Spot height is an exact point on a map or plan with an elevation recorded beside it. In other words, it represents height above a given datum.
The Earth’s surface is naturally uneven (undulating or slopy). In construction works such as framed structures, it is expected that the foundation of columns should be at the same depth especially for undulating grounds irrespective of the nature of the ground surface. Undulating surfaces or fairly uneven surface is applicable here because in slopy grounds, foundations need to be stepped to minimize cost of excavation and materials. If the footing bases are not established at the same level surface, it can introduce moments in the structure due to unbalanced loads. Figure 1 shows picture of uneven fairly flat natural ground surface
In most construction projects, eye judgement are used to establish the depth of footings but errors can be introduced by this method. When these errors become visible in the structure at the latter part of the construction process, it may lead to demolitions. Sometimes, excess concrete and blocks are used to compensate for this which is a lost on economy.
To avoid these errors, it is necessary that Surveyors are invited prior to establishment of foundation depths to use levelling instrument to establish the spot heights of column positions. The same principle used in levelling operation is also applicable here. A benchmark would be established at a stable point. Initial reading is taken to the benchmark and the levels of the spot heights (centre point of columns) are taken as well and used to establish the depth of each point. Figure 2 shows the column positions in a typical framed structure. I would show how to determine spot heights from the levels.
The procedure are described below and results tabulated in Table 1:
- Locate the Column positions and tabulate in column one of Table 1.
- Determine the level of natural ground surface at a given point say a value of 2030 mm
3. Establish a stable level above the point at say 300 mm. New level = 2030 + 300 = 2330 mm.
4.Assuming the proposed depth of excavation = 900 mm, establish the height of benchmark in (5).
5. Height of benchmark (reference point) = New level of reference point + depth of excavation = 2330 + 900 = 3230 mm. This value is constant for all point (Colum 2).
6. Determine the existing levels (spot heights) of the centre point of the column positions (Column 3).
7. Determine the depth of excavation (Column 4).
8. Depth of excavation = (Height/level of benchmark) – spot heights
Table 1; Estimation of excavation depth for framed structures
We can see from column 4 that some points are lower than 900 mm while some points are higher. This is due to the undulation of the earth’s surface. With the established depths above, one can get column footings that lie at approximately the same level under the earth’s surface. One should note that the given depths are the expected depth to be excavated at the column position of each point and not the original given depth of 900 mm. Figure 3 shows the layout of excavated column positions in isometric view.
