What is excavation in construction? Trenches, soil types and Ontario's rules.

Excavation means removing soil or rock from the ground, and the word also names the hole that removal leaves. In Ontario it has an exact legal meaning. The province's construction regulation, O. Reg. 213/91, defines an excavation as the hole that is left in the ground as a result of removing material, and excavating is the work of making it. A trench is an excavation deeper than it is wide. The rules for any dig turn on two things: which of four soil types the walls are cut in, and whether the hole goes past 1.2 metres, where the walls must be sloped back or supported. Before any of it starts, Ontario law requires a locate request through Ontario One Call, and the request is free.

What excavation means.

Section 1 of O. Reg. 213/91 defines an excavation as "the hole that is left in the ground, as a result of removing material". The Canadian Centre for Occupational Health and Safety, the federal agency that publishes workplace safety guidance, uses nearly the same words: an excavation is a hole in the ground as the result of removing material.

On a building project the word does two jobs. It names the hole, and it names the stage when the digging happens: footings and foundations, trenches for water, sewer and electrical lines, drainage work, and grading a site before a build. A basement, a service trench and the hole for a deck footing are all excavations in the regulation's sense.

Excavating or excavation: the work and the hole.

Excavation is the noun and excavating is the activity. The regulation uses both in the same Part. An excavation is the hole, and section 223 says Part III "applies to all excavating and trenching operations", which is the work.

The locate law adds a third word. Ontario's Underground Infrastructure Notification System Act calls anyone who digs an excavator, defined as any person or entity that "digs, bores, trenches, grades, excavates, moves or breaks earth, rock or the materials in the ground", or intends to. A homeowner with a shovel fits that definition as well as a contractor with a machine, so the duty to request locates falls on both.

A trench is an excavation deeper than it is wide.

The regulation defines a trench as an excavation where the depth exceeds the width. A narrow cut for a water service or weeping tile is a trench. A wide, shallow hole for a slab is not.

Some rules apply only to trenches. Under section 225, nobody works in a trench unless another worker is above ground close to the trench or to its way out. Under section 235, a trench deeper than six metres or wider than 3.6 metres needs an engineered support system designed for that location and project. And section 234 lets an excavation that is not a trench go without supports where no worker has to be closer to a wall than the wall is high. A trench gets no such allowance.

Is there a list of three types of excavation?

Ontario's rules do not name three types. The regulation sorts every dig by the soil its walls are cut in, because the soil decides how the walls are cut.

Its nearest thing to a list of three is support systems. Section 222 defines an engineered support system, designed for one project and assembled in place; a hydraulic support system, moved as a unit and pressing back on the walls through hydraulic struts; and a prefabricated support system, meaning a trench box, trench shield or similar structure moved as a unit. Section 236 requires every one of the three to be designed by an engineer. Timber shoring is allowed too, within the limits set by a table in section 238.

Sorted by purpose, the excavation work we take on falls into three groups: trenches for water, sewer, electrical and drainage lines; footings and foundation digs; and clearing and grading a site before a build.

Four soil types decide how the walls are cut.

Section 226 sorts soil into four types. Type 1 is hard and very dense, can be penetrated only with difficulty by a small sharp object, shows no water seepage and can be dug only by machine. Type 2 is very stiff and dense, and looks damp once it is dug. Type 3 is stiff to loose, may crack at the surface or seep water, and includes any soil that has been dug before. Type 4 is soft, wet or muddy, runs or flows unless it is supported, and loses much of its natural strength once disturbed.

Two rules sit on top of those descriptions. The type is set by looking at and handling the soil at the walls, and back from each wall as far as the dig is deep (section 227(1)). Where a dig holds more than one type, the whole excavation is classed as the highest-numbered type present (section 227(3)).

Under 1.2 metres deep, no support system is required. Deeper than that, the walls are supported or sloped back: one vertical to one horizontal in Types 1, 2 and 3, and one vertical to three horizontal in Type 4. In Types 1 and 2 the slope may stop 1.2 metres above the bottom, leaving that last stretch of wall vertical. Our guide to what excavation costs works through what a three-to-one slope does to the width of a trench.

Two ways to stop a cave-in.

The Canadian Centre for Occupational Health and Safety names two basic methods of protecting workers from a collapse: sloping, which cuts the walls back at an angle away from the work, and temporary protective structures such as shoring and trench boxes. Sloping needs room. Two metres down in Type 4 soil, sloped from its base at one to three, each wall reaches six metres back from the bottom edge. Where that space does not exist, beside a house or a property line, the walls are supported instead.

The same agency lists the hazards behind the rules: cave-ins, equipment or soil falling into the hole, people falling in, water collecting, bad air, and contact with buried or overhead lines. Ontario's regulation answers two of them directly. A level strip at least one metre wide along the top of every wall stays clear of equipment, excavated soil, rock and material (section 233(1)). Where someone could fall into an excavation more than 2.4 metres deep, a barrier at least 1.1 metres high goes along every wall that is not sloped (section 233(4)).

Locates come before the first shovel.

Every dig in Ontario starts with a locate request through Ontario One Call. The Act says the corporation shall not charge a fee to any excavator for submitting one. On a standard request, each utility has five business days to mark its lines. Our guide to Ontario One Call locates covers the process and what each paint colour means.

Excavation in Ottawa, where the clay changes things.

Ottawa adds one local factor. A City of Ottawa Planning Committee report from 2017 estimated that approximately 34 per cent of land in Ottawa has the potential to contain sensitive marine clays, the Champlain Sea deposit known locally as Leda clay. The regulation's description of Type 4 soil uses the same words: very sensitive, and significantly reduced in strength once disturbed. Whether one lot sits on it is decided by examining the soil on site or by a soil report, never by a map.

We do excavation in Ottawa and Gatineau: trenches, footings, grading and site prep, with the locate request filed before we break ground and the spoil hauled away on the same visit. Call (613) 852-4059 or email info@ottawasiterestoration.ca with a photo of the site and how deep it needs to go. You get a price before the machine arrives.

Sources

• Government of Ontario, O. Reg. 213/91, Construction Projects: the definitions and Part III, Excavations read 2026-10-01
• Government of Ontario, Ontario Underground Infrastructure Notification System Act, 2012 read 2026-10-01
• Canadian Centre for Occupational Health and Safety, Trenching and Excavation read 2026-10-01
• City of Ottawa, Planning Committee report, Tree Planting in Sensitive Marine Clay Soils, 2017 read 2026-10-01

Related reading

• Excavation in Ottawa
• Ontario One Call locates
• What excavation and dirt removal cost
• Bin and dumpster rental
• Demolition services
• All guides

Page last updated October 1, 2026