Masonry and Tunnel
Related to Brick and Stones
Stone Masonry
Stone masonry can be classified into two types:
- Rubble Masonry & Ashlar Masonry
Rubble Masonry:
- The type of stone masonry in which either undressed or roughly dressed stone are laid in a suitable mortar is called rubble masonry. In this masonry the joints are not of uniform thickness.
Ashlar Masonry:
- It is the type of stone masonry in which finely dressed stones are laid in cement or lime mortar is known as ashlars masonry. In this masonry are the courses are of uniform height, all the joints are regular, thin and have uniform thickness. This type of masonry is much costly as it requires dressing of stones.
Three types of Rubble Masonry are as follows:
Un-coursed Random Rubble Masonry:
- The random rubble masonry in which stones are laid without forming courses is known as un-coursed random rubble masonry. This is the roughest and cheapest type of masonry and is of varying appearance.
- It is used for construction of walls of low height in case of ordinary buildings.
Coursed Random Rubble Masonry:
- The random rubble masonry in which stones are laid in layers of equal height is called random rubble masonry. In this masonry, the stones are laid in somewhat level courses. Headers of one coursed height are placed at certain intervals. The stones are hammer dressed. CRRM is used for construction of residential buildings, boundary walls etc.
Squared Rubble Masonry:
- The rubble masonry in which the face stones are squared on all joints and beds by hammer dressing or chisel dressing before their actual laying, is called squared rubble masonry.
There are two types of squared rubble masonry:
Coursed Square Rubble Masonry:
- The square rubble masonry in which chisel dressed stones laid in courses is called coarse square rubble masonry. It is used for construction of public buildings, hospitals, schools, markets, modern residential buildings etc. and in hilly areas where good quality of stone is easily available.
Un-coursed square rubble masonry:
- The squared rubble in masonry which hammer dressed stones are laid without making courses is called un-coursed square rubble masonry. It consists of stones which are squared on all joints and beds by hammer dressing. All the stones to be laid are of different sizes. It is used for construction of ordinary buildings in hilly areas where a good variety of stones are cheaply available.
Dry rubble masonry:
- The rubble masonry in which stones are laid without using any mortar is called dry rubble masonry or sometimes shortly as "dry stones". It is an ordinary masonry and is recommended for constructing walls of height not more than 6 m.
Quoins
- 1. Quoins are large rectangular blocks of masonry or brick that are built into the corners of a wall and is normally 90o to the horizontal.
- 2. They can be used as a load-bearing feature to provide strength and weather protection, but also for aesthetic purposes to add detail and accentuate the outside corners of a building.
- 3. Quoins are external cornerstones at the edges of stone or brick buildings.
King closer:
- If a brick is cut in such a way that the width of one end becomes half that of a full brick, while the width at the other end is equal to the full width, then it is called as king closer.
- It is obtained by cutting out a triangular portion of the brick between the centre of one end (width side) and the centre of the other end (lay side).
Queen closer:
- When a brick is cut along its length, making it two equal halves then it is called queen closer.
- Squint brick:
- They are cut on one corner at an angle of other than 90 degrees. They are required for giving shape to an exterior or interior corner in a wall.
Thickness of Damp Proof Course D.P.C.
- A cement concrete layer in the proportion 1: 2: 4 is generally provided at the plinth level to work as a damp-proofing course.
- The depth of the cement concrete layer varies from 40 mm to 150 mm. It stops the rise of water by capillary action and it is found to be effective at places where the damp is not excessive.
- Here 4 cm, 5 cm, and 6 cm all are the correct answer for the question as they lie in the desirable range of 4 cm to 15 cm of plinth thickness. However, 4 cm distinct as minimum plinth thickness and hence it is the most appropriate option.
Pointing
Struck Pointing:
This is a modification of flush pointing in which the face of pointing is kept inclined, with its upper edge pressed inside the face by 10 mm. This pointing drains water easily.
Recessed Pointing:
This pointing is done by pressing the mortar back from the edge by 5 mm or more. It gives a good appearance.
Beaded Pointing:
This is a special type of pointing formed by steel or ironed with a concave edge. It gives good appearance but is liable to damage.
Tuck Pointing:
This pointing is formed by first pressing the mortar in the racked joint and finishing flush with the face. While the pressed mortar is green, groove or narrow channel, having 5 mm width and 3 mm depth is cut in the center of the groove.
Weathered Pointing:
This pointing is formed by projecting a V-shaped outward projection from the surface of the masonry wall, so as to shed water readily.
