How do ground conditions influence the design and type of foundations you may use?
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What is meant by the term ‘water table’ and how may it affect construction?
- The level below which ground is saturated with water
- Rises and falls with variations of rainfall, temperature, atmospheric pressure and tide (in coastal regions)
- Usually below the depth required for strip foundations, however when it is not, additional precautions may be needed, such as:
- Change in foundation design to accommodate water levels
- Use of sulphate resisting cement to prevent sulphate attack
- Use of pumps during construction to rid trenches of water whilst concrete is being laid
What is a percolation test and when is it used?
- Test to determine the absorption rate of soil by digging a trial pit, filling it with water to a certain level and observing how quickly the water dissipates
- Often used in conjunction with ground investigations to determine the suitability for a soakaway
- Sandy soils are usually better at absorbing more water than those with a high concentration of clay
What is CBR?
California Bearing Ratio - used to determine how strong the ground is and to calculate the bearing capacity of the soil
What is vane shear testing?
- Used to determine the shear strength of a saturated cohesive soil by inserting a vane into the ground, rotating it until the soil fails and measuring the torque
- Does not require a sample to be taken or additional lab analyse, so can prove beneficial for initial site testing since it takes less time and cost to operate
How are earthworks supported and why?
Temporary shoring (or ‘sheeting’) should be implemented to prevent danger to any person working in an excavation from a collapse of material forming the sides of the excavation
Describe the different ways of dealing with ground water in excavations?
- Pit/sump - excavated below trench level to act as a collection point from which the water can be pumped away
- Cofferdams - temporary enclosures (usually interlocking steel sheets) that form watertight perimeter around the area of work
- Caissons - similar to cofferdams but usually form an integral part of the finished structure
What is the purpose of foundations?
- Safely sustain and transmit the combined dead, imposed and wind loads to the ground so as not to cause any movement that would impair the stability or cause damage to any part of the building
- Accommodate ground movement due to swelling or shrinkage, which may alter the stresses within the foundation
- Withstand erosive elements within the soil
- Be deep enough to be unaffected by climatic changes
- Provide a level base on which building operations can commence
What factors would need to be considered when designing foundations?
- Type of soil/ground
- Depth of suitable strata
- Water table
- Subsoil conditions (i.e. mining)
- Heave
- Trees
Name some different foundation types.
- Strip
- Pad
- Raft
- Piled
- Pier
What is a strip foundation?
Concrete strips (sometimes can be reinforced) under all loadbearing walls
What materials have commonly been used for strip foundations?
- Concrete became common from 1920s onwards (but were used as early as 1875 when recommended by the Public Health Act)
- Prior to this date, stepped brick footings were used, or just brickwork laid on level ground with no steps
Under what conditions would you expect a strip foundation to be used for a substructure?
- 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?
Found in buildings from 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
What is a raft foundation?
- Reinforced concrete slab that often covers the entire footprint of the building
- May incorporate beams or thickened areas to provide additional support for specific loads
Under what conditions would you expect a raft foundation to be used for a substructure?
- Low bearing capacity soils (e.g. soft clay or silt)
- Where subsidence is likely (e.g. above former mining areas or landfill)
- On deep areas of fill where piling would be uneconomic
- Used to spread the load of the superstructure over a large base to reduce the load per unit area being imposed on the ground
Where would you expect to find the main reinforcement in a raft foundation and why?
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What are pile foundations?
- Series of reinforced concrete columns constructed or inserted into ground
- Can be just steel or even timber
- Described as piled when its depth is more than three times its breadth
When did pile foundations start being used?
Rarely found before 1970, except as remedial underpinning
Under what conditions would you expect a pile foundation to be used for a substructure?
- Where a firm layer of ground is at a considerable depth (i.e. where strip foundations would be uneconomical)
- On shrinkable clays with new or felled trees
- Large structures
- Recently placed filling materials that have not sufficiently consolidated
- High water table
What different types of piling are you familiar with?
- End bearing piles - most friction is developed at the toe of the pile, bearing on a hard layer
- 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?
- Replacement/bored piles - poured insitu (if boring and pouring takes place simultaneously they are called Continuous Flight Augured (CFA) piles)
- Displacement/driven piles - prefabricated off site and then driven into the ground
