Description
Jet-grouting is different from other grouting and deep mixing methods as it erodes and loosens the soils with high pressures and completely mixes the soil with cementitious slurry while gradually withdrawing the injection pipe (Mc Carthy, 2007). The resulting material is often referred to as soilcrete, especially when jet grouting is carried out in coarse grained soils.
Jet grouting is carried out as follows:
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An injection pipe is pushed or drilled into the ground to the desired depth.
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Grout is injected laterally at high speed from a nozzle located near the end of the pipe into the soil while the pipe is continuously rotated and gradually withdrawn, either continuously or, preferably, in small discrete steps. The procedure is carried on until the whole unstable layers are covered. Three basic systems may be adopted (Figure 1.): single (grout), double (grout and air) and triple fluid (grout, air and water).
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The procedure is repeated at several locations at a predetermined spacing, usually in a close grid pattern; secant inclusions may be used to form nominally continuous panels where required for stability (see fact sheet 5.0) or for groundwater exclusion.
The addition of air in double and triple fluid systems isolates the eroding jet (grout or water respectively) from the surrounding soil, to achieve greater depths of erosion and thus larger inclusions. Triple jet systems minimize the amount of grout used for erosion.
Jet-grouting may replace a large amount of soil mass; the columns diameter depends on the soil to be treated and on the system used (mono, double or triple fluid); it is typically 0.4 to 2 m for fine grained soils and 0.5 to 3 m for coarse grained soils (Nikbakhtan et al., 2010). Optimization of the nozzle geometry and the use of very high pressure pumps allows the formation of very large inclusions, up to 3 to 5 m wide in the most favourable conditions (Figure 2, Shibazaki, 2003, Mc Carthy, 2007).
In order to achieve the high jet speeds necessary to erode the surrounding soils, the eroding fluid is injected at very high pressure. The pressure is converted into speed at the nozzle and does not materialize in the soil-fluid mix nor in the surrounding soil, provided that a clear outlet is maintained at all times allowing excess fluid and spoil to flow to the surface under low pressure gradients. Severe heaving and/or lateral displacements may occur if this flow is interrupted. To minimize this risk, a cased hole is used in soils where the probehole is prone to instability. The casing is withdrawn simultaneously with the drill string.
Jet grouting inevitably generates large amounts of spoil; in normal conditions the volume of spoil is roughly equivalent to the volume of the inclusion formed. The spoil is a thick soil/grout slurry, not suitable for dry handling (Figure 3).
Jet-grouting is applicable for the whole range of soils and may be applied to any depth down to 50 m (Mc Carthy, 2007); it can be ended at any depth, making it possible to treat only the unstable zone (Jaritngam, 2003).
Very stiff cohesive soils of high plasticity and boulders pose special problems and may limit the applicability of the technique. Active movement may be accelerated by the jet grouting treatment works.
Design methods
Jet grouted columns act as reinforcement having much better mechanical characteristics than the surrounding soil.
Unless mass treatment is carried out, which is highly unusual, the verification of effectivness of the treatment is complex, since it refers to the behaviour of a discontinuous mass. It can only be addressed by applying significant simplifications.
Available simplified methods are based on limit equilibrium (in static and seismic conditions).
The properties of the inclusions are pre-determined from laboratory tests carried out at different confining pressures to determine the strength envelope of the treated soil in terms of both total and effective stress. Bearing in mind that due to inmperfect mixing filed strengths are typically only 35 to 50% of the strength measured in laboratory tests, the actual strength of the treated soil needs to be verified by trial fields and control tests.
Where the surrounding soil is clay, it can be modelled in terms of undrained shear strength, with appropriate reductions in case of cyclic loads (see for example Idriss and Boulanger, 2008).
