Tunnels passing through faults and weakness zones require more extensive support. To secure the tunnel from collapse during excavation, pre-support known as spiling is a frequently applied solution. For more disintegrated rock mass conditions, the preferred technical solution is often thicker steel pipes (hereby referred to as pipe spiles). When pipe spiles are used as pre-support in disintegrated rock masses, a potential failure mode is progressive fall-out of smaller rock blocks between the spiles. In this paper, the critical block size for which pipe spiles can be applied without progressive failure was studied. Small-scale laboratory tests were conducted with the use of a specially manufactured box made of steel plates and plexiglass panels.The width, depth and height of the box was approximately 0.7 m, 0.7 m and 0.6 m respectively. Steel tubes, representing pipe spiles, were inserted into the box through holes drilled in the plexiglass panels. Gravel, representing the surrounding rock mass, of various particle sizes between 2 and 32 mm was filled in and compacted. The locking mechanism was then released, and the subsequent re-arrangement of granular material was observed. The results of the tests are presented and discussed with respect to the required spacing between pipes to prevent this mode of failure. For the specified testing conditions, the critical limit of the ratio between the pipe spacing and block size was determined to be between 3.7 and 4.6 for a rougher steel-rock interface and between 2.8 and 3.2 for a smoother one representing a pure steel-rock interface.