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Chapter 5Subsurface exploration: boring, drilling,probing and trial pittingINTRODUCTIONChapter 4 considered the various indirect methods by which the ground can be investigated, usinggeophysical techniques. Whilst these methods can be extremely valuable for ground investigationpurposes, they are not in everyday use. The bulk of ground investigation is carried out using the directmethods of investigation described in this chapter, coupled with in situ or laboratory tests.The primary functions of any ground investigation process will be one of the following:1.2.3.4.5.6.locating specific ‘targets’, such as dissolution features or abandoned mineworkingsdetermining the lateral variability of the ground;profiling, including the determination of groundwater conditions;index testing;classification;parameter determination.Geophysical methods can be very good at giving information on the location of specific targets, andinvestigating the lateral variability of the ground, but their results are often more qualitative than ispreferred by design engineers. Parameters for engineering design most commonly are derived from insitu tests carried out in boreholes or from self-penetrating probes although, as was noted at the end ofChapter 4, seismic geophysics methods can give valuable information on the stiffness of the ground.Most profiling is done on the basis of soil and rock descriptions, carried out either on samples obtainedfrom boreholes, or on the faces of trial pits or shafts. And the majority of classification and indextesting is carried out on samples taken from boreholes and trial pits.Therefore the direct methods of testing described in this chapter are at the centre of routine groundinvestigation. They provide the opportunity to obtain samples for visual description and index testing,which are the primary ways in which the strata at a site are recognized, and for sampling and much ofthe in situ testing needed for parameter determination, as well as allowing the installation ofinstrumentation such as piezometers.Boring is carried out in the relatively soft and uncemented ground (engineering ‘soil’) which isnormally found close to ground surface. The techniques used vary widely across the world. The mostcommon methods are augering, washboring and (in the UK) light percussion drilling. This lattertechnique is well adapted to stoney soils, and has its origin in water well drilling techniques.Drilling has traditionally been used in the more competent and cemented, deeper deposits (engineering‘rock’). It is now also widely used to obtain high-quality samples of heavily overconsolidated clays,for specialist laboratory testing. Both of the above methods can produce holes to great depths, whichcan be used for in situ tests as well as for sampling, and can allow the installation of instrumentation(for example, to measure groundwater pressures).Probing is increasingly being used as a cheap alternative to boring and drilling. It is used as aqualitative guide to the variation of ground conditions, and is particularly valuable for profiling. The
Boring, Drilling, Probing and Trial Pittingtechniques used are often fast, and are generally cheaper than boring and drilling, but they cannot beused to obtain samples or to install instruments.Finally, examination in situ, by trial pits and shafts, provides by far the best method of recording boththe vertical and lateral ground conditions. Borehole methods generally only take restricted samples,perhaps at every metre or so of depth, for engineering description. Rotary coring normally attempts torecover continuous core, but cannot give a guide to lateral variability, and gives only restrictedinformation on discontinuity patterns in the rock. But trial pitting allows continuous description of soilconditions over the entire face of the pit or shaft, allows measurements of discontinuities in rock, andin addition permits very high quality samples to be obtained.An understanding of these techniques is important not only because they represent the major elementof cost in a ground investigation, and must therefore be used with care, but also because the way inwhich they are selected and used can have a great effect on the quality of site investigation.BORINGA large number of methods are available for advancing boreholes to obtain samples or details of soilstrata. The particular methods used any country will tend to be restricted, based on their suitability forlocal ground conditions. The principal methods used worldwide are: light percussion drilling;power augering; andwashboring.Light percussion drillingOften called ‘shell and auger’ drilling, this method is more properly termed light percussion