Drilling & boring

SpecificationGives an intact core. Standard wireline sizes run AQ, BQ, NQ, HQ, PQ and SQ in ascending diameter. The diameters themselves are set by ASTM D2113 and by drilling-industry convention, and no open-access government or standards-body table of the full series could be opened for this page, so individual core and hole diameters are not asserted here. The one size pair that can be cited is from the ASTM RQD method, which gives NX as 54.7 mm core in a 75.7 mm hole and NQ as 47.5 mm core. 'Wireline' means the inner tube holding the core is winched up through the rod string instead of tripping the whole string — ASTM D6286/D6286M-20, the drilling-method selection guide, notes that 'Wireline rock coring is used in competent rock and results in the best core recovery'. The US federal interagency FRTR reference linked below lists rotary diamond drilling at a maximum depth of '>1000 feet' with a core barrel tube '10 to 26 feet long'. Sampling convention is to saw the core longitudinally, assay one half and retain the other; US National Park Service curation guidance states the retention rule as splitting the core lengthwise 'with at least one half remaining in the box so that a continuous vertical portion of the core remains'.

Where it falls downSlowest and most expensive per metre of the common methods; a federal guide notes the sample 'is available after an extended wait' and that diamond bits cost more than roller bits. Core is lost preferentially in exactly the ground that matters — faults, shears, oxidised and clay-altered zones — and that core loss is not random with respect to grade. Drilling fluid can invade and wash fines from friable core. The West Virginia DEP Geology Handbook, a state surface coal mining regulatory handbook, sets a blunt threshold for coal-quality coring: 'If the recovery is less than 95 percent, the hole should probably be redrilled.' Small-diameter core gives a small sample mass, which matters for coarse-grained or nuggety mineralisation. Monitoring wells completed in diamond-drilled holes 'may be difficult to develop, especially small diameter wells, because of mud or filter cake on wall of the borehole'.

SpecificationGives chips, not core — the rock is destroyed but the sample is captured cleanly and continuously, normally over 1 m intervals and split at the rig with a cone or riffle splitter. The defining advantage, stated in an AusIMM conference paper (Stanley, 1986, available only by subscription and therefore not linked here), is that 'cuttings are free of contamination as they are at no time in contact with the outside formation of the hole'. The same paper divides RC into 'R.C. Roller and Blade for softer formations; and R.C. Percussion for harder formations'; the percussion version is driven by a down-the-hole hammer. USGS, writing about reverse-circulation systems used in water-well and test drilling rather than about dual-wall mineral-exploration RC, notes they achieve 'high ascending velocities (can be several hundred feet per minute)' and give 'the best cuttings samples of any drilling method'. note: typical RC hole diameters (commonly around 5 to 5.5 inches) and per-metre sample masses are widely quoted industry practice, but no government, standards-body or academic source giving those numbers could be opened for this page, so they are omitted here rather than reconstructed.

Where it falls downNo intact core: there is no fabric, no vein geometry, no structural orientation and no RQD. Sample degrades badly below the water table, where wet cuttings smear, stick in the cyclone and split unevenly; a wet RC split is a different sample from a dry one. Fine, soft or high-grade material can be lost preferentially to the dust fraction. The USGS TWRI notes that the conventional (non-dual-wall) reverse-circulation configuration used for water wells has a 'practical minimum diameter (about 16 in.)' which 'almost precludes its being used for test-hole drilling'. Contamination still occurs if the hammer is lifted off the bottom or if the hole caves above the bit.

SpecificationGives chips returned up the outside of the rods. A US federal interagency reference gives maximum depth '>1,000 feet', 'limited only by the capacity of the air compressor to deliver enough air', and a minimum 6-inch hole diameter for adequate cuttings circulation. The West Virginia DEP Geology Handbook, a state surface coal mining regulatory handbook rather than a general drilling standard, specifies an air velocity 'of between 3000 and 4000 ft/minute' for effective cuttings lift, cuttings 'normally be of a size. ½ inch or less in diameter', and sampling 'continuous and in 1-foot intervals, unless a lithology change warrants a smaller increment'. USGS records that 'in some instances, the air-rotary coring method cuts faster and provides less-contaminated core' than mud rotary.

Where it falls downdestructive. Because cuttings return up the open annulus they can be contaminated by material sloughing from the walls at any depth above the bit, so a given 'sample' can contain rock from anywhere in the hole. USGS measured that air-rotary coring dried the sample, with 'moisture decreases ranging from 22 to 45 percent' — so the method is poor for moisture-sensitive or volatile work. Requires casing in soft formations below the water table; carries a cross-contamination risk between zones at different water pressures; compressor discharge can introduce hydrocarbon contamination; down-hole hammer drilling 'may cause hydraulic fracturing'.

