Trench takeoff: 3 measures before pricing shoring

Before pricing shoring, record 3 trench measures: depth, soil type, and the clear strip beside the trench edge.

Key takeaways

Record depth changes along each trench.
Identify soil information gaps before pricing.
Mark the clear strip and any obstructions.
Check protective-system requirements under the legislation for your project.

Before you pick a method or put a number on support, you need to know whether the trench calls for a protective system at all. That answer starts on the plans, with the measurements you take off before anything else.

This post is practical guidance for the takeoff. It sets out what to measure and record, and it links the sources for each figure it uses.

WorkSafeBC says that excavations present a number of risks to workers, including contact with buried facilities and cave-ins. WorkSafeBC's excavation page The takeoff described here covers the measurements that feed the shoring decision, not every hazard on the site.

Why the 3 measures come before any shoring price

The depth measure, and whether the ground is entirely stable rock, can change whether a protective system is needed, and you cannot price that need without them. The third measure, the clear strip, is a site condition you still have to plan around, so it belongs on the takeoff too. Record all three for each trench before you open the pricing sheet, so the shoring decision is made from the same data you show in the bid.

Start with the length. Measure each trench along its centreline on the plan, and mark the start and end of each segment on the drawing so you can find it again. Check the drawing scale before you measure, and check your measured length against any dimension printed on the sheet. A length that is wrong at this stage carries into every line of the estimate, including the shoring line, so it is worth a second look before you move on. For related setup checks, see takeoff scale setup and stated limits.

What depth triggers a protective system?

CCOHS says that, in general, trenches that are 1.2 metres (4 feet) deep or greater require a protective system unless the excavation is made entirely in stable rock. The sentence comes from the Canadian Centre for Occupational Health and Safety's page on trenching and excavation: CCOHS: Trenching and Excavation

Two parts of that sentence matter for a takeoff. "In general" tells you the sentence states a general rule, not a rule with no exceptions. "Unless the excavation is made entirely in stable rock" means you need to know what the trench is cut through before you apply the depth figure.

The same CCOHS page also says: "Do not enter an unprotected trench deeper than 1.2 metres (4 feet), or as specified in the legislation." That last phrase sends you to the legislation. For your bid, the 1.2 metres (4 feet) figure is the CCOHS number, and the legislation that applies to your project is the document to check.

Measure the depth wherever the distance from the ground surface to the trench bottom changes, and split the run into segments at those points. A single average figure can hide the part of the trench that reaches 1.2 metres (4 feet), so record each segment's depth separately. Measure from the existing ground surface down to the bottom of the trench. If the drawings include profiles or sections, use them alongside the plan view when you read depth.

Keep the units consistent as you go. If the plans are in metres, record depth in metres, and convert only once, at the point where the numbers go into the estimate. A depth typed in feet into a metric column would change which segments appear to reach the 1.2 metres (4 feet) figure, so check the units on each row before you flag it.

How should you record soil type?

Record each segment's soil using the classes CCOHS describes, which typically run on a scale of 1 to 4 or on a scale of A to C. The page says: "Soil types typically use a scale of 1 to 4 where 1 is hard and dense to 4 which is loose, soft, wet or muddy soil, or a scale of A to C where A is hard and solid, and C is soft, sandy, filled or loose." CCOHS: Trenching and Excavation

Use the scale that matches the soil description you have, and write that scale in the takeoff so the next reader knows which one you used.

If the project has a soil report, use it to set the class for each segment, and note the report's date in the takeoff. If the project has no soil information for a segment, say so in the takeoff rather than leaving the field blank, so the gap is visible to whoever reviews the bid.

Note any change in soil class along the trench, the same way you note a change in depth. A change in ground along the run is a change in the takeoff, and it should show up as a separate segment. If a segment has no soil class in the documents you have, ask the owner or designer before you price it. Do not fill the gap with a guess from memory.

The soil and depth measures are linked by the stable rock exception. Record rock portions and soil portions separately, but assess whether the excavation is made entirely in stable rock; dividing a mixed excavation into segments does not establish an exemption.

How much clear space does the trench edge need?

CCOHS says: "Do not place the sections of pipes, piles of spoil, unused tools, and timber, and other materials within 1 metre from the trench's edge." CCOHS: Trenching and Excavation

The quote gives a distance from the trench edge, so the takeoff should show the strip on the plan for each trench, measured from the trench edge. Note where the strip crosses a road, a pedestrian path, an existing structure, or another work area, because a strip that is blocked changes how the site is planned. If something sits inside the strip and cannot be moved, flag it in the bid so the client can make a decision about it before work starts.

