06 Aug 2018

Which foundation choice is right for your project? Check out Professor Dr Michael Benfield’s advice

Today the foundations of a building are required to provide the basis for constructing a ground floor that is, or can be, both well insulated and free of thermal bridging. The basic element for this is the means of connecting the floor and the rest of the superstructure with the ground. In many cases the ground floor platform is built as an integral part of this, although it need not be.

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Since the initial decision on how and what to build depends on site-specific ground conditions, there are a range of different intermixable foundation elements. Here, Professor Dr. Michael Benfield of Fast Frame Systems, a Benfield ATT Group company, discusses the different options.

Strip foundations

Although the days of strip foundations for everything are now long gone, this is especially true when developers have the opportunity to use (relatively) lightweight superstructures, like timber frame. In themselves, these offer the opportunity to revisit the design and engineering of foundations, potentially resulting in a variety of time and cost savings.

However, strip foundations remain relevant because they are still a basic means of creating the substructure required for highly insulated over-site slabs, beam and block, and suspended timber ground floor structures.

This applies to all sizes and types of building, from small extensions through to domestic and low-rise residential development, to sizable multi-storey, commercial and industrial buildings.

Piled foundations

With a large body of technical knowledge behind them, the choice of piled foundations for any project is complex to say the least.

As well as sheet steel, tubular and concrete piles, timber is an often overlooked alternative, even though it has perhaps one of the longest histories in providing foundation support for everything from harbours and bridges to commercial, industrial and residential buildings.

In recent years, short bored piling, requiring a concrete or similar ring beam on top to carry the building load, and mini piling has come to the fore both for new buildings and retrofit underpinning requirements.

Helical or screw piling and ground anchors have a robust history having been developed and used for lighthouse foundations.

However, in all cases, the question of insulation has to be addressed in the design and construction of the ground floor elements.

Augured foundations and Walter Segal

These are a simple alternative to piled foundations. A ground auger – usually about 600mm in diameter – is used to create a hole down to a solid ground base. This will itself probably be excavated and extracted by up to 300mm to ensure that this is the true solid base. Being particularly suitable for short or shallow depths, and ideal for low-rise structures, this is the system developed by Architect Walter Segal in the early 1970s.

Avoiding ‘wet’ trades, after the structural design has been engineered and point loads determined, holes are augured out as described previously in positions to receive posts. The holes are then filled up to ground level with non-compressible materials such as lean mix concrete or even crushed rock. These bases are then capped off with pads of concrete or possibly a reinforced concrete paving slab to serve as a base for the above-ground parts of the foundation. Structural posts are then set on top of these. Full details can be obtained from the Walter Segal Self Build Trust (WSSBT).

Simple pad foundations

Where ground conditions are solid and stable, it may be possible to simply remove topsoil and construct a series of simple pads to carry a lightweight structure, such as a single-storey timber frame building, above these.

If it is intended that the building remain in place for a long time or be permanent, then a base below the frost layer is advisable, as is avoidance of high water tables.

In the case of suspended and other floors, the floor is most likely to be a suspended, insulated timber ground floor, but consideration can be given to other forms.

There are other advantages of such a system, like the avoidance of tree roots and the ability to build quite substantial structures very close to trees.

But providing the pad base is sufficiently strong and able to spread the load over a firm ground base, then consideration for such an alternative could lead to considerable savings in time and cost.

Raft and slab foundations

Requiring minimal excavation (and resultant soil away) this highly insulated reinforced concrete raft is proofed against rising damp. With very low U-values in the order of 0.15, and the option of incorporating underfloor heating as part of the construction, this unique foundation and ground floor slab provides a base for the timber frame structure.

Insulslab foundations

These are based on a hybrid-style steel fibre-reinforced concrete (SFRC) raft set on expanded polystyrene blocks interlocked together to give detached house ground floor U-values down to 0.11 W/m2K. Compared with conventional beam and block or raft methods, tests have shown this to be over £1000 per plot more cost-effective for a typical 50m2 house footprint.

Technopor foundations

Using recycled, ground glass to provide a highly insulated base for the concrete slab/raft, developed and used extensively in Germany for Autobahn construction, this is a low-cost, rapid alternative form of foundation.

The glass foam granulate is equally suited to other forms of structure requiring highly load transference such as multi-storey residential buildings, industrial construction and biogas plants.

Suspended floors Timber ground floors

Suspended timber floor ground floors provide a good alternative to concrete slab and raft foundations and can be very economical.

Although this used to be the usual way to build ground floors until the 1970s, they are still widely used in Scotland.

Insulation is fixed either between joist or over floor deck in the conventional way and can deliver equal or better U-values than concrete or beam and block at much lower costs. Care needs to be taken to provide good ventilation beneath the floor, but this may be well worth doing.

Beam and block ground floors

While this form of ground floor construction has become fairly commonplace across the UK, to meet the latest Building Regulations, it must incorporate some form of insulation.

There are various ways of doing this, either under or over the beam and block construction, but Cellecta’s patented system provides a quick and useful way to grasp the fundementals of this.

Further information....

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