What are Rigid Inclusions

What are Rigid Inclusions?

Rigid Inclusions have become increasingly more common in UK ground engineering practice, the technique allows the use of an alternative ground improvement approach on sites and in ground conditions which are not best suited to more flexible techniques such as vibro stone columns.

Most often at present rigid inclusions are formed using displacement augered methods, as this system is very well suited to the nature of ground conditions where rigid inclusions offer their greatest benefit. Although, this need not always be the case, many other traditional piling methods can be used as part of a rigid inclusion scheme, for instance –

  • Soft soils overlain by very stiff soils, where premature refusal of a displacement auger might occur. In this case pre-boring could be carried out prior to installing displacement augered rigid inclusions, or alternatively, replacement augered piles (CFA) could be used to form the rigid inclusions.
  • Commercial floor slab replacement in restricted access, low headroom conditions. For a project of this nature, steel cased bottom driven piles could be used as rigid inclusions, avoiding the need for an expensive suspended piled floor slab.
  • Remote sites without a dependable supply of concrete. For sites outside of the serviceable radius of the nearest concrete plant, pre-formed driven displacement piles could be used as rigid inclusions, removing concerns over material conformity.

Many more examples exist, and the technique is readily adaptable to a wide range of foundation types and structures.

In this blog we will explore some of the benefits of the Rigid Inclusion technique, along with some of the key technical considerations.

Benefits of Rigid Inclusions

Key benefits of the technique can be listed as –

  • Use of Shallow Lightly Reinforced Foundations: When installed to an appropriate area replacement ratio, rigid inclusions below pad and strip foundations can successfully limit settlements to < 25mm under unfactored bearing pressures of up to circa. 250kPa. When detailing foundations over rigid inclusions, consideration must be given to load transfer platform thickness, inclusion spacing, and the maximum permissible load on individual inclusions. These points will be discussed in greater depth below.
  • Reduced Slab Thicknesses: For commercial developments, where the cost of the floor slab represents a relatively large proportion of the overall build cost, the use of rigid inclusions will help to minimise slab thickness and reinforcement content. The preeminent design guide for Rigid Inclusions – ASIRI (2012) – gives a lot of attention to the design of ground bearing slabs, and we will discuss some key points below.
  • Improved Bearing Capacity, and Load/Settlement Performance: When compared with vibro ground improvement alternatives, rigid inclusions can offer bearing capacities up to 100% higher, for a similar settlement performance. Or alternatively, rigid inclusions can offer improved settlement performance compared with vibro stone columns, at equivalent bearing pressures.
  • Cost Efficiencies: Although the overall package costs for Rigid Inclusions, taking into consideration the cost of constructing the load transfer platform, will be higher than the costs associated with vibro ground improvement, use of the technique will result in significant savings when compared to a fully suspended piled slab and substructure.

Key Considerations

There are several aspects of rigid inclusion design and implementation that need to be understood when assessing the suitability of the system, some of these considerations will be discussed in more detail below.

  1. Load Transfer Platform

The Load Transfer Platform (LTP) is arguably the most critical aspect of the successful design and implementation of a rigid inclusion scheme. It must be dimensioned appropriately to the size and spacing of the rigid inclusions, the stiffness of the shallow soils, and the magnitude of the applied loads.

ASIRI 2012 recommends –

 

The load transfer platform must exhibit high shear strength and stiffness to ensure that internal arching effects develop, and may, in the case of an LTP formed after completion of column construction, be reinforced with a geotextile layer.

In UK practice it is commonplace to form the LTP at, or near to, underside of slab level and to use it to support the piling rig during the rigid inclusion construction phase. This requires that the rigid inclusion head levels are reduced by some means after construction is complete. In the case of augered rigid inclusions this is generally carried out while the concrete is still wet, through localised excavation. The LTP is then recompacted over the rigid inclusion heads to bring it back up to the correct level. This process is clearly sub-optimal if it is not strictly controlled, as it leads to an increased potential for localised areas of LTP with reduced strength and stiffness, directly over the RI locations. The optimal solution from a design perspective is to install the rigid inclusions from a working platform of suitable thickness, at some proportion of the required final LTP thickness, to reduce the levels as described above whilst the concrete is wet, and to then build the LTP up to its full design thickness once all RI’s are installed.

When preparing a load transfer platform, or finalising proposals for RI head reduction, the elevation tolerances present in ASIRI (2012) should be recognised –

In short, RI’s must have a depth of penetration into the LTP and should be within 50mm of design level at completion.

The important thing is that this is discussed and agreed before any work begins!

  1. Maximum and Minimum RI spacings.

ASIRI (2012) is explicit in its recommendations for maximum and minimum rigid inclusion spacings, as a function of RI size, and the proposed testing regime.

Minimum spacings –

Maximum spacings –

In UK practice, where rigid inclusions are most often installed at diameters < 500mm using displacement augering techniques, a minimum centre to centre spacing of 4 x diameter would be expected, along with a maximum spacing of 3.0m or 9m2 of foundation area per RI.

  1. Maximum Load on a Single RI.

For small pad foundations supported on a single column, ASIRI (2012) stipulates that –

This, coupled with the minimum spacing criteria noted above, sets some limitations on the maximum bearing capacity that can be applied to small, isolated pad foundations.

Whilst not all the load transferred at top of LTP level will be transferred to the RI, it is clear that at the higher bearing pressure ranges, 250kPa – 300kPa, this upper limit of 150kN unfactored load on a single inclusion could be exceeded.

ASIRI (2012) also places limitations on the minimum required distance from edge of RI to edge of footing as –

Taking all these points into consideration, and as an example, for a typical RI diameter of 320mm formed using displacement augering methods, a single RI could be installed below a pile cap with dimensions of approx. 0.6m x 0.6m. To increase this to an appropriately sized cap over two RI’s, without contravening the minimum spacing constraint of 4 x diameter, would require a pile cap as large as 1.5m x 1.5m.

  1. Foundation and LTP Overhang.

The reaction of the rigid inclusion at underside of LTP level is shown diagrammatically below, with conditions required for development of the full shear failure mechanism.

In most practical cases of isolated foundations, and at the edge of a slab, insufficient overhand may exist for this full mechanism to develop. The ASIRI (2012) document considers the range of conditions encountered in practice, with a procedure detailed for the reduction in the design value of Nq (bearing capacity factor) from a maximum at Lmax, to a minimum in the zero-overhang case, again this is shown diagrammatically below –

This does not limit the use of RI’s but provides an additional factor for consideration and inclusion in the design validation report.

  1. Reinforcement Content

Whilst most Rigid Inclusion projects comprise unreinforced displacement augered concrete inclusions, on occasion it might be necessary to reinforce the rigid inclusions against any shear forces or bending moments which might develop. This is a potential concern for RI’s at the perimeter of the loaded area, below an embankment slope, or for RI’s supporting foundations subject to inclined loading.

Want to know more? Contact us today to find out how we can help you with your rigid inclusion project.