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Soil Uplift on plastic drain pipe

Soil Uplift on plastic drain pipe

Soil Uplift on plastic drain pipe

(OP)
How do I calculate the uplift force on plastic drain pipe shown in the attached sketch? I want to fill above the pipe with polystyrene pellets to minimize the the load on the hanger for the pipe. But I am concerned that the plastic drain pipe might start to rise due to upward soil pressure, if there is no soil above the pipe to hold it down.

RE: Soil Uplift on plastic drain pipe

Very strange detail. Does the pipe have to be so far below the slab?

If the soil is a swelling clay, the pipe will lift whether or not there is soil above it. Swelling pressure is quite variable but 1500 to 2000 psf is not uncommon. The upward force on the pipe is simply the weight of the pipe and its contents plus whatever downward pressure the polystyrene beads can provide.

It might be better to hang the pipe clear of the soil and forget about all that gravel and polystyrene.

If the soil is not a swelling clay, the pipe will not lift so it is safe to let it rest on the bottom of the trench. If the soil below the pipe is "existing fill and loose soil" , the soil may settle over time and the pipe will be carried totally by the hangers to the slab.

BA

RE: Soil Uplift on plastic drain pipe

Two questions:

What is the water table depth? If the pipe is usually empty, that can create uplift.
Will the pellets resist collapse of those vertical trench walls?

RE: Soil Uplift on plastic drain pipe

Ajk1:
The real issue probably isn’t the soil load on the pipe. As BA mentions, these loads can be pretty high, and the pipe really isn’t normally able to, or intended to, resist them in bending. The pipe tends to move with the soil around it. The real question is how much movement can the pipe tolerate without breaking or without the joints failing (leaking, breaking, what are the joint movement limits, the pipe deflection limits, etc.) , and I would ask a few pipe manufactures for info. on these subjects, for their pipe and joining methods/details. Then, talk with your GeoTech guy about what he/she expects the soil movement to be, given the local conditions, the soil properties and makeup, and the details you are using.

Consider a piece of pipe, most any kind...., when you pick up a 4' piece of the pipe, it seems quite stiff and strong, and if you try to bend it over your knee it will bend and buckle at that reaction point, if you can generate enough bending force. Alternatively, a 60' long piece of that pipe acts much more like a piece of wet spaghetti, very flexible, except as joint details concentrate any flexural movement. The longer pipe will assume a very generous, even, flexural curvature, within some product limits. The joints tend to be the controlling detail, expect if you try to bend that 60' pipe over an immovable object (a rock, your knee, another structural element, etc.), a concentrated reaction point.

RE: Soil Uplift on plastic drain pipe

(OP)

I will try to answer all the questions, although if you need the full story it will take about a day for me to write it all out:

- This is an existing 50 year old pipe, so the depth is a given. We know the depth from measuring at the cleanout, and it agrees with the invert given on the original 1956 construction drawings which we have. It is down at this depth because it drains the elevator pit among other things, and the elevator pit is at the far end of the building. Today yu would not be allowed to put drain in the elevator pit, but in 1965 it was routine to do so. The camera put down into the pipe seems to indicate a sag where water collects, and recently a clog of paper of some type, perhaps paper towels collected and had to be snaked out.

- This is not on expansive soil, and it is not below the water table. I am concerned about uplift because there is a buoyant force from the soil which is not counterbalanced if I do not put soil above the pipe. An analogy is an oil storage tank where it is routine to place a thick concrete pad under the tank and strap the tank down to it, to hold it down so that it does not rise when the tank is empty.

- the soil is a sandy silt till. No clay.

- I don't think it matters if the pellets resist collapse of the vertical soil walls or not, does it? The slab is a suspended reinforced slab spanning between drilled piers that are founded on good soil.





RE: Soil Uplift on plastic drain pipe

(OP)

I think my question about uplift force from the soil is more of a geotechnical question, but there do not seem to be geotechnical engineers on here... I used to know a very good geotechnical engineer but he retired a number of years ago and I have lost contact.

RE: Soil Uplift on plastic drain pipe

The buried storage tank is held down to a slab to prevent it from rising when the tank is empty and the water table rises above the tank. If it is known that the water table will never rise above the tank, the hold down device is not necessary.

