Gravel is used all the time by contractors and DIY enthusiasts. Its readily available, and its super easy to use right? Well, kind of!
In this article we will cover the different types of gravel, colors available, wrong excavation depth and design, and all things compaction.
Gravel is classified by 3 main types. Minus, Clear, and Round
Minus material is gravel that is made up of a mix of sizes. For instance, ¾” minus is material ranging in sizes from ¾” down to a powder. This type of material compacts very well, as all of the smaller particles lock in-between the larger ¾” rocks. Nothing in this mix will be larger than ¾”.
This material drains the slowest as there are very few void spaces in it when its compacted.
Clear Material is clean, washed gravel of uniform size. ¾” clear is all ¾” sized rocks. There is no small particles or dust in this mix. Because ¾” clear does not have any fines in it, when its compacted it is easily moved and uncompacted.
Even when compacted there is a lot of void space for water to immediately travel down and through it.
Round material is rocks that are round and oval in shape.
Generally, this product comes in a single size such as 2”, or in an assorted size such as 2-6”. This material is always clear, and free from fines.
It does not compact, and is not used for building. It's either used for drainage systems, or low maintenance River Rock gardens.
Water moves the fastest through round rock because it has the biggest void spaces.
Each type of gravel (Minus, Clear, Round) has pros and cons when using it in an application. From cost, color, drainage ability, compaction requirements, and weight bearing capability.
Here are some typical applications where the wrong gravel is used.
Retaining walls should be built with a clear material like ¾” clear. This includes the base under the block, 12” behind the retaining wall, AND 6" in front of the block. This wide foundation distrubutes the weight of the wall over a greater area of the soil. Failing to do this will put added pressure on the soil below and could cause the wall to sink/settle over time.
Typically, contractors use a minus material as its easier to get it level for stacking the blocks on. If a minus material is used it will settle and erode over time.
Additionally, if minus gravel is used behind the wall, it will not drain very well, and put additional weight on the wall.
Drainage applications like French drains or rock pits should be built with round rock. Clear material has smaller void spaces, and your system will not perform as good.
The worst material you could use would be a minus gravel as there are no void spaces for water to drain through.
The North American standard set by CMHA for pavers is to use a minus material for the base, and a coarse sand for the final leveling prior to placing the pavers down.
The most common mistake made when building a gravel base for pavers is to use the wrong sand.
A fine sand will wash away over time, and this will cause pavers to sink and settle. Likewise using any more than a 1” thick bedding layer will cause the sand to settle over time.
If you have ever seen pavers sink, or settle this is almost 100% due to improper base material.
Gravel parking areas should be built with minus material. This allows the gravel to lock in place, and form a near concrete like surface. Several sizes are available, but the most common is ¾” minus.
For larger vehicles like semis 3” minus is very common.
Clear materials and round rock should not be used as they don’t stay put at all. They will move even under the weight of foot traffic.
Foundations and load bearing applications can be very specific so consult with the geo technical engineer if applicable.
In general, both minus and clear material may be used, however not interchangeably.
In areas where you can achieve full compaction such as under a shop, I would use a minus material such as ¾” or 3” minus. In foundation holes for a patio were getting a compactor in there would be difficult I would use a clear material like ¾” clear.
Clear material is 95% compacted just be placing it in a hole, this makes it more stable when it’s not as easy to compact.
Minus material has to be wetted thoroughly with water to lubricate the material prior to several passes with heavy equipment.
The color of gravel varies based on the source. Gravel can come from mountains, rocks in the earth, or rocks in water.
Mountain rock is only ever minus or clear material. No blasted rock produces round rock. Every mountain has different colored veins so even rock from the same mountain or gravel pit will be different in color.
Typical local colors:
Abbotsford quarries produce a nice blueish grey gravel as most of it is blasted from a mountain. This blasted rock is angular in nature.
Mission gets most of their rock from the earth and crushes the round rock into smaller pieces. This produces a dirtier brown color that is ½ round, ½ angular.
Chilliwack is blasted mountain rock and typically is a nice dark grey and sometimes black. All blasted rock is angular in nature.
River rock is the most colorful and decorative rock. It’s also more expensive.
Round rock comes from the ground, and is browner in color. Its primarily used in underground drainage applications, as it’s not as colorful or decorative.
New to the market is recycled concrete. This is always a minus material, and typically is available in only ¾” minus and 3” minus.
This product is cheaper, and compacts the hardest of every options. It literally almost turns back into concrete.
The downside though is the color is brownish, and there is some garbage/trash mixed in with it from the demo/recycling process.
The wrong color of gravel can be a problem if you are trying to match existing gravel, or make a new gravel/river rock garden and have a vision for how it will look.
