change of hydraulic conductivity of cohesive soils due to consolidation treatments with expanding resins

Settlements of ground subjected to foundation load are often due to both natural and/or anthropogenic causes, such as differential consolidation, seepage, periods of drought alternating with heavy rainfalls, growth of tree roots, sewer leaks, vibrations induced by vehicle traffic, excavations, or construction defects. Techniques of soil consolidation and stabilization by means of injection of polyurethane expanding resins have been efficiently used to mitigate or even solve this issue. The efficiency of the resin injection treatment has been well documented also thanks to the develop of a procedure of planning and monitoring which combines traditional geotechnical tests with the application of electrical resistivity tomography, which is strongly sensitive to the presence of water or voids. The combination of these survey methodologies allows to recognize the effect of injection in terms of soil compaction and water migration, a key factor for understanding the phenomenon and planning of resolutive interventions. The present study follows a previous work aimed at evaluating the increase in the geotechnical performance of cohesive soils treated by injection of expanding resins (same soil, resin, at the same site) and integrates it by focusing on the quantification of the effects of consolidation treatments, by means of expanding resins injection, on hydrologic properties of cohesive soils, focusing on saturated hydraulic conductivity measurements. Laboratory permeability test, performed in triaxial cell at different stress conditions and hydraulic gradients, were performed on undisturbed samples collected on "natural (non-treated)" and "treated soils", before and after the injection of resin (MAXIMA ® by GEOSEC ® ) in a full-scale test site characterized by the presence of continuous and homogeneous cohesive soils, (clay and silts). The investigated soil is classified as CH or CL soil, very firm and apparently overconsolidated. The study shows how the treatment is able to modify the characteristics of hydraulic conductivity of the soil. The injected resins partly follow pre-existing weaknesses and partly impregnate homogeneous and continuous masses of soil in a capillary way, giving it, in addition to the already documented greater mechanical strength, a lower hydraulic conductivity. The electrical resistivity investigations allow to appreciate the volumes affected by the treatment to which the variations of the measured properties can be attributed

Deep geophysical investigation in urban area: Ferrara city example

No abstract available

Time-Lapse 3D Electric Tomography for Short-time Monitoring of an Experimental Heat Storage System

A borehole thermal energy storage living lab was built nearby Torino (Northern Italy). The aim of this living lab is to test the ability of the alluvial deposits of the north-western Po Plain to store the thermal energy collected by solar panels. Monitoring the temperature distribution induced in the underground and the effectiveness of the heat storage in this climatic context is not an easy task. For this purpose, different temperature evolution strategies are compared in this paper: Local temperature measurements, numerical simulations and geophysical surveys. These different approaches were compared during a single day of operation of the living lab. The results of this comparison allowed to underline the effectiveness of time-lapse 3D electric resistivity tomography as a non-invasive and cost-effective qualitative heat monitoring tool. This was obtained even in a test site with unfavorable thermo-hydrogeological conditions and high-level anthropic noise. Moreover, the present study demonstrated that, if properly calibrated with local temperature values, time-lapse 3D electric resistivity tomography also provides a quantitative estimation of the underground temperature

Use of Ground-Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) to Study Tree Roots Volume in Pine Forest and Poplar Plantation

