The International Information Center for Geotechnical Engineers

Review of Polyurethane Resin Grouting for Rock Mass Stabilization


7.0 CONCLUSION                                                                                       

Polyurethanes are a versatile and widely applicable material for civil engineering applications. Regarding stabilization in rock, PUR has been advantageous to the coal mining industry for decades, due to the ease of mobilization, fast set time, and high strength of the PUR material. Only recently has it been extended to above ground applications for rock slope stabilization, where cost considerations and other stabilization methods are more competitive.

For underground mining applications, proper characterization of the rock mass is vital to both estimating the amount of product required for injection as well as targeting injection zones. Additionally, having an expected volume of material allows the contractor to compare to the amount of material injected during the grouting process, which could point to multiple issues such as loss of PUR to the target zone, or unknown void spaces. Targeting the zone for improvement is performed by constructing a grout curtain, after which the targeted zone is filled by injecting PUR in a typical grid pattern. Spacing of the injection boreholes is the design choice of the grouting contractor, as it is sensitive to the fracture patterns and geometry of the project. It has been shown that 100% filling of void space is not required to stabilize mine roofs, however verification of void space filling is important to ensure that the roof is stabilized and other stabilization methods are not required.

For rock slope stability, more recent studies by the CFLHD have focused on stabilization of rock slopes along transportation routes. These studies have concluded that PUR grouting is an effective stabilization technique that can reduce the number of rock anchors or dowels required to stabilize a rock slope against rock falls. They recommend using PUR in fractured rock with fracture apertures greater than 2 mm. The injection sequence should be performed from the bottom up using a staged pumping approach, with borehole spacing approximately 8 ft to 16 ft apart. Pumping should be performed at low pressures (less than 250 psi) to avoid fracturing the rock mass further and causing rock falls. Finally, PUR may not be cost effective in above ground applications if the void space of the fractured rock mass is excessive, requiring an excessive amount of PUR material to fill.

In general, PUR grouting is a proven method for roof and longwall stabilization in the coal mining industry, and is becoming much more prevalent in above ground rock slope stability applications. The ease of mobilization, as well as the chemical control of set time, strength, and viscosity of the PUR material make PUR grouting an attractive option for rock stabilization, however material costs, as well as the difficulty of flow control after injection and the difficulty of verification may make it less attractive than other stabilization methods.


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