Measuring depth of foundation excavations in the field Which instrument is generally used to measure the depth of excavation for foundations with reference to a benchmark or formation level?

Introduction / Context:
Accurate control of excavation depth is essential to ensure the footing reaches design levels, achieves the required bearing stratum, and maintains uniformity across the site. Survey instruments and accessories are used to check levels relative to site benchmarks.

Given Data / Assumptions:

• The measurement is relative to a benchmark or desired formation level.
• Standard site surveying tools are available.
• Practical site procedure is being queried.

Concept / Approach:
Depth is best controlled by taking level readings with a levelling instrument and a levelling staff placed at the excavation bottom. The difference between the reading and the benchmark-based reference gives the excavation depth or the reduced level of the bottom. A steel tape only measures linear distances locally and does not reference elevation unless a datum is established and maintained, which is less robust on uneven ground.

Step-by-Step Solution:

Verification / Alternative check:
Boning rods can be used for trench grade control in small works, but staff+level provides higher accuracy and is standard for foundations.

Why Other Options Are Wrong:

• ranging rod: used for aligning straight lines, not level measurement.
• steel tape: measures length, not elevation relative to a datum.
• boning rod: for trench line/grade by sighting, less precise for depth to benchmark.
• none of these: incorrect because a levelling staff is correct.

Common Pitfalls:
Measuring depth from variable ground points without referencing a fixed benchmark; ignoring instrument errors and staff misreadings.

Final Answer:
levelling staff