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AN INTRODUCTION TO
SUBSURFACE INVESTIGATION (PHASE II)

SUBSURFACE INVESTIGATIONS (PHASE II)
HOLLOW STEM AUGERING - USING THIS TECHNOLOGY TO INSTALL GROUNDWATER MONITORING WELLS

THE OLD TECHNOLOGY: Around the world, many drinking water wells are constructed by the ancient method, employing the simple excavation of a wide pit, often circular, and lining it with stone to prevent cave-ins. Where shallow clean groundwater was once commonly available, and where subsurface soils or rock layers could be excavated easily, this method was and occasionally remains effective in some areas. But where clean fresh water is only found in deeper aquifers where the water supply must be protected from surface contamination sources such as septic systems, spills, or industrial operations, hollow stem auger technology arose as an ideal solution. This method is also a cost-effective solution for many environmental investigations.


A NEW TECHNOLOGY: The well known corkscrew form such as we commonly see associated with any hand drill bit or screw is adapted so that the inner core or shank is a hollow cylindrical space. The base of the hollow center can be secured with a removable plug while drilling proceeds and cuttings are pushed aside and conveyed upward by the corkscrew blades. As the borehole deepens, additional auger segments - usually five feet in length - are bolted to the segments already in the ground.

THE SPLIT SPOON SAMPLER: Soil samples are taken when desired to record the nature of the subsurface material, to locate water-bearing zones, and in the environmental field, to identify, sample and measure soil or groundwater contamination. One common sampling tool is known as the split spoon, a steel tube inserted inside the hollow stem of the auger and pounded into the as-yet undrilled soil beneath the base of the auger. A cap in front - with a slightly narrow opening for the soils to enter - and the attched rod above, hold the two halves of the 'spoon' together. Once filled with soil and retrieved, the rod and cap are removed, and one half of the spoon is removed to expose the sample soil.

COLLECTING SOIL SAMPLES: Samples are described and recorded by a drilling technician or a geologist. In the environmental field, disposable gloves are required, and in some cases, special disposable clothing, respirators or self-contained chemical suits with internal air supply may be necessary, to protect staff from more dangerous contaminants. More commonly, disposable gloves alone are required, both to protect the sampler and to prevent sample contamination by those handling it. Samples are commonly packed into special vials supplied by the laboratory and placed on ice in a cooler to preserve volatile components.

MONITORING WELL INSTALLATION:


Assembly of screen and casing prior to installation.


Placement of the screen and casing into the auger.


Filling the annular space around the screen with clean no. 20 sand.


Sealing the screened zone with bentonite.


Finishing the well head - prior to placement of concrete.

LAB SAMPLES READIED AND DECONTAMINATION OF THE AUGER AND SPLIT SPOON SAMPLER


Samples prepared for delivery and laboratory analysis.


Decontamination of the split spoon sample prior to reuse.


Decontamination of the hollow stem augers prior to reuse.


Soil cuttings and decontamination water are stored in drums pending lab results.


Image credit: USEPA

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