Pile Foundations in Calcareous Sediments

The primary structural components of jacket type offshore structures include the deck, jacket and pile foundations. The design and construction of offshore oil platforms founded in calcareous and carbonaceous materials are major challenges in marine structural and geotechnical engineering. Such sediments are stretched vast regions in the coastal area of Iran. Thus, the necessity of consideration and study of such soils is evident.Calcareous sands are accumulations of pieces of carbonate materials, usually derived from reworked shell fragments and skeletal debris of minute life. Several factors affect the qualities of calcareous sediments, such as grain type and size, cementation, pore spaces, environmental conditions, etc. Their mechanical behaviors mainly differ from terrigenous deposits or other types of sands and sediments like quartz or siliceous sands. Experiences have show that the application of engineering loads within the normal range causes calcareous soil structures to collapse and grains to crush, when conventional pile design theory and experience rely on data developed from terrestrial sediments where particles are not crushed, but instead, are displaced during pile installation, resulting in packing more densely. The crushing characteristics of calcareous soils are very important in defining foundation frictional resistance. In addition, overburden has very little influence on offshore calcareous sands in different manner with quartz sands. For these reasons, conventional soil mechanics strength and identification testing does not provide a good definition of soil properties and engineering behavior of calcareous sediments. Conventional soil mechanics theories should not be directly applied to estimate pile capacity in such cases and field load testing is a more reliable procedure. Due to the above points and other researches the ultimate bearing capacity for embedded piles in calcareous soils is lower than the values obtained for pile tipped in quartz sands and is limited to a certain value, which can be even less than 50% of the values obtained by conventional design procedures. For sands having the same relative density, the skin friction of driven piles in calcareous sands is less than that developed in siliceous sands and is limited to a certain value. There are many other differences in behaviors of piles in calcareous soils such as lateral and cyclic behaviors, which will be presented. This paper contributes to illustrate key features and associated problems for piling in calcareous and carbonaceous soils and sediments. It focuses on providing data obtained both in the field and laboratory. Some case problems for piling in will be described and finally guideline will be given for in such sediments.

Keywords:

pile driving , pile capacity , calcareous soils , skin friction , end bearing , cyclic loading