How to add drainage for a concrete to lawn conversion?

[jacobm]

Hi, i am wanting to remove some of the old concrete that covers half my backyard to make the lawn area bigger, but im concerned about the risk of rain water soaking into the soil and causing more issues. I also dont know how i would put subsurface drainage in because i have no way of connecting the drainage to any stormwater connection?

Things to note:

• The soil is highly reactive P2 clay.
• There is grass lawn on 1/3rd of the width of the property already that doesnt seem to have any issues, no water pooling etc.
• The exisitng concrete sloped towards a stormwater grate drain when it was installed 20 years ago, but the slabs have moved so much water now pools by the bricks in front of the shed.
• The stormwater pit is at the front corner of the house, and the entire side of the house has a surface swale drain, so to access the pit i would have to cut through 20m of concrete thats in reasonable condition. (much newer than the backyard concrete)
  
  Maybe im overthinking things but im not really sure what the approach here should be!? would love some suggestions from peoples experience
  
  to be removedexisting lawnexisting stormwater

[MitchellMc]

Hi @jacobm,

Since the existing lawn area is growing well without drainage, the new lawn will likely be fine as well. However, given the highly reactive P2 clay, you’ll need to improve the soil to prevent compaction and drainage issues.

Start by removing about 200mm of the underlying clay soil and applying gypsum to help break it up. This will improve water penetration and reduce the risk of the area becoming waterlogged. Then, replace the removed clay with a high-quality topsoil blend suitable for lawns, which you can order in bulk from our Special Orders team.

Since your existing concrete has moved over time, leading to pooling near the shed, it might be worth slightly regrading the area to ensure water runs off naturally rather than collecting in low spots. You could also consider incorporating a gentle slope away from structures or installing a shallow surface swale if you notice water pooling after the change.

If you're still concerned about drainage but don’t want to cut into the newer concrete, you could use a gravel trench along the edge of the new lawn to help manage excess water. This would act as a basic soakaway without the need to connect to stormwater. 

Please let me know if you have any questions.

Mitchell

[jacobm]

Hi Mitch, thanks for the advice. Im curious about the gravel trench, could you expand on that a bit more? Where would should it be ussed and how would it need to be installed?

[EricL]

Hello @jacobm 

I believe a gravel trench is a stone-filled channel situated over well-draining soil that utilizes the empty space between stones to act as a temporary reservoir for stormwater as it soaks into the surrounding soil. On the condition that you have a section of your garden that drains really well, this is where you would locate your gravel trench. 

If you need further assistance, please let us know.

Eric