drillingsince the barrel auger is now rarely used with this type of equipment. The drilling rig (Fig. 5.1)consists of:1. a collapsible ‘A’ frame, with a pulley at its top;2. a diesel engine; connected via a hand-operated friction clutch (based on a brake drum system)to3. a winch drum which provides pulling power to the rig rope and can be held still with a frictionbrake which is foot-operated.The rope from the winch drum passes over the pulley at the top of the ‘A’ frame and is used to raiseand lower a series of weighted tools on to the soil being drilled. The rig is very light, and can bereadily towed with a four-wheel drive vehicle. It is also very easy to erect, and on a level site can beready to drill in about 15 mm. Where access is very limited, it can be dismantled, and rebuilt on theother side of an obstruction such as a doorway.In clays, progress is made by dropping a steel tube known as a ‘claycutter’ into the soil (see Fig. 5.2).This is slowly pulled out of the borehole and is then generally found to have soil wedged inside it. Theclaycutter normally has a solid or slotted weight, called a sinker bar, attached to its upper end, the topof which is connected to the winch rope. When the claycutter is withdrawn from the top of the hole,the soil is removed with a metal bar which is driven into it through the open slot in the claycutter side.In granular materials, such as sands or gravels, a shell is used. At least 2 m of water is put in thebottom of the borehole, and the shell is then surged, moving about 300mm up and down every secondor so. Surging the shell upwards causes water to be drawn into the bottom of the hole, and this waterloosens the soil at the base of the hole and forces it to go into suspension. As the shell is dropped onthe bottom of the hole the mixture of soil and water passes up the tube of the shell, past the simple2
Site Investigationnon- return valve (sometimes called a ‘clack’). As the shell is raised, the clack closes and retains thesoil, which precipitates above it.Fig. 5.1 Light percussion drilling rig (Pilcon Engineering Ltd).By repeatedly surging the shell up and down at the base of the hole, soil can be collected and removedfrom the hole. The casing should either be allowed to follow the hole down (if it is loose) or should bedriven so that it is just above the base of the hole, otherwise progress will be slow, and either largecavities will be formed on the outside of the casing or the soil will be loosened for a considerabledistance around the hole. Of course, casing is nearly always used with the shell, because most granularsoils will not stand vertically if unsupported in the presence of water.Casing is not only used when drilling in granular soils, but is also necessary when drilling in very softsoils or when drilling in clays, to seal off groundwater after it is encountered. The presence of water inthe base of the hole will allow samples to swell, but the reason that most drillers seal off water is morebasic: stiff plastic clays become difficult to recover with the claycutter if large quantities of water arepresent and if this water cannot be controlled the driller will usually be forced to drill more slowlyusing the shell.In the UK, where large parts of the South-east have stiff clays which provide ideal drilling conditions,the light percussion rig normally has 1000—1500kg capacity and most commonly uses 150—200mmdia. casing and tools. It will have little difficulty in boring to 45 m depth in a very stiff clay such as theLondon clay, but in sandy soils more casing sizes will often be needed to reduce friction.3
Boring, Drilling, Probing and Trial PittingFig. 5.2 Light percussion drilling tools.The friction transmitted by sand or chalk to the outside of casing will often be too great to allow therig to pull more than 10—20m of casing out of the ground without the use of short-stroke hydraulicjacks. Under these conditions strings of casing of different diameters are used to allow a greater depthof drilling. As an example, if a borehole were to be advanced to 50m in sand, the driller might start theboring using 300 mm dia. casing and tools and drill until the rig began to have problems pulling thecasing, which might occur at 15 m depth. At this stage the driller would insert a string of 250 mm dia.casing and pull back the larger casing 1 m or so to make sure that it would still be loose at the end ofboring the hole. The inner 250 mm dia. casing, of course, would receive no friction on the upper 14mof its length, and the hole could now be advanced until its second string became tight, when a 200mmstring of casing would be inserted at, say 30 m below ground level (GL). At the end of boring the holemight be cased with four different sizes, as in Table 5.1.The minimum casing size possible in Britain is 150mm dia., because this is the smallest size allowingthe use of the British Standard General Purpose 100mm dia. sampler (BS 5930). The casing used inthe UK is square threaded and flush coupled, in contrast to the drive pipe’ in use in the USA which is4