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| Types of Pointing |
Thickness of Plastering
Thicker plaster is not recommended as it may strip off from the wall. Recommended thickness of plaster is 12 mm but it should not exceed more than 20 mm.
Note:
Cement mortar proportion of plaster work is 1:3 to 1:4.
Pans: Used for conveying concrete for very small work such as concreting the sidelines of small-sized drains.
Pumps: It is used to transport to raft foundation, tunnel lining, long concrete member, etc.
Belt Conveyors: It has very limited applications in construction as concrete tends to segregate on steep inclines, at transfer points, and at points where the belt passes over the rollers. Also, the concrete tends to dry and become stiff if carried over the longer distance. Therefore, it is suitable for conveying for shorter distance only.
Transit Mixer: It is one of the most popular equipment for transporting concrete over longer distance particularly in RMC. The capacity of each transit mixer is about 6 m3. The rotating speed of the drum is about 4 – 16 revolutions per minute.
Chute: This is generally provided for transporting concrete from ground level to a lower level. The slope of the chute should not be flatter than 1 vertical to 2.5 horizontal.
Skip and Hoist: This is the most widely adopted technique for transporting at a higher level e.g. construction of the multi-story building, etc.
Buckets are never used to conveying concretes.
Tunnel
Tunnel is defined as an underground passage for the transport of passengers, water, sewage minerals, gas, etc.
Based on alignment tunnels are classified as follows:
i) Off-spur tunnels: These are short length tunnels to negotiate minor local obstacles, which cannot be avoided by permitted curves.
ii) Saddle or base tunnels: These tunnels are constructed along the natural slopes till the slopes do not exceed the ruling gradient.
iii) Slope tunnels: These tunnels are constructed in steep hills for economic and safe operations of roads and railways.
iv) Spiral tunnels: These tunnels are provided in narrow valleys in the form of loops in the interior of the mountain so as to increase the length of the tunnel to avoid steep slopes.
The objective of providing a tunnel with permanent lining are manifold:
1. it gives the correct section to the tunnel
2. it withstands soil pressure when driven is soft soils.
3. it reduces losses in fiction and erosive action, and ensure streamline motion when the tunnel has to carry water by providing a smooth passage at a good velocity, free from turbulence.
4. it forms a good protective covering to a certain type of rocks prone to air slacking.
5. it keeps the inside of the tunnel free from water percolation.
6. it supports a large slab of rock which might have become Ioosened during blasting.
Tunnel Jacking:
Tunnel jacking is the process of making a tunnel in already existing bodies such as road and railway area
Immersed Tunnel:
These types of tunnels are partly or wholly under water.
Tunnel Lining:
Tunnel lining is the wall of the tunnel.
It is usually in the form of a ring of a precast concrete segment.
Shield Tunneling:
A shield tunnelling is a protective structure and trailing support mechanism.
Grouting:
It is a method of providing additional support to the drilled mine.
Drift method of Tunneling:
A drift is a small tunnel measuring 3 m x 3 m, which is driven into the rocks and whose section is expanded in the later processes till it acquire the size of the tunnel.
A number of drill holes are provided all around the drift and these are filled up with explosives and ignited so that the size of the drift expands to become equal to the required cross section of the tunnel.
Advantages:
(a) If the quality of the rock is bad or if it contains excessive water, this is detected in advance and corrective measures can then be taken in time.
(b) A drift assists in the ventilation of tunnels.
(c) The quantity of explosives required is less.
(d) A side drift allows the use of timber to support the roof.
Disadvantages:
(a) It is a time-consuming process, as the excavation of the main tunnel gets delayed till the drift is completed.
(b) The cost of drilling and removing the muck from the drift is high, as the work has to be done using manually operated power-driven equipment.
Types of Tunnel Section
1. Circular section:
It can withstand the pressure caused by water, water-bearing soils or soft grounds. It is best suitable for sewers and water carrying purposes.
It is best suitable for non-cohesive soils and for tunnels driven by the shield method.
2. Horseshoe section:
Horseshoe section has a semi-circular roof together with arched sides and a curved invert. It also is suitable for carrying water or sewage. The section is found to be most suitable for soft rocks. This shape is commonly used for highways and railway tunnels.
When lined, this cross-section offers good resistance against external ground pressure and serves to combine the advantages of both D-shaped and circular sections.
3. Egg-shaped section:
It is commonly used for carrying sewage because it gives self-cleansing velocity even in dry weather flow.
