method of laying Water Bound Macadam (WBM) road

Describe the method of laying Water Bound Macadam (WBM) road.

Solution:-


The following method of laying WBM road are as followed:-

(i) Preparation of sub grade: The subgrade must be strong enough so that it does not settle under the loads and thus soil subgrade is compacted usually by rollers (of weight 80kN) after sprinkling water on to the soil. Any resulting depressions must be filled first before laying the second layer.

(ii) Construction of sub – base: Here in this step, crushed stones or over burnt bricks or boulders in thickness of 10 cm – 20 cm is laid. The sub – base is also compacted by rollers.

(iii) Construction of base: On the prepared surface of sub – base, a layer of base course is laid of specified material. The thickness, grade and cross – section are maintained as per the design and specifications.

(iv) Preparation of wearing course: This layer is the top must surface which undergoes large abrasion due to moving vehicles. This course is laid in one or more layers and thickness of this layer is kept as per the design. Generally this wearing course is of asphalt of tar.
After all these steps, shoulders are constructed by spreading earth on either side of the pavement at the specified slope and width. This is achieved by rolling and tamping.

What is super elevation?

What is super elevation?

Super – elevation :-

 Super-elevation (or banking) is the raising of the outer edge of road with respect to the inner edge in order to counteract the centripetal force arising due to negotiation of a curve by a vehicle. Without super – elevation, it a vehicle negotiates a curve then it will be thrown out of the curve due to outward acting centripetal force.

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Assumptions made in Lacey’s theory of canal design

Enumerate the Assumptions made in Lacey’s theory of canal design.

Solution:-

Assumptions in Lacey’s theory of canal design:-

(i) Lacey assumed that silt is transported due to eddies generated from the entire wetted perimeter of canal section and that the canal section achieves a semi – elliptical (or cup shaped) profile called as regime canal.

(ii) Grain size of the silt material is considered important in this theory which is related to sit factor as f = 1.75 (dmm)0.5

(iii) Kutter’s formula is used lacey’s theory for computation of flow velocity.

(iv) Lacey assumed that canal slope bears a definite relation with discharge which Kennedy didn’t considered

(v) Lacey assumed a constant discharge through the canal.

(vi) It is assumed that canal flows uniformly in an incoherent alluvium which bears the same characteristics as that is being carried by the canal.

drawbacks in Kennedy’s theory

Give brief account of the drawbacks in Kennedy’s theory.

Drawbacks of Kennedy’s theory:-

Kennedy assumed that eddy current in channel arises only due to roughness of channel bed.

•  This theory is applicable only for those channels which carry certain amount of silt.

•  This theory is applicable only for regime channels.

•  The channel width (B) and depth of flow (y) has no significance in this theory.

•  It is a trial and error based design approach.

•  There is no logical basis for assigning the values of ‘m’ (critical Velocity Ratio i.e. CVR) to different types of soil and the phenomenon of silt transportation is no properly accounted for.

•  It assumes that silt carried in the flowing water by the vertical eddies generated from the channel bed only actually eddied get generated from the entire wetted perimeter of the channel.

•  This theory gives no expression for determining the channel bed slope.

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Write a short note on Night irrigation

Write a short note on Night irrigation

what is night Irrigation?

Night Irrigation 

It refers to the practice of irrigation of crop at night so as to lower the loss of water due to evaporation. Since water is a sparse resource and thus it always remains debatable when to irrigate the crop in a day or at night. Many prefer irrigation at the time of early dawn while others prefer afternoon time. Among all, day time irrigation is the most uneconomical due to high evapotranspitation demand of crop.

In night irrigation, it is ensured that irrigation should complete by the morning before sunrise in order to have sufficient soaking of water by the soil. Water lost during night time irrigation is about 25 – 50% less than that during day time irrigation.

