Experimental study of the evolution of pore water pressure and total stresses during and after the deposition of slurried backfill

Mining backfill is increasingly used in underground mines to fill stopes. Its successful application depends on the stability of barricades built to retain the backfill in stopes. The design of barricades requires a good estimation of pore water pressure (PWP) and total stresses during and after the deposition. On this regard, a large number of works have been published on analytical and numerical solutions. There are however very few experimental results with simultaneous measurements of PWP as well as horizontal and vertical total stresses that can be used to validate or calibrate the analytical and numerical solutions. For a specific project, field measurements are interesting in terms of representativeness to field conditions, but the results are very difficult to be correctly interpreted because the treated problem can involve a large number of uncertainties and the obtained results are due to combined effects of several influencing factors. Laboratory tests with simplified and well-controlled conditions are thus preferred. Until now, however, the most previous laboratory tests were conducted with dry backfill or with a tailings slurry instantaneously poured in a confining structure without simultaneous measurements of PWP as well as horizontal and vertical total stresses. Studies on the effects of filling rate and solid content of backfill on the variation of PWP and total stresses during the filling operation are absent. To fill these gaps, a series of column backfilling tests were conducted with simultaneous measurements of PWP as well as vertical and horizontal total stresses during and after the deposition of slurried backfill. When the filling rate is high, the test results showed that the PWP, horizontal and vertical total stresses increase at the same rate and equal to the iso-geostatic overburden pressure during the deposition of backfill slurry. Their peak values appear at the end of deposition. The backfill thus behaves like a liquid with little generation of effective stresses during the deposition. High filling rate and/or high solid content lead to high PWP and horizontal total stresses at the end of deposition. When the filling rate is small, the PWP and total stresses exhibit also peak values at the end of filling operation, but the vertical total stress at the center can continue increasing with time after the end of deposition due to the suspended sensor and occurrence of a phenomenon known as stress shielding effect. The results also showed that the settlement of settled backfill after the end of slurry deposition can generally exhibits a fast evolution rate stage, followed by a slow evolution rate stage. The duration of the fast evolution rate stage and the final settlement of the settled backfill increase as the solid content decreases. The final settlement after the end of slurry deposition is related to the solid content, not to the filling rate.