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High quality remediation wells - what’s best?

Demonstrated benefits of combined cased auger/cable percussion over large diameter hollow stem augers for installation of remediation wells.

Geotron UK were recently retained to install a number of additional remediation wells for product recovery at a site in Greater Manchester following a diesel spill.  The first phase of 100mm ID well installation had been completed by another contractor using large diameter hollow stem augers (HSA) a week or so earlier.  Based on the reported geological sequence and our experience in remediation well installation, we recommended the use of combined cased auger/cable percussion (CA/CP) drilling rather that HSA to achieve higher quality remediation wells in this additional phase.  The Nordmeyer DSB 0/3 multi-purpose rig was subsequently employed to install an additional 11 wells at a progress rate of two 9m wells per day by CA/CP (significantly faster than traditional Shell & Auger drilling).

Cased auger/cable percussion, is a relatively little known drilling technique in the UK, although it is extensively used in The Netherlands and Germany.  A hydraulic track or truck mounted multi-purpose rig rather than a traditional Shell & Auger A-frame is used to advance a borehole by solid stem auger (through cohesive sediments) and a shell/bailer (through non-cohesive/saturated sediments), operated by a hydraulic, automatic cable percussion mechanism.  During both augering and shelling, a temporary steel casing (219mm diameter in this case) is simultaneously advanced by a hydraulic rotary casing table mounted on the rig mast. 

On reaching the target depth, the cased borehole is cleaned out with the shell and the well is then lowered into place.  In this case the wells comprised nominal 100mm ID HDPE/PVC with 0.6/0.5mm slots, complete with centralisers, 1-2mm filter gravel and bentonite pellets, our standard materials for remediation wells. During installation of the annulus fill, the casing is slowly retracted allowing the filter gravel to completely surround the well all the way from the base and enabling the installation of an effective bentonite seal around the upper plain section of the well.

Following installation, all the wells on site were developed.  It was observed that Geotron’s wells quickly discharged clear, sediment free water.  In contrast, the wells installed by HSA initially contained up to 2m of silt within the well which, although could be pumped out during development, continued to enter the well during pumping. 

There could be a number of reasons for this.  In the opinion of our client, who was present during both phases of work, the reason for the silting up of the hollow stem wells is that after the pilot bit was removed from the lead auger (once at the required depth – 9m), there was the obvious impact of blowing sands into the well.  The other contractors were not able to deal with this ingress between the end of drilling and beginning of well installation. They effectively had no way of cleaning out the hole prior to installation.  This was borne out as when well installation was attempted, there was generally a need to hammer the well into the sediments within the lead auger for at least half a metre or, where this was not effective, the excess well was cut off. Essentially, the bottom part of the well then had no surrounding filter pack.

(N.B. when installing through hollow stem augers in geological conditions similar to this, Geotron would generally fill the centre with water, above the rest groundwater level, to raise the pressure within the well  relative to the formation and minimise sediment ingress, however this method was not employed by the other contractor.)

In addition, installing through the narrower diameter centre of the HSA augers as opposed to a larger diameter steel casing, reduces the gravel pack thickness (where a gravel pack is actually installed), precludes the use of centralisers and increases the potential for bridging of the gravel/bentonite resulting in areas of screen directly in contact with the surrounding sediments.  

For the above reasons, finer particles within the formation are pulled directly into the well rather being than pulled through a filter gravel pack during pumping. Development results in internal sorting of the filter pack preventing continued ingress of these finer particles, however, if there is a discontinuous or insufficiently thick filter pack, this cannot happen.

On this site, the on-going silt ingress and build up in the HSA installed wells has caused difficulties during product recovery pump installation (i.e. insufficient effective well depth) with increased potential for blockage and failure of the pump components. No such problems have existed with the Geotron wells.

It is rare that we have been given the opportunity to be able to make a direct comparison between wells, adjacent to each other, supposedly of the same design/construction, where there is such a marked difference in performance, validating our recommended methodologies.  It was clear to our client that CA/CP had resulted in the installation of superior quality remedial wells at this site. Better wells have resulted in savings in both engineer’s time on site and in cost of the well development and product recovery phase of the remediation programme.

We are not denying the usefulness of HSA for certain geological conditions, for enabling the collection of lined undisturbed high quality soil samples by windowless sampler through its centre and for the installation of narrow diameter monitoring wells.  However, when the objective is to install high quality larger diameter wells which will best serve a remediation system on a long term basis, CA/CP wins hands down.

Remediation Strategies were impressed with the performance of cased auger/cable percussion technique over the traditional hollow stem auger method, both in terms of the quality of recovered soil samples and the ability to fully install boreholes to the required depth with a full filter pack. 

Subsequent monitoring has indicated that wells installed using cased auger/cable percussion to be of a significantly higher standard and as such, are likely to remain serviceable for the duration of the remediation works.

We would have no hesitation in recommending Geotron and the cased auger/cable percussion technique offered by the Nordmeyer DSB 0/3 multi-purpose rig for installing high quality remediation wells in difficult ground conditions.

Alex Moore – Remediation Strategies Ltd.