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Barry's Introduction to Construction of Buildings > Foundations > Flashcards

Foundations Flashcards

1
Q

Describe the process of constructing a strip foundation

A
  1. Dig trenches to a depth that reaches the required soil bearing strata.
  2. Erect and place temporary supports
  3. Pour concrete into the trenches.
  4. On top of these concrete trenches, the loadbearing walls of the building can be erected.
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2
Q

For buildings with insulated ground floors what is the general minimum foundation depth that is sufficient against the possibility of damage from ground movement due to frost heave?

A

450mm

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3
Q

Why are strip foundations not commonly used when you have to excavate deep into the ground to reach soil with a higher bearing strata?

A

It is very expensive thus pad foundations (less excavation required) are used.

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4
Q

What does the width of a concrete strip foundation depend on?

A

The Bearing Capacity of the soil and the loads that will be acting on the foundations

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5
Q

What is shear failure?

A

Shear is caused by two opposing forces of the wall and the ground acting on and tearing or shearing the concrete (footing).

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6
Q

Strip foundations on subsoils with poor bearing capacity need to be a lot wider than the wall they support. The alternative to widening the wall is adding…?

A

Steel reinforcement (means the foundation doesn’t need to widen as much).

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7
Q

It is normal when using strip foundations to have a foundation depth of at least what?

A

0.9m minimum

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8
Q

What depth is ‘topsoil’ (the weak top surface of land that needs to be removed before construction can begin)

A

Typically between 100-300mm

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9
Q

Describe the process of constructing a pad foundation?

A

Pits are excavated down to the necessary level (the sides of the excavation are temporarily supported and isolated pads are cast in the bottom of the pits. The ground beams/foundation beams may be just below or at ground level.

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10
Q

Describe the process of constructing a pile foundation?

A

Pits are excavated down to the necessary level, the sides of the excavation temporarily supported and isolated pads of concrete are cast in the bottom of the pits.

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11
Q

What material is most used for temporary support for the sides of excavations in foundations?

A

Rough Sawn Timber

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12
Q

Name the 4 different types of foundation (and describe them in your head)

A
  1. Strip
  2. Pad
  3. Pile
  4. Raft
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13
Q

What is surface water?

A

Rainwater that falls on the surface of the ground

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14
Q

What is ground water?

A

Water that is held in soils at and below the water table

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15
Q

What are subsoil drains be used to improve? (see Titas Library or page 29 of pdf for examples)Reasons for using them?

A

The run of Surface water water and the drainage of ground water to maintain the water table at some depth below the surface for the following reasons:To improve the stability of the ground (sloping sites/poor bearing strata!)To avoid surface floodingTo alleviate or avoid dampness in basementsTo reduce humidity in the immediate vicinity of buildings

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16
Q

Groundwater/Land drains are used generally on what type of land?

A

One way sloping land

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17
Q

Groundwater/Land drains are used generally laid in trenches at what depth?What is the normal hole diameter of the pipes?

A

0.6m and 0.9m in heavy soils, and 0.9 to 1.2m in light soils.75mm and 100mm for main drains, 65 or 75mm for branches ( for more information see tech book page 29 of pdf version)

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18
Q

The concrete slab in a foundation (commercial or private) must be an absolute minimum of?

A

At least 100mm (generally 150/200mm)

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19
Q

A DPC (Damp proof course) should run at least how high in foundations?

A

150mm (used to protect the bottom of the building from splash back from rain)

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20
Q

What is the purpose of a DPC (Damp proof course)?

A

To stop moisture rising through the foundation walls

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21
Q

What is the absolute minimum concrete slab depth when using a concrete based foundation?

A

100mm (generally 150-200mm though)

22
Q

Without a DPM moisture would be able to do what?

A

Rise up through the foundation walls to make the inside walls below the DPC dampServe as a barrier to damp.

23
Q

What is the primary purpose of the use of hardcore on site?

A

To raise the level of the top of the concrete of the foundation (to that of the dpc in walls)

24
Q

Where can a DPM be placed in relation to the concrete slab?

