Rigid Inclusions - the latest trend
What are Rigid Inclusions?
Rigid inclusions are in vogue at present in the UK Geotechnical Engineering industry, and with good reason.
Rigid Inclusions tend to be of most benefit in soils too compressible to be suitable for vibro ground improvement techniques, and where the cost for a full piled foundation is excessive with respect to the overall project value – generally speaking, in difficult ground conditions!
A range of trademarked names describe the technique of forming rigid inclusions using displacement auger techniques, but, in essence, they are all versions of the same thing.
Displacement augering is not the only solution though, most other well-known methods of pile construction can be used in rigid inclusion design – ODEX percussive, Pre-cast Driven, CFA, SFA, Rotary – all these techniques could be used.
How does the RI system work?
If the construction method is not the most important factor, what is the secret, how does rigid inclusion design work?
It cannot be emphasised enough that the key to successful rigid inclusion design is understanding the interaction between the loaded foundations, the rigid inclusions and the load transfer platform (LTP) that is present between the two. The LTP facilitates a complex load transfer mechanism that distributes load to the rigid inclusions and to the host soil at underside of LTP level; the load applied to the soil surface is transferred back into the rigid inclusion as a negative skin friction, and from there it is transferred to more competent soil at depth.
How thick should the LTP be?
The recommendations of the primary design guidance - ASIRI 2012 - is that an LTP must always be present between the loaded foundation and top of rigid inclusion. Although, it seems that current UK practice is to provide a 500mm to 600mm thick LTP below slab areas, with no LTP provided below the isolated pad and strip foundations. Additional design checks are then carried out to demonstrate the ability of the unreinforced concreted core of the RI to resist the bending moments and shear forces that might develop. The correctness of this approach is open to debate, although the ASIRI document is quite explicit in its recommendations – minimum 400mm LTP below slabs, minimum 300mm LTP below foundations.
Can I use a ground bearing slab over RI’s?
Yes, and no. The safe design of a slab over rigid inclusions requires a detailed understanding of the load transfer mechanisms that develop in the LTP. Use of a singular modulus of sub-grade reaction approach is incorrect - ASIRI 2012 devotes a lot of attention to this topic. In essence, your slab will be thinner and more lightly reinforced than a fully suspended slab, but perhaps slightly thicker and more heavily reinforced than a traditionally designed ground bearing slab.
How can RI’s benefit your project?
In common with all ground improvement techniques, the objective when specifying rigid inclusions is the minimise slab thickness and reinforcement content, and allow the use of shallow, lightly reinforced, pad and strip foundations. Helping to keep costs in the ground to a minimum, even in difficult soils conditions.
Successful rigid inclusion design requires an appreciation of all the factors which make the system distinct from other methods, and more. At Suoloconsulting we have many years of experience designing ground improvement solutions, for a range of ground conditions, and using a range of methods.
Come and speak to us at suoloconsulting@gmail.com
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