The classification is the design, not the paperwork
Every protective system on a trench sits downstream of one call: what the soil is. Get that call right and the batter angle, the bench geometry, the shield selection and the strut spacing all fall out of it in sequence. Get it wrong — grade a ground as cohesive and firm because the face stood up overnight — and every number downstream is wrong by a margin no amount of good workmanship recovers. Classification is not paperwork filed after the dig. It is the design input, and it is the only input that the weather, the traffic and the previous crew's backfill can quietly change while the excavator is still parked.
A competent person makes that call, and the definition carries weight in both halves: someone able to identify existing and predictable hazards, and someone with authority to stop the work and have them corrected. The second half gets forgotten on busy sites. An operator who can name the soil but cannot pull a crew out is not competent in the regulatory sense. In the United States the framework sits in 29 CFR 1926 Subpart P, Excavations. Great Britain works to outcomes instead of tables under the Construction (Design and Management) Regulations 2015, which pushes the geometry onto a temporary works designer rather than a wall chart. Australia, Canada and the European states each publish their own regime. Confirm which one governs the site before quoting any angle to anybody.
Depth is the trigger, not the reason. Most regimes demand a protective system from roughly 1.5 m (5 ft) down unless the cut is entirely in stable rock, and demand an engineered design once the excavation passes about 6 m (20 ft). Below the trigger depth the soil still decides; it simply decides through the competent person's judgement rather than through a table. Fatalities keep occurring in trenches shallower than 1.5 m because somebody read shallow as safe. A cubic metre of soil weighs on the order of 1.5 tonnes, which is about 2,500 lb in a cubic yard. Chest-deep is deep enough to fix a worker in place and stop the chest wall moving long before a rescue team arrives.
Grading the face without a laboratory
Start with the visual pass, taken from the top and from a safe standing position, never from inside. Look for layering and the direction the layers dip — beds running down into the excavation are a different animal from beds running away from it. Look for fissures, tension cracks parallel to the edge, spalling off the face, and the tell-tale cone of dry material at the toe. Look for seepage, damp bands, and any change of colour that marks a water table or a perched layer. Look for evidence that somebody has been here before: a soft strip of different-coloured fill, old bedding stone, a service marker tape. Previously disturbed ground never returns to its parent classification, however long ago it was reinstated.
Manual tests confirm what the eye suggests. Thumb penetration gives a working feel for unconfined compressive strength: firm cohesive ground that resists the thumb and only indents under real effort behaves very differently from material the thumb sinks into. A pocket penetrometer or a torvane gives a number rather than an impression, and both belong in the site vehicle. Plasticity is checked by rolling a moist thread — cohesive soil rolls to a thin ribbon and holds together; granular soil crumbles and cannot be rolled at all. Dry strength on a broken lump separates cemented and cohesive material from soil that falls apart between the fingers. None of these replace a laboratory classification to ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes, but they are what is available at the face at seven in the morning.
The grades themselves are banded on strength and then overridden by condition. The strongest cohesive band commonly starts around 144 kPa (1.5 tsf) unconfined compressive strength, the middle band runs from roughly 48 to 144 kPa (0.5 to 1.5 tsf), and the weakest band covers everything at or below about 48 kPa. Overrides matter more than the bands. Granular material — sand, gravel, loamy sand — sits in the weakest band whatever a penetrometer says, because cohesion is what a penetrometer is measuring and granular soil has none to measure. Submerged soil, soil with water freely seeping from the face, and unstable submerged rock all land in the weakest band. So does any soil subject to vibration, and any soil in a layered system where a weaker bed dips into the cut.
What each grade buys you in batter
Slope ratios follow the grade directly. Under Subpart P the simple slopes are three-quarter to one for the strongest cohesive band, one to one for the middle band, and one and a half to one for the weakest, with vertical faces permitted only in stable rock, and simple slopes capped at about 6 m, or 20 ft, of depth before an engineer is required. Those ratios describe horizontal run per unit of vertical rise, and the run is what consumes the site.
