Foundations and Floors

This blog examines the main types of foundations and floors in buildings.

What is a Foundation?

The foundation is the part of the building in direct contact with the earth. It provides support to the building by transmitting the load of the building to the earth in order to increase the stability of the building. If a foundation is not built correctly, it places the entire structure in danger. The Leaning Tower of Pisa (Figure 1) is a famous example of what happens to a building with poor foundations.

Figure 1: Leaning Tower of Pisa

Image from Structuredfoundation.com

Bearing Capacity and Choice of Foundation

The type of soil directly under the footprint of the building has a large impact in the choice of foundation. This is because different soils have a different bearing capacity. The bearing capacity of soil is the soil’s resistance to a force. It is expressed in Newtons or N/mm2 and ranges from 10,000 for solid rock down to 75 N for cohesive soils.

Prior to building a foundation, the soil should be excavated to the level at which the concrete will settle evenly and uniformly under the whole building.

Soil can be divided into three main horizons: topsoil, subsoil and a layer derived from the parent rock (Figure 2).

• Top soil is the top 100 – 300mm of soil. It contains most of the soil's organisms and has a poor bearing capacity. It should be removed from the area and reused for landscaping.

• Subsoil is the layer of soil below the topsoil and is usually lighter in colour. The majority of subsoils are mixes of rock, gravel, sand and clay. The nature and behaviour of these types of subsoil under the load on foundations are similar. Sub soils with a high amount of peat have a lower bearing capacity. An investigation of the nature of subsoil should therefore be undertaken, to determine:

  • its bearing capacity

  • its likely behaviour under seasonal and ground water level changes

  • the possibility of ground movement

Figure 2: Ground Cross Section

Image from Quora.com

Methods used to investigate the site include:

• Check local authority records

• Survey any adjacent buildings by excavating beside the building and check their foundations

• Excavation on site via trail pits or boring: a trench can be dug near the intended foundation to determine what lies below the surface, for instance to check if there is an underlying pocket of deep peat (O'Donnell, 2021)

The layer of soil below the foundation should be compacted and there should be no soft places in the foundation due to items such as roots or a peat pocket; any soft or defective areas should be dug out and filled with hard material (Jeyakumar, 2016).

Building Load

The load of a building will be calculated by a structural engineer. It includes the live load (the load which may vary over time such as occupancy or furniture), the dead load (the permanent structure such as walls, beams) and the wind load (the force that wind exerts on the building) (Figure 3) (Jeyakumar, 2016). Load bearing walls are the external walls, chimneys, and certain internal walls, and they transmit the weight or load of the building to the ground.

Figure 3: Building Loads

Image from theconstructor.org

Foundation Depth

Foundation depth will vary based on the building load and the bearing capacity of the soil. The minimum depth of a foundation is 0.9m. This is to avoid movement due to:

• Frost heave, where the moisture in the soil freezes and changes the density in the soil up to 1m below ground level

• Seasonal and ground water level changes which cause expansion and contraction of the ground in areas up to 0.9m below ground level.

Foundation Thickness

• Part A of the Technical Guidance Document (2020) outlines that the minimum thickness of the foundation should be 200mm or 'P', the projection beyond the face of the wall, whichever is the greatest (See Figure 4)

• The minimum projection beyond the face of the wall on either side should be equal to the thickness

Figure 4: Foundation Dimensions

Image from Technical Guidance Document - Part A

Foundation Walls / Rising Walls

Rising walls (Figure 5) are usually built with concrete blocks up to DPC level. They should be at least the same thickness as the wall they will support above ground level. They are usually solid all the way up, but if the wall is a cavity wall then it should be started 225mm below the DPC level. This is to capture any moisture and debris from the cavities used for the building's walls.

Figure 5: Rising Walls

Image from https://www.youtube.com/watch?v=yzaQJm1ToR0

Types of Foundation

The type of foundation used will depend on the type of building and the building load and the nature of the soil underneath the building.

1. Strip Foundations

Strip foundations (Figure 6) are the most common type of foundation in Ireland and are the least expensive type of foundation. A strip foundation is a strip of concrete laid in a trench under the load bearing walls. The width of the strip foundation depends on the bearing capacity of the subsoil and the load of the building.

• Shallow strip foundation: most homes are built on shallow strip foundations where the concrete strip is laid about 1 metre below the soil

• Deep strip: larger buildings are built on deep strip foundations where the foundation is deeper than 1 metre below the soil and therefore the base of the foundation is on soil with a higher bearing capacity

• Reinforced wide strip: steel reinforcing is used in the strip to prevent shear failure; used in subsoils with poor bearing capacity such as soft sandy clays

• Stepped strip foundation: this foundation is used on sloping sites and is a series of concrete steps following the slope of the ground

Figure 6: Strip Foundation

Image from http://civilconstructiontips.blogspot.com/2011/06/wide-strip-foundation.html

2. Pad Foundations

A pad foundation (Figure 7) is a square pad that supports the structural columns, whereby the building load is transferred to a lower level via the column, to a level where the soil is of sufficient load bearing capacity. They can also be used to support ground beams. Pad foundations are sometimes used in underground car parks.

