The Construction of Buildings 4 [antikvár]

CHAPTER ONE FOUNDATIONS SfB (16) Foundations: General SfB (17) Piles: General In Chapter 1 of Vol. I, subsoils were described as clay, sand and gravel and a table of safe bearing pressures was given. For the very moderate loads imposed by the foundations of small buildings, definition of the safe bearing pressures for clay, sand and gravel is sufficient because no appreciable compaction of the subsoil occurs under these small loads and an appreciable intensity of pressure does not extend to any great depth below the foundation. The foundations of large buildings impose considerable loads on subsoils so that consolidation of the subsoil may be appreciable either during the erection or for some years after the completion of the building. Owing to variations in the subsoil or to different intensities of pressure in the subsoil at various points below the building or to both, unequal settlement of the foundation could occur which may damage the structure. The intensity of pressure in the subsoil below the foundation of a large building may be considerable some depth below the foundation so that a stratum of weak subsoil in this region may give way. If the intensity of pressure below a foundation is sufficiently great it may cause the subsoil to collapse by shear failure, either by forcing a column of subsoil down, or by the displacement of soil each side of the foundation. To anticipate the likely behaviour of a subsoil under the foundation of a large building the Engineer must know the nature of the subsoil for some depth below the surface and have a knowledge of its behaviour under load. The general classification of soils as clay, sand and gravel is not sufficient for this purpose. Soil is defined as sediments and deposits of solid particles produced by the disintegration of rock and it is the size of the particles of a particular soil and the degree to which the particles bind together which is of interest to Engineers. Soils, therefore are defined as: — ^o^oiiMve or coarse grained soils (sand and gravel) — Cohesive or fine grained soils (clay and silt) — Organic soils (peat) This broad division of soils is by itself insufficient as many soils have the properties of both non-cohesive and cohesive soils and a more definitive classification of common soils is set out in the following table. Coarse Grained Soils Type Sile Idemificaiion Value as Foundation Boulders Stones over 65 mm dia. Good. Gravels- -Coarse -Fine Contains stones over 20 mm. in dia. Contains no stones over 20 mm. in dia. Excellent. Good. Sands - -Coarse -Medium -Fine Contains particles over 0-5 mm. in dia. Contains no panicles over 0-5 mm. in dia. Contains no particles over 0 25 mm. in dia. Excellent. Good. Fair to good. Fine Grained Soils Silts - -Sandy -Clayey Very gritty touch but grains not visible Slightly gritty touch Fair to poor. Fair to poor. Clays - -Stiff -Firm -Soft Cannot be moulded by finger pressure Can just be moulded by finger pressure Easily moulded by finger pressure Fair to poor. Fair to poor. Fair to poor. Organic- -Peat Brown or black fibrous nature Very poor.