Site Investigationcoupled via a threaded external collar. This type of coupling can be particularly troublesome in sands,where the coupling considerably increases the difficulty in extracting casing at the end of drilling.Table 5.1 Example of casing for50m boreholeDepth (m) Casing (mm ingAugers may be classified as either bucket augers (Fig. 5.3) or flight augers. Bucket augers are similarin construction to the flat-bottomed Sprague and Henwood barrel auger. They consist of an opentopped cylinder which has a base plate with one or two slots reinforced with cutting teeth, which breakup the soil and allow it to enter the bucket as it is rotated. The top of the bucket is connected to a rodwhich transmits the torque and downward pressure from the rig at ground level to the base of the hole:this rod is termed a ‘Kelly’. Bucket augers are used for subsurface exploration in the USA, but arerarely used for this purpose in the UK. This is probably because they require a rotary table rig, orcrane-mounted auger piling rig for operation, and this is usually expensive to run. Casing alsoprovides some problems, since a single rig cannot drill in cohesionless soil beneath the water table.Fig. 5.3 Bucket auger.Flight augers may be classified as short-flight augers (Fig. 5.4) or continuous- or conveyor-flightaugers. Short augers consist of only a few turns of flight above cutting teeth or a hardened steel edge.A high-spiral auger may contain three or four turns of flight. The hole is made by forcing the augerdownwards at the bottom of the hole, while rotating it. The cutting teeth break up the soil or rock,which is then transferred up the auger flights. When the flights become full, or when the auger hasbeen advanced for the height of the flights, the auger is raised to the top of the hole and the soil flungclear by rapidly rotating it. Once again, the auger is supported by a Kelly rod which transmits thetorque and downward thrust from the drill rig to the auger.The principal limitation of short augering is that the hole depth is restricted to the length of Kelly rodwhich the rig can handle. For many of the rigs commonly in use this is only 3—6m. The use of acrane-mounted auger piling rig will allow holes to be drilled to 20—30m if a telescopic Kelly rod isfitted, but as already noted such rigs are very expensive.5
Boring, Drilling, Probing and Trial PittingThe problems of deep drilling with short augers are largely overcome by the use of continuous orconveyor augers. Continuous augers can be classed as: (i) solid stem continuous-flight augers (Fig.5.5); or (ii) hollow stem continuous-flight augers.Fig. 5.4 Short-flight augers and auger bits.Fig. 5.5 Mobile Minuteman’ small diameter solid stem continuous-flight auger rig.6
Site InvestigationSolid-stem continuous-flight augers allow much deeper holes to be drilled with fewer problems. Withthis type of auger the Kelly never enters the borehole, as the auger flights extend to above groundlevel. As the auger is rotated and pushed downwards the soil removed from the base of the hole travelsup the flights and emerges at the ground surface. Although this type of auger apparently overcomesthe problems found in drilling deep holes with short-flight or bucket augers, it presents a seriousproblem in site investigation because soil moving up from the base of the hole is free to mix with thesoil at higher levels on the edge of the borehole. Thus while auger tailings from short-flight augers orbucket augers may be fairly representative (even if highly remoulded), the soil emerging from the topof a continuous-flight auger will be of no use. In addition, in common with all the auger methodsabove, the need for casing in granular or other collapsing soils presents a problem. In fine-grainedsoils a casing can be inserted when collapsing soil is encountered, and can sometimes be advanced byjetting; but in coarse gravels the continuous-flight auger is unusable because it must be removed eachtime a sample or in-situ test is to be carried out. At this stage the hole will collapse.Hollow-stem augers (Fig. 5.6) consist of an outer spiral continuous flight with a separate inner rodwhich blocks off the base of the hole when the auger is being advanced. Both the outer flights and thecentre plug are furnished with a bit at the base. The auger is forced into the ground in the same way asa solid-stem auger, with the inner and outer sections