Foundations & Floors

Image - www.selfbuild.ie

What are Foundations?

According to Selfbuild Ireland, 2021 foundations are the part of the building directly in contact with the ground, safely distributing the weight of the building. The building's concrete structure can sometimes be referred to as the artificial foundation, with the ground it sits on as the natural foundation. It spreads the weight of the load from the building to the ground. It should be strong enough to withstand dead loads and pressure from the building itself, imposed loads from the internal contents of the building and wind etc., set evenly and uniformly to prevent settlement and ensure stability. Specifications regarding foundations may be consulted in the following Technical Guidance Document: TGD, Part A p.26. 2012. The concrete base of the building is described as the foundation with the part of the building below ground and the horizontal damp proof course (DPC) described as the foundation or rising wall. Foundation design should be such that settlement is minimised and uniform across the building as the soil underneath compresses.

Bearing Capacity is the ground's resistance to force or the capacity of the soil to support loads applied to it. It is expressed in Newtons or N/mm2 and can range from 1000N for solid rock to 75N for cohesive soils. Peat soils are unsuitable for building on as they shrink when pressure is applied. Geological surveys help with determining ground formation to determine its nature, bearing capacity, groundwater levels etc. for suitability for building on and therefore the type and size of foundation which will be required.

Foundation Specifications Minimum depth for foundations is 900mm below ground level to avoid movement due to frost/heave expansion and groundwater level changes. Separate specifications are laid out in TGDs for sloping sites. The minimum thickness (T) for foundations should be 200mm or equal to the projection (P) beyond the face of the wall, whichever is the greatest. The minimum projection should be 75mm beyond either face of the wall which should help prevent any reduction on the bearing of the base of the concrete were it to fail due to shear pressure.

Rising walls are usually built with solid blocks up to DPC level (minimum 150mm above ground), at least the same thickness of the wall they will support.

Figure 3 - Rising Walls - Image - iStock

Steps for setting out a Foundation

1. Timber profiles are required to support the lines marking where the foundation will be. These profiles are set up at least 1m beyond the edge of the trench if digging by hand, further if using a digger.

2. The outline of the trench is marked out square. (Figure 4)

3. The trench is excavated ensuring the sides and base are plumb, level, and square. (Figure 5)

4. The trench is then pegged out using steel bars or timber stakes, driven into the bottom of the trench to the required depth {standard depth is 300mm), evenly spaced and level on top. These serve as a guide for the depth of concrete to be poured and leveled to. A timber screeder board is used to tamp or level the poured concrete to the required height spanning from peg to peg. (Figure 6)

Once the foundation concrete is level the rising walls can be built using solid blocks up to the DPC level.

Figure 4 Figure 5 Figure 6

Figure 4 - Setting the outline - Image www.ndzl.com

Figure 5 - Excavating - Image www.selfbuild.co.uk

Figure 6 - Cement poured & levelled - Image Getty

Types of Foundation

There are four types of foundation the choice of which is usually based on suitability and cost.

1. Strip (or shallow strip) foundation (Figure 7) is used for most domestic dwellings . Minimum thickness is 200mm, standard thickness 300mm. If soils are weak deep strip or

reinforced wide strip foundations may be used instead to create a larger surface area to take the load of the building.

Figure 7 - Image - www.geotech.hr

2. Pad (Figure 8) foundations are generally used in industrial units or portal frames incorporating vertical columns, beams, and roof trusses. The columns are supported on pad foundations which is a more economical method than excavating trenches.

Figure 8 - Image - Quora

3. Raft (Figure 9) foundations are used on single story or low-rise buildings where the soil may be weak. A raft foundation is one continuous slab of concrete laid underneath the building. This type of foundation is also more economical than excavating trenches.

Figure 9 - Image - www.homebuilding.co.uk

4. Pile foundations are usually used on multi-story buildings which are going to be very heavy, particularly if the soil is weak. A series of columns are constructed or inserted into the ground to transmit the loads to a lower level of subsoil. The columns are used with caps or ground beams which support the walls of the structure above ground transferring the load onto the piles.

