GNGTS 2015 - Atti del 34° Convegno Nazionale

152 GNGTS 2015 S essione 1.3 The wells have been realized in rotation with reverse circulation of fluid. The wells, with a diameter ∅ 800 mm, are 45 m deep. The blank casing and the bridge screens, butt welded, have diameters ∅ 406 mm. Extraction well has bridge Johnson screens placed between 16.00 - 40.00 m from ground level; while the injection well has bridge Johnson screens placed between 10.00 - 40.00 m from ground level. The gravel packing (siliceous gravel) was put between 10.00 – 40.00 m from ground level. A monitoring piezometer (Pz in Fig. 1B), with diameter ∅ 127 mm and depth of 30 m, was realized downstream from the well of restitution at a distance of 12 m. The piezometer was equipped with a multiparametric probe (“Schlumberger water service”), placed at a depth of 25 m from ground level, with the aim of monitoring the variability of water table and the temperature of the water. Measurements collected concern the following parameters: static level and temperature of water (Fig. 1C). Geological and geotechnical features of the subsoil. By the stratigraphy drawn up during the drilling of the wells and the piezometer, the subsurface can be so exemplified: - 0.00-21.00 m: gravel and pebbles with very dense sandy-silty matrix, interbedded by cemented gray layers (conglomerate); - 21.00-45.00 m: gravel with moderately compacted brown silty sand and subordinates pebbles. In Fig. 1D is shown a simplified stratigraphic cross section of the subsoil. The granular soil is characterized by a high angle of friction (expressed in terms of effective stress), and then by a high shear strength resistance. These geotechnical features are indicative of a load-bearing capacity of the soil where the high value cannot be reduced in any way as a result of any changes of the water table. Hydrogeological and thermodynamic features of the aquifer. The hydraulic and hydrogeological features of the aquifer are essential tools for the simulations of thermal motion. The aquifer is unconfined and the natural movement of the groundwater is towards NW-SE. The static level of the water table l s is equal to 18.50 m from ground level (value measured in November 2011). The hydraulic gradient i is equal to 0.00476; the horizontal hydraulic conductivity k x,y is equal to 4.3·10 -3 m/s; the vertical hydraulic conductivity k z has been assumed equal to 1/10 k x,y m/s; the radius of influence R estimated and adopted in the simulations is 100 m. The specific storage S s has been assumed equal to 0.0001 (1/m) and the specific yield S y has been assumed equal to 0.2. The hydrogeological characteristics of the aquifer were deducted by performing a pumping test (step-drawdown test). The pumping test was carried out in 4 steps, with the duration of 45 minutes each. The test allowed to obtain the critical discharge Q c 42 l/s according to the Dupuit equation. In particular, for a discharge of Q 12 l/s corresponds a lowering of 0.17 m; for Q 20 l/s corresponds a lowering of 0.30 m, for Q 30 l/s corresponds a lowering of 0.50 m and, finally, for Q 55 l/s corresponds a lowering of 1.60 m. The transmissivity T 2.38·10 -2 m 2 /s was calculated using the equation T 0.183·Q c /s, where s is the lowering of the groundwater during the time interval considered. The permeability K 9.31·10 -4 m/s of the aquifer was obtained by dividing the transmissivity for the thickness of the aquifer. The specific discharge was calculated with the expression q s Q/s, where s is the lowering of the water in the well and Q is the discharge. For Q 20 l/s and s 30 cm, we obtained q s 60 l/s·m. The specific lowering s* s/Q is equal to 15 m/m 3 /s for a discharge of Q 20 l/s and a lowering of water s 30 cm. In conclusion, at the maximum discharge operating of 12 l/s, the loss of linear load imposed by the hydrodynamic parameters of the aquifer (consequent to the laminar flow of the same) is to be considered negligible.