If I was adding on to my gravel driveway, I wouldn’t want half my driveway to be a nice blue color and the new section to be a brownish color.
Gravel quarries are off limits to retail sales.
If picking the perfect color is important then visiting a landscape supply store to see the color prior to loading should be considered.
The challenge contractors face is quarries are significant cheaper than retail. Most of the time though your contractor should be able to guide you on sourcing the perfect gravel color and brightness for your project.
The wrong excavation depth is a big mistake that is made in an effort to save money. Instead of excavating deep enough to provide a stable foundation of gravel, contractors cheat this to save on the soil disposal costs, and gravel required.
Minimum excavation depths can be found here
River Rock excavation should be at least twice as deep as the rock size to ensure 2 layers of rock so you don’t see the filter fabric and ground.
Gravel is used to provide stable weight bearing for driveways, patios, retaining walls, foundations, and many more applications.
Weight is transferred downwards at approx. a 45-degree angle. The deeper the gravel is the wider the weight distribution is.
In ideal conditions follow the above excavation depths. In poor clay like soils make the excavation and gravel base thicker.
The wrong design for gravel is most commonly found in the perimeter of a project.
When excavating it’s important to make the excavation area wider by at least the thickness of the base.
For a driveway application if the base gravel is 6” thick the base material should extend 6” past the edge of the driveway. This ensures that the load on the edge of a driveway is transferred outwards at a 45-degree angle and doesn’t cause the edge to crack.
Most paver, concrete, and gravel pads have failure at the edge because the excavation doesn’t extend outward enough.
Another design mistake we see is not taking into account future plans. We do a lot of gravel driveways and parking areas. If a customer is only ever going to have a gravel parking area the excavation and gravel only needs to be 6” thick.
If they are planning on finishing the area with concrete or pavers in the future than we should prep for that right from the beginning. Instead of excavating 6” for the gravel we would excavate 10” deep, then either fill with 6” or 10” of gravel.
When the time comes to finish the area its either ready to go or a quick removal of 4” of gravel and then its ready to go.
Compaction is one of the biggest mistakes a contractor can make when using gravel for a project.
Note: River rock and Round rock do not get compacted.
Minus material and recycled concrete compact the most, and must be compacted using water to lubricate the fines into the void spaces of the bigger rocks.
To achieve complete compaction (98%) the right size and type of compactor must be used. Often times to small or the wrong style of compactor is used.
Walk behind units that can be lifted by one person are always too small for complete 98% compaction. Larger units can only be moved by machines, and will compact a deeper amount of gravel.
Concrete Masonry Hardscape Association (CMHA North America) lists the smallest compactor to be used at having a minimum Centrifugal Force of 7000LBF.
A typical compactor in this size would be around 400lbs. The minimum # of passes should be established by a geotechnical engineer but it’s usually in the ballpark of 4-6 passes in multiple directions.
Clear materials that are contained in a trench or hole are already 95% compacted upon dumping on the ground. Leveling and wetting clear material and compacting can increase the compaction to 98%
Knowing how to size a compactor for minimum compaction as well as for how deep of a gravel layer (lift) you can do at a time is a mistake contractors commonly get wrong.
Uncompacted gravel will eventually compact over time. As the gravel is compacting it settles downwards.
This can create void spaces under your concrete, and cause it to crack.
How many times have you seen cracked concrete?
Estimating how much soil will be excavated and how much gravel a contractor will need is a mistake every contractor has made.
Soil expands approx. 1.3-1.4x when its unearthed and this takes up more space in the truck.
Likewise, if gravel compacts how much extra material will be needed? Different types of gravel compact at different rates, and the size of the compactor plays a big role in this as well as the stability of the soil under the gravel.
In general, clear gravel compacts 10-15% and minus gravel compacts about 15-25%.
OH, and did I mention gravel is bought at the quarry in Tonnes, not in cubic yards like the landscape center?
To make that conversion multiply the cubic yards by 1.4-1.5.
OK, now that we got that math out of the way, how do you measure an area?
Well for a simple area input the length, width, and depth into an online cubic yard calculator. I use Calculator soup every day.
https://www.calculatorsoup.com/calculators/construction/cubic-yards-calculator.php
For irregular shapes it’s almost impossible to be accurate, and when you add the above-mentioned variables, you will inevitably have not enough or to much material. Both of which are a problem.
For exact measuring we use Moasure one.
I hope you have enjoyed this article, and are more familiar with the top 5 mistakes contractors make when working with gravel and river rock.
If you live in the Fraser Valley and have a gravel or river rock project, please complete our “Get a Quote” form.