Showing posts from September, 2013

Road Materials - Soil as Sub-grade

Road Materials are the materials which are used for the construction of the roads, commonly used road materials are, soil, Aggregates and binders.
Soil is used for the construction of the bottom most layer of the pavement, i.e. sub-grade. Here is a short details of the sub-grade and its function.:

sub-grade is the layer of the pavement whose main function is to support the upper layers of the pavement and to provide the good drainage facility to the infiltrating rain water. It has to act as a single structure along with other layers of the pavement.Soil is compacted to its maximum dry density which can be achieved by using the optimum moisture content and the methods of compaction control. Strength has to be ensured which is required for the given design thickness of the pavement.Strength analysis and the thickness of pavement are inter linked because more thickness of the pavement is needed if the soil is weak but if the soil possess a good strength then less thickness is needed.

Westergaard's theory for rigid pavements

Hi, Rigid Pavements are constructed with some rigid materials like Cement Concrete(Plain, reinforced or prestressed).

Here the load is transferred  through the slab action not like in the flexible pavements. Westergaard's theory is considered good to design the rigid pavements.

He considered rigid pavement slab as a thin elastic plate resting on soil sub-grade, which is assumed to be a dense liquid. So, here the upward reaction is assumed to be proportional to the deflection, i.e. p = K.d, where K is a constant defined as modulus of sub-grade reaction. Units of K are kg/cm^3.
Material of pavement parameters:
Westergaard's modulus of sub-grade reaction:
Modulus of sub-grade reaction is proportional to amount of deflection d. Displacement level is taken as 0.125 cm in calculating K i.e. d = 0.125 cm, so modulus of sub-grade reaction
K = p/d = p/0.125 kg/cm^2

Radius of relative stiffness of slab to sub-grade:
Amount of deflection which will occur on the pavement surface depends on …

Part -2-GATE preparation - Transportation Engg. - One liners

5. What is the value of intermediate sight distance for a highway with a design speed of 65 kmph? Assuming the data for co-efficient of friction f = 0.36 and for reaction time t = 2.5 sec.
Ans: 182.8 m

6. The speed of overtaking and overtaken vehicles are 70 and 40 kmph respectively on a two way traffic road. If acceleration of overtaking vehicle is 0.99 respectively, then safe overtaking sight distance, assuming reaction time of 2 sec, will be  278 m.

7. What is the minimum length of overtaking zone for a design speed of 96 kmph assuming acceleration as 0.69 m/sec^2, reaction time as 2 sec and traffic as one way?   Ans: 1026 m  OSD = 3(d1+d2)

8. The radius of horizontal circular curve is 100 m. If design speed is 50 kmph and design co-efficient of lateral friction is 0.15, then super elevation required if full lateral friction is assumed to develop will be 0.047

9. What is the co-efficient of friction needed if no super elevation is provided for a horizontal circular curve of radius 190 m…

GATE 2014- Transportation Engineering - one liners - part 1

1. The safe stopping sight distance for design speed of 50 kmph two way traffic on a two lane road assuming co-efficient of friction as 0.37 and reaction time as 2.5 seconds is 61.4 m.

2. The stopping sight distance for design speed of 80 kmph for two way traffic on a single lane road. Assume co-efficient of friction as 0.35 and reaction time as 2 seconds, is 232.94 m.

3. What is the minimum sight distance required to avoid head on collision of two cars approaching from the opposite directions at 90 and 60 kmph? It is given that the reaction time is 2.5 seconds, co-efficient of friction is 0.7 and a break efficiency is 50 percent in either case.   Sol: 235.8 m.

4. What is the stopping sight distance on a highway at a ascending gradient of 5% for a design speed of 70 kmph assuming co-efficient of friction as 0.35 and reaction time as 2 seconds?  Sol: 87.14 m.

Reference: GATE 2013 by GK publishers (refer for elaborate answers)

CBR(California Bearing Ratio) Test

Aim: To find out the CBR(California bearing ratio) value of the given soil sub-grade.Apparatus: CBR Apparatussoil sample with known OMC and MDD.Theory: California Bearing Ratio test was invented by California State Highway Department. This test is used to design the thickness of the flexible pavement.  CBR value signifies the strength characteristics of the soil sub-grade which is compacted to the MDD using the OMC. The samples used for the testing are prepared in the laboratory. If a new pavement is to be constructed then, sample are prepared by compacting it with OMC and then sample is soaked in water for four days.
If the test is done for the overlay design then the sample is prepared by compacted it to the density of the soil at the site. Four days soaking is necessary in order to achieve the worst site conditions. Higher the CBR value more is the strength of the soil sub-grade. Empirical charts are prepared by the IRC to find out the thickness of the flexible pavement correspondi…

Aggregate Impact Value Test

Aggregate to be used as road materials must possess some properties or characteristics. Toughness is one of the highly required property which is necessary for the aggregates to bear the impact loads. Road aggregates are applied with the impact loads a millions of times in their life as the road materials. So they must be tested for their toughness before they are used in the pavement layers. Toughness can be tested using a Impact testing machine, which is an arrangement to apply the impact loads on the aggregates just like they are applied on the roads. Materials which gets fractured or crushed into smaller particles are not tough. In this test we will find out the percentage of the weight of the aggregate sample which gets crushed with respect to the total weight of the sample. This percentage is known as the aggregate impact value and more the aggregate impact value less is the toughness of the road aggregates and vice verse.  If impact value is less than 10% then aggregate…

Engineering Surveys and the location of highway alignment

Location of the highway alignment is done after carrying out survey of the area, these surveys are called Engineering Surveys. We have to locate an alignment which fulfill the basic requirements like the path must be short, safe, economic, easy and useful. To check all these basic requirements we can carry out the Engineering Surveys in the following phases:
 Map Study Reconnaissance Surveys Preliminary SurveysLocation Surveys  Map Study:  This is the first step of the Engineering survey, using a topographic map of the area under consideration, which can be availed from the Survey of India, we can propose different alternatives of the road alignment. This topographic map in general have a contour interval of around 30 m to 40 m. We can get the details of the natural and artificial features of the area using the topographic map, and accordingly we can suggest a numbers of alternatives for the road alignment. These routes are further studied in the Reconnaissance survey.  Reconnaissa…

Basic Requirements of a Highway alignment on plain and hill roads

There are some basic requirements of the highway alignment in the plain and hill roads which must be fulfilled. In general the basic requirements are:

(1) Short: The alignment must be the shortest of the various alternatives available. Of course the shortest path between any two points is a straight line but the topography of the area or other factors may necessitate it do divert and take some other route, but as far as possible it should be kept minimum.
(2) Easy: Alignment should be such that the road must be easy to construct and easy to maintain or repair. If curves are of large radius and the gradient is gentle it would be easy to construct the road, rather than opposite.
(3) Safe: Safety is again the basic requirement of the highway alignment and special care must be taken to align the road in such a way that it must have the safe or minimum Sight distances and Radius of the curves, means the geometrical design features like Sight Distance, Radius of the curves and the gradient of…