J.J.K. Daemen

Experimental investigation of creep behavior of salt rock

Abstract The confining pressure and axial pressure effects on the time-dependent stress–strain behavior of salt rock are analyzed quantitatively based on a number of uniaxial and triaxial creep test results in this paper. An exponential function is suggested to model the creep strain from transient to steady-state and found fitting well the creep strain. The strain (or time) hardening effects are analyzed in detail. Calculated values using the proposed functions are compared with experimental results.

A new method of surface subsidence prediction for natural gas storage cavern in bedded rock salts

The surface subsidence above underground natural gas caverns in salt formations is mainly induced by the volume shrinkage of the cavern. In this paper a new method, based on the Mogi model, to predict the subsidence above bedded rock salts gas storage cavern is proposed. Firstly, the equivalent elastic boundary deformation of the cavern volume decrease is assumed to be the first-order approximation of the real subsidence induced by creep shrinkage. Secondly, the cavern shape is simplified to be a spherical one with the same depth and volume, subjected to the pressure change of hydrostatic pressure in the sphere. And then, solving for the subsidence above, the storage is analyzed as the boundary displacement problem of the spherical cavern subjected to pressure change of hydrostatic pressure in the sphere in semi-infinite space. Based on the equivalent elastic model, the Mogi model, which has been developed for subsidence prediction in volcano seismology, is introduced to solve the boundary displacement problem. Then the expressions for subsidence and for horizontal displacement are obtained. The advantage of the new method is that the ground displacements can be calculated directly from the cavern volume shrinkage, thus the calculation has much been simplified. The validity of the method is verified by comparing to numerical simulation results, which show that the Mogi model has high validity for subsidence prediction for the gas cavern in salt formations. In the end, some constructive discussions on the application and improvement of the Mogi model are presented for its further applications.

Variogram characterization of joint surface morphology and asperity deformation during shearing

The morphology of the joint interface is described using geostatistical parameters such as the sill, range and slope of the initial part of the variogram. The methodology is also used to identify the scale effect and to determine the appropriate cutoff length of the sample which is representative for a direct shear test. The anisotropy characteristics of the interface are described using the sill and slope of the initial portion of the variogram. The result is used to assess the flowpath for a hypothetical fluid injection test. The surface compliance of the interface subjected to shear is determined using the mean difference of the interface. The rate of deformation of the interface is represented in two-dimensions using polar coordinates. The practical implication of the rate of collapse of the loop on the stability of the translation motion is given.

Temperature effects on creep of tuff and its time—Dependent damage analysis

Abstract An investigation and explanation are given of temperature effects on creep of tuff. Creep tests have been conducted at room temperature and at elevated temperature (204°C). In order to model the creep behavior of tuff, a new time-dependent damage model and two definitions of stress intensity factor are proposed. The results of experiments and theoretical analysis show that the creep strain of tuff increases with increasing temperature under the same loading condition, and that the stress intensity factor is not only a function of stress states, but also of temperature.

Dynamic compaction properties of bentonite

Abstract Bentonite has been proposed as a primary material for sealing shafts of nuclear waste repositories. It possesses low permeability, chemical and physical stability, and compatibility with most host rock masses and groundwater chemistries. This paper investigates the construction properties of bentonite and evaluates the use of dynamic compaction in constructing an effective bentonite shaft seal. Extensive laboratory tests of dynamic compaction have been conducted to study the densification of granular bentonite mixed with distilled deionized water or with brine from the Waste Isolation Pilot Plant (WIPP). Results of the dynamic compaction investigations delineate the influence of moisture content, compactive energy, mixed brine content, lift thickness, and rammer weight on the achievable dry density. Dynamic compaction can densify bentonite to a dry density of 1.86 Mg/m 3 when mixed with WIPP brine and 1.74 Mg/m 3 when mixed with distilled deionized water. At these densities bentonite exhibits permeabilities on the order of 1.0×10 −19 m 2 .