What is Compaction Grouting?
A recognized structural engineering remedial process of installing grout injection pipes below the foundation’s footings or piles to raise settled foundations and compact poor ground conditions.
Proper Application is Important
Over the years, 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 pressures, to displace, densify, and improve the adjacent soils. The key element in any grouting program is the effective control and placement of 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 “1/2 to 2 1/2″. The use of more fluid mixes, under 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 over consolidated 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 homogenous 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, flyash, 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 footing and floor slabs, treatment and densification of tunnel crown soils during soft ground tunnelling operations to reduce surface settlements and eliminate the need for conventional underpinning. Additional applications include improvement of slope stability using compaction grouting in conjunction with reinforcing techniques.
The success of compaction grouting techniques depends on the correct understanding of the cause of foundation distress or settlements and proper evaluation of the subsurface as well as competent application of the grouting procedure. In addition, a concurrent monitoring program should be carried out during grouting, to observe any potential surface heave and associated movements and to verify the effectiveness of the grouting operation by observation of slight surface heaves.
For assistance in incorporating compaction grouting techniques into design and implementation of underground construction projects, or for remedial solutions to ground stabilization problems contact Gunner.