[BJR]

I have been asked to do a job that requires replacing a 6ft high treated pine retaining wall. Normally not a problem.

The wall is only 12in from the lower house and the top of the wall is only 4 ft from from the upper house. Again, normally not a problem.

The problem here is that the ground is pure sand. There are no records to indicate if the top house is pieered below the wall. We have to excavate to within 14in of the top house so that we can build a vertical segmental block wall with 'no fines' concrete behind it. If the sand is not stabilised in some way it will fall away from under the top house. Also we cannot fit a machine in to excavate this job, so it has to be hand done.

I know a bloke that owns a company that stabilises mine shafts by urethane injection. He has told me that if this is done it would be impossible to excavate the stabilised sand. So urethane injection is out of the question.

I also have an engineer looking into it, with no success yet.

Any ideas please?

[Bill Schwab]

Can you have the house moved until you get the project completed? A good house mover can put it on a trailer, drive it off, and even connect utilities up yo it do the people could live in it while your project is being done, then set it back down.

Unless you can find the bottom and shore the foundation up, that is all I can think of...

[Stonehenge]

I would think that there would be some kind of sonar that could be used to determine if there were piers in place. Or some exploratory boring

Also, does anyone know who built the first wall? Might be some answers there.

As for equipment, my guys saw something a few days ago I've never seen. They were excavating directly in and around utilities - to do this one guy had a pressure water gun, and another had a wide-mouthed suction hose. They just shot it wet like a slurry and sucked it up. The excavation was like a laser - 6" wide, 3' down (roughly15 cm x 1 m) in a couple areas. Either this could be done to pour some kind of footing for the house, or it could be the means for excavation for the retaining wall. Either way, I think a trench box would be in order.

[Paul]

There are companies that use ground penetrating radar that could locate any piers under the house.

With out moving the house you could sink a couple of caissons at each end of the house that needs support, then a proper size steel beam could be placed under the house and rested on the caissons. Then temporary shoring could be constructed to hold the majority of the sand back while deconstruction the existing wall.

One other question, what have soil borings shown as far a slippage?

[BJR]

It is a brick house. So moving is impossible.

As far as underpinning to house goes, you would have to do it by machine. The engineer says that the piers for the underpinning have to go down 8ft. Too far for hand digging and also very difficult to do in sand. the sand would just fall into the hole.

This type of sand is not stable. It would fall away to about a 30 degree batter. I have to take away the wall and dig a 1ft deep x 2ft wide footing. This makes a 7ft sand bank at 30 degrees. the sand would hall away to about 2 ft under the top house. Even if it were pieered the engineer would be reluctant to let this happen.

Stabilising the sand is the only way.

[Paul]

Thats why you use caissons. Caissons are metal tubes driven into the ground while you excavate the material inside them.

After the home is supported, I would start shoring using sheet steel driven with a crane. There are many companies here that do this type of work. I don't know about your area but dredge and dock people do it all the time. Once the wall is built the steel is removed.

The nice thing about sand is it's easy to drive into You might even be able to do it without a crane, they make some really nice hydraulic drivers now.

[PSUscaper]

I'm with paul's idea.

Like when they build bridges, they drive huge sheets of corrugated steel into the ground on each side so they can work.

Also, I believe they ran into similiar problems doing the 'big dig' up in Boston. When they were making the tunnels under the city, they had to be sure the buildings above didn't lose their foundations. They had some sort of trenching machine that would go down like 30 ft, and then pour solid concrete into the trench, to keep the buildings soft under underfoots from collapsing into the new tunnels.

Any way you look at this project, it sounds like a nightmare......and quite frankly, I wouldn't be looking for solutions anywhere else than a engineer. On top of that, I'd be sure my insurance was paid and and covered this sort of work.

[PSUscaper]

Another quick question....

Being there is only 4 feet of room to work with, is a segmental wall system going to be able to be engineered for this applicaation......

the way I see it, you are going to build a wall, and at most, maybe get 2 or 3 feet worth of grid into the soil.

