Substructure

Question 1

How do ground conditions influence the design and type of foundations you may use?

Answer 1

Depending on the soil type, water table etc

Question 2

What is meant by the term ‘water table’ and how may it affect construction?

Answer 2

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

Question 3

What is a percolation test and when is it used?

Answer 3

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

Question 4

Describe the different ways of dealing with ground water in excavations?

Answer 4

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

Question 5

What is the purpose of foundations?

Answer 5

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

Question 6

What factors would need to be considered when designing foundations?

Answer 6

Type of soil/ground
Depth of suitable strata
Water table
Subsoil conditions (i.e. mining)
Heave
Trees

Question 7

Name some different foundation types.

Answer 7

Strip
Pad
Raft
Piled
Pier

Question 8

What is a strip foundation?

Answer 8

Concrete strips (sometimes can be reinforced) under all loadbearing walls

Question 9

What materials have commonly been used for strip foundations?

Answer 9

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

Question 10

Under what conditions would you expect a strip foundation to be used for a substructure?

Answer 10

1. Most subsoils

2. Light structural loadings (e.g. low to medium rise domestic dwellings)

Question 11

What is the difference between a standard strip foundation and a deep strip foundation and why may the latter be advantageous?

Answer 11

With a deep strip foundation, the trench is filled to a higher level than with a standard strip foundation, meaning:

1. It is cheaper (although deeper than 1000mm becomes uneconomic)
2. Fewer man hours required
3. Working space for bricklaying not required
4. Requires less skilled trades
5. Uses ready mixed concrete, so less materials to store on site (clearer and easier to manage)

Question 12

When did deep strip foundations become common

Answer 12

Found in buildings from 1970s onwards.

Question 13

How would strip foundations be constructed on sloping sites?

Answer 13

The strips would be stepped to reduce the amount of excavation and materials required

Question 14

what is a raft foundation?

Answer 14

1. 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.

Question 15

Under what conditions would you expect a raft foundation to be used for a substructure?

Answer 15

1. Low bearing capacity soils (e.g. soft clay or silt)
2. Where subsidence is likely (e.g. above former mining areas or landfill)
3. On deep areas of fill where piling would be uneconomic
4. Used to spread the load of the superstructure over a large base to reduce the load per unit area being imposed on the ground

Question 16

What are pile foundations?

Answer 16

1. Series of reinforced concrete columns constructed or inserted into ground
2. Can be just steel or even timber
3. Described as piled when its depth is more than three times its breadth

Question 17

When did pile foundations start being used?

Answer 17

Rarely found before 1970, except as remedial underpinning

Question 18

Under what conditions would you expect a pile foundation to be used for a substructure?

Answer 18

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
5. High water table

Question 19

What different types of piling are you familiar with?

Answer 19

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

Question 20

How can pile foundations be installed?

Answer 20

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

Question 21

Describe the components of a piled substructure and their functions.

Answer 21

1. Pile shaft - provides friction and stability in the ground
2. Pile cap (aka ground beam) - spans from pile to pile, carrying the floor structure and walls (either formed in situ or pre-cast)

Question 22

What ancillary activities are carried out in connection with piling?

Answer 22

1Site constraints (e.g. headroom limitations of driven piles)
2Monitoring adjacent sites (e.g. vibration of driven piles)
3.Health and safety
4Underground services scan

Question 23

What is a pad foundation?

Answer 23

Usually reinforced concrete pads

Square in plan where possible

Question 24

Under what conditions would you expect a pad foundation to be used for a substructure?

Answer 24

Used to support localised loads, such as columns

Most subsoils except loose sands, loose gravels and filled areas

Question 25

Where would you expect to find the main reinforcement in a pad foundation and why?

Answer 25

The main reinforcement would be in the middle of the pad with reinforcement protruding out of the pad with a base plate for the steel column to be bolted to.

Question 26

What is a pier foundation?

Answer 26

Series of thick concrete piers (usually large concrete rings), formed insitu, supports a reinforced concrete ground beam

Question 27

Under what conditions would you expect a pier foundation to be used for a substructure?

Answer 27

Alternative to piling where ground is full of obstructions (e.g. old basements)
If the ground is too steep for piling rigs