Soil Boring test process service ASTM D1587

Soil boring with wash boring process service active standard ASTM D1587, Casing with a diameter about 50 to 100 mm and to a depth of 1.5 m to 3.0 m is directly driven into the soil surface. The casing is full of chopping bit tied on lower end of a wash pipe inner the casing in mandate to take away the soil. At first water is compulsory through the wash pipe that appears adjacent the chopping bit through a small starting with a high speed. Due to the high rapidity of water, the soil is slashed into oddments and soil water gets up through ring area between the wash pipes and casing. the T-connection fixed at the top aids the assemblage of wash water in a pond made handy.

soil boring test

Soil boring a hole, the wash water again compiled is anew pumped through a hose into the wash pipe. As the method continues, a borehole is advanced and further casing pipe and drill rods are extra. A swivel head equipped between the water hose and water pipe so that the wash pipe and chopping bit can be distorted and pressured down whereby the borehole is advanced further into the soil of soil boring. But, while civil engineer drive the casing into the soil, the swivel head is eliminated. Following the same method frequently, a borehole of essential depth is obtained. This type of method of soil boring test is appropriate for all types of soil other than gravels and boulders.

And, to include more this process is very swift. For clayey soil, casing is unnecessary. Observing the slurry streaming out of the hole, civil engineer can predict the type of soil. Likewise, the change of color in the wash water predicts the change in rock strata layers of the soil.

Soil boring and soil samples are taken from the materials settled in the cesspit. But, the soil samples thus taken would be extremely disturbed and equips only a irregular forecast of the soil type. For adequate accuracy of the samples taken, we follow a suitable sampling procedure at any desired depth by replacing the chopping bit with a sampling tube.

For sufficient exactitude of the samples taken, we follow a appropriate sampling method at any imposed depth by substituting the chopping bit with a sampling tube. Mainly, this method is used for the preparatory soil boring moment of the exploration.

Open standpipe piezometer Service

open standpipe piezometer
Piezometric pressures influence the strength of soil or rock. Critical pore water pressures should be estimated during design of embankment fills and other structures. During construction, piezometers can be installed to monitor the pore water pressures. The dissipation of the pore water pressure over time is used as a guide to consolidation rate. Thus, piezometers can be used to control the rate of fill placement during embankment
construction over soft soils. Piezometers should be placed prior to construction in the strata/zones that contribute to settlement or shear strength. If the strata or zones are more than 10 feet thick, more than one piezometer should be placed to provide adequate coverage with depth. It is possible to install several piezometers in the same borehole; however, this requires more complex installation and introduces the possibility of crossflows from inadequate seals. Single piezometer installations are generally recommended.

The pore water pressure should be measured often during embankment construction. After the fill is in place, and critical conditions have passed, pore water pressures can be monitored at a decreasing frequency. The data should be plotted (as pressure or elevation of water head) as a function of time. A recommended practice is to plot pore water pressure, settlement, and embankment elevation on the same time-scale plot for comparison.

Instruments commonly used to measure groundwater pressures include observation wells, open standpipe piezometers, vibrating wire piezometers and pneumatic piezometers.

Observation Wells

An observation well consists of a perforated section of pipe attached to a riser pipe installed in a borehole backfilled with sand. A pipe cap on top of the riser pipe and, typically, a cement seal around the top of the pipe are used to prevent surface water from entering the monitoring system. A vent is required in the cap to allow pressures in the pipe to equalize.

Groundwater levels in standpipes and wells are read using an electronic dipmeter, which emits an audible (beep) and visible (red light) signal when the surface of groundwater is encountered. The red light signal can be very handy when working around noisy equipment. The depth is measured using a graduated cable.

Observation wells are used for monitoring the groundwater levels. Observation wells are also used to monitor the changes of ground water levels due to conducting a pump test to determine permeability. If an observation well is installed across several zones of permeability, the measurement will correspond to the zone of highest permeability. Observation wells should preferably be installed in materials of high permeability so that the lag time related to changes in groundwater levels is minimized and reasonably accurate results are obtained. Observation wells in silt and clay soils could experience long lag times and therefore should be used cautiously. Where these conditions exist, it is advisable to use an alternative type of piezometer.

