Microporous Loess soils 1

[Myoho]

We propose to excavate in microporous Loess soil around the city of Galati in Romania. The proposed structure is a combined sewage storage tank 25m diameter 15m below ground level.

Does anyone have any experience in excavating in this type of soil and the associated geotechnical hazards.

Also typical soil strength parameters (total stress) would be much appreciated.

Thanks in advance.

 

[BigH]

What do you mean by microporous??? I dealt with loess in Northwestern China. Great material to excavate in - stood many metres vertically with no signs of distress. Could make excavation and use the loess itself as wall formwork!

 

[dicksewerrat]

Why not bore the tank in with a 'well type' machine. then place a concrete floor and seal it all up. Safe and quick. How will the sewage be removed? what is the depth of the inflow pipe?

 

[BigH]

dicksewerat - good point. - this is like a pier well foundation - sink a caisson and then do the bottom. The only part might be on the leakage aspect - you might not be able to get proper waterstops in place. But loess has been cut very high openly. See photos in some references of loess cuts in Iowa.

 

[Myoho]

Firstly thanks for your replies everyone.

I seem to be getting confilting reports about this material. Some papers say that Loess can cause a number of problems associated with its sudden settlement upon wetting. Problems result because Loess undergoes structural collapse when wetted. This happens when the initial dry density is low and the initial water content is low.

However as you say - you can excavate a vertical face in the same material.

The construction method is not a problem, however I would appreciate some typical shear strength and dry density parametets if any has any.

Cheers

 

[BigH]

Myoho - you are correct on collapse of loess soils on wetting - there were quite a few papers out of China and Russia in the late 70s on loess soils. Check them out. I have a copy of something here with me - if you send your email address to bohica@indiatimes.com, I will scan and forward to you. One point with the loess soil that I encountered - it is the most erodable soil I have EVER seen. You could stand at the top of a slope crest, take a leak just behind it and see the rathold develope before your very own eyes in the time of the pssssssssssssssssss. I'll look out for some typical values of dry density and that - my China stuff isn't with me now.

 

[BigRedGeo]

We all know that soil properties are variable, so keep that in mind when reading this. Due to the depositional characteristics of loess, the strength properties will vary greatly. The grain size of loess is coarsest near the source. I do a lot of work in the loess hills of Iowa/Nebraska. In this area, the Missouri River valley was the source, so the first bluffs off the river consist of the coarsest material (primarily silty clay or clayey silt). Moving farther from the source, the loess becomes more clayey (smaller particles travel farther in the wind.) Therefore, identifying your source may give you some insight into the materials you will have, and their strength properties. For long-term conditions, we'll usually use phi of 28 degrees for silty loess materials, and phi of 26 degrees for more clayey materials. Cohesion will vary considerably.

The most critical problem with loess, as already discussed, is collapse (hydro collapse). Because they are wind-deposited, loess soils will usually have a low dry density and high void ratio. There are many rules of thumb to use in evaluating collapse potential. The easiest rule of thumb is dry unit weight - if the dry unit weight is less than 85 pcf, you are likely looking at some collapse potential (unless you are already below the water table). Dry unit weights down to about 72 pcf are pretty common in this area. If you have collapse potential, then take a hard look at potential sources of moisture and leakage.

Considering a sewage storage tank, I would be looking at providing 2-3 feet of compacted cohesive material beneath the tank, and anywhere you might have leaks, if you have determined that collapse could be a problem. This will provide a low-permeability zone and help protect your collapse-sensitive materials.

Notes for sampling loess: Loess is very sensitive to disturbance. SPT won't tell you much - get some Shelby tubes. Use 3" tubes, and only do 1-foot pushes. Dry loess will compress a lot when pushing a tube if you are not careful, which will throw your density through the roof.

Hope this helps...

 

[Myoho]

Many Thanks Big Red. You have covered all I asked for and more. I especially appreciate the advice on insitu testing.

RGds,

Myoho