Reliability of Percussion Drilling as a Mean to Define Rock Surface

In Finnish ground investigation methods percussion drilling has an important role as a mean to define the rock surface. Ground conditions in Finland vary a lot, however, it is quite common that the soil layers are relatively thin and the rock surface lies in around 3...5 meters depth. Besides that the rock is usually very hard, so it has to be blasted away in many cases. Not only is blasting much more expensive than excavation, the level of rock also often determines the foundation depth and the design solutions. Therefore, the knowledge of rock surface is essential before construction work starts. Based on the pointwise ground investigations, like percussion drillings and measurements, the rock surface is normally modeled as three dimensional soil surface in design programs, where the surface geometry is described as triangulated irregular network (TIN). Geotechnical solutions, and the calculation of costs and quantities are often based on this model and thus, it should be as accurate as possible. The aim of this study is to analyze how reliably the rock surface can be modeled with percussion drillings, and also how the density of drillings affects the reliability. The study will be based on the statistical data from four Finnish construction sites, situating in different parts of Finland. Monte Carlo method is implemented in the statistical studies to define the effect of the amount of drillings. The goal is to determine proper drilling densities for different situations. The analysis includes the measurement errors and variability of the ground conditions. The results are studied in terms of relative volume differences, which is useful indicator in quantity calculations. Based on the results, percussion drilling grid density of about 15 m x 15 m provides a sufficient estimate of rock surface profile in most cases. If more accurate results are needed, drilling density should be adopted appropriately.