Preventing settlement from MSE Wall? 5

[Jean_Wong]

Good day!

I am designing 8m high MSE Wall. There is a 0.6m diameter high pressure water supply pipe 1.2m below NGL, along the wall alignment. What could be done to prevent any settlement from the MSE Wall?

 

[JedClampett]

Encase the pipe?

 

[EireChch]

How close is the pipe to the wall? What's the load on the pipe? 10kPa surcharge?

MSE walls with Georgrid layers, typically the layers at the top of the wall don't need to be as long as the layers at the base of the wall. You could maintain a constant geogrid length for the total height of the wall.

 

[Jean_Wong]

It is a railway with MSE walls on both sides of the alignment. As of now, the top of the pipe is 1.2m below the NGL. An 8m high fill with 9m width for a span of 40m would be added on top of the NGL. The fill would be supported by MSE walls.

In short, there's already 1.2m soil on top of the pipe, and an additional 8m would be added. Should the pipe be encased or is there another way to prevent settlement?

 

[EireChch]

Is the pipe running perpendicular to the embankment?

There are things you can do to prevent settlement, like support the pipe on piles etc but these may be a bit extreme.

Concrete encasing the pipe will not prevent settlement, just stiffen the pipe section.

If its a short section of pipe, say 40m, I would replace the section of pipe with a HDPE pipe. Specify a pipe with a low Standard Dimension Ratio (SDR, which means a thick wall compared to diameter) HDPE pipes are more flexible than traditional concrete pipes and can absorb settlement better. Place a manhole either side of the embankment and use a flexible connection at the manholes.

 

[Jean_Wong]

The pipe runs along/parallel to the embankment. The embankment height increases from 3.5m to 8.0m at a distance of around 190m. The pipe curves at some point so it doesn't run along all 190m, but still a significant distance. I would consult with client and the local government if the pipe could be replaced. Anyways, do you have a reference for specifications of hdpe pipe?

 

[PEinc]

Can you install bearing piles along the sides of the pipe and place a pile cap over the pipe to support the embankment load? Can you install stone columns or CMC's along both sides of the pipe?

http://www.PeirceEngineering.com

 

[Pinwards]

You should realign the pipe outside of the fill area. It's going to be nearly impossible to dig up safely if you add another 8m of fill on top of it, and such excavation would probably cut through your reinforcing grid. Water mains do not last forever and some day it will need to be repaired or replaced.

#

 

[oldestguy]

Look up the principles of the "Imperfect Ditch". It is a technique developed in Iowa DOT. In short, the backfill over the pipe is replaced loose with compressible material there. I once was with the Wisconsin DOT and there was an active outfall from a sewage treatment plant that could not be shut down. A new Interstate road was to cross it with a fill about 30 feet high. The plan called for removing the backfill over the pipe. Then the highway fill was placed. It did the job and had been there for about 50 years now.

One could elaborate on this by doing the loose backfill part and then, on the original surface place a wide concrete slab to transfer loads from directly over the pipe to wide adjacent ground, and placing compressible material such as corn stalks as backfill. That would even out the settlement and only require a gradual transfer of this treatment gradually to non-treated areas.

 

[EireChch]

I agree with Pinwards, if it was a perpendicular crossing replacing the pipe with a more flexible pipe material and mn either side might be a solution. Since it's running parallel, but it quite the full 190m, relaying may not be as benisifical. If you're going to relay you might as well relay it outside of the embankment.

As PEinc says piles/ground imporovment are options, might be expensive but it's a 0.6m water pipe so it's might/probably be warranted. Pulling shouldn't require removal of the pipe and they may need to be bored piles sue the potential effects from driving vibrations.

Stone Columns may not require the removal of the pipe either.

If it's both public infrastructure , it may be a lot easier to get funding for a robust solution.

 

[fattdad]

move the pipe!

pile support the reinforcement zone in proximity to the pipe.

then again, you could just move the pipe!

f-d

ípapß gordo ainÆt no madre flaca!

