BTF 19 Flashcards
In which state does water cause problems?
- Flowing
- ponding
- seeping
- migrating by capillary action or
- in the air as vapor.
Water affects negatively the following materials:
- Masonry
- permeable mortar
- timber
- plasterboard
- steel.
Soil is affected by water through…
Absorption or shedding of moisture: erosion, subsidence, swelling or shrinkage.
Water also affects the soil by attracting living organisms such as:
Insects, termites, spiders, dust mites and fungi.
Soil problems result in…
- 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.
What are the two types of soil?
Granular (sand) and cohesive (clay, silt).
What is the problem associated with granular soils?
Erosion
What are the problems associated with cohesive soils?
Saturation and swell/shrink problems.
Problems with rock soils are:
- 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.
Problems with granular soils are:
- 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.
Problems associated with clay soils are:
- 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
Two type of settlement due to construction:
- immediate
- consolidation
What is immediate settlement?
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.
What is consolidation settlement?
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.
What soils are prone to erosion?
All soils are, but sandy soil is particularly susceptible to being washed away. Even clay with a 10% component of sand can suffer erosion.
What soils are prone to saturation?
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.
Do all clays react to the presence of water?
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.
The swelling of soil creates…
an upward force on the footings of the building.
The shrinkage of soil creates…
subsidence that takes away the support needed by the footing to retain equilibrium.
