Groundwater Control – Staged well point dewatering for deep excavations For an excavation 20 m deep, how many single-stage well point systems placed at successive levels are typically required (assume each single stage can practically lower water by about 6 m)?

Introduction / Context:
Well point systems are often used to lower groundwater during shallow to moderately deep excavations. Because suction-based well points have a practical lift limit of roughly 6 m (due to atmospheric pressure constraints and system losses), deep excavations require multistage installation with successive headers as the excavation proceeds.

Given Data / Assumptions:

• Excavation depth = 20 m.
• Single-stage well point drawdown capability ≈ 6 m (practical limit).
• Uniform soil permitting well point operation and staged lowering.

Concept / Approach:
The number of stages is estimated by dividing the total required drawdown by the practical drawdown per stage, then rounding up to the next integer because partial stages are not feasible in practice.

Step-by-Step Solution:
Required drawdown ≈ 20 m.Practical drawdown per single stage ≈ 6 m.Number of stages N ≈ 20 / 6 ≈ 3.33.Round up to the next whole number ⇒ N = 4 stages.

Verification / Alternative check:
Field practice commonly uses 3–4 stages for 18–22 m drawdowns, allowing for operating margins, head losses, and ensuring the water table is kept below the excavation base by a safe margin to prevent heave or piping.

Why Other Options Are Wrong:

• 2 or 3 stages cannot provide the full 20 m drawdown with safe margin.
• 5 or 6 stages are unnecessarily conservative given the 6 m per stage rule-of-thumb.

Common Pitfalls:
Assuming theoretical 10 m lift; practical well point systems seldom achieve this due to losses. Always plan for staged headers and check soil permeability and base stability.

Final Answer:
4