Overburden soils logging and Core drilling


The global standard for the engineering logging and classification of overburden soils is the Unified Soils Classification System (USCS – ASTM D2487, Table 2.7). The basis of the system is that coarse-grained soils are logged according to their grain size distributions and fine-grained soils according to their plasticity. Thus, only grain size analyses and Atterburg Limits tests are needed to completely identify and classify a soil (Holtz & Kovacs 1981).

There are four major divisions in the USCS: coarse-grained, fine-grained, organic soils and peat. The classification is performed on material passing a 75 mm sieve, with the amount of oversize being noted on the drill log. Particles greater than 300 mm equivalent diameter are termed boulders, and material between the 300 mm and 75 mm sieves are termed cobbles. Coarse-grained soils are comprised of gravels (G) and sands (S) having 50% or more material retained on the No. 200 sieve. Fine-grained soils (silt, M, and clay, C) are those having more than 50% passing the No. 200 sieve. The highly organic soils and peat can generally be divided visually.

The gravel (G) and sand (S) groups are divided into four secondary groups (GW and SW; GP and SP; GM and SM; GC and SP) depending on grain size distribution and the nature of fines in the soils. Well-graded soils have a good representation of all particles sizes; poorly graded soils do not. The distinction can be made by plotting the grain size distribution curve and computing the coefficients of uniformity (Cu) and curvature (Cc) as defined in the upper right-hand side of Table 2.7. The GW and SW groups are well-graded gravels and sands with less then 5% passing the No. 200 sieve. The GP and SP groups are poorly graded gravels and sands with little or no non-plastic fines.

The particle size limits given above are those adopted by ASTM D2487, which is published in the USA. Different limits may be adopted in different countries. For example, the Australian Standard (AS 1726-1993) adopts different limits, which are 2–60 mm for gravel, 0.062 mm for sand and less than 0.06 mm for silt and clay. As 60 mm, 2 mm and 0.06 mm sieves are not normally used, the percentage passing these sizes must be identified from a laboratory test using regular sieve sizes.

The fine-grained soils are subdivided into silt (M) and clay (C) on the basis of their liquid limit and plasticity index. Fine-grained soils are silts if the liquid limit (LL) and plasiticity index (PI) plot below the A-line on the Casagrande (1948) plasticity chart in the lower right-hand side of Table 2.7. They are clays if the LL and PI values plot above the A-line. The distinction between silts and clays of high plasticity (MH, CH) and low plasticity (ML, CL) is set at a liquid limit of 50.

Coarse-grained soils with more than 12% passing the No. 200 sieve are classified as GM and SM if the fines are silty, and GC and SC if the fines are clayey. Soils with 5–12% fines are classed as borderline and have a dual symbol. The first part of the dual symbol indicates whether the soil is well-graded or poorly graded. The second part describes the nature of the fines. For example, SW-SC is a well-graded sand with some fines that plot above the A-line.

Fine-grained soils can also have dual symbols. The shaded zone on Table 2.2 is one example (CL-ML). It is also recommended that dual symbols (e.g CL-CH) be used if the LL and PI values fall near the A-line or near the LL = 50 line. Borderline symbols can also be used for soils with about 50% fines and coarse grained fractions (e.g.GC-CL).

Planning and scoping

Planning and scoping the objectives of the drill hole are the most important steps of the drilling investigation. There must be clear primary and secondary objectives to extract the maximum amount of potential information. For example, geotechnical data collection may be the primary objective of the hole, but at the same time it may be possible to gain important geometallurgical and/or geohydrological information and/or use the completed hole for groundwater or other monitoring purposes.

Ideally, before objectives are finalised they should be reviewed by a multidisciplinary team to ensure that all such possibilities have been taken into account.

There are other critical points.

■ Before the location and orientation of the drill hole are finalised, the objectives of the hole must be checked to ensure they are consistent with the current geological, structural and hydrogeological models.

■ When they have been finalised, the objectives of the drill hole must be recorded in a written memorandum that includes alternative actions in case drilling difficulties are encountered and/or it is not possible to complete the hole. The memorandum must be signed-off by all members of the team responsible for preparing the document.

■ Before drilling commences, the rig site should be reviewed to ensure its location is compatible with all current and planned mining activities in the area.

■ When drilling commences, it is essential that the core be photographed and logged by a properly qualified and experienced person at the rig site before it is disturbed and moved from the site to the core shed.

■ Each step in the drilling process must be owned by the appropriate person. For example, the driller must accept responsibility for the core recovery process, the engineering geologist for the core logging and any downhole testing, and the environmental team for decommissioning the site.

■ A plan and geological section showing the drill hole trace and the expected geological/structural pierce points should be available to the drillers and loggers at the rig site.

■ The drilling and logging and any downhole testing must be regularly reviewed using an appropriate QA/QC procedure.

■ The potential of the drill hole for future monitoring and/or downhole testing should be continuously reviewed.

In a landmark paper, Philips et al. (2001) detailed the compilation and interpretation of a number of 3D petrophysical property models over the San Nicholás copper-zinc deposit in Mexico. Figure 2.9 shows a simplified geological cross-section of the San Nicholás deposit as determined from drill holes for comparison with the inverted petrophysical property model sections shown on Figure 2.10. As a next step in the use of these data to derive geotechnical and mining parameters, they need to be segmented into packages with similar properties then calibrated against measured samples from strategically placed drill holes.

Ground penetrating radar (GPR) is an electromagnetic analogue of the seismic method, but with limited depth penetration. GPR in reflection mode performs best in resistive rocks as the waves are attenuated in conductive materials. GPR can be used to detect lithology and structures; it tends to be highly sensitive to clays.

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