Open Standpipe Piezometers

Open standpipe piezometers are similar to observation wells, except that the perforated portion of the pipe and sand filter region (backfill) is sealed in a discrete zone and the riser pipe is much smaller in diameter. The smaller diameter of the pipe reduces the lag time related to changes in ground water levels. Above the sand filter, the remaining backfill should include a seal (either cement/bentonite or bentonite pellets). Surface runoff should be prevented from entering the standpipe by installing a box or monument that does not restrict the pipe from venting. Groundwater levels in standpipes are read using an electronic dipmeter which signals when the water surface inside the pipe is encountered. The depth is measured using the graduated cable.

Open standpipe piezometers have a long successful performance record and are preferable to observation wells in most applications. Open standpipe piezometers should be used only in materials of high to moderately high permeability so that lag time remains relatively short. Standpipe piezometers are less effective in low-permeable silts or clayey soils to measure fluctuations in groundwater levels.

Vibrating Wire Piezometers

Vibrating wire piezometers are pressure transducers that operate using the frequency of vibration of a wire connected to a flexible metallic diaphragm. As the pressure changes, the length of the wire changes, resulting in a different vibrating frequency, which can be correlated to a specific water pressure. Each uniquely calibrated piezometer is contained in a protective housing with a porous filter. There are several manufacturers of vibrating-wire piezometers. Each manufacturer sells readout devices that are generally able
to read other brands of piezometers as well.

Vibrating wire piezometers are installed in a similar manner as open standpipe piezometers. It is recommended that the wire leads be kept taut during installation, especially when using hollow-stem auger drilling systems, to avoid abrasions and breakage. This can be performed by taping the leads to a solid rod or PVC pipe.

Vibrating wire piezometers have several significant advantages over open standpipe piezometers including: (1) require very short lag time related to changes in groundwater levels in all types of soils, (2) cause minimum interference to construction equipment due to flexibility of wire placement, and (3) are easily adapted for use with an inexpensive datalogger for automated readings. Disadvantages include: (1) more care is required to assure proper installation, and (2) the electronic units are susceptible to damage by lightning, transient electricity, and shorting if the leads become abraded and the wires are exposed to
moisture. The power source needs to be maintained (i.e., periodic replacement of batteries).

Pneumatic Piezometers

Pneumatic piezometers consist of a sensor body with a flexible diaphragm, and inlet and outlet tubes. The junction box outlet is connected to a readout unit and pressurized gas is applied to the inlet tube. As the applied gas pressure equals and then exceeds the pore water pressure, the diaphragm deflects allowing gas to vent through the outlet tube. The gas supply is then turned off and the diaphragm returns to its original position. The pressure in the inlet tube equals the pore water pressure and is measured and recorded.
Pneumatic piezometers are installed in a similar manner as vibrating wire piezometers.

Pneumatic piezometers have many of the same advantages that vibrating-wire piezometers have. Disadvantages include: (1) require more equipment, (2) require more complex setup and operator training, and (3) the quality of the readings is more operator dependent.


Field permeability test Service (Open End)

ทำโดยการเจาะฝังท่อลงไปในดิน ถ้าชั้นดินที่ต้องการหาความซึมน้ำ ได้อยู่เหนือระดับน้ำใต้ดิน ให้ใช้วิธีสูบน้ำลงไปในหลุมเจาะ ถ้าชั้นดินอยู่ใต้ระดับน้ำใต้ดิน จะใช้วิธีสูบน้ำเข้าในหรือออกจากหลุมเจาะก็ได้ แล้ววัดอัตราการไหลของน้ำที่จะรักษาระดับความดันคงที่ ค่าสัมประสิทธิ์ความซึมน้ำ สามารถหาได้จากความสัมพันธ์ดังต่อไปนี้

k = q/(5.5rh)


k = สัมประสิทธิ์ความซึมได้
q = อัตราการไหลของน้ำลงไปในหลุมเจาะ เพื่อที่จะรักษาระดับความดันให้คงที่เหนือระดับน้ำใต้ดิน
r = รัศมีของกระบอกเจาะ
h = ระดับน้ำที่รักษาไว้เหนือระดับน้ำใต้ดิน

