The geology of the area also affects route selection to a major extent apart from the topography. Iranian railways built prior to 1938 have a number of 'figures of eight' with spiral tunnels and gradient up to 2.8%. Investigations for tunnels comprise the following aspects: Route location, Topographic Survey or Alignment Survey, Geological Investigations, Hydrological Investigations, and Seismic Studies. A good geological investigation helps in anticipating the type of soil that will have to be gone through and planning construction methodology. Geological investigations perforce must follow a logical sequence, each stage improving upon the accuracy of the former but being more and more complete by itself. As part of preconstruction investigation, the geological nature can also be determined by running horizontal pilot headings between the individual shafts. Tunnels should be avoided in undermined regions, that is, in mining districts, because of the unpredictable nature of settlements and forces to which the tunnels may be subjected.