Engineering and Construction

There are many other methods available for forming deep foundation elements. Examples include earth stabilization columns, such as (NAVFAC DM-7.2, 1982):

Mixed-in-place piles. A mixed-in-place soil-cement or soil-lime pile. The pile is created by forcing a grout through a hollow shaft in the ground. As the grout is forced into the soil, an auger- like head (that is attached to the hollow shaft) mixes the soil to create the mixed-in-place pile.

Vibro-replacement stone columns. Vibro-flotation or other method is used to make a cylindrical, vertical hole that is filled with compacted gravel or crushed rock.

Grouted stone columns. This is similar to the above but includes filling voids with bentonite- cement or water-sand-bentonite cement mixtures.

Concrete vibro columns. Similar to stone columns, but concrete is used instead of gravel.

Another type of deep foundation element is the helical anchor. The principle behind a helical anchor is the same as a corkscrew, except that the helical anchor has fewer blades. The advantages of helical anchors are that they are usually lower cost then other types of deep foundations, they are quick and easy to install, and because they are rotated into place, they tend to produce much less vibrations than driven piles, which could be important if there are nearby settlement sensitive structures.

Helical anchors can be used for new construction, where the helical anchors are attached to the footings or grade beams. It is usually desirable that heavy vertical building loads coincide with the location of the helical anchors. In addition, the tops of the helical anchors should be securely attached to the grade beams or footings. Embedding the tops of the helical anchors within the grade beams and wrapping the steel in the grade beam around the top of the helical anchor can accomplish this. With settling soil, the grade beams, which support a structural slab, can be designed to span unsupported between the helical anchors.

Besides supporting bearing loads for new construction, helical anchors can also be used to resist uplift loads (i.e., tie-down anchors), for foundation underpinning, and as tieback anchors for retaining walls. Figure 5.8 shows an example of helical anchors being used for foundation underpinning.