Drilled Retaining Wall

[mbp]

I need to excavate at the side of my house to a depth of about 2 metres. I want to do this to about 1 meter away from the walls of the existing building and will need to provide a retaining wall so as to prevent slippage of the end wall of the house (stone).

My only option, as I see it, is to drill bore holes (piles) with an auger and fill them with compacted, reinforced, structural concrete.

The soil is rocky and VERY stable. We have tested this by digging down over 1.5 meters till we hit water and then leaving this to collapse over time. It has still not collapsed after about 5 years!!!!!

My question are:
1) If I am excavating to a depth of 2 metres, how much deeper do I have to go down bearing in mind that there will not be a traditional footing. If I go down 3 meters in total (or into rock at least) will this be sufficient?
2) If I hit rock, how far into the rock should I cut to stabilise the foot of the pile?
3) What diameter bore holes, and thus piles, should I create?
4) Should I complete the job by tying the tops with a cast concrete beam to tie all of the piles together?

If anyone can help of point me in the right direction, I would appreciate it.

Thanks
Matthew

 

[Shytalkz]

The method you are proposing is called contiguous piling. Expensive. If the piles are to act like a vertical cantilever, 1m embedment will not be enough: the piles will move outward away from the house, the soil beneath the foundations will loosen and migrate and you'll get some building movement. Not as much as if you just went ahead and dug, granted . You can mitigate against this by using a reinforced concrete capping beam, but that has to be tied back somewhere. It all starts to get complicated as well as expensive.

Why not just underpin the house walls x 900mm wide (into the soil in other words) to a depth of say 600m deeper than the level to which you propose to excavate? This will also lose you less space in front of the existing wall. When completed, this will act like a gravity retaining wall, where its own mass will resist the thrust of the material behind it.

As you seem to have groundwater about 500mm above the level where you want to go digging, you'll have to dewater the excavation long enough to get the concrete in. Don't forget adequate trench shoring as well.

Are you anticipating Armageddon and building a bunker, perchance?!

Whatever you do, you'll also have to think about the water problem in your excavation in front of the house as well.

 

[mbp]

The problem here is that I would therefore need to dig the 'retaining wall' to just over 2 1/2 meters in depth and go under the walls of the house which would be problematic in itself.

One of the reasons why I was thinking of using contiguous piles is that it would allow for water seepage between the piles as I am at the base of a cliff and wish to allow for the water to run off. The spacing between the piles would allow for this.

Bearing in mind that I have a test dig to just over 1.5 meters which has not shown any slump and allowing for the fact that I am able to dewater the bores quite efficiently, is it not viable?

Finally, the retailing wall would be a relatively temporary structure as it would be backfilled against later on once all of the groundworks were completed.

 

[Shytalkz]

If it's only a temporary affair, could you not put some sheet piles in, instead and remove them on completion of whatever it is that you're doing?

Also, contiguous piles are intended to be placed next to one another, not spaced out; but if there is any space, then water percolation through the gap could result in fines removal from the founding soils and possible movement to the structure in any event.

 

[mbp]

Sheet piles would be great if I could get them installed as I guess that this is not something that I can do myself? This is to be done in France and I would want to buy the sheets as I can not be absolutely certain how long I would need them for.

The next problem would be what depth do they need to go to which is very much the same sort of question that we started off with.

As for the spacing, contiguous piles are spaced apart, the spacing being determined by the type of soil so that the gaps are not so great as to allow very much wash out.

Secant piles are interlocking to prevent washout.

I was going to use coniguous as I want water to balance from one side to the other.

 

[Shytalkz]

Contiguous are installed close together (the only gap generally being the mispositioning as they are drilled), just not interlocked as per secant; that is the only difference.

How long is the wall and how long is the intended excavation along the wall ooi? Are you creating this on all four sides of an excavation?

Regarding toe-down (for either sheets or piles), without accurate soil parameters and profile, especially those which are to provide the passive support to the piles and knowledge of loads on the back of the piles from the building, it is not possible to say with any certainty what depth you would need; however, a conservative estimate for a vertical cantilever would be 1.5 x the height of the retaining section. Eek, so that's a 3m toe-down...

On the other hand, if you were installing the same arrangement directly opposite, then installing flying shores (and assuming that you could work around them in constructing whatever it is that you need the hole for)would mean that the soil loads at the base are much less and consequently less toe-down requirement, say 1m, mainly to restrict the flow of groundwater underneath the piles.

This is where underpin could have an advantage: the load from the house increases the frictional resistance at the base, thus there is less reliance on the passive support of the soil on the excavated side; the mass is greater giving greater resistance to overturning; and there is no (or much reduced) water flow and consequent risk of fines migration.

 

[mbp]

The whole excavation is 10 meters long, the wall of the house being about 7 meters of the side of this excavation so the whole of the house wall will be 'flying'.

The whole excavation is 10 meters by 15 meters ( I am using rounded up measurements for illustration but they are near enough ).

It would not be practical to use flying shores across the gap as it is too great.

 

[Shytalkz]

That's a big hole...

Using a capping beam would work and make the wall a propped cantilever and cut down on the required resistance of the soil to the toe, if it was designed to span horizontally between the returns, but that would require some hefty rebar in it to span those distances.

 

[mbp]

Ahhh... bit of news, I have found that SGB have a distribution depot near where I need to do the work and they distribute sheet piles so....

How do I work out what type of sheet piles to use and how deep to drive them?

 

[JohnD]

Another bunker-builder.

 

[mbp]

JohnD, thanks for taking the time to read the posts! What a shame you haven't got anything valuable to add.

For your information, it is not for a bunker but for a swimming pool and the foundations for a pool enclosure which will be built to cover it attaching to the side of the house. As the pool walls will be within a couple of meters of the foundations and the foundations will need to be deeper than the pool so as not to exert pressure on the side wall of the pool, it would seem sensible to simply excavate the whole plot rather than cutting a trench and hole for the pool separately.

Not that this has anything to do with the discussion on how best to construct a retaining wall to hold the side wall of the house up as, after all, this is what the discussion is all about as the end of the pool will be about 2-3 meters from the house foundations.

Now, if you feel that you might have something that you can add to the conversation, I would be very happy to read it and, you never know, there may be other people reading the thread who would appreciate your input also. Otherwise, may I suggest that you give the sarcastic comments a break as there will be very few people reading them who will find them either constructive or amusing.

Thanks
Matthew

 

[mbp]

Hmmm, sheet piles are not going to work for me either by the look of it. Too expensive to purchase at about 150 quid each and I want to hang on to them for too long to make rental worthwhile. Looks more and more like it will be a trench, footings and concrete block retaining wall with buttresses. Any thoughts?