Field Vane Shear Test Service

Standard Test Method for Field Vane Shear Test in Cohesive Soil

field vane shear test

ASTM D 2573 – 94

1.1 This test method covers the field vane test in soft, saturated, cohesive soils. Knowledge of the nature of the soil in which each vane test is to be made is necessary for assessment of the applicability and interpretation of the test.
1.2 The values stated in inch-pound units are to be regarded as the standard. The SI units given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Summary of Test Method
The vane shear test basically consists of placing a four-bladed vane in the undisturbed soil and rotating it from the surface to determine the torsional force required to cause a cylindrical surface to be sheared by the vane; this force is then converted to a unit shearing resistance of the cylindrical surface. It is of basic importance that the friction of the vane rod and instrument be accounted for; otherwise, the friction would be improperly recorded as soil strength. Friction measurements under no-load conditions (such as the use of a blank stem in place of the vanes, or a vane that allows some free rotation of the rod prior to loading) are satisfactory only provided that the torque is applied by a balanced moment that does not result in a side thrust. As torsional forces become greater during a test, a side thrust in the instrument will result in an increase in friction that is not accounted for by initial no-load readings. Instruments involving side thrust are not recommended. The vane rod may be of sufficient rigidity that it does not twist under full load conditions; otherwise a correction must be made for plotting torque-rotation curves.

3. Significance and Use

3.1 This test method provides an indication of in-situ shear strength.
3.2 This test method is used extensively in a variety of geotechnical explorations, such as in cases where a sample for laboratory testing cannot be obtained.

4. Apparatus

4.1 The vane shall consist of a four-bladed vane as illustrated in Fig. 1. The height of the vane shall be twice the diameter. Vane dimensions shall be as specified in Table 1. Sizes other than those specified in Table 1 shall be used only with the permission of the engineer in charge of the boring program. The ends of the vane may be tapered (see Fig. 1). The penetrating edge of the vane blade shall be sharpened having an included angle of 90°.
4.2 The vane shall be connected to the surface by means of steel torque rods. These rods shall have sufficient diameter such that their elastic limit is not exceeded when the vane is stressed to its capacity (Note 1). They shall be so coupled that the shoulders of the male and female ends shall meet to prevent
any possibility of the coupling tightening when the torque is applied during the test. If a vane housing is used, the torque rods shall be equipped with well-lubricated bearings where they pass through the housing. These bearings shall be provided with seals to prevent soil from entering them. The torque rods shall be guided so as to prevent friction from developing between the torque rods and the walls of casing or boring.

NOTE 1—If torque versus rotation curves are to be determined, it is essential that the torque rods be calibrated (prior to use in the field). The amount of rod twist (if any) must be established in degrees per foot per unit torque. This correction becomes progressively more important as the depth of the test increases and the calibration must be made at least to the maximum depth of testing anticipated:

4.3 Torque shall be applied to the torque rods, thence to the vane. The accuracy of the torque reading should be such that it will produce a variation not to exceed ±25 lb/ft2 (1.20 kPa) shear strength.

4.4 It is preferable to apply torque to the vane with a geared drive. In the absence of a geared drive, it is acceptable to apply the torque directly by hand with a torque wrench or equivalent. The duration of the test should be controlled by the requirements of 5.3.

5. Procedure

5.1 In the case where a vane housing is used, advance the housing to a depth which is at least five vane

geometry of field vane

housing diameters less than the desired depth of the vane tip. Where no vane housing is used, stop the hole in which the vane is lowered at a depth such that the vane tip may penetrate undisturbed soil for a depth of at least five times the diameter of the hole.5.2 Advance the vane from the bottom of the hole or the vane housing in a single thrust to the depth at which the test is
to be conducted. Take precautions to make sure no torque is applied to the torque rods during the thrust.

5.3 With the vane in position, apply the torque to the vane at a rate which should not exceed 0.1°/s. This generally
requires a time to failure of from 2 to 5 min, except in very soft clays where the time to failure may be as much as 10 to 15 min. In stiffer materials, which reach failure at small deformations, it may be desirable to reduce the rate of angular displacement so that a reasonable determination of the stress-strain properties can be obtained. During the rotation of the vane, hold it at a fixed elevation. Record the maximum torque. With apparatus with geared drives, it is desirable to record intermediate values of torque at intervals of 15 s or at lesser frequency if conditions require.

