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This paper presents procedures developed for designing caissons bearing on highly consolidated soil mixtures having both granular and cohesive characteristics. The ultimate bearing capacity of these materials approaches that of rock, but testing and design procedures for soil are applicable. Several hundred static loading tests on bearing piles made over the past 20 years have demonstrated that highly consolidated soils will carry loads up to the crushing strength of the sand and gravel particles without displacement. Their ultimate bearing capacity ranges up to several hundred tons per square foot considerably in excess of nominal bearing capacity normally used on rock. These piles have been driven to practical refusal, all but eliminating consolidation settlement which is a controlling factor in caissons. Lacking the opportunity for preconsolidation of the bearing materials, caisson design consequently is limited to the preconsolidation or initial strength of the soil support well below the ultimate capacity available to driven bearing piles. Preliminary design incorporated in plans and specifications for bidding purposes is based on the results of borings and laboratory tests with standard field penetration used as the basic control. The final design is made in the field at the time of construction when bell diameters are adjusted to the field penetration taken at the bottom of the caisson. The field and laboratory testing is described briefly. A general expression for bearing capacity is developed in terms of internal stability and cohesion of soil mixtures. Design charts for rapid determination of bearing capacity in situ and for the selection of final bell sizes are explained.
caissons, bearing capacity, internal stability, cohesion, design procedures, borings, laboratory tests, penetration resistance, construction, evaluation, tests
Housel, W. S.
Professor of Civil Engineeringresearch consultantPersonal member ASTM, University of Michigan, Ann Arbor, Mich.