Consider what the grade costs at ground level. A 3 m (10 ft) deep trench with a 900 mm (3 ft) formation width in the strongest band opens to roughly 5.4 m across the top, or 18 ft. The same trench in the weakest band opens to close to 10 m, which is 33 ft. That difference decides whether the dig fits between a boundary fence and a live carriageway, whether the footpath stays open, whether a lane closure is needed, and whether battering is viable at all. Crews rarely lose a trench to a wrong ratio; they lose the programme to discovering the ratio after the barriers are set out.
Layered ground is where composite judgement bites. Where beds of different strength stack, the safe default treats the whole face as the weakest bed present unless a designer has separated the layers and slopes each to its own requirement. A firm clay cap over a running sand is not a firm clay trench. The cap will stand while the sand runs out from beneath it and drops the cap as a single slab.
The grade is now settled, and the two numbers the crew needs before the excavator repositions — the batter angle and the resulting top width — come straight out of it.
Depth from existing ground to the trench bottom.
Width needed at the base for the work.
Run of trench at this profile, for the total excavation volume.
This calculator does not classify soil. A competent person must.
Excavation top width
19.5 ft
Geometry at the tabulated maximum allowable slope for the soil type you selected. It is not a classification, and it is not a protective-system design.
- Setback each side
- 8 ft
- Slope angle from horizontal
- 45 °
- Spoil volume per unit length of trench
- 3.41 yd³/ft
- Extra volume caused by sloping
- 2.37 yd³/ft
- Top width in feet
- 19.5 ft
- Total bank excavation volume
- 112.44 yd³
They open the calculator with your figures already in it
Trench Sloping & Excavation Width Calculator: 19.5 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
The top width is what the excavation costs you at the surface, and it is the figure people are surprised by: in Type C soil a trench 2.4 m deep opens 7.2 m wide before you have dug anything. The section is drawn to one scale on both axes, so the batter you see is the batter the soil classification requires.
What this calculation does not cover
- Soil classification must be made on site by a competent person using the tests in OSHA Appendix A. This calculator applies the slope for a classification you supply; it cannot make one.
- Excavations deeper than 20 feet (6.096 m) require a protective system designed by a registered professional engineer. The depth input is capped for that reason.
- Layered soils are classified by the weakest layer present, and previously disturbed ground is Type C regardless of appearance.
- Spoil must be kept at least 2 ft (0.6 m) from the edge, and that setback is not included in the top width here. Surcharge from spoil, plant or adjacent structures changes the problem entirely.
What takes the soil down a grade mid-dig
Nothing about a classification is permanent, and the downgrades come faster than the upgrades. Rain is the most common. Water entering the face reduces effective stress, raises the mass being retained and turns fissured clay into something that sloughs. Regimes generally require re-inspection after every rainstorm precisely because the grade may have moved overnight while nobody was on site.
Vibration downgrades ground to the weakest band regardless of what the penetrometer read. Plant tracking along the verge, a roller compacting the haunch, sheet piling on an adjacent plot, a busy road twenty metres (65 ft) away, or a rail line further off all qualify. The test is not distance but whether vibration is transmitted, and a hand on the crest of the spoil heap answers it faster than an argument.
Time works against the face on its own. An open cohesive trench dries, shrinks and cracks; the crack fills with water at the next shower and acts as a hydraulic wedge. Freeze and thaw cycles loosen a face that stood clean in autumn. Adjacent excavation, a leaking main discovered during the dig, or a spoil heap parked too close all change the loading. Any one of these obliges a re-grade, and a re-grade obliges a change to the batter, the bench or the shield before anybody steps back down.
Benching, and the soils that refuse it
Benching is available only in cohesive ground. It cannot be used in granular soil, and it cannot be used anywhere the classification has landed in the weakest band, which rules it out of most utility work in made ground and river terrace gravels. The reason is mechanical rather than bureaucratic: a bench relies on a vertical rise standing unsupported for the life of the works, and material without cohesion has no mechanism to do it.
