Foundations Flashcards
What is the purpose of foundations?
To provide support for structures, to transfer the load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics
What determines the type of foundation used in any given situation?
- Ground conditions; how deep, soil types
- Water table
- Imposed loads (point or uniform)
- Cost
- Durability
- Location
Name different soil types
- Solid rock (high load bearing characteristics
- Sands/granulated soils
- Silts and clays (effect by weather - shrink and heave)
- Peat and organic soils (subject to compression and changes in volume - generally unsuitable for buildings)
Name different types of shallow foundation
- Pad foundations
- Strip foundations (including traditional, deep strips)
- Solid slab raft foundations
What is a pad foundation?
A pad foundation is typically a rectangular or circular pad used to support localised loads, such as columns
What is a strip foundation? Describe the different types
Strip foundations are used to provide a continuous strip of support to a linear structure, such as a wall
- Traditional strip foundation, where the wall extends below ground level to meet the concrete strip
- Deep strip foundation/trench foundation, where the concrete extends to just below the ground level
Both traditional and deep strip foundations have the same overall depth. Deep strip is generally cheaper as ready mixed mass concrete is used for all
What is a strip foundation? Describe the different types
Strip foundations are used to provide a continuous strip of support to a linear structure, such as a wall
- Traditional strip foundation, where the wall extends below ground level to meet the concrete strip
- Deep strip foundation/trench foundation, where the concrete extends to just below the ground level
Both traditional and deep strip foundations have the same overall depth. Deep strip is generally cheaper as ready mixed mass concrete is used for all
What is a solid slab raft foundation?
Solid slab foundations are slabs that cover a wide area, often the entire footprint of a building. They are suitable where ground conditions are poor, or settlement is likely. They are also suitable in situations where it may be impractical to create individual pad or strip foundations for multiple individual loads. Rafts may also incorporate beams and thickened areas where specific loads require additional support
Define deep foundations
Deep foundations are greater than 3m deep, generally pile foundations. They are defined as a series of columns constructed or inserted into the ground to transmit the loads of s structure to a lower level of subsoil
What are to two main categories of pile foundations?
Piles are either displacement or replacement piles
What are displacement piles?
Displacement piles are forced/driven into the ground, which displaced the soil
What are some advantages and disadvantages of displacement piles?
Advantages
- Can have a compacting effect on poor ground conditions
- End bearing - suitable for situations where there is low level firm strata
Disadvantages
- Noisy and cause vibration when being laid
- Piles can crack when being driven in
What are replacement piles?
Replacement piles require the excavation of soils, the installation of a temporary support system (such as steel mesh or bentonite), following which either a cast in-situ or precast pile is inserted (trimming may be required). They are friction bearing
What is bentonite used for in replacement piles?
Bentonite is used to stabalise excavations and support side of bore holes prior to the concrete infill
What is a Continuous Flight Auger pile type?
Continuous Flight Auger piles are replacement piles that do not require any temporary support. The process involves drilling into the ground with a hollow stemmed drill with a continuous flight auger until you reach the point desired depth. Concrete slurry is then pumped through the centre of the hollow stem, while the drill is removed from the hole at a controlled rate, removing the soil with it. Once this process is complete a reinforcing cage is then dropped into the slurry to complete the foundation.
