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Civil Engineering advice needed

Or what is the best thing to do

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Erfolg14/11/2018 11:27:10
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11122 forum posts
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I have moved into this house about two years ago. A little earlier the rear patio was laid. By whom, I have no idea.

The issue is that the area adjacent to a retaining wall has slumped. Causing the flags in that area to subside.

I decided that I would deal with the issue my self, partly because I have no faith that a tradesman would do the job properly.

I have now temporarily relaid a short section. I have found that the ground immediately adjacent to the wall, is still uncompacted. So far I have used the head part of a sledge hammer to use as a tamper to consolidate the ground as I go forward. Onto the consolidated ground i have poured stone chippings/gravel. Laying the the flags directly onto the ground, as a temporary measure.

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From the picture below the extent of the problem can be seen.

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The central area has also moved. Here I intended to lift the worst affected because of rocking and not being well supported and then do a full bed support.

The areas by the wall I am less confident that they will not move in the future. Here I am most unsure of the most prudent way forward. Should I relay a little later in the year on cement dabs. Or just leave them for another year.

Now that I am getting on, I am less sure that in the future I will be physically up to the job. On that basis I am inclined to do the job soon.

I cannot but notice that the whole of the patio is anything but flat. In that when viewing from various angles I can see slight slopes and depressions. I am now thinking this could be the reason the joints have not been grouted, to allow water to drain away.

What do you think?

Simon Chaddock14/11/2018 12:01:32
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5249 forum posts
2738 photos

Erfolg

It all depends on how much of the underlying soil has been disturbed or built up.

Tamping really only compacts an inch or two, the rest underneath will continue to sink until it reaches its 'natural' density.

To produce and maintain a truly flat patio you are likely to have to lay proper foundations, in other words, dig out to the undisturbed ground level and then build back up with a layer of material that does not sag, typically compacted hard core and then finish with thin layers of progressively finer materials.

Alternatively you can just re lay the sunk bits every few years. It will eventually stop sinking! wink 2

Nigel R14/11/2018 12:13:29
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1969 forum posts
366 photos

If it were me, I'd be inclined to lift the slabs and simply put a layer of sharp sand on top, run a 6' board over it to level it, compact it a bit, then repeat as needed to get the compacted sand level.

As above, it'll move again, but, you can always come back and repeat the process in a year or two.

This job is on my to-do list for our front garden area, where the slabs between gate and door have been trodden down by our comings and goings, and have sunk a little, of course this is exactly where the rain forms large puddles...

Broken Prop14/11/2018 12:15:49
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566 forum posts
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Hi Erfolg

The problem that you have is quite common and is caused by long term consolidation of the fill material behind the retaining wall. The area immediately behind the wall is the deepest part of the fill and this will consolidate more than areas close to the house (assumimg that originally the ground sloped up towards the house).

The fill was clearly not laid properly and is in a loose condition, hence the subsequent consolidation. The proper way to lay fill over about 600mm in depth is to lay 225mm of fill, consolidate and then cover with 150mm of very wet concrete, which partially flows into the fill and binds it together. Repeat until the required level is reached.

Short of digging it all out and doing that, whatever you do will only be partially successful as the fill below the level at which you are wotking will continue to consolidate over the years. However your current approach is good and I would suggest that if you wish to do the work yourself, you continue more or less as you have been doing.

I would suggest however that you bed the flagstones on 50mm of soft sand, which makes them easy to level and also to re-level when they inevitably subside in the future. You will no doubt have noticed that the council use this method to lay pavements and that is why they do it.

Do not be tempted to cement between the flagstones as the joints will only break up in the future as the patio continues to move. Instead trickle silver sand into the joints and repeat from time to time. (It makes weeding easier too).

Good luck with the work! Long term consolidation is a pain to deal with effectively!

Pete

Erfolg14/11/2018 12:56:44
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11122 forum posts
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Thanks for your replies.

I have little idea of the ground levels prior to the laying of this patio area. I assume that the area near the house, previously had a flat area. My best assumption is at least half of the present area, was there at essentially the same level. That then would suggest the rest is back fill.

On the other hand, using a long screwdriver as a probe. It is the area close to the retaining wall that is very soft.

