[1] This paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an ''excess water'' technique. In these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space. Results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting. In contrast, the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas, termed in this paper the ''excess gas'' method, exhibits a cementing behavior, while tetrahydrofuran-hydrate sands or where the hydrate is formed from dissolved methane within the pore water, exhibit a pore-filling behavior for hydrate saturations less than 40%. For sands where the hydrate is formed using the excess water method, much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs, although increases in compressional wave velocity are seen at lower hydrate contents. These results suggest that hydrate interaction with the sediment is strongly dependent on morphology, and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms.

[1] Remote seismic methods, which measure the compressional wave (P wave) velocity (V p ) and shear wave (S wave) velocity (V s ), can be used to assess the distribution and concentration of marine gas hydrates in situ. However, interpreting seismic data requires an understanding of the seismic properties of hydrate-bearing sediments, which has proved problematic because of difficulties in recovering intact hydrate-bearing sediment samples and in performing valid laboratory tests. Therefore a dedicated gas hydrate resonant column (GHRC) was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both V p and V s made at frequencies and strains relevant to marine seismic investigations. Thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested. The results show a bipartite relationship between velocities and hydrate pore saturation, with a marked transition between 3 and 5% hydrate pore saturation for both V p and V s . This suggests that methane hydrate initially cements sand grain contacts then infills the pore space. These results show in detail for the first time, using a resonant column, how hydrate cementation affects elastic wave properties in quartz sand. This information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation.Citation: Priest, J. A., A. I. Best, and C. R. I. Clayton (2005), A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand,

This paper reports the development and testing of two independent, innovative techniques for measuring rail displacements. One system combines remote video monitoring with particle image velocimetry, using a webcam and a small telescope. The second uses sleeper mounted geophones that give a voltage output proportional to the velocity of motion, which can be filtered and integrated to calculate displacements. Laboratory validation tests show that the video monitoring system can measure peak-to-peak displacements to within 0.04 mm from a distance of 15 m for frequencies less than 4 Hz. The geophones measure peak-to-peak displacements to within 0.07 mm for frequencies as low as 1 Hz. Data from three different standards of railway track and/or train speeds are used to explore and quantify the limitations of each system in the field.

Recent experience with the operation of high-speed railways in the UK and elsewhere has revealed the phenomenon, termed 'ballast flight', of ballast particles becoming airborne during the passage of trains, potentially causing damage to both the railhead and the vehicle. This article reports the results of an investigation into the mechanical and aerodynamic forces acting on ballast particles that are generated during the passage of a high-speed train and addresses the question whether these might offer a possible explanation for the initiation of ballast flight. As the high-speed trains passed, measurements were made of the air pressure and velocity at various locations across the track, and of the velocity and acceleration of the track system (sleeper and rails) and the ballast itself. The aerodynamic forces exerted on a suspended ballast particle were also measured. An analytical model of the behaviour of small ballast particles was constructed to assist in the interpretation of the measured data. Analysis of the data and modelling suggest that neither mechanical forces nor aerodynamic forces in isolation are likely to be sufficient to initiate ballast flight under the conditions investigated, but that the phenomenon could arise from a combination of the two effects. It appears that the process is stochastic in nature: further work, with an increased number of measurements, is required to explore this.

S U M M A R YCompressional wave (P wave) and shear wave (S wave) velocities (V p and V s , respectively) from remote seismic methods have been used to infer the distribution and volume of gas hydrate within marine sediments. Recent advances in seismic methods now allow compressional and shear wave attenuations (Q −1 p and Q −1 s , respectively) to be measured. However, the interpretation of these data is problematic due to our limited understanding of the effects of gas hydrate on physical properties. Therefore, a laboratory gas hydrate resonant column was developed to simulate pressure and temperature conditions suitable for methane gas hydrate formation in sand specimens and the subsequent measurement of both Q −1 p and Q −1 s at frequencies and strains relevant to marine seismic surveys. 13 dry (gas saturated) sand specimens were investigated with different amounts of methane gas hydrate evenly dispersed throughout each specimen. The results show that for these dry specimens both Q −1 p and Q −1 s are highly sensitive to hydrate saturation with unexpected peaks observed between 3 and 5 per cent hydrate saturation. It is thought that viscous squirt flow of absorbed water or free gas within the pore space is enhanced by hydrate cement at grain contacts and by the nanoporosity of the hydrate itself. These results show for the first time the dramatic effect methane gas hydrate can have on seismic wave attenuation in sand, and provide insight into wave propagation mechanisms. These results will aid the interpretation of elastic wave attenuation data obtained using marine seismic prospecting methods.

No abstract

Transition zones between railway tracks on embankments or natural ground, and fixed substructures such as bridges and culverts, typically require extensive maintenance to retain acceptable track geometry. These high maintenance costs and the potential to cause delays to train services are of major concern for railway infrastructure managers. In view of the importance of the problem, surprisingly little research has been carried out to identify the fundamental causes of the poor performance of transition zones. To better understand the physical mechanisms involved, an extensive monitoring and investigation programme was undertaken on a typical transition zone in the Netherlands, comprising reinforced concrete approach slabs linking the normal track onto a concrete culvert. Accelerations and velocities of the track, soil, and approach slabs in response to passenger trains were measured, from which displacements were calculated. In addition, track settlements and pore water pressures were monitored over a 1-year period. This article presents and discusses the measurements made. The results highlight the problems associated with track quality at a transition zone, including the large dynamic displacements induced during train passage and the tendency for ongoing long-term movement. The implications of these for design and maintenance are discussed.

No abstract