"Running Sands" in geotechnical borings

[theclipper]

Knowing that you will be explaining it to a structural engineer with limited knowledge of geotech - When I see "running sands" noted in a boring log: What are they, why do the slow down borings, and why can they be blamed for inaccurate blowcounts?

Thanks for any input, i've never heard of this phenomena and now have seen it pop up on two jobs in the past two weeks.

 

[VAD]

So called running sands result from sand leaching out of a borehole or testpit under the influence of water/seepage.

The resultant is slurry like. The terminology is often cited by drillers. This sand/silt falls into the borehole when the hole is uncased and on removal and replacement of drilling tool the depth drilled previously is not reached and hence the hole has to be cleaned out.

This is often a battle as the sand keeps running into the hole as the tool is removed for cleaning.In many instances a hollow stem auger has to be used to case the hole through the zone of the running sand.

If the hole is not cleaned properly, yes the sloppy "running sand" material will form part of the thickness through which the SPT will be taken and hence low blow counts will be recorded. The geotechnical technologist would make a note of this problem and this is what you would have seen on the drill hole logs.

If the drilling contractor was not equipped with hollow stem augers, then that would be the end of the drilling.

If rotary drilling is used with drilling mud then this problem can be largely mitigated. Generally the drillers have the experience to apply bran to the mud etc, etc.and with a fluid inside this tends to prevent the running sand condition unless an artesian condition is encountered.

Hope this helps. Others may have input as well on this topic.

 

[CarlB]

Regarding inaccurate blowcounts, I've seen where running sands move up into the bottom of a hollow stem auger. The drillers may be unable to get the sampler to the bottom of the auger, begin driving through the "heave", and get artificailly high blowcounts.

 

[BigH]

We always called this "live sand" - according to French Quebecker drillers. This is one reason that you must always keep (or try to) your hollow stem augers or casing full of water so that the water pressure in the casing is higher than the pressures below forcing the sand up inside the casing. It is not easy to do - as you pull the plug, you sometimes pull a 'vacuum' (in the broadest sense) which sucks the sand up inside. Similarly with other clean-out tools - so it may be a long tedious process to pull the tool or plug out of the hole so that the water you are keeping in the casing can seep in and fill the voids being left below the tool. Doesn't always work, but most of the time, we had relatively good success.

 

[DRC1]

Running sands are as described above and most sands under improper drilling can be made to "run". In a construction sense, running sands are typically loose, clean fine to medium sands that will not stand, or are very unreliable. If they fail it will be a signifcant amount of sand movig, not minor sloughing. As you excavate, more sand flows in. Beach sand is generally running sand. Running sand, because it is loose, is typically low stength. It also complicates underground operations such as pipe jacking or underpining. For operations like that, gel (weak chemical grout)is often required to temporarily hold the sand.