Development of a Sampler for Measurement of Gas Content in Soils

The problem of wave-induced liquefaction is an important feature in coastal and marine engineering, and is one of the main research topics in the European Union project Liquefaction around Marine Structures (LIMAS). In this context, the gas content of a soil is one topic of interest, as the presence of gas influences the pore pressure response and the gradient distribution in the seabed during wave action. The main objective of the work described herein has been to develop a new sampler that enables in situ measurement of gas content in the seabed, and at the same time retrieves a representative sample of the soil. The sampler should be able to maintain the natural soil structure and amount of gas in the pores to a high degree, and should primarily be used in sandy and silty materials. The major challenge in the project was to achieve perfect sealing of the sample container after sample shearing. This was finally obtained by use of a ball valve and backpressurized ball-valve housing in the lower end of the sample container, and a piston in the upper end. The main problem with the ball-valve concept was wear and clogging from sand particles inside the ball valve, and various sampler designs were tested and evaluated before the final, robust design was chosen. The gas content in the soil is determined in situ, using backpressure for compression of the gas in the sample container, enabling deaired water to be injected in the sample. This water volume is recorded on a burette and is used to determine the total amount of gas in the sample. To obtain the degree of saturation $S_r$ in the soil, the weight and volume of the soil grains is determined after dismantling of the sampler. Laboratory calibration tests show that a known gas volume of $100\mathrm{mL}$ may be measured with $\pm 0.75\mathrm{mL}$ accuracy with the new sampler. The new sampler was used successfully at the LIMAS research site in Capbreton, France, where gas measurements were carried out under various wave and tidal conditions in a sandy seabed. The obtained results showed that the gas content could vary significantly in the upper $0.3–0.6\mathrm{m}$ top layer of the soil. The smallest gas contents, representing 0.2–0.65% of the sample volume, were measured under falling tide conditions after repeated cycles of strong waves. The tests showed that the new sampler can be used successfully under field conditions, provided that great care is taken during mounting and preparation of the equipment.