These files span more than fifty years, and three incarnations of Durham County Council. Over the course of this time, the nature of the reports has changed considerably, which has resulted in variation of descriptions as tests were described differently at certain dates. The files are numbered according to their original reference numbers, used by the Highways Laboratory, which is roughly chronological. Until April 1986, this took the form of a sequential number prefixed by H. After that date, they were allocated numbers by financial year. So for example, the files from April 1992 to March 1993 end /92. They are arranged by geographical area. In the case of roads between two named locations, they are listed under the first location and cross referenced to the second. In cases where there are a large number of entries for a location, they have been broken down into categories, usually based around named or numbered roads and types of building (e.g. schools and colleges, public buildings) Where possible, the reports have been grouped together as a number of reports would often be commissioned for a given project. The description will always list the original H reference and these can also be found on the reports themselves. The reports vary greatly in size and detail. The early ones are often no more than a location and a list of figures, while later reports may run to several files. The Civil and Geotechnical Engineering Laboratory was closed in September 2011. For records in this catalogue check the section below.