soil stabilization with lime, cement require water for chemical reaction.

soil stabilization with lime, cement require water for proper chemical reaction to take place.

In road construction, you will have the maximum dry density when you compact soil in optimum moisture content. In this way water increases the strength of the road

Also moisture may prevent washboarding formation on roads

The formost use of water in the soil is for the compaction. When we introduce water (upto OMC) it act as a lubricant and bring the soil particles closer to increase their density. Ultimately you would achieve better shear strength due to improvement of density also reduction in permeabilty of soil as a result in future less water will come to create problem. 

In swelling clays, water causes the closure of the pores and reduction of the permeability, i. e. it enhances the barrier effect of clay liners. This is an advantage for natural and technical clay barriers of municipal landfills helping to protect the groundwater from pollution by leachate. A very special case is the bentonite barrier around the canisters with radioactive waste in a planned deep geological repository, where swelling of smectite (a group of expanding clay minerals) in contact with water should close the free spaces between bentonite segments and make the barrier impermeable.

Water causes also the apparent cohesion of sand, so the wet sand (at limited moisture) will be stable at higher slope angle than the dry sand.

Positive aspects of water:

1) One more comment on bentonite usage as mentioned by Prof. Adamcova. Bentonite slurry (bentonite mixed with water) is commonly used in deep foundation works. It stabilizes deep and narrow excavations before filling them with concrete through tremie pipe. This technology is used during pile formation or execution of slurry walls.

2) Regarding strictly to usage of water in ground improvement techniques, there is a technique called vibroflotation. It is a kind of loose soil compaction by means of a vibrating probe immersed in loose sand with water jet (see e.g. http://www.vibroflotation.com/Vibro/vibroflotation_fr.nsf/site/Vibro-Compaction.Effects-and-Test).

3) Jet-grouting - injection of mortar, mixing it with soil to form cemented soil column. Apart of high pressure injection of mortar, two additional streams may be used to improve efficiency: air and water.

You can find many papers on jet grouting and vibroflotation.

Water is very important for mechanical compaction.The maximum unit weight occurs at the optimum moisure content.Water causes lubrication and larger density.However with increasing moisure content the density decreases onthe wet side of optimum.Water is essential in almost all methods of soil stabilization.

Water flow adjacent to sheet piles could cause heave,boiling and instability. So depending on the situation you could have a positive or a negative effect

Water is also useful in application of certain non- traditional stabilizer like enzymes for soil stabilization. These stabilizers are mixed with water prior to its application to the soil or sub-grade soil. 

Water also plays an important role in preparation of bentonite slurry in ground investigation. The slurry stabilizes the walls of the borehole in case of loose soils and thus helping in retrieval of soil samples during soil investigation.

thank you so much for useful feedbacks, Mr. Ehsan Abdi, Dr. Amit Srivastava, Mr. Shamas Ali, Mrs. Renata Adamcova, Mr. Krzysztof Sternik, Mr. Nabil Ismael, Mr. Sravan Muguda, I was also thinking about application on soil stabilizing, compaction, dry density. But actually what the question asked was what is the advantage of the water presence within the soil.

To be very simple.

To some extent water reduces the shear strength of fine soils and increases the shear strength of granular soils

Increases workability of soil cement mixture