Identification of residual force in static load tests on instrumented screw displacement piles

Occurrence of the so-called residual force of an unknown value significantly disturbs interpretation of static load tests performed on piles equipped with additional measuring instruments. Screw displacement piles are the piling technology in which the residual force phenomenon is very common. Its formation mechanism is closely related to the installation method of this type of piles, which initiates generation of negative pile skin friction without any additional external factors. Knowledge of the value and distribution of a residual force (trapped in a pile shaft before starting the load test) is a necessary condition for the proper interpretation of instrumented pile test results. In this article, a clear and easy-to-use method of residual force identification, based on the analysis of shaft deformations recorded during pile unloading is presented. The method was successfully verified on two pile examples and proved to be effective and practical.

Non-destructive testing of bored piles

Quality control of bored piles is a complex operation aimed at determining possible defects in the pile shaft and the strength of the pile material made by different technologies. The presence of pile shaft defects and a decrease in the strength characteristics of the pile shaft material lead to the development of negative processes at the stage of subsequent operation of the building and structure. It is known that the bearing capacity of the pile material should not be less than the bearing capacity of the ground. Consequently, it is necessary to strictly observe the quality of the concrete of the design strength values to ensure the reliability of the designed building concerning the service life. Nowadays different methods of nondestructive testing such as pile integrity test, cross-hole sonic logging. The paper presents a discussion of the advantages and disadvantages of each of them, experimental data also are given

The Analytical Solution of the Grouting Migration Height for the Post-Grouted Drilled Shaft Based on the Herschel-Bulkley Model

The traditional bored pile technology has some arduous problems, such as the sediment at the pile tip, the mud skin along the pile shaft, and the stress release due to borehole construction. The post-grouted technology at the pile tip of bored pile has emerged because of demand. The grouting migration height (GMH) is of great significance to the strengthen and reinforcement of the pile foundation. This paper derives the calculation formula of the GMH based on the theory of the column hole expansion and Herschel-Bulkley model. The influence of relevant parameters on the GMH is discussed. Aiming at the problem of the grouting migration along the pile shaft in layered soils, the iterative calculation method of the GMH is proposed. The correctness of the GMH is verified by an engineering case, which can guide the engineering practice. The result shows that the GMH increases with the increase of the grouting pressure, the pile diameter and the thickness of the mud skin, and the grouting pressure is positively correlated with the GMH. The GMH decreases with the increase of the buried depth, the consistency coefficient and the rheological index. On this basis, the correctness of the GMH is verified by an engineering case.

MODEL TESTING OF THE "PILE-SOIL" INTERACTION UNDER AXIAL FORCE

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 studied sufficiently yet. 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 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 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 tip. The next series were aimed to study the influence of the flat rigid diaphragm inside the pile shaft. 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 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 tip is followed by decreasing of soil level in the pile 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 diameters) there may be no contact between diaphragm and the soil inside the pile (e) application of the diaphragm may lead to increasing of the pile’s bearing capacity. It was proposed (and checked by our tests) the technological improvement based on sand filling into space under the internal diaphragm to provide constant diaphragm-soil contact and related soil resistance.

Dynamic driving formulas and static loadings in the light of wave equation solutions

The advances in pile monitoring have motivated attempts to support dynamic formulas to estimate pile bearing capacity. Based on numerical analysis of the wave equation and the results of dynamic loading tests in three piles the paper deals with the investigation of the soundness of some of the most used in Brazil, namely, the so called Chellis-Velloso Formula, the Energy Approach Equation and Uto’s Formula. The former gained strength through a misinterpretation of Casagrande (1942) statement that the elastic compression of a pile during driving is a measure of the dynamic force with which the soil is tested, and not of its static resistance. Therefore, the elastic compression and rebound, measured during driving, are generally smaller than the corresponding static values. The second is based on an elasto-plastic load-displacement relationship without physical meaning, besides the fact that the effective energy in driving a pile is related to the work of dynamic forces and has nothing to do with the static resistances. The third was derived from a simplified solution of the wave equation, assuming among other hypothesis that there is no friction along pile shaft. The paper shows the ineffectiveness of attempts to universally validate these formulas with dynamic pile monitoring and the implications in the simulation of static loadings.