SpecificationGives chips, destructive, and the sample return path is the open annulus — the same path described for conventional air rotary above, which is the source of its characteristic sample problem. note: RAB is essentially small, cheap open-hole air rotary; specific RAB hole diameters, per-hole depths and costs are widely quoted by drilling contractors but no government, standards-body or academic source giving numeric RAB specifications could be opened for this page, and none is invented here.

Where it falls downThe worst sample quality of any method still in routine use, for a structural reason: cuttings travel up the open hole and are progressively mixed with, and diluted by, material falling in from the walls above. Depth assignment of a RAB interval is therefore approximate and down-hole smearing is expected. Unusable below the water table. No core, no structure, no reliable grade — RAB results are normally treated as anomaly-detection data, not as resource-quality assays.

SpecificationGives chips. It is destructive but, like RC, uses an inner-tube return path, which is the whole point of the method. That dual-wall return principle is documented by USGS in the source linked below, which does not itself discuss air core; no government, standards-body or academic description of air core drilling as such could be opened for this page: it buys most of RC's contamination control at close to RAB's cost. Sampling is normally on 1 m intervals at the cyclone. note: typical air core hole diameters and depth limits (commonly quoted as roughly 100 to 150 m in soft ground) come from drilling-contractor literature; no government, standards-body or academic source with those figures could be opened for this page, so the numbers are omitted rather than guessed.

Where it falls downRestricted to soft to moderately competent ground — soil, saprolite, weathered rock and soft sediments. It stalls in fresh, hard or abrasive rock and in coarse gravel and boulders. No intact core and no structural information. Sample quality falls off sharply below the water table, as for all air-return methods. Because rates of penetration are high, depth control depends entirely on disciplined interval-by-interval sample capture at the cyclone.

SpecificationGives chips recovered from the returning fluid, usually over a shale shaker; core and split-spoon samplers can be run down the rods. A federal interagency reference gives '>1000 feet is the usual limit, but greater depths are possible depending on the borehole diameter, mud pump capacity, and ability to maintain circulation', over a 'wide range of borehole diameters in consolidated and unconsolidated materials'. USGS Techniques of Water-Resources Investigations 2-F1 records a penetration rate in unconsolidated materials of about '1 ft every 5 min'. ASTM D6286/D6286M-20 lists fluid rotary drilling where groundwater is present among the predominant methods for geotechnical soil investigation.

Where it falls downdestructive, and the sample is the worst-resolved of the fluid methods: 'Collection of representative samples is difficult due to mixing of drill cuttings and sample lag time in deeper holes' — the cuttings you catch at the shaker left the bit minutes ago and are mixed with everything else in the return. The mud itself is a contaminant: 'the presence of drilling mud can contaminate the water samples', and residual fluid in the formation 'may lower borehole permeability'. USGS notes drilling fluid invades core and that 'too much fluid can result in abrasive damage'. Unsuitable for fractured, cavernous or very coarse ground where circulation is lost.

SpecificationGives a continuous core of soil and soft rock, though a disturbed one. A US federal interagency source gives a vibration frequency 'generally between 50 and 120 cycles per second', a maximum depth of '<500 feet', suitability for 'unconsolidated and soft/fractured bedrock', and a rate of penetration 'around twice as fast as air rotary and 8 to 10 times faster than hollow-stem auger and cable tool', though slower than mud rotary.

Where it falls downThe core is structurally disturbed, not undisturbed: the vibration disturbs soil structure along the borehole wall, distorts the sample during extraction, and in rock can create mechanical fractures that affect permeability testing. Drilling-induced heat in dense dry ground compromises sampling for volatile organic compounds. An ASTM practice for sonic drilling exists, but no accessible copy could be opened for this page, so nothing here is quoted from it. The federal source adds that 'driving of material into borehole wall when using a drill bit might create problems for logging, aquifer testing, and may affect monitoring well filter pack'. Drilling rock needs supplementary water or air to clear cuttings. Because the core is compacted by vibration, apparent recovery can exceed the interval drilled — recovery figures from sonic holes need care.

SpecificationSolid-stem gives disturbed cuttings; hollow-stem gives disturbed cuttings plus whatever sampler is run down the stem, so it can deliver an intact tube sample. ASTM gives hollow-stem auger a 'maximum depth of drilling of 200 to 300 ft (60 to 90 m) depending on torque and pull down/retract capacity', and notes that 'in situ testing or other soil sampling methods can be performed through the hollow auger column below the lead auger assembly' — split-barrel, thin-wall tube and vane shear without pulling the augers. USGS notes the hollow stem 'remains in the ground to act as a casing and hold out any potential caving materials'.

