GEOSTATISTICAL ANALYSIS OF DISSOLUTIONING AND VOIDS IN THE FOUNDATION OF MOSUL DAM, IRAQ TO CHARACTERIZE SUBSURFACE CONDITIONS AND FACILITATE SEMI-QUANTITATIVE RISK ASSESSMENT

Karst development in the foundation of Mosul Dam, Iraq was analyzed to facilitate risk assessment in 2018. This involved an analysis of statistics from two similar datasets: 1) initial construction-era drilling and grouting from 1981-1985, and 2) exploratory drilling and remedial grouting in 2017-2018. Datasets used were permeability, cavity aperture, cavity frequency, percent cavity, massive grouting frequency and massive grouting by volume of solids. Statistical analysis was used in tandem with geospatial analysis to aid in engineering judgment of risk resulting from foundation rock behavior and movement of the active dissolution front beneath zones of dam-safety interest. Lateral and stratigraphic divisions of the dam by material properties were made from original exploratory boring logs. Grouting, permeability and cavity data was spatially associated in ArcGIS, and the resulting data was then organized and statistics were run in Access and Excel by horizontal reach and stratigraphy. The statistical and spatial analyses confirmed that a highly-developed network of karst features existed prior to the placement of the dam. It showed that the development of karst and the dissolution of gypsum and limestone has continued during the operational life of the dam to date, consistent with established models of solutioning under increased hydraulic head and cavity development in a multiple confined aquifer setting. The analyses showed cavity formation continued to progress in shallow areas with higher gypsum concentration, but it also showed aggressive dissolution in other rock types in the deep valley and the right bank that were weathered prior to dam placement, consistent with the established hydrogeologic regime. Analyses indicated intersected solution features in the 2017-2018 remedial program never reached the same volume of cavity space or permeability as was initially encountered during the first grouting of the project in the early 1980’s. Using a dataset from dam axis and downstream exploratory holes drilled in 2017-2018, a map approximating the karst surface within key gypsum horizons was interpolated in GIS. This map could be used to estimate the most active locations of karstification to aid in future risk assessment and identification of areas of monitoring or remediation interest.