Trench Design & Construction
Excavation should proceed upgrade, and excessive runs of open trench should be avoided. Principal considerations in trench construction are trench width, stability of the native soil supporting and containing the pipe and its embedment soil, stability of trench walls, and water accumulation in the trench. If the trenching conditions present construction problems such as trench sidewalls that readily slough off or a soft trench floor that will not support workers or compaction, the trench is termed "unstable", and is more likely to occur in trenches excavated below the groundwater level. Stability can be improved by de-watering, such as by deep wells, well points or sump pumps placed in the trench. In very weak ground (such as loose fine sands or silts with low blow counts or soft clays with low unconfined compressive strength) it may be necessary to employ special procedures such as a "wide" trench or permanent trench sheeting to provide proper support to the embedment material.
The trench floor must have sufficient stability and load-bearing capacity to present a firm working platform during construction to maintain the pipe at its required alignment and grade and sustain the weight of the fill materials placed around and over the pipe. The trench bottom should be smooth and free from sloughed sidewall material, large stones, large dirt clods, frozen material, hard or soft spots due to rocks or low-bearing-strength soils, and any other condition that could lead to non-uniform or unstable support of the pipe. The trench bottom must be kept dry during installation of the pipe and the embedment materials. All foundation and bedding materials must be placed and compacted according to the design requirements.
In unstable soils where the trench floor does not support the weight of the workmen, over-excavation and replacement of unstable soil with more stable material may provide a sufficient foundation. Appropriate bedding material should then be placed on the foundation. In extremely weak soils (such as saturated, loose sands, organic silts and peats) further over-excavation or other special treatment may be required.
To control deflection, the embedment material must be placed from undisturbed trench sidewall to undisturbed trench sidewall. Where trench walls are unstable, it may be necessary to use trench shields, bracing, or permanent sheeting to achieve a stable sidewall while installing the pipe. Walls of trenches below the elevation of the crown of the pipe should be maintained as vertical as possible. The shape of the trench above the pipe will be determined by the stability of the trench walls, excavation depth, surface loadings near the trench, proximity of existing underground structures, presence of groundwater or runoff water, safety and practical considerations. When trench walls are shored or otherwise stabilized, the construction scheme must allow for the proper placement and compaction of pipe embedment materials.
Trench safety information is provided in OSHA 29 CFR 1926.650-652, Subpart P, Excavations.