of modified Proctor density, using a vibratory roller and then a pneumatic roller; shape with the grader.
6. Cure for 7 days with moist cover or a bituminous prime coat (bitumen emulsion) to retain moisture; keep traffic off.
7. Provide the bituminous surface or the next layer after curing.
8. Control: cement content (titration), moisture and density, UCS cubes, thickness and level.

Double bituminous surface dressing (DBSD) is a surface treatment of two successive applications of bitumen and stone chips on a prepared WBM or granular base, forming a 20-25 mm waterproof wearing surface (IRC:17; Nepal DoR "Standard specification for road and bridge works", used for low-cost roads).

Materials

• Binder: bitumen 80/100 or 60/70, cutback or emulsion; typical rates are about 1.2 kg/m2 for the first coat (with 13.2 mm chips) and about 1.0 kg/m2 for the second coat (with 11.2 mm chips), as fixed from the IRC:17 table.
• Aggregate: crushed stone or gravel, AIV below 30 percent, flakiness below 25 percent, water absorption below 1 percent, stripping value below 5 percent: first coat 13.2 mm (about 0.15 m3 per 10 m2); second coat 11.2 mm or 6.6 mm (0.09 m3 per 10 m2).
• Prime coat over WBM base if required: 0.6-1.0 kg/m2 of cutback or emulsion.

Construction procedure

1. Prepare the base: sweep and clean; fill all depressions, patch potholes and ensure correct camber. Apply tack/prime coat and let it cure.
2. Spray the first coat of binder at the specified rate and temperature (150-170 C) with a pressure distributor, in a uniform sheet, within one lane width at a time.
3. Spread the first coat chippings (13.2 mm) immediately by chip spreader or manually, to cover 100 percent without piling.
4. Roll with an 8-10 t roller, moving from edge to centre; brush off surplus chips.
5. Apply the second coat of binder with smaller chips (11.2 mm) in the same way, spread them, and roll.
6. Final rolling with a pneumatic roller; sweep off loose chips; open to traffic after 24 hours at low speed (20 km/h) for a few days.
7. Quality control: bitumen temperature and rate of spread (tray test), chip spread rate, grading, cleanness and surface regularity.

Activities in road construction

The main activities in constructing a road are:

1. Survey and setting out - final location survey, fixing of centre line, bench marks, and clearing limits (right of way).
2. Site clearance - cutting trees, removing stumps, boulders and topsoil; demolition of structures.
3. Earthwork - excavation in cut, hauling and filling to form the formation level; compaction of embankment in layers (150-200 mm) to 95-97 percent of Proctor density; benching and slope trimming in hills (use of the mass haul diagram).
4. Drainage and protection works - side drains, catch-water drains, cross-drainage (culverts, causeways), retaining/breast walls, bioengineering for slopes.
5. Subgrade preparation - scarifying, watering, compacting the top 500 mm of soil to specification and checking CBR.
6. Sub-base and base courses - granular sub-base (GSB), WBM/WMM or stabilised base; each layer spread, watered and rolled.
7. Bituminous courses - prime coat, tack coat, binder course (DBM/BM) and wearing course (BC, premix, surface dressing, otta seal) or CC pavement slab.
8. Shoulders and kerbs - construction of earth/gravel shoulders and kerb or drains.
9. Road furniture and finishing - signs, markings, guard rails, kilometre posts, plantation.
10. Quality control and measurement - field tests, levels, thickness, density and payment measurement; keeping to the Nepal DoR "Standard Specifications for Road and Bridge Works" and IRC/MoRTH specifications.

Plants and equipment for bituminous roads

• Aggregate: stone crusher and screens.
• Binder: bitumen boiler/heater, pressure distributor (sprayer) for tack and prime coats.
• Mix production: hot-mix plant (batch or drum) with dryer, mixer and temperature control.
• Placing: paver-finisher, motor grader, chip spreader.
• Compaction: smooth-wheel (8-10 t) and tandem rollers, pneumatic tyre roller.
• Haulage: tippers with covers; sweeper/blower, thermometers.

Plants and equipment for cement concrete roads

• Batching and mixing plant with silos, weigh batchers and a transit mixer.
• Paving: slip-form paver or fixed-form paving train, with side forms, dowel baskets.
• Compaction: needle vibrators and surface (screed) vibrators.
• Finishing: straight edges, floats, texturing (tining) machine, curing compound sprayer.
• Joints: concrete saw (joint cutter), sealant applicator.
• Subgrade/sub-base: grader, rollers and a DLC paver.

