Effectiveness of Shotcrete in Underground Coal Mines

The primary objective of this project is to quantify the effectiveness, applications, and benefits of shotcrete in underground coal mines. Shotcrete refers to a ground-support system that is pneumatically sprayed, typically onto the exposed surface of an excavation face, to produce a compacted self-supporting and loadbearing layer. Quantitative information on shotcrete properties, such as adhesion strength to coal or other rock surfaces and its interaction with other support elements such as mesh, rock and cable bolts, are essential design considerations in the application of shotcrete.

Many researchers have elucidated the performance and failure mechanism of the shotcrete liner. However, many unknowns and misinterpretations remain on the failure process of fibre-reinforced shotcrete (FRS), such as the interactions between shotcrete and rock bolt configuration, the effect of the shotcrete adhesion, and the effect of in-plane confining loading conditions on the shotcrete liner, etc. These factors play an important role in understanding and developing a more practical and suitable shotcrete lining design.

Based on the review of current shotcrete techniques, the applications and testing methods of shotcrete are summarised. Shotcrete's versatility allows it to be applied in challenging environments while providing a strong and durable support structure. Shotcrete is a cost effective solution compared to other support techniques. However, in some complex geological conditions, shotcrete may need to be used in conjunction with other support elements to achieve the desired strength.

Bond strength is a critical parameter in shotcrete, which is influenced by various factors such as the roughness, the cleanliness, and the mineral composition of the substrate. Experimental work was conducted that examined the effectiveness of using smartphone photogrammetry technology to digitise rock surface roughness, as a preliminary step towards investigating its influence on shotcrete bonding strength at the interface. Results show that while errors exist, the accuracy of the technology is comparable to that of structured light 3D scanners. This low-cost and efficient method offers non-destructive evaluation, saving time and eliminating the need for expensive equipment and specialised personnel. Generally, a rough surface can facilitate the penetration of cement paste and enhance mechanical interlocking, thereby improving bond strength between shotcrete and substrate.

To evaluate the impact of substrate mineral composition on the bond strength between the shotcrete and substrate interface, a series of experimental works was carried out. The bond strengths at six different rocks-shotcrete interfaces were evaluated. Although bond strength is primarily provided by the cement hydration products in shotcrete, different chemical reactions occurring at the rock-shotcrete interface can affect the quantity and properties of these hydration products, thereby affecting the structure and bond strength of the interface.

A full-scale load-deflection test was carried out using a numerical analysis, LS-DYNA based on the nonlinear concrete model to represent fibre-reinforced shotcrete (FRS). The numerical input parameters were calibrated to achieve the Uniaxial Compressive Strength (UCS) of 40 MPa and flexural strength of 4.5 MPa. The flexural strength was obtained from Round Panel Tests (RPT) with the inclusion of 6 kg/m³ Barchip BC54 macro-synthetic fibre. A series of LS-DYNA analyses were performed and based on the findings, a practical shotcrete design approach has been proposed.

The main findings of this study are:

• The ultimate capacity of shotcrete liner is primarily governed by a flexural tensile failure mode rather than punching shear or direct shear failures.
• The confining effects on the shotcrete liner, which could be induced by ground in-situ stress conditions, has been investigated under various confining stress conditions.
• The increase of the ultimate capacity of shotcrete panel associated with the confining loading conditions has been investigated using the sectional load diagram (ACI 544.7R-16).
• The effect of shotcrete thickness was evaluated using LS-DYNA.
• The moment capacity, Cflex has a linear correlation with the estimated bending moment, Mo, with the ratio of 0.2, referred to as the Bending Moment Ratio, α.
• To validate the relationship between the shotcrete capacity and the bending moment ratio, a series of MIDAS analyses utilising structural shell elements were separately performed to evaluate the internal force distributions in the shotcrete panel.
• A practical shotcrete design approach was proposed.