ABSTRACT
23 Assessment of Difficult Ground Each ground investigation must be tailored to the local potential problems Overview of possible hazards recognizes the geological conditions which determine the scale of each: for any one site, many potential hazards can be eliminated in the desk study phase, while others demand specific investigation techniques. GROUND SUBSIDENCE Beyond the acceptable limits of structural settlement on soils, this can only occur on certain rock types. Limestone with solution cavities provides the most difficult ground (section 29). Can also occur on clay, peat, loess, chalk, salt, gypsum, basalt (sections 27, 28). Major potential hazard over any mined ground. SOFT GROUND Mostly provided by alluvial clays, lake sediments, organic soils, young clays, artificial made ground. Laboratory testing to find bearing capacity. CAVITY S E A R C H Natural and artificial cavities in rock are notoriously unpredictable in their locations. Local building codes may require probes for 5 m at each column base, with central vertical hole and holes splayed out at 30° on each corner. ROCKHEAD RELIEF May influence foundations and tunnelling. Buried valleys are most likely in areas of glaciation and meltwater erosion; larger features may be traced by seismic survey. Solution features and pinnacles dissect limestone rockhead; may need many boreholes or soil stripping. Tunnelling up through rockhead is major hazard; rely on boreholes from surface and probing in advance of heading. Latter would have averted Lôtschberg tunnel disaster (section 15).
