The impact of ice on the tensile strength of unconsolidated sand - A model for gas hydrate-bearing sands?

Abstract

Tensile strength is an important parameter when it comes to predictions of potential fracturing of sediments by natural processes such as the emplacement of ice or gas hydrate lenses, as well as anthropogenic fracturing or else the stability of engineering constructions such as boreholes. Yet, tensile strength (στ) measurements of unconsolidated ice-bearing or gas hydrate-bearing sands are scarce and affected by a large variability.In the course of the SUGAR project we successfully used ice as a model for pore-filling and “load-bearing” gas hydrate in sand to determine compressional wave velocity. We were thus able to verify comparable formation characteristics and morphologies of ice and gas hydrate within the pore space. As these are important values for the tensile strength of ice/hydrate-bearing sands, ice was also used as a model for hydrate-bearing sands, despite differences in the mechanical behavior and strength of pure ice and gas hydrate. Water-saturated sand cores with ice saturations (Sice) between 0 and 100% were tested at −6.8 °C. The varying Sice were a result of the freezing point depression caused by saline solutions of different concentrations. The στ was directly determined using a sleeve-fracturing test with an internal pressure that was created within the frozen samples. The setup was also adapted to fit a pressure vessel for tests using confining pressure.The correlation of Sice – στ shows an exponential increase of στ with Sice. Whereas at Sice < 60% the increase is small, it is large at Sice > 80%. In conjunction with the change in strength, the viscoelastic behavior changes. A clear peak strength occurs at Sice > 80%. We conclude that given 60% < Sice < 80% the pore-filling morphology of the ice converts into a frame-building habitus and at Sice > 80% the frame gains strength while the amount of residual water decreases. Tensile failure and cracking now exceed grain boundary sliding as the prevailing failure mode. The ice morphology in the sand is non-cementing and comparable to a gas hydrate-sand mixture.