Consumptive use of water

Write a short note on Consumptive use of water

Solution:-

Consumptive use of water:

•  It is the amount of water required by a crop in transpiration i.e. growth of plant tissues and the water lost from the surrounding soil and from the leaves as evaporation. It is also referred to as evapotranspiration.

•  The value of consumptive use of water varies from crop to crop and from soil to soil. Also it may differ for the same crop at different times of a year.

•  Consumptive use is useful in assessing the irrigation requirement for a crop.

•  The factors which affect the consumptive use of crop are temperature, humidity, wing velocity, atmospheric pressure etc.

•  Blaney – Criddle equation is the most commonly used expression for estimating the consumptive water requirement of a crop in a area which is given by:

Cu= kp/40[1.8t+32]

Where Cu = monthly consumptive use (in cm)
k = Crop factor which is determined experimentally for each crop under the given environmental conditions
t= Mean monthly temperature (in ∘C)
p = Monthly percentage of annual day light hours for the period under consideration

Limitations of plate load test

Limitations of plate load test

(i) In no case shall a test plate smaller in width than 30 cm be used, because experimental evidence has indicated that the load settlement behavior of the soil is qualitatively different for smaller widths of the test plates compared to that of larger widths.

(ii) Settlement of a foundation cannot exceed about four times the settlement of a plate of 30 cm width, howsoever large its width may be.

(iii) If the soil plate load test may lead to misleading results. Consider for example, the subsoil conditions illustrated. Where the upper stratum is strong soil like dense sand and the lower stratum is a weak soil like soft clay. In such a situation, the load test reflects the load settlement characteristics of the stronger stratum but does not give any indication of the settlement behavior of the poorer soil below. The foundation being of a much larger width its bulb of pressure of the same load intensity as that on the test plate, will extent into the weaker stratum. Hence, the extrapolated settlement of the foundation will be much smaller than the actual settlement, leading to unsafe design.

(iv) The effect of capillarity in a sand bed is to increase its effective vertical stress or its stiffness. A test plate resting on a capillary sand bed undergoes smaller settlement than a plate resting on dry or submerged sand bed.

(v) A plate load tset is of short duration. The settlement measured is only immediate settlement. In granular soils, immediate settlement can be taken as the total settlement, while in cohesive soils, consolidation settlement, which constitutes most part of the total settlement, cannot be predicted through this test. Hence the plate load test is not of much relevance in clayey soils for which the settlement criterion is very important in the determination of the allowable bearing pressure of a foundation.



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factors which affect the bearing capacity of soils

Explain the factors which affect the bearing capacity of soils.

Solution:

Factors affecting the bearing capacity of soil:

1. Soil type i.e., cohesive or cohesion less.

2. Size of the footing.

3. Shape of the footing i.e., rectangular, square, circular, wall footing etc.

4. Unit weight of soil both above the footing leve and below the footing level

5. Location of water table in the soil.

6. Depth of foundation i.e., shallow foundation or deep foundation.

7. Surcharge load.

8. Ground inclination.

9. Loading eccentricity.

10. Amount of settlement.






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What soil investigations are required for construction

What soil investigations are required for construction of

(i) an embankment and (ii) a building? Give details.?

Soil investigations required for construction of an embankment

I. Bearing capacity of the soil on which embankment is required to be constructed.

II. Seepage properties of the soil on which embankment is proposed to be constructed.

III. Slope stability characteristics of the soil.

IV. Erosion control measures required on the embankment slope.

V. Load expected on the embankment i.e. whether the embankment is for road traffic or is for a railway track.

VI. Consolidation characteristics of the soil and the ultimate settlement expected in the embankment soil.

VII. Geometry of the proposed embankment and shear strength (c, ɸ) characteristics of the soil.

VIII. Water table level in the embankment.

Soil investigation required for construction of a building:

I. Bearing capacity of the soil.

II. Shear strength (c, ɸ) characteristics of the soil.

III. Water table level at the location of building foundation.

IV. Consolidation characteristics of the soil and ultimate settlement expected.

V. Load expected on the soil.

VI. Liquefaction potential of the soil.

significant properties of soil

Write a short note on the significant properties of soil.

