Open-cut trenching and reinstatement

How does Open-cut trenching and reinstatement work, step by step?

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   Step 1: Design the route around the constraints

   The route is chosen from the intake or substation to the charging bays, and the shortest line is rarely the right one. The designer weighs the known and suspected buried services, the surfaces to be crossed and what each costs to reinstate, the trees and their root protection, the drainage falls, and the parts of the site that cannot be closed at all. Crossings of roads and high-value surfaces are identified, since those are the points where a trenchless method may earn its keep. The route is drawn, agreed with the site operator and marked out.

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   Step 2: Establish what is buried before anything is opened

   Service records are obtained for every utility that may be present, and they are treated as an indication rather than the truth. Detection equipment is used to trace and mark what is actually there, and trial holes are dug by hand or by vacuum excavation to confirm the position of anything the route approaches. Marking is transferred to the ground and to the drawings, and it is refreshed as it wears off. This is work for competent people under permit, and no mechanical excavation starts in the vicinity of a known or suspected service without that permit in place.

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   Step 3: Set up the traffic management and the working area

   The working area is separated from vehicles and pedestrians by barriers that suit the site, with signed diversions for both. On a forecourt or a car park that means agreeing with the operator which bays and which routes are lost and for how long, and pedestrian routes are designed to be usable rather than merely marked. Where the works reach the highway, the arrangements follow the permissions granted for that work. The layout is checked at night and in the wet, because that is when the temporary arrangement is tested hardest.

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   Step 4: Excavate the trench

   The surface is cut cleanly along the marked lines and the excavation proceeds to the depth and width the specification requires. Arisings are separated so that material fit for reuse is kept and the rest is removed. Any support to the sides of the excavation, and the arrangements for people working in or near it, are designed for the particular trench by a competent person and are not a matter of site custom. Hand digging is used wherever a service is known to be close. Spoil is kept back from the edge, and the open length is limited to what can be closed the same shift.

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   Step 5: Install ducts and cables with bedding and protection

   Ducts and cables are laid on the bedding material the specification calls for, surrounded and protected as specified, and marked with tape or tiles so that the next person to dig here finds a warning before they find a cable. Draw ropes are left in every spare duct. Separations between services, and the depth and cover for each, come from the specification for the particular route and are checked before backfilling. Duct ends are sealed against water and vermin immediately, because an open duct end fills with silt within days on a working site.

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   Step 6: Backfill and compact in layers

   Backfill is placed in layers of the thickness the specification allows and compacted, with the material appropriate to what sits above - a trafficked car park has different requirements from a grass verge. Compaction is the step that decides whether the reinstatement settles, and a trench that is filled in one lift will announce itself as a trough within a season. Records are kept of material, layer thickness and compaction, since the reinstatement is often subject to inspection long after it has been covered up.

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   Step 7: Reinstate the surface to match

   Reinstatement puts back the construction that was there, in the layers specified, finished to line and level and drained so that water runs where it did before. Where the surface is a highway, reinstatement follows the requirements attached to the permission for the works and is subject to inspection. Where it is a customer-facing forecourt, appearance matters commercially as well as technically, and a patched surface that looks like a patch is a complaint waiting to happen. Line marking, tactile paving and any other surface features are replaced.

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   Step 8: Record what was installed and where

   The as-built position of every duct and cable is surveyed before it is covered, not sketched afterwards. That record - route, depth, what is in each duct, which ducts are spare and where the draw ropes are - is the single most valuable document the civils package produces, because it is what makes the next phase of the site affordable. Duct proving with a mandrel and rope, before the electrical contractor arrives, closes out the work and finds any collapse or blockage while the plant is still on site.