Manhole Design Expansive Soils

[AnotherDirtGuy]

I am working on design and construction recommendations for a series of HDPE manholes installed in expansive clays. As far as I tell, there are many posts on this site about shallow foundations over expansive soils, but nothing yet about buried structures like manholes, pipes, tanks, etc. I am looking for advice on evaluating potential uplift and lateral compression forces as well construction practices to mitigate/eliminate the effects on the buried structures.

So some details...

The project site is north FL. We have a geotech report that indicates layers of plastic clay (LL = 60's, PI = 30-40, fines > 60%) within the local zone of seasonal moisture variation (10 ft bgs). The expansive soils in the the upper 10 ft are randomly interbedded with fine sands, clayey sands, etc. We are designing 40-odd manholes for a storm system which will be installed to depths of 5-13 ft bgs. The manholes will have HDPE risers and cast-in-place concrete bases to resist buoyant forces as well as these possible expansive uplift pressures. The geotech report did not provide reliable indication of the GWT at the site.

Any advice on evaluating the risk of expansion in these soils as well as the magnitude of forces imparted on the buried structures? Also any similar project experience or technical resources about mitigating these risks through design or construction techniques?

Thanks!

 

[fattdad]

is the classification data the only reason you think expansive forces will be an issue?

Do you know whether the soils include any smectite or halloysite minerals?

On the face of the classification data, I would not give a second thought on that basis alone. Then again, if local failures speak to such swelling, then it could be a mineral trait rather than a classification trait.

In Richmond, Virginia we have Triassic basins that include elastic silt with such minerals. I'm not familiar with special design for utilities, but we do lower the footings to 4 ft in these areas to minimize the affect of temporal changes in moisture content. We also have other areas with marine clays where we don't see the shrink-swell problems. Other problems? Yes.

f-d

ípapß gordo ainÆt no madre flaca!

 

[AnotherDirtGuy]

Thanks for the quick response Fatdad!

I do not know the mineralogy of the site soil (perhaps I can track this down somewhere). However, the geotech report I have provides foundation recommendations for the proposed development--all 2-3 story apartment buildings. They recommend shallow foundations with a combination of limited over-excavation of the fat clays and a structural slab and/or integral grade beams to resist differential ground movement due to shrink/swell. So based on that I am assuming it is a risk at this site.

At the projects you mentioned where you lower the footings to 4 ft, do you give any special consideration to the fill material used over and around these excavations? It seems like standard sand or sand/gravel fill might drain surface water to your underlying expansive soils and negate the gains of lowering the footings in the first place?

 

[oldestguy]

Even though the barrels may be different that the "usual" manholes, uplift on the "usual" manholes might or might not have been experienced in the area. I'd check locally with utility companies, public works departments and see if historically there have been any problems. If so what treatments work? A check with other areas, such as in the states where expansion - shrinkage of clays are common, (Denver Co for example). My experience in these areas of Wisconsin has not encountered problems with manholes that I know of.

 

[geomane]

Where in north Florida? I work in Escambia, Santa Rosa, and Okaloosa County.

I could get information from our other branch offices on soils for any county in Florida.

 

[AnotherDirtGuy]

OG - That is a great suggestion. I've reached out to the local utility to see what advice they may have.

jmcc - The job's near Tallahassee. My firm is out of state, so we have little to no direct experience with the local soils. Any insight you have on the clays in this area would be a huge help! Thanks

 

[cvg]

what ever happened to picking up the phone and calling the geotech? he wrote the report, he should be able to answer a few questions without generating another invoice, right?

 

[AnotherDirtGuy]

CVG - You're totally right. Unfortunately the organization of the project team is a little convoluted on this one, so I'd like to wrap my head around the issue before speaking directly to the geotech and eventually to the contractor. I'm not making any final decisions based off what I get from this thread haha... just looking for more info.

On that note... any other thoughts on the OP?

 

[oldestguy]

In case the issue is real as to lifting the barrel section, consider the backfill type and how to use that to prevent the possible lifting.

 

[AnotherDirtGuy]

OG - that's direction we've been leaning. Limited over-excavation of the expansive soils where encountered below the shallower manholes and a low-permeability backfill material to prevent draining surface water and shallow groundwater to the underlying expansive material. The concern I have with this approach is that it does not seem to address possible lateral pressures on the riser section from horizontal expansion of the native materials surrounding the manhole excavation. Perhaps this lateral effect we could attenuate by reducing compaction of the backfill or by widening the manhole excavation to create a larger buffer zone between the structure and the potentially expansive material?

 

[Okiryu]

AnotherDirtGuy, here is an old thread about calculating swelling pressures:

http://www.eng-tips.com/viewthread.cfm?qid=401965