I have no personal experience per se, but I have investigated this scenario before. My two cents...

The right choice will be dependent on particle size and consolidation. An orientation study should be undertaken to include grain size analysis, SPT tests, moisture content etc. A quick ground penetrating radar survey can indicate the base of the pile, slope of retaining structures, well as any hidden surprises (barrels etc).

As you say there are also a range of track mounted geotechnical rigs for unconsolidated sediments with options for triple tube drilling.

Have you considered sonic drilling? I haven't seen this technique in operation so I can't give any more info than can be found on the web.

Also Vibracore rigs For example: http://is.gd/WNG9Ov

Depending on the purpose of the survey, a range of in-situ measurements can be made using cone penetration testing. A direct push rig can quickly cover a large area to depths of up to 30-40m at a penetration speed of up to 1 m per min.

http://is.gd/9tpYrB

I have used BoartLongyear and sonic drills and they are fast and effective. However they face similar challenges in material that is very unconsolidated and wet - you probably have to case the holes. How deep do you need to drill? Augers are still a good choice.

Banka, triple tube diamond, auger and sonic are pretty much the choices. Sonic is probably the best, and can drill quite large diameter holes. We have used sonic in peisolithic gravels, and on salt lakes, the recovery was excellent. Samples were used for assay and bulk density measurements. The barrel had to be calibrated against test pits for density to be accurate. As above casing was only required in the wet holes, generally the holes were shallow.

See Vigar et al 2009 "Resource Estimation for the Aurukun Bauxite Deposit"in 7th International Mining Geology Conference, pg 273 - 279 AusIMM Publication. This provides some detail on Sonicdrilling.

I have been involved in quite a few projects evaluating slimes dams. The simplest and cheapest way to sample fine material in a slimes dam or sand dump is with an auger fitted with a sleeve to hold the sample. This is a small rig, hand portable, often on a frame of some sort and with a hand held motor on top of the rods. They can be put on a rubber mat to distribute the weight so they don't sink into the slimes if the surface is a bit wet.

Main issue is that they cannot collect liquid samples, penetrate the really nasty swelling clays, or very hard layers. The sleeve provides rigidity in drilling minimizes contamination and helps stabilize the hole. Samples are collected at 1.5m intervals drilling stops when the hole cannot stay open due to high water content or encounters very hard material.

This is used successfully throughout Southern Africa and is the method of choice for sand dumps and slimes dams. I don't have any pictures to send but if you follow the links below there are some pictures. I have had similar rigs drill to 60m+ successfully.

http://is.gd/E4EHow Other drillers such as Dump and Dune in SA use the same system.

Aircore could also work as it can penetrate harder material than the auger bit it is more expensive and the rigs are heavier so not good for slimes dams.

Neither of the above is good for stockpiles with coarse material or rock dumps. Sonic can get through the material but like any other technique, there is the question of diameter of the hole vs. coarseness of the material.

I have been involved with the study of precision of the results of drilling heavy mineral sand deposits with sonic drilling as opposed to earlier RC drilling. The sonic drilling data was clearly very much superior. I was not involved with the drilling itself.

Do you have a paper you can send me about those results? I've just joined on a project to drill 36,000 ft. of placer gold beach deposits with a sonic drill. I've worked with one in Arizona and was impressed, although it still had problems with caving in with unconsolidated and/or wet ground. I also am disappointed that you have to trip out for each sample interval - there is no continuous sampling and that slows things down a little.

My personal experience is very much in line with some previous comments on gold slimes dams in South Africa, i.e. relatively fine grained tailings and it works very well. I drilled down to 15m in places using a +- 40mm diameter portable auger rig which we have to our disposal. Dumps can be drilled much deeper though. Then sub sampling every 1.5m 'lifts' for gold assaying under stringent QA/QC protocols. Also, performed similar study for a tailings mineral resource in Zimbabwe for sampling Nickel minerals from previous asbestos mine tailings. Worked reasonably well but then you start finding hole collapses and auger jamming as soon as the particle size becomes too coarse (nominally about +1mm). Please shout if you need any further information or photos.

I have considerable experience in this area having completed Resource Definition drilling (JORC 2012) on a base metals tailings dam which was ground to sun 7 microns prior to original flotation.  The tailings had now been re-mined and processed validating the estimate from these samples.

Conventional drilling methods are very flawed for sampling  in unconsolidated material - and for tailings the moisture content is a very important consideration relating to bulk density.  Any penetration has the tendency to have a piston effect on the material forcing sample away from the drill face, air methods blow sample and moisture out, sonic vibrates and compressed sample losing moisture and compromising spatial accuracy, and augers - well augers are augers and if you think that is a good sample I won’t even elaborate.

A high recovery, repeatable, and valid sample can however be achieved with some basic modifications to a diamond drilling configuration to an almost ‘push tube’ system.  Not quite as basic as it sounds but I’m happy to elaborate based on my experiences if anyone comes to this thread looking for answers in the future…