Water management in hydraulic fill operations by M. Helinski and A.G. Grice is a paper presented in Montreal at the 2007 Mine Backfill symposium. The authors have kindly supplied me with a copy. Here is their abstract followed by my summary:

In the authors’ experience inappropriate water management in hydraulic fill masses has been the root cause of most barricade failures. Inappropriate water management can lead to high water pressures, reduced effective stresses and increased barricade stresses. In addition the high hydraulic conductivity and non-plastic nature of most hydraulic fills makes susceptibility to piping failure a high possibility in many hydraulic backfills where a static water head is available. In order to avert both of these failure mechanisms it is essential to control the elevation of the phreatic surface within the fill mass. This is usually achieved through campaigned filling and resting periods. The duration of filling and resting periods are often estimated using some form of stope drainage simulator. Due to the random nature of many characteristics that influence the fill mass hydraulic conductivity, simulation of hydraulic fill drainage can be difficult. The high consequences of inaccurate phreatic surface estimates combined with the high variability in drainage properties often results in operators taking a conservative approach to this modelling. This conservative approach can lead to significant delays in the filling cycle which can have a major impact on the productivity of many hydraulic fill operations and can often results in an unnecessary movement towards capital intensive paste fill systems. This paper presents a water management technique which combines a simplified drainage model with in-situ monitoring to provide a rational approach to water management.

The authors modeled flow of water from a mass of backfill using MODFLOW. Being a flownet junkie, I make bold to reproduce their results.

On the basis of their flow nets, they decided that if you put piezometers into the backfill somewhere above the opening and outflow point you can reasonably accurately monitor the overall situation and hence decide on when to put in more backfill without risking trouble. In the case of backfill trouble comes in two ways:

  1. The fast flow of water down through the backfill to the bulkhead may induce piping of the backfill; the whole mass of backfill may inrush to the mine workings.
  2. The great head of water in the backfill can put too much pressure on the barricades, and they may fail.

The solution, or at least the way to avoid these failures, is to place the backfill in a series of successive sessions. Allow the water pressures from one backfill placement to dissipate before you place the next. Piezometers in the backfill enable you to monitor water pressures. And if you understand the flownet in the backfill, hence the water pressure distribution, you can use the results of the piezometers to tell you when you can add more backfill. Also read the paper.