Cement concrete pavement (Pavement Quality Concrete, PQC) is built as a slab on a sub-base following IRC:15 (construction of concrete roads), IRC:58 and MoRTH Section 600; Nepal DoR specifications follow the same steps.

Materials: ordinary Portland cement (OPC 43/53, IS:269) or PPC; fine aggregate (zone II sand), coarse aggregate 20-40 mm crushed stone (abrasion below 35 percent, flakiness below 25 percent); potable water; admixtures if needed; dowel bars (mild steel, plain), tie bars (deformed), joint filler and sealant (hot or cold poured), curing compound. Concrete of flexural strength about 4.5 MPa (M40, w/c ratio below 0.45).

Construction steps

1. Sub-grade and sub-base: prepare and compact the subgrade; lay a Dry Lean Concrete (DLC) or granular sub-base with separation layer of polythene sheet (125 micron) on top.
2. Setting out and forms: fix steel side forms (for fixed-form paving) rigidly to line and level, or set string lines for slip-form paver.
3. Batching and mixing: batch aggregates and cement by weight in a batching plant; mix in a transit mixer or central plant, slump 25-50 mm (workability for paver).
4. Placing: place concrete in a uniform layer, slightly above final level; place dowel bars on cradles/baskets at transverse joints and tie bars at the longitudinal joint.
5. Compaction: use needle vibrators along edges and joints, and a screed or surface vibrator; slip-form paver compacts, shapes and finishes in one pass.
6. Finishing: float, straight-edge, texture (broom or tining) for skid resistance; surface tolerance 3 mm under a 3 m straight edge.
7. Curing: spray a curing compound immediately and cover with wet hessian or polythene; keep moist for at least 14 days (7 days minimum), protect from rain, sun and wind.
8. Joints: saw-cut contraction joints (3 mm wide, 1/4 to 1/3 slab depth) within 6-18 hours of laying; form expansion and construction joints; seal with sealant after cleaning.
9. Opening: allow traffic after concrete attains the specified strength, generally 28 days (or 75 percent of strength for light vehicles).

Quality control: cement, aggregate and water tests; slump (every batch/at truck); cube strength (3 cubes per 30 m3 at 7 and 28 days) and flexural beam tests; temperature of concrete (between 5 and 30 C); thickness by levels or cores (+- 5 mm); level and evenness with a straight edge; checking of dowel alignment, joint depth and curing record.

Types of bituminous pavements

Bituminous pavements are classified by the layer, method of mixing and surface treatment.

Construction of asphalt concrete pavement

Asphalt concrete (bituminous concrete, BC) is a dense-graded hot mix of aggregates, filler and bitumen (VG-30/60-70 penetration grade) laid as a wearing or binder course (IRC:29 / MoRTH Section 509, Nepal DoR specifications).

Materials

• Aggregate: crushed, clean, hard and cubical (LAA below 30 percent, AIV below 24 percent, flakiness index below 30 percent combined, stripping value below 5 percent, water absorption below 2 percent), graded to the BC Grading 1 or 2.
• Filler: stone dust, cement or hydrated lime passing 75 micron.
• Binder: bitumen 60/70 or VG-30; 5 to 6 percent by weight of mix as fixed by Marshall design (stability above 9 kN, flow 2-4 mm, air voids 3-5 percent, VMA 12-14 percent).

Plant and equipment: hot-mix plant (batch or drum mix) with dryer, mixing unit and temperature control, tippers with tarpaulin, bitumen distributor, paver finisher with screed, steel-wheel roller (8-10 t), pneumatic tyre roller (15-20 t), small plate compactor, thermometers.

Construction steps

1. Prepare the base: clean, repair defects, and apply a tack coat (0.20-0.25 kg/m2 of emulsion) to the old surface or a prime coat to a granular base.
2. Produce the mix at the plant: aggregate heated to about 160-175 C and bitumen to 150-165 C; the mixed material leaves the plant at not more than 170 C.
3. Transport in covered, insulated tippers; the mix must arrive hot enough to be laid at about 120-165 C depending on the binder grade.
4. Lay by paver with proper level sensors and uniform speed; hand-lay only at small patches.
5. Initial (breakdown) rolling by 8-10 t steel roller at 110-135 C, intermediate rolling with pneumatic tyre roller, final rolling with tandem roller at 70 C or more to remove marks. Roll longitudinally from edge towards centre, overlapping.
6. Joints: cut transverse joints vertical and apply tack coat; stagger longitudinal joints.
7. Traffic opens when the surface cools below 60 C.

