Jurij Karlovsek

Investigation of voids and cavities in bored tunnels using GPR

Tunneling by an Earth Pressure Balance Machine (EPB) encounters a range of ground conditions ranging from hard rock to soft soil. To maintain the stability of routed ground, grouting is required both before and during the advance of the machine. Due to difficult geological conditions and incomplete grouting, cavities and voids can be created around the tunnel excavation. These can cause unpredictable settlements of the ground and peak stresses in the tunnel lining. So far, these hollow /water filled spaces have only been detected manually by boreholes driven through the crown of the tunneling lining. GPR can be used as an alternative method to detect these spaces. Experimental and numerical investigations were conducted to prove the applicability of GPR for this task.

Broadband dielectric measurement methods for soft geomaterials: coaxial transmission line cell and open-ended coaxial probe

Broadband dielectric measurement methods based on vector network analyzer coupled with coaxial transmission line cell (CC) and open-ended coaxial probe (OC) are simply reviewed, by which the dielectric behaviors in the frequency range of 1 MHz to 3 GHz of two practical geomaterials are investigated. Kaolin after modified compaction with different water contents is measured by using CC. The results are consistent with previous study on standardized compacted kaolin and suggest that the dielectric properties at frequencies below 100 MHz are not only a function of water content but also functions of other soil state parameters including dry density. The hydration process of a commercial grout is monitored in real time by using OC. It is found that the time dependent dielectric properties can accurately reveal the different stages of the hydration process. These measurement results demonstrate the practicability of the introduced methods in determining dielectric properties of soft geomaterials.

Radio to microwave dielectric characterisation of constitutive electromagnetic soil properties using vector network analyses

The knowledge of constitutive broadband electromagnetic (EM) properties of porous media such as soils and rocks is essential in the theoretical and numerical modeling of EM wave propagation in the subsurface. This paper presents an experimental and numerical study on the performance EM measuring instruments for broadband EM wave in the radio-microwave frequency range. 3-D numerical calculations of a specific sensor were carried out using the Ansys HFSS (high frequency structural simulator) to further evaluate the probe performance. In addition, six different sensors of varying design, application purpose, and operational frequency range, were tested on different calibration liquids and a sample of fine-grained soil over a frequency range of 1 MHz-40 GHz using four vector network analysers. The resulting dielectric spectrum of the soil was analysed and interpreted using a 3-term Cole-Cole model under consideration of a direct current conductivity contribution. Comparison of sensor performances on calibration materials and fine-grained soils showed consistency in the measured dielectric spectra at a frequency range from 100 MHz-2 GHz. By combining open-ended coaxial line and coaxial transmission line measurements, the observable frequency window could be extended to a truly broad frequency range of 1 MHz-40 GHz.

Frequency-dependant dielectric parameters of steel fiber reinforced concrete

Steel Fiber Reinforced Concrete (SFRC) is increasingly being used in civil constructions, especially precast tunnel segment linings. Fibers are mixed with fresh concrete, which results in a more or less random distribution of fibers within the concrete matrix. Because of this, SFRC demonstrates a great challenge when it is being investigated using GPR. In order to investigate the frequency-dependent dielectric parameters of SFRC; (I) PET/CT scans of concrete were conducted to understand the nature of orientation and density of fibers being distributed within the concrete sample and (Π) a two-port coaxial transmission line technique developed for the determination of dielectric spectra of undisturbed soil samples was used to investigate the concretes complex permittivity with and without steel fibers in the frequency range from 1 MHz to 5 GHz.

Dielectric measurement method for real-time monitoring of initial hardening of backfill materials used for underground construction

The broadband dielectric measurement method based on the vector network analysis technique, in combination with an open-ended coaxial probe, was applied to the determination of the dielectric relaxation behaviour of one- and two-component backfilling grout materials in the frequency range from 40 MHz to 2 GHz. The cement hydration process and the gelling of commercial grouts was monitored in real-time to investigate the application of non-destructive testing methods in the tunnelling industry. It was found that the time-dependent dielectric relaxation behaviour can accurately reveal the different stages of the hydration process and delineate the start of gel hardening. These measurement results demonstrate the practicability of the real-time dielectric measurement method to determine the broadband dielectric parameters of conventional backfill materials used in underground construction to determine construction integrity using non-destructive testing methods.

Estimating propagating velocity through steel fibre reinforced concrete

Steel fibre reinforced concrete (SFRC) is the primary lining in many urban constructions including tunnels. The thickness of the tunnel segmental lining blocks is highly precise: within ±1 mm of the nominated thickness [1]. In tunnels, the deeper layers are backfill material and surrounding ground. For structural integrity it is important that the inner layer is correctly embedded. The integrity of the layers can be explored non-destructively using a ground penetrating radar (GPR). The images produced by a GPR device are called radargrams, and are typically interpreted manually. The goal of this research is to analyse radargrams automatically and provide confidence in integrity of these layers. For this purpose it is first required to accurately determine extrados of the first layer of the tunnel lining built using SFRC. We perform this analysis using three thicknesses of concrete segmental blocks.

Spatial time domain reflectometry for monitoring of the hydrological water balance at a lysimeter test site in Thuringia/Germany

For monitoring of the hydrological water balance in top and subsoil during plant growth accurate knowledge of the spatial and temporal variation of soil water content is essential. In this context, a new full two port spatial time domain reflectometry (Spatial TDR) technique in combination with elongated microwave transmission line sensors were developed for advanced data acquisition and analysis. The technique is tested at a lysimeter station in Thuringia/Germany and compared with neutron moisture meter probes.