 

[3Fan]

EireChch, could you expand on you idea about installing stone columns near the utility? We have a similar situation as the original poster. Out utility lines are 2-20" high pressure gas mains. We have moved the MSE wall outside of the MSE reinforcement, but the pipes will still be in new embankment. Settlements have been estimated in the 6" range. Would like to know more about using stone columns to prevent the existing utility from settling.

Thanks!

 

[Okiryu]

Can you consider the use of lightweight fill? If the water line is critical (which for the size of the pipe I am assuming that it is) and the city does not allow to relocate it, although lightweight fill is expensive, perhaps it can help you in this situation. You may also have some savings in the design of the wall since you will we exerting less pressure to it.

 

[Jean_Wong]

Thank you for all the answers!

Realignment is good considering maintenance issues but can be costly since right of way extension might be needed.

I would consider lightweight fill if it can resist collision and seismic forces.

For the meantime, is there a good reference or sample calculations for imperfect ditch and stone columns?

Will suggest every possible solution to client at next meeting and let them decide.

 

[EireChch]

JW

The settlement assessment of stone columns is pretty empirical for the most part. You need to determine the amount of settlement without stone columns first (i.e. settlement from the MSW wall) and then the settlement improvement factor with stone columns in place, which is based on the replacement ratio - area of stone column (Ac) to area of soil/untreated ground (As).

For example you may have pre-stone column settlement of 85mm. Your assessment indicates an improvement factor of 2.2. Therefore your settlement will be 85/2.2 = 39mm.

See link below for further information, a google search should bring up enough information for a starting point. Sorry cant be of more help as I am swamped at the minute..

http://userweb.eng.gla.ac.uk/MScPos...nt/Ground Improvement using Stone Columns.pdf

 

[PEinc]

Lightweight fill sounds good but you need to consider the effect on MSE wall strap length due to the lighter weight of reinforced fill. The overturning and sliding forces would probably be the same but the MSE gravity wall will not be as heavy when resisting the forces. Therefore, the wall needs to be made larger (longer straps).

http://www.PeirceEngineering.com

 

[BigH]

First off - what is the foundation soil? In India, we had 6 m of Su=15 kPa (N=0,1) and built 10 m high wall and the whole thing settled 1100 mm during construction (we used pvd for quick settlement). So, if your foundation in which the pipe and wall is going to sit on is rock, no problem other than later maintenance. 8 m of fill will cause the soil beneath the pipe to settle and if the pipe takes a turn out of the alignhment somewhere - this may be a problem.

Use of styrofoam as lightweight fill has been used in a number of locations around the world - Utah's I-5 (or is it I-15) used it. Issues include fire - but you would need to surround the styrofoam by fill (on top); on the sides you could easily "hang" siding on the styrofoam - make it look like a MSE wall. Check Horvath at Manhatten University for references and papers. Elastizel is another possible solution.

As for MSE - we had a problem once when the culvert supported on piles was skewed to the roadway embankment's MSE wall . . . the strips were perpendicular to the wall - as is normal; but they went under some weird deformation due to the culvert's non-settlement where the embankment underwent major settlement (and yes, slip joints were used on the walls face at the culvert's ends - but this did no good for movement inside the reinforced mass due to the skew.

A lot depends on the nature of the ground on which you are building the fill. OG's sage advice, coupled with other ideas - but as some say - moving it might be best. Who is responsible for repair of the pipe in years down the line??

 

[PEinc]

One more thought: An MSE wall not settling over a rigidly supported pipe may have similar problem to an MSE wall sitting above imperfect trench backfill. Pick your poison; the wall will probably differentially settle either way, unless fully supported. If the pipe is already in place, and if it is not possible to relocate the pipe, you best solution may be to found the entire MSE wall (including the straps or grids) on stone columns, CMC's (controlled modulus columns, ref: Menard), or on some other type of improved soil.

http://www.PeirceEngineering.com

 

[EireChch]

I am looking into a thesis on stone columns at the minute and came across these lectures notes which may be of use to you.