Tight Filling Challenges - Comparing Uncoupled and Coupled Backfilling of Underground Stopes with Cemented Paste Backfill
Authors: Veenstra R. L. , Grabinsky M. W. , Thompson B. D. 
Published: August 2024

ABSTRACT:
Uncoupled filling is defined as when the underground distribution system (UDS) is not connected to the fill mass whereas coupled (or tight) filling is defined as when the UDS can connect to the fill mass. This paper will explain the development of an early-age undrained-drained analysis methodology based on laboratory testing and in situ instrumentation. This methodology will then be used to numerically simulate the filling of two stope shapes with different backfill recipes to examine the filling regimes within these stopes, with a focus on how the shotcrete barricade wall responds to both filling regimes.
The goal of this paper is to explore the differences between uncoupled and coupled filling, and to present these differences so that the reader can understand the risks of containment wall/barricade failure associated with both types of filling.

1. INTRODUCTION
Cemented paste fill (CPF) is a commonly utilized backfill material used to create artificial pillars that support the rock mass to allow maximal economic extraction of an orebody. This material is composed of full stream mill tailings (usually), water, and cementitious binder, combined into a relatively dense slurry or paste. CPF has several advantages in the material can be engineered (e.g., the paste recipe can be designed to produce a particular strength at a particular time) as well as being able to be reticulated through system of underground pipelines or underground distribution system (UDS) to the UG void or stope that needs to be filled.
Since the CPF is a fluid when it enters the stope, it needs to be retained inside the void. This is commonly accomplished by building a backfill retaining wall across the stope access points that connect the stope to rest of the mine. These walls retain the backfill as the stope is filled. These walls have been built from a variety of material historically, but the most common current types are shotcrete structures (usually arched) or a waste rock berm. This paper will focus on the shotcrete arch type, which are termed "wall" in this paper and occasionally termed barricades elsewhere in literature.
CPF is a versatile type of fill that can be used for many mining methods. CPF filling can be divided into two types: uncoupled and coupled. Uncoupled filling is defined as situation where the fill mass cannot connect to the UDS and the only pressure that is placed on the wall is due to the self-weight loading of the backfill itself. A coupled (or tight) filling situation means that the fill mass can connect to the UDS and the wall load consists of the deposited backfill's self-weight load plus the pressure within the UDS (see Figure 1). Understanding this loading system is critical to assessing barricade failure risks, particularly in the tight filling context, as will be discussed. Tight filling is usually completed in order to maximize the amount of confinement being placed against the crown of the stope, and is particularly common in cut and fill mining applications.