Quality control: tests on aggregates, bitumen (penetration, softening point, ductility), mix design by Marshall method; at site: temperature of mix, binder content (extraction), gradation, Marshall stability and flow, density by core cutter (above 98 percent of laboratory Marshall density), thickness (+-5 mm) and surface regularity with 3 m straight edge (max 5 mm) and bump integrator.

Surface dressing

Surface dressing is a thin wearing surface made by spraying a film of bituminous binder on the road base or old surface and covering it immediately with a layer of single-size stone chippings, which are then rolled (IRC:17). It is single (one layer of binder and chips) or double (two layers). It seals the surface, makes it waterproof, gives skid resistance and renews worn-out surfaces, but adds no structural strength.

Construction method

1. Prepare the surface: repair potholes and cracks, sweep and clean; apply a tack or prime coat if needed. Weather should be dry (surface temperature above 20 C, no rain).
2. Spray binder: heat bitumen (VG-10/30 or 80/100) or cutback to 150-170 C and spray uniformly by pressure distributor at the specified rate (about 1.0-1.5 kg/m2).
3. Spread chips: immediately cover with clean, dry single-size chips (13.2 mm, or 11.2/9.5 mm for the second coat) by chip spreader or by hand in a thin layer without overlap, covering the binder fully.
4. Roll: roll with an 8-10 t smooth-wheel or a pneumatic tyre roller from edges to the centre until chips are embedded (about 50-70 percent of their depth).
5. Brush and cure: sweep off loose chips after traffic has compacted them; restrict speed to 20-30 km/h for the first days.
6. For double surface dressing, the second coat of binder and smaller chips is applied in the same way after the first.

Construction procedure of DBSD

1. Prepare the base: sweep and clean; fill all depressions, patch potholes and ensure correct camber. Apply tack/prime coat and let it cure.
2. Spray the first coat of binder at the specified rate and temperature (150-170 C) with a pressure distributor, in a uniform sheet, within one lane width at a time.
3. Spread the first coat chippings (13.2 mm) immediately by chip spreader or manually, to cover 100 percent without piling.
4. Roll with an 8-10 t roller, moving from edge to centre; brush off surplus chips.
5. Apply the second coat of binder with smaller chips (11.2 mm) in the same way, spread them, and roll.
6. Final rolling with a pneumatic roller; sweep off loose chips; open to traffic after 24 hours at low speed (20 km/h) for a few days.
7. Quality control: bitumen temperature and rate of spread (tray test), chip spread rate, grading, cleanness and surface regularity.

Equipment and plants for road construction activities

Tools, equipment and plants are grouped by the work they do.

Plants (hot-mix plant, crusher, batching plant) are fixed installations that produce materials; equipment is mobile machinery that places and compacts them. Choice depends on quantity of work, haul distance, site access and cost; for small hill roads in Nepal labour-based methods with light equipment are common.

Prime coat, tack coat and seal coat

• Prime coat: low-viscosity cutback bitumen (MC-30 / MC-70) or emulsion sprayed on an unbound granular (WBM/WMM) surface before bituminous layers are laid (rate 0.6-1.0 kg/m2). It binds loose dust, plugs the surface voids, hardens the top, and provides adhesion between the base and the bituminous layer (IRC:16).
• Tack coat: a thin film of bitumen emulsion (RS-1 / SS-1) or cutback (rate 0.20-0.30 kg/m2) sprayed on an existing bituminous or cement concrete surface before overlaying. It gives bond between the old and new layer, preventing slippage and delamination.
• Seal coat: a thin surface treatment of bitumen (0.6-0.9 kg/m2) with 6.6 mm chips or sand on a newly laid or old surface (IRC:16 / MoRTH Section 510). It seals voids, makes the surface waterproof and protects against weathering and skid.

Usefulness: the three coats give the bond, waterproofing and protection that make the layers act together:

• Prime coat gives bond with the granular base and protects it from rain during construction.
• Tack coat prevents slipping between bituminous layers and between old and new surfaces.
• Seal coat protects the surface from water, ageing and wear and improves skid resistance.

