TYPES OF GEOLOGICAL FORMATIONS

With respect to water retention and discharge capacity, geological formations are classified as follows:

1) AQUIFIER

2) AQUICLUDE

3) AQUITARD

4) AQUIFUGE

1) AQUIFIER:-

          Aquifer is a type of geological formation which have sufficient porosity and sufficient permeability.

i.e., Aquifer can retain the water and we can discharge water from these aquifer. 

Example: Formations with fine sand and coarse silt.

2) AQUICLUDE:-

          Aquiclude is a type of geological formation which are highly porous but are practically impermeable.

Due to impermeability of these formations we cannot discharge the water through them.

Example: Formation with clay.

3) AQUITARD:-

          The geological formation which are having sufficient porosity but are less permeable, then these formations are called as "Aquitard".

Due to sufficient porousness the water seeps through them and we cannot take any discharge from them.

Example: Formations with sandy clay and silty clay.

4) AQUIFUGE:-

          These are the geological formations which are neither porous nor permeable. The water does not seeps through it as well as the discharge from these is not possible.

Example: Formations with granite, sand stone, marble.

Types of Spillways - Classification of Spillways

          There are different types of spillways that can be provided depending on the compatibility of the site and other parameters. Typically a spillway consists of a control structure, a conveyance channel and a terminal structure, but the former two may be combined in the same for certain types. The more common types are briefly discussed below:

1. Straight Drop Spillway

2. Ogee Spillway

3. Siphon Spillway

4. Chute or Trough Spillway

5. Shaft Spillway

6. Side Channel Spillway

1. Straight Drop Spillway:

          In a Straight drop spillway, when the water level in the reservoir rises above the normal pool level, the over flowing water falls freely and almost vertically on the downstream side of the hydraulic structure, and hence it is referred as Straight drop spillway (or) free overfall spillway. This sort of spillway is suitable for weirs (or) low dams.

          The crest of the spillway is provided with nose so that the water jet might not strike the downstream base of the structure. To prevent the scouring of downstream bed from falling water jet, an artificial pool (which is known as water cushion) with a concrete apron (horizontal impervious apron) and low secondary dam is made on the downstream side. This cushion serves the purpose of energy dissipater.

          Proper ventilation should be provided on the bottom portion of a falling jet to prevent pulsating and fluctuating effects. 

          Sometimes, an overhanging projection is provided on the crest of the weir to prevent the entrance of small discharges onto the face of the weir wall.

2. Ogee Spillway:

Ogee-spillway-of-Shahghasem-dam

          Ogee spillway represents the shape of the downstream face of the weir. In this case, the bottom face of the weir is constructed corresponding to the shape of lower nappe of freely falling water jet, which is in ogee shape. This is a modified form of straight drop spillway. The Ogee spillway is usually provided in rigid dams and forms a part of the main dam itself if sufficient length is available. 

          The shape of the lower nappe isn't same for all the head of water above the crest of the weir. It differs with the head of water.

          Whenever there is surplus water, it will be freely disposed of through ogee spillway along its ogee shaped crest, the overflowing water just follows the curved profile of the spillway and there is no gap between the water and the spillway surface and the discharge is maximum. Ogee spillways are most commonly preferable in case of gravity dams, arch dams, buttress dams, etc., For gravity dams, it is generally located within the dam body.

3. Siphon Spillway:

          A siphon spillway is a type of spillway where the surplus water is disposed to downstream through an inverted U-shaped conduit. It is generally constructed inside the body (or) over the crest of the dam. On siphon principle this spillway works. This spillways are having high discharge capability.

          In this siphon spillways, air vents are provided at the upper passageway in the bent portion to prevent the entrance of water when the water level is below the normal pool level. Whenever the level rises above normal pool level, water enters into the conduit and by siphonic action discharged to the downstream of the channel. 

4. Chute Spillway:

          The Chute spillway is a type of spillway in which overflowing water from upstream is disposed to the downstream through a steeply sloped open channel. It is generally constructed at one end of the dam (or) separately faraway from the dam in a natural saddle in a bank of the river. The spillway can be provided close to the dam or at an appropriate saddle away from the dam where site conditions permit.

