Rock Coring ObjectivesObjectives

Rock coring is intended to obtain intact cores and a high percentage of core recovery.

Equipment

Rotary drilling machine, drill rods, a core barrel to receive the core, and a cutting bit are needed.

Operations

The core barrel is rotated under pressure from the drill rig applied directly to it, while water flows through the head, down the barrel, out through the waterways in the bit, and up through the rock hole and casing (in soil) to return to the surface. When the rock is first encountered in a borehole, the initial core runs are usually short because of the possibility that the upper rock will be soft and fractured. As rock quality improves, longer core runs are made.

The core barrel is generally rotated between 50 and 1750 r/min; rotation speed is a func-tion of the bit diameter and rock quality. Slow speeds are used in soft or badly fractured rocks and high speeds are used in sound hard rocks. If large vibrations and “chatter” of the drill stem occurs, the speed should be reduced or core recovery and quality will be severely affected.

Bit pressure is also modified to suit conditions. Low bit pressure is used in soft rocks and high pressure is used in hard rocks. When vibrations and “chatter” occur, the pres-sure, which is imposed hydraulically, should be reduced.

Fluid pressure should be the minimum required to return the cuttings adequately to the surface to avoid erosion of borehole walls. If there is no fluid return, drilling should imme-diately stop and the core barrel returned to the surface to avoid overheating the bit, which would result in bit damage (loss of diamonds) and possible jamming in the hole.

Lack of fluid can result from:

● Blockage of the core barrel, which occurs in clayey zones. Continued drilling after a broken core has blocked entry into the core barrel results in core grinding.

Indications of blockage may be heavy rod vibrations, a marked decrease in pen-etration rate accompanied by an increase in engine speed, return fluid more heavily laden with cuttings than normal, and a rise in circulation fluid pressure.

TABLE 1.17 Subaqueous Soil Sampling Without Drill Rigs and Casing DeviceApplicationDescriptionPenetration DepthComments Petersen dredgeLarge, relatively intact Clam-shell type grab weighing about To about 4 in.Effective in water depths to 200 ft. “grab” samples of seafloor100 lb with capacity of about 0.4 ft3More with additional weight Harpoon-type Cores from 1.5 to 6 in. Vaned weight connected to coring tube To about 30 ftMaximum water depth depends gravity corer diameter in soft to firm soilsdropped directly from boadonly on weight. UD sampling (Figure 1.72)Tube contains liners and core retainerpossible with short, large-diameter barrels Free-fall Cores 1.5 to 6 in. diameter Device suspended on wire rope over Soft soils to about 17 ft.As above for harpoon type gravity corer in soft to firm soilsvessel side at height of above seafloorFirm soils to (Figure 1.73)about 15 ft and then releasedabout 10 ft Piston gravity 2.5 in. sample in soft to Similar to free-fall corer, except that Standard core barrel Can obtain high-quality UD corer (Ewing firm soilscoring tube contains a piston that 10 ft; additional 10 ft samples gravity corer)remains stationary on thesections can be added seafloor during sampling Piggot explosive Cores of soft to hard bottom Similar to gravity corer. Drive weight Cores to 1 7/8 in. and Has been used successfully in coring tubesedimentsserves as gun barrel and coring tube to 10 ft length have20,000 ft of water as projectile. When tube meets been recovered in resistance of seafloor, weighed gun stiff to hard materials barrel slides over trigger mechanism to fire a cartridge. The exploding gas drives tube into bottom sediments Norwegian Good-quality samples in Similar to the Osterberg piston About 35 ft Geotechnical soft clayssampler, except that the piston on the Institute gas-sampling tube is activated by operated piston gas pressure Benthos High-quality representative Weighed free-fall plastic core tube droped Up to 80 in. At times, less inRequires minimum water depth of Boomerang corersamples in clays and sandsfrom a vessel penetrates the sea floor. dense sands33 ft. Has been used to depth (Figure 1.74)Floats inflate and rise to surface with of 29, 000 ft the core Vibracore High-quality sample in softApparatus is set on seafloor. Air Length of 20 and 40 ft Maximum water depth of about (Figure 1.75)to firm sediments, diameterpressure from the vessel activates anRate of penetration varies 200 ft 3 1/2 in.air-powered mechanical vibrator to with material strength. cause penetration of the tube, which Samples a 20 ft core in soft contains a plastic liner to retain the coresoils in 2 min

Wire

Tail fin

Nonreturn valve

Dead load

Sample cylinder

Catcher

FIGURE 1.72

Harpoon-type gravity corer.

