What Factors Affect Trench Stability?

The main reason for a trench during the construction excavation stage is the repair or installation of utility lines to connect the site to the public system or to enhance the public system as the surrounding population grows. Prior to the trench excavation, the civil engineer looks to the surrounding slopes; i.e., retaining walls, man-made or natural ground slopes, that will affect the trench stabilization and drainage.

Existing Conditions

Prior to the actual dig, the excavation contractor must study and know the existing conditions of the site’s surface and subsurface. The contractor is responsible for the coordination and scheduling of mark-outs by the designated utility providers via the 811-Before You Dig system to determine the locations of subsurface utility lines and facilities.

These underground lines; i.e., electrical, sewer, water, and gas, are live and will cause harm and injury to those on site and will negatively affect the stability of the trench when the lines are fractured or damaged.

Surface conditions must also be studied. The locations and stability of existing structures and any evidence of slope instability, both on-site and off, need to be evaluated.


A geotechnical engineer, under a specific scope from that of the civil engineer, must be retained to determine the flow and depth of groundwater and the soil types at the site of the planned trench. The depth of the groundwater relative to the depth of the trench and the work within the trench will define the need for any special excavation techniques to deter water run-off or flooding and whether to engage in dewatering. The need for soil analysis is important for the geotechnical engineer to calculate the natural percolation rate of water volume introduced into the trench whether from rain or surface run-off.

Any dewatering at the site must be at the specifications, and under the supervision, of the geotechnical engineer as any manipulation to groundwater levels will affect ground settlement. The integrity of the existing building and road . foundations need to be considered.

Underground Utilities

Surface Drainage

Regardless of the depth of the trench, surface water run-off must be avoided. Consideration must be given to the negative effects of surface water on the integrity of the trench and of the surface slopes as flowing water will cause instability to both.

The grade height of the trench must be compared to the grade of the site, adjacent roads, and neighboring parcels as flowing or collected water could initiate from off-site and terminate in the open trench.

The civil drawings should include the specifications for temporary walls or upstands to be constructed around the open trench to avert surface water.  You must retain a structural engineer for the design specifications of any temporary structures.

Man-Made Slope Features

When planning the trench path, consideration must be given to the proximity and stability of all man-made slope features, such as retaining walls and cut or fill slopes. If any of these slope features are determined to become unstable during the construction excavation stages or vulnerable to ground movement, then proactive measures must be taken to protect these slopes.

Site Loads

If the construction site is tight without allowances for safe-distant stockpiling or staging of the construction equipment, then plans must be in place for the support of the trench walls to withstand the loads and the ground vibrations of the stockpiles and the heavy equipment.

Underground excavation services

If you need construction excavation services at a commercial site in the Milford area, in Southeast Michigan or in neighboring States, then Merlo Construction can provide turn-key solutions for trench excavation.
A family-owned business founded in 1991, Merlo Construction enjoys a reputable safety record, has the wherewithal to maintain the necessary equipment and technological tools in its inventory and the support of local references.