Description
Mechanical deep mixing is the creation of vertical inclusions (columns or barrettes) by blending in-situ the soil with a stabilizing admixture to improve its mechanical characteristics (higher strength, lower compressibility). It is typically performed by specialist rotary equipment with mixing blades, which are inserted into and removed from the ground nominally without soil extraction while the admixture is injected from nozzles in or near the blades (Figure 1). The hydraulic conductivity of the treated soil will be higher or lower than that of the parent soil, depending on the soil type and admixture used. The admixture consists of stabilizing binders that react chemically with water, resulting in cation exchange on the surface of clay minerals or bonding of soil particles and/or filling of voids (Terashi, 2003). The most common binders are cement or lime; other materials like gypsum or fly ash are also used (Moseley and Kirsch, 2004).
The method is best suited to soft fine grained materials of relatively low shear strength and is applicable down to a depth of 30 m. The effect on slope stability depends on the type of soil being treated, the layout and spacing of the inclusions, the type of admixture used and the equipment and method of mixing (Mc Carthy, 2007).
Research and development of deep mixing as it is known today started in Japan and in Sweden in the late 1960’s using blades rotated by a single vertical shaft and lime as a binder, with the first applications being impleneted in the mid 1970’s. Since then there have been significant developments in all aspects of the technology.
Different equipment and procedures have been developed to respond to different soil conditions and performance requirements. Figures 2 and 3 show typical equipment developed in Scandinavia, consisting of relatively lightweight rigs and trailers loaded with dry binder. This equipment is suitable for treating extremely soft, “quick” soil, which can be mixed satisfactorily with very lightweight, fixed blades (Figure 4); dry binder is used, reacting with the soil pore water. Heavier, stiffer soils, possibly mixed with silt or even granular soils, require more robust, heavier equipment with a mix of rotating and fized or even counter-rotating blades to break up lumps (Figure 5). A thick slurry of binder is normally used in these soils (wet method). The selection of dry versus wet soil mixing is normally made on the basis of the natural water content and undrained shear strength of the natural soil, dry mixing being preferred where the natural moisture content of the soil is greater than 60% and its undrained strength less than 70-75 kPa.
A further development in single shaft technology has been the introduction of composite systems which combine deep mixing and jet grouting techniques. The jet grouting nozzles are located on the outer edge of the mixing blade (Figure 6) such that the completed column has a mechanically mixed core and a jet grouted annulus (Figure 7).
Multi-rotary equipment (Figure 8) has been developed primarily to allow simultaneous installation of 2 or more secant circular columns to form wall panels of mechanically mixed stabilized soil for the construction of temporary or permanent walls. These systems have the added benefit that mixing is much enhanced by the action of counter-rotating blades on adjacent, compenetrating, columns (Figure 9). An additional benefit specific to landslide mitigation or remediation is that panels are much better than isolated columns in resisting landslide loads, as discussed in fact sheet 5.0.
The need to form panels has driven the development of radically different approaches, deviating from the technology based on blades rotating around the vertical axis. Discrete panels or barrettes can be formed by two cutter/mixer heads counter rotating around horizontal axes (Figures 10, 11 and 12). Continuous walls can be formed, but only to a limited depth, by a continuous chain cutter/mixer (Figures 13and 14). In all cases the dimensions of the resulting inclusions are the same as those of the mixer (auger or cutter).
The method may be applicable with caution to sensitive clay since probably the installation process does not induce significant pressure in the surrounding material and the temporary change in slope stability may probably be disregarded.
Design methods
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.
The surrounding (clay) soil can be modelled in terms of undrained shear strength, with appropriate reductions in case of cyclic loads (see for example Idriss and Boulanger, 2008).
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Application to landslide stabilization generally mimited 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 |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Most suited to fine soils. Not applicable in coarse debris and rock |
| Debris | 4 | |
| Rock | 0 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 0 | Typically inappropriate in shallow applications. The entire soil thickness needs to be treated, which makes it unsuitable for selective tretament at depth. |
| Shallow (0.5 to 3 m) | 4 | |
| Medium (3 to 8 m) | 8 | |
| Deep (8 to 15 m) | 8 | |
| Very deep (> 15 m) | 6 | |
Rate of movement |
||
| 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 |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 6 | The possibility to operate with a dry binder or a slurry depending on conditions and the fact that the soil is never removed make the technique 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 |
||
| 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 slurry) may impose restriction on construction procedure. No problems once the works are completed, except possibly when the inclusions 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 | 8 | Reliable performance in well characterized landslides; in first time slides it depends on estimate of piezometric regime and apprporiate operational strength parameters of soil, which can be problematic; problems may occur during construction, for example if unforeseen boulders are encountered. |
| Feasibility and Manageability | 6 | The technique is well established and widely used for the preventive stabilization of engineering slopes; less so in the mitigation of natural landslides. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Requires specialist equipment and techniques; implementation may need temporary roads and working platform for safe operation. |
| Environmental suitability | 2 | will be updated |
| Economic suitability (cost) | 4 | Relatively expensive. |
References
- Idriss, I.M., Boulanger, R.W. (2008)., “Soil liquefaction during earthquakes”. MNO-12, Earthquake Engineering Research Institute, Oakland, CA, USA
- Mc Carthy D.F. (2007) ”Essentials of soil mechanics and foundations: basic geotechnics” Upper Saddle River, N.J., Pearson Prentice Hall.
- Moseley M.P., Kirsch K. (2004) ”Ground Improvement” New York, Spon Press.
- Terashi M. (2003). ”The state of practice in Deep Mixing methods”. 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, 25-49.