Assessment of static performance of LKAB´s welded mesh : Laboratory testing and analysis

Sammanfattning: Surface support is an areal support, which is installed on rock excavation surface to prevent bulking of rock mass and retain loose rock fragments. Welded wire mesh is one type of surface support. Literature study indicates that there is a wide range of testing methods on mesh. Different setups regarding mesh configuration, installation, and load applying system are used to evaluate its load-bearing, deformation, and energy absorption capacities. Loads are applied in different ways to simulate both static and dynamic loading conditions. However, there is not any standardized testing method.   Common configuration of a welded mesh sheet in LKAB´s underground mines has the dimension of 2.3 m × 2.5 m and is made of 5.5 mm (in diameter) wires welded with a square grid pattern of 75 mm × 75 mm spacing. It is installed with a square bolt pattern with a bolt spacing of 1.0 m × 1.0 m particularly in seismically active areas. Comprehensive field damage investigations at LKAB’s Kiirunavaara mine have shown localized failure of mesh, i.e., the mesh was cut or torn by rock blocks as a result of a seismic event. This is especially common along the mesh overlap where mesh sheets are joined together. However, the performance of welded mesh used at the LKAB’s mines and its performance along mesh overlap is not well understood.   Laboratory tests of LKAB´s welded wire mesh were conducted at the Mining and Civil Engineering Lab at Luleå University of Technology. A test frame was built to test the mesh under static conditions after literature review. Mesh sheets with reduced dimension of 1.2 m x 1.2 m were mounted at the corner by four bolts on the frame and tested with different loading conditions in test series AA. In test series AB/AD, two mesh sheets with overlap were tested and the load was applied at the overlap between two bolts. For the base case in series AA, a rupture load of 41.4 kN was registered at a displacement of 0.44 m using the loading plate with the size of 0.07 m2 when the load was applied at the center of the whole mesh sheet. The highest rupture load, 60.7 kN at 0.26 m displacement, was reached when the size of the loading plate was increased from 0.07 m2 to 0.5 m2. Load-carrying capacity (by using rupture load) obtained for test series AA was in the range of 32.4 - 60.7 kN with a displacement range of 0.26 - 0.44 m, considering varied loading plate material and size of the loading plate. In test series AB/AD where focus was placed on the overlap, the load-carrying capacity was in the range of 28.9 – 47.5 kN at a displacement range of 0.19 – 0.22 m. A single mesh tested with this loading configuration gave the lowest rupture load, 28.9 kN at a displacement of 0.19 m. The load-carrying capacity of two mesh sheets with three grids overlap was increased to 47.5 kN at a displacement of 0.22 m. Stiffness of the tested mesh also increased with an overlap. There is nearly no difference in load-carrying capacity when the loading mode has changed from pulling to pushing. Reducing the number of grids at the overlap to one grid decreased the load-carrying capacity of the mesh significantly, and the overlap seemed to become open quickly as the load was applied on it.   Two failure modes were observed for the mesh tests: tensile failure of the wires and failure at the heat affected zone (HAZ). Failure at HAZ is caused by weakening of the wires at the welding points. In test series AA, failure at the HAZ was observed in all tests near a faceplate. In test series AB/AD, both tensile failure and failure at HAZ were observed. They were observed close to either a faceplate or the loading plate.   To conclude test series AA, there is a problem with the redistribution of load and failure always occurs at the welding points close to faceplates. Roof mesh with wider wires at the face plates and high energy absorbent mesh have shown good results regarding handling these described problems, therefore these could be tested with LKAB´s bolting pattern and mesh configurations. In test series AB/AD, the observed problem is that the load concentrates on the closest bolts, therefore the load should be redistributed to bolts next to the loaded area. Seismic mesh, straps and improved bolting pattern can help with that, and they could be tested to evaluate them further.