As has been suggested, to do the job immediately properly would take a lot of effort, time and some money. From working experience a substantial part of a build project would be in the ground works. Having missed this part out, the profit would be good and the price apparently keen. The clerk of works seems to have slipped up, as the ballast layer seems to have been a surface scattering from the visual evidence.

I will be taking your advice in blinding the top area with sand. I will be using "builders sand" as that is what I have a bulk bag of. I may even have enough gravel, without getting a bulk bag delivered.

Again as suggested I will then leave the flags, to do their thing.

I had considered digging out a trench down to the original ground level. I quickly dismissed this as it could involve removing approx 18m3 of soil and replaced with concrete. Recently having laid to slabs in the garden, one for a Summer House and another to support a fancy garden bench, pouring concrete gets harder with age.

Simon, and I thought my days of pouring pipes and welding concrete were well behind me.

David Mellor14/11/2018 13:05:48
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1122 forum posts
539 photos

One possibility is that the retaining wall itself is moving (or has moved) slightly, especially if it has been built on little or no founds.

People often underestimate that the side force behind even the smallest of retaining walls can be significant over time - especially with repeated soakings, drying out and freeze-thaw cycles over time. Retaining walls never rotate towards the high ground, or move towards the high ground, they only ever creep or rotate slowly away from it, which has the net effect of allowing the soil behind it to move (in the down hill direction), or deconsolidate.

Thats a long winded way of saying that the "lay the slabs on sand" advice already given by others is the best short to medium term fix, especially as they can be subsequently relaid.  

Good luck... don't put your back out!

Dave

 

Edited By David Mellor on 14/11/2018 13:13:27

Bruce Collinson14/11/2018 13:22:24
179 forum posts

Erfolg,

I'm afraid it's +1 for Dave Mellor. As a Chartered Surveyor, if surveying this for a potential purchaser, I'd start at the outer face of the retaining wall, plumb it with a spirit level and find the vertical cracks.

If it was one of those lucky days and the nosey neighbour appeared I'd find out about the previous ground levels and degree of slope. I endorse the robust approaches above and don't forget, it's a patio, not the envelope of the dwelling.

BTC

Shaun Walsh14/11/2018 13:28:15
69 forum posts
We had a similar problem in our previous house.
It was sorted when the police removed the body that the previous owner had covered with the patio.
Broken Prop14/11/2018 13:40:58
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566 forum posts
1 photos

Erf you are quite right about the builder taking a short cut with the patio works. External finishes are the last thing to be done on a contract by which time the builder is chafing for his money and wants to finish the job quickly. The built up area is also a good place to get rid of some of the general rubbish that accumulates around all building works.

The architect rarely specifies external works in detail and as David quite rightly says, the works get skimped as a result. David is also quite right about the failure of retaining walls, which nearly always fail by leaning outwards. The mechanism is as follows:

During the summer the fill behind the wall dries out and shrinks, opening up a crack between the fill and the wall. Dust and dirt falls into the gap and during the winter the fill gets wet and swells. Because the gap no longer exists, the swelling of the fill exerts a pressure on the back of the wall. The following summer the same thing occurs and over the years the increasing pressure gradually pushes the wall over. Retaining walls on clay subsoils are particularly vulnerable to this action and I have lost count of the number that I have condemned over the years!

However it does take a long time.....

John Tee14/11/2018 13:52:45
693 forum posts
65 photos

I had to have half of my patio relaid about years ago do to alterations to surrounding walls. They were relaid on blobs of cement as opposed to the full layer of a sand cent mix I had used orignally. now they are loose and sinking like yours whereas my originals are still solid. I would have redone it myself but my knees would not let me kneel or bend to do it. Now got a new knee and can bend enough to relay them. Still can't kneel for long though.

John

GONZO14/11/2018 15:33:13
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1080 forum posts
13 photos

Erf,

Get a copy of BRE (Building Research Establishment) GBG (Good Building Guide) No 27, its about the design of retaining walls. Its an easily read and understood document that draws on the information in the relevant British Standards (listed on back cover). Your pics don't show the front of the wall so cant tell if you have weep holes etc as part of water management. A loose fill(encased in semi permeable material) of easy draining gravel just behind the wall could be part of the water management. I can send you an email with a copy of BRE GBG No 27 if you PM me your email address. But, be prepared to be upset as I think it highly unlikely that anything about the wall will be right, from the foundations up!