rotating together. When samples are required, theinner rods and plug are removed and samples can be taken from the material below the base of theauger. Hollow-stem auger drilling would at first sight seem to be the ideal method of producing siteinvestigation holes, because it is often fast and reliable. There are, however, several problems whichshould be considered.Fig. 5.6 Acker ADII drilling rig and hollow-stem auger system.First, fissured clays or soils with fabric require relatively large samples for the determination ofundrained shear strength and consolidation properties. This means that the hollow stem of the augermust have a large internal diameter (typically 140— 150mm to allow the use of U100 sampling). This7
Boring, Drilling, Probing and Trial Pittingin turn means that a relatively powerful and therefore large drilling rig is required. Even if such a rig isavailable, access to the site of the borehole may be a problem.Secondly, there are considerable dangers of disturbing soil ahead of the auger if the driller is overeagerin soft or firm soils. Heavy downward thrust may cause the auger to be forced into the soil, displacingmaterial ahead of it instead of boring through it. The hand auger provides a light, portable method ofsampling soft to stiff soils near the ground surface.At least six types of auger are readily available: posthole or Iwan auger;small helical auger (wood auger);dutch auger;gravel auger;barrel auger; andspiral auger.Figure 5.7 shows a selection of these augers.Fig. 5.7 Selection of hand-operated augers.8
Site InvestigationHand augers are used by one or two men, who press down on the cross-bar as they rotate it thusadvancing the hole. Once the auger is full, or has collected sufficient material, it is brought back to thesurface and the soil removed. Although the method is cheap because of the simplicity of theequipment, it does suffer from several disadvantages.The most commonly used auger for site investigation is the ‘Iwan’ auger. This is normally used atdiameters of between 100 and 200 mm. Small helical augers are quite effective in stiff clays, butbecome difficult to use once the water table is reached.Barrel augers are now rarely seen, but were formerly used with the light percussion rig when progressthrough clays was made using a shell. They allowed the base of the borehole to be very effectivelycleaned before sampling took place. Because they are heavy they require a tripod for raising andlowering them in the borehole. When lowered to the bottom of the hole they were turned by hand (seeHarding 1949).In stiff or very stiff clays, hand-auger progress will be very slow, and the depth of boring may have tobe limited to about 5 m. When such clays contain gravel, cobbles or boulders it will not normally bepossible to advance the hole at all. In uncemented sands or gravels, it will not be possible to advancethe hole below the water table, since casing cannot be used and the hole will collapse either on top ofthe auger (which makes it difficult to recover the auger from the hole) or when the auger is beingremoved. Only samples of very limited size can be obtained from the hole. In addition, it will not bepossible to carry out standard penetration tests without a frame to lift the trip hammer and weight, sothat no idea of the relative density of granular deposits can be obtained.Despite these difficulties, where access for machinery is impossible the hand auger may give valuableinformation.WashboringWashboring is a relatively old method of boring small-diameter exploratory holes in fine-grainedcohesive and non-cohesive soils. It was widely used in the USA in the first half of this century, but hasbeen largely replaced by power auger methods. It is still used in areas of the world where labour isrelatively cheap, for example southern Brazil.A very light tripod is erected, and a sheave is hung from it (Fig. 5.8). In its simplest form there are nomotorized winches and the drilling water is pumped either by hand, or by a small petrol-driven waterpump. Hollow drilling rods are connected to the pump via a flexible hose, and the drilling crew lift thestring of rods by hand, or using a ‘cathead’ (a continuously rotating steel drum, around which amanilla rope is wound).Progress is made by jetting water out of a bit at the base of the rods. These are continuously turnedusing a tiller, whilst being surged up and down by the drilling crew. Cuttings of soil are carried up thehole by the drilling water (the ‘flush’) and emerge from a casing T-piece, being deposited in a sump.Routine identification of the ground conditions at the base of the hole is carried out by the drillerplacing his hand under the T-piece to collect a sample of cuttings.Hvorslev (1949) commented that:Drillers with adequate experience in washboring can determine changes in and estimate the general character ofthe soil with satisfactory accuracy, especially when both the drill rod and the pump are operated entirely byhand. On the other hand very serious mistakes may be made by inexperienced or careless drillers, who often failto recognize changes in the character of the soil, do not clean the borehole properly. and take samples of thecoarse segregated material settled at the bottom, instead of the undisturbed material below the bottom. Theresults of such errors are very misleading soil profiles which often indicate strata of coarse materials at depths9
