IN-SITU SAMPLE CORE SCANNING FOR PREPARATION OF DREDGING WORKS

Transcription

1 IN-SITU SAMPLE CORE SCANNING FOR PREPARATION OF DREDGING WORKS By K. Geirnaert 1, P. Staelens 1, L-R. Cool 1 S. Sargis 2 ABSTRACT Geotechnical information from the seabed is critical for the successful preparation and execution of capital and maintenance dredging works and port construction works. A standard technique that is applied to obtain this information is sample coring. A core sample is a cylindrical section taken from the seabed by intrusion equipment. The type of equipment needed to obtain the sample is dependent on the soil substance. Most core samples are obtained by either vibration or drilling; for example sediment is taken by vibrating a plastic tube by a vibro corer or rock, with a hollow steel tube. In the coring process, the sample is pushed more or less intact into the tube. At the laboratory the core sample is removed from the tube and then inspected and analysed by different techniques and equipment depending on the type of data desired. This paper presents an innovative approach to gain further advanced information using a combination of current and new techniques and instrumentation to retrieve as much information as possible in situ during the coring process by scanning the core during the coring process, eliminating the need to wait for laboratory analysis for critical information on site conditions. This information ranges from substance density like sediment, soil or rock density up to soil classification and soil contamination. This technique creates significant value by reducing the elapsed time of the retrieval of geotechnical information from the site of investigation with no additional the operating costs. The measurement allows also substance measurements in situ under undisturbed and real life conditions. With this information obtained much more rapidly and accurately, important decisions can be made on the dredging strategy such as equipment choice between for hopper and cutter dredger and dredging power. In addition, an optimal sanitation strategy for polluted soil can be set-up. Keywords: Sample coring, Dredging strategy, Soil characterisation, Density profile 1 dotocean NV, Pathoekeweg 9B, 8000 Brugge Belgium, +32 (0) , info@dotocean.eu, 2 IEC International Entrepreneurship Center, 320 Nevada Street Suite 301 Newton, MA 0246 USA, x308, info@iecprograms.com

2 INTRODUCTION Sediment density profiler X-ray detector X-ray source Figure 1: X-ray density profiler The profiler in Figure 1 is the leading density measurement device on the market, the DensX. The sediment density profiler consists out of a source and a detector integrated in two legs of a profiler. In standard operation, the profiler is lowered by a winch and intrudes into the sediment under its own weight. Depending on the resistance of the sediment, the profiler can intrude up to 5m in the sediment. The density of the sediment between the legs of the profiler is measured during intrusion. When photons are sent out by the source into the sediment, a portion of them reach the detector without interacting. Also, a portion of the photons interacts with the electrons of the sediment molecules and deviates or absorbs according to scattering and absorption principles. The big advantage of using radiation scattering for measuring the density is the fact that there is physically a direct relationship between the amount of photons reaching the detector and the density of the sediment.

3 The higher the density, the higher the number of electrons and the greater the chance of interaction. The photons received in the detector are a measure of the density. The signal intensity received by the detector is an exponential function decreasing with the density of the mixture. The relationship between medium density D and the intensity of the signal received by the detector is: where: D = D 0+ D 1.Ln (Ic/Io) D is the medium density Io is the signal intensity of the detector in clear water Ic is the signal intensity of the detector in mud D0 and D1 are calibration constants Figure 2: Detector output signal as a function of the density Radiation attenuation is a physical process that has a direct relationship with density. Due to the direct relation between attenuation and density, accuracies of more than 0.25% can be reached with measurement times of less than 1s. When the medium is denser, less photons reach the detector due to the increased absorption or scattering effect. The decrease of photons is an exponential curve as depicted in Figure 2.

4 Sediment density profiler mounted on a vibro core Worldwide dredging companies, construction companies, geotechnical consultants, ports and waterway authorities are using coring techniques like the vibro core or drill core techniques to investigate the seabed. The accuracy of the density measurement has a direct impact on the dredging effort and construction cost These cores are used to investigate soil characteristics, soil volumes, dredging volumes and ton dry weight of the dredged soil. Once the cores are collected, they are typically sent to a lab for analysis. A new and innovative approach to increasing the accuracy and decreasing the time duration and laboratory costs and analysis has been developed. By installing a density profiler on the vibro core, an in situ measurement can be taken during the coring process. In this case the sample is undisturbed and allows for a very accurate measurement. Furthermore, a fast and cost effective assessment is possible since data is immediately available during the coring. As depicted in Figure 3 an X-ray density profiler can be installed on the seabed frame of a vibro core system. The Figure 3: Vibro core system X-ray profiler is mounted perpendicular on the frame. While pulling the core out of the seabed it is pulled through the scanning beam and the results are immediately available. By measuring the position of the drill head, also the scanning position can be retrieved. The combination of the measured density and the position can provide a density over depth profile as depicted in Figure 4.

