Assessment of layer thickness and uniformity in railway embankments with ground penetrating radar

Abstract

In the aim of a national research project entitled “Interaction soil-rail track for high speed trains”, a protocol was established between the National Railway Network and four national research institutions to develop the knowledge concerning the methodology for the construction and quality control of the railway embankments and railtrack layers for high speed trains. One of the objectives of this protocol is to establish a methodology for quality control of construction layers by different available test methods. Nondestructive testing (NDT) methods are currently very attractive due to their ability to provide information about layer thickness and state condition without causing damage or requiring the removal of material samples. Within the NDT available, ground penetrating radar (GPR) is a very fast and reliable technique, whose advantage is the repeatability and the capability of acquiring continuous data. To reach the proposed goal, a trial embankment was constructed with different materials, layer’s thicknesses, water contents and compaction energy levels. GPR was used in two embankments, in order to detect the thickness of the sub-ballast layer located over the compacted sand layer and its uniformity along the track, but also along the cross-section of the track. In order to control some parameters of the sub-ballast layer, like thickness and uniformity, several metallic plates had been used in the base of the sub-ballast layer, along an alignment. It shows clearly the ability of GPR to detect the sub-ballast layer and its thickness variations along the profile. For applications in civil engineering, usually frequencies from 500 MHz to 2.5 GHz are applied via bow tie antennas. The reflections recorded with the receiving antenna by moving both, transmitter and receiver along a predefined line on the surface can be visualized as 2D images or radargrams, where the intensity of the reflected impulses is displayed in a grey scale. 3 RAILWAY INVESTIGATION The construction of new railway infrastructures and the maintenance and upgrade of existing ones for higher speeds and loads depends greatly on obtaining field information about construction quality, materials characteristics and conditions. For that purpose, ground penetrating radar (GPR) is being actively used and studied for several decades by diverse scientists (Clark et al. 2004, Hugenschmidt 2000, Gallagher et al. 1999, Jack & Jackson 1999, Roberts et al. 2006). This technique has become an accepted method for in-situ monitoring. The georadar allows to obtain a significant number of parameters that are very relevant to analyse and assess the conditions of construction layers and embankments. It includes the assessment of the thickness, elastic modulus, dielectric and magnetic properties and the moisture condition of ballast, subballast and base layers (Narayanan et al. 2004, Clark et al. 2001). It also can help in the detection of base or ballast settlements. GPR has several key characteristics that made it very adequate for the referred purpose. It is a portable and compact equipment, which allows the continuous survey and data acquisition along very large distances and at rather fast speeds (up to 50 km/h). It allows the characterisation of the soil and ballast at large depths (up to 5 m), depending on the antenna frequency used. Furthermore, it is significantly faster, more reliable and precise than traditional techniques based on hole drilling, destructive methods or simple visual inspection. 4 TESTING SITE AND METHODOLOGY To reach the proposed goal, a trial embankment was constructed with different materials, layer’s thicknesses, water contents and compaction energy levels. Two tracks were built, one constituted by a compacted sand layer and a second one constituted by a sub-ballast layer over a compacted sand layer (see Figure 2). Additionally, on the first track, metallic plates were introduced in the base of the subballast layer, along an alignment, which provides an indicator for thickness control. The surveys were carried out in order to detect the thickness of the sub-ballast layer located over the compacted sand layer and its uniformity along the track and the cross-section of the track. Figure 2 – Partial view of the trial embankment with execution of a GPR profile. To accomplish this, two GPR systems were used together with different antenna frequencies. One RAMAC system (MALA Geoscience, Inc.) was used together with three different antennas, each one with a different objective: 500 MHz antenna for deep sounding, 800 MHz antenna for optimum detection of the layers in the first half meter and 1600 MHz antenna for the survey of shallow characteristics. A SIR-10 system (Geophysical Survey Systems, Inc.) was used with an antenna of 900 MHz, as illustrated in Figure 3. Figure 3 – Equipment: System SIR-10 – GSSI, and antennas of 900, 500 and 200 MHz.