When is a wellpoint system the right choice?

It is most effective for lowering the water table by 5-6 m in permeable soils like sands and gravels (Table 1).

What method is used for deep excavations or low permeability soils like silt?

For deep excavations (>6m), deep well systems are used. For low permeability soils, vacuum wellpoints or ejector systems are needed to achieve the required drawdown (Clause 3.4 & 3.5).

What are the main risks associated with dewatering?

Key risks include settlement of adjacent buildings, instability of excavation slopes or base, increased soil consolidation, and damage to timber piles by exposing them to air (Clause 7).

How is the appropriate dewatering method selected?

Selection depends on factors like soil characteristics (especially permeability), required drawdown, size of excavation, and proximity to existing structures, as detailed in Table 1 and Clause 4.

IS 9759

: 1981

Guidelines for dewatering during construction

CurrentFrequently UsedGuidelinesBIMGeotechnical · Soil and Foundation Engineering

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This standard provides guidelines for controlling groundwater during construction through various dewatering methods. It covers site investigation, selection criteria for techniques like sump pumping, wellpoints, and deep wells, and considerations for system design, operation, and potential side effects like ground settlement.

Guidelines for dewatering during construction

Quick Reference Values

Practical suction lift for wellpoint systems5 to 6 m

Typical spacing of wellpoints0.75 to 3 m

Typical radius of influence for a single wellpoint5 to 15 m

Drawdown from deep wellsCan exceed 30 m

Typical drawdown for vacuum wellpoints in silty soils7 to 9 m

Practical Notes

A thorough geotechnical investigation, including permeability tests, is the most critical step; basing a dewatering design on assumed soil conditions is a common cause of failure.

Always monitor adjacent structures and utilities for settlement, as lowering the water table can cause ground subsidence, especially in loose or organic soils.

Plan for the legal and environmentally compliant disposal of pumped water. Unexpectedly high discharge volumes can cause flooding, erosion, or regulatory issues.