Where the surrounding soil is sand, it can be modelled in terms of the least of its drained and its undrained strength; the latter may be evaluated on the basis of empirical correlations as proposed, for example, by Olson and Stark (2002), Olson and Stark (2003a), Olson and Stark (2003b) and Mesri (2007) at pre-liquefaction and post-liquefaction conditions.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Application to landslide stabilization generally limited by need to use relatively heavy equipment. Applicability to spreads and flows to be carefully evaluated on a case by case basis, bearing in mind the risk that installation iteself could trigger movement |
| Topple | 0 | |
| Slide | 6 | |
| Spread | 4 | |
| Flow | 4 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 6 | Most suited to coarse grained soils. Stiff plastic clay and boulders pose special problems |
| Debris | 8 | |
| Rock | 0 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 0 | Typically inappropriate in shallow applications. Selective treatment may be carried out, which makes it potentially suitable for deep lansdslides. |
| Shallow (0.5 to 3 m) | 0 | |
| Medium (3 to 8 m) | 6 | |
| Deep (8 to 15 m) | 8 | |
| Very deep (> 15 m) | 8 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Workers’ safety and end result require construction to take place when movement is extremely slow or very slow (maximum 1.5 m/year or 5 mm/day). Under special conditions and taking due precautions, it may be carried out when movement is ”slow” (up to 1.5 m/month, corresponding to 5 cm/day) . |
| Slow | 2 | |
| Very slow | 6 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 6 | Generally applicable in all groundwater conditions. Severe artesian groundwater conditions or strong underground flows may cause seepage induced leaching of the inclusion before the binder sets. |
| High | 8 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Water courses need to be temporarily diverted or reliably dry during construction. Potential pollution of watercourses during construction (for example by spillage of grout or spoil) may impose restriction on construction procedure. No problems once the works are completed, except possibly when treated columns provide an undesired ”hard bank” to watercourses. |
| Snowmelt | 8 | |
| Localized | 8 | |
| Stream | 2 | |
| Torrent | 2 | |
| River | 2 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 6 | Geometry and mechanical characteristics of inclusion uncertain, especially in landslides where mixed and variable soil profiles are encountered.. |
| Feasibility and Manageability | 6 | The technique is well established, but with limited previous application to the mitigation of natural landslides. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 5 | Requires specialist equipment and techniques; may need temporary roads and working platform for safe operation. Generates significant amounts of spoil |
| Environmental suitability | 2 | will be updated |
| Economic suitability (cost) | 4 | Relatively expensive. |
References
- Mc Carthy D.F. (2007). “Essentials of soil mechanics and foundations: basic geotechnics”. Upper Saddle River, N.J., Pearson Prentice Hall.
- Mesri G. (2007). “Yield strength and critical strength of liquefiable sands in sloping ground”. Géotechnique, 57, n°3.
- Jaritngam S. (2003) “Design concept of the soil improvement of road construction on soft clay” Proceedings of the Eastern Asia Society for Transportation Studies, Satho K. editor, Fukuoka.
- Nikbakhtan N., Ahangari K., Rahamani N. (2010). “Estimation of jet-grouting parameters in Shahariar dam, Iran”. Mining Science and Technology, China, 20, 472-477.
- Olson S.M., Stark T.D. (2002). “Liquefied strength ratio from liquefaction case histories”. Canadian Geotechnical Journal, 39.
- Olson S.M., Stark T.D. (2003a) “Yield strength ratio and liquefaction analysis of slopes and embankments” J. of Geotech. and Geoenvironmental Engineering, ASCE, vol. 129, n° 8.
- Olson S.M., Stark T.D. (2003b) “Use of laboratory data to confirm yield and liquefied strength ratio concepts” Canadian Geotechnical Journal, 40.
- Shibazaki M. (2003) “State of practice of Jet Grouting”. Proceeding of the 3d International Conference on Grouting and Ground Treatment, 10-12 February 2003, New Orleans, Louisiana, L.F. Johnsen, D.A. Bruce, M.J. Byle eds., ASCE, reston, Virginia, 198-217.