Where benching is permitted, the vertical rise of each individual step is capped — commonly at about 1.2 m, the 4 ft of the US tables — and the cap applies to each step, not to the total height of the arrangement. Multiple benching stacks those steps behind an overall slope that must still satisfy the ratio for the grade. Cut the benches level and cut them wide enough to walk; a bench too narrow to stand on becomes a place to fall from, and a bench that slopes back toward the face ponds water straight into the crest.
Combination systems earn their place on deeper jobs. A battered or benched upper section over a shielded lower section keeps the top width inside the boundary while giving the crew a rated box at the working level. The junction between the two needs care: the top of a shield used this way generally has to project above the base of the sloped portion, commonly by at least 450 mm (18 in), so that material off the batter cannot arrive inside the box.
When the boundary says no batter
Boundaries, services and live carriageways rule out battering on most urban work, and the alternative divides into two families that are routinely confused. A shield — a trench box — does not prevent the face collapsing. It protects whoever is inside it when the face does collapse. Shoring is different: it applies load to the face and holds it. Hydraulic aluminium struts and walers, timber and screw jacks, sheet piles with waling frames and props, and slide rail systems all belong to the shoring family and all resist active soil pressure that varies with the grade.
Selection runs off the classification twice over: once to establish the lateral pressure the system must carry, and once to establish spacing. Manufacturer's tabulated data covers standard cases, and it has to be on site, legible, and matched to the actual grade rather than the optimistic one. Outside the tabulated envelope — unusual depth, surcharge from adjacent plant or structures, layered ground, sloping surface — the system needs design by a registered engineer. EN 13331-1, Trench lining systems, covers product requirements for lining equipment in the European market; national codes elsewhere carry the equivalent.
Installation discipline decides whether any of it works. Boxes go in as the dig proceeds, not after. Over-excavation below the base of a shield is limited — commonly to about 0.6 m (2 ft under the US tables), and only where the shield is rated for the full depth and no material is being lost from behind or below. Voids behind sheets or panels defeat the system entirely, so pack them as they appear. Struts are installed and removed in a defined order, top down for installation and bottom up for withdrawal, and nobody works below a strut being released.
Once a batter is ruled out, the system holding the face has to answer to the pressure this grade actually generates — which is a different question from whichever box happened to come off the last job.
The total (moist) unit weight of the retained soil — γ, not its dry density.
The soil's internal friction angle, in degrees, used in the Rankine active pressure coefficient.
The vertical height of the excavation face or shoring wall being checked.
Total active lateral force per unit length of wall
1,900 lbf/ft
This is a preliminary Rankine active-pressure estimate for COHESIONLESS soil under simplified conditions (no water table, no surcharge, vertical smooth wall) — a screening/educational estimate, NOT a stamped design. Cohesive soils, groundwater, surcharge loads (equipment, spoil piles, adjacent structures), sloped ground, braced/strutted multi-level systems (which use Peck's empirical apparent-pressure diagrams instead), and seismic loading all require different methods not captured here. Per OSHA 29 CFR 1926 Subpart P, the actual excavation protective system must be selected by a competent person using the Subpart P tables or designed by a registered professional engineer using site-specific geotechnical data — this calculator is not a substitute for that process.
- Peak lateral pressure at base of cut
- 381.95 psf
- Active pressure coefficient Ka
- 0.33 (dimensionless)
They open the calculator with your figures already in it
Excavation Shoring Lateral Active Soil Pressure Calculator (Rankine): 1,910 lbf/ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1,900 lbf/ft — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- There is no water in this model. Below a water table the soil pushes with its buoyant weight, but the water adds its own full hydrostatic pressure on top, and Ka does not reduce that part — the load on a saturated face is much higher than the figure here. Perched water, seepage through a granular lens, and a table that rises after rain are not checked at all.
- No surcharge is included. Spoil heaped along the crest, an excavator or crane tracking beside the cut, stacked pipe, site traffic, and the footings of any adjacent structure all add lateral load this ignores. On a tight site the surcharge can be the larger part of the total.
