Engineering in practice in Britain: 4 =ncineerinc ceo ocyanc si'e inves:ica:ion Part 3: Investigation methods by E. J. WILSON», BSc,CEng, MICE, FGS

SITE INVESTIGATION is a compromise be- repeatedly. Less disturbance to the sur- This limitation led to the development tween complete excavation, which would rounding is caused by the use of the of the hollow stem continuous flight auger, destroy that being investigated (the ar- auger, which is run on rods and rotated by in which the auger flight runs the full depth chaeologist's method) and inspired guess- hand, a much slower and more laborious of the about a large bore hollow work (the field geologist's method). This process. Submerged or are tube which on removal of a plug from the article discusses the techniques in routine baled out with a shell, which is in effect bottom gives access to the bottom of the commercial use in the United Kingdom, a claycutter with a flap valve on the bot- borehole for undisturbed sampling or insitu offering different levels of information at tom. Obstructions, dense or dry and testing. different levels of cost, and with different rock are broken up with a heavy chisel Solid stem augering is a rapid, crude degrees of site disturbance. prior to removal with the shell. and relatively cheap technique, useful for The objects of borehole investigation are are stabilised in loose ground, such fields as gravel prospecting. It suffers twofold: and groundwater sealed off, by lining from some vagueness in determining depth (1) to create a hole for tubes. These are driven and pulled by per- to changes of stratum. Some refinement is (a) identifying crude boundary con- cussion, or jacked out under diflicult con- offered by the hollow stem technique, but ditions (e.g. rockhead), ditions. Lining tubes are nominated by the large torque and heavy lift required of (b) observing or monitoring insitu their internal diameter, and are of 6, 8, 10 mechanical augering restrict it to the field conditions (e.g. groundwater), and 12in (150, 200, 250 and 300mm). of heavy rigs. (c) perfolttllng Insltu tests Unless difliculty is anticipated boring nor- Hollow stem augering is very fast under (2) to produce samples of material in se- mally starts in 6 or Bin (150 or 200mm) ideal conditions such as unobstructed quence for size, the larger equipment being heavy to , but is not easily able to penetrate (a) identification a n d comparison transport. The larger sizes are used in gravel and cobble. with other boreholes, and coarse gravel and cobble, or when difficult The difficulty of interpreting the sample (b) determination of properties by conditions are anticipated, which could re- recovery from flight auger boring, which 'testing quire reduction. necessitates the full-time attendance of The objects of and shaft excava- Percussive rigs are available in a range an engineer with specific experience in the tion are: of sizes, these relating to the pulling capa- interpretation of those samples, coupled (i) to create an artificial exposure of city of the winch, which will determine the with the logistic problems of the large rigs soil or rock in which its structure and length and diameter of casing that can used for hollow stem augering under UK the spatial relationships of its prop- be used under any particular condition. site conditions, have restricted the use of erties may be observed and record- Rigs are normally towed by Landrover and this technique very severely as a site in- ed, can be manhandled over even ground, or vestigation method. It remains however (ii) to permit the taking of large (e.g. pulled by their own winches from an an- very attractive as a means of producing block) samples for testing, and par- horage. holes quickly, where site and contract con- ticularly to take samples with regard Many other techniques of soil boring ditions permit continuity. to soil or rock structure, as opposed are available, but none enjoys the univer- 4. Piling rigs to the random nature of borehole sal acceptance in the United Kingdom of Small percussive bored piling rigs used sampling, shell and auger boring. Some are limited for bored piling in 19 and 21in (or 500 (iii) to permit large scale insitu testing in the range of conditions over which they and 550mm) diameters have often proved (e.g. plate loading tests), and will work, but may produce better results useful in tackling beds of cobble which (i v) to penetrate material not easily or be cheaper within that range. typify some of the glacial morainic gravels penetrable by boring or , 2. Hand sugaring of South Wales and Scotland. They may either in place of a borehole or to For shallow investigations in stable un- achieve penetration where a 12in (300mm) start one. obstructed this is a valuable low- shell and auger rig would not, but even cost approach, but with limitations of then the going is far from easy. Boreholes total depth (3-4m common, unusually Large diameter auger piling rigs have Methods of producing boreholes divide more than 6m), insitu testing and samp- been used to create man-sized boreholes. themselves naturally into those suited to ling, which is normally limited to the tak- These are considered as shafts. soil and those suited to rock. The first are ing of 38mm diameter undisturbed samples. 5. Dry coring known as boring, the second as drilling. All equipment is light and portable and The dry coring technique can be used rods are short; therefore accessibility is with a rotary rig, by drilling in under feed Soil boring excellent. pressure an open-ended tube with a hard Boring methods in common use are as 3. Mechanical sugaring metal cutting edge, and extruding the re- follows: Mechanical augers are generally design- sulting continuous disturbed . 