Properties of soil

 The various properties of soil depend on the type of soil, Basically there are two types of soil viz. cohesive soil and cohesion less soil.
Cohesion less soil: This type of soil does not possess any cohesion and its shear strength is due to internal friction only. Sand falls under this category.

•  This soil usually possesses good drainage property and its bearing capacity gets improved by vibration.

•  The phenomenon of consolidation is not observed in this type of soil. This soil has the tendency of undergoing liquefaction.

•  This soil generally has high bearing capacity if properly compacted and long term settlement of footing founded on this soil is not a problem.

•  Undisturbed sampling of this soil is difficult.

Cohesive soil
This soil in addition to internal friction also has cohesion between the particles. Thus shear strength of this soil is due to both internal friction and cohesion between the soil particles.

•  Clay falls under this category.

• This type of soil undergoes consolidation and does not have the tendency for liquefaction.

•  Some of the index properties of cohesive soils are plastic limit, liquid limit, plasticity index, consistency index, shrinkage limit, shrinkage ratio, activity etc.

•  Footing founded on this type of soil has to be checked for long term settlement of soil.

•  Undisturbed sampling of this soil is not so much difficult.

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Standard Penetration Test

Explain standard penetration test of measuring the penetration resistance of the soil.

Standard Penetration Test

 It is in-situ that is particularly useful for cohesion less soil (i.e. sand) because undisturbed sampling of cohesion less soil is difficult. This test is used to assess the bearing capacity, shear strength, the angle of internal friction of the soil. It can also be used to determine the unconfined compressive strength of cohesive soils.

•  This test is conducted in a bore hole using a split spoon sampler. The bore hole is drilled to the required depth and sampler is lowered to the bottom of the bore hole.

•  The sampler is then driven into soil by a hammer of 63.5 mass falling from a height of 750 mm at the rate of 30 blows per minute.

•  The number of hammer blows required to drive the sampler 150 mm through the ground is counted.
•  The sampler is again driven further by 150 mm and the number of blows is counted.

•  Once again the sampler is driven further 150 mm into the ground and number of blows is counted.

•  The number of blows for the first 15l0 mm is discarded.

•  The number of blows for the last two 150 mm are added which gives the standard penetration number (N).

•  Thus standard penetration number is equal to the number of blows required for 300 mm penetration beyond the first 150 mm penetration.

•  If the number of blows for the first 150 mm penetration exceeds 50 then the test results are discarded and the test is stopped.

This standard penetration number (N) is corrected for dilatancy and overburden pressure.

Correction of dilatancy

Very fine silty sand and also the fine sand develop pore water pressure which does not get dissipated easily. This pore water pressure increases the resistance of sol against the hammer blow and hence standard penetration number (N) gets increases which otherwise should be less than this value.

When N > 15, the corrected N value for dilatancy is,
NC = 15 + 0.5 (N-15)

Where N = Recorded N value,
If N ≤ 15 then,
NC = N

Correction for overburden pressure

 The overburden pressure on soil affects the penetration resistance of soil, particularly in cohesion less soils. The confining pressure in cohesion less soil increases with depth and thus he penetration resistance measured at shallow depths gets underestimated and at greater depths, it gets overestimated. Thus N value obtained in the field under various overburden pressure is given by, to a standard effective overburden pressure. Thus corrected N Value for overburden pressure is given by, to a standard effective overburden pressure. Thus corrected N value for overburden pressure is given by,

Nc = 350N (𝜎𝑜+70)

Where 𝜎𝑜 = Effective overburden pressure (in kN/m2) ≤ 280 kN/m2

What is compaction? Why is it needed?

What is compaction? 

Why is it needed? 

List the different methods of compaction.