- Active pressure only develops if the wall moves enough to let the soil behind it relax. A stiff or tightly propped wall that cannot yield attracts at-rest pressure, which is substantially higher, and a multi-level braced excavation is designed to an apparent-pressure envelope of a different shape entirely. Size a strut off this triangular distribution and it comes out light.
- The formula is for cohesionless granular soil against a vertical smooth wall with level ground. It carries no cohesion term, no wall friction, no allowance for sloped or benched ground above the cut, and no layering — clay, silt and mixed fill do not behave this way, and a cohesive face standing unsupported for a shift is not evidence the pressure is low.
- This is a pressure figure, not a shoring design. It does not size sheets, soldier piles, wales, struts or anchors, does not set embedment depth below formation, applies no factor of safety, and checks nothing for toe stability, base heave or overall slope failure. Under OSHA 29 CFR 1926 Subpart P the protective system must come from the Subpart P tables, a trench shield manufacturer's tabulated data, or a registered professional engineer's design.
Water reclassifies the ground faster than anything else
Free water is the fastest route to the weakest classification and the hardest condition to argue with. Seepage from a face is not a nuisance to be pumped from the invert; it is evidence that the ground is losing fines and that the classification has changed. Sumping from inside the trench pulls water through the face and accelerates the loss. Dewatering belongs outside the excavation — wellpoints, ejectors or perimeter drainage — and it belongs in place before the dig reaches the water, not after.
Watch for perched water at a layer interface, which arrives without warning at a specific depth and turns a competent face into a running one within minutes. Watch, too, for the trench acting as a drain for a leaking service; a slow rise that does not correlate with rainfall usually has a pipe behind it. Where dewatering runs, somebody has to monitor it, and the excavation has to be re-inspected when it stops, including after an out-of-hours pump failure.
Whether a wellpoint scheme can hold the water table below formation depends on what the ground will actually yield to it, not on the pump sitting on the trailer.
The aquifer's hydraulic conductivity.
The undisturbed saturated aquifer thickness far from the well.
The remaining saturated thickness right at the pumping well, after drawdown.
The radius at which h1 is measured (often the radius of influence).
The radius of the pumping well itself.
Estimated well yield
44.7 gal/min
This is a steady-state estimate assuming a fully penetrating well in a homogeneous, isotropic unconfined aquifer — real dewatering systems are affected by partial penetration, aquifer boundaries, and non-steady flow, and should be verified with an actual pump test.
- Equivalent in m³/day
- 243.51 m³/day
- Equivalent in US gallons per minute
- 44.67 gpm
They open the calculator with your figures already in it
Dewatering Well Yield Calculator (Dupuit-Forchheimer): 44.67 gal/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 44.7 gal/min — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The equation covers one fully penetrating well pumping on its own. It does not add the drawdown cones of neighbouring wells or wellpoints, so it cannot tell you how many wells an excavation needs or how far apart to set them — in a ring or a line each well delivers less than this figure.
- Steady state only, in a homogeneous, isotropic, unconfined aquifer. There is no time in the arithmetic, so the higher rate while the cone is still developing and the total volume pumped over the job both sit outside it, as do partial penetration, layered or anisotropic ground, confined and leaky aquifers, and any recharge or no-flow boundary such as a river, a cut-off wall or bedrock.
- This is a planning-level flow estimate, not a dewatering design and not a pump specification. Screen entrance velocity, filter pack capacity, drawdown available to the pump, suction lift limits, friction losses in the header and discharge run, and standby capacity are all separate calculations, and the yield itself should be confirmed by a pump test before plant is ordered.
- Nothing here covers the consequences of lowering the water table: settlement of shallow-founded structures and services inside the drawdown cone, migration of fines and piping into the well, or the discharge consent and abstraction licensing the pumped water needs. Where drawdown reaches a neighbour's property, an abstraction, or compressible ground, that is a geotechnical engineer's assessment, not a page of arithmetic.
