Calculation for the ultimate bearing capacity of piles based on the principles of soil mechanics.
The materials for the pile construction can be precisely specified, and their fabrication and installation can be controlled to conform to strict specification and code of practice requirements, the calculation of their bearing capacity is a complex matter which at the present time is based on theoretical concepts but mainly on methods based on experience.
The site area of supporting the foundation is exposed & inspection to be carried out by the pile contractors & consultants with the team of soil lab???. Samples should be taken from multiple locations to ensure that its bearing characteristics confirm the results of exploratory boreholes and soil tests.
FDT Test & the constructional techniques are used limited to a depth of only a few centimeters below the excavation level for a spread foundation. Virtually the whole mass of soil influenced by the bearing pressure remains undisturbed and unaffected by the constructional operations.
Safety factor against the general shear failure of the spread foundation and its settlement under the design working load can be predicted from physical characteristics of the undisturbed soil which depends on the complexity of the soil stratification.
The bearing capacity of the piled foundation is quite different. The soil strata or rock bed beneath the toe of a pile is compressed or loosened to an extent which affects its end-bearing resistance. Changes take place in the certain environmental conditions at the pile-soil interface over periods of days, months or years which affect the skin-friction resistance of a pile. These changes are due to the dissipation of excess pore pressure set up by installing the pile, to the relative effects of friction and cohesion which depend on the relative pile-to-soil movement and to chemical or electrochemical effects caused by the hardening of the concrete or the corrosion of the steel in contact with the soil.
Piles that are installed in groups to carry heavy foundation loads, the operation of driving or drilling for adjacent piles can cause changes in the bearing capacity and load/settlement characteristics of the piles.
The general pile construction procedure simple empirical factors are applied to the strength, density and compressibility properties of the undisturbed soil or rock. The various factors which can be used depend on the particular method of installation and are based on experience and on the results of field loading tests.
The basics of the soil mechanics subject are to calculate the bearing capacity of piles.
Pile Construction Tips
The total resistance of the pile to compression loads is the sum of two components, namely shaft friction, and base resistance. A pile in which the shaft-frictional component predominates is known as a friction pile while a pile bearing on a rock or some other hard incompressible material is known as an end-bearing pile.
The need for adopting an adequate safety factor with calculations to determine that we may never be able to estimate axial pile capacity in many soil types more accurately than about 30%’. However, even if it is possible to make a reliable estimate of total pile resistance, a further difficulty arises in predicting the problems involved in installing the piles to the calculated depths by the Structural Engineer.
In the case of driven and cast-in-place piles, the ability to drive the piling tube to the required depth and then to extract it within the pulling capacity of the piling rig must be correctly predicted. say, 20 m to carry safely a certain working load, but quite another problem to decide on the energy of the hammer required to drive the pile to this depth, and yet another problem to decide whether or not the pile will be irredeemably shattered while driving it to the required depth.
Conditions under the environmental changes with time affect the importance in calculating the bearing capacity of the pile in clay; the effects also include the rate of applying a load to a pile and the time interval between installing and testing a pile. The shaft-frictional resistance of a pile in clay loaded very slowly may only be one-half of that which is measured under the rate at which load is normally applied during a pile loading test.
The slow rate of loading may correspond to that of a building under construction, yet the ability of a pile to carry its load is judged on its behavior under a comparatively rapid loading test made only a few days after installation. Because of the importance of such time effects both in fine- and coarse-grained soils the only practicable way of determining the load-carrying capacity of a piled foundation is to confirm the design calculations by short-term tests on isolated single piles, and then to allow in the safety factor for any reduction in the carrying capacity with time.
The effects of grouping piles can be taken into account by considering the pile group to act as a block foundation, as described in ARTICLE LINK!!!!!!.