TRAN3001Highway Engineering Lecture 10

Road ConstructionRoad Maintenance

Road ConstructionIn the public sector, like the Ministry of Works and Transport and stateowned companies, with the necessary personnel and equipment resources, road construction can be carried out in-house, either as an end in itself or as part of the infrastructure (together with water supply, electricity, etc.) for a housing or other development. In the private sector the latter is usually the case when roadwork is carried out in-house with personnel employed and equipment owned/leased by the developer. More often and particularly for major works, specialist contractors are engaged. The role of technical personnel engaged by the employer would be for design and/or administration of the contract and supervision of the construction.

Road Construction (Contd) Set out The scope of works for the construction of the road is defined by the drawings and specifications. The planned horizontal and vertical alignment, pavement cross-sections, drainage and other design features of the proposed road have to be moved from the plans to the ground. This is accomplished by surveying and setting out the construction site.

Road Construction (Contd)Set out (Contd)Wooden or metal stakes/pegs are used to mark various road design points within the work area. They are also used to mark reference points outside the clearing limits in order to relocate the centreline, etc. stakes, which are typically disturbed by the clearing operations. Datum points or benchmarks are stable points of reference, with know elevations, close to the site. They are usually identified on the drawings and are located during the setting out of the works.

Road Construction (Contd)Set out (Contd)All elevations marked on the reference points are set out on site with respect to these benchmarks. On average reference points are set out at least every 70 to 100 metres, at locations where the centreline alignment can be easily re-established, such as points of curvature. The whole or a suitable section of the proposed new road is to be set out to allow construction activities to proceed without delay.

Road Construction (Contd)Relocate Public UtilitiesAny public utilities should be identified and then relocated from the carriageway area. Relocation of public utilities to the shoulder, footpath, etc. prevents damage to the carriageway from inspection/repair of these utilities. Heavy construction and later traffic loads can also damage utilities left in the carriageway and cause disruption in the supply of these services.

Road Construction(Contd)Clear and Grub Site preparation for construction of the road comprises clearing and grubbing. During the clearing phase, trees are felled and the topsoil, which is mostly organic material that will decompose, is removed, generally up to a depth of 300 mm. Particularly in urban areas clearing may also involve demolishing and removing man-made obstructions and debris.

Road Construction (Contd)Clear and Grub (Contd) Grubbing refers to the clearing and removal of tree stumps. All roots have to be removed as, like topsoil, any remaining vegetation will decay and leave voids in the subgrade that can cause eventual settlement of the pavement. Cut and FillAlso referred to as Earthworks, this is carried out after clearing and grubbing, in accordance with the horizontal and vertical alignments shown on the drawings.

Earth-Moving Equipment

Bulldozer LoaderScraper

Dump TruckExcavator

Road Construction(Contd)Cut and Fill (Contd) Excavation involves the use of equipment like bulldozers and particularly on steeper slopes, excavators, to get the required road widths and slopes. Depending on the extent of any rock excavation specialist rock-breaking equipment or blasting (controlled use of explosives) may be necessary. Slopes should be protected/supported with temporary shoring, etc. as required to prevent erosion and landslides during construction. Excavated material suitable for fill is loaded onto dump trucks and hauled to the required areas on site.

Road Construction(Contd)Cut and Fill (Contd) The mineral soil (free of organic matter) used for embankment construction should be built up in layers or lifts each of which is spread and then compacted. While a bulldozer is used to spread the fill it is not a good machine for compacting because of its low ground pressure characteristics, so rollers are required.Typical lift heights should be about 300 mm and should not exceed 500 mm.

Subgrade Compaction and Finishing Equipment

Vibrating RollerSheepsfoot Roller

Grader

GraGGGrader

Road Construction(Contd)Compact Subgrade Proper compaction results in a subgrade that is sufficiently dense and stable to withstand the stresses transmitted by the overlying pavement . This is particularly important for subgrades under flexible pavements, however even in the case of rigid pavements the subgrade must attain the state of compaction anticipated in the design procedure. At least the top 300 mm of embankment and preferably double this depth should be compacted to the specified density .

Road Construction(Contd)Compact Subgrade (Contd)For cut sections the method of compaction to achieve the design density at the appropriate depth will be determined by the soil type. Fine-grained soils, especially those of high plasticity, can be weakened by heavy compaction equipment. Particular attention must be given to sections where the subgrade changes from cut to fill as there should be no abrupt change in the degree and uniformity of compaction.

Road Construction(Contd)Shape Subgrade After compaction the surface of the subgrade will still be fairly rough so at this stage irregularities will be removed and the subgrade surface is shaped to the final road cross-section. This ensures that the pavement will be constructed to its proper shape and any moisture that enters into the subgrade can drain readily.Once the final shape of the subgrade surface has been attained, the subgrade, now at formation level, must be protected prior to the start of pavement construction.

Road Construction(Contd)Pavement Construction Flexible Pavement Drainage, either temporary or permanent, should be in place during construction of the pavement, which requires effective co-ordination of these works. Granular subbase material delivered to the site is spread over the subgrade and compacted in layers to the specified thickness and density.

Road Construction(Contd)Pavement Construction (Contd)Flexible Pavements (Contd) Crushed rock aggregate comprising roadbase material is then spread on the subbase and also compacted to the required thickness and density. Where specified a prime coat (low viscosity asphaltic concrete) is applied to the roadbase. In recent years this has been omitted as there is no real evidence of failure resulting from its omission.