The evaluation of tree root biomass is significant and difficult to survey accurately. Traditional approach used for roots biomass harvest (e.g., soil cores and trenches) provide reasonable accurate information but they are destructive in nature, labour intensive, and limited whit respect to soil volume and surface area that can be assessed. Data derived from traditional root extraction approaches are also generally limited to root biomass averages across plots or treatments rather than information on root distribution. Sampling needed to detect difference among treatments can be expensive as well as time consuming for technical personal. For the above reason test and develop new indirect tools for roots biomass survey appears of leading importance. In this study we have assessed the possibility to use Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) as a root volume indirect survey investigation. Although previous studies have demonstrated the potentiality of these methodology to detect root systems (J. R Butnor et al 2001 J. Hruska et al 1999 L. Wielopolski, et al 2000) up to now few research (Stokes et al, 2002; ) has tried to compare the GPR and ERT response with direct observations of the entire root system. The research showed that Geophysical surveys can reveal a useful approach to roots investigation, both in describing the shape and behavior of the roots in the subsoil and in estimating the volumes of root biomass. To achieve the better results this paper underlines the need of integrating different techniques: GPR method is able of detecting with higher resolution the distribution of the tree roots in the subsoil. Three dimensional ERT can be useful in correlating the recovered resistivity distribution with root volumes. In particular the extraction of volumes of resistivity percent increment between dry and wet conditions in the subsoil around the trees seems a parameter that can be directly related to the volumes of roots. Further studies should focus on two directions: first, the improvement of a standard field-procedure to carry on the geophysical surveys; second, the development a statistical processing tool to relate root biomass to geophysical parameters.JRC.H.2-Climate chang

Deep electrical resistivity tomography for the prospection of low- to medium-enthalpy geothermal resources

SUMMARY The growth of the geothermal industry sector requires innovative methods to reduce explo- ration costs whilst minimizing uncertainty during subsurface exploration. Until now geoelec- trical prospection had to trade between logistically complex cabled technologies reaching a few hundreds meters deep versus shallow-reaching prospecting methods commonly used in hydro-geophysical studies. We present a recent technology for geoelectrical prospection, and show how geoelectrical methods may allow the investigation of medium-enthalpy geothermal resources until about 1 km depth. The use of the new acquisition system, which is made of a distributed set of independent electrical potential recorders, enabled us to tackle logistics and noise data issues typical of urbanized areas. We acquired a 4.5-km-long 2-D geoelectrical survey in an industrial area to investigate the subsurface structure of a sedimentary sequence that was the target of a ∼700 m geothermal exploration well (Geo-01, Satigny) in the Greater Geneva Basin, Western Switzerland. To show the reliability of this new method we compared the acquired resistivity data against reflection seismic and gravimetric data and well logs. The processed resistivity model is consistent with the interpretation of the active-seismic data and density variations computed from the inversion of the residual Bouguer anomaly. The combination of the resistivity and gravity models suggest the presence of a low resistivity and low density body crossing Mesozoic geological units up to Palaeogene–Neogene units that can be used for medium-enthalpy geothermal exploitation. Our work points out how new geoelectrical methods may be used to identify thermal groundwater at depth. This new cost- efficient technology may become an effective and reliable exploration method for the imaging of shallow geothermal resources

Geysers, Boiling Groundwater and Tectonics: The 3D Subsurface Resistive Structure of the Haukadalur Hydrothermal Field, Iceland

Geysers are among the most fascinating geological features on Earth. Yet, little is still known about their hydrogeological structure at depth. To shed light on the spatial relationships between the vertical conduits and the aquifers feeding them, we conducted a 3D geoelectrical campaign in the Haukadalur hydrothermal field, Iceland. We deployed 24 Iris Fullwavers across the hydrothermal field and inverted resistivity and chargeability measurements. Additionally, we measured temperature variations inside Strokkur and Great Geysir geysers showing temperature fluctuations pointing out the oscillatory behavior that characterizes the geysering cycle of the geysers. By combining a semi-quantitative temperature distribution of the thermal springs across the hydrothermal field with the inversion of the geoelectrical data, we highlight the control that extensional tectonics have on the distribution of fluids across the hydrothermal field. We also point out the occurrence of a common deep groundwater reservoir feeding the hydrothermal centers. Induced polarization data show that the geysers are fed by sub-vertical water-filled fracture zones. The geysers are found at the margins of highly resistive regions where we speculate boiling groundwater and vapor is found. Our proposed model suggests that local waters feeding the main groundwater reservoir downwell from the nearby region and then convect upwards, phase transitioning into vapor at about 200 m depth. From here, fluids flow toward the surface through pipes cutting a highly pressurized and hot system. This study shows to the best of our knowledge the first full 3D tomographic image of a hydrothermal field hosting geysers

