Excavation is the process of creating an open face, trench, or cavity to remove soil or rock. It involves layers of procedures done at various stages of the building process. Site preparation, setting out, excavation, and safety precautions depending on the depth of excavation are all part of the excavation process for foundation construction. It can be done using tools, machinery, or explosives. Since there is an ocean of terminologies and procedures, it is necessary to know the basic yet important aspects of excavation of foundation.
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Settlement of Foundation
One area of a building's foundation may settle at a different rate and to a different extent than another area of the foundation as a result of possible ground movements beneath it. To limit the damage to the building's superstructure, this differential settlement must be minimised as much as possible. While a rigidly framed structure with panels cannot allow the same extent of differential movement of the foundation of the structure, some structural forms can do so by allowing minor movement of the small brick units and mortar joints. On clayey soils, settlement is generally greater and more continuous over time.
Basic Excavation Terminology
Shoring is the process where temporary supports are propped in the trench, which prevents the sides from collapsing during construction. It is mainly used for construction that requires up to 20-foot-deep excavation, road construction, underground utilities, etc. Residential projects built on loose soil also require shoring.
Planking is the process of placing temporary timber supports at the sides of an excavated trench. It can be either open planking (intermittent) or closed planking. If the soil is soft or loose, closed planking is preferable, where horizontal planks are placed.
The process of strutting involves laying vertical sheets, also known as sheeting or polling boards, against the trench's two walls and securing them in place with one or more rows of struts. It can be done only if the depth of the trench does not exceed 2 meters. It is mostly done for construction that requires a deep foundation.
Dredging is the process of excavating materials underwater. It is usually done to reshape land and water forms, to excavate minerals, to construct dams, etc.
Grading is done to level the base of a trench or to form a particular slope. It is done before laying the foundation, landscaping, storm water drainage, etc.
Trenching is a narrow excavation below the earth's surface where the depth is greater than its width. The width cannot be greater than 4.6 metres.
The Procedure for Excavating a Foundation
1. Small Buildings
The foundation can be excavated using manual methods and tools for construction such as pick axes, spades, crow bars, etc.
2. Loose Soil/ Deep Excavation
Shoring, planking and strutting are suitable for deep foundations or when built on soil that is unsuitable.
3. Hard Soil
If the depth of excavation is less than 1.5 metres, the sides of the trench do not require external support.
Machinery Used for Excavation
The use of heavy machinery comes for deep excavation, where manual excavation with a pickaxe won’t suffice. The advantage of this machinery is that it is time-saving and flexible. They can reduce the need for labour and require about 1-2 people to operate the machinery.
Tractors are the basic form of a bulldozer, backhoe, scraper, and digger. The machine is flexible in terms of adding parts as needed.
They are heavy, motorised equipment with metal blades, used for pushing earth and rocks for construction and road building.
Backhoes have equipment attached to the front and back. It has a long arm and a bucket-shaped shovel to move earth.
Power shovel has a bucket shaped equipment used to dig and load earth or rocks. It is mainly used to relocate earth after excavation.
Scrapers are used to scrape the ground's surface as part of site preparation for construction or excavation.
Draglines are heavy equipment that has a rope and bucket and can dig earth from greater depths.
Clamshell excavators have buckets with jaws which can be used for collecting clayey soil or water.
Rippers are tractors with an arm attached that is suitable for moving rocks instead of blasting or drilling, which can be time consuming and costly.
Points to Remember in the Excavation and Filling of the Basement
The bottom of foundation pits should be dressed level in all directions before any concrete is put in, and they should be well watered and thoroughly rammed.
The surface to receive the filling should first be cleared of all the roots, vegetation, and spoil before it is thoroughly wetted.
Filling up to plinth level should be done in layers as the building is built, so that the earth filling is thoroughly consolidated by the trampling of the workers.
The basement fill should have an optimum moisture content. It should be well consolidated by ramming it into 15-cm layers using iron rammers and the butted ends of crowbars.
When filling reaches a finished level, the surface should be flooded with water for at least 24 hours, allowed to dry, and then rammed and consolidated in order to avoid any settlement at a later stage.
If necessary, protection against slides and caving should be provided.
Excavation is one of the first steps towards constructing a building foundation, reservoirs, and roads. There are various stages during the excavation of the foundation, which must be done very cautiously so as to get better results. At the same time, using the right kind of equipment and machinery can speed up the process and reduce the time drastically. In addition, the type of soil determines the type of excavation and the procedure used to increase stability. Thus, being aware of the important aspects of excavation can help design a cost-friendly building.
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