typical excavation depths,grade,and accuracy

Offering reliable landscaping services to homeowners in British Columbia since 2013

Picture of Sam Maerz

Sam Maerz

Owner/Foreman

Hello, I’m Sam Maerz from Back 40 Landscaping. Along with my Wife Natasha we own and operate Back 40 Landscaping and are based out of Abbotsford B.C.

We are primarily a landscape excavation company that uses our vast selection of equipment to move soil and gravel to instal Allan Block retaining walls, Concrete pavers, Drainage systems, and compacted gravel pads for foundations, driveways, parking, and storage.

retaining-wall-excavation

I am certified and experienced in Concrete retaining walls, and Interlocking Concrete Paver Systems. This schooling and experience is where I have learned the most common excavation depths, grades, and accuracy requirements.

Typical excavation depths

Typical in this case means in normal or ideal soil conditions. The poorer draining the soil, the deeper excavation will be required. Good draining sandy gravels are best, while blue/silver clay soils are the worst. The most common excavation depths would be based on the soils found between these two types. Normal soil in the Fraser Valley is tan/brown, has some baseball to softball sized rocks in it, and drains moderately well.

Why does the soil type determine how deep an excavation should be?

Poor soils have lower weight bearing capacity. This means that they will rut easier, and sink into the ground more. If this is under a concrete slab, the concrete will crack. This is why concrete paver driveways typically show tire ruts. The base underneath the pavers is not thick enough for the soil type.

The thicker the gravel base is, the wider its able to disperse the weight, and thus transfers less weight to a single spot. Thicker gravel bases kind of float over poor soil types.

Excavation depths are a combination of the thickness of the gravel base, and the thickness of the finish product I.E Concrete, asphalt, concrete paver, Allan Block retaining wall block, etc.

 

The most common excavation depths are as follows

 

Residential excavation depths

British Columbia building code sets the frost line at 18”. This means any infrastructure that shouldn’t freeze like electrical conduit, plumbing, etc. must be below this depth.

excavation-trench

Poured concrete residential house foundation footings have an excavation depth of 20-24” (18” for frost protection, plus 2-6” of gravel)

Gravel depth will be specified by geotechnical engineer based on site specific soils. Even if gravel is not required it is often used to keep the site clean and less muddy for the framers.

 

Allan Block retaining walls will require at a minimum 10” for excavation (6” for gravel, 4” for buried block). The taller the retaining wall, or if there is a slope ABOVE or BELOW the wall, the more blocks need to be buried and the deeper the excavation will need to be.

drainage-pipe-retaining-wall

Interlocking concrete paver systems for driveways in a residential application will require an excavation depth of 8.5”-9” (6” for base gravel, and 1” for bedding sand, and on average 2.5” -3.5” for the paver itself) excavate deeper in poorer soils.

Of NOTE is the actual thickness of the paver being used. Driveway pavers may need to be thicker than typical pavers for foot traffic.

gravel-sidewalk-screeding

Interlocking concrete paver systems for patios and sidewalks in a residential application need an excavation depth of 7.5” (4” for base gravel, and 1” for bedding sand, and on average 2.5” for the paver itself) excavate deeper in poorer soils.

flagstone-paver-sidewalk-prep

Poured concrete driveways in a residential application have an excavation depth of 10” (6” for base gravel, and 4” for concrete) excavate deeper in poorer soils.

Poured concrete for patios and sidewalks in a residential application have an excavation depth of 8” (4” for base gravel, and 4” for concrete) excavate deeper in poorer soils.

Asphalt for driveways in a residential application has an excavation depth of 9” (6” for base gravel, and 3” for asphalt) excavate deeper in poorer soils.

driveway-addition

Asphalt for patios and sidewalks in a residential application has an excavation depth of 6.5” (4” for base gravel, and 2.5” for asphalt) excavate deeper in poorer soils.

Residential gravel areas used for parking, or driveways are 6” thick. Gravel residential roads should be at least 8” thick, and are more commonly 12” thick.

Residential gravel pads for open area storage, under sheds, and shipping containers should be at least 4”, and are more commonly 6” thick (3-5” for gravel, and 1” for bedding sand)

gravel-base-prep

French drains in a residential yard are typically 14-16” deep and 14-16” wide, and should be graded at a 1-2% slope. This slope will affect the overall finished excavation. I.E a French drain sloped at 1% that is 14” at the start will end up 26” deep at the end of a 100’ long run. A French drain sloped at 2% will end up being 38” deep at the end of a 100’ run.

1% slope = 1 unit over a run of 100 units

2% slope = 2 units over a run of 100 units

french-drain-trench

Residential lawns should have at least 4” of topsoil to grow in. Ideally lawns would have 6-8” but this additional depth really starts to add up when factoring in the disposal of the old soil, cost of the new soil, and the labor to place, compact, and grade this much material.

On smaller yards this is achievable, but on larger projects it can be cost prohibitive.

 River rock flower beds should be at least 2x as deep as the thickness of the river rock being used. I.E 2” river rock should be in a 4” deep excavation. This produces at least 2 layers of rock and hides the fabric and soils below it.

Mulch Flower beds should be at least 4” deep and when freshly mulched be 5-6” deep. This accounts for the new mulch to settle, decompose and extends the time in between placing new mulch.

Just be careful not to choke out any existing plants by raising the grade too much, especially around the base of the plant.

Commercial excavation depths

British Columbia building code sets the frost line at 18”. This means any infrastructure that shouldn’t freeze like foundations, plumbing, etc. must be below this depth.

