The Complete Trench Support Guide

Soil is heavy, and an unsupported trench wall will fall in sooner or later. A trench collapse can bury a worker under a load of earth, which is why safe excavation starts with decisions made before anyone picks up a spade. How will the sides be held? Who will inspect them? Which equipment will do the job?

This guide covers what deep trenching involves, why trench support matters, the main ways to support an excavation, how to choose between them, and the equipment Altrad Hire & Sales supplies for hire and sale.

A trench is an excavation where the length greatly exceeds the depth. Shallow trenches are usually considered to be less than 6 metres deep, and deep trenches are those greater than 6 metres.

The law does not wait for a trench to reach 6 metres. Regulation 22 of the Construction (Design and Management) Regulations 2015 requires you to take all practicable steps to prevent any excavation, or any part of one, from collapsing. That includes providing supports or battering where necessary. On deeper work, involve whoever designs your temporary works early.

It prevents collapse. Supports and battering keep the sides of an excavation in place while people work below ground. Plant and vehicles parked close to the edge add extra load and make the sides more likely to collapse.

It protects people from falls and falling material. HSE guidance says the edges of excavations should be protected with substantial barriers where people could fall in, and that edge protection should stop material falling from spoil heaps into the trench.

It controls water. Supports should be designed to control the entry of groundwater and to take any extra water loading into account.

It keeps the job legal. A risk assessment of excavation work should identify buried services before digging starts, and the support equipment, such as trench sheets and props, should be on site before work begins.

Battering back excavations

Battering excavation means cutting the sides back to a safe angle so they support themselves. In granular soils the slope must be shallower than the natural angle of repose of the material. In wet ground you need a considerably flatter slope. Battering takes up space, so it suits open sites better than tight ones.

Stepped excavation

A stepped excavation cuts the sides back in a series of horizontal steps, which breaks one tall vertical face into several shorter ones. Like battering, it needs room on either side of the trench, and a competent person should assess the ground before anyone decides on step sizes.

Trench boxes

A trench box is a prefabricated steel or aluminium structure that supports the walls of an excavation and protects the people working inside. You install trench boxes with a dig and push or excavate and lower technique. The range is designed to meet BS EN 13331 and BS 5975, the UK standards for trench lining systems and temporary works. Depth depends on the model. The Mini Trench Box suits trenches up to 4 metres deep, the Standard Trench Box works to 6.5 metres and the Magnum Trench Box works to 6.6 metres, provided ground pressure stays within the rated limit for each box.

Trench sheets

Trench sheets hold back the ground along the full length of the trench. Overlapping sheets are lightweight and easy to handle, which suits shallow excavations. Interlocking sheets have integrated clutch profiles that lock together mechanically, giving stronger lateral support in deeper or more unstable ground. Heavy duty interlocking steel sheet piles from the Larssen range form a continuous wall with watertight joints.

Waler systems, braces and struts

Where cross services stop you using a trench box, a hydraulic waler system or brace holds the sheets in place instead. Alloy Waler Rails provide two sided hydraulic support in lightweight aluminium. The Double Acting Manhole Brace suits excavations where ground conditions or cross services make a standard manhole box unsuitable. Raptor Brace works with trench sheets or sheet piles to create cofferdams for chambers, tank installations and drive pits. Trench Struts brace opposite trench walls, with a safe working load of 3 tonnes at any extension.

Slide rail systems

A slide rail system uses vertical steel rails and panels that slide together to form the support. The rails and panels are pushed down as the excavation gets deeper, so the walls stay supported as you dig. If you are considering a slide rail system for a job, talk to your local branch about which support options suit your site.

Start with the risk assessment. Ground conditions, groundwater, buried services and plant movements decide what is safe.

Check the space you have. Battering and stepping need clear ground either side of the trench. If you have no room, a trench box or a sheeted system keeps the excavation narrow.

Look at cross services. Pipes and cables running across the trench rule out a standard box. Waler rails or braces get around them.

Match the support to the depth and pressure. Every box and system has a rated depth and ground pressure. If ground pressure exceeds the rating of a trench box, move to a manhole brace or another designed system.

Think about the pipe. Larger diameter pipes need more clearance under the struts. The Magnum Trench Box offers 2,460mm of clearance below the strut for this reason.

Plan the edge and the way in. Barriers, toe boards and safe ladder access belong in the plan from the start.

Altrad Hire & Sales supplies Groundworks equipment for hire and sale, including the following.

Groundwork Boxes covering trench boxes, manhole boxes and drag boxes, including the Mini Trench Box, Standard Trench Box and Magnum Trench Box.

Trench Sheets in overlapping and interlocking ranges, including Standard Overlapping Trench Sheets and Larssen Range Interlocking Trench Sheets.

Waler Systems including Alloy Waler Rails.

Groundwork Braces including the Double Acting Manhole Brace.

Props & Struts including Trench Struts.

Edge and access equipment such as the Trench Guard Panel and Ladder Safe Platform.

Safe excavation comes down to a plan made before digging starts, the right support for the ground and the depth, and regular inspection once work is under way. Altrad Hire & Sales supplies trench boxes, trench sheets, waler systems, braces and struts for hire and sale across the UK, along with the edge protection and access equipment that goes with them. Contact your local branch to discuss the right trench support for your project.

Q. When should an excavation be battered back or stepped?
A. Batter or step an excavation when you have enough clear space around it and the ground can hold a slope. The safe angle depends on the soil. Granular soils need a slope shallower than their natural angle of repose, and wet ground needs a much flatter one. If space is tight, the ground is wet or unstable, or the trench is deep, use supports such as a trench box or trench sheets instead. A competent person should assess the ground before you decide.

Q. How often should excavation inspections take place?
A. Where an excavation has supports or battering, a competent person must inspect it at the start of each shift before work begins. They must inspect it again after any event likely to have affected its strength or stability, and after any unintended fall of rock, earth or other material. If the inspection finds a problem, nobody works in the excavation until it is fixed. Keep a written record of each inspection. HSE guidance lists what it should include, such as the date and time, the location, any matter that could cause a risk and the action taken.

Q. What is vacuum excavation?
A. Vacuum excavation, often called non destructive digging, uses pressurised air or high pressure water to loosen soil while a vacuum system sucks up the soil and debris. Contractors use it to expose buried gas, water, sewer and electrical services without damaging them. It does not replace trench support. If you then dig an open trench, the same excavation rules apply.

Q. What is trenchless technology?
A. Trenchless technology installs, repairs or replaces pipes, conduits and cables underground with minimal excavation. Common methods include horizontal directional drilling, pipe jacking and pipe bursting. Any pit or open excavation the work still needs falls under the same excavation rules, so it still needs a risk assessment, support and inspection.