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Applying electrical resistivity tomography to the identification of endokarstic geometries in the Pleistocene Sites of the Sierra de Atapuerca (Burgos, Spain)

Fri, 01 Oct 2010 00:00:00 GMT

Applying electrical resistivity tomography to the identification of endokarstic geometries in the Pleistocene Sites of the Sierra de Atapuerca (Burgos, Spain) Ortega, Ana Isabel; Benito Calvo, Alfonso; Porres Benito, José Ángel; Pérez González, Alfredo; Martín Merino, Miguel Ángel In this paper we have applied the electrical resistivity tomography (ERT) in order to prospect and to analyse the morphological and geological subsurface of the Torcas-Cueva Mayor endokarst system (Sierra de Atapuerca). These works are essential to establish the development of the sedimentary infills where the Early and Middle Pleistocene archaeo-palaeoanthropological sites of the Sierra de Atapuerca are located. The prospecting was based on the elaboration of 15 ERT sections, which were interpreted using topographic, archaeological, geological and geomorphological data. Through this procedure we have indentified the endokarst morphologies and the main lithological groups. The latter correspond to the Upper Cretaceous limestones and dolostones (> 1500 ohm m−1) and Neogene sediments and Quaternary valley infills (< 400 ohm m−1). The endokarst structures inside the Upper Cretaceous carbonates were related to empty cavities (> 1500 ohm m−1), passages filled with speleothems (400–1500 ohm m−1and with detrital materials (< 400 ohm m−1), such as the deposits of the Dolina, Elefante and Galería sites. The analysis of these subsurface structures shows that the karstic passages present a regular south–north development, starting with subsurface faults detected in the north margin of the main valley (Arlanzón River) and finishing along the Pico valley headwaters. These passages were cut off by the Pleistocene incision of the Sierra de Atapuerca minor valleys (e.g. Propiedad valley), forming entrances to caves that were occupied by hominids and fauna from Early Pleistocene times.

Correlations between p-wave velocity and SPT-N for saturated and unsaturated alluvial clays using dimensional analysis

Thu, 01 Feb 2024 00:00:00 GMT

Correlations between p-wave velocity and SPT-N for saturated and unsaturated alluvial clays using dimensional analysis Ortiz Palacio, Santiago; Ibáñez García, Sergio; Sancho Calderón, Diego; Porres Benito, José Ángel The aim of this research is to establish practical and robust correlations between dilatational wave velocities (p-wave velocities, Vp) and the Standard Penetration Test blow count (SPT-N). Very low levels of correlation are recorded with the conventional single variable approaches, in consequence, a multivariable study is needed for higher precision. A dimensional analysis was conducted including other soil parameters that condition this physical phenomenon, such as the void ratio (e), the degree of saturation (Sr), the effective overburden pressure (σ′v), and the particle density (γs), for greater precision with Vp and SPT-N correlations. Over 100 seismic and SPT tests were performed around the coastal city of Juchitán de Zaragoza (Oaxaca), Mexico, to gather information for this research. As a result, two dimensionless indexes -DSIu (Undrained Dilatational Stiffness Index) and MDSIu (Modified Undrained Dilatational Stiffness Index)- yielded two alternative expressions between both indexes and Vp. In this way, a strong relationship between Vp and SPT-N was obtained with an adjusted coefficient of determination R2 value of 0.93.

An adjusted analytical solution for thermal design in artificial ground freezing

Sat, 01 Apr 2023 00:00:00 GMT

An adjusted analytical solution for thermal design in artificial ground freezing Sancho Calderón, Diego; Ortiz Palacio, Santiago; Ibáñez García, Sergio; Bock, Sven Artificial ground freezing is a widely used, reliable method for excavation in water-bearing ground. The questions posed in the thermal design of ground freezing projects require solving moving boundary (Stefan) problems. Approximate analytical solutions, such as the ones by St¨ ander1 and Sanger and Sayles,2 have been developed for thermal engineering design and are used by practitioners across the industry. For instance, Sanger & Sayles’ solution is widely used for the single-freeze-pipe problem, but it has proven to be of limited accuracy.3 In the present paper, an adjustment to this formula is proposed based on the re-evaluation of their empirical assumption that the ratio between the temperature penetration depth and the phase-change radius equals a constant value of 3 regardless the conditions. A sensitivity study is performed using a verified numerical model as a benchmark to study several problems with different initial and boundary conditions (initial, phase change and freeze pipe temperatures) and thermal properties of the ground (water content, thermal conductivity and heat capacity). This is done for the freezing times of 10 and 365 days, in order to consider the potential change of the ratio with the freezing time. In this way, a calibrated formula is proposed to find appropriate values of this ratio and a suitable adjustment to Sanger & Sayles’ solution is determined. Adjusting Sanger & Sayles’ solution in this manner, a significantly higher and more consistent accuracy is achieved for different boundary and initial conditions. This accuracy improvement was checked for real conditions from an engineering project, which shows that the adjustment can be useful for thermal problems in engineering design of ground freezing.

The energy threshold in dynamic probing

Sat, 01 Oct 2022 00:00:00 GMT

The energy threshold in dynamic probing Ibáñez García, Sergio; Sagaseta, César; Fernández del Rincón, Alfonso In dynamic probing tests, penetration is closely related to the potential energy of the hammer (nominal energy). This energy stems from the mass and free fall of the hammer after being released from a certain height. Penetration depends on energy, although only on a portion of that nominal energy that is effectively transferred to the rods (ENTHRU) and, more precisely, the energy that reaches the cone (ENTHRUcone). ENTHRU can be measured by monitoring the upper part of the drive rods. To calculate ENTHRUcone, ENTHRU needs to be corrected in three ways. Firstly, the energy loss in the energy transmission through the rods has to be subtracted, as well as the energy loss due to the skin friction of the rods along the soil around them. It is also necessary to add the energy due to the rod weight penetrating the soil. The main hypothesis assumed and later experimentally proved in this paper is based on the fact that ENTHRUcone has to be greater than a certain value or minimum energy (energy threshold: Th) in order to be able to cause penetration. After analyzing more than one hundred blows with different hammer mass and drop height, a small but consistent Th and a linear relationship between energy and penetration beyond it were found. The energy that really produces penetration (ENPEN) will be ENTHRUcone, minus Th. This allows for improved energy corrections and correlations between results from various kinds of penetration tests.