Wire clamp

Release mechanism

Weight stand

Piston stop Core barrel

Threaded joint

Plastic liner

Piston Nose cone (core cutter) Pilot corer or

pilot weight Trip

wire Freefall

loop

FIGURE 1.73

Schematic diagram of a typical piston-type gravity corer. (From USAEC 1996, Pub. EM 1110-1-1906. With permission.)

● Loss in caverns, large cavities, or highly fractured zones. In Figure 1.76, a light drilling mud is being used to minimize fluid loss (note the mud “pit”), while cor-ing in limestone with highly fractured zones above the water table.

When the prescribed coring length is obtained the core barrel is retrieved from the ground. The core is removed from the barrel (Figure 1.77) and laid out in wooden boxes exactly as recovered (Figure 1.78). Wooden spacers are placed to divide each run. The depths are noted, the core is examined, and a detailed log is prepared.

Core Barrels

The selection of a core barrel is based on the condition of the rock to be cored and the amount and quality of core required. Core barrels vary in length from 2 to 20 ft, with 5 and 10 ft being the most common.

Table 1.18 provides summary descriptions of suitable rock conditions for optimum application, descriptions of barrel operation, and general comments. The types include:

● Single-tube core barrel (Figure 1.79).

● Double-tube rigid core barrel (Figure 1.80).

● Swivel-type double-tube core barrel, of two types: conventional and Series M (Figure 1.81). These types usually provide the best core recovery and are the most commonly specified for rock coring.

● Wireline core barrel (Figure 1.82).

Pressure resisitant

Benthos boomerang corer (model 1890) Maximum depth

The operating of sequence of the Boomerang Corer. (From USACE 1996, Pub. EM1110-1-1906. With permission.)

● Oriented core barrel contains knives that scribe a groove on the rock core. The compass orientation of the groove is continuously recorded, which enables determination of the strike of joints and other fractures (Figure 1.83). During nor-mal coring operations cores twist in the hole and accurate determination of joint strikes are not reliable.

Coring Bits General

Types of coring bits are based on the cutting material, i.e., sawtooth, carbide inserts, and diamonds.

Waterways are required in the bits for cooling. Conventional waterways are passages cut into the bit face; they result in enlarged hole diameter in soft rock. Bottom-discharge bits should be

Core pipe Air-operated

mechanical vibrator Air hoses

and signal cable

FIGURE 1.75

The Vibracore lowered to the seafloor.

(Courtesy of Alpine Ocean Seismic Survey, Inc.)

used for coring soft rock or rock with soil-filled fractures. Discharge occurs behind a metal skirt separating the core from the discharging fluid, providing protection from erosion.

Common bit sizes and core diameters are given in Table 1.14. The smaller diameters are used in exploratory borings for rock identification or in good-quality rock, but when max-imum core recovery is required in all rock types, NX cores or larger are obtained. In seamy and fractured rock, core recovery improves with the larger diameters, and HX size is com-monly used.

FIGURE 1.76

Core drilling with a Failing Holemaster. Light drilling mud is necessary in the fractured limestone above the water table to prevent drilling mud loss.

Reaming shells, slightly larger than the core barrel diameter and set with diamonds or carbide insert strips, ream the hole, maintaining its gage and reducing bit wear.

Bit Types

Sawtooth bits are the lowest in cost and have a series of teeth cut in the bit which are faced with tungsten carbide. They are used primarily to core overburden and very soft rock.

Carbide insert bits (Figure 1.53) have tungsten carbide teeth set in a metal matrix and are used in soft to medium-hard rocks.

