Establishing Benchmark in Civil Engineering

Benchmark is a point of reference which is used to measure the height or elevation of other points in a given area. In civil engineering, benchmarks are used to ensure that all measurements are taken from a common reference point. The process of establishing a benchmark involves the use of various techniques such as hypsometry, barometric levelling, spirit levelling, and trigonometrical levelling.

Spirit Levelling as the Technique for Establishing Benchmark

Among these techniques, the most commonly used method for establishing a benchmark is spirit levelling. This technique involves the use of a levelling instrument which is mounted on a tripod. The instrument consists of a telescope which is used to sight a staff that is placed on a benchmark or a point of known elevation.

The process of spirit levelling involves the following steps:

1. Setting up the instrument: The levelling instrument is mounted on a tripod and levelled using adjustable screws. The instrument is then adjusted to read zero on a graduated staff.

2. Measuring the elevation of the benchmark: A staff is placed on the benchmark and the telescope of the instrument is sighted on the staff. The reading on the staff is noted and this gives the elevation of the benchmark.

3. Measuring the elevation of other points: The instrument is then moved to other points in the area and the elevation of each point is measured in the same way as the benchmark.

4. Calculating the elevations: The elevations of the other points are calculated by subtracting the elevation of the benchmark from the readings obtained at each point.

Advantages of Spirit Levelling as the Technique for Establishing Benchmark

Spirit levelling is preferred for establishing benchmarks because of the following advantages:

1. It is accurate: Spirit levelling is a highly accurate technique for establishing benchmarks. The errors in the readings are usually very small and can be easily corrected.

2. It is reliable: Spirit levelling is a reliable technique that can be used to establish benchmarks in any terrain.

3. It is fast: Spirit levelling is relatively fast compared to other techniques such as trigonometrical levelling.

Conclusion

In conclusion, the technique of spirit levelling is used to establish a benchmark in civil engineering. This technique is preferred because it is accurate, reliable and fast. It is important to establish a benchmark in any construction project to ensure that all measurements are taken from a common reference point.