The strip sits outside the cut itself. That means it adds to the working area you plan around the trench, even though it does not appear in the trench volume. When you lay out staging for pipe, spoil, tools, and timber, check each staging area against the strip before you commit to a location on the sheet. See also site storage limits before staging.

Other work may also use the ground beside the trench. Before you assume the strip is free, confirm with whoever controls the site which areas are reserved for the excavation and which are open. Write the answer in the takeoff, so the layout reflects what was confirmed rather than what was assumed.

Why does spoil weight matter for the bid?

The HSE gives a figure for the weight of spoil. Its excavation page says: "Depending on conditions, a cubic metre of soil can weigh in excess of 1.5 tonnes." Excavations - HSE

The HSE page is UK guidance, so the figure describes soil in general, not a rule for your project. The quote gives a sense of scale for the weight of soil.

For the estimate, the volume of spoil comes from the trench dimensions, so compute it from the segment lengths, widths, and depths you have recorded. Keep the figure in the takeoff as a volume. Do not turn the HSE weight figure into a hauling weight for the bid unless the soil data you have supports that conversion, and if you do use it, state the assumption and where it came from.

How do the 3 measures feed the shoring decision?

Go through the trench segments one at a time. For each one, write the depth, the soil class, whether the excavation is entirely stable rock, and the strip note. Use those readings to flag protective-system requirements for assessment under the legislation for your project.

A protective system is not necessarily shoring, and these readings alone do not establish a method or price. Do not treat a shallow reading as the end of the question without checking the legislation for your project.

A segment priced for shoring should carry the reading that put it there. If the depth is the reason, the depth reading and its segment number go next to the shoring line. If the soil class is the reason, the class goes there too. That way someone checking the bid can trace the number back to the takeoff without rebuilding your work.

The takeoff is not final until the plans are final. If a drawing revision changes a depth, a soil description, or the layout near a trench, rerun the flags for the affected segments before the bid goes out. Note in the bid which revision of the drawings you used, so the numbers can be traced to a specific set of plans.

What should the bid say about these assumptions?

State the depth, soil class, and stable rock assumptions in the bid, so the shoring line can be checked against the takeoff. A shoring line with no assumptions behind it is hard to check, and it is hard to revise when the site information changes.

List the depth readings you relied on, the soil classes you used, and any stable rock portions recorded. If you asked the owner or designer about a segment's soil and got an answer, record the answer and the date you received it. If the protective-system assessment led to a shoring allowance, record the basis for that allowance in the bid.

Put the 1 metre strip in the same list when it affects the layout or the staging. A blocked strip can change how spoil, pipe, and materials are handled, and the bid should show that the constraint was considered.

Write any clarification you received as a written note, not as a verbal answer. A note that says what was asked, who answered, and what the answer was gives the estimate something to stand on if the site information changes later.

What this takeoff covers

This post does not cover how to design shoring, which system to choose, how to size support, or what a shoring line costs. For those answers, go to the legislation that applies to your project and to the people responsible for the design.

The CCOHS guidance is Canadian, and the HSE page is UK guidance. Neither should be carried into a project somewhere else as if it were that place's rule. Check the rules that apply where the work is.

Illustrative example: recording a depth change

Its first 5 metres are 1 metre deep, and its remaining 5 metres are 2 metres deep. Record the portions separately rather than using an average depth. Add the soil information and strip note to each portion, then use the readings for the protective-system assessment. The example does not select a system or calculate a shoring price.

What to write on the takeoff sheet

For each trench segment, write the length, depth, soil class, rock status, and strip note.

The segment number or name, and its length along the trench centreline
The depth at the bottom of the segment, measured from the existing ground
The soil class and the scale it came from, if the documents give one
Whether the excavation is entirely stable rock, or a mix of rock and soil
The 1 metre strip along each edge, and anything that sits in it or blocks it
A flag for protective-system assessment under the applicable legislation

Today, choose a trench on your current plans and record its depth changes, soil information, and strip constraints before opening the pricing sheet.

What should you check before pricing?

Can an average depth replace separate readings?

Record separate readings wherever depth below the ground surface changes. Use the profiles and sections alongside the plan view.

What if soil information is missing?

Record the gap and ask the owner or designer before pricing. Keep the written answer with the takeoff.