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Selection criteria for foundation for buildings depend on two factors, i.e. factors related to ground (soil) conditions and factors related to loads from the structure. The performance of foundation is based on interface between the loadings from the structure and the supporting ground or strata.
The nature and conditions of each of these varies, so, the selection of appropriate foundation becomes necessary for these variations depending on circumstances.
Foundation Selection Criteria for Buildings
1. Loads of Structure
Loading conditions coming from the structure is one of the factors that govern foundation selection. It is influenced by type of building, type of construction material, environmental factors, earthquake susceptibility.
Selection of material such as bricks, stone, steel, and concrete for construction affect the selection of foundation. Settlement calculation of foundation is another factor that associated with loads of the structure and affects foundation choice.
In case of low-rise building, shallow foundation is preferred. However, for high-rise building, deep foundation is required. Deep foundation is provided because ground at greater depth are highly compacted.
2. Soil Bearing Capacity
Soil bearing capacity is one of the most crucial criteria that affect the selection of the appropriate type of foundation. Decision can be made to choose shallow or deep foundation based on the soil bearing pressure.
An allowable bearing pressure of at least 100kN/m^2 or higher is effective for shallow foundations up to 4 stories. However, higher structures can consider a raft foundation provided that the modulus of subgrade reaction shall not be exceeded when calculated.
3. Soil Types
There are different types of soil like sandy soil, loose soil, clayey soil, and expansive soils. Form the surface to a depth of 3m is termed as top soil, and subsoil begins after 3m depth. Characteristics specifically bearing capacity of both topsoil and subsoil layers would to a great extent controls the foundation selection.
For low bearing capacity soil, stronger and suitable foundation shall be selected compare to with case where the soil has strength bearing capacity. The bearing capacity is the ability of soil to safely support structural loads without suffering shear failure and intolerable settlement.
3.1 Clay Soil
Clay soil has great capacity for water retention that is why great expansion and shrinkage are expected in this type of soil. As a result, foundation structure can suffer from great settlement and uplift pressure that is why clay soil is not desirable. Applicable codes such as British standard recommends minimum depth of 1m for foundation and 3m if there are trees around.
Raft/Mat foundation is the best foundation type to be built on clay soil, and ribs and beams can be incorporated into it to increase its stiffness. If raft foundation is costly, imposed loads are large, or strong soil layer is not available at shallow depth, then under reamed pile should be selected.
In clay soil, it is recommended to collect and drain rainwater, extend foundation to a depth where moisture fluctuation does not occur, remove weak and shallow soil layer such as black cotton soil, execute construction during dry season if possible, and distribute structural loads as uniform as possible.
In the case where a shallow firm soil layer cover soft clay soil layer, it is advisable to use wide reinforced strip foundation. In this manner, the affect of loads on weak soil layer is reduced. Pile foundations are recommended for high rise buildings and whenever uplift is expected.
3.2 Peat Soil
It is considerably porous, easily compressible, and dark brown or black color soil which is commonly present near wetlands. It undergoes expansion and shrinkage due to moisture fluctuation, extremely weak in terms or load carrying capacity.
So, it should be either removed strong strata and strip foundation is good option in this case. If the thickness of peat soil is great and its removal is not economical, then other foundation types should be selected.
For instance, concrete piles extended to the firm soil layer below, pad and beam foundation took to firm strata blow for small projects, or raft foundation for the case where firm strata are not available at reasonable depth but there is hard surface crust with 3-4m thick of suitable bearing capacity.
Silty soil, which is smooth to touch, is generally not suitable for foundation structure because of its expansion which exert pressure against foundation and damage it.
The silt retain moisture and does not drain water easily. Reinforced concrete spread and isolated pad footings are appropriate if silt or silty clay is stiff and extends to a great depth. The depth of the foundation should be greater than the zone of erosion and the zone of swelling and shrinkage.
