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You're So Rigid

It never fails. The highest fill, tallest structure, or heaviest loads always end up located on the part of the site underlain by the softest/weakest soils. Ground improvement techniques such as stone columns and soil mixing can help but have their limitations. Particularly when soils are very soft and weak (think of toothpaste consistency), many ground improvement methods are either difficult to construct or simply will not work.

One exception, and a notable one, is the Rigid Inclusion (RI) method. This is a column-style method for reinforcing a soft soil profile. It's most useful in a soil profile that would otherwise be appropriate for stone columns but for the presence of one or more layers too weak to confine the stone.

Rigid Inclusion concept

The concept is simple enough. Instead of stone columns, use grout columns. For that matter, columns of any material that will hold its shape in weak soils can be used to construct RI. This means timber piles, precast concrete piles, pipe piles, and most commonly grout columns may all be used.

Unlike piles, there is no direct connection to the structure. The structure bears on shallow foundations separated from the RI elements by a "Load Transfer Platform", or LTP. This is typically a compacted granular material having a thickness sufficient to spread the structural load to the RI elements uniformly and prevent point loading of the footings.

The LTP can be continuous, such as beneath a slab or large area load, or can be comprised of discrete "bearing pads" at each footing. The LTP may or may not contain geosynthetic reinforcement to enhance performance. Usually beneath buildings designed for very limited settlement, this reinforcement is not used since there is not enough movement to mobilize its resistance. In other words, it may look good on the drawings, but it's not doing anything to improve performance.

RI FEM Model

There are many different RI arrangements, installation techniques, and design methods used in practice. Most often in design, finite element modeling (FEM) is used to estimate the settlement at the LTP level and the stress within the RI elements. Experience is essential: for the RI designer as well as the RI installer.

When grout is used to construct RI, it must have a low slump so that it will hold its shape even in very soft soils. One of the easiest ways to blow it during RI installation is to "water down" the mix to make it easier to pump. Don't do it!

Designed and installed correctly, RI is a cost-effective, viable ground improvement option for even the softest soils like fibrous peat and normally consolidated fat clay. Ask your geotechnical engineer about this technique and keep your structure on standard shallow foundations. Or, just ask Doctor Ed!

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