Where it falls downUnconsolidated ground only — augers stop at rock, cobbles and boulders. Solid-stem cuttings are not a depth-resolved sample: USGS warns that the 'sample returned to the surface will not represent true lithologic conditions' because of segregation and mixing on the flights, and the hole caves in saturated zones. Hollow-stem drilling in saturated sands suffers heave and piping up the stem; ASTM records that 'saturated cohesionless soils such as clean sands may flow and cave during drilling'. USGS also notes '2-3 in. of material loosened by the pilot bit. has not been moved up the auger flight', which contaminates the top of the next sample. Augers can drag contaminants downward.

SpecificationGives a lined core of soil, continuous or discrete-interval. ASTM describes advancement by 'static push, or impacts from hammers, or vibratory methods, or a combination thereof', and covers both single-tube and dual-tube systems, with dual-tube preferred because 'the outer tube stays in place to protect and seal the borehole and prevents potential cross contamination'. ASTM D6286/D6286M-20 notes that 'Direct Push methods were developed because they develop minimal investigative derived waste (IDW)'. Samples support contaminant analysis, classification to ASTM D2488 and lithologic logging. note: ASTM D6282 does not state a maximum depth or standard sampler diameters, so none is quoted here.

Where it falls downCannot penetrate solid rock, and is only marginally effective in 'partially weathered rock or very dense soils' and in 'compact gravelly tills containing boulders and cobbles, stiff clay, compacted gravel, and cemented soil'. Samples are compressed and sheared by the push, so they may not be suitable for laboratory engineering-property testing (Classes C and D of ASTM D4220). Small sample mass. Depth is limited by the rig's reaction weight and by rod friction, both of which rise quickly in dense ground.

SpecificationGives a disturbed but depth-certain soil sample. ASTM D1586 specifies a '140 lb [63.5 kg] hammer dropped 30 in. [750 mm]' onto a '2 in. [50 mm] diameter sampler'. The N-value is the number of blows to drive the sampler through 'the depth interval of 0.5 to 1.5 ft [0.15 to 0.45 m] of a 1.5 ft [0.45 m] drive interval' — that is, the first 6 inches are seating and are discarded, and the N-value is the sum of the second and third 6-inch increments.

Where it falls downApplies 'to non-lithified soils with maximum particle sizes less than approximately half the sampler diameter' and 'is limited to non-cemented materials' — gravel, cobbles and cemented soils give meaningless, inflated blow counts. The sample is disturbed and unusable for strength or consolidation testing. N-values are sensitive to hammer energy transfer, rod length and borehole cleanliness, so raw N is normally corrected before use. It is an index test, not a measurement of any fundamental soil property.

SpecificationGives chips piled at the collar; destructive. A university mining-engineering course gives bench blasthole diameters that 'typically range from a low of 3" to a high of 15"', with underground work at the low end and surface operations in the middle to upper range. The same course gives the standard design relations as burden B = K_B x D, spacing S = K_S x B and stemming T = K_T x B, where D is hole diameter, with K_B = 20 underground or 25 at surface for ANFO in rock of density 2.5 g/cm3, K_S = 1 to 1.3 and K_T = 0.7 — and it stresses these 'provide guidance rather than absolute precision and require iterative refinement'. Surface blastholes are normally drilled by rotary tricone or by down-the-hole hammer.

Where it falls downThe blasthole cuttings pile is a biased sample by construction: material from the whole hole is mixed at the collar, fines are lost to dust and wind, subdrill below the planned floor contaminates the pile with waste or with the next bench's ore, and the pile is sampled by whatever ad-hoc method the crew uses. Depth resolution within the hole is effectively nil. Blasthole assays are used for short-range ore/waste decisions, not for resource estimation.

SpecificationGives cuttings from the bailer, or a driven sample if a split-spoon or drive barrel is used. A federal interagency reference gives a maximum depth of '<5000 feet', a 'minimum 4-inch casing' for hard tooling, and rates of penetration of '1.5 to 2.5 feet per hour for bedrock and dense tills', '2.3 to 3.3 feet per hour for gravel and tills' and '3.5 to 4.5 feet per hour for silts, clays, and sands'. Hard tooling works 'in any formation including basalt'; the drive-barrel variant suits 'relatively dry, unconsolidated soils such as sand and gravel'. Because no circulation fluid is used, the method does not spread contamination between zones and can be run by one operator.

Where it falls downVery slow — the penetration rates above are per hour, not per minute. Hard-tooling samples are explicitly 'low in quality'. Requires relatively large-diameter steel casing, which corrodes under aggressive contaminant conditions and is hard to decontaminate. 'Heaving of material from the bottom of the casing upward might require special measures' in saturated sands.