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Earthen levees along rivers may break down because of piping; this is especially the case for levees built on two-stratum levee foundations with an impervious stratum and an underlying pervious substratum. However, the initial stage of this piping is difficult to recognize because finding the piping outlets along the whole levee is challenging. Practical experience confirms that weak spots, such as holes or thickness reductions in the blanket stratum, usually occur at the levee toe. Some of these are harmless, whereas others are not. This study takes weak spots into consideration and provides a detailed description and evaluation of the piping features. Experiments were conducted on three models with different pervious conditions in a flume. Weak spots caused by holes, a thickness reduction, or exposure of the substratum at the levee toe were tested. The results show that the thickness reduction influences the formation of continuous sand boiling once the confined water bursts the blanket stratum. The potential for piping is reduced when the pervious substratum is composed of thin and uniform soil. In the absence of a covering stratum, piping happens with sand boiling. Three typical phenomena, namely sand boiling near the levee toe, a pressure decrease of the confined water, and a rise in the hydraulic gradient in the pervious substratum, could be used as indicators of the occurrence of piping.

Internal erosion is one of the most critical contributing factors for levee failures. Contact erosion, as a type of internal erosion, dominants when the seepage flow direction is parallel with the interface between coarse and fine soil layers. As is often the case beneath the foundation of a levee or at the top of clay core in an embankment dam, contact erosion can induce continuous transport of fine particles. Consequently, these water-retaining structures may collapse. To mitigate such potential hazards, the condition for initiation and progression of contact erosion should be investigated. In this research, the characteristics of contact erosion within cohesionless soils are presented visually by means of a newly devised apparatus. Two types of transparent coarse particles, namely quartz rubble and glass bead, are incorporated in two tests to study the effect of hydraulic conditions and particle shape on the initiation of contact erosion. The occurrence of contact erosion is captured qualitatively by snapshots, and the variation of soil sample configuration and flow lines are monitored by a stationary camera. Quantitative measurements are also conducted. Hydraulic gradients, turbidity, eroded mass, and inlet water pressure are recorded. The results provide insight into the influence of flow condition and particle shape on the occurence of contact erosion. The initiation and progression of contact erosion are governed by both hydraulic conditions and pore morphology. The latter is more crucial.

The distribution of the contact stress between a spread foundation and subsoil has been regarded as a classic issue in geotechnical engineering. Previous studies on this problem mainly focused on rigid foundations under axial loads, usually with width-to-height ratios (w/h) of 2.5 or less. However, flexible foundations are much more widespread in engineering applications. To investigate the effect of rigidity on the contact stress distribution and settlement deformation under axial and eccentric compression, two series of model tests on spread foundations (with w/h ratios from 2.5 to 4.0) placed on silty sand and clay were conducted. The results indicate that, under axial load, the contact stress in the central area of the rigid foundation is smaller than that around the edges. However, as the aspect ratio increases, the opposite occurs. Furthermore, in the eccentric compression progress, the w/h ratio (representing the rigidity of a foundation) has an important impact on the pressure distribution. A clear asymmetric distribution of pressure was observed under the eccentric load, and the maximal stress appears around the edges of the eccentric side under the rigid foundations, while under the flexible foundations the maximal stress appears in the central area. These research results can provide a practical reference for engineering applications with flexible foundations.

A series of centrifuge model tests were performed to examine the short-term soil stress responses around closed-ended displacement piles during jacking in Shanghai silty clays. Miniature transducers were placed near the piles to monitor the evolution of the total radial soil stress and pore pressure at different positions, which were closely associated with the progress of pile penetration. The results show that these stresses accumulated when the pile approached and decreased when the tip (or shoulder) passed, which indicates that the transducers experienced a loading and unloading process. The maxima of the soil stress or pore pressure increased with the initial depth of the observation positions and decreased with the horizontal distance to the pile axis. Compared with the total radial stress, the arrival of the peak pore pressure had an obvious hysteresis. Three typical modes to describe the stress variations during the pile installation process were summarized. Furthermore, the cumulative total stresses and pore pressures at a given location because of the installation of two piles were investigated.