- Yield is directly proportional to the hydraulic conductivity you type, and K in natural ground varies by orders of magnitude over short distances and directions. The result carries no uncertainty band around that, and the K box is read as metres per second whichever measurement system the page is set to.
Spoil, surcharge and the zone of influence
Everything stacked near the crest loads the ground the classification was assessed on. Spoil is the obvious case and the usual offender: most regimes set a minimum setback of about 0.6 m (2 ft) from the edge, and on anything other than firm cohesive ground a metre (3 ft) or more is the sensible working figure. The setback exists for two reasons — surcharge on the face, and material rolling into an open trench onto somebody's back.
Plant, pipe stacks, arisings skips, crane outriggers and site traffic all surcharge the face, and none are covered by the tabulated case unless the tables say so. As a working screen, temporary works designers commonly project a line up and out from the trench base to define the zone of influence; anything with a footprint inside that zone loads the excavation. A building foundation inside it needs support designed before the dig passes it, not after the crack appears in the render.
Mark that influence line on the site layout before anybody decides where the spoil heap, the stone stack and the machine will stand — it reaches further back from the base than a walk round the job suggests.
The vertical depth of the excavation.
The OSHA soil type classification (A, B, or C) determined by a competent person's field tests.
Minimum required horizontal setback
10 ft
Soil classification (Type A/B/C) MUST be determined by a competent person using OSHA 29 CFR 1926 Subpart P Appendix A's field tests (not assumed) — misclassifying soil is a common cause of excavation collapse. This simple-slope method is only valid for excavations 20 ft (6 m) or less in depth; deeper excavations, benched configurations, or soil with seeping water require the full Subpart P tables or a registered professional engineer's design. Type A soil has a further exception (1/2:1 for short-term cuts ≤12 ft) not implemented here — use the more conservative 3/4:1 ratio shown unless your competent person confirms the exception applies.
- Slope ratio used (horizontal:vertical)
- 1 :1
They open the calculator with your figures already in it
Excavation Sloping/Benching Setback Calculator: 10 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
The setback is measured from the bottom edge of the excavation out to where the top of the slope breaks. Nothing — spoil, plant, material — belongs inside that line. This page never asks how wide the base is, so the section shows the batter and the depth and leaves the bottom open rather than closing it on a width you did not give.
What this calculation does not cover
- The three ratios held in the lookup — 0.75, 1.0 and 1.5 — are the steepest cuts permitted for each class, so the distance returned is the smallest setback allowed rather than a recommended one; laying the face back further is always acceptable and is frequently what the ground, the weather or the length of time the cut stays open actually calls for.
- A single letter drives the whole answer and it is applied from formation level right up to the crest, so ground that changes character partway up the face — a firm crust over soft alluvium, made ground sitting on undisturbed material, a granular lens between clays — is not handled, and a face like that is governed by its weakest band and not by the class picked here.
- Depth multiplied by ratio gives the horizontal run of one face measured out from the toe. It is not the width of the excavation at ground level, since no trench or base width is asked for, and it produces no spoil volume, no footprint of site consumed, and no allowance for the working room, plant standoff or access route you still need beyond the top of the slope.
- Only the depth and the soil class reach the arithmetic, so nothing in it represents how long the cut stays open or what goes on beside it. Rain, freeze-thaw, drying cracks along the crest, vibration from compaction or passing plant, and ground disturbed by earlier work all degrade a face that stood well when it was first dug, and the setback shown does not move.
- Taking the depth as one vertical dimension presumes the ground above the crest is level; where the site falls away behind the cut, rises toward it, or carries an embankment, an old excavation or a services trench above the face, the section being dug no longer matches the geometry these ratios were written for.
Code thresholds this tool can check
Code thresholds this tool can check
Checked for United States. Each check below names the body that published the limit it uses. Switching market re-runs them. This is not a code review and has no official standing.
These checks cover only the specific numeric limits listed below. They are not a complete code review: fire separation, egress, structural capacity and accessibility provisions are outside their scope, and only the handful of local amendments offered in the selector are modelled — your municipality may have others. Passing every check here does not make a design compliant. Final approval rests with your local building authority.
