Q.
(a) Elaborate four (4) factors influencing design and size of a building foundation. (10 marks)
(b) Using diagrams, explain the types of foundation suitable for the following types of soil.
(i) Soft Clays
(ii) Peat and fill
(iii) Solid chalk and sands (15 marks)
(25 marks, 2016 Q2)
A.
a) 4 factors influencing design and size of a building foundation are:
See earlier posts on the same topic below:
Factors influencing choice of pile
Precautions why designing type of foundation.
Ref:
b)
The question has 15 marks, hence it is expected to write a lot on the each soil type influencing the choice of foundation.
Read here on varied soil and suitable foundation types.
Selection of type of foundation is quite a complex process. This is the expertise of the soil engineer. However, in simplified manner, there are three general conditions of the soil type which influence the selection of foundation. The Sand, Clay and the Silt. Below is a table of types of soil and how foundation selection is done.
As a whole, there are 2 issues to discuss.
- First the particle size, and the bearing capacity that goes with it.
- Second the effect of water when it is affected by cohesiveness.
Sand and gravel would have big particle size and hard without much movement or compression, so it allows water to run through very well. It does not have cohesiveness. Its bearing capacity is high. Its settlement also very fast.
Clay and silt are small particle size and highly compressible. Water cannot run through them easily and they will swell up with water. They have cohesiveness. The load bearing capacity is much lower than sand and graver. They move easily but its settlement takes a long time.
Peat is the worst. Peat are decomposed grounds with lots of organic materials. The definition given in Wikipedia for peat is:
Peat (/piːt/), also called turf (/tɜːrf/), is an accumulation of partially decayedvegetation or organic matter that is unique to natural areas called peatlands, bogs, mires, moors, or muskegs.
So, peat and fill means the ground is decomposed organic materials, and it is filled at top layer with reclaimed ground.
Soil-bearing by description is a below:
*A good source of reference is by homebuilding & renovating - A guide to soil types here.
(i) Soft Clays (Raft or Reinforced Strip Foundation)
Soft clay is medium load-bearing.
The first 900-1,200mm layer of clay is subject to movement due to expansion and shrinkage depending on moisture content, so it is generally necessary to excavate foundations to a depth where the moisture content of the clay remains stable. British Standard 8004 recommends a minimum depth of 1m for foundations But if there are, or were, trees nearby, depths of up to 3m may be necessary.
In clay, prior to concreting the foundations, the trench is often protected from heave by lining it with a compressible layer (e.g. Clayboard).
A traditional strip foundation is sometimes acceptable but it is important not to overdig as this may increase the stress on the softer clay beneath. A common solution is to dig wide strip foundations with steel reinforcement — however an engineered foundation may be necessary.
(ii) Peat and fill (Raft Foundation)
This is the least load-bearing of the three. Peat and fill means it is a consolidated peat, a void which had been filled other soil material on top as to reclaim the land. Peat occurs in low-lying areas and consists of compressed dead vegetation that has been preserved from decay by acidic groundwater. Although the surface of the ground can appear stable and dry, peat may be present below the surface in a deep layer that will compress under the weight of a building.
If the presence of peat is suspected, consult a soil engineer. The extent and depth of the peat will need to be determined by drilling bore holes. Where there is only a thin layer of peat, it may be able to be removed to expose firmer soil below. Alternatively, a specifically designed raft foundation and floor slab may be needed.
Peat and loose waterlogged sand are very poor subsoils. If the peat can be stripped back to find suitable load-bearing ground of at least 1.5m depth, strip foundations may be suitable. A reinforced raft foundation will likely be required.
Ultimately, as it is stripped and fill, some test is necessary. Tests must be carried out to determine the bearing capacity of the fill. Generally, fill is unlikely to meet the required bearing capacity so foundations must pass through the fill down to solid bearing below.
(iii) Solid chalk and sands (Trench or Strip Foundation)
This is the hardest load-bearing compared to the above two.
Strip foundations are commonly used in chalk. Providing the chalk is not too soft, widths of 450mm for low-rise buildings are generally acceptable. The depth of the foundation must be below any frost action (700mm).