5.4 Following the determination of the maximum torque, rotate the vane rapidly through a minimum of 10 revolutions; the determination of the remoulded strength should be started immediately after completion of rapid rotation and in all cases within 1 min after the remoulding process.

5.5 In the case where soil is in contact with the torque rods, determine the friction between the soil and the rod by means of torque tests conducted on similar rods at similar depths with no vane attached. Conduct the rod friction test at least once on each site; this shall consist of a series of torque tests at varying depths.

5.6 In apparatus in which the torque rod is completely isolated from the soil, conduct a friction test with a blank rod
(Note 2) at least once on each site to determine the magnitude of the friction of the bearings. In a properly functioning vane apparatus, this friction should be negligible.

NOTE 2—In some cases it is not necessary to remove the vane for the friction test. As long as the vane is not in contact with the soil, that is, where it is retracted into a casing, the friction measurement is not affected.

5.7 Conduct undisturbed and remoulded vane tests at intervals of not less than 2|n$ ft (0.76 m) throughout the soil profile when conditions will permit vane testing (Note 3). Do not conduct the vane test in any soil that will permit drainage or dilates during the test period, such as sands or silts or in soils where stones or shells are encountered by the vane in such a manner as to influence the results.


Soil boring with Rotary drilling

soil boring rotary drilling

Rotary drilling uses a rotary action combined with downward force to grind away the material in which a hole is being made. Rotary methods may be applied to soil or rock, but are generally easier to use in strong intact rock than in the weak weathered rocks and soils that are typically encountered during ground investigations. For a detailed description of equipment and methods the reader is referred to Heinz (1989).

Rotary drilling requires a combination of a number of elements:

1. a drilling machine or ‘rotary rig’, at the ground surface, which delivers torque and thrust;
2. a flush pump, which pumps flush fluid down the hole, in order to cool the mechanical parts and lift the ‘cuttings’ of rock to the ground surface as drilling proceeds;
3. a ‘string’ of hollow drill rods, which transmit the torque and thrust from the rig, and the flush fluid from the flush pump to the bottom of the hole; and
4. a drilling tool, for example a corebarrel, which grinds away the rock, and in addition may be designed to take a sample.

rotary core drilling
Layout for small-scale rotary core drilling


Rotary methods may be used to produce a hole in rock, or they may be used to obtain samples of the rock while the hole is being advanced. The formation of a hole in the subsoil without taking intact samples is known as ‘open-holing’. It can be carried out in a number of ways, but in site investigation a commonly used tool is the ‘tricone rock roller bit’ (or roller core bit). In site investigation such methods are usually used to drill through soft deposits, which have been previously sampled by light percussion or auger rigs. Sampling during open-holing is usually limited to collecting the material abraded away at the bottom of the borehole, termed ‘cuttings’, as it emerges mixed with ‘flush fluid’ at the top of the hole.

bits for roatary
Bits for rotary open holing


The most common use of rotary coring in ground investigations is to obtain intact samples of the rock being drilled, at the same time as advancing the borehole. To do this a corebarrel, fitted with a ‘corebit’ at its lower end, is rotated and grinds away an annulus of rock. The stick of rock, the ‘core’, in the centre of the annulus passes up into the corebarrel, and is subsequently removed from the borehole when the corebarrel is full. The length of core drilled before it becomes necessary to remove and empty the corebarrel is termed a ‘run’.

Coring equipment
The manner in which the rock is abraded away, and by which the cuttings formed by this process are taken to ground level having been discussed, it becomes necessary to discuss the machinery used for the job. At the base of the borehole a bit is rotated against the rock, thus advancing the hole. This bit can be either solid or annular, depending on whether a sample is required. Annular corebits are screwthreaded to the bottom of a ‘corebarrel’, of which is a typical example in use in site investigation. The corebarrel is screwthreaded to a ‘string’ (i.e. several lengths screwed together) of ‘drill rod’, which is generally of smaller diameter than the corebarrel. The function of the rods is to deliver torque and downward force to the bit (via the corebarrel) while at the same time providing the flush fluid to the bit. The drill rods are therefore hollow.