WITHIN LIMIT — Simple slope method valid only to 20 ft (6 m) depth; deeper cuts need an engineered design.
Depth 3.0 m is within the 20 ft (6 m) simple-slope range, provided a competent person has classified the soil.
OSHA · 29 CFR 1926 Subpart P, Appendix B · 29 CFR 1926 Subpart P App. B
The volume the classification just created
Cutting to a batter or a bench does not simply make the trench safer, it makes it bigger, and the volume arrives on the muck-away schedule and again on the backfill schedule. A battered trench is a prism, not a box, and the extra material is proportional to the square of the depth — which is why a modest depth increase on weak ground can double the arisings against the estimate drawn from the pipe trench on the section.
Backfill follows the same arithmetic in reverse, with two adjustments the take-off has to carry. Excavated material bulks on the way out, typically in the range of fifteen to thirty per cent depending on the soil, and compacts back down on the way in, so the volume leaving site and the volume of imported fill required are not mirror images. Bedding, surround and any capping layer occupy part of the trench and are usually imported to specification, so they come out of the general backfill quantity rather than adding to it.
Shields change the backfill method as well as the quantity. Backfill and compact in lifts as the box is drawn up, because pulling a box out of a completed backfill leaves a void the width of the panels along the full length of the run, and that void will find the surface as a trench-line settlement trough. Compaction in layers of the specified thickness, generally in the region of 150 to 300 mm (6 to 12 in) depending on the plant and the material, is what keeps the reinstatement out of the defect list.
What goes back into the hole
- Surface reinstatement — measured against the top width of the cut and whatever overband the street authority insists on, never against the width of the pipe underneath it
- Granular sub-base — crushed stone beneath the surfacing, ordered by the tonne because weight is the unit the quarry ticket and the haulier both work in Gravel Base Layer Tonnage Calculator
- General backfill — goes back as a stack of thin layers rather than one drop, so the compaction plant on site decides how many passes the run costs as much as the void decides the tonnage Backfill Compaction Lift Count Calculator
- Cover over the crown — placed and consolidated by hand, with nothing tracked over it until there is enough depth above the barrel to spread a machine's weight
- Pipe — laid between two fixed invert levels, and the fall between those two figures is what drives the dig deeper at the downstream end Storm Drain Pipe Slope Calculator
- Bedding and haunch — imported stone tamped in under the springings so the barrel bears along its whole length rather than on a few high points of dug formation
The moment the slope is fixed is the moment the arisings and the imported fill stop matching the pipe trench on the section, so price the prism here rather than discovering it at the weighbridge.
The total length of the trench.
The width of the trench.
The depth of the trench.
The outer diameter of the pipe being laid; 0 for a trench with no pipe.
Depth of bedding material under the pipe; 0 if the pipe sits on the trench bottom.
Depth of the same bedding material over the top of the pipe; 0 for none.
Extra loose material needed to achieve full compaction in the void.
Loose backfill material needed
50.9 yd³
Assumes the excavated soil itself isn't reused as backfill (e.g. importing clean granular fill) — if reusing native soil, account for its own swell factor separately.
- Excavation volume (the depth entered)
- 48.89 yd³
- Pipe volume (subtracted)
- 4.6 yd³
- Compacted backfill void
- 44.29 yd³
They open the calculator with your figures already in it
Trench Excavation & Backfill Volume Calculator: 50.93 yd³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
The pipe is drawn to scale inside the trench. Backfill is everything else in the section, which is why the pipe diameter changes the answer at all.
What this calculation does not cover
- The trench is modelled as a plain rectangular prism — vertical faces, one width and one depth over the whole run. Battered or benched sides, the extra width a trench box needs, over-break outside the drawn line, and a bottom that falls with the pipe's gradient are all outside it. A run cut back to a safe slope holds considerably more than this figure, and the shortfall rises with the square of the depth rather than in proportion to it.
