Most tunnel constructions were carried out under the ground conditions of igneous rocks (such as granite) and metamorphic rocks (such as gneiss), since these are the most abundant rock types in Korean peninsula. The results of ground surveys including physical explorations performed in design stage and of drilling speeds recorded during tunnel excavation have been accumulated; which might be greatly helpful when designing and constructing new tunnels in those rock conditions. Recently, on the other hand, tunnel construction has often been carried out in areas where sedimentary rocks (especially, conglomerate) exist such as Gyeonggi and Sobaeksan constituting 20 % of Korean geology), and in areas of sedimentary basins such as Okcheon and Gyeongsang. However, data accumulation and/or researches on geological characteristics of sedimentary rocks with a viewpoint of design and construction of infra-structures are still lacking. Therefore, this study focused on studying the variation of elastic wave velocities depending upon rocks types (especially focused on sedimentary rocks) to be used for tunnel design, and on studying the variation of tunnel drilling speeds according to rock types.
The geotechnical investigation reports collected at 66 construction projects were reviewed which were prepared over the past 20 years; and 214 sets of drilling core data of granite, gneiss and sedimentary (conglomerate) rocks were evaluated. The effect of rock types on the elastic wave velocity was analyzed using these collected data; i.e. collected data of elastic wave (P wave and S wave) velocity were analyzed and correlated with uniaxial compressive strength (UCS) values and RQD (rock quality index) values depending upon rock types. Analysis results showed that the Poisson’s ratio was the smallest in gneiss and the largest in conglomerate. The elastic wave velocities in the conglomerate were lower than any other rock types even with the same RMR-UCS term values and RQD values. This tendency should be taken into consideration when classifying the rock mass using P-wave velocity in case of conglomerate and thereafter even in design and construction.
The blasting hole drilling and charging work in tunnel excavation using, so called, “NATM” is an important process that occupies more than 30% of the total tunnel excavation time. Recently, efforts were made to predict the ground condition and RMR value in front of the tunnel face utilizing the hitting pressure and the drilling speed obtained during the drilling process in the NATM tunnel job sites. However, most of the previous studies have been conducted only on the igneous rocky ground represented by granite. In this thesis, the drilling speed of igneous rocks with similar range of RMR values was compared and analyzed with that of sedimentary rocks (especially conglomerate, sandstone, and shale). Even with the similar RMR values, it was found that the drilling speed could be greatly changed according to rock types. The drilling speed in sedimentary rocks was faster than that in igneous rocks. While the drilling speed in igneous rocks was not much dependent upon RMR grades; in sedimentary rocks, it was found to be increased as the RMR grade was lowered.
In summary, it was found that the conglomerate shows a little bit different ground characteristics (in terms of elastic wave velocity, Poisson’s ratio, RMR value, tunnel drilling speed, etc.) compared to igneous rocks and metamorphic rocks; further collection of field data may be required to improve and update the currently evaluated characteristics of sedimentary rocks, more specifically, that of conglomerate.