Dewatering removes groundwater or stormwater from an excavation, or lowers the water table around it, so crews can work in a dry, stable cut. Done well, it keeps soil from softening and boiling. Done badly, it can wash out soil behind shoring, undermine nearby structures, and send muddy water to a stream.
Why water matters for trench safety
- Soil classification drops: submerged soil or soil with freely seeping water is Type C under OSHA Appendix A.
- Walls weaken as water adds weight and pressure and reduces cohesion.
- Bottom instability: upward seepage can cause boiling or heave in the trench floor.
- Washouts behind or under shields and shoring can collapse the system.
- Drowning and electrical hazards with standing water and pumps.
Dewatering methods
| Method | How it works | Best for |
|---|---|---|
| Open sump pumping | Water collects in a low point or sump in the excavation and is pumped out | Small inflows, cohesive soils, stormwater after rain |
| Wellpoints | Closely spaced small wells around the excavation connected to a header and vacuum pump lower the water table | Sands and silty sands, shallow to moderate depths |
| Deep wells | Larger wells with submersible pumps around the site | Deep excavations, high-permeability soils, large drawdowns |
| Eductor (ejector) wells | Wells using a jet-pump principle | Fine-grained soils with low permeability |
| Cutoff walls | Sheet piles or slurry walls that block water from entering | Where pumping alone isn't practical or would affect neighbors |
Larger or deeper dewatering is usually designed by a geotechnical engineer, because lowering the water table can cause settlement of nearby buildings and utilities.
OSHA requirements
Under 29 CFR 1926.651(h):
- Employees may not work in excavations with accumulated or accumulating water unless adequate precautions are taken, such as special support or shield systems, water removal, or safety harnesses and lifelines.
- Water removal equipment and operations must be monitored by a competent person.
- If the excavation interrupts natural drainage, use diversion ditches, dikes or other means to keep surface water out.
- Excavations subject to runoff from heavy rains require inspection by a competent person, and daily inspections must follow every rainstorm (1926.651(k)).
Where the water goes
Pumped water carries sediment. Construction stormwater permits regulate it. Under EPA's 2022 Construction General Permit, for example, dewatering water must pass through a sediment control such as a filter bag or sediment basin designed to prevent visible turbidity, can't create a sheen, must discharge onto stable surfaces, and requires daily inspection and turbidity sampling against a 50 NTU benchmark. State permits have their own rules, and discharges to sanitary sewers need the utility's approval.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.