At ground level, the rods emerge from the hole and pass through the ‘chuck’ of the rig. The chuck grips the drill rods or ‘Kelly’ and transfers longitudinal and rotational movements to the rods. The Kelly continues upwards and is connected to a ‘water swivel’ or ‘gooseneck’, which connects the water or flush hose from the flush pump while allowing the rods to rotate and the hose to remain stationary.

Drill rigs may vary considerably in size and design. Some of the smallest (for example the Acker 1200 PM) mount directly on top of 2.5—4 in. drill pipe (casing) installed by other means to rockhead. They consist of a small four-stroke petrol engine, typically of less than 10 h.p., which connects via a gearbox to the top of the rods. The water swivel is built into the machine, and feed is controlled by a mechanical system operated by a handturned wheel. Quite clearly, such a rig has a very limited capability. The load applied to the bit cannot be controlled, and the rig has no inbuilt hoist for lifting the drilling equipment out of the hole.

Most rotary drilling rigs used in site investigation tend to be rather small, when compared with the very large rigs used for oil exploration. They usually incorporate:

1. hydraulic feed control;
2. multispeed forward and reverse rotation;
3. cathead, wire drum hoist, or both;
4. a mast or tripod; and
5. variable mounting options, for both the rig and the drilling head.

Hydraulic feed control is used to vary the pressure between the corebit and the rock being drilled. In soft rocks, the use of excessive pressure will fracture the rock before it can enter the corebarrel, while in hard rocks the use of low feed pressures will result in very slow drilling progress. In very soft deposits the weight of the rods and barrel may be sufficient to fracture the rock and the hydraulic feed may need to be reversed to hold up the rods.

Multispeed forward and reverse rotation is important both from the point of view of good drilling and convenience. Slow speeds of the order of 50 r.p.m., are required for augering, and open-holing with the tricone. Faster speeds, of up to 1000 r.p.m. may be used for rotary coring, depending on the rock type and bit in use. Reverse is useful either for unsnagging or backing out auger tools or ‘breaking’ rods.

For shallow rotary work, or where augering is being carried out, a cathead is used to lift the drilling tools in or out of the hole. A rope attached to the tools or rods requiring lifting is taken up the rig to the top of the mast or tripod, passed over a pulley, and then brought down to the cathead. The cathead consists of a drum which rotates at constant speed. The rope is given two or three turns around the cathead, but because the drum is smooth it does not grip and pull on the rope. The cathead is made to lift the tools by the operator pulling on the free end of the rope. This tightens the rope on the drum, and the friction then acts to pull the rope and lift whatever drilling tools are attached. The cathead normally has limited lifting power, but perhaps more importantly, fine control requires considerable skill.

In situations where greater lifting capacity or finer control of lifting are required a wire drum hoist is normally used. This is particularly necessary when long strings of drill rods or augers are being lifted.

Smaller rigs, such as the Craelius D750 or Boyles BBS 10 provide the rotation of the rods via a bevel gear, which drives an octagonal spindle. Hydraulic feed is then developed by a piston on each side of the spindle, which pulls the spindle down by acting on a crosshead. Rigs with this configuration usually have a limited stroke:

Acker ADII 1.80 m.
Acker Hillbilly 600 – 900 mm.
Craelius D750 500 mm.
Mobile B31 1.73 m.
Mobile B53 1.98 m.

Since the corebarrels normally used for rotary work in site investigation are 1.5 m or 3.0 m long these rigs cannot drill the complete length of the corebarrel without having to rechuck; that is to undo the chuck, move it up the rods and reclamp it. To do this, rotation of the corebarrel must be stopped and restarted. This inevitably leads to the exposure of the rock being drilled by the bit to the flush fluid for a longer period than during drilling, and any bad effects of the flush fluid will be emphasized at points on the core where rechucking has taken place.

It can therefore be argued that a long-stroke rig will give much better results when coring soft rocks than the type of rig described above. One type of machine which provides a very long stroke for core drilling is the Acker MPIV hydraulic top drive rig. The rotary action is provided by an hydraulic motor, connected to the engine by flexible hose, which can travel long distances up the mast. The feed is provided by a mechanical system. The Pilcon Traveller 30 and Traveller 50 rotary drilling rigs are examples of lightweight machines capable of drilling a 3 m run without rechucking.

