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Evaluation of sample disturbance due to the exsolution of dissolved gas in the pore
water of deep lake bottom sediments
Évaluation du remaniement des échantillons dû à l’exsolution de gaz dissous dans les eaux
interstitielles des sédiments de fond de lacs profonds
Yamashita S., Miura R.
Kitami Institute of Technology, Kitami, Japan
Kataoka S.
Hakodate National College of Technology, Hakodate, Japan
ABSTRACT: Core samplings were conducted in the Lake Baikal, Russia, where shallow gas hydrates existed. To examine the
mechanical properties of lake-bottom sediments, the handy vane shear and cone penetration tests were performed for the core samples
on board. To evaluate the sample disturbance due to the exsolution of dissolved gas during sampling, the relations between the
concentration of dissolved gas in the pore water and the strength were also examined. Moreover, laboratory tests which simulate the
stress relief from bringing the samples to the lake surface were also performed. Test results showed that the strength of soils becomes
lower with the increase in degree of gas concentration on both on-board and laboratory tests. It was also showed that the in-situ
strength can be estimated roughly from the disturbed samples.
RÉSUMÉ : Un prélèvement d’échantillons stratigraphiquement représentatifs (carottes) a été mené dans le lac Baïkal, en Russie, où il
existe des réservoirs d’hydrates de gaz peu profonds. Afin d’examiner les propriétés mécaniques des sédiments de fond de lac, on a
procédé à des essais de cisaillement avec un scissomètre portatif et à un test de pénétration au cône sur ces échantillons
stratigraphiquement représentatifs, ceci sur place. Dans le but d’évaluer le degré de remaniement des échantillons dû à l’exsolution de
gaz dissous au cours du prélèvement, on a également examiné le lien entre la concentration de gaz dissous dans les eaux interstitielles
et la résistance de l’échantillon. On a, en outre, procédé à des tests en laboratoire qui simulent le relâchement de contraintes résultant
de l’apport des échantillons à la surface du lac. Les tests menés sur place tout comme ceux menés en laboratoire, indiquent que la
résistance des sols diminue plus le degré de concentration en gaz augmente. Ils ont également démontré que l’on pouvait obtenir une
estimation approximative de la résistance in situ du sol à partir de l’échantillon remanié.
KEYWORDS: dissolved gas, gas hydrate, sample disturbance, lake bottom sediment
1 INTRODUCTION
Gas hydrates (GH) are attracting attention as a next-generation
energy source. In Japan, survey and test drilling of GH for
resource development have been conducted in around the
Nankai Trough. On the other hand, GH are also attracting
attention in relation to the global environment, because methane
gas contained in the GH has approximately 20 times the
greenhouse effect of carbon dioxide. There are concerns that
dissociation of GH and exsolution of dissolved gas, from the
GH distributed in submarine surface layers, due to rising ocean
temperatures or leakage at recovery of the hydrates for energy,
may contribute to global warming. Moreover, these phenomena
or seismic activities can reduce the stability of seabed and may
induce seafloor landslides. Therefore, it is necessary to clarify
the mechanical properties of the GH-bearing ground and the
strength change of seabed sediments due to the dissociation or
exsolution.
In generally, to assess the in-situ strength, laboratory tests on
samples retrieved from the site or in-situ tests have been
performed. However, it is difficult to perform the in-situ test in
the deep lake or seabed. On the other hand, samples for
laboratory tests taken from lake or seabed soils in deep waters
are subjected to a large stress (back water pressure) relief. Even
a small amount of gas dissolved in the pore water will come out
of solution and cause disturbance to the soil structure due to the
stress relief. As a result, laboratory tests may not give
appropriate results for the in-situ soil conditions.
In this study, to evaluate the effects of the exsolution of
dissolved gas in the pore water on the strength properties, core
samplings were conducted in the Lake Baikal, Russia, shallow
GH province, and some kinds of on-board tests were performed.
Laboratory tests which simulate the stress relief from bringing
the samples to the lake surface were also performed.
2. SURVER AND SOIL SAMPLING
The sampling of lake-bottom sediments contained GH were
conducted in the Lake Baikal, Russia, September 2010, by the
survey ship ‘
Vereshchagin
’ of the Russian Limnological
Institute. It is already reported that GH in the Lake Baikal is
formed at immediately beneath lake-bottom of the mud volcano
that is observed the eruption of cool spring water contained
gases by the echo sounder (e.g., Matveeva et al. 2003). Thus, at
locations where mud volcanoes were confirmed by the echo
sounder images, lake-bottom sediments were collected by using
a gravity core sampler (sampler length is about 5 m, diameter is
110 mm and weight is about 700 kg). Sampling sits are
Novosibirsk site (water depth is about 1450 m) and Kukuy site
(water depth is about 800 m) at the central parts of the lake, as
shown in Figure 1. The lake-bottom sediments cores were
retrieved 40 sample cores (24 cores in Novosibirsk site, 16
cores in Kukuy site).
After core samplings, these cores were cut into 1-m interval.
Then, each section was cut up longitudinally on two parts for
subsequent processing. To measure the strength of sediments
immediately after recovery, the handy vane shear test (blade
diameter is 10 mm, height is 20 mm; 10 to 40 cm interval) and
cone penetration test (diameter is 9 mm, length is 16.8 mm,
apex angle is 30 degrees; 10 to 40 cm interval) were performed
on board. For measurement of the water content, 10 to 40 cm
interval samples were taken. Additionally, the dissolved gases
in the pore water were taken by the headspace gas method.
