Foundation

Question 1

What is a foundation?

Answer 1

It’s the part of the building that is in direct contact with the ground, and which transmits the load of the building into the ground.

Question 2

What is settlement?

Answer 2

Apart from solid rock, the ground on which a building is founded consists of various types of soil, all of which are compressible in varying degrees so that under the building load, foundations on such soils will, to some extent, move in a downward direction.
It is mainly due to the consolidation of the soil particles.

Question 3

How does excessive settlement form?

Answer 3

It results from overloading the soil to such an extent that the loaded area of soil shears past the surrounding soil in what is known as plastic failure of the soil.

Question 4

Does settlement cause damage?

Answer 4

Provided that the settlement is uniform over the whole area of the building and is not excessive (e.g. uniform settlement or tipping settlement), the movement does little damage.

Question 5

Elaborate on differential settlement.

Answer 5

Differential settlement leads to distortion of the structure. If the differential settlement is so great, it may damage the fabric and finishes or cause the possible failure of the structure.
Thus, differential movements must be kept within limits.

Question 6

What are the major types of loading?

Answer 6

Dead load - big
Live load - small
Wind load - can be huge
Seismic effects - can be extremely huge

Question 7

What are dead load and live load?

Answer 7

Dead load of a building is dependent on the size of the building, its height, its structural form etc. (roof, wall, floor)
Live load (or superimposed load) of a building is largely determined by its intended use. (people, furniture, collected snow, collected water)

Question 8

What is the load path of a building?

Answer 8

Slabs –> Beams –> Columns and Walls –> Foundation

Question 9

What are wind load and seismic effects?

Answer 9

Wind load of a building is dependent on the height and planned shape of the building and its geographical location. Wind load causes tendency to overturn from the transformation of downward loads into uplifting action and downward loads.
Allowance for seismic effects is largely dependent on the importance of a building and its form of construction.

Question 10

What is the function of a foundation?

Answer 10

It is to transmit all of the dead, superimposed (live) and wind loads from a building to the soil on which the building rests in such a way that settlement, particularly uneven or relative settlement of the structure, is limited and failure of the underlying soils is avoided.

Question 11

What must be provided in the foundation’s design and construction to perform its function efficiently

Answer 11

Adequate strength and stability.

Question 12

What does the stability of the foundation depend on?

Answer 12

It depends on the behaviour under load of the soil on which it rests, which is affected partly by the design of the foundation and partly by the characteristics of the soil.

Question 13

It is necessary for the design of the foundation to take into account the __________________ of the materials to be used and also the __________________ on which the foundation will rest.

Answer 13

nature and strength
nature, strength and behaviour under load of the soil

Question 14

How do cohesive soils affect foundations? (soil characteristic)

Answer 14

In general, cohesive soils have great compressibility so they bear a higher risk of settlements;
they are more prone to suffer from shear failure;
variation of moisture content will change the volume of the cohesive soils more significantly.

Question 15

What are the three types of shear failures?

Answer 15

General shear failure: Most common type of shear failure, occurs in strong soils and rocks.
Local shear failure: Intermediate between general and punching shear failure.
Punching shear failure: Occurs in very loose sands and weak clays.

Question 16

How do droughts affect foundations? (soil characteristic)

Answer 16

The significant effect of seasonal changes doesn’t extend to a depth of more than about 1 m except in times of long drought. Therefore, if foundations are placed at or below this depth, the structure is unlikely to be affected by settlement due to drying out.

Question 17

How do trees affect foundations? (soil characteristic)

Answer 17

Tree roots extract water from soil at depths of up to 4.5 m or more. Trees will tend to dry up the soil and cause movement of the building by 25 to 100 mm even if they are planted 20 m away from the proposed building.
As a rough guide, buildings on foundations not deeper than 1 m should not be erected closer to a single tree than its mature height or to groups of trees not closer than 1.5 to 2 x their mature height.

Question 18

How do shrinkable clays affect foundations? (soil characteristic)

Answer 18

Shrinkable clays which have supported trees for many years will have dried out, and when the trees are cut down to permit building, the clay will swell over a long period as moisture returns to the soil.
This swelling can be substantial, and the consequent uplift can cause considerable damage to buildings erected on the site.

Question 19

What is shallow foundation?

Answer 19

Shallow foundation transfers the load to the soil at a level close to the lowest floors of the building, including spread foundation (i.e. strip, pad and raft).

Question 20

What is deep foundation?

Answer 20

Deep foundation includes piles and various piers which transfer their loads to the soil at a considerable distance below the underside of the building.

Question 21

When should strip and pad foundations be used?

Answer 21

Low-rise & medium-rise buildings, strip fnd under cont. walls & pad fnd under isolated piers or columns are used on sites where
sufficient depth of reasonably strong soil exists near surface of ground.

Question 22

When should raft foundation be used?

Answer 22

No firm bearing strata of soil exists at a reasonable depth below the surface &
a max area of fnd is required to bring imposed pressures within the low bearing capacity of the weaker soils & of some made up ground.

Question 23

Functional requirements of strip foundation?

Answer 23

1. Placed symmetrically with the wall it supports.
2. Provide working space for bricklaying.
3. Proper backfilling to the trench.
4. All fnds must bear horizontally on soil.
5. Enough thickness and reinforcement to guard against bending and shear stress.

Question 24

Functional requirements of pad foundation?

Answer 24

1. Isolated piers/columns carried on independent slab of concrete. Pier/column bears on center point of slab.
2. Placed symmetrically with the column it supports.
3. In a framed structure, loads on diff columns are diff, size of pads must also diff to maintain equal soil pressure under each, so it eliminates differential settlement.

Question 25

Functional requirements of raft foundation?

Answer 25

1. Distribute the building loads over large area of weak soil may be flexible or rigid.
   2a. Light (flexible) raft can be constructed of concrete ground slab w/ light reinforcements.
   2b. Soils of low-moderate compressibility, reinforcement required at top + bottom throughout the slab.
2. Raft must extend 300 mm beyond perimeter walls to spread load and protect soil under the walls from frost action.
3. Stiff raft can use under building prone to differential settlement.
4. Stuff raft used to overcome difficulties of subsidence (sinking) sites.
5. Stiff raft required on more compressible soils, and edge beam is reinforced & bonded to slab by linking w/ top lsab reinforcement.
6. When slab thickness need > 300 mm, may be cheaper to use beam and slab construction.
7. When high raft stress and needs great rigidity, beams must be deep and when depth > 900 mm, will adopt cellular form of construction.

Question 26

What is pile foundation?

Answer 26

Form of foundation that takes loads to a low level by columns in the soil that the building rests on.

Question 27

When should pile foundation be used?

Answer 27

On sites where no firm bearing strata exists at reasonable depth
and applied loading is uneven, making raft inadvisable.