BTF 19

Question 1

In which state does water cause problems?

Answer 1

• Flowing
• ponding
• seeping
• migrating by capillary action or
• in the air as vapor.

Question 2

Water affects negatively the following materials:

Answer 2

• Masonry
• permeable mortar
• timber
• plasterboard
• steel.

Question 3

Soil is affected by water through…

Answer 3

Absorption or shedding of moisture: erosion, subsidence, swelling or shrinkage.

Question 4

Water also affects the soil by attracting living organisms such as:

Answer 4

Insects, termites, spiders, dust mites and fungi.

Question 5

Soil problems result in…

Answer 5

• falls in floor levels
• cracking in floor tiles, concrete slabs, walls and ceilings
• squeaky flooring
• binding doors and windows
• deflecting roof slopes
• cracked mortar bedding, ridge and hip caps.

Question 6

What are the two types of soil?

Answer 6

Granular (sand) and cohesive (clay, silt).

Question 7

What is the problem associated with granular soils?

Answer 7

Erosion

Question 8

What are the problems associated with cohesive soils?

Answer 8

Saturation and swell/shrink problems.

Question 9

Problems with rock soils are:

Answer 9

• footings founded on bolders or “floaters” can move due to erosion of soil around them.
• rock is susceptible to water migration via faults and between strata.

Question 10

Problems with granular soils are:

Answer 10

• they can be susceptible to local shear failure when not confined.
• are prone to erosion
• organic material left in soil eaten by termites can leave a void that will be filled by surrounding soil thus reducing bearing capacity of foundation in that area
• sand expands when damp. Ensure it is well-compacted when constructing footings.

Question 11

Problems associated with clay soils are:

Answer 11

• most clays react significantly to the introduction of water
• local shear failure is not uncommon
• when saturated, virtually all clay loses its bearing capacity
• slower to compress under load than other soil types
• a small amount of water can have a significant effect on clay
• clay absorbs and sheds water slowly

Question 12

Two type of settlement due to construction:

Answer 12

• immediate

- consolidation

Question 13

What is immediate settlement?

Answer 13

It takes place when a building is first placed on its foundation soil. Cohesive quality of clay mitigates against this, but granular (particularly sandy soil) is susceptible.

Question 14

What is consolidation settlement?

Answer 14

It is a feature of clay soil and may take place because of the expulsion of moisture from the soil or because the lack of resistance to local compressive or shear stresses. It largely takes place during the first few months after construction but has been known to take many years in some cases.

Question 15

What soils are prone to erosion?

Answer 15

All soils are, but sandy soil is particularly susceptible to being washed away. Even clay with a 10% component of sand can suffer erosion.

Question 16

What soils are prone to saturation?

Answer 16

Particularly clay soils, it creates a bog-like suspension of soil that causes it to lose virtually all of its bearing capacity.
To a lesser degree sand is affected because it may undergo a reduction in volume (sand fill for bedding and blinding layers), usually occurring as immediate settlement.

Question 17

Do all clays react to the presence of water?

Answer 17

Yes, by slowly absorbing it, making the soil increase in volume. The degree of increase or decrease varies considerably in various clays.

Because of the low absorption/expulsion rate, this phenomenon will not be usually significant unless there are prolonged rainy or dry periods of weeks or months, depending on land and soil characteristics.

Question 18

The swelling of soil creates…

Answer 18

an upward force on the footings of the building.

Question 19

The shrinkage of soil creates…

Answer 19

subsidence that takes away the support needed by the footing to retain equilibrium.