1. Shell attd auger, or percussive rig ed on the plate auger or short flight prin- It gives a fully representative sample in This is the mainstay of British site in- ciple, and are normally mounted on a lorry all soils with any , including clay- vestigation, accounting for probably or tractor, the engine of which provides ey gravels, but difficulties arise in com- over 95 per cent of all soils boring. the motive power. These are descended pletely non-cohesive submerged soils. Four tools are used to advance the from the agricultural and services field, The use of the rotary rig gives this borehole. In clay a percussive clay cutter, where they were used for drilling holes for method versatility in that obstructions or a heavy open-ended tube, is suspended on fence or telegraph posts. Mechanical rock bands can be overcome by diamond a wire line and dropped down the hole augers offer no facility for lining the bore- core drilling, then dry coring continued, in hole, so boring is restricted to the depth cogent diameters without the need for to which the borehole will stand unsup- reaming or breaking out. ~ Technical Director, ported, plus the length of the auger Quality of disturbed soil samples Ltd., Glouceater flight itself. Most boring techniques recover recog- September, 1975 21 nisable soil from the borehole. in a more known of these is the U-70 tube (70mm 1. Rotary percussive drilling or less disturbed state. I.D.), which has an area ratio of 19 per This technique is borrowed from the Shell and auger boring produces a con- cent and is contructed of high tensile steel, quarrying industry, and represents the tinuous disturbed core while claycutting, and designed to pass down a diamond lowest cost approach. The bit is a hard which is of too large a diameter to be core-drilled hole of 76mm diameter. metal chisel or cross chisel which forms preserved in its entirety, and samples are The piston sampler is designed to re- the hole by repetitive hammering accom- normally selected by the driller at soil cover high quality samples from soft soils. panied by slow rotation in order to the changes or arbitrary intervals. While shell- A thin walled tube is jacked down over a full face, the chippings being blown back ing in granular soils a largely represent- stationary internal piston, which is then to surface by high pressure air flush. In ative but completely disturbed sample is locked in place and the whole assembly deeper boreholes energy losses in the produced. When chiselling in hard soil or pulled. This may be used with any soils string become significant and improved weak rock, the chippings produced bear boring rig, but the equipment is expensive performance has resulted from the intro- little relationship to the parent material, and specialised, and consequently seldom duction of the "down-the-hole" hammer. particularly when water has to be used to used although often specified. Rotary percussive drilling is fast and assist in cutting and recovery. This is par- An alternative type of piston sampler cheap, but is also very noisy (particularly ticularly evident in materials susceptible has a piston with a cone point which ex- the surface driven type), and dust control to mechanical breakdown in the presence tends below the tube, so that the whole can be a problem. Since the drilling tech- of water, such as the more friable facies assembly may be jacked or driven down nique relies on chipping rock it is most of Keuper Marl, or the softer Middle and to the required sampling depth in soft soil effective in brittle materials, and least Upper Chalk, where it is possible to pene- without a pre-bored hole. effective in sticky cohesive materials. trate with a chisel or even a clay cutzer The work of Butler, Marsland and others Many rotary percussive rigs have no facili- to any depth in a competent rock and has shown that, other factors being equal, ty for casing the borehole. produce nothing but slurry as a sample. sample diameter has a significant effect on 2. Rock bit drilling Flight augers will produce continuous the laboratory measurements of shear This technique is borrowed from the sample in clays, but sampling becomes strength of fissured clays. Overconsoli- oil industry. It is essentially a rotary drill- difficult in gravel, and the admixture of dated clays have long been recognised as ing process, using a tricone bit in which material from adjacent strata makes inter- fissured soils, but the work of Rowe on toothed wheels run around the full face of pretation difficult. normally consolidated clays and of the borehole, removing rock chippings by Dry core drilling in clays in a smaller McGown and others on glacial tills has repeated compressive failures. Compress- diameter using a rotary rig will produce a shown that virtually no clay can be con- ed air, water or mud may be used as a continuous disturbed core sample which sidered to be free of macro fabric. flushing medium. Fishtail bits or drag bits can be presented in core boxes in the The experimental work of Professor may also be used. same way as diamond drill core. Similar Rowe produced the 250mm diameter sam- Rock bit drilling, being amenable to air, sampling is possible in dry or damp ple, which is available for specialised water or mud flush, may be used through and fine to medium gravel with some work. These samples are taken in custom- all materials, but does require a heavy rig or clay matrix, but in submerged sand or made galvanised steel piston tubes, jacked to drill through rock. It is sometimes used clean gravel sample recovery is low. This into the ground against . The as a means of advancing a borehole at low technique does have the advantage that it hole is kept full of water to minimise dis- cost and high speed between intervals is used in combination with core drilling turbance by pressure relief. Modified shell where core drilling is required. It is also as a single rig technique, and when en- and auger rigs are used with underwater useful as a "brute force" means of pene- countering rock there is no temptation to augering between undisturbed sampling trating difficult obstruction eg., slag tips, destroy the weakest weathered rock by intervals. This is a very specialised tech- prior to core drilling. The larger cuttings chiselling before diamond core drilling. nique offered by very few contractors. In offer better identification of strata than in rotary percussive drilling. There is no ideal technique for all con- view of the kentledge required, such samp- These two techniques suffer from ditions. All techniques perform well in ling is extremely expensive. The remark poor directional stability with clays; all have difficulty in coarse gravels has been attributed to Professor Rowe compared core and and cobbles. that a 250mm sample is one thousand drilling, difficulty has sometimes been found in following with diamond drilling Undisturbed soil sampling times as expensive to take and to test as a 38mm casing, which needs a straight hole. Undisturbed soil samples are taken in sample, but the results are one thousand 3. Full-face diamond drilling boreholes by hammering an open-ended times as useful. This is a modification of diamond core tube into the ground at the bottom of the Block samples are taken by hand from drilling in which the bit is made with borehole. Since this displaces soil some trial pits and shafts. Careful hand trimming a full is cutting face