5 Core distance Density CORE SCANNING WITH DENSITY PROFILER Core scanning lab set-up Figure 4: Core profile: density over position A lab experiment was set-up to prove the working principle. For the test, 3 cores were taken out of different sediment tanks. The sediment tanks consisted of mud, clay mud mixture and sand mud mixture. The cores were taken by a tube of 5cm diameter as depicted in Figure 5. A cover was made in lead to shield the source and the core tube was lowered in equidistant steps via an opening in the shield. The scanning principle was conducted as depicted in Figure 6. The standard density profiler as depicted in Figure 1 is used as scanning device. The core is moved through the legs of the profiler and is scanned. On the core, there are distance markers for every centimeter.

6 Figure 5: Manual cores taken out of a mud tank Figure 6: The test set-up

7 Core height in cm Proceedings of the 2015 Dredging Summit & Expo Core density scanning To verify the scanning results and the accuracy of the measurement the total core is weighted. The weight is compared with the integrated density over the core length. Several errors need to be taken into account: A simple scale was used with a low resolution. A scan does not take the variation over the volume. Even under these conditions a deviation of less than 3% is seen. In Figure 7 a sample of 34cm is scanned where about 16 cm of water and 18 cm of soil is included. Core 1 Density by weighing Total Weight Total length of the core Weight casing Weight sample Density by scanning Sum weight Diff weigth 1,1 kg 34 cm 0,312 kg 0,788 kg 0, kg -0, kg 40 Core height vs Density Density in kg/m³ Figure 7: Core 1 scan

8 In Figure 8 a sample of 50cm is scanned where about 30cm of water is included and 20cm of soil material. Core 2 Total Weight Total length of the core Weight casing Weight sample Sum weight Diff weigth 1,45 kg 50 cm 0,312 kg 1,138 kg 1, kg -0, kg 60 Core height vs Density Core height in cm 30 Series Density in kg/m³ Figure 8: Core 2 scan

9 In Figure 9, a sample of 102 cm is scanned where about 25 cm of water is included and 20cm of soil material. Core 3 Total Weight 2,7 kg Total length of the core 102 cm Weight casing 0,312 Weight sample 2,388 Sum weight Diff weigth 2, kg -0, kg 120 Core height vs Density Core height in cm Density in kg/m³ Figure 9: Core 3 scan

10 CORE IMAGING WITH CT SCANNER From core 1 and core 2, X-ray imaging scans were taken. As can be seen in Figure 10, the heterogeneity0. of the material becomes visible. By applying in situ imaging techniques also soil composition, contamination and build up can be retrieved. Core 1 In Figure 10, three types of scans were performed. On the left an outside scan, on the middle a cross section and on the right a material scan. Figure 10: Core 1 CT scan

11 Horizontal CT slices In Figure 11 horizontal slices are depicted. Figure 11: Horizontal CT slices

12 Core 2 In Figure 12 three types of scans were performed. On the left an outside scan, on the middle a cross section and on the right a material scan. Figure 12: Core 2 CT scan

13 Horizontal CT slices Figure 13: Horizontal CT slices

14 CONCLUSION A novel density scanning method based on an X-ray profiler in combination with a vibro core is presented. The advantages of this system are The X-ray is switchable and can be turned off when outside the water. Therefore the usage does not suffer from strong legislation restrictions. The X-ray profiler serves two applications. It can be used as a profiler in soft soils and as scanning device for vibro cores, drill cores and drillings in hard material. The system can be deployed to measure fluids and solids ranging from fluid mud till compact sand or rock, all with the same equipment. By deploying it on the core scanner the measurement is in-situ and undisturbed and the soil is measured under the same pressure conditions. Dewatering is not taking place on the sample. The technology opens perspectives for taking the scan a step further then a pure density measurement. Also chemical, structural or composition analysis are within the possibilities. References K. Geirnaert, P. Staelens, S. Deprez B. Dierikx, M. van der Sluijs, G. Poot (2014) Novel Sediment profiler for preparing and evaluating dredging works and determining the nautical depth. PIANC World Congress San Francisco, USA 2014