Comprehensive geotechnical study is carried out in northern part of Iraq for a site in Duhok Governorate / Kurdistan District, where an earth fill dam supposed to be constructed on Gomel River with discharge ranging between (3 – 4) and (40 – 50)m3/s in dry and rainy seasons respectively. The preliminary design of the dam is of a small type with (15) m. height. Geomorp hologically, the area consist of high, wide and comparatively large extended mountains separated by large synclines. Stratigra phically, seven formations ranging in age from Upper Campanian - Late Ma'astrichtian to Late Miocene - Pliocene in age with Quaternary sediments are exposed in the area. The site is evaluated by studying the core of eight boreholes with total length of (193) meters, four of them along the dam axis, one in the downstream, two in the reservoir area and one in the area where infrastructures related to the dam is to be construct. Each borehole is divided into geotechnical horizons depending upon the similarity of the engineering parameter values which reflect the intensity of discontinuity planes within the rock mass, beside core state. For rock mass evaluation, three engineering parameters, namely Rock Quality Designation (RQD), Fissuration factor (C-Factor) and Number of fractures per meter (F/m) are calculated, each one of them classify the rock mass into five categories from weak to excellent with numerical values (1–5) respectively, mean value of these ranks are used by equal weight with rock rating according to Uniaxial compressive strength to get final evaluation of the rock mass for each individual geotechnical horizon. Evaluation process identifies three rock mass states which are weak, moderate and good, with mean discontinuity spacing (20, 26, 44) cm, respectively, and with mean uniaxial compressive strength (19 and 47.4) MPa for moderate and good rock states. The main ruling factor is the intensity of the discontinuity plains rather than intact rock strength, where the whole rock mass shows relatively uniform intact rock strength.
Consistent and appropriate site investigation techniques were used to ensure that accurate reliable and representative data to be gained during site assessment processes where a small earth fill dam of (15) m. height is to be construct on Gomel perennial river with discharge range from (3 – 4) m3/s in dry seasons, and (40 – 50) m3/s during rainy seasons. The site is located in a mountainous area in the northern part of Iraq / Kurdistan district, Figure 1. The foundation rocks material consist mainly of claystone, marl, limestone and rare gypsum at depths, while the bulk of the abutment rock material is of limestone. The dam axis is located on Pilaspi Limestone Formation (Late Eocene), dipping about 12° towards south west. Subsurface geotechnical evaluation was conducted through studying the core of eight drilled boreholes with total length of (193) m., four of them along the dam axis, two in the upstream, one in the downstream and one in the area where the infrastructures related to the dam is to be constructed.