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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-07          155900


I need to build a series of stepped retaining walls. One of the walls needs to be built right where four plastic Sched. 40 pvc electrical conduits are run. The conduit crosses the path of the wall at an oblique angle, maybe 15 degrees. Each wall will be 4 feet or less in height and the conduit would pass through the wall in the second or third layer of vertical blocks.

Is there a reasonable way to resolve this without pulling the wires and running new conduit?

Maybe a few blocks could be cut to allow the conduit to pass through and the voids filled with compacted sand?

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Working around retaining wall obstruction

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hardwood
Join Date: Dec 2002
Posts: 3583 iowa
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2008-08-07          155902


Mayber split some Sched 80 the next size biggeer then clamp it around the erxisting conduit woth stainless hose clamps??? Just a thought. Frank. ....

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Working around retaining wall obstruction

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-08-07          155915


Ken, we run into this quite a bit.

The two favourite methods of dealing with it are to either move the conduit or wall slightly so as to prevent the two from ever meeting. This is best since eventually one will need to be worked on and the other will create a PITA. The other method is to cut (VERY gently) through the conduit and install a couple of 90 elbows and a short piece of straight pipe so the conduit goes through the wall at as close to 90 as possible.

Don't forget, the bigger and especially longer the hole is, the less connection there is between the layers in that location. That will cause a weak spot that might cause if not accelerate the failure of the wall in that location.

If they must cross each others path at a shallow angle like that, try to make one go over the other instead of through. Often you can dig up enough of the conduit in each direction to lower it enough to clear the wall.

If at all possible, when a conduit crosses under a wall, protect the conduit with a 'bridge' consisting of sand around the conduit, and especially above, then a solid layer, concrete or preserved wood, then gravel. This will spread the load out and prevent the wall from settling and breaking open the conduit.

Best of luck. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-07          155917


Thanks, Murf, that's what I feared. What if we were to build
a small concrete form in place of one of the blocks and pour
some concrete around it? This would be in a layer of block
that would be buried so appearance isn't an issue.
....

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Working around retaining wall obstruction

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earthwrks
Join Date: Dec 2003
Posts: 3853 Home Office in Flat Rock, Michigan
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2008-08-08          155929


KW are you using geogrid between the lifts of backfill? ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155930


Quote:
Originally Posted by earthwrks | view 155929
KW are you using geogrid between the lifts of backfill?


No plans for geogrid. I'm planning on a base of compacted sand with drainage (gravel,drain pipe,fabric) at the rear base of the wall. ....

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Working around retaining wall obstruction

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earthwrks
Join Date: Dec 2003
Posts: 3853 Home Office in Flat Rock, Michigan
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2008-08-08          155931


KW have you heard of the enemy to construction projects? His name is Gravity.

If I understand your description of the job, you might want to consider using the geogrid to keep the walls from bulging over time. And compacted sand is fine providing there is resistance i.e. walls to keep it in place. As soon as the reatineing system moves it's no longer compacted. And it depends on the type of sand you're using too--some sand does not ever compact; others can compact beyond belief. We call the latter around here "sharp sand" which has jagged sharp edges that interlock. For road building and backfilling basement digs, we use a Class 2 sand. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155932


According to the block manufacturer geogrid is not necessary for walls less than 4 feet high. Have you found that not to be true? ....

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Working around retaining wall obstruction

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earthwrks
Join Date: Dec 2003
Posts: 3853 Home Office in Flat Rock, Michigan
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2008-08-08          155933


Here's a case in point: A local developer--who should have known better-- built a big home in the middle of wet lands on a mound of dirt that was just big enough to place the home. The driveway was about 20 feet wide and 150 long and made a lazy curve toward the home on each side. Both sides were formed of 90 lb. retaining blocks 4 feet high. The 20' x 4' x 150' drive was filled with crushed stone. No geogrid was used. Then an 80,000lb. concrete truck drove about 5-6 feet from the edge and took out about 75 feet of wall on one side.

