1、Reinforcing Coal Mine Roof with olyurethane Injection:4 Case StudiesGregory MolindaReceived: 16 November 2006 / Accepted: 13 March 2008 / Published online: 12 April 2008_ Springer Science+Business Media B.V. 2008Abstract:NIOSH has recently completed a study of the interaction between polyurethane (P
2、UR) and coal mine roof in order to determine the mechanism of reinforcement, in both highly fractured rock and unfractured rock. Four case studies of PUR reinforcement are presented. At a West Virginia site, a borehole camera revealed the location of roof voids and guided the PUR injection. By injec
3、ting polyurethane into a zone from 0.6 to 1.8 m (26 ft.) high in the roof, a roof beam was created and ongoing intersection falls were halted. In another highly fractured roof in a western Pennsylvania mine, a total of 5.8 cm (2.3 in.) of rubbleized rock was found in a zone up to 3 m (10 ft.) into t
4、he roof. Untargeted PUR injection filled approximately 1/2 of the fractures. These two case studies showed that it is not necessary to fill up 100% of the void space to create stability. In the other two field sites, polyurethane was injected into weak, but unfractured roof. Post-injection video mon
5、itoring showed that weak bedding planes were hydraulically wedged open and polyurethane injected along bedding. The reinforcement value of this injection method is limited because of the wafer-thin layer of PUR introduced along bedding, and the lack of a PUR webbing which would serve as a structural
6、 framework to provide strength.Keywords: Coal mining Ground control Roof falls Roof support Polyurethane1 BackgroundCoal mine roof strata has been successfully reinforced with polyurethane (PUR) for over 40 years。Applications have included headgate stabilizations,rock consolidation over coal panels
7、in advance of mining, and roof reinforcement for shield recovery.Polyurethane injection for ground stabilization in coal mines was first developed by the German coal mine research organization Bergbau-Forschung GmbH in the early 1960s (Jankowski 1972). It became a standard stabilization method in Ge
8、rmany after its commercial introduction in 1971 (Knoblauch 1994). With the introduction of the RokLok binder system in 1977, polyurethane stabilization, particularly in longwall recovery, has become common in the US (Stewart and Hesse 1985).Polyurethane injection in coal mines is most commonly used
9、in difficult ground conditions including fractured rock in headgates and tailgates, and as a stabilization remedy to prevent longwall face caving. It may also be used as a replacement for roof meshing in shield recovery, and as a sealant to prevent groundwater inflow, but often it is applied as a la
10、st resort where conventional roof reinforcement andsupport has failed.Polyurethane is typically a two component system that has several advantages over conventional support.It has the ability to chemically bond to the rock, unlike other supports which rely on frictional contact.Because it is injecte
11、d under pressure, it inherently targets fractures, which are the paths of least resistance. It also has a low viscosity which allows it to penetrate cracks as small as 0.05 mm (0.002 in.) wide (Knoblauch 1994). It has engineered expansion properties (1:1 to 1:12) which also allow for penetration.(Sh
12、aller and Russell 1986). It is both strong and plastic, preserving its integrity under load and racking-type deformations (Micon 2003). Finally, itdoes not obstruct roadways like standing support.2 Current PUR Injection Design ProcessInjection designs currently have a one size fits allapproach, with
13、 drilling patterns and chemical grout volumes decided in advance, often for the sake of convenience rather than engineering design. A design process is needed to determine the optimum location for injecting polyurethane grout in order to maximize the reinforcement benefit and prevent wasting large v
14、olumes of chemical.There are a number of variables which must be considered:1. The location of fracturesThis information will help determine the zone to target for polyurethane injection.2. The extent of the fracture zoneAn estimation of the total void space could be used to calculate the volume of
15、PUR needed. In highly fractured roof, more test holes may be required.3. Character of the fracturesA determination of the nature of fractures, whether they are beddingseparations or rubbleized zones, will indicate the permeability of the zone (Molinda 2004). Upholemapping of fractures will help defi
16、ne permeability.4. Injection pressuresOften the injection proceeds until a pre-determined injection pressure isachieved, indicating that the fractures are filled If no back pressure is ever achieved the indicationis that the fracture zone is infinitely large.Conversely, if a high back pressure is reached immediately or very quickly, then the roof is considered to be unfractured, and further pumping may hydrofracture the
