Collapsible soil exhibits considerable strength when it is dry, but when it is inundated, it loses its strength and experiences significant volume reduction. Foundations on collapsible soils subjected to inundation may experience sudden settlement without any increase in the in situ stress level. Pile foundations are often used to penetrate layers of collapsible soil to transfer the load to a lower, stronger soil layer. Nevertheless, because of indentation, these piles may experience negative skin friction, which may lead to significant reduction in the pile capacity, or perhaps separation of the pile from its cap. In the literature, although several theories can be found dealing with the design of pile foundations and the prediction of negative skin friction on these piles, little to none was found to estimate the negative skin friction because of the sudden collapse of the surrounding soils. This is mainly because of the difficulties associated in modeling collapsible soil experimentally or nume...

Introduction: Negative skin friction that develops on the pile surface when the soil (adjacent to the pile shaft) settles but the pile is in static condition, causes an additional load (known as drag load) induced on pile. Substantial settlement of collapsible soil occurs only because of inundation. In such a case, the magnitude of drag load is influenced by several parameters, such as the collapse potential, inundation pressure, pile roughness, pile diameter, radius of wetting, depth to the neutral plane, and thickness of collapsible soil subjected to inundation.Methods: A numerical model is developed to simulate the case of a single pile in collapsible soil to predict negative skin friction and drag load by giving consideration to kinetic friction angle between the pile and the settling soil. The comprehensive interdependent relation among the parameters influencing the magnitude of drag load was revealed based on numerical results. Analytical models are developed for predicting the average negative skin friction and depth of neutral plane to quantify drag load because of the inundation of collapsible soil adjacent to the pile shaft.Result and Conclusion: The model coefficients were determined by analyzing the numerical results. The drag loads obtained from the analytical model developed and those from previous experimental studies are found in good agreement. For a given soil profile, the magnitude of drag load is found to vary widely (e.g., between 252 and 925 kN) because of the variations in the pile diameter, pile roughness, and radius of wetting. Thus, this study provides a design guideline for choosing the design pile diameter considering the magnitude of drag load due to inundation of collapsible soil adjacent to the pile shaft.

Seven dynamic load tests were performed on six large diameter piles, installed near the Gumti river Bangladesh. The diameter of five cast in situ piles was 1.5 m, and that of the steel pile was 1 m. The steel pile was tested twice: under both drive and re-drive conditions. The pile lengths varied from 45 m to 71.8 m. The distribution of pile bearing resistance under static load were evaluated using the program DLTWAVE by signal matching. In addition, standard penetration tests were conducted in the vicinity of all the six piles. The results of dynamic load test were calibrated by comparing the static capacity obtained from static and dynamic load tests, conducted on two identical cast in place piles. Therefore, the skin frictional resistance and pile tip resistance are calculated using the conventional formulae of static analysis utilizing the developed correlation for the clay layers. The consistency of clay soil is observed to play an important role in the development of shaft resistance. For very stiff to hard clay, DLTWAVE indicates the development of shaft resistance smaller than that obtained from the formula (static analysis). Greater skin frictional resistance was obtained in DLT re-drive, as compared to that during initial drive.

This paper explores whether delay in construction after compaction can benefit from the gain in soil's strength and stability point of view. An experimental investigation has been carried out to examine the gradual development of soil's shear strength by ageing of mechanically compacted soil at three relative densities. In order to separate the gain in strength due to ageing from that occurring from the reduction in soil moisture, the soil samples prepared in moulds were kept in desiccators for different periods of time (1, 9 and 17 days) before testing unconfined compressive strength test. The soil in densely compacted state is found to gain in strength due to ageing faster than that in medium compacted state. Only due to ageing of 9 days or more, unconfined compressive strength of compacted soil is found about 1.7 to 2.4 times of that attained in day 1 after compaction.