The most common mounting options for site investigation are skid mounting, trailer mounting and lorry mounting. In the UK access is normally poor and many contractors use either trailer or skid mounting. In the Middle East and the USA many more rigs are lorry mounted.

The corebarrel is the normal equipment for recovering samples of rock in site investigation. In its simplest form (as used, for example, to obtain cores of concrete), the corebarrel consists of a single tube with an abrasive lower edge which is loaded and rotated while a flush fluid is passed around the bit under pressure. In this process, first the core inside the barrel is subjected to rotative forces due to the friction of the inside of the barrel against the outside of the core, because the core (being attached to the parent material) does not rotate. Secondly, the flush fluid passes over the surface of the core continuously while it is inside the barrel during drilling.

The effect of the first mechanism is to tend to rotate the core at any points of weakness, such as bedding planes in rock. When rotation of the upper part of the core occurs at such a discontinuity a considerable length of core may be ground away, and a distinctive pattern of circular striations (often called a ‘rotation’) can be seen on the end of each stick of core.

When the flush fluid passes continuously over the core inside the barrel, erosion will occur. This will be particularly serious in soft rocks, where the flush fluid (particularly if water) will tend to soften the outside or along fissures in the stick of core and may well lead to total disintegration of a rock such as soft shale. To counteract these two effects the double-tube, swivel type corebarrel is now used as standard in the UK. shows a typical example. It consists of the following.

1. An outer barrel, connected to the drill rods and drilling rig above.
2. An inner barrel connected to the outer tube at the top via a swivel which allows the inner barrel to remain stationary while the outer barrel is rotated. Flush passes down the barrel between the inner and outer barrel.
3. A reaming shell attached to the base of the outer barrel. This is intended to enlarge the hole produced by the corebit, so that wear on the upper part of the barrel is reduced.
4. A corebit attached to the lower end of the reaming shell. The corebit can be one of many different types, and the illustration shows a face discharge bit.
5. A core lifter or catcher. This device prevents the core from dropping out of the base of the barrel as it is lifted at the end of the run. It consists of:
i the catcher box, which is an open-ended cylinder which tapers downwards, and is of slightly greater diameter than the rock core; and
ii the catcher spring, which fits inside the catcher box and is fluted or grooved so as to grip the rock core. The catcher spring is cylindrical in shape and has an inside diameter slightly smaller than the diameter of the rock core. The wall of the cylinder is cut through at one point, to allow the spring to expand. When the core tries to drop out of the barrel the spring travels down, and is compressed against the core by the inside taper of the catcher box. Thus the greater the downward force, the more friction is developed between the core and the spring.

Triple-tube barrels are identical to double-tube barrels except that a tight-fitting liner tube is used inside the inner barrel. This may be made of stainless steel, or of brass, but in recent UK practice has been composed of clear plastic tube (‘Coreline’). A previously used alternative was Mylar sheet, a thin clear plastic sheet which was held at the lower end of the barrel by a special retainer clip. When using a triple tube, the internal diameter of the catcher and corebit must, of course, be reduced to suit. The advantages of using a third barrel are primarily that the core can easily be withdrawn from the corebarrel, at the end of a run, by pulling the inner liner while holding the barrel horizontally, and that the core can be stored in the liner without disturbing it from its position when drilled. Disadvantages are that the driller cannot immediately see how much recovery he has achieved, and that the engineer or geologist logging the core must cut the liner (usually with a disc cutter) before he can start work. On balance, the use of Coreline seems to have produced a significant improvement in the quality of core available for logging.

Double-tube swivel type corebarrel
Double-tube swivel type corebarrel with face discharge bit

Retractor barrels have inner barrels which are spring-mounted, and protrude ahead of the kerf of the bit, in order to provide some protection for the core from the flush. Notable examples are the Mazier and Triefus barrels

Wireline drilling is a technique which has been widely used for deep mineral drilling for many years, principally because it reduces the trip time (i.e. the time necessary to extract the corebarrel from the bottom of the hole, empty the core and replace the barrel). This technique has, in the past ten years, become well established on high quality ground investigations, and has proved particularly effective in the coring of relatively difficult deposits, such as overconsolidated clays, chalks, and interlayered sands, gravels, limestones and clays.

