Site investigations of foundation failures.

From time to time it is necessary to make investigations of failures or defects in existing structures The approach is somewhat different from that of normal site investigation work, and usually takes the form of trial pits dug at various points to expose the soil at foundation level and the foundation structure, together with deep trial pits or borings to investigate the full depth of the soil affected by bearing pressures A careful note is taken of all visible cracking and movements in the superstructure since the pattern of cracking is indicative of the mode of foundation movement, e g by sagging or hogging It is often necessary to make long-continued observations of changes in level and of movement of cracks by means of tell-tales Glass or paper tell-tales stuck on the cracks by cement pats are of little use and are easily lost or damaged The tell-tales should consist of devices specially designed for the purpose or non-corrodible metal plugs cemented into holes drilled in the wall on each side of the crack and so arranged that both vertical and horizontal movements can be measured by micrometer gauges Similarly, points for taking levels should be well secured against removal or displacement They should consist preferably of steel bolts or pins set in the foundations and surrounded by a vertical pipe with a cover at ground level The levels should be referred to a well-established datum point at some distance from the affected structure, ground movements which may have caused foundation failure should not cause similar movement of the levelling datum

The Building Research Establishment in Britain has developed a number of devices such as tiltmeters and borehole extensometers for monitoring the movements of structures and foundations

A careful study should be made of adjacent structures to ascertain whether failure is of general occurrence, as in mining subsidence, or whether it is due to localized conditions

The past history of the site should be investigated with particular reference to the former existence of trees, hedgerows, farm buildings, or waste dumps The proximity of any growing trees should be noted, and information should be sought on the seasonal occurrence of cracking, for example if cracks tend to open or close in winter or summer, or are worse in dry years or wet years Any industrial plant in which forging hammers or presses cause ground vibrations should be noted, and inquiries should be made about any construction operations such as deep trenches, tunnels, blasting, or piling which may have been carried out in the locality