This method can be used where weak sub-soils are encountered so that the basement wall act as permanent limbering whilst the mound or dumpling is excavated and the base slab cast. Perimeter trench excavations can also be employed in firm sub-soils and it may be possible to excavate the perimeter trench completely after placing timbering in position. The sides of excavation to the perimeter of the basement can be supported from the formation level using raking struts or by using raking struts pitched from the edge of the basement slab. They are suitable for most sub-soils and their installation generates only a small amount of vibration and noise making them suitable for works close to existing buildings. The high cost of these walls makes them uneconomic unless they can be incorporated into the finished structure.
Diaphragm walls are suitable for basements, underground car parks and similar structures.
The choice of actual pieces of plant to be used in any construction activity is a complex matter taking into account many factors. At this stage it is only necessary to consider basic types for particular operations. In the context of basement excavation two forms of excavator could be considered. Backactors - these machines are available as cable rigged or hydraulic excavators suitable for trench and bulk excavating. Cable rigged backactors are usually available with larger bucket sizes and deeper digging capacities than the hydraulic machines but these have a more positive control and digging operation and are also easier to operate.