Where there are gaps in site knowledge it is likely that some form of physical investigations will be undertaken, whether to fill these gaps or to investigate a virgin area. While there any many possible means of site investigation, as outlined in the previous article, boring a hole and testing the properties of the surrounding ground and/or any samples extracted, is still the usual and arguably the most informative approach.
Drilling information
To the uninitiated, drilling a hole to find out what lies beneath may seem a simple matter once the location of the head of the bore is know, but there are many factors affecting the accuracy and relevance of data that can be obtained by this apparently simple act. The choice of equipment will be greatly affected by factors such as:
• Location of the borehole (determined accurately by surveying or GPS) and the terrain there,
• Depth from which information required,
• Type of ground likely to be encountered e.g. hard, soft, abrasive, fractured, waterlogged,
• Physical property data required,
• Accuracy required including hole orientation,
• Whether a permanent sample is required (e.g. a core) for lab testing or records, and from which horizon(s),
• Speed information required
Location
If the required drilling location is on relatively flat, accessible terrain, it should be no problem using a high-capacity, powerful rig, perhaps truck-mounted for mobility. If a mud drilling system is required this will reduce mobility to allow for the additional mud treatment system etc.
On rougher terrain a drill rig with a crawler carrier will be necessary. Even so the capacity and other capabilities of the rig will be roughly proportionate to the size of the rig, which, in turn, will affect its ability to access the desirable drilling site on rough terrain and remote locations.
Sometimes information will be required from beneath a stretch of water, typically for planned tunnelled crossings of a river or other waterway, or for special major structures such as the planned Thames Water’s Thames Tunnel in London. If there is room on the waterway, taking into consideration such factors as required navigability, a barge with jack-up legs can be used to house, transport and act a platform from which vertical or near-vertical drilling can take place. If the water is not accessible, then the strata from which information is required will have to be reached from a ‘dry’ location. This can be achieved by an inclined hole from the surface, a horizontal bore from an underground or open-cut excavation, or by directional drilling (see below). In all cases it follows that borehole surveying, whether post-drilling or by measurement while drilling (MWD), will need to be accurate to determine the correct location and orientation of the borehole, and hence the information being sourced.
Rigs
There are many different types of drill rigs suitable for some form of site investigation, with associated borehole equipment too numerous to mention all here. Prime movers can be from (on the surface) internal combustion engines with mechanical, hydraulic, electric or compressed air drives, and usually electric supply to hydraulic or air drives underground.
Most modern site investigation drilling operations will use lengths of metal or plastics drill pipe connected together to form the drill string, but much slower traditional cable drilling may be used for shallow holes or where the labour costs are low and there is little urgency. Casing will be used to support unstable bore walls.
Most modern, accurate site investigation rigs will have a rotary drive, either top-drive or a table Kelly drive, but non-rotary direct-push rigs and vibratory ‘sonic’ systems have limited applications.
Trends in the design of modern site investigation rigs have generally kept up with trends in all, chiefly surface, rigs for more efficient, environmentally acceptable drives, better safety as regards operator protection, and the prevention of excessive load handling by the introduction of mechanical drill-pipe handling devices etc.
Erik Mattsson, product line manager for Mustang geotechnical rigs at Atlas Copco comments that a rig’s chief function is as a tool-holder only, but one that must be capable of providing the right drilling data through accurate control of speed (rev/min) and proper drill string support to hold back the weight on the bit for accurate coring. Versatile rigs will be expected to be capable of auger-boring as well, in which case provision of the correct torque from the drill head will more important than speed.
"Very important in site investigation is easy set-up and transportation’, says Mattsson. This enables the drill carrier to visit a number of widely spaced sites, perhaps under difficult surface conditions, rather than the case with blast hole drill rigs that tend to operate in one area for longer periods.
"Another important rig design trend is ergonomics for a good drilling position", Mattsson points out. "The operator needs to have a great view of operations, but not too close to moving parts. Having the controls on an adjustable arm helps this, to give a suitable distance between the operator and the rig itself. Remote control systems are getting more popular but not yet for tramming. Normally a rig should be easy to use in awkward places, but we find that drilling location access problems are not a major issue with users. In fact we have several requests for truck-mounted rigs. Oscillating crawler carriers for rough terrain also make the rigs heavier for transportation or steep slopes."
Atlas Copco Mustang geotechnical rigs for site investigation are generally supplied on comparatively light crawler carriers for operation on rough terrain with rapid tramming and ease of transport between sites. The manufacturer’s Diamec rigs dedicated to coring with diamond or tungsten carbide coring bits are in smaller units, facilitating their use underground, chiefly in the mining industry, and so lending themselves to portability. Atlas Copco also supplies larger reverse circulation rigs for deeper bores with sampling of the drill chippings.
A rig capability for using the overburden drilling with eccentric bit (ODEX) method, or similar, for drilling below a casing in unconsolidated ground should aid the penetration of loose overburden preventing its collapse into the hole, and also preventing contamination of any sampling.
Coring
In many ways the key to high quality site investigation through drilling lies with what happens in the borehole in the correct position that is of interest. To achieve the maximum site investigation efficiency the equipment must be capable of testing as many physical properties as possible within the holes coupled with accurate indexing of hole orientation so that, where necessary, the results can be presented on the correct orientation. For tunnelling this may be particularly important to access jointing and fractures that may affect suitability for mechanical cutting in tunnel excavation, or the necessary support characteristics.
