Abstract: A filter for molten metal having a high melting point is in the form of a net woven of glass yarns formed by twisting a multiplicity of glass fibers together. The glass fibers contain at least 75 wt. % of SiO.sub.2 and are thermally cured at a thermal shrinkage of 2.5 to 15%. The thermal shrinkage is expressed by (l-l')/.times.100 % wherein l is the length of the fiber before the thermal cure, and l' is the length of the fiber as thermally cured.

Abstract: A rock bolt to be inserted into a bore formed in the ground from the inner periphery of a tunnel and anchored to the ground with grout filled in the space in the bore around the bolt to reinforce the ground. The rock bolt has two grooves extending axially thereof and an elastic tubular member fixedly fitting around the bolt close to one end thereof. A tube having an opening close to the other end of the bolt is provided within one of the grooves. Another tube provided in the other groove has an opening close to one end of the tubular member remote from the above-mentioned one end of the bolt. Before the injection of the grout, the rock bolt can be fixed to the ground easily and reliably by pressing the elastic tubular member into the bore. The grout is filled into the space through one of the tubes while displacing air through the other tube.