24.8 Resonance drilling


Resonance drilling, also known as sonic drilling, employs adjustable high frequency sinusoidal vibrations of generally between 50 Hz and 150 Hz to advance drilling tools in soil and rock. The drill head contains an oscillator, which generates resonant energy that is transferred down the drill string to the drill bit. In soil the vibratory action causes the soil particles to fluidize and/or displace at the bit face and immediate surrounds thus reducing friction. The extent of the fluidized zone internally and externally around the bit depends on the ground conditions and can be significant. With application of feed force there is very fast penetration. Casing the hole as it advances is an integral part of the method.

Simultaneous rotation of the drill string is possible on some rigs which assists penetration enabling larger diameter holes to greater depths. The hole is usually advanced without flushing although small amounts of water can be added to assist penetration and cuttings removal in difficult soil conditions and rock. Borehole diameters generally range from 80 mm to 200 mm and depths in excess of 100 m can be achieved by the larger rotary-vibratory rigs.

The procedure should be as follows: to sonically advance the sampling barrel (with simultaneous rotation if appropriate where the capability is available) and then sonically override this with a larger diameter casing before returning the barrel to the surface for sample extraction. This process is repeated until the required depth of hole has been reached.

The sonic barrel can be lined with a rigid plastic liner in which the recovered material is retained after being brought to the surface. Alternatively, if the sonic barrel is unlined, the recovered material should be "extruded" by resonance into a flexible polythene "sausage" bag for examination and subsequent taking of disturbed samples. Recovery of approaching 100% of the soil or rock profile can be achieved in favourable circumstances. However, in some instances, resonance can induce high temperatures in the ground leading to a change in water content in the soil sample and loss of volatile organic compounds. The quality classes of the samples is discussed in 25.9.

NOTE Some proprietary systems allow for the use of conventional tube samplers or standard penetration tests between sonic runs or for rotary core drilling with the sonic element turned off. The "downtime" (time spent tripping out one set of equipment for another) associated with using a sonic rig combined with conventional rig sampling needs to be borne in mind when selecting this type of exploratory hole making equipment. The equipment can also be configured to drill open holes for example to facilitate the installation of instrumentation.

24.9 Cable percussion boring


Cable percussion boring is an adaptation of standard well-boring methods, and normally uses a mobile rig specially designed for ground investigation work. For most investigations, the rig has a winch of 1 tonne to 2 tonne capacity, which is driven by a diesel engine and a derrick of about 6 m in height. With many types of rig, the legs of the derrick fold down to form a simple trailer that can be towed by a light vehicle.

The drill tools, which are worked on a wire rope using the clutch of the winch for the percussive action, consist of the clay cutter for dry cohesive soils, the shell or bailer for granular soils and the chisel for breaking up rock and other hard layers. The sizes of borehole casings and tools are usually 150 mm and 200 mm. For special projects 250 mm and 300 mm are available. This gives a maximum borehole depth of about 60 m in suitable strata.

The clay cutter should be used in cohesive soil in a damp or dry borehole. Where the borehole contains water, it might be necessary to use a combination of the clay cutter and the shell in cohesive soil. The shell should be used in granular soils and there should be sufficient water in the bottom of the borehole to cover the shell (about 2,5 m); it is, therefore, necessary to add water to a borehole in order to bore through dry granular strata that require the use of the shell. When the boring advances below the water table granular strata, water should be added to the borehole to sufficient maintain water in the hole at a level above that of the surrounding groundwater. This reduces, but might not eliminate, the tendency for loosening the ground below the base of the hole by 'blowing'. The shell should be withdrawn slowly and, if necessary, an undersized shell should be used.

Cable percussion boring may be used for soil and weaker rocks. The clay cutter and shell bring up disturbed material which is usually sufficiently representative to permit identification of the strata.

NOTE 1 The method can conflict with the geotechnical objects of the investigation, which might require the borehole to be drilled either in a dry condition or with the water level in the borehole maintained at or above the natural groundwater level. In such cases it might be necessary to adopt a less efficient method of boring with the cable percussion rig, e.g. boring a stiff clay under water might need the use of the chisel and shell, or to adopt a different method of drilling such as mechanical augering.

NOTE 2 Cable percussion drilling can result in significant disturbance and mixing of soil samples. Often, it is not possible to clearly attribute returned soils to a specific geological horizon within the borehole.

Where there is a material need to identify, characterize and delineate contamination, the use of this method should be evaluated to ensure any samples collected meet the data quality objectives required to perform robust subsequent assessment of land contamination risks.

24.10 Mechanical augers


Mechanical augers for ground investigations normally use a continuous-flight auger with a hollow stem and these are suitable for augering in cohesive soils. When augering, the hollow stem is closed at its lower end by a plug, which can be removed so that the sampler can be lowered down through the stem and driven into the soil below the auger bit. The use of hollow-stemmed augers in cohesionless soils often presents practical problems because it might be difficult to prevent material from flowing into the hollow stem on removal of the plug. When rock is encountered, boring can be extended by core-drilling through the hollow stem. Typically, augers with hollow stems of approximately 75 mm and 125 mm diameter produce boreholes of about 150 mm and 250 mm diameter respectively, to a depth of 30 m to 50 m in suitable materials.

Continuous-flight augering needs considerable mechanical power and weight so the machine should be mounted on a heavy vehicle. The debris from drilling is brought to the surface by auger flights and gives only a very rough indication of the levels and character of the strata. A precise intermittent identification of the strata might be obtained from driven samples taken through the hollow stem of the auger.

In self-supporting strata, solid rods and a suitable auger tool may be used, the auger tool being drawn up to the ground surface each time it has to be emptied. Drive sampling and testing can be carried out in the borehole.