Compaction

 It is a process in which the soil particles are brought close to each other due to expulsion of air from the voids present in the soil on application of an eternal stress. Thus the volume change I the soil is brought about by removal of air from the soil. It is almost an instantaneous process.

Need for compaction

 Compaction, in general, increases the shear strength of soil due to closer arrangement of soil particles. This in turn increases the stability and bearing capacity of soil.

Different methods of compaction

(i) Rollers: 
Various types of rollers are used for compaction of soil. The selection of a particular type of roller depends on the contact pressure required; thickness of soil layer, number of passes required, speed of roller etc. rollers may be smooth wheeled, pneumatic tyred or sheep foot rollers.

(ii) Tampers:
 Tamper (or rammer) is a block of iron about 3 to 5 kg in mass which is attached to a wooden/steel rod. The tamper is usually lifted to about 300 mm and dropped freely on to the soil thereby compacting the soil. Mechanical or machine operated rammers are muchheavier (around 30 to 150 kg) and are also dropped from a larger height.

(iii) Vibrators :
 Vibrators or vibratory rollers bring about the compaction of soil by transmitting vibrations to the soil. Vibratory rollers are particularly useful for compacting sandy soils.








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What is Contouring? Methods, Maps and Uses of Contours in Surveying

What is Contouring? Methods, Maps and Uses of Contours in Surveying?

or,

Define and explain contour, contour interval, necessity of contour plotting. Discuss factors affecting the choice of contour interval ?

Solution:-

Contour

 A contour is a line joining pints of equal elevation on the ground. All the point lying on a contour will have equal elevation. e.g. A contour of 60.00m indicates that all points lying on this contour are having an elevation of 60m.

Contour interval

It is the difference of elevations between two successive contours.

Necessity of contour plotting Contours :

 Plotting of contours gives the topography of the area surveyed. It gives a general idea about the topographic features of the ground which is helpful n deciding the location or alignment of an infrastructure project.

Factors affecting the choice of contour interval :

The following factors govern the choice of contour interval :

1. Purpose of map :- The contour interval should be so selected that it serves the purpose of map. Too large contour interval may omit the necessary details of the map while a too small contour interval will add to the cost of the project.


2. Nature of ground :- The contour interval depends on the nature of ground. For a flat ground, a large contour interval may serve the purpose but for mountainous terrain, a small contour interval is required.


3. Scale of the map :- If the scale of the map is small, contour interval is kept large to avoid over-crowding of contours. However, if the map scale is large then contour interval is kept small.


4. Extent of survey :- Contour interval largely depends on the extent of survey. For surveying a large area, a large contour interval is required but for a small area of survey, contour interval may be kept small.


5. Availability of time and funds :-
 Too small contour interval will increase the survey work and thereby cost of the project. However, large contour interval will reduce the amount of time and cost

Some Important Questions About Contour 

1) What does on contour mean?

A contour is a line joining pints of equal elevation on the ground. All the point lying on a contour will have equal elevation

2) What is a contour in surveying?

In surveying Contouring is the determination of elevation of various points on the ground and fixing these points of same horizontal positions in the contour map.

3) What is direct method of contouring?

In the direct method of contouring, the R.L (Reduced Level) of various selected points on a contour line are obtained and their positions are located. The contours are then drawn by joining these points. It is a very accurate method but it is slow and tedious.

4) What are the methods of contouring?

There are two methods of contour surveying 
1) Direct method.
2) Indirect method

5)Where are contours used?

Contour lines are used to signify a 3D (three-dimensional image) of a flat surfaces. 
They are commonly used on topographic maps. Here contour lines connect continuous points of equal elevation. There is a connection between the change of height and the relative change of distance

6) What is purpose of contouring?

A contour is a line joining pints of equal elevation on the ground so

contour lines connect continuous points of equal elevation. There is a connection between the change of height and the relative change of distance.

this is the main purpose of contouring.