The builder had me dig out half the width of the driveway down 4 feet and rebuild the wall this time with geogrid every other course.

After it was all done, later another cement truck was able to drive right next to the block wall and didn't even move the blocks, according to the builder who was ecstatic it worked. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155934


Thanks for the info. There's no way a cement truck could drive in this area but the upper wall will be about the minimum allowable distance from a shed and a tractor may be driven parallel above the lower wall. I'll see about putting in some geogrid too. ....

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Working around retaining wall obstruction

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-08-08          155935


Ken, the manufacturers statement is pendant one one single circumstance.

The earth on BOTH sides of the wall must be stable, and hopefully undisturbed.

If you are building a series of terraces, each will have the potential to shift, if only a little bit, that will progressively affect the next one.

For example, the highest wall shifts 2", the next 5" (2" + 3"), the next under the increased load moves maybe 9" (5" + 4"), by the time you get a few terraces down it's moving more than a foot, if it hasn't fallen over.

IMHO, you need to use either GeoGrid (as Jeff mentioned) or a deadman every few feet to tie it back into the existing earth.

As EW also mentioned, gravity is the problem, if you join two things together, i.e. cement in a conduit, and the wall settles, the conduit will not last long. I think you'll find it's a LOT less work in the long run to either move the conduit or the proposed wall location.

Best of luck. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155937


Relocating the wall is not really an option - the location is dictated by the need to control hillside erosion so the shed doesn't slide down the hill in a couple of years :) It already has settlement cracks in the concrete floor and the doors don't close right anymore.

Moving the conduit will add another couple of days to the job but it looks like it has to be done. I'll have to disconnect all power and control wiring at the generator in the shed, pull the wires out, dig deeper (and hopefully avoid the geothermal ground loop), rework the conduit, fish the wires back through and then reconnect everything. Yech. Oh well :) ....

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Working around retaining wall obstruction

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-08-08          155943


Ken, why can't you bring the conduit to the surface and go over the wall?

Or for that matter, put the shed (and generator) in a new location?


Best of luck.

....

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Working around retaining wall obstruction

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kthompson
Join Date: Oct 2005
Posts: 5219 South Carolina
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2008-08-08          155944


Ken, seeing the concrete floor has cracked and the problem with the door makes me think of an ad on radio here often. No idea how well they work, cost or anything other than commerical. But they put steel piers down beside the foundation and jack the foundation level and solid based upon the ad. No idea if that would solve the problem or not. If you would like me to try and find out more on the procedure let me know. I will not be on line until Monday. Hope you well, kt ....

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Working around retaining wall obstruction

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kthompson
Join Date: Oct 2005
Posts: 5219 South Carolina
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2008-08-08          155945


Ken, Just as I hit send the name of the company popped into my memory. Link is below. kt ....


Link:   Ram Jack

 
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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155946


Murf, going over the wall would be nearly be the same amount of work as going under and not nearly as attractive in a finished landscape. Moving the shed would be a major job - the generator is bolted to a concrete floor, plumbed to a 1000 gallon propane tank and wired to the house. I know where the geothermal loop runs so I could go over that, drop below and under the wall, then up to the shed. The part I mostly wanted to avoid was disconnecting and pulling the wires and then fishing them through and reconnecting again but it's for a good cause.

KT - Thanks for the link. The shed is really in pretty good shape now with just the signs of trouble ahead. If another year or two goes by then I'm afraid some real damage will occur. ....

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Working around retaining wall obstruction

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-08-08          155947


Ken, if the only real reason for these walls is to stabilize the shed, there is another option. It might be less work than what you are proposing.

Drill holes with a PHD as deep as you can go a foot or so out from the existing shed. Then pour a footing in the bottom of each that contains small steel I-beams standing up, then back fill the holes with compacted sand. After they are ready have a steel beam bolted or welded to each forming a ring in a trench around the shed. Once that is done excavate (gently) all the soil between the ring beam and the shed, and a little ways under it's slab, then pour a new slab that runs under the existing one and around the ring beam.

