Setting out of building foundation trenches is the process of laying down the excavation line and centerline on the ground based on the foundation plan. The setting out process is also called as ground tracing that is performed before commencing the excavation process.
Once the design of foundation is complete, a setting out plan or foundation layout is prepared for a suitable scale and the plan is dimensioned accordingly. Procedure and requirements in setting out foundation trenches are explained below.
Procedure for Setting Out Building Foundation
The basic steps involved in setting out the foundation trenches are:
- The initial step is to mark the corners of the building. After which, the lengths of the sides are checked by diagonal measurements.
- The axial lines (center lines) of the trenches are marked with the help of profiles, sighting rails, strings, and pegs.
- The trench positioning is controlled by outline profile boards. Profiles are set 2m away from the outline so that they do not interrupt the excavation process.
- The offsets are measured from axial lines and the frontage lines are placed in their correct position relative to local requirements.
- The cross walls positioning is performed by measuring along the main walls and squared from these walls as required. The total width of trenches must be carefully outlined during this process.
Requirements in Setting Out Foundation
The setting out playout must establish the following requirements:
- The size of the excavation
- The shape of excavation
- The direction
- The width of the walls
- The position of the walls
The following points should be observed while setting out trenches:
- In order to set out foundation plan, nails, pegs, profiles, strings, and lime are used.
- In order to correctly determine the position of trenches, the sight rails have to be properly erected at the corners of the building.
- Accurate center lines or axial lines can be determined and marked by using a theodolite.
- To the nails or pegs on the profiles, strings are tied and stretched to achieve horizontal control of dimensions.
- At a distance of 1 meter from the edges of excavation vertical reference pillars are erected. Hence vertical control is achieved during building construction.
- A standard datum is previously determined and marked by the surveyor, based on which the levels on the site are obtained. The depth of trenches and other levels should also be regulated by measurements from this point.
- Before placing the concrete into the trenches, the bottom must be properly rammed and compact.
- The width is marked by means of lime powder when the excavation is performed by hand. These markings give accurate cutting.
- Centreline is marked when the excavations are performed by a machine.