Level 1 Flashcards
What are foundations?
Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity.
What often causes unsuitable ground conditions for foundations?
Poor soil types - peat or filler ground.
High water table
What considerations should be taken into account before designing foundations?
The nature of the load requiring support. Ground conditions. The presence of water. Space availability. Accessibility. Sensitivity to noise and vibration.
What soil types are you aware of?
- Solid rock = high load-bearing, e.g. sedimentary rock
- Clay and silts = prone to shrinkage and expansion through water absorption, leads to swell and heave of ground.
- Peat and organic soils = subject to compression and volume changes. Generally unsuitable.
Difference between cohesive soils and non cohesive soils
Cohesive soil means clay (fine grained soil), or soil with a high clay content, which has cohesive strength
Cohesive soils consist of large or irregular-sized soil particles with little to no clay content.
what are the different types of foundation
Strip
Pad
Raft
Trench fill
Pile
Diaphragm wall
Pier
Sheet Pile
What is a shallow foundation?
Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.
What is a deep foundation?
Deep foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.
What is a strip foundation?
Strip foundations provide a continuous strip of support to a linear structure such as loadbearing walls.
What materials have commonly been used for strip foundations?
1) Concrete became common from 1920s onwards (but were used as early as 1875 when recommended by the Public Health Act).
2) Prior to this date, stepped brick footings were used, or just brickwork laid on level ground with no steps
When would you use a strip foundation?
Most subsoils
Light structural loadings (e.g. low to medium rise domestic dwellings)
What is the difference between a standard strip foundation and a deep strip foundation and why may the latter be advantageous?
With a deep strip foundation, the trench is filled to a higher level than with a standard strip foundation, meaning:
It is cheaper (although deeper than 1000mm becomes uneconomic)
Fewer man hours required
Working space for bricklaying not required
Requires less skilled trades
Uses ready mixed concrete, so less materials to store on site (clearer and easier to manage)
When did deep strip foundations become common?
1970s onwards
How would strip foundations be constructed on sloping sites?
The strips would be stepped to reduce the amount of excavation and materials required
How wide are strip foundations?
The size and position of strip foundations is typically related to the wall’s overall width. The depth of a traditional strip foundation is generally equal to or greater than the overall wall width, and the foundation width is generally three times the width of the supported wall. This results in the load being transmitted at 45º from the wall base to the soil.
What is the Building Regulations Document that sets out minimum widths for strip footings?
Approved Document A- Structure
defines minimum widths for strip footings based on the type of ground and load-bearing wall (although advisable to consult SE)
What unit of measure is total load of load-bearings wall calculated?
kN/linear metre
What types of ground & Condition of Ground are referred to in Table 10 Minimum width of strip foundations?
1 Rock (Not inferior to sandstone, limestone, firm chalk
2 Gravel/sand- medium dense
3 Clay/Sandy Clay- Stiff
4 Clay/Sandy Clay- Firm
5 Sand/Silty Sand/Clayey Sand- Loose
6 Silt/Clay/Sandy Clay/Clay or Silt- Soft
7- Silt, Clay, Sandy Clay, Clay or silt- Very Soft
What Field Test is applicable for each type of ground (1-7)
1- Pneumatic/mechanically operated pick for excavation.
2- Pick for excavation/wooden peg 50mm sq in cross section to be driven beyong 150mm.
3-Thumb test (slight indent).
4- Thumb test (easy impression)
5- Can be excavated with spade/ wooden peg easily driven.
6- Finger pushed- 10mm
7- Finger pushed- 25mm.
How to prevent Frost Action in strip foundations?
At least 450 mm unless they are bearing on rock, and at least 1 m on high shrinkage clays.
For Wide Strip Foundations what is usually included?
Reinforcement.
Limitations of strip foundations?
Higher localised loads, ground conditions are poor or settlement is likely or impractical for individual strips.
What is a Pad foundation?
Pad foundations are rectangular or circular reinforced concrete pads used to support localised loads such as columns.
When would you use a Pad foundation?
Used to support localised loads, such as columns
Most subsoils except loose sands, loose gravels and filled areas
What would be some potential limitations with Pad foundations?
They can be large in plan shape and may not be effective against differential settlement, uplift forces or wind forces.
What is a Raft foundation
Reinforced concrete slabs that cover a wide area, often the entire footprint of a building, Raft foundations may incorporate beams or thickened areas to provide additional support for specific loads.
When would you use a Raft foundation?
1) Poor ground conditions are poor, settlement is likely
2) Where impractical to create individual strip or pad foundations.
3) Where subsidence is likely (e.g. above former mining areas or landfill)
4) On deep areas of fill where piling would be uneconomic
5) Used to spread the load of the superstructure over a large base to reduce the load per unit area being imposed on the ground
What is a Trench fill foundation?
Trench fill foundations are a variation of strip foundations, in which the trench excavation is almost completely filled with concrete (avoids bricklaying below ground).
When would you use a Trench foundation?
Trench fill is often used when soil is loose or in areas with a high water table, although in loose ground it can be difficult to predict the quantity of concrete required, and the quantity can be quite high if trenches meet or cross at right angles.
When constructing trench foundations, what would you need to do if there is the presence of trees or heavy?
Trench fill foundations can be taken deeper in areas with heavy clay or in the presence of trees, to a level where the subsoil moisture content is unaffected. In these situations, mesh reinforcement is often required. In addition, one or both trench faces may need to be lined with a compressible batt. This can also mitigate against the tendency in some situations for the trench strips to pick up longitudinal or lateral ground strains which may occur in the strata immediately around the foundation.
What is a common issue with trench foundations?
Another issue to consider is that the height of the concrete can create access problems for drainage and other services, as well as issues with landscaping and planting.
What is a Rubber-Trench fill foundation?
A further variation of trench fill foundations, and are a traditional construction method which uses loose stone or rubble to minimise the use of concrete and improve drainage.
Where there are higher localised loads, such as columns- what foundation would you recommend?
Pad.
Where ground conditions are poor, settlement is likely, what foundation would you recommend?
Raft.
When did pile foundations start being used?
Rarely found before 1970, except as remedial underpinning
When would you use a Pile Foundation?
1) Where a firm layer of ground is at a considerable depth (i.e. where strip foundations would be uneconomical)
2) On shrinkable clays with new or felled trees
3) Large structures
4) Recently placed filling materials that have not sufficiently consolidated
High water table
What different types of piling are you familiar with?
1) End bearing piles - most friction is developed at the toe of the pile, bearing on a hard layer
2) Friction/floating pile - most of the friction is developed by shear stresses along the sides of the pile, where harder layers are too deep to reach
How can pile foundations be installed?
1) Replacement/bored piles - poured insitu (if boring and pouring takes place simultaneously they are called Continuous Flight Augured (CFA) piles)
2) Displacement/driven piles - prefabricated off site and then driven into the ground