The Circular section offers greater resistance to external pressure and highly suitable for unstable ground such as sand or soft clay. They are not used as traffic tunnels because they require more filling to make flat base, hence used for carrying water and sewage under pressure.
Elliptical section have narrower cross section at bottom and top.They also resist external pressures. Due to compactness from bottom and top they better serve as water and sewage conveyance.
Egg shaped section properties are somewhat similar to elliptical section but here the section is narrower at bottom. Due to narrow section at top and bottom both of them are difficult to construct and are used for soft subsoil.
Horse shoe section forms a semi circular section at top and curved trapezium shape at bottom. Hence most preferably used as traffic tunnels in both roads and railways.
Segmental section is mainly used by railways in some countries, because of its vertical walls and flat floors at bottom side and also forms a semicircular section at top.
The content shown above is for study purpose only.
The shapes of tunnels mainly depends upon the purpose (they follow in future) and second to resist internal and external pressures.
These are most common Cross-sections widely used by construction companies. The Circular section offers greater resistance to external pressure and highly suitable for unstable ground such as sand or soft clay. They are not used as traffic tunnels because they require more filling to make flat base, hence used for carrying water and sewage under pressure.
In the next section we will explain about them.
The modes of failure of gravity dam are :-
- OverTurning or Rotation about toe :- When resultant force acting at any section if it passes outside the toe, the dam shall rotate or overturn about toe. In other words, it means that if horizontal forces dominate over vertical forces the dam will definately rotates. At such condition we mainly use factor of safety , it is the ratio of righting moments(anticlockwise) about toe to the overturning moments(clockwise) about toe. The factor of safety should not be less than 1.5.
- Crushing :- It generally happens when the compressive stress exceeds the allowable stress of the material of dam, hence the dam may fail. The total normal stress = Direct stress + Bending stress Pmax will be produced on the end which is near to the resultant force, if Pmin is negative then tension is produced at that end.
- Sliding or also called shear failure :- When horizontal forces responsible for sliding dominate frictional resistance of dam at that level. To prevent sliding the horizontal forces(∑H) should be less than shear resistance(μ∑V). i.e. ∑H <μ∑V
The explanation is for 1st year students, as the word shows ‘seasoning’ means removal of water in presence of proper air circulation under controlled conditions and while seasoning, water does not exceed 15% of its dry weight.
Following benefits are:-
- to make it durable against cracks, warp and sharp.
- to make it fit for receiving treatments of paints, varnishes, preservatives, etc.
- to make it workable for various operations while construction work.
- to increase strength, hardness and stiffness against rough use.
- increase electrical resistance power.
- to make it suitable for joining with other types of timber.
- to make it suitable for burning as a fuel.