Physical modeling of steel tubular piles installation into sandy soil

Abstract

Modern marine structures (berths, breakwaters, offshore platforms, etc.) often include steel tubular piles of essential length (80-100 m and more) that should provide high bearing capacity in case of external axial loads application. Interaction between elements of the system “piled structure – soil media” is not yet studied sufficiently. It relates also to the bearing capacity of the long steel tubular piles of large diameter. One of the interesting peculiarities of long tubular piles’ behavior is the formation of soil plug at the piles’ tip. There are a lot of suggestion and methods aimed to increase piles bearing capacity under static pressing load. One of them relates to use of the additional structural element, i.e., the internal diaphragm welded to the internal surface of the pile’s shaft. Such approach has been applied in some practical cases of marine construction and demonstrated its effectiveness. At the moment there are no researches focused on study of the peculiarities of internal diaphragm application. So proposed research aimed to study two connected processes during steel tubular pile driving: soil plug formation at the tip of the open-end pile and soil behavior under the internal diaphragm fixed inside the tubular pile’s shaft. To study mentioned processes we provided several series of laboratory experiments fulfilled at the Geotechnical laboratory of the Department “Sea, River Ports and Waterways” in Odessa National Maritime University. In these experiments the model of steel tubular pile has been driven (pressed) into fine sand by mechanical jack. The first series was devoted to determination of the conditions related to the soil plug formation at the pile’s tip (results are presented in this paper). The next series were aimed to study the influence of the rigid diaphragm inside the pile’s shaft (to be presented in the further publications). Obtained experimental results allow to conclude that (a) in the fine sand the plug is formatted at the comparatively early stage of pile installation (in case of our modeling - at the penetration depth of some 4-5 pile’s diameter); (b) our empirical assessment of the conditions of soil plug formation corresponds to the approaches based on PLR and IFR characteristics; (c) formation of soil plug at the pile’s tip is followed by decreasing of soil level in the pile’s shaft relatively its initial value (on completing the plug formation the soil level in the shaft become stable); (d) regarding above mentioned, we may note that in case of use of internal diaphragm on the recommended depth (5-7 pile’s diameters) there may be no contact between diaphragm and the soil inside the pile and the diaphragm does not come up with the soil. So, for the next series of our experiments, it should be foreseen assured contact of the diaphragm’s surface with soil underneath.
 As proved by previous studies, one of the interesting features of the behavior of long tubular piles is the formation of a soil plug at the lower end of the pile. From this point of view, it is important to study the effect of soil plug not only on the bearing capacity at the lower end of the pile, but also on the behavior of the soil inside the pile. It is shown that in fine-sandy soils a plug is formed at a relatively early stage of pile immersion (in this case - at a depth of immersion of about 4-5 pile diameters). The process of forming a soil plug at the lower end of the tubular pile during its immersion is accompanied by a decrease in soil surface level in the pile trunk relative to its initial value (upon completion of plug formation the soil surface level in the pile trunk stabilizes).