The operations that are carried out for constructing an embankment are clearing, stripping and storing top soil, placing of embankment, rolling and finishing.
The process of earthworks is to excavate the existing land to a suitable level so that road construction may begin. The earthworks can take the form of either excavation in the form of cuts or the construction of embankments to carry an elevated highway. Excavation increases the volume of material. It is therefore necessary to use a bulking factor to determine the volume of material that will be created by excavation.
Bulking factor is defined as:
Bulking Factor = Volume after Excavation/Volume before Excavation.
Similarly a shrinkage factor is defined for the compaction of a soil at its final destination:
Shrinkage Factor = Volume after Compaction/Volume before Excavation.
PRINCIPAL EARTHMOVING EQUIPMENT AND THEIR AREAS OF APPLICATION
Bulldozer – This is used primarily for pushing soil. Vehicles are generally tracked and require large amounts of traction.
Drag Line – This vehicle allows excavation below its own level. It is essentially a bucket on the end of a jib and is used solely for bulk excavation as it is relatively uncontrolled.
Dump Truck – These are wheeled vehicles and as such are able to move much faster. But due to offset by a lack of traction, dump trucks are always the first to get stuck.
Shovels – These are normally wheeled vehicles and are used to fill up the dump trucks. Typically they take 2-3 loads to fill an average dump truck.
Hydraulic Excavators – These can be either wheeled or tracked and are used again to excavate below truck level. They have a very small capacity and are extremely flexible.
Grader – Graders are used to level out deposited fill, ready for compaction.
Rollers – There are many different types of roller and they are used for compaction. Different types include vibratory, sheepsfoot and grid.
The road pavement is the actual surface on which the vehicles will travel. Its purpose is twofold, to provide friction for the vehicles and to transfer normal stresses to the underlying soil.
Pavements are classified according to their structural behavior into-
WEARING COURSE- It is the top layer of the road pavement and as such is (usually, but not always) designed to be impervious to the ingress of water. It should have an even running surface, durable, and a high resistance to skidding.
BASE COURSE-This is the layer of material below the wearing course and above the roadbase. The basecourse is a load spreading layer, spreading the load imposed on the wearing course over a wider area of the roadbase.
ROADBASE-The roadbase is the main load-bearing / load spreading layer in the road structure and is usually 100mm or thicker depending on the loading of the traffic for which the road is designed. The roadbase is usually a bituminous material or hot rolled asphalt.
GRANULAR SUB-BASE-Granular sub-base is usually the material used in the sub-base layer, but materials other than granular sub base can be used in the sub-base layer. Granular sub-base (gsb) – is a graded granular material, usually a “hard rock”. SUBGRADE-The subgrade is the layer of naturally occurring material the road is built upon, or it can refer to the imported fill material that has been used to create an embankment upon which the road pavement is constructed.
The pavement is constructed entirely with soil present at site. The camber provided is steep ranging from1 in 20 to 1 in23 which allows water to drain off quickly. The width of an earthen road is taken from outside to outside of shoulders, i.e. carriage plus two shoulders. A width of 6 m is sufficient but that of 7.5 is desirable. The side drains are provided to carry the rain water to a stream.
SOIL STABILISED ROADS
Earthen roads can be upgraded by stabilizing the soil which can be used as a base course or sub-base course. Soil stabilization increases the bearing power of soil by adopting controlled compaction, proportioning, and addition of admixtures or stabilizers.
WATER BOUND MACADAM
General- The W.B.M consists of clean crushed coarse aggregates which are mechanically interlocked by rolling and the voids in the aggregates are filled with screening and binding materials with the help of water. This is laid on a prepared sub-grade, sub-base or base or on an existing pavement. It can be used as a sub-base, base course or surfacing coarse.
-Crushed or broken stone, over burnt broken brick, kankar and laterite.
CONSTRUCTION PROCEDURE OF W.B.M:-
(I) Preparation of foundation for receiving the Water Bound Macadam coarse.
(ii) Provision of lateral confinement of aggregate.
(iii) Spreading coarse aggregate.
(v) Application of screening.
(vi) Sprinkling and grouting.
(vii) Application of binding material.
It consists of one or more courses of crushed aggregate which are bonded and keyed by bituminous materials and filling aggregate. A seat coat is applied on top, if required. The thickness of each course should not exceed 75mm.
Materials Used:-The coarse aggregate consists of crushed stone and crushed gravel, and should be clean, strong, durable and fairly cubical in shape. It should be free from salt and dust. The crushed stone should be graded from 63 to 2.36mm. The key aggregate should also be graded from 38 to 2.36mm.
Construction:-The following are the stages in construction-
-Preparation of Base.
-Spreading and Compacting Coarse Aggregate.
-Application of Bituminous Material.
-Application of Key Aggregates.
-Application of Seal Coat.
CEMENT CONCRETE PAVEMENT-
General-Cement concrete pavements are used on important highways.
Cement:-Ordinary Portland cement and high strength Portland cement are most widely used. However if the road is required to open at an earlier date, rapid hardening Portland cement is used.
Aggregate:-Aggregate should comply with specifications laid down in IS: 383-1970. The Los Angeles abrasion test value should not be greater than 35% and 50% for wearing course and subsurface course.
The coarse aggregate should be graded such that the aggregate is collected in two size ranges. If the grading is from 5-88mm then, one range will be below 20mm and other above 20mm.
Fine Aggregate:-Fine aggregate should be preferably natural. Sometimes crushed stone sand may also be used together with natural sand.
Proportioning of concrete – The mix required is designed in the laboratory so that the minimum structural strength in the field is achieved. The mix is proportioned so that a minimum flexural strength of 4 N/mm2 is developed in the field.
The following stages are involved in the construction of cement concrete pavement:-
1. Preparation of sub-grade and sub-base.
2. Placing of forms.
3. Batching of materials and mixing.
4. Transportation and placing of concrete.
5. Compacting and finishing.
7. Removal of forms.
JOINTS IN CEMENT CONCRETE PAVEMENTS
Changes in temperature cause expansion or contraction in cement concrete pavements. Hence joints are provided in slabs to break the continuity and save it from getting damaged. There are three types of joints:-
SEALING OF JOINTS
After curing period is over and before the pavement is opened to traffic, the temporary seal and other materials in the joints should be removed completely and the grooves filled with approved joint sealing compound.
The edges of the joints should be first primed with primer, generally a thin bituminous paint, which should be allowed to dry before the sealing compound is applied.
The line of separation between adjacent slabs of concrete is cleaned and painted with 200 penetration grade bitumen.
REINFORCEMENT IN CEMENT CONCRETE PAVEMENT
Cement concrete pavement cracks when subjected to changes in temperature and moisture. Hence, reinforcement is provided not to increase flexural strength but to withstand the tensile stresses caused due to shrinkage and contraction. When the slab is provided with joints spaced properly to control cracking, reinforcement is not essential at all. The reinforcement consists of welded wire fabric or mats formed by small diameter bars.
Since the cracks due to high tensile stresses start at the top and tend to open, the reinforcement is preferably placed 50mm below the surface.