In my opinion it would be a serious mistake to lay the planings over soil/grass.
You will find that a lack of preparation will cause problems in the not-too-distant future and you will be faced with removal of the planings and the soil/grass and starting again. (Problems such as uneven grade, ponding of water leading to pot holes of ever increasing diameter, grass growing through the planings.)
The correct approach would be to engage a civil engineer to design a pavement specification, that will identify exactly what you need to do to properly prepare your ground for the car parking area.
Maybe your budget does not allow for this extra cost...?
However you will need some type of mechanical equipment to spread the “road planings”, so I’d suggest at the very least to ask whoever is operating that machinery, to arrive a few hours before your trucks arrive to prepare the site beforehand.
A skilled operator will know what to do, however if they do not, follow these basic guidelines...
- Remove all grass and topsoil from the car parking area and place in a stockpile for use elsewhere at your sports field at a later time;
- Excavate into the subsoil between 150-300 mm (0.5-1.0 feet), or maybe more, depending on the type of soil;
- Consolidate (mix together) this subsoil to create a consistent material;
- Spread this consolidated material back across the excavated area - making certain to include a slope / gradient in suitable direction/s to ensure rainwater runs off the surface of the subsoil to an area where the collection of water is of no consequence (otherwise you may need to install a sump to collect the water and pipe it to a storm water drain);
- Use a mechanical vibrating roller to compact the consolidated material;
- Unload and spread the road planings across the compacted and consolidated material, to a depth of at least 50mm (2 inches) - again making certain to match the slope / gradient of consolidated material to ensure rainwater runs off the car park surface to an area where the collection of water is of no consequence;
- Use a mechanical vibrating roller to compact the road planings.
This method should provide a car parking area with a reasonably long service life.
I’d recommend leaving a small pile of road planings to one side, resting on and covered by plastic, to patch future pot holes, should they start to develop. This may help to extend the service life of your car park area.
In my previous response and this update, I have assumed you would like to complete this task as cheaply as possible and that the car park area is small, providing space for 20 or so light vehicles (family cars & SUVs). I have also assumed your subsoil is of reasonable quality and can be consolidated (made uniform) and compacted.
(If the subsoil in its natural state cannot be consolidated and compacted - for example, sand or thick clay - there are materials that can be added to augment the soil to make this process possible, however you may need a geotechnical engineer to provide some formal advice.)
Based on these assumptions, you will only need two pieces of equipment...
- some form of excavating machinery that can excavate the grass and topsoil, excavate the subsoil, mix the subsoil to consolidate (make uniform), spread the subsoil back out, finally spread the road planings;
- a vibrating roller to compact the subsoil layers and then compact the road planings top layer.
I would suggest talking to a few contractors who have smaller scale earthmoving equipment and have undertaken car park preparation work previously - there should be a lot - so contact a few and have a serious conversation with two or three who seem confident about the work you’re asking them to undertake.
Having selected a contractor who has some experience and some equipment, you will perhaps be surprised what they can do with one piece of equipment. Maybe they suggest a large skid steer loader, or an excavator, or a front end loader?
Keep in mind that the larger the equipment, the faster the work can be completed. So a smaller skid steer loader at $150 per hour may take four hours to complete the work, but a larger excavator at $200 per hour may take only two hours, therefore being the cheaper option.
Regardless, the grass and topsoil should be removed first (Step 1). With that layer removed, you can excavate the required depth of subsoil and place in a pile in the centre of the excavated area (Step 2).
Step 3 is consolidation of the excavated subsoil. The desired outcome is that the excavated subsoil is as consistent as possible... large rocks are removed and set aside, the subsoil that is shale is mixed in with the subsoil that is sand, the clay fill that came from the old drainage ditch you didn’t even know was there is mixed in too. The idea being, that when the consolidated subsoil is spread back across the area, it will all behave in the same way, expand evenly when wet and shrink evenly when dry, because it is all the same type of material.
If you are able to successfully consolidate your subsoil, spread it back across the area, then compact it using the vibrating roller, you may not need the water permeable geotextile membrane, because the compacted subsoil should have formed a very hard surface.
In fact, the compacted subsoil should exclude most storm water and that is why you should incorporate a gradient - any water that permeates through the planings should sheet off the top surface of the compacted subsoil to an area you designate for collecting storm water. It may even be necessary to create a swale drain or similar, to catch and divert storm water during heavy rainfall events.
Whether to use the permeable geotextile membrane will depend on the type of subsoil, the success you have compacting it and to an extent your budget. I do not see it as essential, but it may become essential if you cannot achieve suitable compaction of the subsoil.
Regarding compaction - refer to California Bearing Ratio (CBR) - used as far as I am aware around the world to determine the mechanical strength of soil layers. The more compacted the subsoil, the higher the CBR, therefore the longer your car park will survive without maintenance.
Again, any questions feel free to ask in the comments.