Road Construction(Contd)Pavement Construction (Contd)Flexible Pavements (Contd) Asphaltic concrete is premixed and delivered to site at very high temperatures (generally over 120o C) in covered dump trunks.The asphaltic concrete is discharged directly into the paver which spreads and provides initial compaction to the mix which is compacted in accordance with the specifications. Where asphaltic concrete comprises a basecourse and a wearing course a tack coat (asphalt emulsion) is usually specified to improve adhesion.

Road Construction(Contd)Pavement Construction (Contd)Rigid Pavement Both subgrade and subbase are prepared as specified and formwork, dowel bars and reinforcement are then placed on the subbase and secured in position. If dry the subbase is wetted shortly before placing the concrete to prevent it absorbing water from the bottom of the concrete mixture, which can produce warping stresses and eventual cracking in the slab.

Road Construction(Contd)Pavement Construction (Contd)Rigid Pavement Ready-mix trucks are used for both mixing and delivery of the concrete and typically have up to 90 minutes from initial mixing to delivery. Concrete is placed and compacted, using small, towed machines or larger self-powered equipment. The finished concrete surface is textured (for skid resistance). Wire brushing is a common example of this. Curing is the final operation and a curing compound is often used.

Highway Surface FeaturesHighway Lighting Installation of lighting facilities on roads has served to reduce night accidents, particularly those involving pedestrians. Fringe benefits include crime prevention and late evening shopping. The primary purpose of the road-lighting system Is to promote the safe and efficient movement of pedestrian and vehicular traffic.

Highway Surface Features (Contd)Highway Lighting (Contd) Lamps used for road lighting include tungsten and fluorescent. At one time the tungsten lamp was the most commonly used but now its use is largely limited to residential streets. Despite its low initial cost, its low light-producing efficiency and rated life limits its use on heavily- trafficked roads where lighting standards should be high.

Highway Surface Features (Contd)Highway Lighting (Contd) A fluorescent lamp emits a white light which gives good visibility and is the least dazzling to drivers. It meets the requirements for use on heavily trafficked roads and is particularly suited to locations where illuminating the true colour of a feature is important. However It has a high cost. Another disadvantage is the bulk and weight of its luminaire (fixture which holds the lamp).

Highway Surface Features (Contd)Traffic Signs Traffic signs give information, warnings and instructions. An example of an informatory sign is one giving the distance to a town. A warning sign is typically used at dangerous bends and near schools and pedestrian crossings. Signs can also giving instructions such as Stop and No Entry.

Highway Surface Features (Contd)Traffic Signs (Contd) To get the greatest efficiency from traffic signs they must be designed for the anticipated traffic conditions and road speeds. Among the features of road signs are that they should be of adequate size, legible from a sufficient distance, contain only essential information and be located so that they will remain visible. To be easily read letters are typically a light (reflective) colour with a dark background.

Highway Surface Features (Contd)Road Markings Centre-lines, lane-lines and other delineators are effective traffic management tools. Centre-lines indicate the division of the carriageway with traffic flowing in opposing directions. Usually denoted by broken single lines they function as guide lines to be crossed at the drivers discretion. Lane lines serve to organize traffic into the proper channels, which increases road efficiency. Reflective paints are widely used as marking materials since they are relatively inexpensive and can be rapidly applied. They are often supplemented by reflective road studs, which are particularly effective in wet conditions.

Road MaintenanceRoad maintenance comprises routine/preventive maintenance as well as emergency maintenance and annual budgets for this recurrent expenditure must cater for both. Re-painting road markings is an example of the former whereas replacing a traffic sign or cable median barrier after a crash is typical of the latter. Scheduled maintenance of roads, including maintenance of road surface features, has been described as doing inexpensive repairs on good roads to keep them good.

Road Maintenance (Contd)Maintaining Surface Features (Contd)From about five (5) years after construction the result of weather and traffic is that road markings are generally faded and would need to be repainted at regular intervals over the design life of the road. Lamps in street fixtures and even the fixtures/poles, especially if wooden, need to be inspected and replaced as required. Any vegetation that can interfere with overhead lines must be trimmed and the integrity of underground lines confirmed by inspection.

Road Maintenance (Contd)Maintaining Surface Features (Contd)Traffic signs must remain unobstructed and legible. Vegetation and man-made obstructions must be removed. Particularly in urban areas with a higher risk of vandalism, these signs must be maintained in good order for the safety of motorists and pedestrians.

Road Maintenance (Contd)Maintaining Flexible PavementsIt is essential that drains be kept clear and functioning to preserve the life of the pavement. Narrow isolated surface cracks can be sealed to prevent the intrusion of water and debris. Deeper, more extensive cracking indicates distress/failure in layers underlying the asphalt and the need for reconstruction of that section of the road.

Road Maintenance (Contd)Maintaining Flexible Pavements (Contd)A localized failure or pothole should have been corrected before progressing to that stage on a well-maintained road. However, patching can be a cost-effective method of repair if properly carried out. Straight boundaries are marked and cut as these are easier to compact. Cutting should extend at least 300 mm beyond the cracked area into sound material adjacent to the patch. The area of the patch should accommodate the compaction equipment.

Road Maintenance (Contd)Maintaining Flexible Pavements (Contd) The damaged asphalt should be removed and the underlying areas inspected and repaired/ replaced as required, before placing and compacting new asphalt. Loss of skid resistance, bleeding and similar surface defects in the asphalt can be remedied with a minimum 25 mm thick overlay.

Road Maintenance (Contd)Maintaining Rigid PavementsIt is equally important for rigid pavements that the support provided by the subbase and/or subgrade not be weakened if drains are not kept clear and functioning. Cracking is the most common defect in rigid pavements and these should be sealed prior to the asphaltic concrete overlay that is used to maintain a good travelling surface, to prevent reflective cracking in the overlay.

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