CIVL1810 Chap.4 Foundations: Pads, Strips, Rafts and Piles
Foundations: Pads, Strips, Rafts and Piles
A foundation is the lowest level of a building, the substructure, and its job is to take the whole weight of the building and pass it down into the ground beneath. The choice follows from the soil, the building load, the design, the cost and the available technology.
The shallow family spreads load at the base of the column or wall, reducing the load per unit area imposed on the ground, while a piled foundation penetrates through incompetent upper layers to competent material below. Lean concrete sits under all of it as a working platform rather than as structure.
What this chapter covers
- 01
The substructure and its function of sustaining and transferring the entire building load
- 02
The five selection criteria: type of soil, building load, design, cost and technology
- 03
Shallow foundations that deliver load into the earth directly beneath a column or a wall
- 04
Deep foundations penetrating incompetent soil to reach competent load bearing soil or rock
- 05
Pad foundations: usually reinforced concrete and square in plan, unsuitable in loose sands, loose gravels and filled areas
- 06
Strip foundations for nearly any subsoil carrying modest load, the classic case being houses and low rise flats
- 07
Raft foundations spreading load over a large base where bearing capacity is low or column loads are heavy
- 08
Grade beams gathering wall load and delivering it to supports set at intervals such as pile caps or caissons
- 09
Lean concrete or blinding: a mix of about one to three to six, 50 to 75 mm thick, five to ten megapascals
- 10
Why blinding protects specified cover and is used to fill soft spots in the formation
- 11
The driven pile sequence of excavation for the pile cap, lifting on the lug, and hammering
- 12
The two stopping criteria: refusal, or a specified depth
- 13
Cast in situ bored piles compared with driven piles
Justifying a foundation type from a ground report
- +1Read the soil first. The upper 2.5 m is soft silt, which is incompetent material, but competent sand lies immediately beneath it and continues to depth.
- +1Test the shallow options. A pad is unsuitable in filled or loose material at founding level, and a raft spread across the footprint would still be bearing on soft silt, so spreading the load buys no capacity here. A strip has the same problem.
- +1Consider excavating through. Because the silt is only 2.5 m deep, one legitimate option is to excavate through it and found pads directly on the medium dense sand, which uses the shallow family without relying on the poor material. This is the cheaper answer where the excavation is manageable, and the tight budget makes it attractive.
- +1State the alternative and the deciding factor. If groundwater or excavation support made that dig impractical, short piles taken to the sand with a grade beam under any bearing walls would be the answer. The deciding factors are cost and technology, the two project criteria, since both options satisfy the soil and load criteria.
Key terms
- Substructure
- The lowest level of a building, which has to take the whole weight of the building and pass it down into the ground beneath.
- Pad foundation
- A usually square, usually reinforced concrete footing under a single column. It suits most subsoils but is excluded from loose sands, loose gravels and filled areas.
- Raft foundation
- A foundation that opens the load of the structure above out across a wide base, so the pressure the ground actually feels is smaller. It suits weak ground and heavy loads arriving at single columns.
- Grade beam
- A reinforced beam that gathers the load of a wall standing on it and delivers it to supports set at intervals, typically pile caps or caissons. It shares load out rather than bearing it.
- Lean concrete
- Known on site as blinding, or sometimes as a mud mat. A weak mix of about one cement, three sand and six aggregate, taken by volume, typically 50 to 75 mm thick and about five to ten megapascals, laid to give a clean, level, dry working platform.
- Pile cap
- The element cast over a group of piles to tie them together and receive the column load. Excavation on a piling job is set by the level needed to construct the cap.
Foundations: Pads, Strips, Rafts and Piles FAQ
When does a raft stop being the right answer?
When the weak material continues below it. A raft opens the load out across a wide base so the pressure the ground feels is lower, so it helps where a poor layer is thin or where single column loads are heavy on otherwise adequate ground. If the ground is weak all the way down, spreading the load changes the pressure but not the material carrying it, and the answer becomes piles.
What is blinding for, if it is not structural?
It gives you somewhere clean, flat and dry to work on top of dug ground before any steel goes down, and it keeps bars and fresh structural concrete off the soil so the cover you designed survives. It is also used to fill soft spots in the formation. Strength is explicitly not the objective, which is why a mix of about five to ten megapascals is adequate.
Why does driving stop at two different kinds of criterion?
Refusal is a resistance criterion, in which the ground tells you the pile can go no further and thereby gives evidence that competent material has been reached. A specified depth is a geometric criterion set by the design. Naming both, and explaining that the first is evidence rather than an instruction, turns a description of piling into an explanation of it.
How do driven and bored piles differ on site?
A driven pile is pre formed and hammered in, and the subsoil around the shaft is displaced. A cast in situ bored pile is formed by making the hole first, then placing and installing the pile, and repeating the process along the line, so material is removed rather than displaced. Displacement matters near existing structures, because it moves the ground around neighbouring foundations.
Exam move
Build a single comparison table with a row for each foundation type and columns for what it is, where it suits, and its limit, then learn the sentence that unites the shallow family: each of them lowers the pressure the ground actually feels, at a point, along a line, or across the whole footprint. Memorise the four blinding numbers, because they are cheap marks and they recur in the slab sequence.
For piles, rehearse the three stage sequence with a sketch for each stage and label the lifting lug, the drop hammer, the displaced subsoil and the pile cap. Practise selection questions by writing one ground reason and one project reason for every recommendation you make.
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