Geotechnical parameters evaluation using Multichannel Analysis Surface Waves (MASW) in the upstream basin of Chaq-Chaq Dam in Sulaimani, NE Iraq

Abstract

The velocities of the compressional and shear waves (VP and VS) were determined for seven parallel Multichannel Analysis of Surface Waves (MASW) traverses within the upstream basin of Chaq-Chaq Dam in Sulaimani, Kurdistan Region- Iraq. These traverses were executed with a spacing of 2 meters (m) between the geophones. The objective of this study was to investigate subsurface geological features, determine the depth to bedrock, assess bedrock strength, calculate primary and shear wave velocities, and explore potential causes for the collapse of the constructed Chaq-Chaq Dam. Data was collected using Geometrics Seismodule Controller software and processed using ParkSeis Software. The velocity results indicate that the study area comprises three geological layers: a surface soil layer on top, a middle layer of rock fragments, and a limestone layer at the bottom. The calculated Poisson’s ratio based on VP and VS values for these three layers ranges between 0.35 and 0.39 for the first layer, 0.30 and 0.33 for the second layer, and 0.24 and 0.25 for the third layer. These results suggest that the first layer has relatively high values of Poisson’s ratio, indicating an incompetent soil layer. The second layer exhibits moderate values, suggesting a fairly competent layer of rock fragments, while the third layer has low values, indicating a well-competent limestone layer. The findings also highlight deficiencies in the dam’s design, particularly in the southern parts where most of the traverses show the presence of sinkholes. These sinkholes are likely caused by water infiltration during the rainy winter season. When water pressure is applied to the dam, these sinkholes in the southwestern portion could be attributed as the cause of the dam collapsing. Additionally, the dam side was not properly intertied with the rocks forming the right bank of the dam.