Shallow, direct or shallow foundations are built on superficial soil layers at a shallow depth (up to 1.5 m), however they support structural loads and the depth of the foundation is less than its width.
Types of Shallow Foundations
The various types of foundations can be grouped into two broad categories: shallow foundations and deep foundations.
The classification indicates the depth of the foundations in relation to their size and the depth of the soil that provides most of the support.
According to Terzaghi, a foundation is shallow if its depth is equal to or less than its width, and depth when it exceeds the width.
The “floating foundation”, a special category, is not actually a different type, but represents a special application of a principle of soil mechanics to a combination slab-caisson foundation.
A brief description of these types of foundations will now be given.
Straight or extended footings
the base of unza running or extended footing it is basically a fill used to “spread” the loads of the walls or columns over a large enough area of subgrade.
These are built as close to the ground surface as possible in accordance with design requirements and with factors such as frost penetration depth and potential for soil erosion.
Foundations for permanent structures are rarely located directly on the ground surface.
A stripped or extended footing does not necessarily have to be at shallow depths; it can be located deep in the soil if soil conditions or design criteria require it.
The extended footing required to support a wall is known as a continuous, wall, or strip footing, while that required to support a column is known as single or isolated footing.
A isolated footing it can be square, circular, or rectangular in shape in plan, depending on factors such as the shape of the column plan and space limitations.
If the footing supports more than one column or wall, it will be a combination footing or a slab foundation.
The common types of spread footing mentioned above are shown in the figure below.
Two different types are also shown: the monolithic base, used for a watertight base (also to resist lifting), and the grid base, used for heavy loads.
Insulated footings
The isolated footings they are built under an independent column, they can have a square, rectangular or circular shape and they are applied where the bearing capacity of the soil is high; the isolated footings they form a thick slab that can be flat, stepped, or sloped.
Combined footings
The combined footing It admits two columns and is used when these columns are very close such that it would mean that their individual foundations overlap. This type of footing can be rectangular or trapezoidal and has the advantage that it distributes loads evenly.
You can learn more about combination footings by clicking here.
Cantilever footing
The cantilever footings They consist of two isolated footings connected to their bases by a cantilever (rigid beam) in order for them to behave as a single unit. A cantilevered footing is more economical than a combined footing when the pressure supported by the soil is high and the distance between the columns is wide.
Foundation slabs
A Foundation slab (slab foundation) is a combined slab foundation that covers the entire area under a structure and supports all walls and columns.
This type of foundation is more appropriate and suitable when the allowable ground pressure is low, or the load is heavy, and the spread footings would cover more than half of the plan area.
You can learn more about foundation slabs by clicking here.
Difference Between Shallow and Deep Foundations
| Characteristic | shallow foundation | deep foundation |
|---|---|---|
| Definition | Foundations that are placed close to the earth’s surface or transfer loads to shallow depths are called shallow foundations. | Foundations that are placed deeper or transfer loads to deeper strata are called deep foundations. |
| The depth of the foundation | Between 1.5 to 3 meters on average | It is greater than the surface base |
| Cost | It is cheaper | It is more expensive |
| Feasibility | They are easier to build | Its difficulty is much greater |
| Advantage | Construction materials are readily available, less labor is needed, the construction procedure is simple at an affordable cost, etc. | Foundation can be provided at greater depth, provides lateral support and resists uplift, effective where shallow foundation is not possible, can carry enormous load, etc. |
| Disadvantages | Possibility of settlement, generally applicable for light structure, weak against lateral loads, etc. | More expensive, need skilled labor, complex construction procedures, it may take a long time, and some types of deep foundation are not very flexible, etc. |
| Types | Foundation footings and slabs | Piles, pillars, floating slabs, caissons |