Excavation of Earthwork for Building Foundation

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You can’t build a great building in a weak foundation. You must have a strong foundation if you want a strong building. For every engineering structure, the foundation must withstand and overcome possible loads.

In this article, you will learn step by step process of excavation of earthwork for the building foundation.

A) Preliminary works before excavation of Earthwork

i) Clearing the site

The whole area where the excavation work is to be done shall be cleared first. The materials like grass, roots of trees, loose stones, rubbish, stumps, and other organic material should be cleaned. The possible useful materials are collected and kept. And the useless materials are burned or disposed as per the direction of the site engineer.

ii) Setting out

Once the foundation site is cleared, the setting out work is done on the guidance of the Engineer. The Contractor is responsible for establishing reference marks, benchmark, and to see that it is maintained as long as required. Establishing references in the site is performed by the contractor’s own cost.

B) Excavation work for building foundation

i) Excavation

The excavation work is carried cut exactly in accordance with the dimensions shown by the Engineer in the plans. Excavation shall be taken to the exact width of the lowest step of footing and sides shall be left plumb where the nature of soil admits it. When the soil conditions do not support vertical sides it shall be sloped backward or protected with timber shoring. The necessary shoring cr cutting of slopes to a safe angle or both shall be done by the contractor himself.

ii) Preparation of foundation for footing

Keeping the sides of the trench perfectly vertical, the bottom of the foundation shall be well leveled. Where the soil is soft, loose or slushy, the trench shall be widened for allowing steps on either side or sides sloped or shored up.

The final surface shall be lightly watered and rammed before footing is laid. If the excavation is made deeper by mistake or when soft patches being dugout then no filling shall be done to bring the foundation to the level but the extra depth shall be made up with concrete masonry of the foundation grade.

Engineer shall be notified once the excavation is completed, and footing shall be laid only after the Engineer’s approval.

iii) Shoring

To resist side pressure and ensure safety from steps and blows and to prevent
damage to work and property and injury to a person, shoring shall be done. The design of shoring shall have strength enough to withstand pressures.

Unless mentioned in the contract shoring will be totally on the contractor’s cost.

C) Clearing on completion of excavation of earthwork

i) Protection

For the safety of excavation, workers and other people in the vicinity, basic and suitable precautions should be taken. Foundation pits, well pits other excavations shall be properly fenced and marked so that it is visible even at night.

Similarly, it shall be seen that excavation for the foundation does not affect or damage the adjacent structure. All measures shall be taken by the contractor at his own cost and he shall bear the responsibility of any damage to property or any harm to human life.

ii) Disposal of excavated materials

The waste material producing by excavation shall not be placed even temporariƂy nearer than 1.5m. Useful materials shall be stacked conveniently for refilling. For backfilling, materials should be placed in 15 to 20 cm layers, moistened and well compacted.

The rubble produced by excavation if found useful by the contractor for other work shall be paid at the rate fixed in tender and if not provided, at the rate which both partners mutually agreed.

iii) Dewatering

Water in trenches must be pumped out during the progress of work, which may be accumulated by rain, seepage, etc. Pumping shall be done such that foundation trenches, concrete, masonry or any adjacent structure won’t be adversely affected.

Excavation shall be free from water:

-when the inspection or measurement is being carried out.
-till the concrete or masonry in progress comes above the natural water table.
Care shall be taken that no water enters into the trench.

iv) Backfilling

After concrete or masonry is fully set, all timber work and formwork shall be removed and the space in between foundation and excavation is refilled by the suitable backfilling material. The refiling shall be done in layers of 15 to 20 cm, watered and rammed to compact and done such that it does not exert pressure on any part of the structure.

D) Measurement and payment of excavation

The measurement in the excavation shall be made at the contractor’s rate cubic meters (cubic feet). When dug in excess it shall not be paid and shall be filled with concrete or masonry of foundation grade. Small drill holes made to explore the nature of substratum shall be considered incidental.

The excavation of the additional depth of 1.5 m shall be paid as per tender rate and for excavation beyond 1.5 m payment shall be done as per the general conditions of the contract.

Video Courtesy: 360 Construction

Also Read: Basic Skills Required for Civil Engineers

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