Wireline drilling is a technique which has been widely used for deep mineral drilling for many years, principally because it reduces the trip time (i.e. the time necessary to extract the corebarrel from the bottom of the hole, empty the core and replace the barrel). This technique has, in the past ten years, become well established on high quality ground investigations, and has proved particularly effective in the coring of relatively difficult deposits, such as overconsolidated clays, chalks, and interlayered sands, gravels, limestones and clays.

Wireline drilling does not use any outer casing, but instead uses an outer barrel which extends at full diameter to ground level (Fig. 5.12). The inner barrel is lowered through the full length of the outer barrel, on a wire line. When it reaches the bottom of the hole it latches inside the outer barrel, in the correct vertical position. The outer barrel is then turned by the rig, as flush is pumped down it. The latching mechanism holds the inner barrel down, but does not fix it so that it must rotate with the outer barrel. When the outer and inner barrels have been drilled for the length of the run, the wire line is winched upwards, and the latching mechanism automatically disengages the inner barrel from the outer. The inner barrel and core are hoisted to ground surface, where the core is extracted and a new length of outer barrel is added to the string.

In principle, wireline coring is considerably simpler than conventional double-tube swivel type coring. No casing is used, and there is no swivel to become jammed. In practice, however, the rig used must be considerably heavier than for conventional drilling, because of the torque required to turn the outer barrel, which is in contact with the ground for the entire depth of the hole. Lorry-mounted rigs are the norm. In addition, the bit on the outer barrel can only be changed at the expense of considerable loss of production. It is preferable to use a single bit for the entire length of the hole. Therefore bit wear, and the choice of a type of bit appropriate to the ground conditions are important factors.

Scarrow and Gosling (1986) describe the extensive use of SK6L wireline drilling (producing a core diameter of 102 mm) in the alluvial valley at Baghdad, Iraq. As might be expected, the soils encountered were very variable, consisting mainly of clays, silts and sands, with some gravel being present. Wireline techniques were used in conjunction with polymer drilling mud (see below). Care was taken to restrict the pumping rates, to keep erosion of granular soils in the bit area to a minimum. A constant fluid level was maintained in the borehole at all times, and especially when the corebarrel was being returned to the surface, and the corebarrel was raised and lowered slowly, in order to minimize suction effects and pressure surges, and therefore the chance of piping and base heave

The selection of the right corebit for the job is a rather difficult task. The variables in a corebit design are:

. face contour;
. cutting material;
. diamond types, grades and sizes;
. mounting matrix;
. waterway size, shape and position; and
. ‘kerf’ width.

The face of the bit may vary from a ‘flat’ surface to a ‘full-round’ surface, where the radius of the surface is equal to half the ‘kerf’ width (i.e. half the thickness of the diamond inset part of the bit). In practice most bits are semi-round or semi-flat in design.

The cutting material may be tungsten, diamond impregnate, or hand-set diamonds. Tungsten bits usually have large tungsten inserts mounted radially across the kerf. This type of bit can only be used for drilling very soft formations, such as soft shale or coral. However, the coarseness of the inserts

increases the bearing pressure on the rock, and may well lead to a disturbance and fracture ahead of the bit. This type of bit is also used for casing.

Principal components of the Longyear NQ-3 wireline drilling system
Principal components of the Longyear NQ-3 wireline coring system

Diamond impregnate bits consist of a sintered powder metal matrix with fragmented or fine ‘Bortz’ (i.e. low grade industrial diamonds) embedded uniformly throughout it. As the matrix wears down, new sharp diamonds are exposed. This type of bit is suitable for hard rocks, and may often be used for casing shoes where casing has to be advanced into the rock.

The best quality diamond bits contain hand-set selected Bortz. The diamonds are of selected size and grade and are placed in the matrix by hand, with the hardest vector of each diamond facing in the direction of the work. This type of bit differs from tungsten or impregnate bits because with the former the bit is used until the ‘crown’ (i.e. the part of the bit formed of the matrix, and set with diamond or tungsten) is consumed. A Bortz-set bit is only used until either the diamonds become polished, or the matrix is abraded around the Bortz to the point where they are over-exposed. At this stage the diamond bit is returned to the manufacturer, where the diamonds are removed and reset.

