Development of ‘rockhead’ computer-generated geological models to assist geohazard prediction in London

Abstract

Abstract Much of London is founded on Quaternary river terrace deposits, consisting mainly of sand and gravel associated with the course of the River Thames. The base of these Quaternary river terrace deposits, the ‘rockhead’ surface, is generally planar and rests on London Clay. Locally, however, the terraces may include steep-sided, gravel-filled hollows, some of which penetrate other solid formations below the London Clay, such as the Lambeth Group (formerly known as the Woolwich and Reading Beds). Unforeseen engineering problems involving deep foundation construction or tunnelling beneath London have resulted from failure to predict irregularities in the base of Quaternary deposits. Detailed knowledge of the three-dimensional shape of the Quaternary deposits can assist in predicting ground conditions. In order to provide such a model more than 22 000 borehole records from London have been encoded into a digital, relational database. Contour plots of specific geological surfaces, including the top of the Chalk, the base of the London Clay and the base of the Quaternary deposits have been generated from the database using computer-modelling software. Quality assurance is an important part of the databasing procedure and suspect borehole information is discarded at an early stage. As computer modelling proceeds, further data may be eliminated if considered anomalous. The computing infrastructure provides graphical presentations of borehole logs, contour plots, isopachyte (thickness) plots of the major geological units together with cross-sections, fence diagrams, block diagrams and perspective views, giving an enhanced, three-dimensional understanding of the geology of London.