1.Purpose and scope
This note sets out the essentials of selecting, designing and controlling excavation support for construction excavations — trenches, pits, basements and service excavations — under a BS 5975:2024 temporary works procedure. It is aimed at TWCs, site engineers and designers.
It does not cover tunnelling, cofferdams in open water, or the detailed geotechnical design of embedded retaining walls (see CIRIA C760); those require specialist design input.
2.The ground rules
No safe unsupported depth
There is no depth below which an unsupported excavation is automatically safe. The old “1.2 m rule” has no standing: people have been killed in excavations shallower than that. Every excavation that a person will enter needs a decision — based on ground, water, surcharge and time — to batter it, bench it, support it, or positively exclude entry.
- Excavation collapse is rarely survivable: a cubic metre of soil weighs around two tonnes, and burial to chest height alone can prevent breathing.
- Ground that stands vertically when first cut can collapse without warning minutes, hours or days later as pore pressures adjust, tension cracks open, or it dries/wets.
- CDM 2015 requires excavations to be prevented from collapsing, supported or battered as necessary, and inspected by a competent person before each shift and after any event likely to have affected their stability.
3.Key standards and references
| Document | Relevance |
|---|---|
| BS 6031:2009 | Code of practice for earthworks — the primary UK reference for excavation and trench support. |
| BS 5975:2024 | Procedural control: design brief, design check category, permits, inspection and loading/unloading of support systems. |
| CDM 2015 (Regs 22 & Sch. of inspections) | Legal duties for supporting excavations and inspection/reporting requirements. |
| CIRIA C760 | Guidance on embedded retaining wall design (sheet piles, king posts, secant/contiguous walls). |
| HSG47 | Avoiding danger from underground services — permit-to-dig practice. |
| BS EN 13331 / manufacturer data | Proprietary trench lining systems — certified capacities and use limits. |
4.Support options and selection
| Option | Typical use | Watch-outs |
|---|---|---|
| Battering / benching | Open sites with space; any depth if slope is designed for the soil and water conditions | Needs geotechnical input for slope angle; surcharge and water rapidly degrade stability; weather sensitivity |
| Trench box / drag box | Service trenches in reasonable ground; progressive excavation | Boxes protect rather than tightly support — ground can still move outside; certified depth/pressure ratings must not be exceeded; ends often open |
| Vertical sheets + hydraulic walers/frames | Pits and shafts, poorer ground, closer support of faces | Sequence matters: support installed with excavation, not after; frame levels from the design, not convenience |
| Steel sheet piling (with frames/anchors) | Deeper excavations, water-bearing ground, adjacent structures | Embedded wall design to CIRIA C760; driving vibration and clearance; extraction effects |
| King post (soldier pile) walls | Mixed ground above water table, phased basements | Lagging installation exposes ground temporarily; base stability and toe fixity are design-critical |
5.Design inputs the designer must have
- Ground information — GI report, trial pits or at minimum a competent description of the strata and any made ground. State what was assumed if information is limited, and require verification on excavation.
- Groundwater — standing level, seasonal variation, and any perched water. Water pressure frequently exceeds earth pressure as the dominant load on support systems, and inflow can destroy trench stability.
- Surcharge — plant, spoil, materials, traffic and adjacent foundations. A nominal allowance (commonly a minimum of 10 kN/m² for incidental surcharge) covers routine loads only; cranes, piling rigs and loaded wagons need their real loads and positions assessed.
- Adjacent assets — structures and their foundation depths, highways, railways, and buried services (HSG47 applies: locate, mark, permit, safe-dig).
- Duration and season — how long faces stay open, and winter water conditions.
Zone of influence
As a first filter, treat any load or foundation inside a line drawn at roughly 45° up from the base of the excavation as capable of loading the support system — and any excavation inside that line from an existing foundation as capable of undermining it. This is a screening rule for when to involve the designer, not a design method.
6.Site controls
- Permit to dig before breaking ground; service drawings, CAT & genny survey, trial holes, and marked exclusion corridors.
- Permit to enter the excavation, tied to the support design's stated limits (depth, open length, time faces may stand unsupported, dewatering in operation).
- Access/egress — ladders or ramps at the required intervals so nobody climbs the support or the face.
- Edge protection and spoil — barriers to stop people and plant; spoil and materials set back from the edge (keep the design's stated clear distance — commonly at least 1 m, more for deep or weak-ground excavations); stop blocks for vehicles tipping or reversing.
- Inspections — competent-person inspection before each shift, after rain, frost, thaw, dewatering changes, adjacent works, or any partial fall; recorded, with defects actioned before entry.
- Installation and removal sequence — support goes in as digging proceeds and comes out as backfilling proceeds, per the design method statement. Removing frames early to ease pipe-laying is a classic fatal shortcut.
7.Illustrative example — checking a trench box (concept)
A drainage trench 3.5 m deep in firm clay with the water table below formation, plant kept outside the zone of influence, and a 10 kN/m² incidental surcharge. A proprietary box is proposed:
- Confirm the box's certified depth and panel pressure rating against the design earth + surcharge pressure diagram for the actual soil — not just “it's rated to 4 m”.
- Confirm strut/spindle capacity against the pressure on the panel area, end-panel or end-sheet requirements, and the lift/installation method (dig-and-push vs. place-in-pre-dug).
- Confirm clear internal working width with the pipe and bedding in place, and ladder access position.
If any input changes on site — deeper, wetter, softer, plant closer — the check is void and goes back to the designer.
8.Site control checklist
- Design brief issued; support design and check completed to the correct category before excavation starts.
- Permit to dig in place; services located, marked and, where needed, exposed by hand.
- Support system on site matches the design (type, rating, condition) — damaged or mixed-system kit rejected.
- Installation sequence and maximum unsupported face/time stated and followed.
- Dewatering (if any) running, monitored, and with a failure response plan.
- Access ladders, edge protection, spoil set-back and vehicle stop blocks in place.
- Pre-shift and post-event inspections recorded; entry controlled by permit.
- Backfill and support-extraction sequence follows the design; voids behind sheets grouted/filled where required.
9.References
- BS 6031:2009 — Code of practice for earthworks.
- BS 5975:2024 — Temporary works procedures.
- Construction (Design and Management) Regulations 2015.
- CIRIA C760 — Guidance on embedded retaining wall design.
- HSE HSG47 — Avoiding danger from underground services.