From residential homes to commercial buildings to large industrial structures, the stability, strength and lifetime of such projects depend absolutely on a firm foundation, which in turn depends on favorable properties of the soil below.
Over the last couple of centuries, geotechnical engineers have developed an array of investigative techniques to determine ground characteristics relative to their ability to support large, heavy structures. In parallel, they also created several ground modification or stabilization procedures to improve the load-bearing capacity of poorer granular soils without the expense and disruption required for deep foundations installation.
One such technique is the insertion of low mobility aggregate grout beneath the surface using a technique known as compaction grouting.
What Is Compaction Grouting?
In short, compaction grouting is a specialized technique for increasing the density of otherwise weak granular soil structure without the use of deep foundations. It does so by displacing, by stages, the existing soil at optimum depths with a high-density aggregate cement mixture. This mixture is directly injected via a drilled or driven injection casing.
Conditions Corrected by Compaction Grouting
Compaction grouting is suitable to stabilize weak or loose granular soils or reinforce fine-grained soil especially when the subsurface presents sinking or settling formations. It can both repair and prevent sinkholes, especially where Karst topography is present, such as is commonly found in Florida. It is often used to compensate for inadequately compacted fills as well.
There are several uses for compaction grouting beyond increasing load-bearing capacity of granular soils for new construction:
- Improving soil bearing capacity when additional load is added to existing structures
- Densifying ground beneath settled structures in preparation for lifting and re-leveling them
- Stabilization of ground where pipes or conduits are or will be installed
- General remediation or preparation of sites with sinkhole activity
In terms of installation and performance, compaction grouting presents many benefits for a variety of construction and remediation projects:
- Compaction grouting can be installed in a short time
- It is suitable where access space is restricted
- No spoils are generated
- Produces minimal site vibration
- Provides precise pressure and placement of aggregate grout structures
- No connections to existing foundation structure are required
- Almost immediately increases the bearing capacity of the ground
- Halts or significantly reduces existing foundation settling
- Minimizes soil liquefaction potential due to earthquakes
- Remedies formation of sinkholes
Overall, compaction grouting is an exceptionally economical alternative to using piles or soil removal and replacement procedures.
Why Compaction Grouting Is Effective
The application of compaction grouting is particularly effective for the stabilization of loose, low-density soil structures that tests have determined to possess an inadequate load-bearing capacity. It does so by displacing these soil structures laterally with low mobility, low slump aggregate cement blend at exact depths. The mixture is usually placed in stages from the bottom to the top of the drilled or driven casing.
Each aggregate grout stage forms a bulb-shaped structure that forces out both air and water in the earth in all directions. Because pressure and grout injection rates are precisely managed, the risk of pore water pressure increasing, or any post-application settlement is eliminated.
Compaction Grout Installation
Equipment used to accomplish compaction grouting varies widely depending on the application. Towable machine pumps are typically used for most applications.
For most projects, the placement of grouting follows a grid pattern. Various factors determine the exact grid spacing and placement procedures such as required bearing capacity, grouting depth, the soil’s blow count and whether any re-leveling of an existing structure is needed.
Most compaction grout installations are “bottom-up.” This technique is particularly applicable to projects that involve little or no re-leveling and when soft granular soils are greater than 15 feet in depth.
In bottom-up installation, the equipment’s injection pipe is inserted into the ground to the maximum depth specified to increase ground strength. High modulus, low mobility aggregate grout is then injected until a pre-determined pressure and/or grout volume is reached.
Each grout bulb or grout cavity is ordinarily one to three feet in height and pushes out the surrounding earth in three dimensions. After placing one grout bulb, the pipe is withdrawn a set amount and the process is repeated.
In this manner, a vertical column of overlapping bulb structures is created to a pre-determined height below the surface. The result is increased density, stiffness and friction angle compared to the surrounding granular soil.
Limitations of the Compaction Grout Technique
There are situations for which compaction grouting is not suitable:
- It should not be used where the ground above the grouting area has limited overburden as it may heave the ground around the bulb column. Typically, an overburden of at least five feet is needed.
- Although it can be used to reinforce fine-grained soils, it is not applicable to soils with low permeability such as clays.
Economical, Low-Impact Soil Strengthening for Over Six Decades
Compaction grouting has been used since the 1950s. Initially, it was used as a remedial method of arresting foundation settlement of existing residential, industrial or commercial buildings.
Today, this specialized procedure is a popular and economical approach for modification of the load-bearing capacity and strength of a variety of granular soils, improperly compacted rubble fills, ground subject to liquefaction or to mitigate Karst formations found in locations such as Florida.
Its many advantages, including short installation time, immediate structure support and the lack of spoils, make it a convenient, versatile and cost-effective approach to add ground strength for both new construction and remedial support projects.