Compaction grouting is the injection of a (stiff) cement grout into the ground beneath a foundation in order to densify or compact the supporting soil beneath. Densifying soil increases its bearing capacity—its' ability to support a structure. Injection is done to a depth where stable soil is found, at times far beneath the foundation. Injecting a "bulb" of stiff grout into a soil displaces and densifies it and pushes it upward in a controlled manner. High pumping pressure of over 400 p.s.i. is used and carefully regulated to first stabilize and then, if desired, raise the structure in a controlled manner. The procedure densifies weak soil, fills voids, and drives out perched water, stabilizing the foundation.
Steel injection pipes are inserted to the bottom of the zone to be stabilized, or to a depth of refusal below the foundation to be stabilized or raisedThe pipes are then retracted a few inches to open the end of the injection pipe and allow a low volume, high pressure pump to inject the stiff grout. As the grout expands in a homogenous bulb, radial forces exerted displace and thus compact the surrounding soils. Slow injection prevents soil fracturing and allows water to dissipate as the structure is gradually lifted uniformly.
Foundations are designed to be uniformly supported by soil. Compaction grouting maintains this contact between foundation and soil as intended and without changing the design function of the structure. Alternative methods such as jacking a structure with pilings or piers causes point loading of the foundation, a situation for which it is not designed. Piers and pilings, as they raise a structure, cause foundation cracking. Also as a foundation is raised using point loading methods, voids are created beneath the foundation that later must be filled.
In most grouting applications, all of the repairs are performed from the exterior and completed grouting operations are undetectable within hours and a typical project's duration is one to two days. Finally, compaction grouting does not require excavation, whereas piles and piers require excavation and/or the removal of portions of the footing, driveways, sidewalks, patios, decks, or landscaping—increasing costs, disruption and project duration for the homeowner.