Geophysical anomalies detected by electrical resistivity tomography in the area surrounding Tutankhamun's tomb

Electrical resistivity tomography (ERT) of the area surrounding Tutankhamun's tomb (KV62) in the Valley of the Kings (Luxor, Egypt) reveals the presence of two anomalies located a few meters from Tutankhamun's funerary chamber. The strategy for ERT data acquisition and the adopted methods for data analysis are discussed in detail in this article, together with the possible archaeological significance of the detected anomalies

Preliminary Use of Ground-Penetrating Radar and Electrical Resistivity Tomography to Study Tree Roots in Pine Forests and Poplar Plantations

In this study we assess the possibility of using Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) as indirect non destructive techniques for root detection Two experimental sites were investigated: a poplar plantation (mean height of plants 25.7 m DbH 33 cm) and a pinewood forest mainly composed of Pinus pinea and Pinus pinaster (mean height 17 m, DBH 29 cm). GPR measures were taken using antennas of 900 and 1500 MHz applied in square and circular grids. ERT was previously tested along 2D lines, compared with GPR sections and direct observation of the roots, and then using a complete 3D acquisition technique. Threedimensional reconstructions using grids of electrodes centered and evenly spaced around the tree were used in all cases (poplar and pine), and repeated in different periods in the pine forest (April, June and September) to investigate the influence of water saturation on the results obtainable. The investigated roots systems were entirely excavated using AIR-SPADE® Series 2000. In order to acquire morphological information on the root system, to be compared with the GPR and ERT, poplar and pine roots were scanned using a portable on ground scanning LIDAR. In test sections analyzed around the poplar trees GPR with a high frequency antenna proved to be able to detect roots with very small 41 diameters and different angles, with the geometry of survey lines ruling the intensity of individual reflectors. The comparison between 3D images of the extracted roots obtained with a Laser scan data point cloud and the GPR profile proved the potential of high density 3-D GPR in mapping the entire system in unsaturated soil, with a preference for sandy and silty terrain, with problems arising when clay is predominant. Clutter produced by gravel and pebbles, mixed with the presence of roots, can also be sources of noise for the GPR signals. The work performed on the pine trees shows that the shape, distribution and volume of roots system, can be coupled to the 3-D electrical resistivity variation of the soil model map. Geophysical surveys can be a useful approach to root investigation in describing both the shape and behaviour of the roots in the subsoil.JRC.H.2-Air and Climat

THE CONSOLIDATION AND STABILIZATION OF FOUNDATION SOILS THROUGH THE INJECTION OF EXPANDING POLYURETHANE RESIN UNDER A NON-INVASIVE DIAGNOSTIC CHECK BY 3D-4D-ERT

Vertical differential settlement of buildings is produced by both natural and/or anthropogenic disturbances of the foundation soil, such as, for example, differential consolidation, circulation of water due to rupture of water pipes, seepage, suction, vibrations induced by vehicle traffic, excavations, construction defects and an incorrect design/execution of the work. Techniques of soil consolidation and stabilization by means of expanding resin injections are being advantageously employed to mitigate or even solve this issue. The resin used is a two-component polyurethane compound that, once its chemical reaction is completed, reaches an inert and lightweight solid state so that it is environment-safe and perfectly compatible with the injected soil. The resin is injected directly into the critical volumes by means of small diameter pipes. After the injection, the resin expands rapidly in the soil through an irreversible exothermic chemical reaction, so that it actively counteracts the settlement and its causes. Indeed, the observed effects are threefold: i) filling voids; ii) compacting soil; iii) reducing/removing interstitial water. Among the possible procedures to monitor soil consolidation and stabilization by injections of expanding resins, a procedure is hereafter described which involves the use of electrical resistivity tomography (ERT). The rationale for the use of this non-invasive imaging technique is that electrical resistivity is a physical property sensitive to grain size, porosity and water content of sediments and to voids. Therefore, it can both acquire information about the subsurface structure, also from volumes of difficult access (e.g.,, below the foundations and the floor of the building), which is necessary for the consolidation project, and it can monitor the injection work in progress, in addition to traditional geotechnical testing. Specifically, the process is divided into several steps. A 3D-ERT is acquired first, covering both the volumes affected by the settlement and nearby stable volumes of soil underlying the footprint of the building, to accurately map the different resistivity that characterizes the anomalous volumes. The 3D resistivity model is integrated, wherever necessary, by geotechnical investigations, mainly penetration tests and sample identification, so as to optimally design the injection points, in terms of position, depth and number. During injections, the 3D-ERT and some geotechnical tests are repeated to check progressively the effects obtained by the treatment, so as to allow, if necessary, to modify the injection scheme until reaching the predetermined target, i.e., obtaining the greatest uniformity of chemical and physical characteristics between the stabilized volumes and the nearby stable ones, which are used as a reference. In some sites, where the settlement was stabilized by the above procedure, it was also possible to repeat a 3D-ERT after several years: results confirmed that the situation of the subsurface just after the injections had remained almost unaltered and the effectiveness of the procedure over time was demonstrated. The above approach is described hereafter and is accompanied by the description of the results from a specifically planned test site and from two work sites, selected as examples