(a) According to The Constructor, 2021 End bearing (Figure 10) piles are driven through the weak soil and rest on the strong soil layer, behaving like a column. This transfers the buildings load to the strong stratum layer.

Figure 10 - Image www.theconstructor.org

(b) Friction (Figure 11) or floating piles have rough and jagged edges. The friction created between the pile and the ground helps support the building.

Figure 11 - Image Pinterest

(c) Replacement (Figure 12) piles involve digging out the soil and replacing it with reinforcing steel and filling with concrete.

Figure 12 - Image - www.basiccivilengineering.com

(d) Displacement (Figure 13) piles are load-bearing columns installed without first removing the soil. Hydraulic equipment is used to drive the columns into the ground compacting the soil into the surrounding ground.

Figure 13 - Image - www.basiccivilengineering.com

Floors

Image - Wix

Floors provide a building with strength and stability providing resistance to weather and ground moisture, together with resistance to the passage of fire, heat, and sound. Details of which are foung in Technical Guidance Document C. TGD, Part C, Section 3, 1997. The choice of material is dependent on the width of the floor and performance required but generally concrete or timber are used. Suspended timber floors are generally found in older houses with concrete in newer ones. The strength of the floor is dictated by the choice of the materials as the floor needs to safely support its own dead load plus all the finishes, services and fixtures, while also including the weight of the contents and occupants of the building.

Ground Floor Construction Types

1. Ground-bearing concrete slabs (Figure 14) are most common. The rising walls are built up to DPC level before clean, graded impurity-free hardcore is levelled and compacted to a depth of 150mm. Damp proof membrane is placed over the hardcore and up over the rising walls and a layer of rigid insulation added, the concrete is laid on top to a minimum depth of 150mm. The surface is screeded off and polished using a float.

Figure 14 - Image - Structuralengineercalcs.com

2. Screed finished floors (Figure 15) are used in conjunction with raft foundations for ground floors. 50-75mm of a sand/cement screed is laid on top of rigid insulation and damp-proof membrane.

Figure 15 - Image Kilsaran

3. Suspended concrete slab floors are used in sloped areas, on ground that has poor bearing capacity or prone to changes due to seasonal temperature or moisture fluctuations. This method uses either precast reinforced slabs, block and beam (where ventilation underneath the blocks and beams is necessary to allow moisture to evaporate) (Figure 16) or in-situ reinforced slabs.

Figure 16 - Block & Beam system - Image - thermohouse.co.uk

4. Suspended timber floors (Figure 17) are more common in older houses. They are constructed from timber floorboards which are attached to joists spaced 400-600mm apart running from an external wall to an internal load-bearing wall just above the foundations (compacted hardcore and concrete slab) of a house. These joists are supported by dwarf (sleeper) walls and and wall plates, This creates a small gap and allows ventilation and air movement to prevent dry rot developing in the timber. Insulation can be placed between the joists.

Figure 17 - Image - btlpropertyltd.co.uk

References

1. Selfbuild Ireland. 2021 Available at www.selfbuild.ie/advice/basics/site-foundations-explained/ [Accessed 26.03.2021]

2. Government of Ireland. 2021 Available at https://www.gov.ie/en/publication/3d58c-technical-guidance-document-a-structure/ [Accessed 27.03.2021]

3. The Constructor. 2009-2021 Available at https://theconstructor.org/geotechnical/end-bearing-pile-foundation/37517/

[Accessed 28.03.21)

4. Government of Ireland. 2021 Available at https://www.gov.ie/en/publication/1aa81-technical-guidance-document-c-site-preparation-and-resistance-to-moisture/ [Accessed 29.03.21]

5. BTL Property Services Ltd. 2021. Available at https://www.btlpropertyltd.co.uk/blog/build/concrete-vs-timber-floors/ [Accessed 29.03.21]