Boring, Drilling, Probing and Trial Pittingwhere soft soils of low bearing capacity actually exist. The method should not be used above ground-water levelwhen undisturbed samples are desired of the soil above this level, since the water will enter the soil below thebottom of the hole and change its water content.Fig. 5.8 Washboring rig (based on Hvorslev 1949).DRILLINGRotary drilling uses a rotary action combined with downward force to grind away the material inwhich a hole is being made. Rotary methods may be applied to soil or rock, but are generally easier touse in strong intact rock than in the weak weathered rocks and soils that are typically encounteredduring ground investigations. For a detailed description of equipment and methods the reader isreferred to Heinz (1989).Rotary drilling requires a combination of a number of elements (Fig. 5.9):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 partsand 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 flushfluid from the flush pump to the bottom of the hole; and4. a drilling tool, for example a corebarrel, which grinds away the rock, and in addition may bedesigned to take a sample.10
Site InvestigationFig. 5.9 Layout for small-scale rotary core drilling.Open-holingRotary methods may be used to produce a hole in rock, or they may be used to obtain samples of therock while the hole is being advanced. The formation of a hole in the subsoil without taking intactsamples is known as ‘open-holing’. It can be carried out in a number of ways, but in site investigationa commonly used tool is the ‘tricone rock roller bit’ (or roller core bit) (Fig. 5.10). In site investigationsuch methods are usually used to drill through soft deposits, which have been previously sampled bylight percussion or auger rigs. Sampling during open-holing is usually limited to collecting thematerial abraded away at the bottom of the borehole, termed ‘cuttings’, as it emerges mixed with‘flush fluid’ at the top of the hole.11
Boring, Drilling, Probing and Trial PittingFig. 5.10 Bits for rotary open holing.CoringThe most common use of rotary coring in ground investigations is to obtain intact samples of the rockbeing 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 theborehole when the corebarrel is full. The length of core drilled before it becomes necessary to removeand empty the corebarrel is termed a ‘run’.Coring equipmentThe manner in which the rock is abraded away, and by which the cuttings formed by this process aretaken to ground level having been discussed, it becomes necessary to discuss the machinery used forthe job. At the base of the borehole a bit is rotated against the rock, thus advancing the hole. This bitcan be either solid or annular, depending on whether a sample is required. Annular corebits arescrewthreaded to the bottom of a ‘corebarrel’, of which Fig. 5.11 is a typical example in use in siteinvestigation. 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 todeliver torque and downward force to the bit (via the corebarrel) while at the same time providing theflush 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 chuckgrips the drill rods or ‘Kelly’ and transfers longitudinal and rotational movements to the rods. TheKelly continues upwards and is connected to a ‘water swivel’ or ‘gooseneck’, which connects thewater or flush hose from the flush pump while allowing the rods to rotate and the hose to remainstationary (Fig. 5.9).Drill rigs may vary considerably in size and design. Some of the smallest (for example the Acker 1200PM) mount directly on top of 2 12 —4 in. drill pipe (casing) installed by other means to rockhead. Theyconsist of a small four-stroke petrol engine, typically of less than 10 h.p., which connects via agearbox to the top of the rods. The water swivel is built into the machine, and feed is controlled by amechanical system operated by a handturned wheel. Quite clearly, such a rig has a very limitedcapability. The load applied to the bit cannot be controlled, and the rig has no inbuilt hoist for liftingthe drilling equipment out of the hole.Most rotary drilling rigs used in site investigation tend to be rather small, when compared with thevery 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;12
