Landscaping drainage

hillsi39562 · 10 March

Hi everyone.
Would really appreciate any advice. As per pics, we have lush grass during heat waves and subsidence following course of sewer pipes. Drains have been inspected and no cracks or defects. Flow is away towards the sleepers. Ground now subsiding at the high end due to water collection (but does drain away). Soil is generally poor/ clay but no other issues in the garden. I have dug down to pipe and soil fairly saturated. Minimal pea grit covering the pipe. Not a great fall on the pipe length either. Property now 10 years old. Suggestion has been compaction of backfill over the pipes not allowing adequate drainage? Wondering best way to sort. Backfill along the pipe course again? Thanks very much for any help.

Ceres · 11 March

Welcome to the forum.

That looks deeply serious and OH reckons the whole lot needs digging up. I'm hoping someone here can come up with a less radical solution.

McRazz · 11 March

If the drains have been inspected and signed off then I wouldn't concentrate on that for now.

Looking at the state of your lawn in the summer - and knowing how these resi gardens are landscaped - I'd suggest that the wider garden is severly compacted and the drainage pipe run is the only area that isn't, hence surface water collecting and manifesting as lush green grass.

If your foul water was backed up you'd know about it.

hillsi39562 · 11 March

Thanks very much McRazz. That certainly makes the most sense to me.

Remedy ?deep auger drilling or replacing with free draining soil down to ?pipe level +\- additional drainage.

Really appreciate your help. Thank you.

McRazz · 11 March

How easy is it to dig in the lawn where it browns off?

You can try decompacting it by hand, which may yield results, but you'd need to go deeper than conventional hand tools could practically reach.

Ideally when weather conditions are better the soil should be decompacted to around 600mm depth then ripped through with a toothed bucket to approx. 300mm depth. Then follow standard cultivation methods to re-lay your lawn.

Unfortunately the main decompaction/ripping would involve the use of an excavator, but a 1.5ton machine in the right hands should be more than capable. If you're handy and fancy a laugh a weekend hire would be long enough to learn the ropes and get the job done.

Careful of your drains and any other services that may be running below your lawn.

hillsi39562 · 13 March

Thanks again. Really helpful info.

I just dug a few inspection holes.

The centre hole as look at first pic runs directly above the drain pipes and I can get down to almost 600mm with nothing but clay soil. Didn’t seem particularly compacted (clumps together if squeezed but fairly free otherwise) and was easily removed. Damp all the way down.

The other two holes either side (where lawn browns off) have around 20cm soil, again didn’t seem hugely compacted then sand and shingle.

Maybe the rest of the lawn (where browns off) is draining ok and the issue is poor backfilling above the drain pipes?

Option to dig down to the pipes and backfill properly with shingle/type1/ compacting in layers then sand/ soil + replacing top 300mm of top soil for entire lawn?

Really grateful for your expertise.

Thank you.

hillsi39562 · 13 March

Update. Hit thick clay 400-500mm down on both side holes. Can dig easily down to pipe in centre hole no clay.

Thinking water from rest of lawn drains down to the clay level @ 400mm then finds its way to the poorly compacted back fill above the pipe work causing saturation and ground sinking.

Back filling above drain pipes should stop sinking but the water then needs a place to go. ?french drains above 400mm level into pipe.

Landscaping drainage

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