The result will be that you have a) put deep footers under the building, and b) greatly increased the size of the slab and so increased the area that bears it's weight.

If you want more detailed info. just whistle. I'm in the midst of doing something similar under a 120+ year old log cabin right now.

Best of luck. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155948


Thanks for the idea Murf. That sounds more expensive than building a new shed! :-)

The shed is the main reason for building the retaining walls this year, but long term we wanted them anyway to convert a steep slope adjacent to the house to level ground that can be used for a dog run and small greenhouse in the future.

I'll post some before and after pics. ....

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Working around retaining wall obstruction

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hardwood
Join Date: Dec 2002
Posts: 3583 iowa
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2008-08-08          155949


I was always told that to not ask a question is the only dumb question, so here goes. What is Geogrid? Is it the same thing as we call a dead man or anchor buried bebind a wall to hold it in place??? Can't keep up on construction slang. Frank. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-08          155950


My understanding is that it's sort of a woven mesh material that is buried to stabilize the soil. The soil locks onto it and it helps to hold it in place.
....

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Working around retaining wall obstruction

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earthwrks
Join Date: Dec 2003
Posts: 3853 Home Office in Flat Rock, Michigan
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2008-08-09          155953


The mesh comes in various thicknesses; fabric widths; diameter of the "threads" in the mesh; what the threads are made of; what the mesh is coated with; and the open spaces between the threads themselves--to name some characteristics. This shouldn't be confused with "GeoTextiles" which are a mesh, but are usually tightly woven and used priimarily for road building. The theory behind the GeoTextile is similar to how a trampoline works, except there's road building material covering it.

(I'm kinda needing more rest--so my description needs some polish--but here goes:)

The GeoGrid I've used has openings of about an inch and the fabric roll is about 4-5' wide. It looks like wire fencing but coated black. Essentially you are building a dirt-and-GeoGrid sandwich many layers tall. The GeoGrid lays between the stacked courses of retaining wall and is usually held in place by the weight of the blocks and interlocking vertical pins.

If you have ever tried to pull a section of downed cyclone fence that has weeds growing up through it, you'll have a basic understanding how the GeoGrid works. But multiply that pulling force many times with each successive layer of fabric. ....

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Working around retaining wall obstruction

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hardwood
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2008-08-09          155957


Over the last 40 years I've been on and off our parish council several times. About 30 yrs. ago I was put in charge of hiring a cement contractor to tear out and replace an aprox. 5 ft. high retaining wall in front of the church, the old one was probably 80+ yrs. old and was shot. Anyhow I hired a young fellow from a family of cement type people. I knew very little about retaining walls, so the young fellow explaind to me that a solid cement retaining wall will never stand straight for very long unless you make provisions for drainage of the water from behind it. When they poured the wall they put 2 inch PVC pipe thru the wall from front to back at the base just above the ground on the bottom side spaced about 6 ft. apart then filled behind the wall with coarse crushed rock to drain the water from behind it. His comment was that if there is water behind the wall that the freezing and thawing of that water over the years wiil eventually push a wall to a leaning position. He must have been right, the wall is just as plumb today as it was 30 yrs. ago. Frank. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-09          155958


The manufacturers design book explains all about drainage on these stacked walls. A perforated pipe is wrapped in landscape fabric and surrounded by gravel at the rear base of the wall and it drains to daylight. I'm not yet sure exactly where that daylight will be but it will be found somewhere. ....

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Working around retaining wall obstruction

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-08-11          156032


OK, I'm taking Murf's advice and moving the wall. I was trying to avoid rebuilding the steps on the deck but that seems a lot easier than moving the conduit. The steps have 7-8 risers now and with the wall they will be down to only two or three, which is less deck to maintain (always a winning strategy!). ....

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