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According to the Irish Association of Self Builders, the most important part of the building structure is the foundation, because it can cause severe problems in the future if not done properly. I don't know much about building foundations before I started this blog, but after reading about it, I know now how crucial it is that we build it properly because of the big role it plays in keeping our shelter safe.
On this blog, I would discuss about the definition of foundation, usage, types and the steps on how to build a foundation. A small section on flooring would also be discussed as it is related to the building foundation.
Before we proceed, let us define what a foundation is.
The building foundation is the surface directly in contact with the ground. According to Civil Engineering blog (Sarma, 2017), “ transmits the weight of the structure together with live loads, seismic and wind pressure to the ground surface on which the structure rests, ensuring its safe bearing capacity”. It should be strong enough that it can withstand pressure from construction equipment, wind, and earthquakes. It also must set evenly and uniformly to ensure stability and prevent cracks.
5 Steps in setting a foundation
It is important that we build the foundation properly from the start in order to ensure that we the unit we are building can withstand time and weathering. Below are the steps on how to set our foundation.
Timber profiles are set-up at least 1m beyond the edge of the trench if digging by hand and further if using machineries. Setting the timber profiles are needed in order to support the lines marking the area where the foundation is going to be.
The outline of the foundation trench is marked out square. It means the angle of the foundation should be set out correctly at the start before digging.
The trench is excavated by hand or using a digger. The depth sides and bases must all be plumb (perfectly vertical), level and squared. This is to make sure that when we pour the concrete, it will settle and dry out evenly and uniformly.
The trench is then pegged using steel bars or timber stakes. They are driven into the trench about 300mm evenly spaced and the top must be levelled. They serve as guides for the depth of the concrete and to level-off when the concrete is being poured.
A timber screed spans from peg to peg and is used to level the poured concrete. Also known as ‘tamping’. Tamping is done to compact wet concrete mix to increase its strenght and durability.
Now that we know how to set-up our foundation, we now need to identify the suitable foundation type based on the purpose of the unit we are building (e.g. warehouse, dwelling etc.) and the type of soil we are building in. One of the factors that we should consider when deciding which type of foundation to use is the bearing capacity of the ground we are building on.
Bearing capacity is defined as the ground’s resistance to force. It is expressed in Newtons per mm square (N/mm2). Different materials comprising the ground have different bearing capacity for example, a solid rock would be 10,000 N while a cohesive soil (clay soil) would be 75 N. Peat, organic soils and made up ground are not suitable for building because they are not compacted that when pressure is applied on top, it will shrink. Various site investigations such as geological surveys can help determine the ground formation.
After learning more about the area we are building on, it is also important to check the building regulations to see if your measurements and building plan coincides with the building regulations in the area. In Ireland, the following minimum requirements apply.
The minimum depth of a foundation for dwellings is 900mm below ground level to avoid movement due to frost-heave expansion of the ground and ground water level changes. This depth is recommended as it is deep enough to protect the foundation from ground changes brought by ground contracting due to low temperature or expanding due to increase moisture.
The minimum thickness (T) of a foundation should be 200mm or equal to its projection (P) whichever is greatest. The minimum projection (P) should be 75mm beyond both sides of the wall according to the (TGD Document A (p.26- 27). The minimum measurements provided would help prevent any reduction on the bearing of the base whenever the concrete fails due to shear pressure.
Types of foundation
Listed below are the types of foundation and when it is used in construction.
Strip foundation
Strip foundation diagram (Engineering Feed)
- Also known as ‘shallow strip’ foundation
- Usually used in dwellings
- Formed under load-bearing walls and the stronger the bearing capacity of the ground, the lesser the width of the foundation.
- Minimum thickness is 200mm (TGD, Part A p.27) but the standard is 300mm.
- Has 2 variations, deep- strip for weaker soils and more economical or reinforced wide strip also used in weak soils particularly on soft, sandy soils. It is done by increasing the width of the foundation and steel reinforcing to prevent shear pressure due to lack of tension.
