pipeline protection slab 2

[waseem19]

we got a 17m deep trench ,and must provide protection for the pipe ( GRP 1200mm ) at that depth ,

now, is it better to compact the soil between the slab and the pipe ??

i think it's better not to ,cause if we did, the compacted soil will transmit the load on the slab to the pipe ,so the slab will be useless specially that the pipe and the slab are only 5 meters apart ,,

please tell me what do you think ?

 

[DMWWEngr]

You must compact the soil. If you don't you risk settlement issues later. The compacted soil will actually help distribute the load outward. ---
Andrew

 

[waseem19]

Settlements of which soil? The slab is supported on rocks ,both side of the trench ? Why should i care about settlement of the soil between the slab and the pipe?

and again ,i don't want to distribute the load ,i want the slab ( reinforced concrete to carry the hall load without letting anything for the pipe to carry .

 

[dlew]

waseem19,

Are you serious about the 17m deep trench? Or is it 1.7m?

If the pipe is placed in a trench (rather than under a fill), and the depth of the pipe is beyond a critical depth, the vertical soil pressure on the pipe will not increase because of the soil will arch between the two sides of the trench.

The formula for the critical depth could be found in any drainage and piping handbook.

If the pipe is place in a shallow trench in compressible soils, and a concrete slab is poured on top to protect the pipe against surface loads, you could consider the placing of a one or two inches of some soft material (as urethane foam) immediately on top of the pipe, as a cushion in case the soil at each side of the pipe settles. The farther the slab above the pipe, the better. And as DMWWEng said, it would be better to compact the soil (on top and on both sides of the pipe) to not less than the natural density of the site.

Regards

AEF

 

[waseem19]

dlew ,

yes dlew ,I’m very serious about the 17 ,, one seven ..

i know it's very deep ,, it's a trunk sewer ,i used to think that at that depth it is cheaper to do an NDM ( non disruptive method ) but the prices for open excavation came much lower, so why not do it , it's an open area .

we will use the foam you talked about , but it serves the same purpose that i want to reach by not compacting the soil cause this foam will allow the slab to deform under the load and carry it instead of passing it on to a hard compacted soil beneath it and then to the pipe.


thanks a lot for both of you.

 

[mhnds]

may be i am late,but i wanted to share my opinion.In any case the load above the pipe will be caried by the pipe itself unless the slab is supported by the sides,or by the soil at the bottom of the trench.Slab will help us by distributing the load a respectively larger area.
regards
umitk2211

 

[BigH]

This open cut - is it sloped open or combination of sloped and thin vertical cut with shoring??

Have dealt with situation where we have a pipeline in place and we are putting an embankment above - so we end up protecting this pipe too. In soft soils, the embankment will add load; so we build a "box" about the pipe leaving 1m or so headroom above the pipe. Then with any settlement, that might occur, we won't have worry about contact of slab above to pipe.

Now - if vertical cuts with shoring (to minimize very large excavations, don't forget that there will be some arching likely in the zone above the pipe. In fact, if I remember right, you don't really compact the soil well in the narrow trench up until you get near ground surface. This permits the arching effects.

Hope these thoughts are of some use. Best regards.

 

[waseem19]

ukarahan ,
the slab is supported on it's end on a very dense material, it's working like a column.

bigh ,

it's a nearly a perfect vertical slope , it got two steps on the way down, the slab will be on the last step ,the slab is 5m above the pipe ( even though the pipe can handle 12m of dense soil ,so actually a better design could’ve been achieved by putting the slab 10m above the pipe thus having 2m as a safety factor for the pipe and reduce the slab thickness )

i'm convinced that the soil shouldn't be compacted (perhaps a little bit to minimise the risk of settlement but near the surface only )
but if compacted a lot :1- transmit the load from the slab to the pipe , 2- increase the density and therefore the load on the pipe.

 

[Adso]

Regarding your believings that the soil don't have to be compacted near the pipe, you'll be really surprised how this little compaction work (compared to 17m trench excavation) change the pipe response. I'm analyzing a pipe installed with no compaction at all which has failed as if it was made of paper. Get the AWWA code and follow it as much as you can.

 

[Rawand]

I think That The backfill should consist of gravel 5mm-20mm
it requires minimum compaction, besides it helps the drinage of underground water around the pipe, and detrebutes the loads uniformly and easy to dig out the pipe in the future for maintenance porpuses. but since it is sewage pipe, the float is probably not under preassure, so you have to check the preassure above the pipe for noncohesive soils not exceed the pipes resistance specially that the loads are permanent (pipe deformation may occure and Hydrolic characteristics are desturbed.

 

[mhnds]

waseem19,
if the slab is supported at its ends,then load transmission to the soil below is possible in two ways:
1)Deformation of the dense material that works like a column.Degree of the deformation determines the load transmitted.Failure of this dense material will cause the all load to be transmitted.
2)Deformation of the slab itself.All above is valid for this case.
In your problem,the two deformations will occur,but probably they will be very small(certainly depends upon the dense material and slab quality).
In addition to those,assume a significant amount of load was transmitted,the compaction degree ,i think, would not affect the load carried by the pipe.As i understand, all you expect is from the absorbtive behavior of an uncompacted fill.It may work for impact loadings,but in the other cases after the soil is compacted to the last degree by the effect of the transmitted load,the pipe will be under the effect of this load.
I think it will be better for you to follow the related codes,too.
Regards, umitk2211