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Structural Components of a Road. Typical Road Structure. Structural Components. Subgrade Subbase Base Surfacing/Wearing Course. Subgrade. Subgrade consists of compacted earthwork
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Structural Components • Subgrade • Subbase • Base • Surfacing/Wearing Course
Subgrade • Subgrade consists of compacted earthwork • It is the soil foundation of the road which receives traffic loads from pavement and transfers to the natural ground safely • Entire road structure rests on subgrade, therefore its failure affects the performance of entire road
Subgrade • Although a pavement's wearing course is most prominent, the success or failure of a pavement is more often dependent upon the underlying subgrade - the material upon which the pavement structure is built. • Subgradesbe composed of a wide range of materials although some are much better than others.
Subgrade Performance • A subgrade’s performance generally depends on three of its basic characteristics (all of which are interrelated): 1.Load bearing capacity. The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good. 2. Moisture content. Moisture tends to affect a number of subgrade properties including load bearing capacity, shrinkage and swelling. Moisture content can be influenced by a number of things suchasdrainage, groundwater table elevation, infiltration, or pavement porosity (which can be assisted by cracks in the pavement).
Generally, excessively wet subgrades will deform excessively under load. 3. Shrinkage and/or swelling. Some soils shrink or swell depending upon their moisture content. Additionally, soils with excessive fines content may be susceptible to frost heave in northern climates. Shrinkage, swelling and frost heave will tend to deform and crack any pavement type constructed over them.
Improving Subgrade Performance Poor subgrade should be avoided if possible, but when it is necessary to build over weak soils there are several methods available to improve subgrade performance: Removal and replacement (over-excavation). Poor subgrade soil can simply be removed and replaced with high quality fill. Although this is simple in concept, it can be expensive.
Stabilization with a cementitious or asphaltic binder. The addition of an appropriate binder (such as lime, portland cement or emulsified asphalt) can increase subgrade stiffness and/or reduce swelling tendencies.
Additional base layers. Marginally poor subgrade soils may be compensated for by using additional base layers. These layers (usually of crushed stone – either stabilized or unstabilized) serve to spread pavement loads over a larger subgrade area. when designing pavements for poor subgrades the temptation may be to just design a thicker section with more base material because the thicker section will satisfy most design equations. However, these equations are at least in part empirical and were usually not intended to be used in extreme cases. In short, a thick pavement structure over a poor subgrade will not necessarily make a good pavement.
Subbase Course • It is layer of granular material provided above subgrade generally natural gravel. It is usually not provided on subgrade of good quality.
Function of Sub base in Road Cross Section • It enables traffic stresses to be reduced to acceptable levels in subgrade in the Road Cross Section. • It acts as a working plate form for the construction of upper pavement layers. • Acts as a drainage layer, by protecting the subgrade from wetting up. • It intercept upward movement of water by capillary action. • It acts as a separating layer b/w subgrade and road base. By this it prevent the two layers from mixing up.
Base Course • It is a major structural component of the road. • It is the layer immediately under the wearig surface (Applies whether the wearing surface is bituminous or cement concrete and or more inch thick or is a thin bituminous layer). • As base course lies close under the pavement surface it is subjected to severe loading. The material in a base course must be of extremely high quality and its construction must be done carefully.
Types of Base Course 1.Granular Base Course: • A mixture of soil particles ranging in size from coarse to fine. Processing involve crushing oversized particles and screening where it is necessary to secure the desired grading. The requirements of a satisfactory soil aggregate surface are; • Stability • Resistance to abrasion • Resistance to penetration of water • Capillary properties to replace moisture lost by surface evaporation upon the addition of wearing course requirement change. 2. Macadam Base: • Successive layers of crushed rock mechanically locked by rolling and bonded by stone screening (rock duct, stone chips etc).
3. In-water bound Macadam: • The crushed stones are laid, shaped and compacted and then finer materials are added and washed into surface to provide a dense material. 4. Treated Bases: • Compose of mineral aggregate and additive to make them strong or more resistant to moisture. Among the treating agents is bitumen.
Surfacing/Wearing Course • The top layers of pavement which is in direct contact with the wheel of the vehicle. Usually constructed of material in which bitumen is used as binder materials.
Surfacing Layer provides smooth riding surface • It protects the base, subbaseand subgrade from adverse weather conditions • It minimizes the abrasion of road due to traffic • Presence of bitumen also protects any seepage from the surface to underlying layers.
Camber • Camber is the convexity of road cross section and is defined as the slope of the line joining edge of the carriageway with the crown. • The middle highest point of the road cross section is Crown. • Transverse slope given to the road is called Cross fall.
Function of Camber • The main object of providing the camber is to drain off rainwater from the surface of the road as quickly as possible. Hence a hard smooth surface will require less camber than a soft and rough surface.
The amount of cross fall or camber depends on the rainfall of threat particular locality in which the road is to be constructed and the permeability of the road surfacing material. The steeper the camber is, the more inconvenient it is for the traffic. • For permeable road surfacing camber will be higher • Normally camber is 2%
Disadvantages of ProvidingExcessive Camber • Edges of the road surface will wear out faster than the central part because of more load transfer on outer wheels • Due to high velocity of drainage water shoulder may get eroded easily • Heavily loaded vehicles may topple over • Tendency of side slippage is increased especially when road is wet.
Gradient • It is the longitudinal slope of the road, or the rate of rise and fall of the road surface in a given length.
Factors Affecting Gradient Nature of Topography • Flat gradient in plain area and steep • gradient in mountainous area Type of Traffic • If traffic is light and fast (cars), steeper gradients can be provided. • Passenger cars speed is not much affected by grades upto 6 or 7%, but trucks speed is affected by even less grades • As far as possible, easy gradient should be provided.
Types of Gradients • Ruling Gradient • Minimum Gradient • Limiting Gradient • Exceptional Gradient
Types of Gradients Ruling Gradient • This is the maximum gradient adopted under the normal site conditions • In plain areas 1 in 30 (3.33%) • In hilly areas 1 in 20 (5%)
Types of Gradients Minimum Gradient • Perfectly horizontal roads may give rise to the problem of drainage. The minimum gradient in plain areas is recommended as: • 1 in 250 for open earth drains • 1 in 500 for pecca side drains
Types of Gradients Limiting Gradient • It is provided where lots of saving in earthwork is to be done • 1 in 20 in plain areas • 1 in 15 in hilly areas
Types of Gradients Exceptional Gradient • In unavoidable circumstances, exceptional gradient is provided but only for a maximum road length of 50 to 60 meters • 1 in 15 in plain areas • 1 in 12 in hilly areas