The quality of diamonds used in the bit varies. Diamonds are also sometimes classified on a geographical basis, such as ‘West Africans’, ‘Congos’, ‘Brazilians’, ‘Angolans’, etc. This means only that the diamonds resemble the typical products of these areas. Congos and West Africans are commonly used in drilling bits, with a preference for West Africans. According to Boyles Bros Diamond Drilling Terms and Equipment Standards, Congos were previously only considered for use in broken form, but more recent applications seem to use them as large stones.

The size of diamond in use in the bit should be tailored to the soil or rock being drilled. In soft material or fractured and weathered near-surface rock large Congos may be used, because the large size has good clearance and allows good washing without blocking the bit. Large diamonds are also apparently more capable of surviving the shocks administered during drilling fractured rock. As the rock becomes harder, smaller and more numerous diamonds are necessary to provide more cutting edge and therefore keep progress at a reasonable level. In addition, the use of more diamonds provides an even load distribution on the bit. The weight of a stone is measured in terms of its ‘carat’ where 1 international metric carat = 200 mg. The ‘carat weight’ is the total weight of diamond set in the bit, which may be between 5 and 50 carat depending on bit size.

The matrix must hold the diamonds in the required position, resist shock, and transfer heat away from the diamonds. The property often used to classify the abrasion resistance of the matrix is hardness, sometimes measured on the Vickers or Rockwell scale. This is not a perfect classification method because hardness is not directly related to either abrasion resistance or the other properties mentioned above.

The design of the waterways also depends on the type of rock to be drilled, and in addition on the flush fluid. Air or mud flush require larger size or more passageways. Soft formations require multi-waterway bits to allow the quick removal of cuttings before blocking occurs. Once the waterways have blocked then not only will the bit overheat, and therefore undergo excessive wear, but the core will also be seriously damaged. In hard rock the cuttings are of finer size and are more granular in nature. Fewer waterways need to be incorporated, and in some cases when drilling very easy materials no waterways are used.

Two types of bit are available: normal or face (bottom) discharge. In normal discharge bits all the flush passes down between the inner and outer barrels, outside of the catcher box, and out of the barrel between the core and the bit. The contact of the flush water with the core, even for this short distance, can have a serious effect and in soft deposits face or bottom discharge bits are commonly used. The drilling bit has ports in the lower end (the face) and the majority of the flush fluid is therefore discharged away from the core, flowing to the outside of the bit.

The face discharge bit represents an improvement on the conventional bit, but suffers from some disadvantages:

1. flush fluid is still allowed to make contact with the core; and
2. over-eager drilling may lead to the ports becoming blocked, especially when drilling in soft rocks or hard clays. Under these conditions it may be necessary to apply no downward pressure to the rods, or in extreme cases even to hold the rods up to reduce the pressure on the face.

One method of overcoming the problems of over-stressing and port blocking may be to use a step-taper bit.

Flush fluid
Flush fluid is passed around the bit while drilling proceeds. The purpose of the fluid is:

1. to remove the cuttings from the borehole;
2. to cool the drilling bit, and drill rods;
3. to reduce mechanical and fluid friction; and
4. to help to retain an open hole wherever possible, without the use of casing.

At the same time, the flush fluid should not encourage the softening or disintegration of the cores, which are the purpose of drilling. A large number of different types of flush fluid are in use, but they are generally classed as:

. water-based (for example water, bentonite/water (drilling mud));
. oil-based;
. air (or mist); and
. stable foam.

The most common flush fluid in use in British site investigation is water, with air being used when water causes serious softening of the formation being drilled. Water is, however, by no means the ideal fluid.

Most drill rigs use normal circulation; that is the flush fluid is pumped down through the drill rods, passes outwards over the bit and travels upwards in the annular space between the drill rods and the outside of the hole carrying the cuttings with it (Fig. 5.9). The requirement of removing cuttings from the base of the hole requires either viscous flush fluid or high flush velocity to maintain the cuttings in suspension.