The geological map should be consulted at an early stage in any project analysis, but in view of the limitations of the standard map we in the Geological Survey of Northern Ireland have felt the need to orientate the product more towards the requirements of the engineer. The first result, the Belfast Three-Inch-to-One-Mile (1:21120) Map—Engineering Edition—is scheduled for publication in 1970. The choice of scale was limited to that of the only suitable topographic base-map available, but as the base is reduced from the six-inch map the topographic detail allows sites to be precisely located. The topographic map is overprinted with a conventional coloured geological drift map, and the front of the sheet also bears a horizontal section. On the reverse side is a table of rock and soil characteristics, a brief account of the geology, a rock-head contour map of the city area and a selection of borehole records.
Although a geological map is, of necessity, always an interpretation and not strictly a record of fact, Belfast is a suitable area to attempt a new approach. Here a wealth of borehole and excavation evidence, and recent geological remapping of most of the area, make possible a higher degree of accuracy than is normally possible.
The main foundation engineering problem in the city of Belfast is the widespread occurrence of soft post-glacial Estuarine Clays which have a very low bearing strength. Isopachytes for these deposits are superimposed on the geological map, and show that thicknesses of over 15 m occur