Takeoff & Estimating — Concrete, Sitework & Utilities

Trench

A narrow excavation whose depth and sides drive both safety and cost.

Quick Answer

A trench is a narrow excavation made below the surface, where OSHA says the width at the bottom is not greater than 15 feet. Contractors dig trenches for pipe, conduit and footings. In takeoff it is measured in linear feet and cubic yards, and depth sets whether sloping, shoring or a trench box adds quantity.

The Full Picture

OSHA defines a trench as a narrow excavation made below the surface of the ground, in which the depth is greater than the width and the width measured at the bottom is not more than 15 feet. Trenches carry pipe, conduit, duct banks and utilities, and continuous footings are often dug as trenches too. A trencher is the machine that digs a narrow trench in one pass, usually a chain or wheel type, and it is chosen for long, shallow, repetitive runs such as irrigation, small conduit and direct-buried cable. Wider or deeper work is normally done with an excavator or a backhoe.

Depth drives most of the estimating decisions. Under OSHA's excavation standard, an employee in an excavation needs a protective system unless the ground is stable rock or the excavation is less than 5 feet deep with no sign of a cave-in. Protection means sloping or benching the walls, supporting them with shoring, or working inside a trench shield or box. Trenches 4 feet or deeper also need a safe way out, such as a ladder, within 25 feet of lateral travel. Soil type, which a competent person classifies, decides how steep a slope can be.

How it is quantified: trench is measured in linear feet by depth range and width, and excavation is converted to cubic yards by multiplying length, average width and average depth, then dividing by 27. When walls are sloped, the top width is larger than the bottom width, so the volume is the trapezoidal cross-section times length. Shoring and trench boxes are carried as a separate line, often by the day, or by the square foot of shored wall. Bedding, backfill, spoil haul-off and dewatering are separate quantities as well.

The pricing mistakes are consistent. An estimator who prices a 9 ft deep trench as if it were a straight-sided 3 ft wide cut will undercount the dirt that sloping adds, and the pavement, fence or building line that limits the top width may make sloping impossible. Others forget that spoil has to be kept at least 2 feet back from the edge, which can mean double handling in tight sites. The plans show trench profile in the civil details and utility sheets, and the specs in Division 31 set the bedding, compaction and backfill requirements.

Real Examples

→Worked takeoff: sloped trench vs. trench box: 150 LF of trench, 8 ft deep, with a 3 ft bottom width. If the soil is Type C and the walls are sloped at 1.5:1 (horizontal to vertical), the slope per side is 1.5 x 8 = 12 ft, so the top width is 3 + 12 + 12 = 27 ft. The cross-section is (3 + 27) / 2 x 8 = 120 SF, so volume = 120 x 150 / 27 = 667 CY. In a 5 ft wide trench box, the same trench is 5 x 8 x 150 / 27 = 222 CY, and the box becomes its own rental line. The difference is why the method has to be decided at takeoff.

→Trencher for a shallow run: A site needs 1,800 LF of 4-inch wide, 24-inch deep trench for irrigation sleeves and low-voltage conduit. A chain trencher makes sense here because the run is long, narrow and shallow. The estimator prices it by LF with the trencher as a rental line and does not convert to a bulk excavation quantity.

→Trench at a property line: A sewer lateral must cross 40 LF between a building and a property line with only 6 ft of working room. Sloping would exceed that width, so the estimator carries shoring or a trench shield for that segment even though the rest of the run is sloped.

Common Misconceptions

People assume: A trench only needs protection once it is over 20 feet deep.

Actually: Protection generally starts at 5 feet, unless the ground is stable rock or a competent person finds no indication of a cave-in in a shallower cut. Excavations over 20 feet require a registered professional engineer to design the protective system.

People assume: Trench quantity is just length times width times depth.

Actually: Sloped walls, benching, over-excavation for bedding and working room around structures all increase the cubic yards beyond a simple rectangular box.

Does MeltPlan Solve This?

Partially — adjacent

Trench lengths, depths and widths are quantities estimators measure from civil and utility plans. MeltPlan's takeoff uses AI to pull quantities from PDF plans, and US-based experienced estimators review and adjust them into a verified takeoff. Choosing the protective system and means of excavation remains the contractor's call.

Measure trench LF and depth from plans →

Frequently Asked Questions

What is a trencher?

A trencher is a machine with a chain or wheel cutting system that digs a narrow, continuous trench in one pass. It is typically used for long, shallow runs such as irrigation, small conduit and direct-buried cable. Deeper or wider trenches are usually dug with an excavator or backhoe.

How do you calculate trench volume?

Multiply the length by the average cross-section area (average of top and bottom width, times depth for a sloped trench) and divide by 27 to get cubic yards. Use bank measure for the quantity, then apply swell separately if the spoil is hauled.

What are the OSHA sloping options?

OSHA's excavation standard lets an employer use sloping, benching, shoring or a shield. A default option uses Type C slopes of 1.5 horizontal to 1 vertical, and Appendix B gives maximum slopes by soil type, such as 1:1 for Type B and 3/4:1 for Type A in cuts of 20 feet or less. A competent person classifies the soil.

Where are trench requirements shown in the documents?

Utility and civil plans show alignment, depth and details such as typical trench sections. Division 31 specifications set bedding, backfill and compaction, and the contract or safety plan covers the protective system.