A study of Mine Fill Grouting: Characteristics of deposited grout and the factors that influence them

A trial was undertaken to simulate the emplacement of grout into flooded mine voids, to study the phenomenon of segregation and evaluate its significance for the properties of the hardened grout. The trial, which employed a 1.65 m wide x 2.35 m high x 24 m long container to simulate a coal mine bord, involved the deposition of 22 m3 of lower viscosity permeation grout into a 0.5 m deep layer of rubble on the first day, followed by deposition of 18 m3 of higher viscosity bulk fill grout on the second day.

Observations during grout deposition indicate that for workings with rubble on the floor, a high degree of permeation is possible, but it is likely to be dependent upon the presence (or not) of fines in the rubble. For the relatively coarse rubble used in the trial, the grout permeated the rubble on the floor of the container in preference to flowing over it.

Observations of the hardened grout removed from the container confirm that a form of segregation, affecting the strength of the deposited grout, does take place in the grouting of mine workings, at least under certain conditions. Whilst there is some tendency for particle segregation to occur, causing coarser average ash particle sizes closer to the deposition point, entrainment of water by grout flowing through flooded workings is likely to be the most significant cause of heterogeneity in the hardened grout. The hardened grout samples recovered from the trial showed a wide range of water content and density, with clear trends for reduced strength with increased water content and decreased density, with greater distance from the point of deposition.

A critical factor in the entrainment of water during deposition, with direct consequences for the lateral flow/spread of the grout, is whether or not the grout is discharged through water at some height above a surface (floor or rubble), or whether it is discharged directly onto a surface such that it immediately engulfs the end of the tremie, so that there is no opportunity for the grout to mix with the water as it emerges. Grout deposited into water tends to mix turbulently with the water as it falls, and to travel longer distances down the grout beach slope as a turbidity front, tending to create beach slopes as flat as 44H:1V. Grout that is deposited directly into pre-existing mounds of fresh grout, is excluded from mixing with and entraining additional water, and instead, tends to cause the grout mound to swell, and spread slowly and gently away from the point of deposition. Slopes of bulk fill grout deposited into mounds may be as steep as 15H:1V

It was found that the wettest deposited grout, which resulted from permeation grout (25 second flow cone) deposited through water, tended to consolidate under its own self weight so as to reduce its water content, after it had stopped flowing. On this basis, it is possible that a limit exists for the maximum water content that can prevail in 16% cement-flyash grout, and that this limit was reached in the grouts deposited in this trial, corresponding to the upper bound water content values of about 65%. As such, the low strengths measured in this study may be considered as a lower bound to the strengths likely to be achieved when cement-flyash grout is used to remediate mine voids, found to be of the order of 1 MPa after 28 days.

Based on these observations, strategic positioning of the height of the tremie at either the top or the bottom of water-filled voids can be used to improve the outcomes of permeation and bulk fill grouting, respectively.

Layers of suspended ash and froth which settle onto deposition surfaces between grout deposition events appear to result in weak grout interfaces and are significant for subsequent attempts to verify the integrity of grout remediation through diamond core drilling.