{Re}habitat

Excavation refers to the removal of soils below the grade elevation (ground level) at a project site. An excavation is usually a temporary requirement for the placement of foundation structures, utilities, and basements below grade. Considerations for excavation include the characteristics of the soil at the work location, the required depth and amount of soils to be removed, and any site constraints including adjacent structures and property lines. Depending on the height of the water table and the permeability of the soil being excavated, dewatering may be required if a dry work environment is necessary.

Dewatering is associated with excavations where there is a need to work in dryer soils without being affected by the water table or moisture at a project site. A decision to perform dewatering at an excavation site is determined by the elevation of the bottom of an excavation, the height of the water table, the permeability and saturation of the soils, the degree of stabilization required along the edges of the excavation, and the duration and nature of the work being performed. The type of equipment and pumps selected for a dewatering effort are also determined by these factors.

Soil stabilization is the improvement of site soil characteristics to better facilitate a specific construction activity. Soil stabilization can be a mechanical technique using construction equipment, or a chemical application to change the soil’s inherent properties. Examples of mechanical soil stabilization include compaction and placement of (geo-textile) filter fabrics. Chemical examples include treating soils with cement, lime, or asphalt to better stabilize them in order to receive applied loads. Grouting is another technique often used to stabilize soils.

Underpinning is the temporary support of a structure’s foundation while permanent repairs can be made to it. Since the requirement for underpinning is normally predicated on an existing foundation’s failure or compromise, it involves installing more capable foundation elements than those which currently exist for a structure. A need for underpinning could be caused by unplanned or excessive soil settlement, particularly over a structure’s long service life. Underpinning could be required if the foundation soil’s strength characteristics were over-estimated, or the original foundation’s depth and dimensions were under-designed for the loads actually experienced. Underpinning also may be required for an existing structure’s foundation if adjacent site work will disturb it. An example would be an unforeseen excavation project adjacent to a historical building which has only a shallow foundation.

Anchor tiebacks are the lateral support structure for a bulkhead (or wall).  Bulkheads are most commonly made of steel, concrete, or wood sheet pile.  Sheet piles are long flat sections of construction material that interlock longitudinally but are set vertically to form a thin wall.  Marine bulkheads retain landside soils along their back face, and serve as a shield against erosion from water action along their front face.  Bulkheads known as "fill bulkheads" can also be completely constructed on land.  These may separate one type of soil or work site from an adjacent one, often at different elevations, or brace one side of an excavation, to provide soil slope stability.  The bottom edge of a bulkhead’s sheet piles are sunk or driven into the underlying soil.

When site constraints do not permit cheaper and more traditional excavation methods, constructing slurry walls provides an alternative method of casting concrete in place below the surface elevation. Slurry walls, used for the construction of basement walls and other below grade foundation elements, do not require formwork and excavation side bracing.  Instead, a thin trench is dug into the site soils which exactly matches the thickness and depth of the new below grade wall.

Concrete piles are a type of building foundation system often designed and specified when the sub-grade soils required to support a structure are located too deep below the project’s grade elevation for less expensive foundation designs to be considered. The final tip elevation of concrete pile foundations are generally installed many times deeper than the bottoms of traditional concrete footings. Piles of all material construction, lengths and cross-sections are considered either to be friction piles or end-bearing piles. Friction piles, as their name implies, get most of their support strength through the frictional force that develops between the surface area of each pile face and the sub-grade soils that the pile is placed in.