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Will adding soil to the lawn affect drainage?

So my house is built a meter or so above street level. The front lawn near the house slightly slopes away, then has a steeper slope near the property lawn. I am slowly converting the entire lawn into a native garden but the ground is pretty hard clay with crappy drainage. I've sheet mulched a strip near the property line and it was fine to just add extra compost and dirt on top to plant into while the organics slowly work at the clay underneath. But between mulch, compost and soil, I added about 30cm of height. Im worried if I do the same to the "flat area" that I will be redirecting water towards the house rather than down the hill? In a perfect world I would dig out the top layer of lawn before adding anything to keep it at the same level, but not a lot of extra money for landscaping so it would have to be by hand and will take forever.

How much could I build the area up before affecting drainage? If I keep the dirt layer smaller, I'm guessing the mulch shoudnt affect drainage as much? If I spend the next 6 months topically applying clay breaker is that going to do enough to help the existing soil?

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Comments

EricL

Hello @Snakeytail

Thank you for sharing your question about your lawn. It would definitely help if we could see your lawn and slope layout. I have a general idea of what you are describing but would like to see the actual slope of the soil and how adding soil to your lawn might push rainwater back to the house. Would it be possible for you to post a photo of the garden and lawn? Once we see how it's situated we'll be able to offer recommendations on how to best layout your vegetable garden.

Generally speaking, I sometimes suggest transforming the slope into a terrace configuration which staggers the rainwater and slows its flow down the slope. I've placed an image below to give you an idea of how it can be done with sleepers.

Let me call on our experienced members @Dave-1, @Nailbag and @AlanM52 for their recommendations.

If you need further assistance, please let us know.

Eric

Snakeytail

https://www.workshop.bunnings.com.au/discussion/comment/61963#Comment_61963

Photo will give you a better idea of what I mean. So I basically sheet mulched the slope already; the top edge of the mulched area is there the yard starts to slope. So adding some addition dirt on top really didn't change anything.

What I'm talking about now is the area between the mulched strip and the house. It's essentially flat (very slight slope) and pretty much level with the house so I'm worried if I add another 30cm of material that water could pool next to the foundation. I can't plant in the ground as is, even if I kill the grass first which is why I though if I add a layer of dirt and mulch on top, I could plant tubestock right away.

I just don't know how much affect adding like 15 cm of dirt and 10cm of mulch is going to have or if I'm overthinking. If all else fails I could spend the next couple years trying to amend the current soil to make it plantable, but that will make for a lot of digging!

AlanM52

Hi @Snakeytail,

As suggested by @EricL please post some photos.

Thanks

EricL

https://www.workshop.bunnings.com.au/discussion/comment/93479#Comment_93479

Hello @Snakeytail

If you are worried about back flow from the garden, I suggest installing sleepers at the very top to serve as a border to prevent the soil from eroding and prevent water from flowing to your home. You also have the option of putting in an agi-drain at the top to re-direct the water back down.

Eric

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/93479#Comment_93479

Hi @Snakeytail

It simply comes down to gravity. If you can maintain a small steady slope away from the home then water will run away naturally. If you make it dead level, then you're still good as long as you don't cover any weep holes that allow the sub-floor to breath and prevent mould and moisture traps. So, essentially either of those and obviously avoid any fall back to the house and you will be right.

Nailbag

Snakeytail

https://www.workshop.bunnings.com.au/discussion/comment/113011#Comment_113011

So if I were to build a small wall about 30cm tall next to the cement that surrounds the house (like Eric suggested) and added 30cm of material at that edging that slowly sloped away from the house, would that small wall be enough to keep the water back and direct it down the hill? I really like the look of some of those stone walls but with the gaps wasn't sure if that would contain water.

I guess if all else fails I get a jack hammer to help breakup the clay then mix in some amendments in thr top layer!

EricL

https://www.workshop.bunnings.com.au/discussion/comment/124070#Comment_124070

Hello @Snakeytail

I propose doing a mock up and see if you are ok having a 30cm wall close to your house wall. I recommend using cardboard so that you have a visual reference. I would suggest having the wall at least a meter away from your house wall or in line with the concrete walkway. I would also suggest putting in agi pipe at the bottom of the wall to make sure the water is redirected back to the bottom as your clay soil will definitely not absorb it.

If you need further assistance, please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/131421#Comment_131421

Good afternoon @Snakeytail

I am still a little confused as to the 30cm of raised height. Thats a resonable raising of the slope, even if you slope the added height it will still want to move downhill. Is there a chance of a photo from the street so we can see the height from where is to the height you want?

