Highway construction, servicing and road traffic can generate noticeable effects on the surroundings if left unaddressed. Renovation of roads, highways, railways, bridges, drainage ditches, drains, and other works ancillary to building roads or highways all come under road construction. Granular Sub Base (GSB) is a naturally occurring or artificially created building material used as a sub-base layer for roads. Because crushed aggregate is employed in granular form and is used above the subgrade layer below the base, this layer is referred to as a granular subbase. The full breakdown of GSB materials and building techniques is provided below.
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See also: Road construction: Materials used and stages of construction
GSB material: What is it?
A common layer used in highway building is a granular sub-base, which is positioned above the subgrade and beneath the crust materials. For GSB, the largest particle size of up to 65 mm may be used, and the material is naturally available in a variety of grades.
As the particle size is designed so that the capillary action ends and can’t move beyond GSB, it is often a substance that prevents capillary water from rising to top road layers. The second need is that it serves as a conduit for water to travel through without endangering other road layers. Even in a compressed state, the GSB particles contain enough spaces between them, enabling water from one side of the road to soak through and reach the other side without harming any other layers.
GSB material: What is the GSB’s source?
Commercially speaking, GSB should have the quality of being affordable because it is needed in large quantities for road construction. The GSB material is typically found in dry river banks as coarse pebbles that are left over after sand is removed by water. Where river mining is prohibited, crushed stones of the appropriate gradation are used, which is an expensive alternative when such material is not readily accessible locally.
The material composition of GSB
By laying and compacting well-graded material on a subgrade that has been prepared in line with specifications, the granular sub-base can be made. It sits on top of the crushed subgrade layer of the road foundation. As a sub-base, the material may be spread out over one or more layers. The GSB material must be natural river bed material with the right gradation or crushed stone aggregate devoid of organic and other harmful components.
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- Sand and gravel, crushed stone, and shattered rock make up GSB material.
- Crushed stone often makes up more than 50% of GSB material.
- Material with a CBR of over 30% and a PI of about 6%
- The material must have a minimum of 30% CBR after four days of soaking.
- Less than 2% of GSB absorbs water.
- AIV, or the average impact value, will be about 40%.
GSB material: Characteristics
The hydraulic conductivity and grading of the granular sub-foundation, which provides dissipation for the concrete structure, are crucial. For typical pavement construction, the fines content is typically capped at a maximum of 10% and at 6% when a free-draining subbase is necessary.
The GSB particles must be strong enough to withstand disintegration or deterioration during installation, under crushing, or under traffic.
For traffic up to 2 MSA (million standard axles), the average thickness of the granular base is advised to be 225 mm, and for traffic over 2 MSA, it should be 250 mm.
Notably, in a pliable asphalt pavement construction, a granular sub-foundation should have great stability. For stability, a large, angular aggregate with dense grading composed of tough, resilient particles is desirable.
GSB material: Purpose
To spread the surface wheel load over a large region and prevent shear and consolidation deformations, the Granular Sub Base (GSB) layers are utilised in flexible pavements.
GSB serves as a load-bearing and pavement-strengthening element. The subgrade layer is subjected to less stress as a result. Pavement gets drainage from GSB, which also prevents rutting.
It works well as a drainage layer to remove water that enters the pavement layers. It disperses the wheel load stresses that are transmitted through the granular base course and surface course of flexible pavement.
GSB material: Construction of Granular Sub Bases
Before beginning work, it must be confirmed that the GSB material being used as the sub-base complies with CBR specifications and has other physical attributes that have been compacted and finished.
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GSB layer spreading
- After the subgrade layer has been certified, the granular sub-base layer is fixed using line and level techniques.
- Before beginning construction, the toe line and fastening of pegs to designate the limits of the granular sub-base must be completed.
- On the finished subgrade, use lime granules or pegs to mark the GSB layer’s outer boundaries.
- Before GSB construction, the necessary subsurface outflow and waste outlet at the depth of the subgrade/sub-base must be finished.
Dumping and tipping
- The GSB materials will be transported there by tippers and deposited on the bed in a preset pattern.
- The current subgrade will be polished by clearing away all plants and other sub-extraneous material, softly misting with water if necessary, and rolling with two passes of an 80-100 KN smooth wheeled roller before the sub-base is laid.
- Transported the stuff to the GSB location after loading it onto the dumpers with a loader’s assistance.
- Between the previously specified restriction lines, which should have been marked with lime powder, the necessary material must be poured.
GSB layer compaction
- The granular sub-base must begin being compacted as soon as the required moisture content is reached.
- A vibratory compactor must be used for the compaction.
- Starting from the lower edge, the compaction must proceed width by width to the higher edge.
- Before the next layer is applied, quality control checks must be completed.
Rolling
For the segment possessing a single-direction cross fall and slope length, rolling is performed longitudinally from the bottom edge to the upper edge.
A minimum of one-third of the track laid down during the preceding pass must be consistently overlapped by the roller during each pass. When rolling, the slope and cross fall must be examined, and any high spots or depressions that are discovered must be filled in or removed with new material.
GSB material: Applications
GSB is a sub-base course and is typically not taken into account when designing a segment of the road. The drainage system is the main task performed by GSB, with load transfer support being its secondary task.
The main purpose of the sub-base layer in the flexible pavement is to increase load-carrying capacity by dispersing the load to the layer with a finite thickness and susceptibility to frost.
The GSB typically contributes the most thickness to the pavement structure and gives it bearing strength and drainage.
FAQs
What is the GSB's thickness?
A GSB can be 45 mm to 65 mm in size.
What exam is necessary for GSB?
To evaluate the pattern of particle sizes of coarse aggregates in GSB, a sieve analysis or gradation examination is used.
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