A shallow foundation is a type of building foundation that transfers building loads to the earth very near to the surface, rather than to a subsurface layer or a range of depths as does a deep foundation.
Shallow foundations include spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations and earthbag foundations.
Shallow foundations are also called spread footings or open footings. The ‘open’ refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing.
Define Shallow Foundation
What is Shallow Foundation in civil engineering? A shallow foundation generally is defined as a foundation that bears at a depth less than about two times its width. There is a wide variety of shallow foundations. The most commonly used ones are isolated spread footings, continuous strip footings, and mat foundations.
Many shallow foundations are placed on reinforced concrete pads or mats, with the bottom of the foundation only a few feet below the ground surface.
The engineer will select the relatively inexpensive shallow foundation for support of the applied loads if analyses show that the near-surface soils can sustain the loads with an appropriate factory of safety and with acceptable short-term and long-term movement.
A shallow excavation can be made by earth-moving equipment, and many soils allow vertical cuts so that formwork is unnecessary.
Shallow Foundation Design
The three basic requirements or shallow foundation design criteria are as follows:
Placement of the foundation, which involves the depth and location of foundation.
Safety against bearing capacity is a requirement that includes suitable proportioning of the footing to avoid a catastrophic failure of the soil beneath the foundation.
Every soil settle due to load. Such settlement should be uniform and permissible. Tolerable foundation settlement includes keeping a check on the excessive settlement of a structure.
Shallow Foundation Pros and Cons
We use shallow foundation to distribute the structural loads over a wide horizontal area at shallow depth below the ground level. Shallow foundation is favorable for foundations having depth as equal as foundation width or where depth is less than the width.
Advantages of Shallow Foundation
- It requires less excavation hence reduce labour cost of excavation work.
- Construction of shallow foundations is simple as the depth involved in placing the foundation is less.
- Equipment required for construction of shallow foundation are simple and also less costly.
- Shallow foundations can be constructed in a short time, which also helps in reducing the cost of hiring equipment and labor.
- Construction of shallow foundations would cause lesser disturbance to geo-surface and hence good for ecology and environment.
- It helps to reduce settlement, if soil is compressive.
- No piling is required, which reduce the cost.
- There is less uncertainty in the prediction of behavior of shallow foundations and supporting soil. For deep foundations as the depth of soil involved is more, the uncertainty will be also more.
Disadvantages of Shallow Foundation
- There is a chance of scouring if the structure is near river or sea. Shallow foundation cannot be used at such places.
- If sub-soil water level is high and it is uneconomical to pump out the water from the pit or canal then the shallow foundation cannot be used.
- It cannot be used where the bearing capacity of top surface soil is less.
- It cannot be used when weight of structure is high and load of the structure is distributed unequally.
During the early stages of work, the entire footing is visible to the eye, and is therefore called an open foundation.
The idea is that each footing takes the concentrated load of the column and spreads it out over a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of the soil.
There are several kinds of shallow footings: individual footings, strip footings and raft foundations.
In cold climates, shallow foundations must be protected from freezing. This is because water in the soil around the foundation can freeze and expand, thereby damaging the foundation.
These foundations should be built below the frost line, which is the level in the ground above which freezing occurs.
If they cannot be built below the frost line, they should be protected by insulation: normally a little heat from the building will permeate into the soil and prevent freezing.
Shallow Foundation Types
The different types of shallow foundations are:
- Spread or Isolated Footing;
- Strip Foundation;
- Mat or Raft Foundation;
- Combined Foundation.
Spread footings, as the name implies, widen the base of the element delivering load to the earth to distribute the weight over a larger area. The several types of spread footings are as follows:
- Column footings
- Inverted arch footings
- Reinforced concrete footings
- Wall footings
Two or more columns in a row can be supported by a combined footing. If both columns carry equal weights, the combined footing can be rectangular, but if space is limited, it can be trapezoidal. Read the article ” Combined Footing – Basic to Advanced Guide ” to learn more about combined footing.
Mat or Raft Foundation
A raft or mat foundation supports all of the columns and covers the entire constructed area beneath a structure. Spread footings would cover more than half of the building area when the structure loads are strong or the permissible soil pressure is low, then a raft foundation may be more cost-effective. When the soil mass comprises compressible lenses and differential settlement is difficult to manage, raft foundations are used. A raft foundation is used when hard soil with good size is not available between 1.5 to 2.5 m deep. Read the article “WHAT IS PILED RAFT FOUNDATION & 2 GREAT EXAMPLES OF IT” to learn about the many forms of raft foundations.
Grillage foundations are utilised to carry heavy loads from steel columns to low-bearing-power soils. This solution avoids deep excavations while still providing the necessary area at the base to lower the intensity of pressure while keeping it within the soil’s safe bearing capacity. Grillage foundations can be classed into two kinds based on the materials used in their construction.
- Steel Grillage
- Timber Grillage
Eccentrically Loaded Footing
Eccentrically Loaded Footing is made up of two separate footings that are joined by a structural lever or a strap. The strap joins the footing so that they function as a single unit. The strap’s sole purpose is to serve as a connecting beam. An eccentric loaded footing is more cost-effective than a combined footing when the permitted soil pressure is comparable high and the distance between the columns is large.
As a result, a shallow foundation is a type of building foundation that transfers loads to the soil relatively close to the earth’s surface. Shallow foundations have the goal of distributing structural loads over a large horizontal area at a shallow depth below ground level. High-rise constructions may or may not be acceptable for shallow foundations. It will be determined by the building’s height and the type of soil.