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Selection of Foundation Type
Selection of Foundation Type
There are a number of foundation types available for geotechnical engineers: shallow foundations, mat foundations, pile foundations, and caisson foundations.
13.1
Shallow Foundations
Shallow foundations are the cheapest and most common type of foundation. See Fig. 13.1.
Figure 13.1 Shallow foundation Shallow foundations are ideal for situations when the soil immediately below the footing is strong enough to carry the building loads. In some situations, the soil immediately below the footing could be weak or compressible. In such situations, other foundation types need to be considered.
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Geotechnical Engineering Calculations and Rules of Thumb
13.2
Mat Foundations
Mat foundations are also k n o w n as raft foundations. Mat foundations, as the name implies, spread like a mat. The building load is distributed over a large area. See Figs. 13.2 and 13.3.
Figure 13.2 Mat foundations
Figure 13.3 Rebars for a mat foundation Source: http://www.integer-software.co.uk
13.3
Pile Foundations
Piles are used w h e n the load-bearing soil is at a greater depth. In such situations, the load has to be transferred to the load-bearing soil stratum. See Fig. 13.4.
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Figure 13.4 Pile foundation
13.4
Caissons
Caissons are nothing but larger piles. Instead of a pile group, a few large caissons can be utilized. In some situations, caissons could be the best alternative. See Fig. 13.5.
Figure 13.5 Caissons
13.5
Foundation Selection Criteria
Normally every attempt is made to construct shallow foundations. This is the cheapest and fastest foundation type. The designer should look into bearing capacity and settlement when considering shallow foundations. The geotechnical engineer needs to compute the bearing capacity of the soil immediately below the footing. If the bearing capacity is adequate, settlement needs to be computed. Settlement can be immediate or long term. Both immediate and long term settlements should be computed.
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Geotechnical Engineering Calculations and Rules of Thumb
Figure 13.6 Different foundation types
Figure 13.6 shows a shallow foundation, mat foundation, pile group, and a caisson. The geotechnical engineer needs to investigate the feasibility of designing a shallow foundation, due to its cheapness and ease of construction. In the above situation, it is clear that the weak soil layer just below the new fill may not be enough to support the shallow foundation. Settlement in weak soil due to loading of the footing also needs to be computed. If shallow foundations are not feasible, then other options need to be investigated. Mat foundations can be designed to carry large loads in the presence of weak soils. Unfortunately, cost is a major issue with mat foundations. Piles could be installed as shown in Fig. 13.7, ending in the bearing stratum. In this situation, it is important to be careful of the second weak layer of soil below the bearing stratum. Piles could fail due to punching into the weak stratum.
Figure 13.7 Punching failure (soil punching into the weak soil below due to pile load)
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Figure 13.8 Negative skin friction The engineer needs to consider negative skin friction due to the new fill layer. Negative skin friction would reduce the capacity of piles, as shown in Fig. 13.8. Due to the new load of the added fill material, weak soil layer 1 would consolidate and settle. The settling soil would drag piles down with it. This is known as negative skin friction or down drag.
There are a number of foundation types available for geotechnical engineers: shallow foundations, mat foundations, pile foundations, and caisson foundations.
13.1
Shallow Foundations
Shallow foundations are the cheapest and most common type of foundation. See Fig. 13.1.
Figure 13.1 Shallow foundation Shallow foundations are ideal for situations when the soil immediately below the footing is strong enough to carry the building loads. In some situations, the soil immediately below the footing could be weak or compressible. In such situations, other foundation types need to be considered.
172
Geotechnical Engineering Calculations and Rules of Thumb
13.2
Mat Foundations
Mat foundations are also k n o w n as raft foundations. Mat foundations, as the name implies, spread like a mat. The building load is distributed over a large area. See Figs. 13.2 and 13.3.
Figure 13.2 Mat foundations
Figure 13.3 Rebars for a mat foundation Source: http://www.integer-software.co.uk
13.3
Pile Foundations
Piles are used w h e n the load-bearing soil is at a greater depth. In such situations, the load has to be transferred to the load-bearing soil stratum. See Fig. 13.4.
Chapter 13 Selection of Foundation Type
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Figure 13.4 Pile foundation
13.4
Caissons
Caissons are nothing but larger piles. Instead of a pile group, a few large caissons can be utilized. In some situations, caissons could be the best alternative. See Fig. 13.5.
Figure 13.5 Caissons
13.5
Foundation Selection Criteria
Normally every attempt is made to construct shallow foundations. This is the cheapest and fastest foundation type. The designer should look into bearing capacity and settlement when considering shallow foundations. The geotechnical engineer needs to compute the bearing capacity of the soil immediately below the footing. If the bearing capacity is adequate, settlement needs to be computed. Settlement can be immediate or long term. Both immediate and long term settlements should be computed.
174
Geotechnical Engineering Calculations and Rules of Thumb
Figure 13.6 Different foundation types
Figure 13.6 shows a shallow foundation, mat foundation, pile group, and a caisson. The geotechnical engineer needs to investigate the feasibility of designing a shallow foundation, due to its cheapness and ease of construction. In the above situation, it is clear that the weak soil layer just below the new fill may not be enough to support the shallow foundation. Settlement in weak soil due to loading of the footing also needs to be computed. If shallow foundations are not feasible, then other options need to be investigated. Mat foundations can be designed to carry large loads in the presence of weak soils. Unfortunately, cost is a major issue with mat foundations. Piles could be installed as shown in Fig. 13.7, ending in the bearing stratum. In this situation, it is important to be careful of the second weak layer of soil below the bearing stratum. Piles could fail due to punching into the weak stratum.
Figure 13.7 Punching failure (soil punching into the weak soil below due to pile load)
Chapter 13 Selectionof Foundation Type
175
Figure 13.8 Negative skin friction The engineer needs to consider negative skin friction due to the new fill layer. Negative skin friction would reduce the capacity of piles, as shown in Fig. 13.8. Due to the new load of the added fill material, weak soil layer 1 would consolidate and settle. The settling soil would drag piles down with it. This is known as negative skin friction or down drag.