Question

Discuss the preparation/construction process in different layers of highway foundation. How

determination of optimum moisture content for a given soil will ensure the load carrying capacity of

highway foundation?

Answer 1

The pavement foundations mainly consist of two layers: the sub-base and the base course:-
Sub-base provides a level surface with sufficient load-bearing capacity and acts as a temporary wearing surface course during construction. It is prepared by cutting / Filling (embankment) to the required level. Generally approx. 98% of the maximum dry density of the soil should be the achieved compaction and a minimum of 8% CBR. Sub-base material shall be placed and compacted in layers not exceeding 150mm, to the desired level, to achieve the required compaction. A layer of granular subbase shall be laid in max. the thickness of 150mm per layer, it shall be watered and compacted by roller to the required level. The camber and profile of the road have to be maintained during this period.
After the pavement construction process is completed, base course is done which provides the pavement enough strength and thermal protection to withstand the traffic loads and weather conditions. On the complete layer of Wet mix macadam, the prime coat shall be applied using a bitumen sprayer at a rate of 0.8 – 1.2 liters/m2, it cut back bitumen is used then curing period is 24 hrs is there before laying DBM. The bitumen is heated to 180o C and mixed with well-graded aggregates of size 20 mm. the temperature of asphalt base course at the time of laying should not be less than 120o C. above this base course, a layer of tack coat is sprayed with bitumen sprayer at the rate of 0.4 - 0.6 liters/m2.

To determine the optimum moisture content of a soil to ensure the load-carrying capacity of the soil, Modified Poctor Test in accordance with AASHTO is done.
In this, an air-dried sample of approx. 4 kg of soil which had been passed through No. 4 sieve is mixed with the optimum quantity of water, weigh it and then fill it in a metal mould of an internal diameter of 4 inches and an outer diameter of 6 inches in five layers with 25 blows per layer followed by compaction done by a rammer of 4.5 kg with a free fall of 45 cm. Weigh the sample again and then take out soil from the mould and break up the same on a large tray mix it with the rest of the sample. Add a suitable amount of water again and repeat the exact same process at least 6 trials.
Now plot a compaction curve on a graph with dry density on the y-axis and moisture contents on the x-axis. The degree of soil compaction is measured in terms of its dry density. The maximum dry density must be at the vertex of the curve and optimum moisture content (OMC) at which the maximum dry density is obtained.