SETTING OUT FOUNDATION TRENCHES.

Setting out or ground tracing is the process of laying down the excavation lines and centre lines etc. on the ground, before excavation is started. After the foundation design is done, a setting out: plan, sometimes also known as foundation layout plan, is prepared to some suitable scale (usually 1:50). The plan is fully dimensioned.

For setting out the foundations of small buildings, the centre line of the longest outer wall of the building is first marked on the ground by stretching a string between wooden or mild steel pegs driven at the ends. This line serves as reference line. For accurate work, nails can be fixed al the centre of the pegs. Two pegs, one on either side of the central peg, are driven at each end of the line. Each peg is equidistant from the central peg, and the distance between the outer pegs corresponds to the width of foundation trench to be excavated.

Each peg may project about 25 to 50 mm above ground level and may be driven at a distance of about  2 m from the edge of excavation so that they are not disturbed.

When string is stretched joining the corresponding pegs (say 2-2) al the two extremities of the line, the boundary of the trench to be excavated can be marked on the ground with dry lime powder. The centre lines of other walls, which are perpendicular to the long wall, are then marked by setting out right angles. A right angle can be set out by forming a triangle with 3, 4 and 5 units long. These dimensions should be measured with the help of a steel tape. Alternatively, a theodolite or prismatic compass may be used for setting out right angles. Similarly, outer lines of the foundation trench of each cross-wall can be set out, as shown in Fig. 2.27.

 FIG. 2.27 SETTING OUT WITH THE HELP OF PEGS.

For a big project, reference pillars of masonry may be constructed as shown in Fig. 2.28. These pillars may be about 20 cm thick, and about 15 cm wider than the width of the foundation trench. The top of the pillars is plastered, and is set at the same level, preferably at the plinth level. Pegs are embedded in these pillars and nails are then driven in the pegs to represent the centre line and the outer lines of the trench. Sometimes, additional walls are provided to represent plinth lines.

 FIG. 2.28 SETTINGS OUT USING MASONRY PILLARS.

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