Strength analysis of the frozen-ground enclosure of a low-temperature storage reservoir for liquefied gases

The geometrical dimensions of a frozen-ground enclosure for a low-temperature storage reservoir are determined from a static designstorage reservoir are determined from a static design on the assumption of a safe excavation of ground in digging the trench. Neglecting the combined work of the enclosure and surrounding thawing rock mass, the problem is reduced to the design of the frozen-ground shell, stressed along its outer contour by the thawing ground pressure.

To estimate the tension of the wall of an enclosure it is sufficient to use the solution of the problem of a thick-walled tube subjected to ambient pressure. It is difficult to find a sufficiently simple analogue for the problem of the bottom. The solution of the plate-bending problem locates the neutral line in the middle of the section when there are equal tensile and compressive stresses at the inner and the outer contour. If the combined work of the bottom of the section and the wall is taken into account, the neutral line shifts from the centre of the bottom to the inner contour of the enclosure, i.e., the compressive stresses exceed the tensile ones, which is of particular importance for materials with different resistance to compression and tension.     

Memory Effects in Rock Salt Under Triaxial Stress State and Their Use for Stress Measurement in a Rock Mass

Summary Regularities of memory effects in rock salt specimens under triaxial stress state were investigated. Each specimen was subjected to two loading cycles. The first cycle was axisymmetric triaxial compression (σ₁ >σ₂=σ₃). The second cycle was uniaxial compression in the direction of σ₁ of the first cycle. Distinct acoustic emission (AE) and deformation memory effects were observed in the second cycle at the stress value equal to a linear combination of the first cycle principal stresses given by σ₁− (k + 1) σ₃, where k is about 0.5–0.6 for rock salt. Anomalies in deformation curves were found to be more reliable than the AE methods in distinguishing memory symptoms. The necessary pre-requisite for memory formation in the first cycle was that σ₁ exceeded the elastic limit, corresponding to the given confining stress σ₃. Inflections in uniaxial stress versus axial strain and lateral strain curves, in the second cycle, were observed at equal stress values if in the first cycle σ₁ exceeded the elastic limit and memory-forming damage was induced. If there was no memory-forming damage, those inflections were seen at different stress values. This characteristic was used to distinguish between true memory effects and natural characteristic points in deformation curves derived from rock salt testing. A new memory symptom was established, namely a turn point in curve “uniaxial stress versus differential coefficient of lateral strains”. The results form a basis for application of the memory effects for stress measurement in rock salt masses.     

Observation of Post-failure Kaiser Effect in a Plastic Rock

—?Cyclic uniaxial and triaxial tests of rock salt samples were carried out in pre- and post-failure regimes. In all tests, a distinct Kaiser effect was observed in the post-failure region. The slope of the curve “cumulative AE counts versus axial strain” increased dramatically (several times to an order) when the strain attained its peak value achieved previously. Neither preliminary hydrostatic compression nor preliminary uniaxial cyclic pre-failure loading influence the post-failure Kaiser effect. The results for the plastic rock are opposite to the familiar absence of the Kaiser effect in brittle rocks at stress values approaching the ultimate strength. The results obtained during laboratory tests of a plastic rock are of importance for the proof of a connection between the Kaiser effect and foreshocks preceding earthquakes.