For soil sampling, different systems are available, including Geoprobe Macro-Core, Geoprobe Dual-Tube and open window sampler. To collect environmental samples geo-log recommends liner systems like Dual-Tube or Macro-Core. Both systems work in the unsaturated and saturated zone and guarantee an undisturbed high quality soil core without any risk of cross contamination.
When the preliminary site investigation shows that the geology of the unsaturated zone is favourable, the use of open window sampling is an appropriate investigation methodology as well. Every drilling step provides an up to 2 meter continuous sample. The outer diameter of the open window sampler casing will be reduced from top to bottom (e.g. 80 mm / 60 mm / 50 mm) to avoid cross contamination.
In the saturated zone the use of the two possible continuous soil coring methods, Geoprobe Macro-Core or Geoprobe Dual Tube sampling technique, is recommended. The retrieved liners represent samples of approx. 1200 mm length and 38 mm (1.5”) in diameter for Macro Core System respectively 1200 mm length and 29 mm (1.125”) for Dual Tube system.
To collect soil samples, the Macro Core sampler will be driven into the subsurface and then pulled from the ground to retrieve the first core. In stable soils, an open-tube sampler is advanced back down the same hole to collect the next core. In unstable soils, the sampler is equipped with a 1.25” center rod closed point assembly, able to retrieve the liner and soil core. The core catcher alleviates the fall of the loose soils from the bottom of the sampler.
The alternative Dual Tube sampling uses two sets of probe rods. One set will be driven into the ground as an outer casing. These rods receive the driving force from the hammer and provide a sealed hole from which soil samples may be recovered without the threat of cross contamination. The second, smaller set of rods will be placed inside the outer casing. The smaller rods hold a sample liner in place as the outer casing is driven one sampling interval. The small rods will then be retracted to retrieve the filled liner. The Geoprobe DT21 system uses a 2.25” (57 mm) outer diameter (OD) probe rods as an outer casing and light weight centre rods for the inner rod string. The 2.25” probe rods provide a cased hole through which to sample. The main advantage of sampling through a cased hole is that there is no side slough to contend with. In addition, the outer casing effectively seals the probe hole when sampling through perched water tables. These factors mean that sample cross contamination is eliminated. The liners, with a length of 1.2 meter, have a core catcher permanently fused to each liner. Integrated core catcher prevents soil cores from falling out of the liner during sample retrieval.
