Help with lawn preparation - Adding top soil and turf to bed of clean limestone

Question

We have had issues with drainage in the back garden and so have laid land drains (perforated pipe, wrapped in geotextile terram t1000 backfilled with clean limestone 20mm - we were planning on putting a 50mm layer of clean limestone over the entire area before adding top soil and turfing. would there be any concerns with doing this? and if not how deep would top soil need to be?

The purpose of the 50mm layer of limestone over the area was whilst building work is being carried out around the site. We are planning to top soil and turf the area in Feb time next year so the limestone was an idea to keep the area clean and useable rather than adding the topsoil in now.

Before we do this, I just wanted to check if this would cause any issues to the lawn in future when we do add top soil and turf it. We were planning to add at least 6inches of top soil before turfing next year. The original ground conditions are heavy clay, we are located on the Wirral, United Kingdom

Answer 1

I've done a bit of research and found some interesting anecdotes, but not much that's useful in your case. I'll make a couple of assumptions - please let me know in the comments if any are incorrect and I'll change this answer accordingly.

Assumptions:

• The 50mm of limestone you're thinking of putting on top of the native clay soil is crushed, with sharpish edges, as opposed to rounded edges (such as you'd see with pea gravel).
• Equipment of various kinds may be run over the limestone layer during construction on the site.
• Because of the heavy clay on-site, drainage is a concern.

Assuming my assumptions are correct, then you may be setting yourself up for a very poorly drained lawn. Why? Because the limestone will compact, either from being driven over or simply over time. As it compacts, the limestone knits together to form a thin layer of what is essentially a driveway/car park. This will act as an semi-permeable or impermeable barrier to water soaking into the ground from above, creating a perched water table.

Now, this is not necessarily bad. In the US, golf courses intentionally create perched water tables to aid in both drainage and water retention for their greens. In these cases, the water table is perched by using sand over a gravel base, which is similar to but of course not identical to your case. It is also something that requires expertise to create. See here and here for brief discussions about this. The difference in pore sizes between the sand and gravel in this setup are not, however, that significant (both are considered to be "coarse" materials, but one is much coarser than the other).

In your case, the topsoil will have medium-sized pores at best, so you're looking at a three-layer cake of medium-very coarse-extremely fine pore sizes. This generally leads to poor drainage as the water hits the first pore differential (topsoil to gravel), slows and then slows further when it hits the gravel-clay interface. As the water slows, it backs up into the topsoil. If the topsoil is not higher than the depth this water reaches, then you're looking at, essentially, a swamp instead of a lawn until the water finally makes it through the three layers.

You indicate that you're planning on putting at least six inches of topsoil on top of the gravel. This is probably too little, as this soil will compact to about three-four inches. I've seen recommendations of at least a foot of topsoil in similar situations, which was expected to compact down to about six-eight inches.

So, would I put the gravel in? Absolutely not. To improve drainage after the construction, I would first use deep-tine aeration (most often used for sports fields and new construction on former farm fields) to break up the clay's compaction down to about a foot. I'd then use regular aeration to break up the surface clay even more. Following that, I'd lay down a mix of compost and topsoil at least eight inches deep (to allow for the inevitable compaction as the compost decays and the normal compaction of topsoil). I'd then seed or sod, as appropriate.

As an alternative, I might lay down two-four inches of compost directly onto the clay (after aeration) and rotovate that in before adding the topsoil-compost mix. The more organic material in clay, the better-draining it becomes.

For ongoing maintenance, I'd use compost instead of fertilizer yearly - again, to build up the organic matter in the soil.

Note:

Perched water tables were already excellently discussed in this forum in a post regarding container planting.

Answer 2

I would lay down plywood where you want people to walk and roll machines, and ask the workers to avoid compacting the soil elsewhere.

I'm concerned that weeds would sprout and grow through gravel.

I think you were maybe asking if the [crushed?] limestone would have a deleterious effect on the pH. Is that part of your question? (I don't have a definitive answer.)

I would go dumpster diving and collect a lot of cardboard, and put cardboard on all areas not designated as walkways. This would help prevent weeds getting a foothold. You have to weigh the cardboard down with bricks or rocks or something. Overlapping is best, to prevent weeds coming through. I suppose you're thinking that between now and February, no weeds would get established, but "better safe than sorry." Also, cardboard will help preserve any soil fertility you currently have.

Refrigerator, stove, washing machine boxes are easy to work with because they're BIG. I would pick those up at a big box DIY store or a small appliance store. Talk to someone and ask them to save the cardboard for you.

When the construction is done, I would then fork or rototill the walkways. Then I would put half an inch of topsoil on top and then seed or have sod put in. Grass can grow in clayey soil. Sprinkle something like straw on the new seed and water twice a day with the big oscillating sprinklers -- buy several hoses and Y-connectors so it doesn't take you forever to water the whole thing. Again, establish temporary walking lanes with the same plywood you used before. Once the main lawn areas are established, then you can loosen those temporary walking lanes and scatter seed there.

BUT if you do decide to go ahead with the crushed limestone, then please put cardboard down first, so the stuff doesn't get mixed into the soil.

To prevent people from putting their big feet where they shouldn't, you can put it little stakes with orange tape tied around them, to remind people. But you also have to explain it to each individual.