Activities in road construction

The main activities in constructing a road are:

1. Survey and setting out - final location survey, fixing of centre line, bench marks, and clearing limits (right of way).
2. Site clearance - cutting trees, removing stumps, boulders and topsoil; demolition of structures.
3. Earthwork - excavation in cut, hauling and filling to form the formation level; compaction of embankment in layers (150-200 mm) to 95-97 percent of Proctor density; benching and slope trimming in hills (use of the mass haul diagram).
4. Drainage and protection works - side drains, catch-water drains, cross-drainage (culverts, causeways), retaining/breast walls, bioengineering for slopes.
5. Subgrade preparation - scarifying, watering, compacting the top 500 mm of soil to specification and checking CBR.
6. Sub-base and base courses - granular sub-base (GSB), WBM/WMM or stabilised base; each layer spread, watered and rolled.
7. Bituminous courses - prime coat, tack coat, binder course (DBM/BM) and wearing course (BC, premix, surface dressing, otta seal) or CC pavement slab.
8. Shoulders and kerbs - construction of earth/gravel shoulders and kerb or drains.
9. Road furniture and finishing - signs, markings, guard rails, kilometre posts, plantation.
10. Quality control and measurement - field tests, levels, thickness, density and payment measurement; keeping to the Nepal DoR "Standard Specifications for Road and Bridge Works" and IRC/MoRTH specifications.

Construction procedure of Otta seal

Otta seal is a low-cost bituminous surfacing made of a single or double layer of graded natural gravel/crushed stone (maximum size 16-19 mm) rolled into a soft binder (cutback or 150/200 penetration bitumen) sprayed on a base. It originated in Norway and is used on low-volume roads in Nepal (DoR "Surface Dressing and Otta Seal" guidelines, based on Norwegian/ERA guidelines).

Materials: graded aggregate (continuous grading, max size 16 or 19 mm; fines below 10 percent; flakiness below 30 percent) from crusher or river source; soft binder (150/200 penetration bitumen cut back with kerosene, or emulsion); prime coat of MC or emulsion on the base.

Construction

1. Prepare the base (WBM, crushed gravel, or stabilised base) to a smooth, tight surface; sweep and apply prime coat; patch defects.
2. Spray binder at the specified rate (about 1.4 to 2.0 litre per m2 for the first layer, depending on the grading and base) at 120-150 C using a distributor.
3. Immediately spread the aggregate (about 12-16 kg per m2, slightly damp) by chip spreader or tipper to cover the binder.
4. Roll with a pneumatic tyre roller (at least 12 t) in several passes (at least 4); the pressure forces the binder up through the aggregate.
5. Brush lightly after 24 hours, give the second layer (if double Otta) of 9.5 mm aggregate, roll again, and re-brush.
6. Allow traffic at slow speed. Curing: the seal improves with traffic during the first weeks.

Typical pavement structures

 Flexible       Rigid (CC)     Low volume
 (highway)                     (Otta seal)
 +-----------+  +-----------+  +-----------+
 | BC  40-50 |  | PQC slab  |  | Otta seal |
 | DBM 80-170|  | 200-300   |  | 16-19 mm  |
 +-----------+  +-----------+  +-----------+
 | WMM   250 |  | DLC   150 |  | Base      |
 +-----------+  +-----------+  | 150-200   |
 | GSB   300 |  | GSB   150 |  +-----------+
 +-----------+  +-----------+  | Sub-base  |
 | Subgrade  |  | Subgrade  |  | Subgrade  |
 +-----------+  +-----------+  +-----------+

Thicknesses in mm (IRC:37, IRC:58, Nepal DoR low volume guidelines).

Construction methodology of Otta seal

Otta seal is a low-cost bituminous surfacing made of a single or double layer of graded natural gravel/crushed stone (maximum size 16-19 mm) rolled into a soft binder (cutback or 150/200 penetration bitumen) sprayed on a base. It originated in Norway and is used on low-volume roads in Nepal (DoR "Surface Dressing and Otta Seal" guidelines, based on Norwegian/ERA guidelines).

Materials: graded aggregate (continuous grading, max size 16 or 19 mm; fines below 10 percent; flakiness below 30 percent) from crusher or river source; soft binder (150/200 penetration bitumen cut back with kerosene, or emulsion); prime coat of MC or emulsion on the base.