Erfolg14/11/2018 15:41:12
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11122 forum posts
1086 photos

The retaining wall is dead plumb. The brickies have done that part very well. I believe it is a different crew that laid the patio.

The comments about the wall moving, could provide a clue as to why that strip has not been consolidated. Knowing the wall was newly built, there could have concerns that the cement had not fully cured. As has been suggested it could well have been as the brickies finished the wall, the flagging company could have been pressed immediately to get the flagged area around the house done.

I will am now going to finish of filling with stone ballast, finally to get everything reasonably flat I will level all the ballast, so that a layer of sand can be used to lay the slabs, Although where the steps are I will adopt a more engineered approach, to produce a stable area.

Erfolg14/11/2018 15:53:36
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11122 forum posts
1086 photos

Simon

I have just re-read your comments. As you know or remember, we never had these problems, as we always did a scrape, then piled if necessary. No backfill would move our build.smiley

Gonzo, Unfortunately there are no weep vents in the wall, which would have been useful. On the positive side, the ground is very free draining. The trouble is I will now be thinking of retro fitting some, or at least drilling some holes through on a vertical joint. Then I start thinking, it is unlikely that the joints line up, so it becomes hard work, and will they block with time. Life is to short, it will out last me I am guessing. By then there will be a new fashion and it will be torn down. sadsmileycrying

Tom Sharp 214/11/2018 20:51:57
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3243 forum posts
17 photos

My advice is, tread carefully while on the patio. It only really matters if you are moving house. wink

Erfolg15/11/2018 14:49:36
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11122 forum posts
1086 photos

I woke up this morning thinking about broken props concept of how it should have been done. In the now dim and so distant past, which is circa 40-50 years back. I may have seen a variation of the concept. From what I think I saw and remember, As part of site preparation the site had been scraped to level. This had lead to one hill side being partially removed with a now steep embankment. The solution seemed to be the use of multiple steel strip laid horizontally into the hill side fastened onto concrete blocks which formed a retaining wall. a layer of earth was then dumped on, and the process continuously repeated, until the height of the unsupported hill top was reached. I guess it was a lot cheaper (even using the onsite batching plant) and perhaps quicker than multiple concrete pours etc.

Any way, I have started the implementation of the gravel and topping with sand method suggested. I have managed 1/2 of one side.

I may leave the other side, as it is not obviously as poor.

Just concentrating on completing the side started and then a fully supported central area, where there are obviously issues that cannot be left.

Thanks again, for good guidance.

Erfolg10/12/2018 14:47:32
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11122 forum posts
1086 photos

Well, I have finally made progress. Phase one is complete. Undertaken as suggested, in that the back fill is gravel/ballast, topped with Builders Sand then the slabs laid on top. An awful lot of gravel went in and not a small amount of sand. The general settling has not been completely uniform, resulting in having to average the differences, to obtain a flattish surface.

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I have run out of sand, so I am thinking of another Bulk Bag, although it is more than I probably will need in the near future.

Phase 2 is more of a problem in that it is a much more heavily trafficked area. and it has settled in a hollow. With what appears to be a lot of missing material beneath the slabs.

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Plus I need to buy a new slab to replace the broken one.

The so far good news is the concrete slab (with flags on top) I laid to fasten our ecclesiastical type garden bench has not budged in the gales this year. The previous year the flags were lifted, leaving an anything but flat flagged area. Here is hoping it stays like that,

It gets so windy here that my wives hanging basket hook failed by metal fatigue. Something else I would not have believed in it had not happened.

Erfolg11/12/2018 13:22:52
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11122 forum posts
1086 photos

I have started where the plant pot sat. I have used the same gravel then sand technique.

In this area it looks like the original ground layer was never disturbed. It then looks like the slabs are resting on a number of massive piles of mortar. Never the less the ground has settled.

I am guessing the consensus would be to carry on as before and not to bed on mortar after back filling with gravel?

My intention was to retro grout the whole area, but I am guessing that due to the whole of the patio area not being a single flat plane, with a slope. That any grouting could cause puddling from rain. Also as been mentioned there is a suspicion that the ground will continue to settle for some time?

It seems that producing a truly flat long term area will be some one elses task in the Future?

Edited By Erfolg on 11/12/2018 13:23:51

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