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Backyard sinkhole

zarniwoop

zarniwoop

hoopy frood
A bit of an exaggeration but...

Just walking across my back yard and one of the paving blocks gave way. I've lifted a few and found a hole around 10 cm across and 40 cm deep. Erosion? Rats? Seems odd that a cavity that size could develop over time and only suddenly make itself known - I'd expect more gradual sinking as I have in a few other places with the same blocks.

I'm hoping for some advice. Should I just fill it in, tamp it down and hope for the best (the blocks have been down for 23 years)? It is at least 1m away from the nearest drain.


I can see from the second view that the erosion continues (or starts) in a way that looks like the sand has just been washed away into the hole.
 
stevec67 said:
Rodent.
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Extremely unlikely to be, or a mole, unless there is other evidence, not least spoil from the hole/tunneling.

What has gone from the hole has to have gone somewhere and from what you have said, the most likely seems to be downwards. Try running a hose in it.
 
Exactly - it’s the natural draining point for the stone patio/driveway. Not bad after all those years..

buy a bag of suitable sand (kiln dried) and fill the holes back up as much as you can.. then sell the house as it’s most likely about to collapse. :D
 
I've just been reading the lower back pain thread, and the attendant side-effect of codeine, so I was a bit apprehensive when I saw this thread title.
 
Now everyone else has set your mind to rest

Found this one recently on the tube, have not found any follow up footage, I would imagine there is more to explore down there.


An oldie but a goldie!

 
Judging by your pics it looks like it's been caused by water running, lift back a few more bricks, get a bag of sharp sand (not kiln dried), fill the areas of erosion, tamper down hard with a wooden float, float in with the wooden float getting it level with the surrounding area, re-lay your blocks (good idea to clean the sides of the blocks with a pointing trowel or old kitchen knife, spread some kiln dried sand of the relaid area and tap the relaid blocks with a rubber mallet, this lets the kiln dried sand form the friction joint. Brush up, job done.
 
Mike Reed said:
How did/do you do that? Used to have magnification on Win 7 but not now on Win 10 unless I've missed it. Useful sometimes. Sorry, o.p. but didn't know one could do that.
Click to expand...

In this case, double click the image, which opens it in imgbox, in this case full screen.
 
Considering all the responses and what I can see by poking around there is unquestionably water erosion, and since I exposed the sand and it rained there has been more of this. The puzzle for me that remains is where is the sand going and why just at this point and not over a wider area?

It could be, as Rug Doc suggested, a natural drainage point. The blocks have been down for over 20 years and 3 kids have grown up here, running around on this, yet only this week did the hole make its presence felt. If this had been happening steadily over that period I would have expected local sinkage to be evident sooner than this - like when the hole was maybe just a few inches deep. Underneath all of this is thick clay so I usually have the reverse problem with drainage.

I tried running water into it as Vinny suggested and it all just drained away immediately. The bottom of the hole seems to be hard core rather than clay so I guess that is expected.

There does seem to be some extension of the cavity under the patio at a couple of points. For one of these I was able to push a hose pipe into a tunnel about 15 inches laterally from the centre of the hole. The blocks above this do move a little when stamped on. This seems to be in the clay below the layer of hardcore that was laid for the patio. On this basis my current theory is tunnels below the hardcore, which eventually collapsed at this point, which fits with the apparent suddenness of its appearance. These could be rats, or they could be old plant or tree roots which were there when the patio was laid but have since rotted away.

I've a feeling that the only way to be sure would be to pull up a much wider area of blocks and hardcore but I'm reluctant to turn this into a bigger job than it might need to be - the whole patio is about 20 square metres. My current plan is to use sand as recommended above, pushing as much of it as I can into any cavities in the sides of the hole and then topping up and relaying the few blocks I've lifted. In a month or so I'll lift these blocks again and inspect.

Thanks again everyone.
 
zarniwoop said:
The puzzle for me that remains is where is the sand going and why just at this point and not over a wider area?
Click to expand...

Go back a step - why is it draining that much now, through that point? There are flow lines in the sand, so it was no slight trickle and if it had been that much water for over 20 years..................

Do your guttering downpipes drain via a gulley with grating, or are they sunk into the ground? Where are they compared to this erosion?
 
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