Versatility in testing for ground properties in the hole also minimises the need for time-consuming removal of the whole drill-string.
Coring tends to have limited use in site investigation (as distinct from mining sampling) due to the additional expense, but it does provide a physical sample that can be subjected to more accurate testing in a lab, and provide a physical record if required. The same criteria of accurate recording of the coring horizon and orientation apply as far in-hole testing. Unless the ground consists of competent rock with few fractures, special equipment may be required to recover a useful core in soft or incompetent ground.
Peter Hedenstedt, product line manager for Terracore products at Atlas Copco explains, "There is a bit variation in the type and material of site investigation bits depending on the expected rock type including those made of tungsten carbide, diamond, and polycrystalline diamond (PCD). But what you want is undisturbed core sampling, with the material as close as possible to the conditions of its natural environment. The trouble is a core is usually disturbed as soon as it starts to be hoisted to the surface, especially in unconsolidated ground. The wireline system with core catcher was developed to allow cores to be recovered without moving the drill string tubes, which would otherwise causing further disturbance and delay. The drill rods and the outer tube thus serve as casing and remain in the hole during round trips."
There have been a number of developments since wireline was first introduced, all aimed at improving the quality and performance of core recovery. There have and continue to be efforts aimed at increasing the versatility of equipment used. One of these is the Terracore Geobor S system that can be used in seven different modes of operation (representing different testing and sampling techniques), and another soon for release. The system can be used in a wide range of ground types with various rotary drill rigs provided the torque capacity is sufficient for the type of rock formation and the required hole depth, available pull down force is 4-5t, pullback force 6-8tf, and available rotary speed 25-300 rev/min. The rig must also have a wireline hoist and suitable flush pump. Core barrels are available in lengths of 1500mm and 3000mm.
The Geobor S employs are larger than usual core barrel (146 – 150mm hole diameter) that can accommodate a 4-inch (102-mm) diameter core. Using a triple-tube core barrel in the Geobor S, the inner tube has a liner to hold the core, and with the wireline just attached to the liner. Glands are often installed at each end of the liner to hold in any moisture, and the wireline used to recover the core-holding liner. In unconsolidated ground this achieves the aim of core recovery as near as possible to its natural composition and with minimal or no disturbance. Sampling can be carried out throughout the horizontal 360 deg. If underground drilling requires inclined holes, the inner tube/sampler can be pumped into the hole with the flush pump.
The bottom of the Geobor S core barrel can be equipped with different tools (forming the seven to eight different methods of operation), selected according to the formations to be penetrated. "One method," says Hedenstedt, "is to obtain a sample by punching a hole in soft enough ground, but this is considered rather old fashioned now, with more sophisticated and accurate methods being available such as the Devico methods and those of Reflex Drilling in Australia."
The Geobor S system has been used extensively, by various site investigation contractors, in the construction of the German DB high-speed railway network with its numerous tunnels. Drilling depths have been typically around 120m, but down to 200m maxima. There have also be some horizontal holes of up to 130m length. Tectonic movements have damaged the sedimentary ground with some basalt layers in several locations.
Although not site investigation for tunnelling, Fugro Seacore employed its 8-leg jack-up platform for investigations in the Irish Sea for proposed salt-dome gas storage caverns. The contractor used an oversize (197mm diameter) Geobor S bit inside a 9 5/8-in. casing to produce 660m of core with a 97 per cent core recovery rate.
The smaller rigs that may be associated with TBM probe drilling processes, or for some other underground duties, tend to have smaller coring equipment of around 60mm diameter.
Where?
A development of coring systems developed by Viktor Tokle of Norway is the Devico directional coring system.
The principles of directional drilling were originally developed in the oil and gas industry, chiefly for exploiting fields across a wide area from one location, or to access reserves where vertical drilling would be problematical.
It has now been developed in to a range of applications including site investigation, but necessitating a very accurate method of hole position monitoring, which is available with the use of Devico borehole surveying instruments.
The system offers multiple ‘sidetracks’ for investigating in different directions and depths, with full control of deviation. Less set-up positions reduce exploration time and costs, and reduce environmental impact.
Testing
Apart from coring and other sampling techniques, modern in-bore instrumentation systems owe much to developments for the oil and gas industry.
While this industry can more readily afford to investment in such sophisticated technologies, some filter through to tunnelling projects, especially when they are on a grand scale.
One of the leading developers in this context is Schlumberger which, incidentally, has just purchased the Smith drill tool company. Its latest introduction is MicroScope 475 that provides ‘key’ information for formation evaluation and fracture identification. It represents a high-resolution resistivity and imaging-while drilling service for logging data.
Probing ahead
Once tunnelling has commenced, whether the main project excavation or in the form of a pilot bore, it may be found that the ground conditions are not what was previously expected. This may be due to a previously undiscovered fault zone or fissure allowing water into the excavation.
In such cases probe drilling ahead of the tunnel face may be required. To some extent probe drilling can be carried out by conventional blast-hole drilling rigs, using the performance indicators available on modern rigs to assess the ground conditions ahead. Similarly, with TBM drives, the TBM itself may carry small probe drills to investigate the ground ahead.
For more extensive investigations at greater hole lengths the normal excavation equipment will have to be withdrawn and a specialist rig installed, this will give the additional facilities of coring and longer holes, probably with larger diameters.
In some cases such additional site investigation may be accompanied by grouting operations to consolidate any poor ground found in advance of the tunnel.
