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Designing a Geotechnical Core-drilling Program by Vinay Sahay

by Vinay Sahay

A case study on the importance of understanding a site’s geology for exploration work

Geology plays an important role in the success or failure of a core-drilling program. Many times, due to lack of geological information, the designed drilling system – the drilling rig capacity, mud pump capacity, type of core barrels, selection of casing point, type of bits, selection and properties of drilling mud – and the operational drilling parameters – weight on bit (WOB), rotation per minute (RPM), mud weight, borehole cleaning rate or drilling fluid circulation rate – do not fulfill the desired objective, resulting in various geological and geotechnical problems.

Examples of such problems include reduced recovery of cores, core washing, core fracturing, borehole caving, borehole deviation, drill rod sticking, blow out, wear and tear on drill bits, circulation loss or flooding of borehole by formation ground water, etc. With proper understanding of the geology of the area under investigation, however, such geological and geotechnical problems can be avoided and minimized.

Many factors are responsible for good core recovery. Among these, two factors can be broadly classified. The first factor is geological; that is, involving stratigraphy, lithology, rock mechanics, structure, ground water, mineralogy, rock mass composition and weathering. The second one is geotechnical; that is, involving the appropriate rig, core barrel, coring bits, type of drilling fluids, weight on bit (WOB), revolutions per minute (RPM), and borehole-cleaning drilling fluid pressure.

The geotechnical factor is intrinsically dependant on the geological factor. Core exploration drilling experience shows that utilizing and integrating geological information while designing a drilling system and program is useful to optimize drilling. Doing so helps achieve the desired core recovery to be used in core study, testing and various geotechnical projects, like foundation evaluation and selection, as well as tunnel, underground cavern and shaft-site evaluations and their design and development.

The basic objective of core drilling in geotechnical work is to document, classify and analyze the rock mass conditions in terms of their suitability for various geotechnical projects as described above. Among the various rock mass classifications, rock mass rating (RMR) system, Q-system and rock quality designation (RQD) are useful and commonly utilized methods in geotechnical work.

The RMR system uses six parameters to classify a rock mass. These are uniaxial compressive strength, rock-quality designation (RQD), spacing of discontinuities, condition of discontinuities, ground water conditions, and orientation of discontinuities.

The Q-system classifies a rock mass using six parameters – rock-quality designation (RQD), joint set number, joint roughness, joint alteration, water inflow and stress reduction factor (SRF). The RQD was proposed by Deere as a measure for the degree of jointing or block size, for use with NX-size core (2.15 in.). The RQD measurement is one of the common parameters determined during the logging of borehole cores in any geotechnical investigation program. It helps ensure the maximum core recovery so that the presence and extent of weak parts, such as weathered zones, faults, shear zones, shale or clay layers, etc., can be determined, and appropriate measures can be taken to improve it, as per rock mass classifications.

This article presents a routinely utilized approach in core-drilling work as a case study from one of Maheshwari Mining Pvt. Ltd.’s geotechnical exploration projects.

Geotechnical Core Exploration: Vizag Cavern Project, Visakhapatnam, India

The objective of this project was to conduct horizontal core drilling for geotechnical investigation. The cores were to be used for a petrograhic study, like lithology, rock-quality designation (RQD), joints and other discontinuities, and carry out tests to determine the geo-engineering and geo-structural properties of rocks like uniaxial compressive strength, tensile strength, triaxial shear test, specific gravity, bulk density, water absorption, porosity, etc. The results were to be used in the cost-efficient development of Vizag cavern, and also to design appropriate rock support measures during cavern work.

The borehole depth and size were 164 feet and HW size, respectively. For designing a suitable drilling program, the site was visited to obtain the geological information. Here, the lithology consists of khondalite suite rock, comprised of garnetiferous quartzo-feldspathic gneisses. This rock has higher hardness and uniaxial compressive strength (UCS). It is of higher abrasive grade due to the presence of quartz, garnet and coarse grain size.

Based on this geological information, the firm decided to use a GD 48 rig with a double-tube core barrel (HWT). As rock was hard and self-supported, there was not any specific requirement for casing. Here, higher hardness and UCS with higher rock abrasiveness favored the use of impregnated bits with harder and abrasion resistance matrix. The life of a bit, in general, was 262 feet to 328 feet of footage. The drilling fluid used was only water, and a good borehole circulation was maintained, apart from the high RPM. This helped to achieve good core recovery (98%-100%), as well as drilling production rate (average 66 ft./day).  

In approaching a site’s geology and drilling system with understanding and integration, a core-drilling program can be effectively planned, operationalized and completed. 
Vinay Sahay
Vinay Sahay is a geologist with Maheshwari Mining Pvt. Ltd., India.

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