Figure 7: Pad Foundation

Image from http://civilconstructiontips.blogspot.com/2011/06/pad-foundations.html

3. Raft Foundation

A raft foundation (Figure 8) has one continued slab of steel reinforced concrete under the whole building, not just the load bearing walls, and therefore spreads the load over a bigger area. This type of foundation is used on soil with a lower bearing capacity, such as soft clay, where strip, pad or build foundations would not provide a stable foundation without excessive excavation.

Figure 8: Raft Foundation

Image from https://www.diydoctor.org.uk/projects/rafts.htm

4. Pile Foundation

Pile foundations (Figure 9) are a series columns of concrete pushed deep into the ground to transmit the loads to a lower level of subsoil; this is the strongest type of a foundation and is used on buildings that have high load such as a tall building or a bridge. They are also used for weak soil. There are several types of pile foundations:

• End bearing: the pile is deep enough to hit load bearing soil

• Friction: the pile is rough and jagged pile and the jagged bits cause friction and resistance

• Replacement: a hole is drilled and filled with concrete

• Displacement: the pile is driven up to 40m into the ground by hydraulics

Figure 9: Pile Foundation

Image from https://www.slideshare.net/Parth1023/pile-foundation-142943032

Ground Floors

A ground floor gives strength and stability to the building and must safely support the dead load of the floor and the live load of its finishes, furniture and occupancy.

The Technical Guidance Documents also require that floors, walls and roof of a building shall be constructed as to prevent the passage of moisture to the inside of the building or damage to the fabric of the building (Technical Guidance Document C - Site Preparation and Resistance to Moisture, 2020).

Figure 10: Ground Floor

Image from Stonhard.ie

Types of Ground Floor Construction

Ground Bearing Concrete Slab

A ground bearing concrete slab (Figure 11) is a concrete slab which sits on top of the ground. It is currently the most common type of ground floor construction. Ground supported floor slabs must have clean, well compacted and graded hardcore beneath them, eg crushed stone or concrete. The hardcore should be a minimum of 150mm and level.

Figure 11: Ground Bearing Concrete Slab

Image from http://build-diy.blogspot.com/2017/11/concrete-ground-bearing-floors.html

Screed Finished Floor

A screed finished floor (Figure 12) is where a thin layer of concrete is poured over a base floor, such as a concrete slab, to help form a smooth floor coating.

Figure 12: Screed floor

https://www.becosan.com/difference-between-concrete-and-screed/

Suspended Concrete Floor

A suspended floor is a ground floor with a void underneath the floor, and which is supported from the external walls of the building. It is used on ground that is sloped, or has poor bearing capacity or is prone to volume change.

Methods used for a suspended concrete floor are:

• Block and beam (Figure 13): Beams are supported on external and internal load bearing walls; blocks placed in between the beams (see image)

• Pre-cast reinforced slabs

• In situ reinforced slabs: constructed with reinforced mesh and supported by timber or steal

Figure 13: Block and Beam Suspended Concrete Floor

https://thermohouse.co.uk/blog/suspended-floors/

Suspended Timber

A suspended timber floor (Figure 14) is the traditional method of building floors and is most common in older houses. A raised timber platform is constructed inside the internal and external walls. Timber floors need more care than concrete floors as there is a possibility of rot.

Figure 14: Suspended Timber Floor

Image from https://www.practicaldiy.com/

REFERENCES:

Jeyakumar, J. (2016) What do you mean by foundation in a building, Basic Civil Engineering, [online] Available from <https://basiccivilengineering.com/2016/05/mean-foundation-building.html> [accessed 17 March 2021]

Technical Guidance Document A - Structure (2012) Government of Ireland [online] Available from <https://www.gov.ie/en/publication/3d58c-technical-guidance-document-a-structure/> [accessed 17 March 2021]

Technical Guidance Document C - Site Preparation and Resistance to Moisture (2020) Government of Ireland [online] Available from <https://www.gov.ie/en/publication/1aa81-technical-guidance-document-c-site-preparation-and-resistance-to-moisture/> [accessed 17 March 2021]

O'Donnell, L. (2021) Soil Types. Self Build [online] Available from <https://selfbuild.ie/advice/lay-of-the-land/> [accessed 27 March 2021]