Assessing the dam's stability of the pond at the 'El Bosque' renaissance garden (Bjar, Spain)

The renaissance botanical garden of 'El Bosque' in Bejar (Salamanca, Spain) presents a pond bounded by a dam in its western part. The latter is formed by two masonry walls interconnected by buttresses. Cubic spaces in between are filled with a variable grain-size material (silty sand) that allows limited water flow. In recent years, the southern part of the dam has experienced localized and random subsidence that jeopardizes the entrance to part of the garden. To regain access, a proper and reliable diagnosis of the origin, magnitude and relevance of the subsidence must be made. In this regard, we have undertaken a microgravity survey in the dam area to identify places with an anomalous distribution of the filling material in order to foresee further sinking or potential collapsing areas. The precise positioning (2 mm resolution) and accurate terrain correction needed in this kind of high-resolution gravity surveys (points every 1.5 m) were achieved by creating a detailed digital terrain model (cm resolution) with a remotely piloted aircraft. In addition, we performed three electric resistivity tomography (ERT) profiles at different levels of the garden: (i) on the dam itself; (ii) right on the foot of the dam and parallel to it (5 m below and similar to 17 m to the W); and (iii) a bit farther, but also parallel to the dam (8 m below and similar to 27 m to the W). The ERT profiles identified high conductivity in water-saturated areas and determined the paths that rainfall and pond's seepage water follow in the dam and its underground, formed by granites. The geophysical studies were paired with geotechnical analyses of the sunk materials. The study concluded that the thinnest fraction of the dam's filling material (i.e., silts) is being washed away, leaving behind sand with less density and stability, susceptible to collapse. Thus, the observed sinking is related to soil piping, that is to soil internal erosion and compaction issues that force the soil material to re-adjust geometrically and volumetrically.