The net upward force on the buried storage tank is the weight of water displaced by the tank less the weight of tank. Similarly, in your case, the upward force is the weight of displaced water less the weight of pipe, but it becomes a consideration only if the water table rises above the pipe.

BA

RE: Soil Uplift on plastic drain pipe

(OP)
To BAretired -ok, thanks. I did not quite realize that. So that is very helpful. Thanks.

RE: Soil Uplift on plastic drain pipe

(OP)
...my thinking had been that since the soil exerts a lateral pressure (similar to water, but a lesser pressure) that it acts like a fluid, and a fluid exerts a pressure in all directions including upward. But you are saying that although the soil exerts a lateral pressure, it does not exert an net upward force on an empty tank (if not below the water table). I suppose you are right, since the bottom of an excavation does not heave upward (assuming not a clay and not below the water table). So thanks again. Very much appreciated.

RE: Soil Uplift on plastic drain pipe

I fear this may muddy the waters, but depending on the excavation and the shoring you can most definitely experience base heave at the bottom of an excavation

RE: Soil Uplift on plastic drain pipe

Granted, I usually work outside of the building, but I don't follow why the pipe is being hung from the slab in this instance; or for that matter, why it is being backfilled with packing peanuts.

Why are you not simply digging up the existing pipe and replacing it with new using a standard trenched pipe detail? That is, place and grade to the desired slope your compacted granular bed, lay the pipe, backfill to the required depth around the pipe with compacted granular material, and backfill the remainder of the trench with the compacted spoils (if suitable) from the excavation.

RE: Soil Uplift on plastic drain pipe

ajk1,

This issue which exists now in three separate threads, is quite strange. Therefore your solution will need to be bespoke also.

The reality is that there is no uplift as you describe in soils that are not water logged.

The issue you face is more one of settlement below the pipe and then with these strange vertical hangers, you have point loads which are then trying to support the soil load above. You can probably work on the basis of the soil load being the diameter of the pipe x density x height of soil

IMHO, what you really need to do is supply a horizontal beam made of something sufficiently strong to carry this load either under your pipe to support it or above your pipe and then provide supports every 150mm or so.

If you're replacing a steel or CI pipe with a plastic one, and your current pipe has low points, this will be nothing compared to a plastic pipe of the same size. The strength is much less and PE tends to creep also under constant load.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Soil Uplift on plastic drain pipe

(OP)
to Spartan5; the soil was very weak and inadequate to support the pipe without large settlements, so the original designers 50 years ago decide to hang the pipe from the slab and make the slab a structural spanning slab supported on drilled piers founded on good soil at depth. This is a standard solution for the very weak soil case, that was used 50 years ago and today as well. I don't like it because it makes access to the pipe for repairs relatively expensive, but people don't feel it is worth spending the money up front to make the pipe more accessible when it breaks 50 years after installation.

to Littleinch: pipe manufacturers have tables of recommended hanger spacing. As I noted to Spartan5, hanging the pipes is very common so I am puzzled why you call them "strange". I have never seen a beam provided under the pipe. It would be expensive compared to placing the pipe hangers at the appropriate spacing so that the pipe can safely resist the weight of the soil above, in accordance with pipe manufacturer's recommendations.

The reason that I am giving a pelletized polystyrene option for the back fill is because the excavated soil would have to be stored somewhere in the building if it is to be used as backfill...that storage may be disruptive and messy, and we would have to spread the soil over a large enough area that is does not overload the existing suspended slab.

I will go to my dictionary when I get off the computer and look at what "bespoke" means. LOL

RE: Soil Uplift on plastic drain pipe

How is the pipe supported once it gets outside of the footprint of the building?

RE: Soil Uplift on plastic drain pipe

I'd like to seed tables of support spacing of pipes with soil above. Supports in air yes, with soil above them? Surely depends on how much soil.

If such data exists then follow it. WoulD be good to sse it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Soil Uplift on plastic drain pipe

Spartan5, In these type of situations with very weak soil, at least in localized areas near me, services such as sewer and water, etc... have to be supported on piles outside of the footprint of the building. Makes the installation of servicing quite a bit more expensive.

RE: Soil Uplift on plastic drain pipe

(OP)
To Spartan: Good question, but in my case, once it gets outside the building it is very clearly in very good soil (8 tons per square foot) and support there is not an issue. No need for hangers. But in other areas it is in weak soil (4 to 10 blows per foot).

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