Determination of soil and rock boundary and permeability in geotechnical studies of bed-rock

During geological and geotechnical studies in exploratory and grouting boreholes, field permeability tests are performed in soil sections using the Lefrance method and in rock sections using the Lugeon method. What kind of test should be done at the border between soil and rock, which has characteristics between soil and rock? In action, based on the determination of the geological log of the cores obtained from the boreholes, the type of materials of these boundary sections is determined as soil or rock, and based on that, the specific permeability test of soil or rock is determined and implemented. If these sections are neither soil nor rock, and unfortunately at present, there is no special permeability test for soil and rock boundary sections, which causes errors in determining the permeability of the layers. Usually, the layers from the surface to the depth are changed from soil to rock, and at the border of soil and rock, there is a gradual zone of weathered rock or soil with intermediate characteristics. This range, which is usually several centimeters to several meters thick, has a different permeability compared to its upper and lower layers, and it is not possible to determine it simply and with acceptable accuracy. Depending on the opinion of the logger and technician, the tested section is considered as soil or rock, and the permeability test is performed in the soil or rock at that section, and the error in determining the type of soil or rock of the section, the error in determining the type of permeability test creates, and as a result, an error occurs in determining the suitable bed-rock for the foundation of the structures, and it leads to additional excavations or the determination of weak rock as a strong foundation for the structures. Also, in determining the thickness of soil layers and their permeability in hydrogeology studies and investigations of foundation land subsidence, sufficient accuracy is not provided. It is suggested that in these sections, if possible, both methods of permeability testing are performed and compared with each other, and considering the geological log, one is selected as the permeability number of the section. In fact, with accurate control of the geological log, it is possible to get a better estimate of the type of layer in the direction of soil or rock or the boundary between soil and rock. In the meantime, the geological logger with extensive experience in drilling the boreholes should also consider the crushing and washing of the layers. Of course, based on the current criteria, it is not possible to obtain an acceptable impression of the boundary layers of soil and rock, and it seems necessary to invent more efficient methods at these stages. The results of this research are based on geological and geotechnical studies of several dam construction projects in Iran.