Infiltration Systems near Moisture Sensitive Structure Bearing Soils 2

[ocgeo]

Here is an interesting topic. Where can water be infiltrated into the building load bearing moisture sensitive soils?

My thought is that the load extends at a 1:1 (+ or - with soil type) from the bottom of the footing, so not a great idea to introduce water within this area, until you get to a certain depth, as a function of some % load. And even then, still risky.

Interested if anyone else has some ideas when the site does not have much room for a required infiltration system?

 

[MTNClimber]

Depending on the type of sensitive soils, a cut-off wall may be considered. Either dig a deep trench between the building an infiltration system and fill it with concrete or install sheeting.

 

[fattdad]

The design strength of all soils is based on saturated conditions. The state of practice for unsaturated strength is not quite ready for prime time.

you are implying that if the soil gets wet something bad will happen. Please explain your failure mode?

If the soil is compacted below the critical void ratio (or resides that way in the natural state), in what way will the design strength or compressibility change when it gets wet?

'Cause, we'd have to really worry about our embankment dams!

f-d

ípapß gordo ainÆt no madre flaca!

 

[ocgeo]

Good points, if the soil cannot densify, and has the overburden to resist expansion, that would help. I would be concerned about collapsible soil at depths that were not overexcavated and reprocessed, or (to relate to dam terms) 'piping' vertically into a non-natural open graded soil filter layer, especially with large volume infiltration systems with significant pressure heads. There are also many other variables such as utility trenches and the unknown subsurface soil that make me not a huge fan of the idea of saturating the soil beneath the structure. Part of the issue is the geotechnical design is completed well before the civil, which makes it difficult to go back and redesign.

 

[oldestguy]

Is this a real job? If so so details will help. Mother nature likes to bring water table levels to equal elevations in the long run.

 

[MTNClimber]

Part of the issue is the geotechnical design is completed well before the civil, which makes it difficult to go back and redesign

If grading plans and FFEs are not finalized at the time of your geotechnical report, you should be requiring the engineers to provide those items when they are available so you can see if they impact your recommendations. Only then is your “design” (potentially) complete.

Im with OG. Is this just a theoretical question or does this pertain to a specific situation? Because they’re are too many variables in your opening post.

 

[aeoliantexan]

If you are talking about loess, and it has been demonstrated to be collapsible, be careful. A side issue is that the location of your infiltration facility may be rendered useless for future construction.

One thing that may work in your favor, however, is that loess may contain old small root holes that increase the vertical permeability and the wetted zone may have very steep sides, reducing the lateral effect. On the other hand, if there is an underlying low permeability formation, a groundwater mound that spreads out is likely to develop.

There are enough uncertainties that care is needed. I would resist a "required infiltration system" when building on loess. I have seen very serious building settlement problems caused by a water line leak some distance away.

 

[BigH]

Look at the experience with trees near buildings . . .

 

[ocgeo]

That is a good point with loess (silts) and piping. That is a concern with the levees in Sacramento. The roots, active or dead, create a piping potential.

BigH, I also have had many issues with trees (I think the PTI has a lot of info on the soil moisture gain/loss potential in expansive soils). A good read on it also is simply explained in Forensic Geotechnical and Foundation Engineering, by Robert Day,

 

[BigH]

For references with tree influence from a previous post
BigH (Geotechnical)16 Jun 07 12:44

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd184_e.html

http://www.lga.sa.gov.au/webdata/re...ban_Environment_-_2005_Report_-_Section_1.pdf

http://www.golder.com/Archive/AssessingConstruction.pdf