Compaction Grouting-Sinkhole Repair

Grout is a thick mixture of water, cement, and sand, which hardens over time.  Compaction Grouting is a method of sinkhole stabilization in which grout is pumped or injected into the soil under pressure.  The grout is pumped at a given point until either a maximum pressure is reached, a certain grout volume is injected, or the home is lifted up to the desired level.  When pumped into the earth, the grout fills holes and compacts loose soil.  Compacted grout maintains a distinct interface between the soil and the grout material.  It will also form bulbs of grout where soil is loose.  It does not penetrate between small soil particles, but rather compacts them.  Compaction grouting may be used effectively to stabilize soil in a wide range of geological conditions, including some clay soils, as well as silt, sand, gravel, and boulder.

Compaction grouting works best in soils that are granular and loose, possibly alluvial, which means they were deposited by water.  The mass of grout pumped underneath the surface takes on an irregular shape reflecting the different densities and consolidation of the soil.  Soils with clay are less permeable and result in less soil compaction with this technique.

Generally, grout is pumped under sustained pressures of approximately 500 to 1,000 pounds per square inch.  The tip of the injection pipe is raised into the section of earth requiring improvement.  It is essential that compaction start at the top of the layer of soil that is capable of supporting the load that will rest upon it once the soil has been improved.  The grout pipe is extracted during pumping.

Grout compaction is sometimes combined with slurry grouting in certain soil types, and this is called Modified Compaction Grouting.  This approach is mainly used in karst areas, where water soluble rocks such as limestone, dolomite, or gypsum predominate.  Slurry grouting, also called high mobility grouting, injects flowable, finer-grained grout that permeates the pores of the soil to cement it.  Slurry grouting binds finer soil particles together. 

In difficult soils, slurry grouting may be applied at injection points on a grid to compact and harden soft soils on top of a stable limestone layer, while filling voids and fissures that exist because portions of the limestone have dissolved.   Modified Compaction Grouting can help to seal off a layer to reduce water contamination from upper levels and inhibit the dissolving process.

Grout compaction can be used to develop point support for a structure, and it can be further reinforced with steel pipe or bar.  The grout mass distributes the load weight from the surface to the compressed soil.

Mini Piles may be used in a Compaction Grouting scenario to transfer loads down to strata that are better able to bear the load.  The steel pile may be attached to the home, while the pile rests on the more competent layer.  In another scenario, the column of compaction grout that has been injected will be reinforced with a steel bar that extends further to a more stable layer.

Compaction Grouting Methods

Delivery of grout under pressure according to specification is critical regardless of the drilling equipment and methods used.  Therefore, the grout pumps used by Earth Tech for compaction grouting are very specially designed, not like a typical concrete delivery pump.  The latter type of pump usually allows minor leakage of water through its valves, and this characteristic would cause grout to freeze up in the pump mechanism as water is forced out of the grout.  Rather, the grout pump must be designed for variable pressures up to a very high level (1,000 pounds per square inch) with a constant and generally slow rate of grout flow. 

The compaction grout pump must be leak free.  This is accomplished by using two small cylinders with a bore of 2-4 inches and a long stroke of 40-50 inches.  Usually, a diesel engine runs a hydraulic motor that powers the grout cylinders.   A high degree of pressure and flow rate flexibility makes this pump highly desirable for compaction grouting work.

The source of grout is also crucial to successful grout compaction.  Earth Tech has its own grout production plant and its own fleet of mixer trucks to assure quality and availability of material.  Other companies use an external concrete supplier.  Having a fleet of mixer trucks allows Earth Tech to create discrete batches and time deliveries proactively.

For large scale commercial projects, mobile, on-site grout mixing equipment may be used to support compaction grouting.  Bins of sand, flyash, cement, and other materials are supplied in controlled amounts to the mixing auger.  After the dry materials are blended, water is added, and further mixing occurs.  Materials can be produced on demand.  Complete control of grout mix proportions and slump is available in these circumstances.  The blended grout then flows to the compaction grout pump.

Sand characteristics are very important to effective compaction grouting.  Sand particle size distribution—gradation—influences the density and homogeneity of the grout mix.  The presence of a proportion of fine sand and the presence of rounded edge particles both help to produce a dense and homogenous mix that allow for needed pumpability.  Use of coarse sand of uniform gradation leads to “sand block,” which means the injection line is plugged. 

Flyash is commonly added to many grout mixes to achieve geotechnical goals.  Flyash is the result of burning crushed coal.  The effect of the flyash varies depending on the fineness of the coal particles.  In some cases, the coal adds cementitious (cement-like) properties; other fineness degrees do not.  Flyash helps the grout to fill voids, increases density of the mix, and reduces friction in the grout line.  Flyash gives strength and abrasion resistance to concrete.

Bentonite, a type of clay, can sometimes be helpful as an additive to increase lubrication and thereby, pumpability.

Following is a list of typical components in a compaction grout mix with their quantities and purposes:

1,800-2,200 pounds of sand
Meets ASTM C-33 standard; well graded, rounded edge, minimum 15% passing through a #200 sieve.

250-500 pounds of cement
Meets ASTM C-150 standard; this helps to control the strength of the mix and increases the density of the mix.

20-50 gallons of water
The amount of water helps to control the slump or workability/consistency of the grout.

Other ingredients
Added at a level of 1%-2% of the amount of cement for the purpose of controlling the set time or shrinkage.