Selection of Foundations Based on Different Types of Soil

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Commonly, the selection of foundation type for a given structure is controlled by number of factors for example soil type, past site usage, adjacent construction, size of development process, limitations. Among these factors, types of soil play significant role, that why this article discuss the selection of foundation based on different types of soil.

Selection of foundation based on different types of soil

Foundations are recommended based on the different soil types which are provided below:

1. Rocks
2. Uniform firm and stiff clay
3. Soft clay
4. Peat

1. Rocks

This category involves rocks, hard sound chalk, sand and gravel, sand and gravel with little clay content, and dense silty sand.

Recommended foundation types

The following types of foundations are suitable for this type of soil:

1. Strip foundation
2. Pad foundation
3. Raft foundation.

Factors need to be considered

The following factors shall be taken into account while foundation type is selected:

• Minimum depth of 450mm shall be used for foundation if the area prone to frost so as to protect the foundation.
• Strip or trench base shall be kept above groundwater table if possible.
• Engineer shall be aware of running sand conditions.
• Sand slopes possibly eroded by surface water, so protect foundation by perimeter drainage.
• Weathered rock needs to be evaluated upon inspection
• Engineer shall be aware of swallow holes in chalks

2. Uniform firm and stiff clay

There are three cases which are discussed below:

Case I

where the foundation is not close to vegetation or existing vegetation is not important.

Recommended foundations

1. Strip foundation
2. Pad foundation
3. raft foundation.

Factors to be considered in this case are as follow:

• Minimum depth to underside of foundation shall be 900mm.
• When strip foundation is construction in desiccated clay in dry soil, then the foundation shall be loaded with building prior to the return of rains.

Case II

where trees, hedges, and shrubs are close to foundation location, or there is a plan to plant these trees near the structure in the future.

Recommended foundation

One of the following types of foundation may be selected:

1. concrete piles supporting reinforced concrete ground beams and precast concrete floor
2. concrete piles carrying in situ concrete slab
3. specially designed trench fills in certain clay soil based on the foundation location relative to trees
4. Raft foundation

Factors to be considered

• Engineer should be aware that size and type of pile foundation is controlled by economic factors.
• When in situ concrete ground slab is utilized, avoid must be created under the slab if it is laid in dry weather in desiccated clay.
• If the distance between recently planted tree and foundation position is at least one to two times mature tree height, then it might be suitable to construct strip foundation.
• In few cases, reinforced trench fill could be employed. For example, in clay of low to medium shrinkage potential, or in perimeter zone of the tree root system.

Case III

where trees are cut down shortly before the foundation construction begin

Recommended foundation

1. Reinforced concrete pile in previously tree root zone
2. Strip foundation
3. Raft foundation

Factor needed to be taken into accounted

• Piles shall be tied properly into suspended reinforced concrete slabs or ground beams.
• Sufficient length of slab shall be provided to withstand clay heave force. Added to that, top section of pile may be sleeved to decline friction and uplift.
• Special pile design may be needed for clay slopes greater than 1 in 10 because creep possibly occur. Consequently, lateral thrust and cantilever effect shall be taken into consideration in the pile design.
• In few cases, reinforced trench fill could be employed. For example, in clay of low to medium shrinkage potential, or in perimeter zone of the tree root system.

3. Soft clay

This category includes soft clay, soft silty clay, soft sandy clay, and soft silty sand.

Recommended foundations

The following types of foundations are valid option for this type of soil:

1. Wide strip footing
2. Raft foundation
3. Pile to firmer strata below
4. For smaller projects use pier and beam foundation to firm strata

Factors to be considered

• wide strip footing is used when bearing capacity is sufficient and predicted settlement is acceptable.
• Strip footing shall be reinforced based on the thickness and projection beyond wall face.
• Services entries to the buildings should be flexible.
• Frequently, Sub-soil can be improved using vibro treatment, and it would an economical solution if employed in conjunction with strip or raft foundation.

4. Peat

Recommended foundation

1. Concrete piles extended to the firm soil layer below
2. For small projects, pad and beam foundation taken to firm strata blow.
3. Raft foundation for the case where firm strata is not available at reasonable depth but there is hard surface crust with 3-4m thick of suitable bearing capacity.

Factor to be considered

• Pile types include bored cast in place with temporary casing, driven cast in place, and driven precast concrete.
• Allow for peat consolidation drag on piles
• If raft foundation is used, entries to the building shall be flexible.
• Special high grade and protection is likely to be required in aggressive peats.
• If peat layer is shallow over firm layer of soil, dig it out and replace it with compacted fill. For this, use raft or reinforced wide spread foundation dependent on anticipated settlement.
• Frequently, Sub-soil can be improved using vibro treatment, and it would an economical solution if employed in conjunction with strip or raft foundation.