of diamonds. The applications disturbance is inevitable, and in general used to produce a regular block of the of this technique in site investigation the thicker the tube walls the more dis- required size, normally a cube of about are limited, but it does permit open hole drill- turbance is created. BS CP2001 recom- 250mm side. Larger samples tend to be ing with a medium weight at a mends that the area ratio should be less too heavy to handle without damage. rig, higher Block rate of overall progress and lower cost than 25 per cent (that is the wall or cutting samples are waxed, with reinforcing than core drilling, although the penetra- shoe sectional area expressed as a per- layers of thin cloth such as butter muslin tion is still much slower and centage of the sectional area of the sam- or cheese cloth. Speed in preparation is the cost particularly important in warm weath- higher than rock bit or rotary percussive ple). The best samples are obtained by dry drilling. One advantage is that it thin-walled tubes which have an area ratio er. Block samples are particularly relevant can be integrated with core drilling when of 10 per cent or less, but these are rather to directed sampling, that is sampling for using a medium weight rig, avoiding the use of delicate and restricted to use in softer un- testing of specific layers such as shear obstructed soils. surfaces or joints, and to orientated samp- more than one driller on the smaller site. ling. It is also useful in forming regular holes The routine sample tube is the 4in for insitu rock mechanics testing, e.g. (102mm) tube. The origin of the diameter Rock drilling, open hole insitu stress measurements. Air, water or was accidental but it has remained stand- The major division is between low-cost mud flush used. ard ever may be since, and the design has changed non-coring techniques, which rely for their little, for the con- except use by some usefulness on rapid progress, giving an ex- Core drilling in rock tractors of plastic liners. The typical area tensive cover of information at a low level, Core drilling consists of drilling out an ratio is 34 per cent, hence the sustained and slower more costly core drilling. annulus to leave a core. The bit is the criticism of the level of sample distur- Open hole drilling will determine depth leading part of the tube known as the bance. to rockhead where there is an obvious barrel which is a receiver for the core. In hand auger work and for hand samp- change, the presence of cavities and the Bits are set with diamonds or hard metal ling in trial pits 1/in (38mm) tubes are presence of harder strata, by difference in teeth and a flushing medium, either water used. These vary in wall thickness having penetration rate. It may also provide fine with or without additives, or compressed area ratios down to 15 per cent, but the chippings for a crude assessment of air, is used to cool the bit and wash cutt- small sample volume is rather limiting, for lithology, although these do not often ings up the hole, either to surface or to most purposes. reach the surface quickly enough to gauge loss in the formation. When the barrel is In minority use a wide range of inter- their depth with any accuracy, and this full or earlier if a core blockage occurs, the mediate sizes is available. The Shelby type margin of error is increased with depth of core is retrieved. It is broken in tension by tube is a scaled-up version of the U-38 drilling. lifting the barrel which locks the core lifter tube, having an integral cutting edge and The techniques in most common use spring on to the core. constant internal diameter. Perhaps the best are:— In common with other rock drilling September, 1975 23 Shell and auger rig (courtesy, wimpey Laboratories Ltd) Rotary-percussive drilling with Atlas Copco Lindo drill, part of a drill-and-grout contract for shallow mineworkings

techniques, diamond core drilling per- Lightweight rigs are portable, mounted of drilling tools is required for air or water forms best in stronger brittle rocks with on a frame or tripod. The most widely flush. few discontinuities. Drillability of hard used in this field is in the form of an Water is normally used on its own, but rock relies on micro-discontinuities or attachment to a shell and auger rig, which additives are used sometimes. Bentonite, changes in hardness; therefore extremely clamps on the lining tubes or runs on an known as mud, is the most common. This fine grained monomineralic rocks such as auxiliary frame. Medium weight rigs (up is used to ease the running and pulling of massive chert or flint may prove virtually to two tons or so) are mounted on skid casing by lubrication, to keep sand and undrillable. frames, two wheeled trailers or self pro- cuttings in suspension, and to promote Most rocks are jointed, but occasionally pelled tractors, occasionally on vehi- borehole stability by increasing flush joints or bedding planes may be so ori- cles. Heavy rigs, from about two tons returns through the formation of a filter entated as to cause wedging of fragments, upwards, are mounted on lorries or large skin on the borehole walls. In the same or blocking, in the core barrel. There is no tractors, mostly with large derrick masts. way it can improve core recovery in substitute for removing and emptying the Attitudes to the use of lightweight rigs some weaker friable rocks. Brine is used barrel every time this occurs, but no justi- vary from company to company. At least as a flush to prevent solution while drilling fication for an arbitrary shortening of the one major site investigation company is in saliferous deposits. next drill run in anticipation. Overall pro- largely committed to their use for nearly 3. Drilling diameter gress may be halved by short drill runs. all diamond drilling: others use them only Available core sizes vary between In its extension from the field of for proving rock for a short distance below 16mm and 165mm. Others factors being hard rock mineral prospecting to the geo- shell and auger holes, whilst others rely equal, the larger the core the better the technical field, core drilling has adapted to on medium weight rigs for the bulk of core recovery, up to about 100mm. Here counter new problems. In particular, their drilling. it is impossible to generalise, since core engineering problems are concerned most The medium weight rig is probably the recovery will depend on the condition of with the weaker and more friable members most versatile drilling rig in site investi- the drilling equipment, the driller's experi- of a rock