Site Investigation4. a mast or tripod; and5. 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. Insoft rocks, the use of excessive pressure will fracture the rock before it can enter the corebarrel, whilein hard rocks the use of low feed pressures will result in very slow drilling progress. In very softdeposits the weight of the rods and barrel may be sufficient to fracture the rock and the hydraulic feedmay 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 andconvenience. Slow speeds of the order of 50 r.p.m., are required for augering, and open-holing withthe tricone. Faster speeds, of up to 1000 r.p.m. may be used for rotary coring, depending on the rocktype 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 drillingtools in or out of the hole. A rope attached to the tools or rods requiring lifting is taken up the rig to thetop of the mast or tripod, passed over a pulley, and then brought down to the cathead. The catheadconsists of a drum which rotates at constant speed. The rope is given two or three turns around thecathead, but because the drum is smooth it does not grip and pull on the rope. The cathead is made tolift 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 catheadnormally has limited lifting power, but perhaps more importantly, fine control requires considerableskill.In situations where greater lifting capacity or finer control of lifting are required a wire drum hoist isnormally 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 bevelgear, which drives an octagonal spindle. Hydraulic feed is then developed by a piston on each side ofthe spindle, which pulls the spindle down by acting on a crosshead. Rigs with this configurationusually have a limited stroke:Acker ADIIAcker HillbillyAcker TeredoCraelius D750Mobile B31Mobile B531.80m600—900mm600—900mm500mm1.73m1.98mSince the corebarrels normally used for rotary work in site investigation are 1.5 m or 3.0 m long theserigs cannot drill the complete length of the corebarrel without having to rechuck; that is to undo thechuck, move it up the rods and reclamp it. To do this, rotation of the corebarrel must be stopped andrestarted. This inevitably leads to the exposure of the rock being drilled by the bit to the flush fluid fora longer period than during drilling, and any bad effects of the flush fluid will be emphasized at pointson 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 rocksthan the type of rig described above. One type of machine which provides a very long stroke for coredrilling is the Acker MPIV hydraulic top drive rig. The rotary action is provided by an hydraulicmotor, connected to the engine by flexible hose, which can travel long distances up the mast. The feedis provided by a mechanical system. The Pilcon Traveller 30 and Traveller 50 rotary drilling rigs areexamples of lightweight machines capable of drilling a 3 m run without rechucking.13
Boring, Drilling, Probing and Trial PittingThe most common mounting options for site investigation are skid mounting, trailer mounting andlorry mounting. In the UK access is normally poor and many contractors use either trailer or skidmounting. In the Middle East and the USA many more rigs are lorry mounted.BARRELSThe corebarrel is the normal equipment for recovering samples of rock in site investigation. In itssimplest form (as used, for example, to obtain cores of concrete), the corebarrel consists of a singletube with an abrasive lower edge which is loaded and rotated while a flush fluid is passed around thebit under pressure. In this process, first the core inside the barrel is subjected to rotative forces due tothe friction of the inside of the barrel against the outside of the core, because the core (being attachedto the parent material) does not rotate. Secondly, the flush fluid passes over the surface of the corecontinuously 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 asbedding planes in rock. When rotation of the upper part of the core occurs at such a discontinuity aconsiderable length of core may be ground away, and a distinctive pattern of circular striations (oftencalled 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 willbe particularly serious in soft rocks, where the flush fluid (particularly if water) will tend to soften theoutside or a
As the shell is raised, the clack closes and retains the soil, which precipitates above it. Fig. 5.1 Light percussion drilling rig (Pilcon Engineering Ltd). By repeatedly surging the shell up and down at the base of the hole, soil can be collected and removed from the hole. The casing shoul
