13.3 Equipment

13.3.1 Basic requirements for samplers

(1)P A sampler shall be adequatly technical detailed in order to:

  • get the rock mass into the sampler and keep it there during breaking off the sample and retrieval of the sampler;
  • obtain samples with the accepted degree of disturbance of the actual rock mass after the sampling operation.

13.3.2 Techniques for sampling

(1) The most suitable methods for obtaining rock mass samples is related to the structure and the decomposition grade of the rock mass and the required laboratory quality class:

  • category A or B: rotary sampling in which a tube with a cutter at its lower end is rotated into the rock mass, thereby processing a core sample;
  • category A or B: drive sampling in which a tube or a split-tube sampler having a sharp cutting edge at its lower end is forced into highly or completely weathered rock mass either by a static thrust or by a dynamic impact. Drive samplers are usually piston samplers or open tube samplers;
  • category C: shell or auger sampling, where the sample is taken from the actual drilling tool;
  • category C: cuttings sampling in which the rock mass, remoulded and crushed, by cable or rod handled percussion or cutting tools is brought to the surface by means of bailer or circulation of a transporting substance;
  • category A: block sampling made by hand cutting from a trial pit, shaft or heading or using specially made block samplers.

(2)P Selection of the technique shall be made in accordance with the sample quality required for classification of the rock mass and for the laboratory test to be performed.

(3) For rock masses decomposed to completely weathered rock or residual soil all the sampling techniques described in 13.3.2 may be applied.

(4) Requirements and sampling procedure for the drive sampling technique for use in highly or completely weathered rock are similar to the requirements and procedures described for soil sampling in 12.3.2 and 12.4 to which reference is made.

(5) For fresh rock and masses decomposed less than to completely weathered rock or residual soil the following techniques may apply:

  • rotary sampling;
  • cuttings sampling;
  • block sampling.

13.3.3 Requirements for rotary samplers

(1)P Rotary samplers for category A sampling in rock masses shall fulfil general requirements related to discontinuities in the rock mass and the grade of weathering as presented in (3) to (16).

(2) Fulfilling these requirements cannot guarantee that undisturbed samples will always be obtained. On the contrary it is normally not possible to core out undisturbed samples from rock masses decomposed more than slightly, especially when discontinuities are dominating the rock structure. However, fulfilling the requirements should at least minimize the disturbance of the samples.

(3) Single-tube core barrels rotate directly against the core and this sampling method may only be category A in case the rock mass is fresh and without discontinuities.

(4) Double-tube core barrels with ball bearing swivel between the outer tube and the inner tube which do not rotate against the core may, when used in fresh and slightly weathered rock masses, constitute a sampling method corresponding to category A and at least category B.

(5) Triple-tube core barrels containing a liner inside the inner tube remaining stationary will, when used in moderately and highly weathered rock masses with discontinuities, normally constitute a sampling method corresponding to category B. When the rock mass is fresh, slightly weathered or converted into a hard soil with apparent cohesion, this sampling method may be category A.

(6)P The minimum internal diameter of the core cutter (equal to the core sample diameter) shall be based on the rock mass structure.

(7) Rock masses with discontinuities should not be cored out using an internal diameter of the cutter smaller than 76 mm.

(8)P The minimum length of the inner tube or liner if used shall be such that one core run contains a quantity of rock mass sufficient for identification and performance of required laboratory tests.

(9)P The maximum length of the inner tube or the length of the actual core run shall be such that the inside wall friction does not have too great influence on the results of the laboratory tests to be performed.

(10) Normal lengths of inner tubes are 1, 1,5, 3 and 6 m.

(11)P The inside clearance shall be sufficient to allow for the lateral movement of the sample during the continued core run, but it shall not be so much that it permits excessive deformations and causes additional disturbance of the sample.

(12)P The inside wall of the inner tube or liner shall be clear and smooth without any protruding edges or irregularities which may disturb the sample.

(13) Friction at the inside wall of the inner tube or liner may be reduced by using a cutting edge with an internal diameter slightly less than the diameter of the tube or liner.

(14) When sampling highly weathered rock masses or masses with dominating discontinuities inner tubes or liners split longitudinally may be used to minimize the risk of blocking of the core sample.

(15) A core catcher placed in the sampling equipment just behind the cutter should normally keep a cored sample of fresh to moderately weathered rock inside the sampling equipment and mobilize the necessary resistance for breaking off the core from the rock stratum beneath the cutter face.

(16) For core sampling of highly or completely decomposed rock masses a core retainer such as a spring basket on the core catcher may be used.

Eurocode 7 Geotechnical design — Part 3: Design assisted by fieldtesting