Decking over removed pond

[KevstarUK]

I’ve just removed a rather large sunken pond from our patio in preparation for a raised deck which will cover where the pond was. I’m currently monitoring to determine how the drainage is after removing the fibreglass lining. The base appears to be a mix of sand and clay. Although filling the hole isn’t a priority as it will be hidden I need to host a number of posts for the deck bearers so need to form a good base. I have a number of concrete breeze blocks sitting spare which I thought might be a good base, I can also use these to build an enclosure for posts and their post crete and then pack around them.

My two concerns are constructing something for the posts which won’t move or be wet.
That I don’t end up with a stagnant pit under the decking.

I obviously would like to get this right before the decking is down, what would be the best approach? Thanks in advance.

 

[zanzibar]

I would probably suggest that filling the pond is the most sensible thing to do. Try to get some old rubble backfill, you could use your breeze blocks and then tamp it down so that that it is reasonably level and the put a lining over to prevent weeds and grass and top it with soil. The post should not be a problem if you ensure that they are fully treated by a vac vac style treatment or use tanalised preferably. Hire an auger then ensure the the post are set at a good depth and secure them using "postcrete" or a fine gravel and cement mix allowing two to three days to cure. You should not have any problems with this then.

 

[KevstarUK]

Thanks Zanzibar. Looking to take advantage of what I have already so this works, just not sure how much backfill I have so may need to buy some in to pack out the hole. The posts will be tanalised so good from that perspective.

 

[zanzibar]

Okay, I suppose as it is going to be under the decking it will not matter and I suggested putting some soil on top so if you can see under it at least it will look tidy.

 

[Trazor]

The main thing is to have good drainage, to ensure it does not fill with water.
In time it will smell, and then it will be difficult to do it properly.

 

[zanzibar]

The main thing is to have good drainage, to ensure it does not fill with water.
In time it will smell, and then it will be difficult to do it properly.

I entirely agree, which is why I suggested filling with backfill and covering this with a weed control or other such lining on the top, before applying a layer of soil.

 

[WabbitPoo]

Dont fill, then sink your posts into the fill, though. Make sure they are into solid ground (or span the hole entirely with your joists)

 

[KevstarUK]

Thanks for the responses, appreciate the input, as trezor said I want to get it right first time and not need to come back to it.

On Sunday I removed the lining and as a result the remaining 6 inchs of sludge which I couldn't pump out was left in the bottom. It then rained quite heavily Sunday night and last night the majority of water has gone and now just boggy and even better again this morning. So in terms of drainage it looks like it shouldn't be issue but going to see how it gets on over the week, I'm sure more rain will be due.

In terms of the frame and joists these will span completely over the hole with sunken and set posts either side of the hole with posts in what was the pond to support the middle. Will be using 6x2 and the posts will be around 1m apart.

I intend to set the posts prior to filling the hole, the depth is around 60-70cms. Once the base is dried out I'll do some investigation to see whether to dig down further or whether I can use some of the blocks as bases. I'll look to then build a surround out of further blocks for the surrounding post create before backfilling around these. Will then place membrane on top and soil to then level it with the patio.

The key is obviously the base, will see how its looking in a week.

 

[zanzibar]

I can absolutely see no reason at all why that will not be a good sound structure and I'm sure it will be fine.