New method for selecting the location of additional drillings based on indicator kriging; case study Semilan dam

Defining indicator is used for classification of data, based on a threshold and converting qualitative to quantitative data. One of the applications of the indicatoring in estimation is indicator kriging. In the present study, two geotechnical parameters (Lugeon and RQD) from 27 boreholes, along with various structures of the Semilan dam were studied. Based on indicator functions, Lugeon parameter divided to four, RQD to three and various structures of the dam site to four indexes. Then, indicator variogram of each parameter was calculated and their variogram characteristics were extracted. The geotechnical parameters were estimated, by 3D kriging. Based on the average of kriging error (Lugeon and RQD) were also divided into four indexes. The function to locate additional drillings is based on maximizing parameters such as Lugeon, average of error estimation and index of dam structures; and minimizing RQD. The function to locate additional drillings is found by dividing the result of multiplying Lugeon, estimation error, and index of dam structures, by RQD. In this paper, based on the function of additional drilling locations, two extra boreholes were suggested for the studied dam site. It should be noted that new boreholes in early stage of drillings can more effectively reduces error estimation, compare to those drilled in the later stages. Hence, although adding two boreholes to 27 existing boreholes of Semilan dam cannot considerably decrease the error estimation, but effectively decrease the uncertainty around them.