Month: April 2020

# 9 Objectives of Railway Signalling – Railway Engineering

## 9 Objectives of Railway Signalling

Railway signalling is a system or device through which train movement is safely and efficiently regulated. Following are the 9 purposes or objectives of railway signalling:

1. To prevent a moving train from getting into contact with another train or obstruction and thus to provide safety to the passengers, the staff and the rolling stock.

2. To maintain a safe distance between two trains on the same line in the same direction to avoid accidents.

3. To provide protection against train collision and derailment at converging junctions and to give a directional indication at diverging junctions.

4. To provide facilities for safely and efficiently carrying out shunting operations in marshalling yards.

5. To allow trains to run at a limited speed while maintaining and repairing the track.

6. To facilitate the flow of traffic and to increase the carrying capacity of the track.

7. Signalling provides a safe and secure facility to regulate the arrival and departure of trains from the station.

8. To provide facilities for the efficient movement of trains and maximum utility of the rail track.

9. Signalling system helps to avoid accidents at the point between two or more trains that cross each other’s path.

Advantages of Automatic Signalling System in Railway

# Relation Between Submerged Unit Weight, Specific Gravity, Void Ratio and Unit Weight of Water

## Relation Between Submerged Unit Weight, Specific Gravity, Void Ratio and Unit Weight of Water

In this article, we shall make a formula or relation between the submerged unit weight(${\gamma }'$), specific gravity(G), void ratio(e) and unit weight of water/ density of water($\gamma_{w}$).

We know,

$\gamma _{sat} = \frac{W_{sat}}{V_{sat}}$
Or, $\gamma _{sat} = \frac{W_{s} + W_{w}}{V_{s}+V_{v}}$

Or, $\gamma _{sat} = \frac{G \gamma_{w} + e\gamma_{w} }{1+e}$

Or, $\gamma _{sat} = \left (\frac{G + e}{1 + e} \right )\gamma_{w}$

From the definition of submerged unit weight, we have,

${\gamma }’ = \gamma_{sat} – \gamma_{w}$

Or, ${\gamma }’ = \left (\frac{G + e}{1 + e} \right )\gamma_{w} – \gamma _{w}$

Or, ${\gamma }’ = \left (\frac{G – 1}{1 + e} \right )\gamma_{w}$

Relation Between Porosity and Void Ratio

Relation Between Dry Unit Weight, Specific Gravity, Percentage of Air Voids and Water Content

# Typical Values of Permeability for Different Soils

## Typical Values of Permeability for Different Soils

The permeability or coefficient of permeability can be defined as the rate of flow of water through a unit cross-sectional area of the soil under a unit hydraulic gradient at a temperature of $10^{\circ}$C. Typical values of permeability or coefficient of permeability for different soils are as follows:

Permeability of Soil

Factors Affecting Permeability

5 Empirical Formula for determination of the coefficient of Permeability

# Differences Between Open Caissons and Pneumatic Caissons

## Differences Between Open Caissons and Pneumatic Caissons

Differences between well or open caissons and pneumatic caissons are as follows:

### 1. Structure

➤ Open caissons are box-type structures having their top and bottom open.

➤ Pneumatic caisson consists of an enclosure open at the bottom and closed at the top.

### 2. Cost

➤ Open caissons are relatively cheaper than pneumatic caissons.

➤ Pneumatic caissons are relatively costlier due to the use of compressed air in the working chamber.

### 3. Inspection

➤ Inspection and cleaning process at the bottom of the caisson can not be carried out in case of open caissons.

➤ But in the case of pneumatic caissons cleaning and testing process can be easily carried out, as the working chamber is dry.

### 4. If boulders are faced during construction

➤ The construction process will be slow and difficult if the boulder met during construction.

➤ No problem arises in the case of Pneumatic Caissons.

Box Caissons

Use of Cofferdams

# Relation Between Dry Unit Weight, Specific Gravity, Percentage of Air Voids, and Water Content

## Relation Between Dry Unit Weight, Specific Gravity, Percentage of Air Voids, and Water Content

In this article, we shall make the formula or relation between the dry unit weight($\gamma _{d}$), specific gravity(G), percentage of air voids, and water content(w).

