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The allowable loads on caissons shall be subject to the provisions of Section 13-132-020 of this Code with the following additions:
(a) Loads may be increased from a maximum of 200,000 pounds per square foot (100 tons per square foot) to a maximum of 400,000 pounds per square foot (200 tsf) but only when the base of the caisson is one foot or more below the surface of solid rock. The additional capacity may be calculated as 20 tons (40,000 pounds) per square foot for each foot of depth greater than one foot that the caisson is embedded in solid rock. Solid rock is defined as sound, unweathered rock without visible voids known to have a minimum thickness of eight feet below the bearing level which is free of detrimental fissures, cracks, clay pockets or disintegrated zones.
(b) Caissons founded on hardpan overlying a compressible soil layer shall be designed on the basis of the calculated settlement due to compression of this material.
(c) Piers or caissons bearing on hardpan may be belled to increased load carrying capacity, provided that such bell shall be at least 12 inches thick at its design edge and that the sides shall slope at an angle of not less than 60 degrees with the horizontal.
(d) The load used in determining the areas of the pier or of the belled bottom for soil bearing pressure calculations shall be the load supported at the top of the pier. Where indicated, negative skin friction or down drag forces resulting from settlement of soil above the bottom of the caisson must be considered as additional load on the caisson.
(Prior code § 70-12.1)