sequence, often interspersed gation, having adequate capacity in depth ence and the formation. with stronger material. This has led to a and diameter for most jobs, yet being There are two main size standards for general increase in drilling sizes, sophisti- very mobile. Heavy rigs are at an advan- core drilling, the DCDMA American cation of bit and barrel design, and to tage in deeper drilling and larger contracts, standards, now adopted as BS 4019, and much experimentation in general. In core where their speed of drilling can make up the Craelius metric standard. The BS 4019 drilling perhaps more than any other part for the heavier logistic costs. range uses fairly thick casing and barrels, of site investigation, however, one relies 2. Flushing medium which are consequently quite robust, and for results on the experience and capa- Water is used probably exclusively on the metric range uses thinner casing of bility of the driller, and on the rigorous lightweight rigs, and dominantly on medi- high tensile steel, giving a closer nesting maintenance of equipment. um weight rigs. Compressed air is com- of sizes. For example in reducing from The principal variables in core drilling monly used on heavy rigs. For medium rigs 101mm diameter (core) to 22mm dia- equipment are: (1) weight of rig, (2) there has always been some disagreement meter (core), the metric standard lists flushing medium, (3) drilling diameter, (4) over the relative merits of the two, but nine sequential sizes against six for BS design of bit, and (5) design of barrel. water is in more general use than a few 4019. Details are given in Table I. 1. Weight of the rig years ago. Occasionally the choice is in- 4. Design of bit Whilst rigs exist in an unbroken range fluenced by constraints such as the lack Bits may have cutting edges of hard of sizes from the smallest to the largest, it of water (unusual in UK) or the pro- metal inserts, set diamonds, or impreg- will nonetheless be convenient to divide hibition of noise or dust associated with nated diamonds. Impregnated bits have a them into light, medium and heavy weight compressed air. The two are not freely matrix impregnated with diamond dust. machines. interchangeable, since a different design These have a grinding action and are used 24 Ground Engineering in hard and broken formations. Set bits tal. Inclined boreholes are inherently less Casing have individual diamonds set on the sur- stable than vertical ones, and the prob- Boreholes are normally stabilised in face of the matrix, most of which can be lems of drilling through broken formations loose ground by casing, equivalent to lin- recovered for re-use by leaching when the are greatly magnified to the extent that ing in soil boreholes. Diamond drilling cas- bit is taken out of service. The coarser completion depth cannot always be ing is however different from lining tubes surface-set diamond and tungsten carbide guaranteed. Directional stability of drilling in a number of respects. Casing is never tipped bits are used in softer formations. also deteriorates. In deeper drilling, the driven into the ground: it is drilled in Coarser bits are generally used with air sag of the rods tends to deflect the bore- using a hard metal toothed or diamond flush than with water flush. hole towards horizontal. tipped casing shoe with water, mud or air Bits may be thick or thin walled. Thick Borehole deflection can be measured by flush. The outer clearance of the shoe walled bits are more robust but carrying a variety of borehole survey instruments, leaves an annulus outside the casing. This more diamonds are more expensive and developed in the oil drjlling field. These normally closes in unconsolidated de- drill more slowly. Thin walled bits drill rely on the photographic or mechanical posits, but in rock the casing does not faster, and produce larger core for a given recording of a compass card and plum- form a watertight seal as lining tubes do. hole size. This is important where several bob. Inclination alone can be measured Since drilling casing is not subjected to reductions have to be made. Flush dis- more simply and less accurately with a percussive driving it has much thinner walls charge may be internal or face discharge. hydrofluoric acid etch tube. than lining tubes. Face discharged flush does not wash Core orientation, Rocha sampling Casing may be installed in one of two directly against the core, but this design The orientation of cores is a difficult ways. It may be inserted down a hole is restricted to thick-walled bits. procedure. Several methods have been drilled to a larger diameter, to act as con- 5. Design of barrel developed but none is known to be in ductor casing when reducing and drilling Barrels may have one, two or three regular use. ahead in a smaller diameter, or it may be concentrically nesting tubes, each free to The Rocha technique is used for the in- drilled or reamed in a larger diameter than swivel inside the next. Single tube barrels tact recovery of jointed and broken rock. the initial hole, to allow continued drilling are cheapest, but the flushing medium A small diameter pilot hole is drilled in the same diameter. The first, known as washes over the core throughout its ahead of the borehole and a metal bar running casing, is normally included in length. They are therefore used in hard grouted in using a resin grout. When this drilling rates, but the second is usually rocks where core recovery will not be is set the grouted section is overcored. quoted as an extra over drilling rates, since affected or where core recovery is not im- This does not need any specialised equip- it involves a duplication of the initial drill- portant. They are available in short lengths ment, but incurs heavy standing time, and ing process. and are used as starter barrels. is not in regular use. Borehole stabilisation may also be In double tube barrels the flush passes Wireline drilling achieved by filling the unstable section of between inner and outer tubes, touching Wire line core drilling is a means of re- the borehole with cement grout, then re- the core only at the cutting face. Double trieving and running the inner barrel on a drilling through the plug. Grout- tube barrels are used in nearly all site in- wire line through large diameter drill rods, ing enables the borehole to be continued vestigation core drilling. leaving the outer barrel in place. Core dia- in full diameter, but can involve heavy Triple tube barrels are designed to pro- meter is small in relation to hole diameter, standing time while the grout