zones, potentially resulting in a considerable decrease in aquifer hydraulic conductivity along

the wall of the borehole. The smearing of clays and silts along the borehole wall may,

depending on the site specific properties of the geologic materials, significantly reduce well

yield or produce unrepresentative ground water samples even after the well has been

developed; and

Management of drill cuttings    Control of contaminated drill cuttings is

difficult with the auger method, especially when drilling below the water table.

6.1.2

Solid Stem Augers

Drilling with solid stem augers is similar to drilling with hollow stem augers except

solid stem augers are made of solid steel, and therefore need to be removed from the borehole

to collect "undisturbed" split spoon or thin wall samples and to install casing. Boreholes

drilled in unconsolidated and poorly consolidated deposits in which solid stem augers are used

will typically not remain stable after saturated materials are encountered, and will collapse

after the augers are removed. Consequently, "undisturbed" samples of the unconsolidated

materials can generally be collected only above the water table. An alternative drilling

method is generally used below the water table once the borehole is advanced through

unsaturated deposits.

6.1.3

Cable Tool

Cable tool drilling is a versatile method for sampling and well installation. When the

drill rig is equipped with fishing jars and a sampling barrel, continuous samples are retrieved

and there is minimal disturbance to the borehole wall. Drilling progresses by raising and

dropping the upper half of the jars (the jars are an interlocking set of steel hammers which

slide independently of each other) while the lower half rests on the bottom of the borehole.

There is a sampling tube attached to the bottom of the lower half of the jars. The hammering

action of the jars drives the sampling barrel into the ground. This method will not work in

consolidated bedrock but is applicable to virtually all overburden materials. Borehole

instability can be overcome by using the jars to drive casing ahead of the sampling zone.

Sand heaving can often be overcome by filling the casing with water to maintain a positive

head.

The advantages of cable tool drilling include versatility, applicability to both hard and

soft formations, minimal smearing, suitability for identifying thin subsurface zones, and

usefulness over a wide range of depths. However, problems involving heaving may occur

with cable tool drilling.

November 1992

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