Even if you drive casings in, they will have to be driven right next to the the upper houses' footing, and then the entire area dug out (by hand, yikes) for the little bit of grid. Most likely, I'd imagine a engineered wall would include some sort of driven support cables/bars drilled under the house and connected to the wall to hold the wall........but if you have the casings in, how do you drill support anchors in under the house????

Being that a mishap could not only cause damage to one house, but to two, this is starting to sound like a extreme liability. I'm not sure a srw is up to a task like this....

I'd be thinking a reinforced poured concrete structure of some sort.

[Dale Wiley]

Originally posted by pennscapes

Any way you look at this project, it sounds like a nightmare......and quite frankly, I wouldn't be looking for solutions anywhere else than a engineer. On top of that, I'd be sure my insurance was paid and and covered this sort of work.

I have to highly agree here. I would not trust one engineer on this, I would require the property owner to get at least 2 or 3 engineers to agree on how to do this.

[Paul]

Given the specs that we have it's really not that bad of a wall to construct. Sand is well draining, the load that it carrying isn't bad. There is little changing of the load. Most house foundations can be exposed without a problem (like walk out basements) if it's a slab foundation it's even less of a problem. We do know that it's a brick house which should have a thickened wall or slab. 3' of grid would cover the load fine.

[Green4Me]

How about pressure grouting with cement slurry pumped down into the sand several feet into multiple holes. Possibly a thin slurry of cement would permeate the sand enough to provide a binder. Then use a jack hammer with a spade bit to assist with excavation.

Maybe there are easier more profitable jobs with less risk? You don't have to swing at every pitch!

[BJR]

This is the type of work I specialise in. I decided 2 years ago not compete against the average contractor. I started specialising in large retaining walls in difficult locations. The type that most contractors look at and run from. And so far it has paid off. My turnover is 80% and my net profit is up almost 90% up from last year. Also, don't forget that I have been doing this for 22 years now. So I know what I am doing.

I find most of the time I am the only contractor willing to quote.

Believe me when I say it is worth while.

As far as engineering the wall goes, easy. We use 'No Fine Concrete' behind the blocks to make it a gravity wall instead of a soil reinforced wall. I haven't used geogrid for years now. The beauty of living where I do is no ground freeze like most of you. No Fines Concrete is 6 parts aggregate to 1 part portland cement. For a wall this size the blocks and no fines will have a total thickness of 800mm. We get the no fines in a concrete truck the same as normal concrete. We stack the blocks than fill behind with the no fines. It seta like honey comb, drails exceptionally well, and doubles as the drainage layer behind the wall.

I agree with Paul about using caissons, however, I doubt if you could drive them 10ft deep by hand. I originall looked at using smaller pipe to be driven next to each other against the house then filled with reo & concrete to form a makeshift wall. I came across the same problem of a machine being needed to drive them in and it not fitting.

I do like the idea of injecting cement slurry. But again, how do you get it 10ft deep by hand.

[agla]

Why don't you just start digging it out until you find out how far down the footing is for the house? Then you'll know how much of the wall is surcharged by the house and how much deeper you have to go while having to stabilize the house.

It seems to me that up to a certain point you'll only be removing sand from between a timber wall and a foundation wall 4' away. Untill you get below the footing it will be like emptying a sand box. I have no idea about how things are constructed down there, but I assume there is at least some depth to the foundation and footing before you will be undermining it.

Somebody built each house and I would be very surprized if they relied on a small timber wall to hold up the top house if it were built last. If the top house was built first, someone excavated close enough to build that timber wall without it failing.

Maybe you can find out which was built first. That should help answer a few questions.

[Green4Me]

The cement slurry is injected into the ground on a 10ft peice of pipe connected to the pressure hose. Just pull/push down on the pipe until the desired depth is reached.

[Stonehenge]

BJR, I was thinking the same as Pennscapes, but I know that the difficult projects are your bread and butter.

Paul's a sharp guy when it comes to this kind of thing. I'd run his idea past an engineer there to see what he thought.