Shrouding is provided in case of 

Explanation:

1) Strainer type tube well:

• It is the most commonly used type of tube well in our country.
• It is so common that whenever we talk of a tube well, it is automatically understood to refer to a strainer-type tube well.
• This tube well consists of blind pipes and strainer pipes or screen pipes.
• This tube well is generally unsuitable for very fine sandy strata, because in that case, screen openings are likely to get choked easily.

​2) Cavity type tube well:

• A cavity-type tube well consists of a pipe sunk into the ground and resting on the bottom of a strong clay layer. It does not utilize stainers and hence it draws its supply from the bottom and not from the sides.
• In the initial stages of pumping fine sand comes out with water and consequently, a cavity is formed at the bottom.
• The water from the aquifer enters the pipe through the cavity and as the pumping continues, the spherical area of the cavity increases outwards with result that the velocity flow being reduced and consequently sand stop entering the well pipe.
• Hence, in the beginning, sandy water is obtained, which clears out with time.

3) Gravel pack/ Slotted type tube well:

• A  Gravel pack type tube well (slotted type tube well) consists of a pipe that is slotted for part of its length at one end and for the rest of the length it is plain pipe
• In order to prevent the fine sand particles from entering the well pipe, it is surrounded by a mixture of gravel and coarse sand (bajri). The mixture of gravel and coarse sand placed around the well pipe is called shrouding, and it is poured from the top into the annular space between the well pipe and the casing pipe before withdrawing the casing pipe.