25.7 Rotary core samples
COMMENTARY ON 25.7.1
Guidance on the various methods of rotary core drilling and information on which soil and rock materials each core sampling type is suited are given in BS EN ISO 22475-1:2006, 22.214.171.124, 126.96.36.199, 7.1, 7.3 and Table 2 and Table 5. The procedures for this sampling by drilling method are described in 24.11. The quality of core samples recovered, and consequently any sub-sample of the core, can vary considerably depending on the character of the ground and the type and size of coring equipment used.
Core can be recovered from a variety of material types. The majority of core, however, is likely to be obtained within fine soils of any strength and in rocks in various states of weathering and also having any strength. Some core might be obtained by using double tube core drilling and some by triple tube core drilling; either of these systems can include the use of a semi-rigid plastic liner.
After the recovery of the core barrel to the surface, handling should be carried out so as to ensure that the recovered core is maintained in a condition as near as possible to its natural state until it is finally stored (see 25.11); it is at this stage that the expense and effort to recover high quality cores can be negated by careless or inappropriate handling.
The core barrel should be in a horizontal position when the core is extruded, otherwise it is likely to be disturbed as it is removed from the barrel, and then placed into the core box.
NOTE 1 This is also preferable even in relatively strong and massive rocks where the barrel might be in a vertical position when the core is removed but this can affect the quality of the core presentation.
When using a double tube core barrel, the core should be extruded with the tube in a horizontal position using a purpose-made core extruder. Extruders should be of the piston type, preferably mechanically activated, since water or air pressure type extruders can lead to water contact with the core, impulse stressing of the core and uncontrolled explosive ejection of the core from the barrel. The core should be extruded from the core barrel in the same direction as it entered the barrel.
When using a triple tube core barrel, including double tube barrels fitted with a plastic liner, the core should be retained in the liner which is removed from the barrel. Where split steel liners are used, the core may be retained in the bottom half while being logged and then placed in the core box. Where a seamless steel liner is used, the core may be extruded (as per the double tube barrel) before being placed in the core box. Where plastic liners are used, the core should be transferred to the core box still retained in the liner.
NOTE 2 In weak or fractured rocks and soils, extrusion can lead to core disturbance, however carefully it is done.
Core should be carefully transferred into core boxes which have receiving channels appropriate to the diameter of the core. Where a plastic liner has been used it should be cut longitudinally with appropriate slitting blades or proprietary coreline cutter tools or vibrating saws, ensuring that the sample is not cut into or disturbed, so that the upper half of the liner can be removed when logging. However, the preparation of core for photography and description should be achieved without compromising the requirements for preservation of sub-samples for laboratory testing.
COMMENTARY ON 25.7.3
The sampling category cannot be improved once coring has taken place, but inappropriate handling can markedly reduce the quality.
Soil or rock material recovered as cores should be divided into:
- a) cores not sensitive to water content change: category A status should be maintained by either the use of sealed core liner or by immediately wrapping all extruded core with suitable impermeable material, e.g. cling film, plastic sheeting or film; or
- b) cores sensitive to moisture change: category A status should be maintained by sealing the core immediately with the use of water sealing material, e.g. cheesecloth soaked in wax, waxed sealed tubes. Several layers of the material might be necessary to ensure a complete watertight seal.
NOTE The use of a soil lathe or similar immediately after recovery to remove the outer layer of the core which has been in contact with the flushing medium is advantageous in maximizing the sample quality.
Where the core is to be sub-sampled for specific laboratory testing, this should be done immediately after the core has been placed into the core box. When core liner has been used, this should be done by cutting both the liner and core laterally in two places in order to remove the sample.
The sub-sample's category A status should be maintained as described in b). Each sample should be clearly labelled and top and base marked. The sample should be placed in an appropriate container to protect the sample during transport.
All core obtained from the borehole should be preserved for the period of the main works contract to which the core drilling relates; this can be achieved with wooden or plastic core boxes, usually between 1 m and 1,5 m in length and divided longitudinally to hold a number of rows of core. The box should be of such depth and the compartments of such width that there is minimal movement of the cores when the box is closed. The box should be fitted with stout carrying handles, a hinged lid and strong fasteners and should be designed so that two persons can lift the box when it is full of core.
In removing the core from the barrel and placing it in the box, great care should be taken to ensure that the core is not turned end for end but lies in its correct natural sequence with the shallowest core to the top left hand corner, the top being deemed adjacent to the hinged section. Core retained in the core catcher box should also be placed in the core box at the correct relative depth. Depths below ground surface should be indicated in indelible marker on small spacers of core diameter size that are inserted in the core box between cores from successive runs. Where there is failure to recover core, or where specimens of recovered core are removed from the box for other purposes, this should be indicated by spacing blocks of appropriate length. Both the lid and the box should be marked to show the site location, borehole number, range of depth of the core within the box, in addition to the number of the box in the total sequence of boxes in the borehole. Core box marking should be such as to facilitate subsequent photography (see Annex H).