CONTIGUOUS AND SECANT PILE WALLS
A contiguous pile wall is simply a row of concrete soldier piles installed so that each pile is in contact, or near contact, with piles on either side of it. There is, however, a relatively large increase in cost for this system in comparison with concrete soldier piles. The technique is therefore generally only used in unstable soil profiles (soft saturated clays or sands) which do not have an ability to arch between adjacent piles.
This wall is not watertight, unless specific steps are taken to achieve this, so leaching and / or piping of non-cohesive soils through gaps between the piles can be a problem below the water-table.
A special case of a contiguous pile wall is called a secant wall. Here the piling is carried out in a sequence in which subsequent piles are cut into the previously installed piles, thereby affecting a seal between the units.