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Use of SPT's in V.Soft Soils and Peats

Use of SPT's in V.Soft Soils and Peats

Use of SPT's in V.Soft Soils and Peats

I'm trying to compare Geotechnical Properties derived from In-situ Testing and Lab Testing to determine how reliable/comparable the respective methods are.

The main two properties I'm looking at are Undrained Shear Strength (Cu) and Coefficient of Volume Compressibility (Mv)for some very soft Alluvial Clays and Peat.

SPT's, CPT's and Lab Testing have been carried out on a site. Calculating parameters for the latter two methods is not an issue. However the problem is for the SPT's many of the "N" Values are recorded as Self Weight (SW/0) or 1. Making deriving parameters impossible or parameters that have no variation across the site.

Are there any papers out there that detail the derivation of Geotechnical Parameters in soft soil? (I have used Google). Or that show that using SPT's on soft soil is unreliable?


RE: Use of SPT's in V.Soft Soils and Peats

Have you ever run vane shear tests in place? I have found that the vane results are much better than any other field testing method.

RE: Use of SPT's in V.Soft Soils and Peats

I forgot to mention. Small dimension vanes are difficult to use for precise results in very soft materials. Make your own vanes of much larger dimensions than what is used in bore holes or casing., etc. As I recall mine were roughly 12 inches high by about 6 or 8 inches (diameter), but of the ratios generally recommended.

RE: Use of SPT's in V.Soft Soils and Peats

Only Hand Shear Vanes have been used on site but this data is very limited. Unfortunately I no longer have access to the site but thanks for the heads up I will keep it in mind for the future.

RE: Use of SPT's in V.Soft Soils and Peats

Any correlation between SPT and clay strength is a guess. So the answer to your OP is don't even try to correlate the strengths.

I'm not aware of any correlations to compressibility, but luckily I don't have to work in peat areas.

Good luck.

Mike Lambert

RE: Use of SPT's in V.Soft Soils and Peats

C-sub-alpha for peat correlates to natural moisture content.
Su for normally-consolidated clay correlates to sigma V'
Peat is pretty frictional, but the settlements!


ípapß gordo ainÆt no madre flaca!

RE: Use of SPT's in V.Soft Soils and Peats

For peats - a lot depends on if amorphous or fibrous peat. But as fattdad says - settlements can be very high. We did a lot of peat testing for a Saskatchewan Tailings Dam years ago - studying the decrease in the coefficient of permeability due to height of tailings dam . . . it was rather interesting.

RE: Use of SPT's in V.Soft Soils and Peats

If SPT result is SW or 0, than you should estimate the same for your structure.

Assume that there is no soil at all. For example, if you improve this soil, do not add any bearing capacity at all from the soil. Assume all loads are carried by rigid inclusion. (Or you can try wick drains if you have time.)

What I wonder is how did site crew obtained samples from a peat that has 0 SPT value? It does not seem logical. And they tested it! What did you obtain from the lab? I would question the reliability of one of the results, SPT or lab results.

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