7 Rock sampling methods
7.1.1 Techniques for obtaining rock samples can be divided in the following groups:
- a) sampling by drilling (see Table 5)
- b) block sampling;
- c) integral sampling.
Combinations of these sampling methods are possible and sometimes required due to the geological conditions.
7.1.2 Rock samples are of the following types:
- a) cores (complete and incomplete);
- b) cuttings and retained returns;
- c) block samples.
7.1.3 The quality of the rock recovery is achieved by applying the following three parameters (see also Figure 2):
- total core recovery, TCR (see 126.96.36.199);
- rock quality designation, RQD (see 188.8.131.52);
- solid core recovery, SCR (see 184.108.40.206);
7.1.4 After recovery of the corebarrels to the surface, the core recovery shall be assessed. In cases where core samples are extruded from the corebarrel and placed in a core box, the sample shall be logged. If liners are used, it shall be decided in advance where and when they shall be opened for examination of the core. Core losses shall be filled with a dummy. The drilling direction has to be marked on the core boxes or samples by arrows. The depths of the cores also have to be marked.
7.2 Categories for rock sampling methods
7.2.1 There are three categories of rock sampling methods, depending on the best obtainable quality of rock samples under given ground conditions:
- category A sampling methods;
- category B sampling methods;
- category C sampling methods.
7.2.2 By using category A sampling methods, it is intended to obtain samples in which no or only slight disturbance of the rock structure has occurred during the sampling procedure of the samples. The strength and deformation properties, water content, density, porosity and the permeability of the rock sample correspond to the in situ values. No change in constituents or in the chemical composition of the rock mass has occurred. Certain unforeseen circumstances, such as varying of geological conditions, can lead to lower sample quality being obtained.
7.2.3 By using category B sampling methods, it is intendedto obtain samples that contain all the constituents of the in situ rock mass in their original proportions and the rock pieces have retained their strength and deformation properties, water content, density and porosity. By using category B sampling, the general arrangement of discontinuities in the rock mass can be identified. The structure of the rock mass has been disturbed and thereby the strength and deformation properties, water content, density, porosity and permeability for the rock mass itself. Certain unforeseen circumstances, such as varying of geological conditions, can lead to lower sample quality being obtained.
7.2.4 By using category C sampling methods, the structure of the rock mass and its discontinuities have been totally changed. The rock material may have been crushed. Some changes in constituents or in the chemical composition of the rock material can occur. The rock type and its matrix, texture and fabric can be identified.
|Line||Drilling method||Equipment||Drilling method less suitable for a||Samples||Achievable sampling category b||Remarks|
|Flushing medium||Extraction of sample by||Designation||Sampling tool||Guideline for borehole diameter range a mm||Cores a||Cuttings|
|1||No||Drilling tool attached to drill rods||Rotary dry core drilling||Single-tube corebarrel||70 c to 200||Rock of medium to high hardness||Soft, erodable, water-sensitive rock; short core runs||None||B (A)||To prevent overheating of the bit, core runs should not exceed 0,5 m.|
|2||Yes||Drilling tool attached to drill rods||Rotary core drilling||Single-tube corebarrel||70 c to 200||Rock of medium to high hardness||Jointed, soft rock||Sieve residue and suspended matter||B (A)||Flushing medium can cause disturbance of core material|
|3||Yes||Drilling tool attached to drill rods||Rotary core drilling||Double-tube corebarrel||70 c to 200||Erodable, water- sensitive rock||All types of rock||Sieve residue and suspended matter||A (B)||—|
|4||Yes||Drilling tool attached to drill rods||Rotary core drilling||Triple-tube corebarrel||70 to 200||—||All types of rock||Sieve residue and suspended matter||A||—|
|5||Yes||Drilling tool attached to drill rods, with wireline extractable inner barrel||Wireline core drilling||Wireline corebarrel, or triple-tube corebarrel||70 to 180||—||All types of rock||Sieve residue and suspended matter||A||—|
|6||Yes||Drilling tool attached to drill rods||Open hole drilling||Solid bit, roller bit, DTTH||50 to 350||—||None||Sieve residue and suspended matter||C||—|
|a Guideline values considering the possible use of a casing.
b The sampling categories given in parentheses can only be achieved in particularly favourable or unfavourable ground conditions, which shall be explained in such cases.
c In some crystalline rocks, a minimum borehole diameter of 30 mm may be sufficient for the identification and description of rock.
NOTE The sample diameter is smaller for the same borehole diameter when a triple-tube corebarrel is used, instead of a single-tube corebarrel.