Soil hydraulic fracturing

[GATV]

Does anybody know how to calculate the stress at which soil hydraulic fracturing takes place? I'm trying to check if jet grouting could have cause fracturing of a soft clay.

Thanks.

 

[Focht3]

Hmmm,

The practical answer is that the soil will hydraulically fracture when the pressure exceeds the depth (feet) times one psi. It doesn't take into account shear strength, density, etc. - but it's a pretty good estimate.

But you asked about jet grouting - pressures above 1,000 psi. In that circumstance the soils are eroded, not fractured.

Why do you ask the question? Perhaps we can help if you give us more to go on -



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[GATV]

Thanks Focht.

When a test pit was performed near the jet grout wall, almost vertical soilcrete seams were observed within the soft clay. We couldn't go deep enough to see their depth of origin. In jet grouting the high pressures are at the nozzle, by the time you've eroded the soil to the column diameter can't really know what the pressure is. Also, the only way that hydro fracturing could have happened is if the spoils flow got blocked at some point, even temporarily, which would have increased the pressures within the "confined" space.

My first thought was that it would have to overcome the vertical stress of the clay layer, but some papers I read indicate it depends also on the water content, shear strengh and whether NC or OC. I just haven't found a correlation to use and was wondering if anybody knew more. The equation for hydrofracturing in rock does include the tensile strength and lateral stresses.

 

[Focht3]

...almost vertical soilcrete seams were observed within the soft clay...

This doesn't sound like "classical" hydraulic fracturing to me, which I would expect to be mostly horizontal. I think it's pretty likely that the grout found vertical weaknesses in the soil - probably joints from shrinkage cracks that formed during a prolonged drought. How deep did these features extend? As a point of reference, I have observed infilled shrinkage cracks as deep as 22 or 23 feet (roughly 7 meters) in central Texas.

If the vertical fractures are following old shrinkage cracks, then the soil stress state won't really help you predict their occurrence...



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[GATV]

I said almost vertical when what I should have said is steep - the soft clay was saturated, below the water table.

 

[Focht3]

Where is the site? GWT level? Depth at which the "fractures" seem to form?



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[GATV]

Hi Focht3. Here are some answers to your questions:

1. The site is in New England, groundwater levels are about 15 ft below grade, about 2 feet above the soft clay.
2. About 17 feet of overburden on top of the clay -- wouldn't take much to overcome it.

Some more reading I've done mentions that in the case of NC clays you would expect fairly steep fractures since the fractures would follow the same direction as the maximum stress.

 

[Focht3]

Are the "fractures" originating in the soft clay? Do they pose a problem, or are you 'just' intellectually curious?



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[GATV]

We couldn't extend the test pits deep enough to see where the origin was; we do know they extend through the clay though.

They don't pose a problem now, but did since settlement of a structure nearby was observed -- trying to see if lateral soil movement caused by the fracture displacement could have created some tilt in the piles (pretty short piles - 12 ft, mostly through soft clay). I think we can explain it, but if I could come up with some "numbers" to walk through the cause and effect it would be great.

 

[jdmm]

geotechgal

In reference to the fractures you have observed, in sensitive (2-4), firm clay we frequently see very extensive, sub-vertical fractures in the clay. My theory is that in sensitive clays they behave somewhat like brittle materials resulting in fractures developing along stress concentrations, not unlike that in bedrock. The sensitivity is likely important.

On large construction sites we have seen these 'joints' continuous over 50 metres. They frequently curve and change in dip. There are also frequently intersecting joints.

In terms of your original question. Hydraulic fracturing occurs when the pressure exceeds the effective stress at that depth. Hydraulic fracturing normally occurs horizontally but nothing prevents the grout from following routes to areas of lower effective stress.

 

[soilfraccer]

It is possible to hydraulically fracture clay with grout. Hydraulic fracturing in soils is a tensile parting that is created when fluid is pumped into an isolated zone at a faster rate than it leaks off. Leak off occurs as the fluid moves through the pores spaces in the soil. Leak off is primaily controlled by soil permeability and fluid viscosity. If the fluid is pumped faster than the surrounding soil can take it, the pressure will build until the soil parts (fracture is started). In soils, fabric appears to have a very strong influence on fracture orientation, this influence can be greater than that of stress. Fracture orientation is soils varies from horizontal to vertical and most of the fractures I have seen in test pits, core samples and from tiltmeter analysis have a significant amount of dip. There is no general orientation for soil fractures. Conjugate fractures can also form during fracturing, these can have dip angles and orientations much different from the main fracture. I have seen hydraulic fractures cross the natural vertical fractures in clay till (shrinkage fractures), but these were fractures propagated using a very viscous fluid. Grout seen in vertical sheet could have been placed by vertical fracture propagation or might just be leak off into an existing natural vertical fracture that is filled and possibly extended.

 

[Focht3]

[blue]jdmm[/blue]:

To me, soils with a sensitivity of 2 to 4 have a low to moderate sensitivity. I don't normally refer to soils as 'sensitive' until the ratio of undisturbed to remolded strength exceeds 5 or 6, and if the ratio approaches 10 I'm looking at 'quick' clays (which I very seldom encounter around here [Texas], except in dredge ponds and some alluvial deposits along the Gulf coast.)

Any thoughts?



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[clayR]

Geotechgal,
Interesting thread. What did you determine, and what became of the issue?

 

[dirtguy4]

Geotechgal,
This is a very interesting thread. I'm wondering if the fractures in the clay occurred prior to the placement of the overburden fill? It sounds to me to like the usual cracking that occurs from drying conditions and increased matric suctions when clay is at the surface. Following the placement of the "Fill" the water table may have risen, saturating the clay as well as part of the fill above it and the fractures in the clay haven't had a chance to close or seal (it's possible). If you would like to see how deep the clay fractures are, try test pitting where there isn't any fill in the same clay strata, close to the site. Measure the depth of these cracks and there is your answer. Hope this helps.