Few conditions change an excavation as quickly as water. A trench that was dry and stable in the morning can have seeping walls and a soft, flooded bottom by afternoon. Across New England, from coastal areas with high water tables to inland sites saturated by spring thaw, groundwater and surface water are among the most common and most underestimated excavation hazards.
Water affects how soil behaves, how protective systems perform and what OSHA requires before work can continue. This guide explains how water destabilizes trenches, the OSHA requirements that apply when water is present, common dewatering approaches and how to plan trench protection for wet conditions.
How Water Weakens Trench Walls
A trench wall stands because of the strength of the soil, including friction and cohesion between soil particles. Water works against that strength in several ways.
Reduced Soil Strength
When water fills the spaces between soil particles, it reduces the friction and cohesion that hold the soil together. At the same time, saturated soil is heavier, increasing the pressure on the trench walls and on any protective system. The combination of reduced strength and increased load is what makes wet trenches particularly prone to failure.
Lower Soil Classification
Under OSHA’s soil classification system, soil that is submerged or from which water is freely seeping is classified as Type C, the least stable classification. Soil that would otherwise be Type A or Type B may also need to be classified as less stable when water or other factors affect it. A lower classification reduces the allowable depth for a given trench box or hydraulic shoring system and increases the slope required if sloping is used.
Loss of Soil Behind and Below the Walls
In sands and gravels, which are common in many coastal and river valley areas of New England, flowing groundwater can carry soil particles into the trench. This can create voids behind the walls or beneath a protective system that are not visible until the ground above settles. In some soils, water pressure at the bottom of the excavation can cause the floor to soften, heave or boil.
Hidden Hazards
Standing water conceals the trench bottom, sloughed material, exposed utilities and changes in conditions. Crews working in water have far less ability to see what is happening around them.
What OSHA Requires When Water Is Present
OSHA’s excavation standard addresses water accumulation directly.
Employees may not work in excavations where water has accumulated or is accumulating unless adequate precautions are taken to protect them. Depending on the conditions, these precautions can include special support or shield systems to protect against cave-ins, water removal to control the level of accumulating water, or the use of a safety harness and lifeline.
When water is controlled with water removal equipment, the equipment and the water removal operations must be monitored by a competent person.
Where an excavation interrupts natural drainage, diversion ditches, dikes or other suitable means must be used to prevent surface water from entering the excavation. Excavations subject to runoff from heavy rains must be inspected by a competent person.
These requirements work alongside OSHA’s general rules for excavations, including protective systems for excavations 5 feet or deeper unless they are entirely in stable rock, daily inspections by a competent person and inspections after rainstorms or other events that could increase hazards.
Common Approaches to Managing Water
Water problems vary, and so do the methods used to control them. The appropriate approach depends on the soil, the depth of the excavation relative to the water table and the volume of water involved.
Sump Pumping
The most common method on utility work is pumping from a sump, a low point in the excavation where water collects. A pump removes water as it enters. Sump pumping is effective for rainwater, minor seepage and modest groundwater inflow. Its limitation is that water is removed only after it has entered the excavation, so it may already have passed through and weakened the trench walls.
Wellpoint Systems
In permeable soils where a trench extends below the water table, wellpoints can lower groundwater before excavation reaches it. A series of small wells is installed along the trench alignment and connected to a header pipe and pump, drawing the water table down below the bottom of the excavation. Wellpoint systems require advance planning and are typically designed and installed by contractors experienced in dewatering.
Deep Wells
For larger or deeper excavations with significant groundwater, deep wells with submersible pumps can lower the water table over a wider area. These are engineered systems generally used on major infrastructure projects.
Keeping Surface Water Out
Often the simplest step is preventing surface water from reaching the excavation. Grading the area so runoff flows away from the trench, building small berms along the edges and limiting the length of open trench all reduce the amount of water that needs to be removed.
Two Important Considerations
Discharge: Water pumped from an excavation often carries sediment. Many municipalities and state agencies regulate where and how that water can be discharged, which may require settling tanks, filter bags or other treatment. Discharge plans should be confirmed before pumping begins.
Effects on nearby structures: Lowering the groundwater level can cause the surrounding ground to settle. When dewatering is planned near buildings, roadways or utilities, a qualified engineer should evaluate the potential for settlement.
Planning Trench Protection for Wet Conditions
Water affects not only how the trench is dewatered but also which protective system is appropriate and how it is used.
Plan for Weaker Soil
Because water can lower soil classification, protective systems should be selected for the conditions expected when the soil is wet, not when it is dry. A trench box rated for a given depth in Type B soil may be rated for a shallower depth in Type C soil. The manufacturer’s tabulated data should be checked for the actual conditions.
Match the System to the Conditions
Steel trench boxes are often well suited to wet, heavy soils and longer runs. Hydraulic shoring can be used in many conditions, but in soils that ravel or have water seeping through, it may require sheeting between shores or a different configuration as specified by the manufacturer. Where conditions are especially difficult, the system and approach should be reviewed with the equipment supplier and, where required, an engineer.
Watch the Bottom and Sides
In wet trenches, a box can settle into a softened bottom, and voids can form behind panels as water carries soil away. Placing stone at the bottom of the excavation where appropriate, backfilling voids beside the box and monitoring the protective system throughout the shift help keep it performing as intended.
Do Not Rely on the Pump
Dewatering supports the protective system but does not replace it. If a pump fails or a generator runs out of fuel, water can return quickly. The trench must remain protected regardless of whether the dewatering system is operating, and the competent person should confirm conditions before crews re-enter after any interruption.
Planning for Water Before Excavation Begins
The most effective way to manage water is to anticipate it. During planning, contractors should review boring logs and test pits for groundwater levels, consider the season, since spring groundwater in New England is often higher than in late summer, and note nearby wetlands, streams and drainage paths. Building pumping capacity, discharge arrangements and suitable protective equipment into the plan is far easier than responding after water appears at depth.
Trench Protection for Wet Excavations
North East Shoring supplies Kundel steel utility shields, mainline shields and manhole shields, aluminum trench boxes, TrenchJack, HydroJack and HydroShore hydraulic shoring and A36 steel road plates to contractors throughout New England from its location in Norton, Massachusetts.