13 ROCK SAMPLING
13.1 General
(1)P The aim of rock sampling is to obtain adequate samples for rock identification and for laboratory testing for reliable rock mechanical information on the stratum.
(2) Important rock properties needed in geotechnical design are structure, strength and deformation properties. These properties may be appropriately obtained by description of and laboratory testing on high quality samples representing the actual stratum.
13.2 Categories and concepts
13.2.1 Sampling categories
(1)P Adequate samples shall contain all the mineral constituents of the strata from which they have been taken. Such samples shall not have been contaminated by any material from other strata nor from additives used during the sampling procedure which distinguished from the sample.
(2) There are three categories of sampling methods for description and laboratory testing:
- category A sampling methods;
- category B sampling methods;
- category C sampling methods.
(3) 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 or in handling 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 chemical composition of the rock mass has occurred.
(4) By using category B sampling methods the samples 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.
(5) By using category B sampling the general arrangement of discontinuities in the rock mass may 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.
(6) By using category C sampling methods the structure of the rock mass and its discontinuities has been totally changed. The rock material may have been crushed. Some changes in constituents or in chemical composition of the rock material may occur. The rock type and its matrix, texture and fabric may be identified.
13.2.2 Visual rock identification in the field
(1)P Visual rock identification shall be based on examination of the rock masses and samples including all observations of decomposition and discontinuities.
(2)P Weathering classification shall be related to the geological processes and shall cover the grades between fresh rock and rock decomposed into soil.
(3) A simplified weathering classification subdivided in six grades is given in annex L.
(4)P Discontinuities are bedding planes, joints, fissures, cleavages and faults and shall be quantified with respect to pattern, spacing and inclination using unambiguous terms.
13.2.3 Rock recovery
(1)P Rock quality designation, RQD is the sum length of all core pieces that are 100 mm or longer, measured along the centre line of the core, expressed as a percentage of the length of the core run.
(2)P Solid core recovery, SCR is the length of core recovered as solid cylinders, expressed as a percentage of the length of the core run.
(3)P Total core recovery, TCR is the total length of core sample recovered, expressed as a percentage of the length of the core run.
13.2.4 Area ratio and inside clearance of the sample tube
(1)P The area ratio and inside clearance defined in 12.2.2 are determining the mechanical disturbance during sampling of rock strata in which a noticeable part is decomposed.
