Shallow foundations (open foundations)

SHALLOW FOUNDATIONS (Open Foundations)

A Shallow Foundation is a foundation in which depth is equal to or less than its width. It is built by open excavation of the soil. Hence, it is also known as Open Foundation. The base of the structure is enlarged or spread to provide good and individual support to the load.

This foundation is provided for structures of moderate height, built on sufficiently firm dry ground. It is used up to a depth of 3 m to 4 m. It is convenient above the water-table.

Types of Shallow Foundations: 1. Spread Foundations and 2. Raft or Mat Foundations.

 

1. SPREAD FOUNDATIONS

Spread foundation is the foundation in which the load of the structure is spread over a large area. Intensity of the soil pressure induced should be less than the safe bearing capacity of the soil. Spread foundation is constructed of masonry, plain concrete or reinforced cement concrete.

Uses: This is the cheapest type of foundation. It is largely used for ordinary buildings.

Types: 1. Wall Footings, 2. Isolated or Column Footings, 3. Combined Footings and 4. Continuous Footing.

1. Wall Footings

Wall Footing is provided throughout the length of the wall in case of load bearing wall. It is used where soil of good bearing capacity is available at a depth of equal to or less than 3 m from Ground Level.

The wall footings are of two types:

(i) Simple Footing: See Fig. 2.

It is provided to carry light loads. It has one projection (called Offset) on either side of the wall. The depth of concrete bed is about twice the Offset offset.

In this type, lime or plain cement concrete (1:3:6 or 1:4:8 mix) is used in the foundation bed. The wall (ii) Elevation may be of brick or stone masonry.

Uses: It is used in small residential buildings such as tiled houses, pump house, watchman cabin, etc.

(ii) Stepped Footing: See Fig. 3.

When the foundation width is considerably more than the thickness of the wall, then the footings should be stepped for transmission of the load. It is called Stepped Footing. Also, stepped footings may be provided when the ground has a slope. Otherwise, it becomes uneconomical to provide foundations at the same level on the sloping ground.

In each stepping, the projection (offset) of the step should be about one-fourth of the length of the brick. The footings may be two or more steps of brick or stone masonry. At the bottom of the steps, a concrete bed is provided.

Width of Footing: Width of Footing = Total Load per meter nu in N / Safe Bearing Capacity of soil in N/m²

where Total Load = Dead Loads + Live Loads + Wind Load, etc.

Depth of Footing: Minimum depth of footing for the load bearing wall should be limited to 900 mm for the stability criteria.

Use: Using R.C.C. concrete bed in the stepped footings, buildings of maximum number of floors up to three may be built.

2. Isolated or Column Footing

A footing that supports a single column to transfer the load of the structure safely to soil bed is known as Isolated or Column Footing.

Column Footings may be square, rectangular or circular in shape in plan, depending upon the shape of the column and constrains of space. Square footings are economical for square and circular columns. Under rectangular column, rectangular footings are considered to be more appropriate.

Column footings are (i) Simple, (ii) Stepped and (iii) Sloped types. In the case of heavy loaded columns, steel reinforcement is provided in both the directions in the concrete bed.

Main Reinforcement Rods: These are placed parallel to the width of foundation bed.

Distributors: These are placed perpendicular to the main rods.

(i) Simple Footing: See Fig. 4. If the load on the column is light, a spread is given under the base of the column. This spread is defined as Simple Footing.

(ii) Stepped Footings: [Fig. 5] For heavy loaded column, the total width of the footing is very high. This is attained in three or four steps. This arrangement is called Stepped Footings.

(iii) Sloped Footing: See Fig. 6. Concrete can be moulded to any shape. Therefore, a concrete footing may be constructed as a sloping one to provide sufficient spread under the column. This arrangement is called Sloped Footing.

3. Combined Footings

A footing which supports two or more columns is termed as Combined Footing. Combined footings are proportioned such that the center of gravity of the loads of the structure coincides with the center of gravity of the foundation.

By this arrangement, the load of the two columns will be evenly distributed to the soil. Therefore, the combined footings have either a Rectangular Shape or Trapezoidal Shape in plan.

Rectangular Combined Footings

See Fig. 7. If the column loads are equal ( P = Q), or the interiror column carries greater load (P>Q), then Rectangular combined Footings are used.

Trapezoidal Combined Footing

See Fig. 8. If the column loads are unequal and the external column near the property (boundary) line is heavily loaded, Trapezoidal Combined Footings are used.

Combined footings are used under the following circumstances:

• When the space between two columns is so small that separate footings for individual columns will overlap.

•Very often, a column is to be provided near the edge (boundary) of some property. It may not be permissible to extend the footing beyond a certain limit. In such a case, the load on the footing will be eccentric. This will result in uneven distribution of load to the sub-soil. Alternatively, a combined footing to support the edge column and an interior column close to it may be provided.

• When the bearing capacity of the soil is so low that individual column footing is of uneconomic size.

4. Continuous Footing

See Fig. 9. In Continuous Footing Type, a single continuous R.C.C. slab is provided as the foundation of two or three or more columns in a row.

This footing prevents differential settlement in the structure. Also, it is suitable at location liable to earthquake.

 

2. MAT or RAFT FOUNDATIONS

Mat is like a large spread footing, covering the entire building area. All building loads are supported on a common mat. Mat is the common foundation provided for all the columns of the building.

Types of Mat Foundations

See Fig. 10(i). When the columns occur closely and the supporting soil is not two compressible, mat foundation consisting of only a flat R.C.C. slab of uniform thickness is adopted.

See Fig. 10(ii). When the columns are heavily loaded, the slab may be thickened below the columns. In addition, Pedestals may also be constructed as shown.

See Fig. 10(iii). This type of mat foundation consists of Mat (Raft) Slab, Main Beams, Secondary Beams and Columns Columns transmit the loads to the Main Beams. The Main Beams transmit the loads to the Secondary Beams.

The Secondary Beams transmit loads to the R.C.C. Slab. The R.C.C. Slab ultimately transmits the loads to the supporting soil. By combining all individual footings into one large mat, the unit pressure in the sub-soil is reduced. Hence, the bearing capacity of soil is increased.

Uses of Mat Foundations

1. It is used where the soil has low bearing capacity.

2. The mat reduces the possibility of differential (unequal) settlement.

3. It provides a condition of uniform settlement.

4. It is more economical.