Canal-side Subsidence

[TheFew]

Hi,

I am presuming that the material won't just disappear into the canal as the mooring is bounded by some form of Aluminum roofing sheets that go into the water and are held in place by sunk scaffold poles. Clearly I don't know how deep these roofing sheets go.

I will have to Google "cantilevered deck"

Ta

 

[theprinceofdarkness]

What I would do (if I owned the place), because I have done it once already and have the kit. would to use a metre long section of 9" sewer pipe. Stand the pipe in the centre of where you want your path/bank to be. With a decent wet and dry vacuum cleaner suck up the mud from inside the pipe and tap the pipe down. Continue until you have reached solid ground, then with some bits of rebar and or a 9" post borer, loosen the soil, suck it up. If you are lucky you will have got into solid ground before you run out of your 1m depth. Get two black bin bags, cut one so its a tube, tape the tube one inside the other so you have a 1m+ length tube. Poke it inside your plastic tube. Make a pillar out of 8mm rebar 4" across out of 4 lengths and prop it in the pipe so its central and 4" above the base of the hole. Fill the pipe with concrete and vibrate it so the air is removed, wriggle the sewer pipe out - the concrete will settle, so make the initial height about 3" too high. Repeat every metre or so when all the concrete pillars have gone off, cast a reinforced concrete path across the top of them.
FWIW I built a 22' X 11' garage about 12" from a 4'6" drop using 22 pillars as above, garage had not move at all in the intervening 30 years.
Frank

 

[TheFew]

Over the weekend I started the repair work, following the C&RT's advised approach. I dug out about 2 tonnes of soil and random hardcore to expose the areas that had been undermined. The trenches were lined with a mixture of hardcore and stone to create a base layer in the mud, with some geotextile used to protect the bank. I then inserted a U-shape of geotextile into the trench and added a layer of scalpings/hardcore. The geotextile was folded over on top of the scalping and then the dug out soil put back on top.

So, so far I removed around 2 tonnes of material and replaced that plus about another 1.5 tonnes of material.

The jobs still needs finishing with a further circa 2 tonnes of soil/gravel to bring the level of the walkway back up to where it need to be! So it shows just how much had been washed away.

I just hope it lasts!

 

[TheFew]

What I would do (if I owned the place), because I have done it once already and have the kit. would to use a metre long section of 9" sewer pipe. Stand the pipe in the centre of where you want your path/bank to be. With a decent wet and dry vacuum cleaner suck up the mud from inside the pipe and tap the pipe down. Continue until you have reached solid ground, then with some bits of rebar and or a 9" post borer, loosen the soil, suck it up. If you are lucky you will have got into solid ground before you run out of your 1m depth. Get two black bin bags, cut one so its a tube, tape the tube one inside the other so you have a 1m+ length tube. Poke it inside your plastic tube. Make a pillar out of 8mm rebar 4" across out of 4 lengths and prop it in the pipe so its central and 4" above the base of the hole. Fill the pipe with concrete and vibrate it so the air is removed, wriggle the sewer pipe out - the concrete will settle, so make the initial height about 3" too high. Repeat every metre or so when all the concrete pillars have gone off, cast a reinforced concrete path across the top of them.
FWIW I built a 22' X 11' garage about 12" from a 4'6" drop using 22 pillars as above, garage had not move at all in the intervening 30 years.
Frank

Frank, many thanks for the advice. I think I will be using this method in some other places!!

 

[bourbon]

I would still be inclined to push for them to do the work seeing that you have stated that you rent the bank from them

 

[Himaginn]

I suspect, given your specific location, that bank will be subject to a lot of backwash from boats.
I can imagine boats going into reverse approaching that pinch point, and others powering away after passing that point.
You could probably gauge the backwash effect by measuring the depth of the canal immediately canal-side of your bank. I suspect it'll be full depth.

I'd be tempted to try to reduce that backwash somehow. Whether with old cut-up sections of crash barrier, or corrugated galvanised metal sheets sunk vertically between the bank and your wooden planks, additional wooden posts, vertical sleepers.
Signs saying slow down, whatever, to reduce the backwash on your bank.

