Engineering Geology and Site Investigation Part 3
Total Page：16
File Type：pdf, Size：1020Kb
Engineering geology 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 soil 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 borehole about a large bore hollow work (the field geologist's method). This process. Submerged sands or gravels 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 gravel 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 Boreholes 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 clay, 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 trial pit 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 drilling rig, 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 soils 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 core sample. 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 cohesion, including clay- vestigation, accounting for probably well 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 Geotechnical Engineering 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 cut 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 drill 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.