Diamond bits (Figure 1.53) are the most common type, producing high-quality cores in all rock types from soft to hard. Coring is more rapid, and smaller and longer cores are retrieved than with other bit types. The diamonds are either surface-set in a metal matrix, or the metal matrix is impregnated throughout with diamond chips. There are various designs for cutting various rock types, differing in quality, size, and spacing of the dia-monds, matrix composition, face contours, and the number and locations of the water-ways.

Core Recovery and RQD Reporting Methods

Percent core recovery is the standard reporting method wherein core recovery is given as a percentage of total length cored. Rock Quality Designation (RQD) was proposed by Deere (1963) as a method for classifying core recovery to reflect the fracturing and alteration of rock masses. For RQD determination, the core should be at least 50 mm in diameter (NX) and recovery with double-tube swivel-type barrels is preferred.

FIGURE 1.77

Removal of HX diameter limestone core from the inner barrel of a double-tube swivel-type core barrel.

RQD is obtained by summing the total length of core recovered, but counting only those pieces of hard, sound core which are 10 cm (4 in.) in length or longer, and taking that total length as a percentage of the total length cored. If the core is broken by handling or drilling, as evidenced by fresh breaks in the core (often perpendicular to the core), the pieces are fitted together and counted as one piece.

FIGURE 1.78

Core recovery of 100% in hard, sound limestone: very poor recovery in shaley, clayey, and heavily fractured zones.

TABLE 1.18 Types of Rock Core Barrels Core BarrelSuitable Rock ConditionsOperation Comments Single tube (Figure 1.79)Hared homogeneous rock which Water flows directly around the core. Simple and rugged. Severe core loss in soft or resists erosionUses split-ring core catcherfractured rock Double tube, rigid type Medium to hard rock, sound to Inner barrel attached to head and Water makes contact with core only in reamer (Figure 1.80)moderately fractured. Erosion-rotates with outer barrel as water flows shell and bit area, reducing core erosion. resistant to some extentthrough annular spaceHoles in inner tube may allow small flow around core Double tube, swivel typeFractured formations of average Inner barrel remains stationary, whileTorsional forces on core are eliminated (conventional series)rock hardness not excessively outer barrel and bit rotate. Inner barrel minimizing breakage. Core lifter may tilt and susceptible to erosionterminates above core lifterblock entrance to inner barrel, or may rotate with the bit causing grinding of the core Double tube, rigid typeBadly fractured. Soft, or friable rock Similar to conventional series, except Superior to the conventional series. (series M)(Figure 1.81)easily erodedthat core lifter is attached to inner Blocking and grinding minimized. barrel and remains oriented. Erosion minimized by extended inner barrel Inner barrel is extended to the bit face Wireline core barrelDeep core drilling under all rock See Section 1.3.4Retriever attached to wireline retrieves (Figure 1.82)conditionsinner barrel and core without the necessity of removing core bit and drill tools from the hole Oriented core barrelDetermine orientation of rock Similar to conventional core barrels. Orienting barrel has three triangular hardened Oriented core (Figure 1.83)scribes mounted in the inner barrel shoe that cuts grooves in the core. Ascribe is aligned with a lug on a survey instrument mounted in a nonmagnetic drill collar. The instrument contains a compass-angle device, multishot camera, and a clock mechanism. About 30 cm of coring the advance is stopped and a photograph of the compass clock, and lug is taken. Geologic orientation is obtained by correlation between photographs of the core grooves and the compass photograph

Causes of Low Recovery

Rock conditions: Fractured or decomposed rock and soft clayey seams cause low recovery, for example, as shown in Figure 1.78. Rock quality can vary substantially for a given loca-tion and rock types as illustrated in Figuers 1.84 and 1.85, which show a formaloca-tion of gran-ite gneiss, varying from sound and massive to jointed and seamy. Core recovery in the heavily jointed zone is illustrated in Figure 1.86.