If they ever lower the water level, that would be the chance to jump in quickly (not literally)and do some permanent work along theprinceofdarknes's lines, but with a line of concrete posts along the water edge.

 

[TheFew]

I would still be inclined to push for them to do the work seeing that you have stated that you rent the bank from them

The line is that the contract states that we accept the land in the condition in which it is in and that they are not responsible for repairs. Which I can see, given that the rent is so low. Being reasonable, if I was expecting them to maintain the land I would expect to pay a lot lot more.

 

[TheFew]

I suspect, given your specific location, that bank will be subject to a lot of backwash from boats.
I can imagine boats going into reverse approaching that pinch point, and others powering away after passing that point.
You could probably gauge the backwash effect by measuring the depth of the canal immediately canal-side of your bank. I suspect it'll be full depth.

I'd be tempted to try to reduce that backwash somehow. Whether with old cut-up sections of crash barrier, or corrugated galvanised metal sheets sunk vertically between the bank and your wooden planks, additional wooden posts, vertical sleepers.
Signs saying slow down, whatever, to reduce the backwash on your bank.

If they ever lower the water level, that would be the chance to jump in quickly (not literally)and do some permanent work along theprinceofdarknes's lines, but with a line of concrete posts along the water edge.

Hi,

Thanks for the advice. I suspect that you are correct that the bridge is the cause of the wash!

I will indeed try to measure the waters depth by the mooring out of interest. I think the previous owner said that there was a step just below the water line somewhere, so perhaps it is not that deep? I will investigate.

With regards to sheeting, the mooring is made from sunk scaffold poles and aluminium roofing sheets (and the land side) will the scaffold boards on the outside (if that makes sense). The aluminium sheets are laid horizontally, so whilst they may provide some protection to a limited depth, the won't be as effective as galvanised crash barrier sheets sunk vertically. I may have to look into this option to provide some extra security.

with regards to draining the canal, I cannot image that this section will be drained anytime soon. Unfortunately I am on a section that is 16 miles long between locks, so it would a lot of water for them to get rid off - unless they have specific methods for draining small sections of a canal?

Thanks

 

[Himaginn]

I'd normally expect a couple of feet of mud on the bottom. But in your case I suspect there may be little or no mud because the wash has 'washed' it away. If the bottom is hard clay the wash may be washing underneath any protection for the bank.

They do have methods for draining short sections, by building a scaffold covered in water-proof material to effect a temporary dam.
However they're unlikely to do this for just one or two repairs.
I'd email them with a request to be kept informed if and when they are contemplating draining any sections, and explain that you have some repairs to the bank to carry out.
Hopefully they'll add that to any other reasons that they have for a temporary draining, which won't be during the peak season, and may be many years in the future.

 

[TheFew]

Hi,

Well i just used my trusty pointed metal bar to do some soundings.

If i push the metal bar into the water next to the mooring it sinks about 1.5 feet before encountering mud. I can then push it into the mud for about 4 feet (the length of the bar). Removing the bar causes bubbles. At some point along the mooring the surface is a lot firmer than others. Perhaps some hardcore down there?

If i lean out and put the bar into the water 1.5 feet from mooring. It sinks about 2.5 feet before encountering mud. Pushing the bar into the ground to 4 feet soon produces clay on end.

So it seems that there is material down there and that most of the wash issues may be being caused by breaches in the defenses at the water level.

 

[Himaginn]

Yes, i tend to agree with you, the bottom does appear to be normal. Has there been a boat moored there in the past? That would explain the slightly harder bottom in one area, the boat washing away the mud.

I also agree with you, it would appear to be wash at the surface, rather than the bottom, that is causing the problems.

 

[TheFew]

Yes. We moved in in Oct and prior to that two boats totalling 42ft were moored here. So hopefully they were the issue and hopefully our repair will last a good 10 years or so.