بررسی ویژگی‌های زمین‌شناسی مهندسی ساختگاه سد زیرزمینی اسلام‌آباد فارس

Introduction and Goal 
Underground dams are water structures that are built to block or divert underground and subsurface flows. The use of this technology is recommended for the management, control, protecting and developing small water resources in arid and semi-arid areas such as Iran. The watershed of the Islamabad Underground Dam, due to its mountainous location, lacks an alluvial groundwater aquifer, and groundwater resources are limited to subsurface water in the alluvial deposits of the streambeds or hard formations water. Therefore, the underground dam is one of the solutions for providing water in such conditions. The engineering geological characteristics of the rock mass and alluvium of the streambeds have a direct impact on the success, safety, and sustainability of underground dam projects. Therefore, in the selection of the dam site, geological and geotechnical investigations and studies are carried out with the aim of determining the geological structures, stratigraphy and lithology of formations, discontinuity and faults, grain size, hydrodynamic characteristics and thickness of bed alluvium, the location and conditions of underground water at the dam site and its reservoir area, in order to estimate the volume of the reservoir and its sealing. The inefficiency and failure of dam projects are often caused by insufficient engineering and geological investigations in the pre-construction stages.
Materials and Methods
The catchment area of the Islamabad Underground Dam is located in Arsanjan County, Fars Province, in the high Zagros zone. The current shape of the basin and the numerous fractures in the study area depends on the characteristics of this zone and are affected by the state of the geological structures of the region. In this research, the hydrological groups of the soil in the watershed were evaluated with the aim of estimating the subsurface water and investigating the geological characteristics of rock mass engineering and bed alluvium at the dam site in order to estimate the volume and storage of water by the underground dam. Therefore, Initially, baseline information related to the region, especially its engineering geology, was collected and based on that, a proposal for the construction of the underground dam was considered. Using the results obtained from boreholes, field sampling, in situ tests, laboratory investigations and geology of the area, the geotechnical status and characteristics of the Islamabad underground dam construction site were investigated and evaluated.
Results and Discussion
In the study area, groundwater resources are limited to subsurface water in the alluvium of the streambed or hard rock formation water (fractured rock mass). At the site of the Islamabad underground dam, the most important geological factors affecting water storage was the condition and density of discontinuity surfaces (such as: faults and existing joint sets in the rock mass, as well as the grain size and density of the underlying alluvium). The result of the engineering fracturing investigations indicates the presence of several categories of fractures in the rock mass of the study area, particularly in the right abutment, with water flow potential. Field surveys and drilling results estimate the thickness of weathered rock at the site to be between 1 to 2 m. The alluvium at the dam site exhibits layering and has a thickness ranging from 6 to 10 m. The initial alluvium foundation, with a thickness of 1 to 1.5 m was coarse-grained, while the remaining thickness of the alluvium consisted of sand and gravel with a percentage of fine particles. The permeability of the alluvium foundation was due to moderate to relatively low compaction (approximately 10^-4 cm/s). The cohesion of the alluvium varies between 0.3 and 0.3 (kg/cm²), and the internal friction angle ranges from 28 to 39 degrees. The results of soil mechanical tests on alluvial samples indicate that the alluvium is coarse-grained, and its shear strength parameters suggest adequate soil stability concerning excavation and settlement.
Conclusion and Suggestions
The results of the fracturing analysis and statistical of the engineering characteristics of fracturing not only reveal the potential for water escape in the right abutment but also indicate that the behavior of the rock masses on the sides of the dam foundation is somewhat different in response to the tectonic stresses of the region. This phenomenon was due to the thickness of the layering, the degree of compaction, and the percentage of clay in the body of the limestone rock masses, as well as the alternation of hard layers with pliable and relatively soft marl or shale interlayers. The results of the engineering classification of the rock mass using the RMR method show that the rock masses at the dam site fall into the category of suitable rocks for construction purposes. The Atterberg limits and the cohesion of the soil at the dam site indicate that the clays present in the alluvial foundation primarily exhibit the characteristics of kaolinite and illite, with limited swelling and cohesion potential. The results of the chemical analysis of the water shows that sodium was dominant cation in the chemical composition of the groundwater in the region. Therefore, this finding indicates the potential for divergence in the fine-grained alluvium foundation. The thickness, density, and grain size of the alluvial foundation, along with the results from the Luferan test, indicated that the alluvial material foundation has moderate quality in terms of storage coefficient, storage volume, and water yield for the construction of an underground dam. Therefore, based on these findings, the most suitable method for utilizing the water catchment is the sub-drainage method by creating drains along the catchment, and the well pumping method would not be appropriate.