Poured concrete commercial foundation footings have an excavation depth of 30” (18” for frost protection, plus 12” of gravel) Gravel depth will be specified by geotechnical engineer based on site specific soils. Even if gravel is not required it is often used to keep the site clean and less muddy for the framers.

Allan Block commercial/reinforced retaining walls will require at a minimum 12” for excavation (6” for gravel, 8” for buried block). The taller the retaining wall, or if there is a slope ABOVE or BELOW the wall, the more blocks need to be buried and the deeper the excavation will need to be. Hydro vac excavation may need to be done if there are utilities in the area.

hydrovac-excavation

Interlocking concrete paver systems for driveways and roadways in a commercial application has an excavation depth of 12.5”-14.5” (8-10” for base gravel, and 1” for bedding sand, and on average 3.5” for the paver itself) excavate deeper in poorer soils.

Of NOTE is the actual thickness of the paver being used. Driveway and roadway pavers MUST be thicker than typical pavers for foot traffic.

Interlocking concrete paver systems for patios and sidewalks in a commercial application has an excavation depth of 9.5” (6” for base gravel, and 1” for bedding sand, and on average 2.5” for the paver itself) excavate deeper in poorer soils.

Poured concrete driveways in a commercial application have an excavation depth of 14” (8” for base gravel, and 6” for concrete) excavate deeper in poorer soils.

Poured concrete for patios and sidewalks in a commercial application has an excavation depth of 10” (6” for base gravel, and 4” for concrete) excavate deeper in poorer soils.

Asphalt for driveways in a commercial application has an excavation depth of 13.5” (6” for base gravel, and 7.5” for asphalt) excavate deeper in poorer soils.

Asphalt for patios and sidewalks in a commercial application have an excavation depth of 9” (6” for base gravel, and 3” for asphalt) excavate deeper in poorer soils.

Commercial gravel areas used for parking, or driveways are 10-12” thick.

gravel-parking-lot

Commercial gravel pads for open area storage, under sheds, and shipping containers should be at least 6”, and are more commonly 8” thick (5-7” for gravel, and 1” for bedding sand)

 French drains on a commercial project are typically 14-16” deep and 14-16” wide, and should be graded at a 1-2% slope. This slope will affect the overall finished excavation. I.E a French drain sloped at 1% that is 14” at the start will end up 26” deep at the end of a 100’ long run. A French drain sloped at 2% will end up being 38” deep at the end of a 100’ run.

1% slope = 1 unit over a run of 100 units

2% slope = 2 units over a run of 100 units

Commercial lawns should have at least 6” of topsoil to grow in. Ideally lawns would have 6-8” but this additional depth really starts to add up when factoring in the disposal of the old soil, cost of the new soil, and the labor to place, compact, and grade this much material. On smaller yards this is achievable, but on larger projects it can be cost prohibitive.

 Commercial river rock flower beds should be at least 2x as deep as the thickness of the river rock being used. I.E 2” river rock should be in a 4” deep excavation. This produces at least 2 layers of rock and hides the fabric and soils below it.

Commercial mulch Flower beds should be at least 4” deep and when freshly mulched be 5-6” deep. This accounts for the new mulch to settle, decompose and extends the time in between placing new mulch. Just be careful not to chook out any existing plants be raising the grade too much, especially around the base of the plant.

Excavation grades

Excavation grades refers to the slope of the excavated area, and/or the elevation of the excavation in relation to the surrounding area.

The slope of an excavated area can be flat (level), or if you would like water to move away from the area, can be sloped to move that water to a desired area.

This is an often-overlooked step when making a gravel parking area. usually the excavation is flat (level), and then gravel is used to fill it back in. These create a large void space  for water to collect, and be trapped. It’s especially a problem if the soil is poor draining.

The excavation grade as it relates to elevation simply means that if you want to build a raised gravel area that is 2” higher than the surrounding soils a 6” thick gravel layer only needs 4” of excavation.

Determining site elevations, prior to calculating excavation requirements is a crucial step in a successful project.

It is very common to fix or improve the grades of a property prior to actually doing the excavation. This ensures that water will run where you want it to, and that all site services are buried below the frost line.

retaining-wall-laser-level

Excavation accuracy

Excavation accuracy has a direct impact on the cost and performance of an excavation project. If an operator excavates an additional 1” of soil over a large project this can greatly affect the budget, and cost thousands of dollars to the customer or business.

Likewise, if an additional 1” is removed from a small driveway project it may still fit in the same number of dump trucks planned if you were expecting to fill 1.4 dump trucks. The additional gravel required will still fit in 2 trucks, and the only thing really noticed is a few more minutes load/unloading, and a 15% deeper and stronger base.

In residential excavation projects its very common to balance the customers budget with the truck load capacities to minimize costs, and maximize strength.

Excavation accuracy is only as good as the methods being used. Some operators can grade very well by eye and “feel”, while some operators claim they can, but can’t! Mechanical checked like levels, and stringline can be used by a “ground” worker.

Back 40 landscaping implements a laser on our machine to assist the operator in confirming accuracy, while not incurring the cost of a “Ground” worker. Lasers can be set to accuracy of .002”-.5”. They can be set to show a slope on 2 planes and up to 15%. This means we can grade away from the house, and toward one side of the yard if we want to.

laser-level-on-excavator

I hope you have found some value in this article, and can use it to make the right decisions for your next excavation projects. Please fill out our “Get a Quote” form if you would like to work with us!