Rotary Core Extrusion and Preservation

After recovery of the core-barrel to the surface, every effort should be made in subsequent handling to ensure that, as far as possible, the quality of the core is maintained in its natural state until it is finally stored.

Except in relatively strong and massive rocks, core is almost inevitably disturbed if it is removed from the barrel held in a vertical position and then placed into the core box. The barrel should be held in a horizontal position, and the core extruded into a tray in such a manner that it is continuously supported. Rain-water guttering or other conveniently available rigid split tube can be used for this purpose. When it is required to preserve the core such that it does not dry out, a convenient method is to extrude it from the core-barrel into sleeving formed of thin-gauge polyethylene, again supporting the core with rigid split tube.

Where selected lengths of core are to be preserved at their natural moisture content for laboratory testing, any drilling mud contamination and softened material should first be removed; the sample should then be wrapped in foil, coated with successive layers of waxed cheese cloth and labelled as described in Section 19.10.2.

In the extrusion process, the core should preferably be extruded in the same direction as it entered the barrel. Extruders should be of the piston type, preferably mechanically activated, since water-pressure type extruders can lead to water contact with the core, and to damage by impulsive stressing of the core. It should be noted that in weak, weathered or fractured rocks, extrusion can lead to core disturbance, however carefully it is done. The use of a low-friction transparent plastic liner in the inner tube of a modified conventional double-tube swivel core-barrel overcomes the majority of the problems encountered in core extrusion, and facilitates preservation of the core in the condition in which it is recovered. The general practice is to tape the outside of the sleeved core every 200 mm, and lengthwise along the overlap in the plastic sheet, and then, with the aid of plastic guttering for extra support, the core can be boxed without too much disturbance to the fabric. However, the presence of abrasive and fractured rocks may preclude the use of such liners.

The difficulties of extrusion and preservation can be overcome by the use of triple-tube core-barrels with low-friction liners (see Section 19.8). Split liner tubes are an ideal method of examining the recovered core without further damage after the drilling process. On the other hand, seamless metal liners and plastic liners are particularly useful where core is to be

removed from site for logging or where confined, undisturbed samples are required for sample preservation and subsequent laboratory testing.

It is usual to preserve all core obtained from the borehole for the period of the main works contract to which the core drilling relates. This is conveniently 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 (Geological Society, 1970). The box should be fitted with a hinged lid and strong fastener, and should be designed so as not to be too heavy for two persons to lift when the box 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.

Depths below ground surface should be indicated by an 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 and range of depth of the core within the box, in addition to the number of the box in relation to the total sequence of boxes for that borehole. Core box marking should be done so as to facilitate subsequent photography which, if required, should be carried out as soon as is practicable after recovery of the core, and before description, sampling and testing.

19.10.6 Block Samples

Sample cutting should be carried out as quickly as possible to prevent excessive moisture loss, and the sample should be protected from rain and direct sunlight. The sample should be trimmed to size in plan while still connected at its base (Plate 5A). The sides should be protected with aluminium foil or grease-proof paper, and then coated with a succession of layers of microcrystalline wax, reinforced with layers of porous fabric (e.g. muslin), if required. A close-fitting box with the top and bottom lids removed should then be slid down over the sample (Plate 5B). The top of the sample should be trimmed flat, marked with location and orientation, coated as described above, and the top lid attached to the box, ensuring a close fit. The sample may then be cut along its base, and turned over slowly and carefully for trimming and coating of the bottom prior to attachment of the bottom lid. A strong, rigid, close-fitting box is required to minimize sample disturbance during transport and to prevent discontinuities from opening.

20. Groundwater

20.1 General

The determination of groundwater pressures is of the utmost importance since they have a profound influence on the behaviour of the ground during and after the construction of engineering works. There is always the possibility that various zones, particularly those separated by relatively impermeable layers, will have different groundwater pressures, some of which may be artesian. The location of highly permeable zones in the ground and the measurement of water pressure in each is particularly important where deep excavation or tunnelling is required, since special measures may be necessary to deal with the groundwater.

For accurate measurement of groundwater pressures, it is generally necessary to install piezometers. The groundwater pressure may vary with time owing to rainfall, tidal, or other causes, and it may be necessary to take measurements over an extended period of time in order that such variations may be investigated. When designing drainage works, it is normally desirable to determine the contours of the water table or piezometric surface to ascertain the direction of the natural drainage, the seasonal variation and the influence of other hydrological factors.

The monitoring of groundwater levels and pore pressures, and their response to rainfall, is carried out routinely in Hong Kong, as this information is vital to the design and construction of slopes, excavations in hillsides, and site formation works. The choice of piezometer type depends on the predicted water pressures, access for reading, service life and response time required. Open-hydraulic (Casagrande) piezometers are often used in soils derived from insitu rock weathering and colluvium, which are generally relatively permeable. Other piezometer types may be used for specific projects; the available types are described in Sections 20.2.3 to 20.2.6, and their advantages and disadvantages are summarized in Table 10.

Slope failures in Hong Kong are normally triggered by rainstorms. The response of the groundwater regime to rainfall varies widely from site to site, ranging from virtually no response to a large immediate response. The measurement of transient response is therefore very important (see Section 20.2.8). In order to provide design data, groundwater monitoring should extend over at least one wet season; this wet season should ideally contain a storm that has a return period of greater than ten years. For site formation works which involve substantial modifications to the hydrogeological characteristics of the site, the period of monitoring may need to be extended to beyond the end of the site formation works. Ground conditions in Hong Kong may produce perched or multiple water tables which must also be considered when installing and monitoring piezometers (Anderson et al, 1983).

It may also be necessary to measure negative pore water pressures, or soil suction (see Section 20.2.9). In many cases, existing groundwater data in the vicinity of the site will be available in the Geotechnical Information Unit (see Section 4.2), and may be useful in planning an appropriate groundwater monitoring scheme.

An additional consideration in urban areas is the contribution of leakage from water-bearing services to the overall groundwater regime. This contribution can be significant at some sites. Hydrochemical analysis of groundwater may aid the identification of the leak, e.g.

the presence of fluoride attributable to leakage from fresh water mains. Advice on chemical

analysis of groundwater and related interpretation techniques such as trilinear plotting of cation and anion contents are given in ICE (1976).

Borehole permeability tests are described in Section 21.4, packer, or Lugeon, tests are described in Section 21.5 and large-scale pumping tests are described in Chapter 25.

20.2 Methods of Determining Groundwater Pressures