Execution of soil borings

General safety instructions

Before performing any drilling it is important to make sure that:

  • No utilities are present in the selected drilling points. Special attention must be paid to gas lines, electric wires and steam or hot water pipes. Beware that clashing of such lines may cause deaths. For this reason a utility survey must be carried out at each drilling location using a pipe locator and if necessary pre-drilling must be conducted.
  • Personnel is familiar with these tasks and the equipment used.
  • Personnel is using appropriate protective equipment.
  • Every person/organization that could be affected by the drilling activities knows about them.
  • In some cases, permits issued by appropriate authorities might be required in order to perform drilling activities.
Types of soil borings
  • When carrying out soil sampling very close to the surface, borings may be replaced by excavations with a trench digger. Only in rare cases digging by hand will be advantageous. Excavating by machine provides a good overview of the soil layers and how the contamination varies along the face of the excavation. Excavations are particularly advantageous in cases where the contamination is distributed unevenly, like for example in landfills.
  • Localization borings are shallow investigation borings of up to 3-4 m in depth, which are made in order to determine contamination in the upper soil layers and in the aquifers close to the surface.
  • Investigation borings are borings of more than 4-5 m depth, where contamination in deeper soil layers and deeper groundwater is described.
Drilling tools and operation

Soil borings can be completed by using drilling rigs, portable motor-driven augers, percussion gouge equipment or hand augers, and they should be correctly identified with a specific identification number.

Hand augers, portable motor-driven augers and percussion gouge equipment. In connection with shallow soil borings hand augers, portable motor-driven augers and percussion gouge equipments are usually used. Drilling can be performed up to a maximum of 3-4 m depth, but this will depend very much on the soil consistency. These equipments are recommended rather for loose soil and unpaved areas, because the rigs can’t drill through asphalt and concrete.

If asphalt or concrete is present, it has to be drilled previously by means of a drill. If geological conditions permit, the borings can be executed without the use of casing, since this method can provide adequate level-specific soil samples in connection with shallow borings. Shallow localization soil borings can be filled back with excavated material if the borings do not exceed a depth of 3-4 m.

Drilling rigs. Normally rotary and auger drilling rigs are used for drilling of environmental investigation borings:

  • Auger rigs (equipped usually with augers of 62 - 135 mm diameter). Drilling can be performed up to 5 - 30 m depth. 
  • Rotary rigs. Drilling of boreholes by rotating a hollow core barrel (76 - 151 mm diameter) for soil and water sampling. Drilling depth is not limited.


One of the targets of investigation soil borings is to ensure water sampling at specific depths through a screen. For this reason, the dimensions of the borings must be selected according to the desired screen size and gravel pack. The following table shows typical associations between borehole and screen diameters.

Download table: Borehole and screen diameters

Borings with this depth (> 4-5 m) should always be carried out using casing in order to ensure representative soil samples and in order to prevent cross contamination. Usually the method of boring used is dry rotation soil boring with casing, which will be removed during either well development or borehole abandonment activities. For drilling of deep soil borings, a double casing system will be used. One larger diameter will be used to drill down to the first layer. A smaller diameter casing will then be used inside the first casing to drill down to the second layer. This will prevent vertical cross contamination from the shallow soil to the aquifer. At conclusion of each soil boring, the hole will be filled with grout to ground surface to prevent cross-contamination between stratigraphic layers. Piezometers can be installed to investigate deeper aquifers, to monitor and remediate groundwater.

Furthermore, it is also important to decontaminate drilling equipment which comes into contact with soil before each drilling operation in order to avoid cross-contamination between different drilling locations and to ensure that collected samples are representative (see Chapter III “Management of Investigation Derived Waste and Equipment”).

Finally it is important to consider the following information, which should be collected during drilling operations:

  • Name or identification number of soil boring
  • Start and end date of works
  • Observed lithology
  • Soil appearance and color
  • Presence of humidity
  • Water levels and non-aqueous phase liquid levels, if present. Once drilling a soil boring is concluded, water level and possible presence and depth of non-aqueous phase liquids shall be measured by means of an interface probe.
  • Drilling company
  • Drill typology
  • Boring depth
  • Drilling device diameter
  • Collected samples, with relative sampling depth and identification code
  • Stratigraphy, with possible visual exam notes
  • Photo of soil boring

All field works must be performed in accordance with the corresponding procedures and security measures.

In case no groundwater is present in the study area, the best scenario will be studied depending on each specific case: boreholes might be abandoned, or drilling operation might be continued in order to find groundwater at deeper depths, or in order to study contamination in the soil.

 


    Drilling rig