Trench Boxes and Trench Shields

A trench box, also called a trench shield, is a pair of steel or reinforced-plastic panels held apart by adjustable struts, assembled into a complete unit and lowered into an excavation to resist the earth pressure of the surrounding ground. Each box is rated to a specific permissible earth pressure in kN/m², and SEL's range runs from lightweight utility boxes to heavy-duty systems for deep drainage and sewer trenches.

A static trench box is installed using the "dig and push" method: the assembled box is lowered into a shallow pre-excavated pit, then an excavator digs from inside it and pushes the panels down incrementally until the required depth is reached, with a top box added if needed. Drag boxes use "excavate and drag" installation, where the assembled box is pulled forward through the trench by an excavator as work progresses along stable, self-supporting ground.

The difference between a trench box and a manhole box is the number of sides supported. A trench box supports the two long faces of a linear excavation, while a manhole box adds end return panels to support all four sides, which is needed for square or rectangular excavations such as manhole rings, chambers and pumping stations.

SEL's Standard Trench Box suits a typical drainage or utility trench. It shores excavations up to 5.30m deep with 1.50m of pipe clearance, making it the default choice for most sewer and utility works. For shallower trenches with less lifting capacity, the Lightweight Trench Box or Mini Trench Box are usually more suitable.

The choice of which type of trench box to use comes down to a combination of access, ground conditions and the presence of any crossing services. Stable, self-supporting ground with a clear run suits a drag box, since an excavator can pull it forward continuously as you lay the pipe, without lifting and repositioning a static box for every section.

Unstable or water-bearing ground calls for a static trench box, which you dig and push down incrementally so the trench walls stay supported throughout. A live service crossing the line typically rules out a solid-sided box such as a drag box; a system that lets you place sheets individually around the service is usually the safer choice there. Site access plays a part too: a smaller excavator with limited lifting capacity points towards one of the lighter static boxes in the range, while a drag box needs a machine capable of pulling it forward through the trench.

SEL's Hi-Clear Box System gives the most clearance for large pipes, with up to 2.29m beneath the lowest strut when set to its high position. This is designed for large-diameter pipes, culverts and tanks that need more room than the 1.50m clearance of a Standard Trench Box.

Edge protection is often needed alongside a trench box to guard the top of the excavation, in order to prevent a fall into a trench from ground level. SEL supplies guardrails and ladder access to fit alongside every box in our range.

SEL's trench boxes are designed and manufactured to BS EN 13331, the British and European standard for trench lining systems. Our in-house engineers also provide Temporary Works Design to BS 5975 where ground conditions call for a project-specific calculation.

You can ensure safe access to a trench box excavation by using purpose-built access equipment that clamps directly onto the box panels. Our Tube Steel Barriers provide secure guardrails around the perimeter, while our Ladder Access Platforms pin securely to the trench box channel to give personnel a fixed, self-closing gated entry point with a slip-resistant ladder base. For confined space rescue or retrieval, a Davit Safety System can also be mounted directly to the access platform socket.