Oroville Dam Chute Spillway

          Chute spillway is preferred when the width of the river valley is very narrow. This spillway is mostly suitable for gravity dams, rockfill dams, earthen dams. These spillways are also termed as "Trough spillway" (or) "Open channel spillway".

          The slope of spillway is designed that it should maintain a supercritical flow. Energy dissipators can be provided on the bed of chute spillway to dissipate energy from the falling water.

5. Shaft Spillway:

          A Shaft spillway is a type of spillway which consists of a vertical shaft connected by a horizontal conduit. The overflow water in the reservoir enters into the vertical shaft and then flows to the horizontal conduit and eventually reaches the downstream of the channel.

          The shaft constructed is either artificial or natural. Only when the hard rock strata is present then it is suitable for natural shaft otherwise we should choose artificial shaft constructed with heavily reinforced concrete. The horizontal conduit either passes through the dam body or through the foundation of the dam.

Ladybower Reservoir East spillway shaft

          In the case of major projects, the inlet hole of the vertical shaft is specially shaped (a stepped circular rings shaped) which is called as "Glory hole of the spillway". Hence, shaft spillway is also referred as "Morning glory spillway" (or) "Bell mouth spillway". Shaft spillways are preferred when there is no space to provide for other sorts of spillways such as Ogee spillway, Straight drop spillway, etc., 

6. Side Channel Spillway:

          Side channel spillways are located just upstream and to the side of the dam. The water spilling from the crest is turned 90 degrees and flows parallel to the crest of side channel spillway. Sometimes a tunnel could also be used rather than a chute.

          When flanks of sufficient width are not available, side channel spillways are preferred over chute spillways, to avoid heavy cutting. The angle of turn of water flow after passing weir crest can also be kept between 0⁰ and 90⁰.

Types of Spillway Gates

Spillway gates are classified into many types, they are:

1. Dripping shutters or permanent flashboards

2. Stop logs and needles

3. Radial gate or tainter gates

4. Drum gates

5. Vertical lift gates or rectangle gates 

DIFFERENT TYPES OF FITS

Fits:

          The relationship between two parts, which are to be assembled, with respect to difference of their sizes, before assembly is called 'Fit'.

          It is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in  mating parts.

There are four types of fits between parts, they are

          1. Clearance Fit

          2. Interferance Fit

          3. Transition Fit

          4. Line Fit

1. Clearance Fit:

          When an internal member fits in an external member (as a shaft in hole), in clearance fit there is always an air space or clearance between the shaft and hole. Such type of fit give loose joint. A clearance fit always have a positive allowance.

Clearance fit also sub-classified as

a) Slide fit

b) Running fit

c) Slace running fits

d) Loose running fits

2. Interference Fit:

          When the internal member is larger than the external member then there is always an actual interferance between those materials. This negetive difference between diameters of shaft and hole  is called interference. An interferance fit always have a negetive allowance.

          From the below example, the smallest size of the shaft is 2.2313" and the largest hole is 2.2305" so that least interference among the materials is 0.0008". Under maximum material condition the interference is 0.0018".  

Interferance fit can be sub-classified as

a) Shrink fit (or) Heavy force fit

b) Medium force fit

c) Tight fit (or) Press fit.

3. Transition Fit:

          It may result in either Clearence fit or Interference fit condition depending on the actual value of the individual tolerances of mating parts. This type of fit is used for applications where accurate location is important but either a small amount of clearence or interference is acceptable. 

Transition fit can be sub-classified as 

a) Push fit

b) Force fit

c) Wringing fit

4. Line Fit:

          The limits of size are so specified that a clearance or surface contact may result when mating parts are assembled.

ex: Shrink fit   

** credits: some definitions taken from ANSI text book.

                    Slideshare.net

DEFINITIONS OF LIMITS, FITS & TOLERANCES

Limits: 

          A useful definition is provided by McGraw Hill: "Two extreme permissible sizes of a part between which the actual size is contained, are called Limits".

Fits:

          The relationship between two parts, which are to be assembled, with respect to difference of their sizes, before assembly is called ''Fit'.

          It is the general term used to signify the range of tightness or looseness which will result from the application of a specific combination of allowances and tolerances in  mating parts [ANSI Y14. 5M-1982].

Tolerance:

          The total permissible variation of a size is called ''Tolerance''. It is the difference between maximum and minimum limit of size.

** credits: some definitions taken from ANSI text book.