I have a heavily clay yard, not great for working with but still doable. I have installed berms during the last drought / rain instances and they have served their purpose so its time to dismantle them in a large way (so the berms height is only 5cm if there at all) I was thinking of maybe 5cm of topsoil across the yard to get rid of any bumps and then mulch by 10cm. My yard slopes like yours. I will dig out the holes where I will plant anything much like trees. So double width of pot plus a pot and a half deep at least of the depth. The plants will break up the clay slowly and i can add more as time goes on.

Will wait for a pic from the road, I am thinking of a retaining wall at the front of the yard and less overall raising of the height.

Dave

Snakeytail

https://www.workshop.bunnings.com.au/discussion/comment/135902#Comment_135902

Not sure if a pic from the road will help as it's face on, and there is another garden bed in the way to view from the side. Hopefully this little sketch will help.

So you can see the house and the small concrete path surrounding and the original slope of the lawn. Earlier this year I added the 15cm of compost and 10cm of mulch along the slope. I planted shrubs on the slope for screening so I'm not really worried about levelling that part.

The reason I put a thick layer of compost was to be able to plant directly in good soil while the microbes do their work on the clay underneath. So now I'm looking at the upper section. If I used the same thicknesses that's about 25cm of added height. I assume water would still run downhill but worried about potential pooling where the lawn meets the concrete.

I suppose if I waited to plant for like 12 to 24 months then I could just spread a thin layer of compost to add the organic material to help break down the clay and mulch over? I assume mulch won't affect drainage? I just really struggled to get a hole even deep enough to plant tubestock in the current soil.

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/138982#Comment_138982

Good Evening @Snakeytail

The sketch helps out heaps 🙂 So the fall looks resonable. Im heading towards that first sketch up that @EricL drew. Even if its just one sleeper half way up the slope it will hep the drift of material down hill. I would still go less mulch/soil near the house and then towards the depth you want as you go towards the slope.

I use a mattock, a 6 foot crowbar and a square nosed shovel to dig holes in clay. They work well, usually its just the crow bar and the shovel to remove the clay. I generally try and make it liek a big bowl and fill them with decent soil, not compost. I also make sure I dont overwater the plants which is hard to judge. Definently the crowbar is the best tool.

Dave

Snakeytail

https://www.workshop.bunnings.com.au/discussion/comment/141048#Comment_141048

I think I've exaggerated the slope a bit, the material I've put down thus far has stayed pretty much in place even through our Brisbane storm season. But I did pickup some nice bush rocks to put along the bottom of the slope. And it would be easy enough to put a sleeper at the top edge of the current mulch patch before I do anything else. Just need to figure out where the sewer pipe is in the yard before I drive stakes! I like the look of stones would a one or 2 layer stone wall work as well for this scenario as a wood sleeper? I have watched a couple videos of installing a raised garden bed wall, which assuming this install would be similar. Most of them seem to put a trench of rock behind the wall for drainage and support, would a shorter version need this?

I was also planning to run a path on the flattish area parallel to the current garden, so those edges would also be another line of defence against erosion.

Might have to try a crowbar, I have a tool that's more of a hoe than a mattock that is pretty much useless. I saw someone use a jack hammer to breakup clay which sounds like fun!

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/141048#Comment_141048

Hi @Snakeytail,

Yes, you could use stones as your retaining wall as long as they are sufficiently heavy and they sit on a solid, stable base. This video from our supplier Adbri Masonry has a good example of how to create a stable foundation for a block or stone retaining wall - How to Build a Retaining Wall | DIY Made Easy | Adbri Masonry.

Regardless of the height of your retaining wall, it is a good idea to install a trench of drainage gravel behind it, as this helps to reduce the hydrostatic pressure applied to the back of the wall by giving it a channel that water can easily travel through to a drainage point elsewhere. It is also worth installing some socked agi pipe in this drainage gravel as it will help collect and move the water elsewhere.

If you wanted to give @Dave-1's method a try, then a Fencing Crowbar is the specific type you'd be looking at. They are great for breaking up clay, which you can then remove with a shovel.

Let me know if there is anything else we can assist with.

Jacob

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/142588#Comment_142588

Good Evening @Snakeytail

As an alternative retaining wall that incorporates its own drainage have a look at these small gabion baskets. They only need a firm base to sit on (I scalp the grass so its undug soil) and then just rest the cages on it. Fill with your style of rocks you like and bingo, water drains easily, they can be moved if you want to later on. Have a look at my bookmarks and you will see a fair range of different gabion baskets.

To make life easy Bunnings stocks a few different sizes and I am thinking of Jack 65 x 30 x 15cm Landscaper's Plant Support Garden Gabion with some weed mat behind it so the fine soil dosnt filter through. You could have the wall 150mm high or 300mm high depending on what you like.

Dave

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