Foundation Defects

Question 1

What is subsidence?

Answer 1

The vertical downward movement of a building foundation, caused by loss of support.

Question 2

What is settlement?

Answer 2

The downward movement of a structure as a result of the initial load being too much for the ground bearing capacity.

Question 3

What is heave?

Answer 3

The upward movement of the ground as a result of an increase in soil moisture content or the removal of loads on the foundation.

Question 4

What is shrinkage?

Answer 4

The downward movement of the ground as a result of a reduction in soil moisture content or the addition of loads on the foundation.

Question 5

What is the remedy for a tree causing soil shrinkage but cannot be removed?

Answer 5

Pollarding can be carried out over a period to slowly reduce the trees capacity to remove water from the soil.

Question 6

What is differential Movement?

Answer 6

When part of the building remains stable while the remainder moves.

Question 7

What is considered a leaning wall?

Answer 7

One that is out of plumb by more than 25mm or bulges more than 12mm in a storey height.

Question 8

What is considered a dangerous amount of lean for a wall?

Answer 8

If the amount of lean in the full height of the wall exceeds one third of the thickness of the wall at the base.

Question 9

What causes leaning/bulging walls?

Answer 9

It is often caused by decreased stability of the wall caused by vibrations from traffic and machinery, increased floor loads or the addition of floors, inadequate original walls, and a lack of restraint between walls; floor joists; and partitions.

Question 10

What factors should be considered when assessing a crack?

Answer 10

It’s important to consider the crack’s width, position, direction, the nature of the brick and the season.

Question 11

How can cracks me monitored?

Answer 11

Using tell tales (fixed across the crack with epoxy resin) or laser measures.

Question 12

What causes overloading?

Answer 12

Excess floor loads, often due to change of use. In addition, forming additional openings or enlarging existing ones in older buildings will transfer the load to smaller areas of brickworks.

Question 13

What are signs of overloading?

Answer 13

Bowing, crushing and connections between walls and joists showing signs of movement.

Question 14

What causes brick fractures?

Answer 14

A combination of external weathering and structural issues. Causes include no/little ties betweeen inner and outer leaves in old buildings, the outer leaf being exposed to harsh conditions and poor bonding.

Question 15

What is a tie rod?

Answer 15

Lateral restraints holding the walls in a vertical position by providing lateral restraint. Their use was abandoned towards the end of the nineteenth century due to brick damage caused by the expansive corrosion of the iron.

Question 16

Why are wall ties necessary?

Answer 16

The outer leaf of a cavity wall may move independently due to roof thrust or sulphate attack on the mortar.

Question 17

What are usual signs of wall tie corrosion?

Answer 17

Horizontal cracks, several millimetres wide, corresponding to wall tie positions.

Pagoda effect to the roof eaves caused by the external leaf expanding vertically.

Outward bulging of the outer skin facing the prevailing wind.

In severe cases, the inner leaf of a cavity wall may show horizontal cracking causing floors and roofs to lift.

Question 18

What causes wall tie corrosion?

Answer 18

Calcium chloride and sulphates in cement, lime and sand mortars, along with sub-standard galvanising of steel ties.

Question 19

What are remedies for wall tie corrosion?

Answer 19

Corroding ties should be encapsulated with foam and new resin.

Cementitious or mechanical ties should be supplemented and tightened to the required torque.

In severe cases, brickwork can be partially removed and replacement ties installed.

Question 20

What causes expansion/contraction of brickwork?

Answer 20

Bricks will shrink when its cold and expand when hot. Moisture changes also shrink and expand bricks.

Question 21

What is used to prevent cracking due to movement of the brickwork?

Answer 21

Expansion joints.

Question 22

At what intervals should expansion joints be installed?

Answer 22

At a maximum of every 12 metres of brickwork.

Question 23

Apart from cracking, what else may be observed in a building with no expansion joints?

Answer 23

Brickwork can thrust outwards if an arch abutment is inadequate in size.

Excessive deflection of a lintel can cause converging cracks to run upwards.

Lengths of wall either side of openings can cause arches to lift as they expand.

Question 24

What is frost attack in brickwork?

Answer 24

Spalling and sometimes complete disintegration of brickwork, caused by water in saturated brickwork producing ice in exposed conditions, creating an expansive force within the crevices. This can be exacerbated by strong mortar which constrains the brickwork.

Question 25

What is the remedy for frost attack in brickwork.

Answer 25

Remove the water source if possible. Localised replacement or repair of damaged brickwork and repointing.

Question 26

What is efflorescence?

Answer 26

The white surface deposit left by the evaporation of natural salt solution.

Question 27

What can affect the amount of efflorescence?

Answer 27

The higher the porosity in the masonry, the greater the change of efflorescence as it’s easier for water to pass through.

Additionally, it can be seen as a seasonal problem to an extend. In low humidity, the evaporation of water may occur prior to reaching the surface, so the salt deposit remains unseen.

Question 28

What causes sulphate attack?

Answer 28

Leaching of the bricks soluble sulphate content or sulphates from pollution react with the portland cement in mortar joints, causing expansion of the cement.

Question 29

What are the signs of sulphate attack?

Answer 29

Cracking between bricks and some spalling. Not to be confused with wall tie failure.

Question 30

What are the remedies for sulphate attack?

Answer 30

Remove the water source if possible. Localised replacement/repair of damaged brickwork and repointing. Rendering over the affected brickwork can exacerbate the issue as you are adding more cement and water.