Arch spreaders in trench shoring: what they do and when you need one
By Marc Griffin
Built for Clearance, Not Just Capacity
The strut in a standard trench box has to sit somewhere, and it's not always somewhere convenient. The Griffo Arch Spreader clears the space underneath instead, giving up to 2400 mm of working room for large diameter pipe and box culverts. Here's the detail.
More working room for large diameter pipe, box culverts and twin pipes
We’ve just added an arch spreader to our range. The Griffo Arch Spreader is a steel high clearance system designed to work with the Legend Pro Aluminium Shoring Box range.
An arch spreader replaces the standard cross struts in a trench box with a raised steel frame, so the working area underneath stays clear instead of having a strut sitting through the middle of it. If you’re laying large diameter pipe, dropping in box culverts, or working a boring pit, that clearance is often the difference between a box you can work in and one you fight all day.
How a trench box normally works
A standard trench box is two panels held apart by struts. The panels take the soil load, and the struts resist it by spreading the panels apart. Simple, proven, and it’s why the system is as light as it is.The catch is that the struts sit across the trench at set heights. On our Legend Pro base box you get 1400 mm of under-strut clearance, and 900 mm on an extension box panel. For a 225 mm service main that’s plenty. For a 1500 mm RCP with bedding under it, it isn’t.Crews work around this in ways that cost time. Threading pipe in at an angle. Lifting over the strut and swinging it into place. Sending someone in to work a joint in a space that’s too tight to swing a bar properly. None of that is efficient, and some of it is worse than inefficient.
What the Griffo changes
An arch spreader swaps the strut for a portal frame. Two steel end posts sit on the panels, and a bolted beam spans between them above the working zone. It carries the same spreading duty, just at a higher level.Our Griffo system gives up to 2400 mm of under-strut clearance. It suits all our Legend Pro Aluminium Shoring Box sizes from 1.8 m to 7.2 m, and it’ll achieve internal trench widths from 1400 mm up to roughly 6500 mm depending on the beam combination you run.The beams are modular. End posts plus 125 mm, 250 mm, 500 mm, 1000 mm and 2000 mm sections bolt together to hit the width you need. End post to beam and beam to beam connections are 10 × M20 bolts each, with an M36 bolt through the end post itself. It’s steel throughout, with a raw galvanised finish.
It’s steel, and the weights matter
Each end post is 380 kg. Beam sections run from 55 kg for a 125 mm up to 306 kg for a 2000 mm. Add that to the panel weights and the total lifted assembly gets heavy fast, particularly on a 7.2 m box.That’s not a reason to avoid the system, but it does mean the lift needs checking rather than assuming. Confirm the total system weight before you pick it up, and confirm your chains or slings are rated for it. Our guidance is that lifting chains and slings sit at 45 degrees to horizontal.
Watch it go together
We’ve built the whole system to be assembled flat on the ground and lowered into place with a single excavator. Beams bolted to width, end posts on sleepers, panels lowered on and pinned while everything’s still horizontal, then one lift into the trench. We’ve laid out the full seven-step sequence visually on our product page. If you’re briefing a crew who haven’t used one before, that page and this video are the right things to put in front of them.Two things to hold onto regardless: get all pins fully home with keepers in before the assembly leaves the ground, and inspect the lot once it’s positioned. Check plumb, check the connections, check for any movement or deformation. If something looks off, stop and get a competent person to assess it before anyone works in there.
When an arch spreader is the right call
Reach for one when clearance is your constraint rather than depth or width. Typical cases:
Large diameter pipe where the OD plus bedding leaves nothing under a standard strut
Box culvert installation
Twin pipes running side by side in the one trench
Boring pits, where the strut usually lands right on the bore line
Long pipeline runs where the time your crew loses working around struts compounds across every joint
When a standard strut is enough
Plenty of work doesn’t need this. If you’re running small to medium diameter service pipe and 1400 mm of clearance is comfortable, a standard strut is lighter, faster to set up and cheaper to hire. Don’t buy clearance you won’t use.The trade-offs are real. An arch spreader adds significant steel, which pushes up your lift weight and can push you into a bigger excavator. It adds ground assembly time and bolted connections that need tightening and checking. And it doesn’t solve every problem: if your issue is width beyond the box range, staged excavation, or ground that won’t hold for a lower-in-place install, we recommend slide rail shoring instead.
What to sort before you hire
Loading: Panel working load limits vary by panel length and by whether the load case is triangular or uniform. This is a design input, not a catalogue choice.
Surcharge: Plant, spoil stockpiles, traffic and adjacent structures all change the numbers.
Lift capacity: Total assembled weight against your excavator and your rigging.
Target width: Check the beam combination that gets you there before the gear turns up.
Ground and access: You need somewhere firm and level to assemble flat, with room for the machine to work.
Certification: Our Griffo Arch Spreader and Legend Pro panels are certified to AS 4744.1:2025, the Australian Standard covering design of shoring and trench lining equipment. Every panel carries a traceable serial number. Ask us for the documentation and keep it with your site records.
Get it Engineered
We have explained how the system behaves. It isn’t a temporary works design and it shouldn’t be treated as one. Trench support design depends on your soil, your depth, your surcharge and your sequence, and it should be signed off by someone competent to do it.That’s where our in-house temporary works engineering team comes in. We’ll work through the load case, confirm the configuration, and produce the drawings your principal contractor is going to ask for. It’s a different proposition to hiring a box off a rate sheet and hoping the numbers work out.
Have a Chat
Send us the pipe size, the trench depth and width, the soil report and the site constraints, and we’ll tell you whether an arch spreader is the right tool or whether you’re better off with something else. Call 1300 890 890 or email legends@legendhire.com.au. You can browse our shoring and trench safety range online, and our Legend Pro Aluminium Shoring Box has the panel data if you want to check the numbers first.