A

Under or over (or in the slab)

25
Q

Why do we link the DPM to the DPC in walls?

A

To provide a continuous seal against moisture/damp in the foundation

26
Q

What is hardcore made of?

A

Broken brick, crushed rock / stone or concrete rubble

27
Q

Where the foundation has trouble settling (on relatively unstable land), what should be added to the concrete slab and footings?

A

Steel reinforcement

28
Q

Why do we use insulation around or under the edges of a solid floor?

A

As it will significantly reduce heat losses

29
Q

How do we avoid a cold bridge across the foundation wall and the edges of the slab?

A
  1. Fit insulation around the edges of the slab2. Continue the insulation down inside the cavity
30
Q

Give three reasons why we use blinding above the hardcore layer?

A
  1. To prevent the hardcore penetrating the DPM
  2. To prevent the wet concrete running down between the lumps of broken brick or stone
  3. To provide a clean, level and dry working surface.
31
Q

What depth will blinding be?

A

Will almost always be 50mm thick

32
Q

What is a second purpose of blinding other than to protect the DPM from being punctured?

A

To prevent the wet concrete running down between the lumps of broken brick/concrete (hardcore) as this would make it easier for water to to seep up through the hardcore and would be wasteful of concrete

33
Q

When your building utilises underfloor heating where should your DPM be placed in relation to the concrete slab?

A

Under it

34
Q

What is one advantage of having the DPM under the concrete slab?

A

It will be protected from damage during subsequent building operations.

35
Q

What is one disadvantage of having the DPM under the concrete floor slab?

A

It will delay the drying out of the concrete as the concrete will only be able to lose moisture in the upwards direction

36
Q

What must the materials in a DPM be impervious to? (resistant to)

A

Water in both liquid and vapour form

37
Q

When using a DPM under the concrete slab it should be laid on what to prevent puncturing from the hardcore?

A

A bed of sand

38
Q

Where the level of the DPM is below that of the DPC in walls it is necessary to do what?

A

To turn the DPM up against the side of the walls so that it can overlap the DPC (forming a continuous barrier against moisture)

39
Q

What is a DPM impermeable to?

A

Water in both liquid and vapour form

40
Q

Can a DPM be laid directly on top of hardcore?

A

Yes. But only if it has been thoroughly compacted first.

41
Q

DPC’s should be placed at least how high in cavity walls?

A

150mm

42
Q

Why is a screed used over the concrete slab?

A

It is practice to lay floor finishes to solid ground floors once the slab has dried out (a screed on top). This finishing layer is called a screed.

43
Q

What is a screed made of?

A

Sand &Cement

44
Q

As the screed does not strongly bond (stick) to dry concrete, what depth is the minimum used to ensure it does not dry too quickly and crack?

A

65mm thick

45
Q

What type of heating system is most commonly used in conjunction with the screed?

A

Underfloor heating (buried within the screed)

46
Q

What is a bonded screed and what is the minimum thickness for this type of screed?

A

When the screed is laid on a concrete base within 3 hours of placing the concrete. Min thickness 12mm.

47
Q

What is a semi-bonded screed and what is the minimum thickness for this type of screed?

A

When the screed is laid on a concrete base that has set and hardened. Min thickness 40mm.

48
Q

What is an unbonded screed and what is the minimum thickness for this type of screed?

A

When the screed is laid on an impermeable DPM (no bond between screed + concrete base). Min thickness 50mm with reinforced light mesh.

49
Q

Describe a floating floor screed. And what is its minimum thickness?

A

When the screed is laid on top of insulation. At least 65mm thick for domestic & 75mm for other buildings.

50
Q

Supports for foundations / trenches are typically made using what two materials?

A

Timber or Steel beams and posts

51
Q

If the hardcore is thoroughly compacted / consolidated, do we still require the blinding layer?

A

Theoretically no, as the hardcore should not penetrate the dpm

Barry's Introduction to Construction of Buildings (5 decks)

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