The excavation procedure for constructing a foundation typically involves several steps, which include site clearance, setting out, excavation, and safety measures.
The first step in the excavation process is site clearance. This involves removing any obstructions or debris from the construction site to ensure that the area is clear and safe for excavation work to begin.
The second step is setting out, which involves marking out the boundaries of the foundation using pegs and string. This helps to ensure that the excavation is carried out in the correct location and to the correct dimensions.
The third step is excavation itself, which involves digging out the soil or rock from the marked-out area to create the foundation trench. The depth of the excavation will depend on the specific requirements of the building design and the type of soil or rock present at the site.
Finally, safety measures must be implemented throughout the excavation process to ensure the safety of workers and site visitors. This may involve using protective equipment, such as hard hats and safety harnesses, as well as implementing measures to prevent collapses or other accidents, especially if the excavation depth is significant.
Site Clearance Before Excavation for Foundation
Prior to commencing excavation for the proposed foundation, it is necessary to clear the site of any vegetation, brushwood, or stumps of trees that may be present. This is important to ensure that the excavation work can be carried out smoothly and without hindrance. Additionally, any tree roots that may be present must be removed to a depth of at least 30 cm below the foundation level to prevent any future damage to the foundation.
It is also important to fill in any pits or depressions that may have been caused by tree roots or old foundations. These pits must be filled with soil and then compacted to ensure that the ground is level and stable. This is important to ensure that the foundation is placed on a solid and stable base, which will help to prevent any future problems with the foundation. Therefore, it is essential to carry out these preparatory measures before beginning any excavation work for the proposed foundation.
Setting out Foundation Layout for Excavation
To establish the layout of a foundation excavation, it is necessary to create a benchmark on the site using a masonry pillar and connect it to the nearest standard benchmark. The levels of the site should be measured at intervals of 5 to 10 meters, depending on the terrain and the importance of the building. To mark the center lines of the walls, strings are stretched across wooden pegs driven at the ends. The center lines of perpendicular walls can be marked by setting out a right angle using steel tapes or, preferably, a theodolite.
To facilitate the setting out of walls, it is useful to have a permanent row of pillars that are parallel to the building’s periphery and located at a suitable distance beyond it. These pillars should be at least 25 cm on each side and located at the junctions of cross walls and external walls. They should be bedded deeply enough to avoid disturbance during excavation for the foundation. The center lines of the walls should be extended and marked on the plastered tops of the pillars, which should be kept at the same level, preferably the plinth level.
If the setting is rectangular or square, it is important to check the diagonals to ensure the accuracy of the setting out.
Excavation Procedure for Foundation
Excavation for small buildings is typically done manually using tools like pick axes, crow bars, and spades. However, for larger and deeper excavations, mechanical earth cutting equipment is often necessary. In cases where the soil is hard and the depth of excavation is less than 1.5 meters, external support for the sides of the trench is not required. But, if the soil is loose or the excavation is deeper, shoring is necessary to prevent the sides from collapsing.
To provide support, planking and strutting can be used intermittently or continuously depending on the nature of the soil and the depth of excavation. In the case of intermittent or “open” planking and strutting, only specific areas of the trench sides are covered. This involves placing vertical boards, called poling boards, that are 250 x 40 millimeters in size and spaced about 50 centimeters apart. The vertical boards are kept apart by horizontal wailings made of strong timber that are 100 x 100 millimeters in section, with a minimum spacing of 1.2 meters. The horizontal wailings are strutted by a cross piece that is 100 x 100 square or 100 millimeters in diameter.
For soft soils, continuous or “close” planking is adopted. This involves placing the vertical boards close together without any gap between them. This method provides more support for the trench sides.
Fig: Wailings and Poling Boards – Excavation Procedure for Foundation
Open Planking for Excavations in Soft and Loose Soils
When the soil in an excavation site is extremely soft and loose, it is necessary to take certain precautions to avoid any collapse of the trench. In such cases, boards should be positioned horizontally against the sides of the trench and held in place by vertical waling boards. These waling boards should be reinforced by similar timber pieces on the opposite side of the trench.
It is crucial to be cautious when removing the timber members after the foundation work has been completed. Any carelessness during the removal process could cause the trench to collapse, resulting in significant damage and possible injury. Therefore, it is essential to handle the removal of the timber members with care to ensure the safety of the workers and the stability of the site.
Dewatering of Excavation
Construction of foundations below the subsoil water level can lead to waterlogging, making it necessary to dewater the excavation area. This can be achieved by lowering the water table at least 0.5 meters below the bottom of the excavation. The most suitable and economical method of dewatering can be decided by conducting site and soil investigations.
For shallow foundations in fairly dense soil, the simplest and most economical method is to install drains along the edges of the excavation and collect water in sumps, which can then be removed by bailing or pumping. This method requires simple equipment and can be executed with unskilled labor.
However, for larger excavations, such as those for rafts, a wellpoint system can be employed. This system involves installing perforated pipes with valves and screens, spaced at about 1 meter along the periphery of the excavation at the required depth. These wellpoints are surrounded by sand and gravel filters and connected to a high-capacity suction pump through riser pipes of 5 to 7.5 cm diameter. The groundwater is drawn out by the pumping action and discharged away from the excavation site. The exact spacing of the wellpoints can be decided based on the type of soil.
Fig: Lowering Water Table by Wellpoint
Concreting of Foundation in Excavation
The footing for a masonry wall typically consists of a mixture of cement and concrete with a ratio of either 1:4:8 or 1:5:10 (cement: sand: coarse aggregate). Coarse aggregate used in the mix should not exceed 40 mm in size. Lime concrete can also be used as an alternative material. When dealing with important projects, it is recommended to use a mechanical mixer to ensure a well-mixed concrete. The concrete should be laid in layers, not exceeding 15 cm, and should be properly compacted rather than thrown. To protect the concrete from drying out too quickly, it should be covered with moist gunny bags after around 1-2 hours of laying. Regular curing should be initiated after 24 hours and should be continued for at least 10 days.
For RCC column footings and raft foundations, a leveling course of lean concrete measuring 75 mm is laid to provide an even and soil-free surface for the placement of reinforcement. After laying the concrete, masonry work can commence after 3 days, but curing should continue alongside that of the masonry.