Foundation Repair

Over the years, literally hundreds of structural settlement problems have been solved using compaction grouting techniques to stabilize faulty support soils. Frequently, subsurface densification and improvement of inadequate support soils can be accomplished economically, via an important geotechnical tool termed compaction grouting.

Compaction grouting involves the injection of a stiff granular mortar grout into subsurface soils under high pressure, to displace, densify, and improve the adjacent soils. The key element in any grouting program is the effective control and placement of the grout. In compaction grouting, this control is accomplished by the use of a very stiff, granular grout mix with concrete cone slumps in the range of ½” to 2 ½”. The use of more fluid mixes, under the pressures required for effective compaction usually results in loss of control of the grout location, the splitting or fracturing of the ground, and migration of the grout away from the intended treatment zone. Injection of grout mortars with slumps of 3” to 5”, although normally considered a stiff mix when pumping concrete, has been shown to easily fracture, even overconsolidated soils, and to run several feet away from the grout pipe tip, even under pressures as low as 100 or 200 psi. In contrast, pressures in excess of 500 psi have been used with the very stiff compaction grouts resulting in a  homogeneous grout bulb that grows somewhat symmetrically about the grout pipe tip and remains generally within the intended treatment area.

The selection of the mortar aggregate and water-retaining agents such as cement, fly ash, clay, etc. is extremely important to achieve a pumpable mix that still exhibits high internal shearing resistance to flow and can avoid sand blockages in the lines under high pumping pressures. Special pumping equipment is obviously required to mix and inject such materials under high pressure.

Widely variable conditions that can be treated using compaction grouting techniques include the stabilization of poor backfill material behind retaining walls and under footings and floor slabs, treatment and displacement of peak areas underlying structures, the elimination of convoy settlements in soft natural materials loaded by subsequent man-made loads, and most recently the replacement and densification of tunnel crown soils during soft ground tunneling operations to reduce surface settlements and eliminate the need for conventional underpinning.