is setting. duce the absolute maximum core re- with very thick walled bits. Penetration covery, and have produced excellent is therefore slower, but advantage is felt Insitu testing results in unconsolidated sediments. Un- in very deep drilling in the reduction of The Standard Penetration Test fortunately they are extremely expensive, "round trip" time. This advantage is offset There can be few engineers who are not and the di%cult drilling conditions which in unstable ground where casing is re- familiar with the standard penetration test, indicate their use may at the same time quired. Wire line drilling therefore has no or SPT. It is probably the oldest insitu preclude it on the basis of cost risk. particular feature which could lead to its test, having been used in the United States Inclined drilling specification in terrestrial site investigat- in conjunction with washbo ring tech- Most machines will at any ion, and it should be regarded as a con- niques, when it was first calibrated by inclination between vertical and horizon- tractor's option. Terzaghi and Peck with the bearing capa-

TABLE I TABLE II STANDARD CORE DRILLING SIZES APPROXIMATE RATES OF PROGRESS AND RELATIVE COSTS BS 4019 FOR DIFFERENT BORING AND DRILLING TECHNIQUES

Designation core holes casing casing full die cl I a i.d.min 0.(I. Technique Relative costf Progress m/shift 201.6 219.1 Shell and auger 6"/8" 8 ZX 165.1 199.3 176.2 193.7 10"/12" 2 k UX 139.7 173.9 149.2 168.3 Hand auger 0.8 20 SX 112.7 145.2 123.8 139.7 Open hole rock drilling: PX 92.1 120,0 100.0 114.3 rotary percussive 0.3 70 HX 76.2 98.9 76.2 88.9 rock roller 0.7 70 NX 54.0 75.4 60.3 73.0 Diamond core drilling in range BX 41.3 59.5 48.4 57.2 BX to HX (vertical) AX 30.2 47.5 38.1 46.0 lightweight rig 2.5 6 EX 20.6 37.4 30.2 36.5 medium 2.5 8 XRT 17.5 29.5 heavy 2.5 20 'Average figures within normal toleiances. inclined to horizontal drilling (medium rig) 5.0 5 CRAELIUS METRIC STANDARD Static cone penetrometer 0.1 30 (e.g. Dutch cone) Thinwalled bits (T,B)f Thickwalled bits(K>)f Casing tRelative cost is the cost/metre quoted by a contractor expressed as a multiple of the cost/metre quoted by the same contractor for shell and core hole core hole auger boring in stiff clay. 'Laroe diameter equipment is not used under the normal conditions assumed cl I a dia dia die I.d. o.d. in this table. 116 146 134 143 Both relative cost and rates of progress are the mean or estimated norm 101 131 119 128 for a range of contractors, working under good to normal conditions with 86 116 104 113 continuity. Under the normal range of conditions progress may rise or drop by 50 per cent, but under extreme conditions it could be improved by a 84 101 72 101 89 98 factor of uo to 3 or reduced by a factor of uo to 4. 72 86 58 86 77 84 Some limitations on the above figures are explained in the article. 62 76 48 76 67 74 52 66 38 66 57 64 42 56 34 56 47 54 32 46 24 46 37 44 22 36 29 35 16 28 tT—thin walled double tube barrel. B—thin walled single tube barrel. K3—thick walled, face discharge, double tube barrel. September, 1975 25 Inclined diamond core drilling with a medium Air flush drilling with a heavy (Reich) rig weight Craelius D-750 rig (modified) city of sands and the undrained cohesion respect, it is better not to carry out SPT Shear vane apparatus comes in three size of clays. That this was a very empirical in sand below the water table. ranges. The biggest size consists of a heli- test was recognised from the outset, but The SPT may be carried out using any cal-spring torque head mounted on a the fact that the test will produce a num- rig with a wire or rope hoist. frame or small tripod. The rods carrying erical result in any soil under any test con- Static penetration tests the vane are pushed into the ground for a ditions has at the same time proved an In static penetration tests the cone is distance normally 300mm below the unending challenge to correlators and a pushed into the ground by static pressure. bottom of the borehole, or a retractable pitfall for the unwary. usually without a prebored hole. The best sleeving may be used to enable the test Briefly the test records the number N of known technique in this country is the to be carried out without a prebored hole. blows of a 63kg weight dropping freely Dutch Cone method in which the pene- It may be used with any type of drilling or through 760mm to drive the standard tool trometer rods are sleeved, and point and boring rig capable of pushing the rods in, 300mm below the bottom of the borehole sleeve can be advanced alternately. The although this can be done by hand in very after an initial seating penetration of pressure required for each can be plotted soft soils. The operation of the head is 150mm. The standard tool is a thick walled against depth, giving a continuous resis- manual. tube split longitudinally in two with a tance profile. The rig used may be small A smaller apparatus uses a torque single piece cutting edge. In gravel it is and skid-mounted, in wl ich case screw wrench and is more appropriate to hand customary to replace this with a solid anchors are used to resist uplift, or it may auger or probe investigations. The small- 60 deg cone point, the test being then be mounted on a heavy lorry. est shear vane, developed for laboratory referred to as the dynamic cone test. The process is extremely rapid, up to 1 use, is frequently used in trial pits to de- The obvious reliance on the driller in metre per minute. It is however restricted termine the variation of cohesion over an hoisting the weight carefully to the full to fairly soft unobstructed soils, and the area of exposed soil. height, often under grisly site conditions, equipment can easily be damaged in The torque head equipment is used with and the dependence of the free drop on gravel. a vane 150mm long by 75mm wide, or his quick release of the rope, or the free Static penetrometers are in widespread 100mm long by 50mm wide for use in running of a wire winch drum, led to the use in Sweden, Denmark and Holland, firmer soils. A range of intermediate sizes introduction of the automatic trip hammer. where a site investigation of an area for a is used with torque-wrench apparatus, and When in good condition this equipment structure which in this country might be the laboratory vane is normally used with ensures a free 760mm drop, and has done investigated by say a dozen boreholes, a 28mm by 19mm vane. much to encourage the correlation of re- would consist of perhaps sixty probeholes Since the vane test is a direct measure- sults obtained by different