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Add 4 Parts Soil, 1 Part Binder, Stir Vigorously.

Let's face it, there are some really sloppy sites out there upon which we are trying to build. Really sloppy! Usually when faced with such a challenge, engineers default to either (1) removing the "bad" soils and replacing them with clean fill material, or (2) bypassing the bad soils using a deep foundation system. Both options, while technically feasible, often come with a very large price tag.

There is a ground modification technique known globally as Mass Stabilization, and in the US simply as Mass Soil Mixing, that offers an alternative to these other options. This approach entails leaving the in-situ soils in the ground, and then mixing in a specially formulated binding agent using a purpose-build tool.

Mass mixing tool

This tool mounts onto an excavator and is powered by the rig's hydraulics, so it's a compact and quiet operation. The site requiring modification is subdivided into smaller areas of known volume ("mix cells"), and then a predetermined dosage of binding agent (often Portland cement with admixtures) is mechanically blended until a specified mixing energy and material uniformity have been attained.

The binder dosage and mixing energy are determined in advance of construction using a comprehensive laboratory bench scale study. The goal is to achieve the desired strength and stiffness of the mixed mass usually within a period of about 28 days (cement industry standard).

Bench scale test results

Design often includes comprehensive Finite Element Modelling (FEM), and numerical analysis of local and global stability. This gives the design engineer the required strength and stiffness properties that the production mix must achieve.

Global stability analysis results

Once all of this is complete, construction commences. Quality control consists of monitoring mixing energy and binder dosage for each mix cell on the project, as well as the depth of treatment. The in-situ shear strength of the material is measured with a specially calibrated penetrometer, and cylinders are cast for laboratory testing.

This technique requires that the full depth of the problem soils is mixed and cemented. For this tool, there is a practical depth limitation of about 21 feet. This can be adjusted slightly, but efficiency is sacrificed at greater depths.

So next time you are faced with a sloppy site, either due to the physical soil properties or because of contamination, consider mass soil mixing. It eliminates the need for costly soil exchange, can replace or be used in conjunction with piles and other deep foundation systems, and can support tremendous loads with minimal compression and no long-term settlement.

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