Coring equipment: Worn bits, improper rod sizes (too light), improper core barrel and bit, and inadequate drilling machine size all result in low recovery. In one case, in the author’s experience, coring to depths of 30 to 50 m in a weathered to sound gneiss with light drill rigs, light “A” rods, and NX double-tube core barrels resulted in 40 to 70% recoveries and 20 to 30% RQD values. When the same drillers redrilled the holes within a 1m distance using heavier machines, “N” rod and HX core barrels, recovery increased to 90 to 100%

and RQDs to 70 to 80%, even in highly decomposed rock zones, layers of hard clay, and seams of soft clay within the rock mass.

Coring procedure: Inadequate drilling fluid quantities, increased fluid pressure, improper drill rod pressure, or improper rotation speed all affect core recovery.

Integral Coring Method Purpose

Integral coring is used to obtain representative cores in rock masses in which recovery is difficult with normal techniques, and to reveal defects and discontinuities such as joint openings and fillings, shear zones, and cavities. The method, developed by Dr. Manual Rocha of Laboratorio Nacional de Engenheira Civil (LNEC) of Lisbon, can produce cores

Core barrel head

Core barrel Reaming shell

Core lifter

Coring bit

FIGURE 1.79

Single-tube core barrel. (Courtesy of Sprague and Henwood, Inc.)

Core barrel head Outer tube Inner tube

Reaming

shell Core lifter

Coring bit

FIGURE 1.80

Rigid-type double-tube core barrel. (Courtesy of Sprague and Henwood, Inc.)

Core barrel head

Swivel-type double-tube core barrel, series M. (Courtesy of Sprague and Henwood, Inc.)

Retriever − Lowered on wireline cable − spreads latch and grips retrieving spear for inner tube recovery Simple multifinger spring latch holds inner tube securely in position Hardened landing seat prevents inner tube from striking the bit One heavy-duty bearing − designed to take both thrust and hanging loads

Compression spring − transfers core breaking load to the outer tube Swivel connection for surface handling

Locking balls cammed into positive locking position on the retrieving spearHeat-treated retriever lock body Latch spreader − opens latch fingers to release inner tube assembly Compression spring force engages locking bulls on retrieving spear

Jar rod has provision for jarring in both directions

Teflon ring

Teflon ring

Shut-off load cell − provides for indication of core block and resets automatically

Hardened pump-in washer − opens when inner tube is seated and latched Hardened retrieving spear

Retriever Hard surfacing strips Core lifter

Grease fittingBit

Reversible inner tube − core can be removed from either end FIGURE 1.82 Wireline core barrel and retrieval assembly. (Courtesy of Sprague and Henwood, Inc.)

of 100% recovery with the orientation known. Defect orientation is an important factor in rock-mass stability analysis.

Technique

1. An NX-diameter hole or larger is drilled to where integral coring is to begin.

2. A second, smaller hole (nominally about 1 in. [26 mm] in diameter) is drilled coaxially with the first through the desired core depth, although usually not exceeding 1.5 m in depth.

3. A notched pipe is lowered into the hole and bonded to the rock mass with cement or epoxy resin grout, which leaves the pipe through perforations.

4. After the grout has set, a core is recovered by overcoring around the pipe and through the cemented mass.

5. During installation of the pipe, the notch positions are carefully controlled by a special adapter and recorded so that when the core is retrieved, the orientation of the fractures and shear zones in the rock mass are known.

Large-Diameter Cores by Calyx or Shot Drilling Purpose

Calyx or shot drilling is intended to allow borehole inspection in rock masses in holes up to 6 ft (2 m) in diameter.

Method

Calyx drilling uses chilled shot as a cutting medium. The shot is fed with water and lodges around and partially embeds in a bit of soft steel. The flow of freshwater is regulated carefully to remove the cuttings but not the shot. The cores are recovered by a special core-lifter barrel, wedge pins, or mucking after removal of the core barrel.

Limitations

The method is limited to rock of adequate hardness to resist erosion by the wash water and to vertical or nearly vertical holes.

Scribe mark

90°



90°

δ α Joint surface

FIGURE 1.83

Oriented core with the joint surface intersecting the core wall at the joint dip angle. The boundaries of a horizontal line across the joint are located at angle ø providing the strike angle.

1.4.6 Sample and Core Treatment

In document Geotechnical Investigation Methods-1420042742 (Page 111-123)