DEWATERING & GROUNDWATER CONTROL
Cut-off & Exclusion
Temporary or permanent groundwater cut-off or exclusion can be accomplished by a variety of methods. These fall into the following two general categories:
Vertical physical barriers (cut-off structures), for example slurry trenches, slurry walls or sheet piling, that are typically excavated or keyed into a soil or rock stratum to provide a complete barrier to vertical and horizontal groundwater flow.
Geotechnical methods such as grouting or ground freezing that modify the characteristics of the ground in order to reduce the hydraulic conductivity and/or improve the strength of the soil. Unlike physical barriers that require the installation of vertical elements from ground surface to an impermeable stratum, geotechnical methods can be used to target specific vertical zones within the soil profile, to create horizontally oriented cut-offs such as bottom seals, and in situations where access to the target soils is obstructed by surface or underground structures and angled drilling is necessary.
Moretrench specializes in the use of the following techniques for a variety of cut-off and exclusion applications:
Slurry trenching techniques are typically used in conjunction with construction dewatering, for environmental containment or cut-off, and to control seepage under dams and levees.
Trenches are generally two to five feet wide and typically keyed into an underlying aquitard or acquiclude to form a complete barrier to horizontal or vertical groundwater flow. Excavation is by backhoe or clamshell, depending on design depth, and is accomplished under thixotropic slurry to maintain trench sidewall stability. Engineered backfill of low hydraulic conductivity, most commonly a soil-bentonite (S-B) mixture, is placed in the trench as excavation progresses to form the continuous cut-off. Where backfill strength is also required, a soil-cement-bentonite (S-C-B) mixture may be used. Self-hardening slurries, such as those composed of a cement-bentonite (C-B) mixture, can also be used.
Several grouting methods are in use for groundwater cut-off and exclusion. Selection of the appropriate method is a function of the subsurface conditions and the requirements of the project.
Jet Grouting can be used to infill around subsurface obstructions to ensure continuity of conventional barriers walls; provide the main groundwater barrier where more conventional methods cannot be used; seal gaps in steel sheetpiling; create a horizontal groundwater barrier (bottom seal); and provide an economical and effective groundwater barrier when installed as a thin panel.
Permeation Grouting may be used for site perimeter cut-off for construction purposes; exclusion of contaminants on environmental projects; in situ containment of contamination; and closure of windows in bathtub excavations.
Structural Seepage Grouting, as the name implies, is the introduction of a penetrable grout into the cracks and gaps of subterranean structures to seal off groundwater flowpaths. Water-reactive polyurethane grouts that expand rapidly on contact with water are most commonly used.
Ground freezing is an effective method of groundwater cut-off and earth support for difficult, disturbed, or deep ground conditions.
The freezing process converts in situ pore water to ice through the circulation of a chilled liquid through a system of small-diameter pipes placed in drilled holes. The ice acts to fuse the soil or rock particles together, creating a frozen mass of improved compressive strength and impermeability. Brine is the typical cooling agent, although liquid nitrogen can be used in emergency situations or where the freeze is only required to be maintained for a few days.
Peripheral ground freezing is primarily used for access shaft and tunnel excavations, and for creating frozen earth cofferdams for excavations.
Mass freezing has been used to where ground control in difficult subsurface conditions was crucial to the success of the project.