THE CONSOLIDATION AND STABILIZATION OF FOUNDATION SOILS THROUGH THE INJECTION OF EXPANDING POLYURETHANE RESIN UNDER A NON-INVASIVE DIAGNOSTIC CHECK BY 3D-4D-ERT

Vertical differential settlement of buildings is produced by both natural and/or anthropogenic disturbances of the foundation soil, such as, for example, differential consolidation, circulation of water due to rupture of water pipes, seepage, suction, vibrations induced by vehicle traffic, excavations, construction defects and an incorrect design/execution of the work. Techniques of soil consolidation and stabilization by means of expanding resin injections are being advantageously employed to mitigate or even solve this issue. The resin used is a two-component polyurethane compound that, once its chemical reaction is completed, reaches an inert and lightweight solid state so that it is environment-safe and perfectly compatible with the injected soil. The resin is injected directly into the critical volumes by means of small diameter pipes. After the injection, the resin expands rapidly in the soil through an irreversible exothermic chemical reaction, so that it actively counteracts the settlement and its causes. Indeed, the observed effects are threefold: i) filling voids; ii) compacting soil; iii) reducing/removing interstitial water. Among the possible procedures to monitor soil consolidation and stabilization by injections of expanding resins, a procedure is hereafter described which involves the use of electrical resistivity tomography (ERT). The rationale for the use of this non-invasive imaging technique is that electrical resistivity is a physical property sensitive to grain size, porosity and water content of sediments and to voids. Therefore, it can both acquire information about the subsurface structure, also from volumes of difficult access (e.g.,, below the foundations and the floor of the building), which is necessary for the consolidation project, and it can monitor the injection work in progress, in addition to traditional geotechnical testing. Specifically, the process is divided into several steps. A 3D-ERT is acquired first, covering both the volumes affected by the settlement and nearby stable volumes of soil underlying the footprint of the building, to accurately map the different resistivity that characterizes the anomalous volumes. The 3D resistivity model is integrated, wherever necessary, by geotechnical investigations, mainly penetration tests and sample identification, so as to optimally design the injection points, in terms of position, depth and number. During injections, the 3D-ERT and some geotechnical tests are repeated to check progressively the effects obtained by the treatment, so as to allow, if necessary, to modify the injection scheme until reaching the predetermined target, i.e., obtaining the greatest uniformity of chemical and physical characteristics between the stabilized volumes and the nearby stable ones, which are used as a reference. In some sites, where the settlement was stabilized by the above procedure, it was also possible to repeat a 3D-ERT after several years: results confirmed that the situation of the subsurface just after the injections had remained almost unaltered and the effectiveness of the procedure over time was demonstrated. The above approach is described hereafter and is accompanied by the description of the results from a specifically planned test site and from two work sites, selected as examples