How to Find Excavation Depth from NGL – Step-by-Step Guide

Excavation is one of the most crucial stages in any civil construction project. Whether you are constructing a building, a road, or any other infrastructure, knowing the depth of excavation from Natural Ground Level (NGL) is essential. It ensures proper foundation placement, structural safety, and cost control.

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Excavation Depth From NGL RL

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In this blog, we’ll understand what NGL is, the process to calculate excavation depth from it, and a real-life example to clarify the concept.

What is Natural Ground Level (NGL)?

NGL stands for Natural Ground Level, which is the original, undisturbed surface level of the ground before any construction activity or excavation begins.

Think of NGL as the ‘zero level’ or reference level on site before you dig or build.

Why is Excavation Depth Important?

Knowing the correct excavation depth helps in:

• Achieving the exact required foundation level (formation level).

• Ensuring the stability and strength of the structure.

• Avoiding over-excavation, which increases cost.

• Maintaining design accuracy in civil works.

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Tools Required for Measuring Excavation Depth

To find the excavation depth, you’ll need:

• Auto Level or Dumpy Level

• Levelling Staff

• Benchmark (BM) or known reference level

• Field Book for noting readings

• Tripod for setting up level instrument

How to Calculate Excavation Depth from NGL

Follow these steps to calculate the excavation depth from NGL:

1. Establish a Permanent Benchmark (PBM)

A Benchmark (BM) is a fixed reference point with a known Reduced Level (RL). This could be a nearby road level, plinth level of an adjacent building, or any government-established point.

Example: BM = 100.00 m (assumed RL)

2. Take Back Sight (BS) on the Benchmark

Using the auto level, take the first staff reading (Back Sight) on the benchmark.

Example: BS = 1.250 m

3. Calculate Height of Instrument (HI)

Use the formula:

HI = RL of BM + BS

HI = 100.00 + 1.250 = 101.250 m

4. Take Fore Sight (FS) on the Natural Ground

Now, take the staff reading on the Natural Ground Level (NGL) where excavation is to be done or where excavation work is going on.

Example: FS = 1.900 m

5. Calculate RL of NGL

RL of NGL = HI - FS

RL of NGL = 101.250 – 1.900 = 99.350 m

This is the actual level of the existing ground.

6. Know the Formation Level (Final Level)

The formation level is the bottom level after excavation, decided by the design drawings.

Example: Formation Level = 97.850 m

7. Calculate Excavation Depth

Excavation Depth = RL of NGL – Formation Level

Excavation Depth = 99.350 – 97.850 = 1.500 m

✅ Final Answer: You need to excavate 1.5 meters from the natural ground level.

Tips to Ensure Accurate Excavation

• Always carefully or re-check benchmark and formation level before starting.

• Use minimum two staff readings at corners and center of the plot.

• If slope exists on site, take multiple readings at various points.

• Ensure the instrument is properly leveled and calibrated.

• Record readings in the field book for verification and reports.