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Jet grouting is used to improve soil
Jet grouting is a construction technique used for improving ground conditions or modifying them so that the soil can be stabilized and this stabilized condition used to increase the load bearing capacity of soils. In the long run this can save a lot of money for foundations, which quite often can constitute about 30 to 40 percent of the cost of a building structure. It can also be used to change the condition of the soil and its permeability, so that other work like tunneling can be undertaken under conditions that can be controlled. In this technique a jet of fluid with high kinetic energy is used to break up the soil and mix it with thin slurry that has cement or other agents mixed in it. In reality it is a process that produces a soil cement material with a hydro-dynamic mix in place method. Other grouting techniques in use for soil remediation are permeation, compaction grouting, and by fracturing the soil before grouting the fractured soil.
Image source: Wikimedia: Jet grouting equipment
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The Process of Jet Grouting
Soil investigation is a necessity before deciding on any jet grouting. Cores of the soil are taken to various depths that may be governed by the foundation depths or other considerations. Laboratory tests are carried out on the cores to determine the shear strength of the soil, which is important for deciding the bearing capacity of the soil. In case this is inadequate, one of the methods suggested to improve the soil could be jet grouting.
There are three physical processes that take place during the process of jet grouting. A very high speed jet with the fluid first loosens the soil. This jetting fluid brings some of the soil to the surface. The slurry that has the binder gives the soil strengthened properties.
The sequence of work involved in jet grouting starts with an initial hole of up to 200 mm being drilled up to the depth required. This will be decided according to the final loads that are to be borne by the soil and the depth of the foundation from where the load dispersal to the soil will start, and the further depth required to ensure that loads on the soil are within the limits suggested by the laboratory tests.
A drill of 100mm diameter with nozzles of about 10mm diameter is then lowered into this hole and a high pressure fluid with cement or other additives is pumped through them. The nozzles are positioned so that they create the high pressure in the horizontal direction thus increasing the diameter of the drilled hole and affecting a much larger area. This fluid gets mixed with the soil and some of the soil is displaced and comes up. This excess is removed as it emerges. The drill is raised and rotated constantly to form a column of the soil and cement mix. The fluid used is normally neat cement grout.
Some jet grouting systems use a combination of air and water to break up the soil while a separate nozzle underneath the air and water nozzle lets in the binder at a slightly lower pressure. Bentonite is added to the cement slurry when it is necessary to also increase the impermeability of the soil. By using a jet grouting overlap thickness which has been predetermined, a continuous wall can be created which can help to seal areas so that no water enters the treated area. Such overlapping would also be an effective method in guaranteeing that the soil bearing capacity is increased over the entire area of a proposed structure.
Image Source: Wikimedia