Water Pressure Injection
Across the United States, soil heaving due to expansive clay is a major construction issue. Moisture conditioning by water pressure injection is a process used to pre-swell expansive soils prior to construction when the use of in-situ soils and slab on grade are planned. The objective of pre-swelling is to mediate potential post construction structural damage resulting from soil heave. Water pressure injection has a proven successful track record for over 40 years and is typically the most cost effective means to deal with a soil’s potential swell.
An alternative school of thought when dealing with expansive soils is to deny expansive clays’ access to water. Most agree that it is nearly impossible to exclude moisture from expansive soil. Outside sources such as changes in drainage, under slab plumbing leaks or rising water tables are all likely sources of intruding moisture which increase the potential for post-construction movement. On the other hand, a pre-swelled subgrade can be kept moist by excluding points of desiccation and is more manageable than precluding moisture. Once a slab of concrete structure is in place, the moisture will only wick from the exposed perimeters. With the use of foundation maintenance best practices, such as excluding large trees away, maintaining the structure’s perimeter with an automatic sprinkler or installing horizontal or vertical moisture barriers, pre-swelled expansive clay can be indefinitely maintained in a moist state.
The mechanics of swelling is the result of hydrating ion’s affinity for water. Hydrating ions, which exist in between clay particles, adsorb water until charge equilibrium is achieved. In this state, little or no swell potential remains in the clay soils. The purpose of moisture conditioning is to simply supply enough water to the ions within clay particles to create equilibrium. Simply stated, the basic premise behind pre-swelling expansive soils by moisture conditioning is that once the sub-grade is saturated to a high enough level, the expansive soil no longer has the ability to apply heave pressure on a structure.
Moisture conditioning clay soils via the water pressure injection process involves specially adapted equipment which advances injection rods throughout the desired treatment area. The rods inject a solution of surfactant and water into the soil mass. Injection points are spaced on a five foot grid pattern. Injection rods typically descend at 12-18 inch intervals, waiting for refusal at each injection increment. Refusal is achieved once the injected water begins to return to the surface. Injection depths range from five feet to twenty feet deep.