drillers at diff- with one or two control boreholes. ment of shear, the results in clay are erent times. Several variations of the technique are fundamental insofar as any undrained The use of the SPT is principally in available, but comparisons are difficult to can be, although in non-cohesive soils, where undisturbed make since test conditions vary, and there and fine sands the results are open to sampling is impracticable. Testing in non- is still some latitude in interpretation of question. It cannot be used in gravel. cohesive soils is subject to two major the results. Development in this field has Each machine is calibrated by the manu- limitations —lack of overburden restraint at been very active in recent years, and its facturer, and the calibration should be shallow depth, and disturbance due to commercial use appears to be on the in- checked periodically. seepage forces below the water table. The crease. Pressuremeter tests first is a matter of interpretation rather Vane tests In the pressuremeter test an inflatable than test conduct, and has been allowed The vane test is an insitu cylindrical element is placed in a borehole and filled for in correction tables published by Gibbs shear test, in which a cruciform vane is with oil under pressure. The yield of the and Holtz (1957), and by Thorburn pushed into the ground, then rotated to ground is measured by the volume change (1963). The second can be a serious shear a cylinder of soil against a torque in the oil tank, giving a stress/strain re- source of error, since the loosening may measuring device. Its chief use is to lationship. The best known such test is reduce the blow count by many times. measure the undrained cohesion of soft the Menard pressuremeter, which has The only acceptable remedy is to keep and sensitive clays which would not sam- been available in Britain for many years. the borehole filled with water above ple satisfactorily. The test may be contin- The test may be carried out in most soils, standing water level at all times. This ued beyond the initial failure (known as or in weak rocks, but requires a stable makes very awkward demands on the the undisturbed strength) to measure the and tolerably smooth-sided borehole. driller, and boring should be supervised remoulded strength, and thus to establish The test has never received widespread under these conditions. Unless the results in a single test a measure of soil sensitivi- acclaim in this country. The principal ob- of the test can be relied upon in this ty. jections have been that it stresses the 26 Ground Engineering ground horizontally which rarely corre- joints, often with a tip of sin- where Dh is the change in level during sponds with the design requirement, and tered porous ceramic (the Casagrande the time interval Dt, A is the internal area that its interpretation rests on the as- piezometer) or porous plastic. of the casing, H is the mean height during sumption that the ground yields isotropi- Sealing of the response zone in a large the test of the water level in the hole cally. In practice, its widespread use has diameter (150mm or more) borehole is above standing water level, and rO'he been inhibited by the franchise system carried out by tamping bentonite balls to mean radius of the borehole below the under which it has been operated, with form an impermeable layer, then gravity casing C is a dimensionless coefficient re- the result that the fund of experience grouting or backfilling with spoil. In smal- lated to borehole shape and penetration necessary to its promotion has been slow ler diameter drillholes or deep boreholes into the pervious stratum, and may be to accumulate. this is not practicable, and cement grout- deduced from charts given by Terzaghi Currently the test is used in conjunction ing by tremie is normally adopted, using an and Peck. with the process of dynamic consolidat- intermediate sand layer to prevent grout In order to minimise the effects of soil ion, as a "before and after" test under penetration of the gravel. When setting a smearing on the borehole walls, and to controlled conditions. response zone part way up a borehole, this prevent the formation of a filter skin from Other pressuremeters and directional involves a delay while waiting for the first suspended fines, the test may be reversed devices such as flat-jacks have been de- grout pour to set. Rapid hardening cement by baling the hole and measuring the rise veloped for use in boreholes and sawn should never be used in boreholes, since in level. slots, but their use has largely been re- the time saved is never worth the risk of Alternatively the constant head test stricted to specialised applications of rock grouting in the casing. may be used, where water is added at a mechanics. Water level may be measured with an constant rate to maintain a steady level. Permeability is given the following Plate bearing tests electrical dipmeter, but unless the contact by formula derived from electric analogue ex- Where a representative undisturbed breaker probe is designed to shed water and in the USBR Earth cannot recovered, insitu and break contact even when hanging periments, quoted sample be and an Manual:— borehole test does not cover a large against the access tube wall, it is easier in enough area or volume of soil to be re- the long run (and just as accurate) to use Q presentative, a plate bearing test a small bottle on the end of a piece of (PBT) 5.5rH will give an indication of load-bearing string, provided the access tube is wide capacity. PBT are therefore particularly enough. In of low there where is the rate of flow into the appropriate to fill materials. They are also clays permeability may Q radius of the used for more accurate measurement of be insufficient volume of free water to fill hole, r is the internal casing, settlement on granular soils than can be a standpipe or piezometer access tube up and H is the head of water above standing level. For above the water estimated from standard penetration tests. to the piezometric surface, that is the water tests which atmospher- table H is the depth of water in the hole. In recent years they have acquired new level at pore water is at ic In this Constant head testing is also used in status. in measuring the stress/strain pressure. case the piezometric drillholes, where higher pressure heads properties of fissured clays, by the use level can be established by measuring the use of packers of large(up to 1.0m diameter) plates so as . A conical piezometer can be developed by air value which expand to seal against the