Subgrade Preparation for interlocking paver pavements and retaining wall systems.
The durability and longevity of your interlocking paver pavement and/or retaining wall is dependent upon the design, its structural design and load-bearing capacity of the subgrade soil. Anything that can be done to increase the structural support or load-bearing capacity of the subgrade soil will improve its strength and overall performance.
Checking the subgrade density is very important
in order to guarantee maximum structural support. The subgrade soil must be compacted to an adequate density. If not the subgrade will continue to compress and lose its structure after construction, causing the interlocking pavement to shift over time.
The proper density is specified as a relative density for the top 6 inches of subgrade. In order to achieve these densities, the subgrade must be at or near its optimum moisture content. Usually compaction of clay, muddy or sandy soils will result in adequate structural support that must be excavated and replaced.
If once you compacted the subgrade the structural support is inadequate, here are 3 options that can be applied to increase its resistance.
Stabilization. Usually, lime is used with highly plastic clay soils. Lime changes the nature of the adsorbed layer and provides pozzolanic action. Plasticity index of highly plastic soils are reduced by the addition of lime with soil. Also, cement can be used for less plastic soils, this is called CCS (Cement Stabilized Soil). The appropriate amounts of cement needed for different types of soils:
- Gravels – 5 to 10%
- Sands – 7 to 12%
- Silts – 12 to 15%
- Clays – 12 – 20%
The binding characteristics of these materials generally increase subgrade load-bearing capacity, durability, and longevity of the pavement.
Over-excavate. The general idea is to replace poor load-bearing subgrade with a much better load-bearing material. Generally, 1 to 2 ft. of poor soil can be excavated and replaced with better load-bearing materials such as gravel.
You can add a base course with a subbase course over the subgrade. A base course offers additional load-bearing capacity. New pavement structural designs often use some sort of granular base course unless subgrade structural support is extremely good and expected loads are extremely low. Base courses are subjected to the same compaction and elevation requirements as subgrade soils unless you’re building a permeable interlock pavement.
Establish the area where you wish to install your pavers and mark the area with orange spray paint. Example: if you’re building a 12’x 12′ patio, you trace your lines on the ground and make sure they’re symmetric.
Excavate the area to the proper depth. Keep in mind that the depth of excavation varies depending on the use. Pedestrian use will require a minimum aggregate base of 6″ to 8″, while a vehicular use will require a minimum depth of 10″ to 12″ (Here at Milton Stone Group, we like to go well above those numbers for optimum durability base strength).
Compaction of the sub-grade. For this part, you will need a compactor (a Jumping jack) to compact the sub-grade. Pass it a minimum of two times going in the opposite direction. Once done, lay the geotextile fabric on the ground (overlapping each piece of geotextile fabric if multiple pieces are used).
Put stakes at each corner of your area with a string line and line level at the desired height of your gravel, this will let you know how much gravel you will need. Add crushed 0 3/4 gravel to the compacted area please not: If using more than 6″ of gravel we like to add it in layers of 6″ and compact it with a compactor as we build it up). Compact the area the same way you did it with the sub-grade. The more time you spend compacting, the better the longevity.
Raise your line 1″-1.5″. this is for the stone dust that goes over the compacted area. Fill the area with stone dust, give it a quick level and compact it using a vibratory compacting plate. Now you’re ready to set your pipes on the ground to give it a final and perfect grading. Set 2 pipes parallel on the ground (preferably 1″ in diameter), the top of your pipes should be at the desired height you wish for your stone dust base to be at. And simply use a long screeding tool or long level and drag it on your set pipes. As you pull, you will leave the area perfectly smooth, add more stone dust where is needed and repeat the process.
Before installing pavers, make sure you square the area. The diagonal corners should be equal length to each other. Now you’re ready to start laying down your pavers. Start by laying the majority of your pavers and keep all the cuts for the end. You’ve laid your pavers, this is when you would put in place your plastic edge restraints with spikes every 12″, make sure you place them nice and tight against your pavers as this will retain them. Next, mark all the cuts and start cutting them. We suggest a water table saw to reduce the amount of dust.
Your pavers are all installed, all your cuts are done, it’s now time to apply the polymeric stabilizing sand and compact the area one last time using a vibratory plate to fill in the joints and stabilize your pavers into the stone dust. Once completed, broom the entire area extremely clean (make sure there’s no residue from the sand. Sprinkle the pavers lightly until it is all lightly wet, repeat the process of lightly wetting your pavers once a day for the next week. Congratulations, your project is now complete!