Raft foundation
Raft Foundation by Alan Toomey
- Usually used in single story buildings
- Also used when soil is weak
- It is built by using one continuous slab of concrete placed underneath the structure.
- More economical than digging trenches.
- Thicker concrete rafts can also be used with steel mesh reinforcements.
Pad foundation
Pad foundation (Langley Design)
- Also known as ‘portal’ frames
- Generally used in industrial units (e.g. factory, warehouses) where the building would be framed in vertical columns or ground beams.
- More economical than digging trenches all around.
Pile foundation
- Used in multi-story buildings or extremely weak soils.
- More economical than excavating huge amount of soil and paying for its disposal.
- Utilizes piles driven or made below ground with a pile cap (made of concrete) on top of the pile (as seen on the picture below) and a ground beam can be built in between piles.
- There are 4 types:
Pile foundation from Live Enhanced
1. End bearing- involves digging into the ground until load-bearing soil is reached and building a concrete column or implanting a column down into load-bearing soil.
2. Friction or floating piles has rough and jagged edges and the friction created from the jagged piles and the soil helps support the building. Friction between the jagged pile and the ground helps support the building. The diagram below shows the difference between the end bearing pile and the floating pile.
3. Replacement involves digging and reinforcement bars are applied before pouring concrete. The concrete, or steel, simply replaces the soil which has been excavated.
Replacement pile using steel from Abilene Foundation Repair Experts
4. Displacement is using a concrete cast that is driven down into the soil by force using hydraulic-pressure machines until it hits solid ground. As you can see on the diagram below, the pre-made casts are driven down using pressure.
Displacement pile (Live Enhanced)
Floor
The floor in buildings, like the foundation, supports the weight of the inhabitants, furniture, etc. It should be able to provide strength, stability, resistance to weather and ground moisture, durability, fire resistance and resistance to passage of sound. There are 4 ways on how to build floors.
Ground bearing concrete slab
Concrete slab marked in red (Pinterest)
- Concrete slab sitting on top of a layer of insulator and the dashed lines represent the DPM (damp proof membrane).
- most common
- minimum thickness of the concrete slab should be 150mm
- The top is polished using a hand float or power float.
- Must have clean, graded and well- compacted hardcore (150mm thick) beneath the slab. It should also be free from sulphates because it causes a chemical reaction with concrete which can cause degradation.
2. Screed finish floor
Screed finished floor (The Screed Scientist)
- Usually resting on top of raft foundation
- Sand and cement about 50mm- 75 mm thick on top of DPM.
3. Suspended concrete slab
Suspended concrete slabs (ABS Building Systems)
- Used in sloped areas, ground that has poor bearing capacity and prone to changes due to temperature and moisture.
- Uses either pre-cast concrete, block and beam (as shown in the picture above) or in situ reinforced slabs.
4. Suspended timber
Suspended timber floors (The Heritage Portal)
- Traditional, mostly seen in older houses.
- Timber joist, spaced at 400mm- 600mm apart, from one external wall to the other or from an external wall to an internal load-bearing wall.
- Covered with floor boards on top.
- DPM are applied on the timber wall plates to absorb moisture.
- As you can see on the picture above, layers of brick called the 'sleepers' are in place to support the timber floors. It is on top of an oversight concrete which is on top of compacted hardcore (minimum thickness 150mm).
- Ventilation must be provided underneath the timber floors to prevent dry rot.
Thank you again Robbie. The videos you made are very useful and so are the worksheets. It makes the assignment a lot manageable, otherwise I wouldn't know where to start.
Hi Anne, a lovely overview of foundations and floors, including great practical examples as well as theoretical concepts such as concrete strength and durability of materials. Well done for including some references to the specific TGDs. Again, great photos used to help get your points across. Well done Anne, and keep up the good work.
Thanks
Robbie