[Note: Dry unit weight means dry density]

Soil three-phase diagram is shown in the above picture. From this diagram, we can write,

Total Volume = Volume of solids ($V_{s}$)+ Volume of water ($V_{w}$) + Volume of air ($V_{a}$)

$V = V_{s} + V_{w} + V_{a}$
[Both sides divided by V]
Or, $\frac{V}{V}= \frac{V_{s}}{V} + \frac{V_{w}}{V} + \frac{V_{a}}{V}$

[We know, $\frac{V_{a}}{V} = n_{a}$]

Or, $1 = \frac{V_{s}}{V} + \frac{V_{w}}{V} + n_{a}$
Or, $1 – n_{a} = \frac{V_{s}}{V} + \frac{V_{w}}{V}$

[ We know, $\gamma_{s} = \frac{W_{s}}{V_{s}}$, and Specific Gravity(G) $= \frac{\gamma_{s}}{\gamma_{w} } = \frac{W_{s}}{V_{s}\gamma_{w} }$ ]

[From this equation, $G = \frac{W_{s}}{V_{s}\gamma_{w} }$, we can write $V_{s} = \frac{W_{s}}{G\gamma _{w}}$. Now, place the value of $V_{s}$ in the above equation.]

Or, $1 – n_{a} = \frac{W_{s}/G\gamma_{w}}{V} + \frac{W_{w}/\gamma _{w}}{V}$

[We know, Density of solids$(\gamma_{d}) = \frac{W_{s}}{V}$]

Or, $1 – n_{a} = \frac{\gamma_{d}}{G\gamma_{w} } + \frac{wW_{s}/\gamma _{w}}{V}$

Or, $1 – n_{a} = \frac{\gamma_{d}}{G\gamma_{w} } + \frac{w\gamma_{d} }{\gamma_{w} }$

Or, $1 – n_{a} = \frac{\gamma_{d}}{\gamma_{w} }\left [ w + \frac{1}{G} \right ]$
$\gamma_{d} = \frac{(1 – n_{a})G \gamma_{w}}{1 + wG}$

Relation Between Dry unit Weight, Bulk unit Weight & Water Content

Relation Between Void Ratio, Water Content, Degree of Saturation & Specific Gravity

Relation Between Porosity and Void Ratio

# 5 Characteristics of Fire-Resisting Materials

## Characteristics of Fire-Resisting Materials

The planning and designing of the buildings should be such in a way that the structure should offer sufficient resistance against fire to protect the occupants in the event of a fire. It should be possible when the materials used in the construction have well-fire-resistant properties. The following 5 characteristics of any fire-resisting material should be exhibited.

1. The material used in the construction should be a bad conductor of heat. When subject to high temperatures, the material does not significantly lose its strength.

2. Expansion and contraction of the material due to the rise and fall of temperature, respectively should not be excessive.

3. The contraction due to sudden cooling is more dangerous than the expansion effect. A good fire-resisting material should not be cooled rapidly as it may break into pieces.

4. The composition of the material should be such that it does not disintegrate or crumble under the effect of high temperature.

5. The material should be non-combustible as far as possible. It should never be understood that non-combustible materials are good in heat resistance. For example, mild steel is non-combustible but does not have good resistance against heat. Similarly, timber is a combustible material but it is better heat resistant than mild steel.

Characteristics of a Good Timber

Safety Against Fire in Theatres and Cinemas

# What are the Uses of Cofferdams?

## Uses of Cofferdams

Following are the 8 uses of cofferdams:

1. The cofferdams are used to enclose the workplace for preventing the entry of water into it.

2. To provide a safe water-enclosed platform for the workers while working on laying the foundation in water.

3. To provide a space for foundation work without affecting the safe conditions of adjoining structures.

4. To facilitate pile-driving operations.

5. To place the raft foundation.

6. To provide space for carrying out the foundation work and superstructure work of concrete dams.`

7. To facilitate the construction of foundations for piers and abutments of bridges, locks, etc.

8. To lay the grillage foundation.

Box Caissons

# Box caissons & their Construction Procedure

## Box caissons

In the case of box caissons, the top portion is open and the bottom portion is closed. It may be made from reinforcement cement concrete, steel, or timber. Box caissons are preferred under the following conditions.

1. When the depth of water is not more than 6 to 8 m.

2. When bed material consists of soft and loose material which can be easily dredged out to form a levelled bearing surface.

3. When the velocity of flow is not so large that it affects the stability of the caisson against the scour.

4. When the excavation work for preparing the bed of the foundation is not required or bed of the foundation may be prepared with a group of piles.

### Construction Procedure of Box Caissons

The following procedure is followed in the construction of the box caissons.

First of all, a level bearing surface is made to receive the bottom of the box. The surface can be made by dredging all the loose and soft soil.

In case the caisson is to be rest on piles, all the pile’s heads must be kept at the same level before placing the box caisson over the piles. Sometimes, piles cap may be used to group all the piles together which acts as one structural member.

Then the empty space in the box caisson is filled by using sand, gravel, or concrete. Then the top portion of the caisson is sealed and further work is started above the water surface.

The box caissons are used in the construction of quay walls