bore- to stress a large volume of soil in relation tip of high entry (the Bishop pie- in under to the fissure spacing. Circular plates are zometer) is seated at the bottom of the hole walls. Water is pumped the pressure being added also used in large boreholes and bored borehole in a shaped cavity produced by pressure, pump pile shafts. a special tool, carefully sealed in and con- to the head difference H in the above nected to surface with twin nylon tubes. equation, having made allowance for any The load is transferred to the plate The is filled with de-aired water pressure losses in the system. from a loading yoke, which derive its system may Packers are designed to expand or "set" reaction either from static kentledge or and the pressure monitored at surface. against the borehole walls pneumatically, from anchor piles or ground anchors in Permeability Most soil and rock masses contain hydraulically or mechanically. The latter tension. Loading is applied in increments discontinuities such fissures, are very simple to operate. The test length to failure, which is normally considered to as joints, bedding surfaces or sedimentary may be adjusted in increments depending be a settlement of 10 per cent of the layers of on the equipment. Tests may be carried plate dimension. PBT must be carried out different composition. The internal per- meability of the blocks of virgin material out as single packer tests, over the inter- over the full depth range to be stressed by pri- val between the packer and the bottom the . It is normal to use a de- between discontinuities is known as permeability. The overall permeability of the borehole, or as double packer tests, sign load of one half of the ultimate load ary of the entire soil or rock mass including over the interval between two packers. as measured in the PBT. discontinuities is known as secondary per- The former are normallly carried out at Groundwater measurement meability. The ratio between the two will intervals during drilling, and the latter Measurements of standing groundwater determine the pattern of groundwater flow, after completion of the borehole. Double level, and ground permeability, are fun- and often the stability of open cuts under packer testing can therefore reduce super- damental to engineering problems. many seepage. visory time. In ground of high permeability( gravel, In most soil or rock masses there will The accuracy of single-borehole per- sand, open-fissured rock) the true ground- be several sets of discontinuities, various- meability testing is severely limited by the water level establishes itself very quickly ly spaced and orientated, each of different constraints already mentioned, and at- in boreholes, and measurements made permeability. Any attempt to measure per- tempts to sophisticate existing methods during drilling will be reliable. Where the meability on the interface between a should be resisted. The use of pumping- formation is less permeable, or where borehole and the formation is therefore out tests to measure drawdown is a much fluctuations in groundwater are to be fraught with many uncontrollable varia- more effective means of measuring sec- measured, an hole must be main- open bles, and the value measured may be any- ondary permeability, and will be dis- and this is most conveniently tained, done where between primary and secondary. cussed in the ninth article in this AEG inserting a tube, porous over a given by A further limitation lies in the distur- series. The chief limitation in site investi- To choking length. prevent by slurry bance to the soil or rock fabric on the gation terms is the large d'iameter of bore- formation are from the these normally set walls of a borehole. This is evidently se- hole required to accommodate a sub- in or sand. pea gravel vere in all techniques of soil boring, mersible pump of sufficient capacity. The length of the borehole over which where the fissures of a clay formation may the gravel pack extends and which there- be completely filled with remoulded clay. Trial excavations fore is open to the formation, is known as The effects in rock drilling with a flushing The use of trial pits in site investigat- the response zone. Where the response medium are probably much less. ion has escalated with the cheap and zone consists of the entire borehole depth The simplest first approach to measuring ready availability of hydraulic backacter the installation is known as a standpipe permeability is by the falling head test, excavators, and with the improved de- or observation well. Where the response where the borehole (or a permeameter sign of pit and supports for quick zone is restricted usually by a cement tube extending above ground level) is installation. grout or bentonite seal, it is known as a filled with water and its rate of fall Nonetheless hand-dug pits are still piezometer. measured. The permeability is given by used, and have the following advarrtages: Standpipes may be constructed from a the formula, quoted by Terzaghi and Peck (i) The sides and bottom of the pit are wide range of available plastic tubes ac- much less disturbed, hence much cording preference. are 1 A less cleaning up is required to reach to (Dh/Ltd'=— normally constructed of rigid 19mm I.D. undisturbed soil for sampling. PVC tube with cemented watertight C rO' (ii) The reduced disturbance to sur- September, 1975 27 (Above). Selection of bits, showing (from left to right) 131mm face discharge bit, surface set diamonds; 84mm casing shoe, hard metal inserts; 76mm thin walled bit, surface set diamonds; 76mm thin walled bit, impregnated diamond dust; BX full face (non-coring) bit, surface set diamonds; and 4)in (114mm) rock roller bit (Right). Core samples, the upper box containing 85mm dry drilled, and the lower 73mm diamond drilled

