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Finland

Fig. 9. Percussion sounding.

When using light rock drilling machines as percussion scunds, the rods belonging to the Swedish weight sounding eąuipment are usually used and a sharpened rod is used as a bit. The rods dc net rotate during the drilling.

6. Piston percussion drilling

6.1    General

Stony and dense noraines are extremely common in Finland. Neither the dynamie penetrometer nor the Swedish weight sound can usually per.etrate said deposits. The penetratior. depth of percussion sounding is also very limited. Therefore the use of piston percussion drilling has great-ly inereased particularly in the last few years also in the investigation of soil deposits. This method has also proved to be relatively reasonable in its expenses.

6.2    Description of method

Usually ordinary rock drilling for blasting devices operating on the percussion or rotation principle are employed in drilling in hard soil layers. The drilling units are madę for the most part as precision work. Agricultural and forestry tractors are usually used as a base. The drilling machines most in use are Atlas Copco BBE-53 and BBE-57 as well as Tair.rock L-^00 and ES-300 with relevant feeding devices. These are fastened to the base sc that they can be directed to differer.t inclinations either hydrauli-cally or even mechanically in older devices. In some units the compressors and rig boxes are placeó or.to the same base (fig. 10). They car. also be placed on a base of their own, in which case they are being towed by the tractor during transportation.

Fig. 10. Piston percussion drilling

6.3 Test procedurę and recording of test results

When the thickness of the soil layers and the ievel of bedrock is determined, the investigation is usually carried out with rods 0 1”, 1 IM" or 1 1/2" used in rock-drilling. The corresponding sizes of the drilling crow are 0 ^5, 51 or 6H mm. The crowns are eąuipped with hard metal pieces. When drilling deeper boreholes (> 20 m) or when drilling in very dense deposits, a casing pipę should be instal-led for the upper part of the hole. The jamming of the rods into the holes is prevented by means of the casing pipes. The size of the casing pipes is chosen to suit the rod eąuipment and type of crown used at the time. Wash water or air is employed in the drilling to lift the soil from the hole. The water or the air is fed to the borehole's bottom through a hole in the rods or through the casing pipę when such a pipę is used.