- This is a quantity take-off, not an excavation safety assessment. Nothing here classifies the soil, checks the depth against sloping, benching or shoring requirements, or sizes a protective system — that comes from a competent person on site, and past the depths the rules set, from an engineer.
- Only the single pipe you enter is deducted. A second pipe or duct sharing the trench, cable bundles, manholes, chambers, valve boxes, thrust blocks and concrete surround all displace backfill and are not subtracted. No check is made that the pipe fits the trench you described either: where its volume exceeds the excavation, the answer is floored at zero rather than reported as impossible geometry.
- At most two materials: a bedding and surround zone when you enter a bed or a cover, and one backfill above it at one flat percentage. The bedding row is an in-place volume across the full trench width, capped at the trench depth, with no compaction or waste allowance, so add your own for a graded bedding that is compacted. Marker tape or protective tiles, and the sub-base, blacktop or topsoil at the surface are further materials in further thicknesses and are not split out. The percentage is a loose-volume allowance on the backfill and nothing else — it is not a density or Proctor specification, and it says nothing about lift thickness or how many passes the plant makes.
- Nothing is said about the spoil. The excavation row is a bank volume measured in place, not the loose volume that leaves in the truck, and the calculation does not judge whether the arisings can go back, how much of the void they would fill, or what has to be carted away. Rock, groundwater and dewatering, and over-excavation to remove unsuitable ground are all excluded.
Getting in, getting out, and what is in the air
Access sits at 1.2 m (4 ft) in most regimes: at that depth and below, a ladder, ramp or stair must be within roughly 7.6 m (25 ft) of lateral travel for anybody in the trench. Ladders are tied and extend about a metre (3 ft) above the landing. Inside a shielded run the ladder goes inside the box, not beside it, because the exit route has to sit within the protection.
Atmosphere is the hazard crews under-rate. Where a hazardous atmosphere could reasonably exist — contaminated ground, made ground and landfill, work near sewers, fuel stations or gas mains, or any deep excavation with poor natural ventilation — testing is required before entry and monitoring during the work, covering oxygen, flammable gas and toxic contaminants such as hydrogen sulphide and carbon monoxide. Engine exhaust from a pump or a generator set on the crest will find its way into a trench.
Inspection is a daily obligation, not an event. The competent person inspects before each shift, as needed through the shift, and after every rainstorm or other occurrence that could increase the hazard. Anything found is either corrected before entry or the excavation stays closed. Edge protection, crossing points for pedestrians and lighting for night work belong to the same inspection.
Recording the call, and recording the change
Because the classification is the design input, changing it is a design change. Record the grade with the date, the time, the tests performed and the person who made the call, and photograph the face while doing it. When the grade moves — after rain, after a piling rig arrives next door, after a layer of gravel shows up at formation — record the new grade the same way and tell the crew before they go back in.
Two failures recur on incident reports. The first is a face that was classified once, on day one, and never reassessed through six weeks of open trench. The second is a crew that knew the grade had changed and had no route to stop the work. Both are failures of the same mechanism: the soil kept deciding, and nobody was listening for the decision.
On the truck before the first cut
A trench take-off starts with the classification kit, not the pipe schedule — the grade determines both the geometry and the quantity of everything that follows.
- Pocket penetrometer and torvane — Turns a thumb impression into a number at the face; keep both in the vehicle, not the site office.
- Manufacturer's tabulated data for every box and shoring set — Must be legible, on site, and matched to the grade actually found rather than the one assumed at tender.
- Bedding and surround to specification — Imported and placed inside the trench, so it comes out of the general backfill quantity rather than adding to it.
- Spoil setback markers — Around 0.6 m or 2 ft minimum in most regimes; a metre or more on anything below firm cohesive ground.
- Four-gas detector with oxygen, LEL, hydrogen sulphide and carbon monoxide — Pre-entry test plus monitoring wherever made ground, sewers or fuel infrastructure are in play.
- Ladder or ramp for each 7.6 m (25 ft) of lateral travel — Tied, extending about a metre above the landing, and positioned inside the shield rather than alongside it.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.