rounding ground imparts greater mineshafts, more open excavation is re- available. These can pick up spontaneous stabil'ity. (As an extreme example quired. Unless the location of the shaft is electro-magnetic fields from electric or the meticulously excavated archaeo- known with certainty, a dragline should telephone cables, and detect their posi- logical pit will stand vertically with- be used in order to keep the machine well tion in plan w'ithin a few centimetres. Gas out support for months in materials clear of the area of potential collapse. and water mains give fainter signals but can and under weather conditions which Trial adits are in fact and be activated where they are accessible would have led to the collapse of a should never be attempted on a do-it- (e.g. at hydrants). Sewers and culverts machine-dug pit 'in hours or days) . yourself basis. can be traced with equal accuracy by fol- (iii) A pit may be dug by hand anywhere Trial shafts were originally overdeep- lowing at surface a probe transmitting a a man with his shovel and pick will ened trial pits. Large diameter power radio signal which is floated or rodded go. auger piling rigs can now excavate man- from the nearest manhole. Inaccessible (iv) Excavation round existing founda- sized shafts to depths of over 30m. This sewers or culverts. and non-metallic tions or services requires too deli- permits insitu inspection of weathering, water lines, are difficult to detect. These cate a touch for machine work. bedding and jointing —particularly im- techniques can detect services at depths in like of 10m. (v) Hand can proceed beneath portant materials chalk which are up to highly sensitive to disturbance in drilling strutting, or to a depth beyond Con dusions machine reach. and sampling. It also enables small block samples to be taken at depth. Unstable Boring techniques in the UK are re- The principal advantages of mechanical stricted almost exclusively to shell and excavation are ground near surface is retained by casing, but this obscures totally the interval it auger work, after some short-lived ex- It is quick and therefore inexpensive, (1) supports. Below the casing, the man is cursions into other methods. Despite the and protected by an inspection cage in which extensive early development work in the In unstable ground a quick view of (2) he is lowered, with a removable side panel United States, described in detail by conditions can obtained from sur- be for working. With the very heavy on-site Hvorslev, relatively few sampling tech- face (including bucket samples) and costs for an auger piling rig, this technique niques of any sophistication are in general the hole then backfilled without risk is only economical where a large number use. Notably increasing use is being made life. to of shafts is contemplated. of probing techniques, both in static pen- Machines for shallow excavation, down etrometers and in the field of groundwater to 3—4m depth, are available "off the measurement, the importance of which is shelf", at any plant yard, at very cheap Buried services becoming generally realised. rates. In order to excavate down to about Site investigation specifications usually Diamond core drilling accounts for the 7m or to tackle weak rock conditions place the onus for protection of buried ser- vast majority of rock drilling, and here (such as may be required in exposing the vices on the contractor, without providing the tendency has been toward the use of outcrop of a coal seam) larger machines an item for billing what is often a very larger diameter and advanced design to are required which become much rarer time-consuming exercise. In effect this achieve maximum core recovery in weaker and more expensive. Larger machines are often means that the contractor who rocks. A notable development is the use track mounted and have to be transported wants the contract badly enough will take of lightweight core drilling attachments to to site by low-loader. Such is the extra the risk. If tendering for site investigation shell and auger rigs for proving a few cost, that the need for a large machine work has to be thrown open to competition metres of rock. must be fully justified before it is speci- in this way, then it is irresponsibility on the Tnal pitting and shafts are used increas- fied. After all, an isolated pit can be deep- part of the client or his engineer to present ingly with the availability of mechanical ened econom'ically with a small machine documents which allow the investigation equipment, and electronic detection of by digging out a stance at 8 lower level. of buried services to go by default. buried services has done much to reduce With the advent of the Safety at Work Statutory undertakers are notoriously one of the risks of work'ing in developed Act (1974), the question should no longer uncertain of the exact whereabouts of their areas. arise whether it is necessary to support property, and in most cases will guardedly an excavation. There are now at least two recommend that their information should References trench shoring systems on the market not be taken for granted. Gibbs, H. J. and Holtz, W. G., 1957, "Research on determining density of sends by spoon penetration which can be installed from surface. Even The digging of starter pits is often testing". Proc. 4th Int. Conf. Soil Mech. and without these, expanding screw props adopted, but has been known to stop Found. Eng. London. Vol. 1 pp. 35-39. Hvorslev, M. J., 1949, "Subsurface exploration snd against scaffold boards can produce very short of deeper services where shallow sampling of soils for civil engineering purposes". effective support. ones are found, and also involves re- New York: The Engineering Foundation. Roche, M., 1967, "A method of integral sampling For very deep or wide excavations re- instatement of paved areas. Often how- of rock masses". Rock Mechanics. Vol. 3 pp. 1-12. maining open a long time, however, or ever, it is not possible to provide the con- Rows, P. W., 1972, Twelfth Rankins Lecture. "The relevance of soil fabric to site investioation prac- for excavations in very unstable ground, tinuous employment required for a special- tice". Geotechnique. Vol. 22 pp. 193-391. the more expensive trad'itional timbering ist gang, and pitting is left to the driller. Scrota. S., 1972, "Site investigation rigs: design and use. GROUND ENGINEERING. Vol 5 pp. 18-23. is preferred. Ladders must always be pro- Even the most expert driller may be only Terzaghi, K. and Peck, R. B., 1967, vided: men should never ride the bucket. an indifferent navvy, and under these cir- in engineering practice. (Second edition) Wiley. Thorburn, S., 1963, "Tentative correction chart for Using quick supports, it is possible in firm cumstances services may be at greater risk the standard penetration test in non-cohesive to stiff clay to dig, support and backfill up during pitting than when drilling blind. soils". Civil Engineering snd Public works Re- view. Vol 58 pp. 752-753. to 10 pits to 3 or 4m in a working day. Recently techniques have been de- USBR, 1968, Earth Manual. Appendix E-18, pp. 541- Trial are simply laterally ex- veloped for the electronic detection of 546. (United States Department of the Interior, Bureau tended trial pits. When excavating for old buried services, which are commercially of Reclamation). 28 Ground Engineering