Construction

1. Prepare the base (WBM, crushed gravel, or stabilised base) to a smooth, tight surface; sweep and apply prime coat; patch defects.
2. Spray binder at the specified rate (about 1.4 to 2.0 litre per m2 for the first layer, depending on the grading and base) at 120-150 C using a distributor.
3. Immediately spread the aggregate (about 12-16 kg per m2, slightly damp) by chip spreader or tipper to cover the binder.
4. Roll with a pneumatic tyre roller (at least 12 t) in several passes (at least 4); the pressure forces the binder up through the aggregate.
5. Brush lightly after 24 hours, give the second layer (if double Otta) of 9.5 mm aggregate, roll again, and re-brush.
6. Allow traffic at slow speed. Curing: the seal improves with traffic during the first weeks.

Road construction technology

Road construction technology is the set of methods, materials, tools, equipment and procedures used to build a road from the survey stage to the finished pavement, so that the road meets design standards in strength, durability, safety, economy and time. It covers the planning of operations, the choice between labour-based and equipment-based methods (in Nepal, the former is common for rural roads), and quality control as per the specifications (Nepal DoR "Standard Specifications for Road and Bridge Works", IRC and MoRTH).

Activities in road construction

The main activities in constructing a road are:

1. Survey and setting out - final location survey, fixing of centre line, bench marks, and clearing limits (right of way).
2. Site clearance - cutting trees, removing stumps, boulders and topsoil; demolition of structures.
3. Earthwork - excavation in cut, hauling and filling to form the formation level; compaction of embankment in layers (150-200 mm) to 95-97 percent of Proctor density; benching and slope trimming in hills (use of the mass haul diagram).
4. Drainage and protection works - side drains, catch-water drains, cross-drainage (culverts, causeways), retaining/breast walls, bioengineering for slopes.
5. Subgrade preparation - scarifying, watering, compacting the top 500 mm of soil to specification and checking CBR.
6. Sub-base and base courses - granular sub-base (GSB), WBM/WMM or stabilised base; each layer spread, watered and rolled.
7. Bituminous courses - prime coat, tack coat, binder course (DBM/BM) and wearing course (BC, premix, surface dressing, otta seal) or CC pavement slab.
8. Shoulders and kerbs - construction of earth/gravel shoulders and kerb or drains.
9. Road furniture and finishing - signs, markings, guard rails, kilometre posts, plantation.
10. Quality control and measurement - field tests, levels, thickness, density and payment measurement; keeping to the Nepal DoR "Standard Specifications for Road and Bridge Works" and IRC/MoRTH specifications.

Prime coat vs tack coat

Surface dressing method

1. Prepare the surface: repair potholes and cracks, sweep and clean; apply a tack or prime coat if needed. Weather should be dry (surface temperature above 20 C, no rain).
2. Spray binder: heat bitumen (VG-10/30 or 80/100) or cutback to 150-170 C and spray uniformly by pressure distributor at the specified rate (about 1.0-1.5 kg/m2).
3. Spread chips: immediately cover with clean, dry single-size chips (13.2 mm, or 11.2/9.5 mm for the second coat) by chip spreader or by hand in a thin layer without overlap, covering the binder fully.
4. Roll: roll with an 8-10 t smooth-wheel or a pneumatic tyre roller from edges to the centre until chips are embedded (about 50-70 percent of their depth).
5. Brush and cure: sweep off loose chips after traffic has compacted them; restrict speed to 20-30 km/h for the first days.
6. For double surface dressing, the second coat of binder and smaller chips is applied in the same way after the first.

Interface treatment is the preparation of the surface between two pavement layers (for example, between a granular base and a bituminous layer, or between an old and a new bituminous layer) to give a good bond. The surface is first swept clean and then sprayed with a prime coat on granular bases or a tack coat on bituminous or concrete surfaces. A good interface bond prevents slippage and delamination, helps the layers act together as one structure (reducing tensile strain at the bottom of the bituminous layer), and stops water entering between layers. Interface treatment is done just before laying, with the correct rate of spray (prime coat 0.6-1.0 kg/m2, tack coat 0.20-0.30 kg/m2) and sufficient curing/breaking time.

Surface dressing is a bituminous surface treatment in which a thin film of binder (bitumen or emulsion, 1.0-1.5 kg/m2) is sprayed on a prepared base and covered with single-sized chippings (13.2 mm, 11.2 mm), then rolled so that the chips are firmly embedded (IRC:17). It may be single or double. It gives a waterproof, skid-resistant and wear-resistant surface and seals the pavement, but does not add structural strength. It is cheap and quick, so it is widely used for low-volume roads and for renewal of old surfaces in Nepal.