4. D-section or the segmental roof section
The risk of failure or collapse caused by external pressure from water or loose or unstable soil conditions on tunnel lining is practically non-existent and it is then convenient to have a section with an arched roof and straight side, Which is called D-section.
It is suitable in hard rock for subways or navigation channels.
Note
Cross-section of the tunnel must be checked at regular interval of 2 - 3 m for maintaining the shape of the tunnel.
When the tunnel diameter is more than 8 m, it is advisable to do the excavation in two stages by heading and bench method.
Certain factors that must kept in mind in the tunneling procedures are:
Like Gradient
1. The best and economical alignment was chosen must be straight in nature.
2. Tunnel should have a grade, which is less than the outside. It is observed that in the railway tunnels, constant slipping of the wheels takes place due to the wetness of the rails. This reduces the hauling capacity of the locomotives.
3. Gradient of 0.2% must be provided to ensure proper drainage.
4. When it comes to long tunnels, two grades at either ends must be provided.
Advantages of Tunnels:
- a) Tunnels are more economical than open cuts beyond certain depths.
- b) Tunnels avoid disturbing or interfering with surface life and traffic during construction.
- c) Tunnels prove to be cheaper than bridges or open cuts to carry public utility services like water, sewer, gas, electricity and telephone lines.
- d) There is an overall reduction in cost because of shortening the distance as compared to bridges or open cuts.
- e) Tunnels avoid interference with surface and air rights.
- f) Its maintenance cost is low.
Disadvantages of Tunnels:
- a) The initial cost of construction of a tunnel is high as compared to an open cut.
- b) It is necessary to have skilled labour and technical supervision of high order for the construction of a tunnel.
- c) It takes a long time for the successful completion of a tunnel under normal conditions.
- d) The construction of the tunnel requires specialized and sophisticated equipment.
Various Methods for tunnelling through the rocks are as follows:
- i) Full face method
- ii) Heading and bench Method
- iii) Cantilever car dump Method
- iv) Drift system
- v) Pilot tunnel Method
Various Methods for tunnelling through the soft ground are as follows:
- i) Forepoling Method
- ii) Needle beam Method
- iii) Five-piece set Method
- iv) Liner plates Method
- v) Casing Method
- vi) Square set and lagging Method
- vii) Horse cups Method
Tunnel Jacking:
- Tunnel jacking is the process of making a tunnel in already existing bodies such as road and railway area
Immersed Tunnel:
- These types of tunnels are partly or wholly under water.
Tunnel Lining:
- Tunnel lining is the wall of the tunnel.
- It is usually in the form of a ring of a precast concrete segment.
Shield Tunneling:
- A shield tunnelling is a protective structure and trailing support mechanism.
Grouting:
- It is a method of providing additional support to the drilled mine.
Methods of tunnelling in hard rock | Methods of tunnelling in soft soil |
Drift Method | Fore-poling method |
Heading and benching method | Needle beam method |
Full face method | Army method or case method |
Cantilever car dump method | American method |
Pilot tunnel method | English method |
Perimeter method or German method | Belgian method |
| Shied tunnelling method |
| Linear plates method |
Most suitable shape of the tunnel for noncohesive soils is a circular section.
- The circular section of a tunnel offers greater resistance to external pressure caused by water, water-bearing soils or soft grounds.
Advantages of Circular section:
- i) It is the best theoretical section for resisting internal and external section.
- ii) It provides the greatest cross-sectional area for the least perimeter.
- iii) It is the best suited for noncohesive soils.
- iv) It is most suitable for sewers and water carrying purposes.
Disadvantages of Circular section:
- i) Not suitable for roadways or railways as more filling is required.
- ii) The shape is more difficult for the concrete lining.
California Bearing Ratio (CBR) test:
California Bearing Ratio (CBR) test is a method of classifying and evaluating soil-subgrade and base course materials for flexible pavements.
CBR test, an empirical test, has been used to determine the material properties for pavement design.
This test measures the strength of the material and is not a true representation of the resilient modulus.
It is a penetration test wherein a standard piston, having an area of 3 in 2 (or 50 mm diameter), is used to penetrate the soil at a standard rate of 1.25 mm/minute.
The pressure up to penetration of 12.5 mm and it’s ratio to the bearing value of a standard crushed rock is termed as the CBR.
Types of Explosive
Type of Explosive | Suitability |
Blasting Powder | In large quarrying blocks |
Dynamite | Small boreholes and quarries |
Cordite and Gelignite | Under water |
Lithofracteor | Tunnels |
