Soil improvement is a central issue to many of today’s engineering projects where land is scarce, good quality materials are in short supply, and developed sites are often times congested or contaminated.  New and innovative research is based on classical geotechnical engineering and facilitated by approaches with supporting interdisciplinary expertise including solid mechanics, numerical methods, geology, and chemistry.

Ground modification is the in-situ or in-place, controlled manufacturing of ground materials to form part of a geotechnical construction system.  Among the many techniques available to improve soils, grouting is perhaps the most common.  Tremendous strides have been made to advance grouting from an empirically-based “art” form to the much improved, although not yet fully understood or predictable, more scientifically oriented and better controlled technique that it is today. Several of the most common types of grouting methods are shown in Figure 1, and their associated predominant contribution for modifying soils.

                                                                     Figure 1

Permeation grouting, in particular, is the injection of pumpable material, which is either a suspension or a solution, into a soil or rock formation to change the physical characteristics of the formation.  The effect of the process is schematically represented in Figure 2.

                            Figure 2                                              Figure 3

The process of permeation grouting is shown in Figure 3. First, a borehole is drilled and used to lower the grout line into the subsurface where the grouting process will begin. The grout is pumped down hole through the line and ejected from a nozzle at the end to permeate the surrounding soil. As more grout is injected, the resistance to permeation increases because the intergranular space (space between the soil particles) is being filled with grout and the necessary pressure to inject additional grout increases accordingly. Once the design pressure limit has been reached, the rod is raised, and the injection process continues.

The primary role of grouting is to improve the strength, permeability, and stiffness of soil or rock formations.  The process is quite flexible, it can be designed to cause minimal disruption at the surface and therefore, is advantageous for use in urban areas or areas of limited access.  It is suited for a wide variety of applications, such as foundation retrofitting, dam rehabilitation, subsidence and liquefaction mitigation, contaminant containments and barriers, tunneling and mining operations, offshore construction, etc.  Applications can be categorized into the following general areas, site improvement, foundation rehabilitation, excavation support, groundwater control, and contaminant/pollution control.  Site improvement involves increasing the bearing capacity of soils and fractured rock and thereby reducing future settlement of structures due to loading, earthquakes, etc.  The process of strengthening subsurface materials will permit construction on marginal soils such as loose sands, fills, mine spoils, collapsible soils, and expansive soils.  Increasing the load-carrying capability of soil is also beneficial to improving the stability of slopes.  Rehabilitating existing foundations includes underpinning and renovation.  Excavation support concerns maintaining structural support, controlling tunnel/excavation settlements, and excavation retention in both soil and rock.  Applications for controlling groundwater flow include controlling seepage into and around excavations and tunneling projects, underground waterproofing of structures, and facilitating subsurface pipeline rehabilitation.  Contamination/pollution control involves containment of contaminated soil, water, and soil-gas and subsurface plume movement.  Containment of contaminated leachate, encapsulating nuclear waste, and initiating or enhancing in-situ biodegradation of harmful substances gives added importance to grouting as a ground modification technique.