The mechanical behavior of engineering materials, whether of natural or artificial origin, varies with their internal structure. As the uniaxial strength σ_uis commonly used to characterize basic responses to external loading, it is of value to evaluate the effect of porosity n on the magnitude of σ_u. The authors first review some existing relationships developed and applied to quantify the influence of n on uniaxial strength, in compression (σ_c) and in tension (σ_t). A new general formulation, applicable to various materials over a wide range of porosities, is then proposed. Using experimental data from different sources, the validity of this new relationship is demonstrated and briefly discussed.Key words: uniaxial strength, compression, tension, porosity, porous media, experimental results, soil, rock, concrete.

Evaluating the stresses in backfilled stopes constitutes a critical step for insuring underground mine safety and for analyzing backfill response. In recent years, the writers have presented several methods for conducting numerical and analytical investigations to assess the stress state in backfilled stopes. The results have shown significant stress transfer to the rock mass along stope walls which induces an arching effect. To date, most solutions have been developed for (sub-) vertical openings. In this paper, the results of an extensive numerical investigation on inclined stopes are presented. Emphasis is placed on the influence of stope geometry, backfill properties, and filling sequence on the stress distribution in the backfill. Results indicate that for a given stope geometry, the most influential factors are the backfill shear strength parameters (cohesion $c^{\prime }$ and friction angle ${\varphi }^{\prime }$ ), Poisson’s ratio $\mu$ , and dilatation angle ${\psi }^{\prime }$ . Some of these parameters not only affect the stress magnitude, but also the distribution pattern which is shown to also depend on the filling sequence. One of the main outcomes from this study indicates that increasing the value of $c^{\prime }$ tends to change the mechanical response of backfill from that of a particulate material to that of a consolidated medium, with each layer responding somewhat like a bending beam under vertical loads. The numerical results are compared to others obtained in previous investigations and also to analytical solutions and experimental test results obtained on a physical model. These are discussed in relation to the effect of the influence factors and their implications for the analysis of backfilled stopes.

Underground backfilling offers significant economic and environmental advantages to mining operations. There is however a limited knowledge and understanding of how the backfill behaves within mine stopes, which creates some concern regarding the risk of accidents with potentially serious consequences. It is thus important to investigate further the response of backfill to ensure safe working conditions and optimize the filling sequence. This paper presents key results from a numerical study aimed at analyzing the hydrogeotechnical response of backfill in a narrow vertical stope. The simulations illustrate how stresses are influenced by stope geometry, water drainage, and filling rate. Three main cases are presented here to illustrate these effects; namely, (i) simulation of dry (or drained) backfill, (ii) a rapidly filled stope with progressive drainage and consolidation, and (iii) sequential backfill placement with different filling rates. The third case includes a simulation with evolving properties due to the binder added to the backfill. The results from the numerical analyses show that arching effects develop within narrow backfilled stopes because of the stiffness contrast between the rock and the fill material. This can produce a significant reduction of the stresses (horizontal and vertical) in comparison with the overburden pressure. The simulation results also show the development of excess pore-water pressures after the placement of the saturated backfill within the stope. Drainage tends to reduce these pressures and increase the frictional stresses along the rock walls. The sequentially filled stope simulations show that a rapid filling rate produces much higher total stresses and excess pore-water pressures, compared to slower rates. The simulation of the cemented backfill, with evolving properties, indicates that the progressive changes can have a significant effect on the total and effective stresses in the stope. A discussion follows on the implications of these results.

Mining backfill is widely used in underground mines worldwide. The stresses in backfilled stopes must be correctly evaluated because they influence the ground stability and the required strength of exposed (horizontally or vertically) cemented backfill. The stress estimation can be accomplished by numerical modeling with or without the use of interface elements between the backfill and rock walls. To date, most numerical models have been developed without the use of interface elements, and only a few have used interface elements with the same shear strength as the backfill. Stress changes resulting from the addition of interface elements or from the variations in the shear strength of interfaces have never been investigated. In this study, these aspects were analyzed with two-dimensional (2D) (plane-strain) numerical models. The stress changes resulting from the variation of the backfill’s shear strength were also analyzed. The results show that the stresses decreased as the interfaces’ shear strength increased but were relatively insensitive to the variation of the backfill’s shear strength. When the interfaces have the same shear strength as the backfill, the consideration of interface elements is not necessary in numerical models because the stress distributions with or without consideration of interface elements are almost identical. When the interfaces are weaker than the backfill, the stresses obtained by the two numerical models can differ considerably, and the interface elements should be used in numerical models.