Foundation Construction [PDF]: Depth, Width, Layout, and Excavation

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Procedure for construction of foundation starts with a decision on its depth, width, and marking layout for excavation and centerline of foundation. Foundation is the part of the structure below the plinth level in direct contact of soil and transmits the load of superstructure to the ground.

Generally, it is below the ground level. If some part of the foundation is above ground level, it is also covered with earth filling. This portion of the structure is not in contact with air, light, etc., or to say that it is the hidden part of the structure.

Footing is a structure constructed in brickwork, masonry, or concrete under the base of a wall or column for distributing the load over a large area.

Depth of Foundation

Depth of foundation depends on following factors:

1. Availability of adequate bearing capacity.
2. Depth of shrinkage and swelling in case of clayey soils due to seasonal changes, which may cause considerable movements.
3. Depth of frost penetration in case of fine sand and silt.
4. Possibility of  excavation nearby
5. Depth of groundwater table
6. The minimum practical depth of foundation should not be less than 50 cm. To allow removal of topsoil and variations in ground level.

Hence the best-recommended depth of foundation is from 1.00 meter to 1.5 meter from the original ground level.

Width of Foundation / Footings

The width of footings should be laid according to structural design. For light loaded buildings such as houses, flats, school buildings, etc, have not more than two storeys, the width of the foundation is given below:

1. The width of the footing should not be less than 75 cm for one brick thick wall.
2. The width of the footing should not be less than 1 meter for one and a half brick wall.

Procedure for Construction of Foundation

The processes executed in the foundation works are given below:

1. Excavation of earthwork in trenches for foundation.
2. Layout cement concrete.
3. Lay the footing in case of raft or column construction.
4. Lay Anti termite treatment.
5. Lay Brickwork up to the plinth level.
6. Lay damp proof course on the walls.
7. Refilling of earth around the walls
8. Refilling of earth in the building portion up to the required height according to the plinth level.

Foundation Design Precautions

• A foundation should be designed to transmit combined dead load, imposed load, and wind load to the ground.
• The net loading intensity of pressure coming on the soil should not exceed the safe bearing capacity.
• Foundation should be designed in such a way that settlement to the ground is limited and uniform under the whole of the building to avoid damage to the structure.
• The whole design of the foundation, superstructure, and characteristics of the ground should be studied to obtain the economy in construction work.

Concrete and Mortar Ratio for Foundation

• The cement concrete 1:8:16 is generally used in the foundation of walls in construction work.
• In the case of column raft cement concrete, 1:4:8 is the best-recommended ratio for it in the foundation.
• For brick masonry, cement mortar 1:4 to 1:6 is used as a loading condition.

In the case of column and raft footings up to plinth level, cement concrete 1:2:4 or1:1.5:3 are used.

Safe Bearing Capacity Soil

Dry coarse and well-graded dense sand have maximum shear resistance and maximum bearing capacity. In general, submerged soil and clay have less bearing capacity.

Foundation Excavation Precautions

The depth and width of the foundation should be according to structural design.

• The minimum depth of the foundation is 1 meter in case the design is not available.
• Check the length, width, and depth of excavation with the help of centerline and level marked on the marking pillars.
• Dump the excavated material/ earth at a distance of 1 meter from the edges.
• Start excavation work when the soil is dry.
• Arrange a water pump to pump out rainwater.
• Compact the bottom layer of the foundation.
• There should be no soft places in the foundation due to roots etc.
• Dugout any soft/ defective spots, and fill the excavated area with concrete/ hard material

Demarcation/layout Procedure

The following procedure is recommended for the demarcation of a building:

1. Mark the baseline on the ground from the centerline of the road, or a permanent building nearby. This line helps to mark out the front of a building.
2. Use a side structure, road, first baseline, or plot boundary to mark side baselines of the building.
3. Fix temporary pegs at the centerline of walls/columns on both sides of walls and columns in front and backside.
4. Fix peg at the centerline of walls/columns on both sides of the walls and columns on the left and right side of the building front.
5. Check diagonals of the square or rectangle formed after fixing pegs.
6. Construct marking pillars with pegs at a distance of 1.5 meters to 2 meters, and plaster their top surface.
7. Mark center line on the top of marking pillars with the help of thread (Soot) or with the Theodolite in big projects and the diagonal, and check other dimensions.
8. Level marked pillars on all corners of the building.
9. Mark the foundation of walls/columns according to drawing on the ground with the help of centerline marked on the marking pillars.
10. Use chalk powder to mark the foundation trench on the ground.
11. Excavate the foundation of the walls /columns up to the required level, and check the excavation with the help of centerline and level marked pillars to avoid any complication later on.

Advantages Marking Pillars to Layout Buildings

• It saves time for measuring and setting the point again and again at the time of construction.
• It increases the efficiency of mason and foreman for doing their jobs.
• Accuracy can be checked at any time at any step.
• If any mistake is found, it can be easily be rectified at an early stage. It is very difficult to rectify the mistake later.
• Cross-check can be done by a senior engineer in minimum time.
• Qualitative work is maintained.

Disadvantages Construction Execution without Layout

At some sites of work the contractor brings steel pieces, erects the same on the ground, and start the work of excavation. In due course these steel pieces are just discarded. Thus no proper reference point is available while doing further jobs.

• It involves extra time for measuring the offset again and again.
• Accuracy can not be checked at an early stage and it will be very difficult to rectify the same in later stages.
• It involves wastage of time and money while doing rectification. This also leads to bad quality of work.

Equipment For Layout Setting

1. Leveling Instrument
2. Long Nails
3. Hammer
4. Right Angle
5. Steel Tape
6. Thin Cotton Thread
7. Bricks
8. Cement
9. Screen Sand
10. Lime Powder
11. Theodolite

FAQs on Foundation Constructions

What is the layout of the foundations?

Layout is a process of marking the location of foundation of new buildings on the ground.

What is the standard depth of foundations?

Standard depth of simple footing or foundation is 1.5m.

What are the factors influence the depth of foundations?

1. Adequate bearing capacity.
2. Frost penetration depth.
3. Groundwater table
4. Depth of shrinkage and swelling.
5. Nearby excavations.

What are the materials tools and equipment used in building layout?

1. Leveling Instrument
2. Long Nails
3. Hammer
4. Right Angle
5. Steel Tape
6. Thin Cotton Thread
7. Bricks
8. Cement
9. Screen Sand
10. Lime Powder
11. Theodolite

What are the benefits of the foundation layout?

• Saves time for measuring and setting the point again and again at the time of construction.
• Increases the efficiency of mason and foreman.
• Check accuracy at any time at any step.
• Rectify mistakes, if present, at an early stage.
• Cross-check can be done by a senior engineer in minimum time.
• Qualitative work is maintained.