3.4 Sand and Gravel
Sand and gravel allow water drainage that is why do not cause structural movement. Moist compaction of soil and sand make good support for foundation structure.
Dry compact gravel, or gravel and sand subsoils are adequate for spread and strip foundations. Generally, a depth of 700mm is acceptable, as long as the ground has adequate bearing capacity.
If gravel is submerged in water, the bearing capacity is declined by half. That is why it is important to keep the foundations as high as possible. A shallow, reinforced, wide strip foundation may be suitable.
Sand holds together reasonably well when damp, compacted and uniform, but trenches may collapse and so sheet piling is often used to retain the ground in trenches until the concrete is poured.
If loose sand is extended for great depth, then it is recommended to compact it and use spread footing. Alternatively, raft, driven pile, augured pile, or cast in place pile can be selected without the use of compaction energy.
Generally, rocks such as limestone, bedrock, and sandstone have substantially high bearing capacities. This makes it suitable for supporting foundations of commercial and residential buildings.
Loam is the best option to support foundation because of its uniformly balanced characteristics. It maintains water at balanced rate and hence neither shrink or expand to an extend that damage foundations. Loam is a combination of clay, silt, and sand, and dark in color and soft; dry; and crumbly to the touch.
Isolated footing is the most desired type of foundation for loam soil. The depth and area of foundation is dependent on bearing capacity, depth of groundwater table, and depth of load bearing stratum.
3.7 Previously Used Soil
If the site previously used for buildings such residential, mining, industrial, or other structures, then obstruction such as ground contamination can hinder immediate excavation works. Such considerations need to be taken into account while the type of foundation is selected.
If for instance loose sand extends to a large depth, then raft foundation can be selected to evenly distribute the load, and pile foundation would be required for heavy structures such as multi story buildings.
3.8 Mountain Soils and Forest Soils
Mountain soils and forest soils are most likely to have boulders at various depths, so pile foundations are not suitable. For residential buildings with one or two floors, Isolated reinforced pad foundations are suitable. For single storey buildings, reinforced concrete strip foundations are suitable.
4. Ease of Construction
The choice of foundation should be made keeping in mind the ease with which it can be executed/constructed at the site. The construction of different types of foundation requires labor of different skillset and different level of skills.
5. Water Table Level
Water table is another significant criterion that affect the foundation selection. foundation should not be placed on soil that undergo expansion and contraction due to water table fluctuation. So, it should be construction on fully dry soil or on fully wet soil.
If the groundwater table is located below the formation level of the foundation, then a shallow footing like isolated or combined footing should be chosen. Moreover, for higher water table, raft/mat footing should be proposed.
This is to counteract uplift pressure and counter the effect caused by water to avoid any overturning moments at the early stage of construction. If this option cannot be executed, then deep foundations like micro piles or bored piles should be considered to provide the necessary uplift resistance.
6. Adjoining Structure/Water Bodies/Slopes
Adjoining structures, water bodies, slopes are parameters that need to be considered for selection of foundation types. When foundation of the neighboring structure is very close the foundation to be construction, it may influence the choice and the safety of the adjoining structure.
If the foundations of an adjoining structure are very near to the target foundation, it may affect the choice as the safety of adjoining structure is of paramount importance. the selection of proper foundation type would be more challenging it the neighboring structure is high rise building or an old property.
Factors such as closeness to a river, lake, natural drain, or vicinity to a slope should be taken in counting. Isolated RCC pad foundations under stilts are usually preferred for buildings on sloping grounds. No residential buildings should be constructed on slopes steeper than 25 degrees.
7. Natural Disaster and Extreme Weather
If the area has a history of severe natural events or extreme weather, then these parameters should be selected taken these parameters into considerations. The past record or data of natural disasters and extreme weather should be employed as a base for foundation selection.
8. Economic Design
When there are number of feasible foundation choices for the given project site, economical factor may influence the selection of the foundation. Nonetheless, choosing an economical foundation should not compromise the safety, workmanship, strength, and durability of the foundation.