Conventional reinforcement as a potential prevention measure against piping

Piping is one of the main failure mechanisms that can affect the safety of water-retaining structures. A phenomenon that can occur when a local disruption of water structure caused sand erosion and concentration of seepage flow at that location. This entails sufficient hydraulic gradient resulting in the formation of shallow pipes in the sand layer. There are number of methods to increase the factor of safety against piping. An effective technique is soil reinforcement. The soil reinforcement can be performed by the inclusion of elements (strips, bars, etc) within the mass of soil in a preferred direction. Geogrids can be used as a convenient reinforcement material for improving the behaviour of the soil because of the high tensile resistance and significant friction in the soil. This paper presents laboratory experiments that were performed on unreinforced and reinforced soil samples. Reinforcement was done using different types of geogrid in different layers of the soil sample to investigate the effect of this method and arrangement of the geogrid sheets on the critical hydraulic gradient and resistance against piping. Subsequently, the results demonstrate that reinforcement increased the critical hydraulic gradient up to 75% compared to the value in unreinforced soil. The amount of improvement of the critical gradient is dependent on the arrangement and type of the geogrid

On the mechanism of backward erosion piping in a CSB

Backward erosion tests on vertically layered sand samples, as for example are present in a coarse sand barrier, result in failure patterns that are different from the ones found in tests using one homogeneous sand layer. In this paper, it is explored what are the underlying erosion mechanisms for vertically layered sands. It appears that there are at least 2 erosion mechanisms caused by the hydraulic gradients in the soil: instability of a sandy slope due to an outward directed gradient and high horizontal gradients due to flow line contraction. Both these mechanisms are present in different erosion conditions. This paper describes the mechanisms and quantifies them to some extent

Analysis of development and depth of backward erosion pipes in the presence of a coarse sand barrier

Backward erosion piping (BEP) is a failure mechanism that can affect the safety of water-retaining structures. It can occur when a local anomaly on the downstream side of an embankment causes a concentration of seepage flow at that location. Shallow pipes may then form, progressing in the upstream direction and leading to a collapse of the water-retaining structure. A novel and economically appealing measure against BEP is the coarse sand barrier (CSB), which is now being developed in a multiscale experimental programme in the Netherlands. The method involves placing a trench filled with coarse sand below the blanket layer on the downstream side of the embankment. The CSB prevents the upstream progression of the pipe and significantly enhances resistance to BEP. This paper presents medium-scale laboratory tests involving a range of sands, barrier depths and relative densities. The piping process and the observations of pipe progression in the presence of a CSB are presented, followed by a conceptual model. The presence of a CSB changed the erosion pattern. It resulted in pipe formation perpendicular to the flow direction over the entire width of the barrier before the barrier was damaged. The findings also demonstrate the effect of material properties on pipe initiation, progression and pipe depth. Measurements of the pipe depth are presented and analysed, revealing the significance of pipe depth for understanding the piping process. This analysis shows considerable erosion in the downstream background sand and demonstrates that erosion profiles and measured pipe depths are significantly larger than in BEP tests without a CSB

Scale effects in coarse sand barrier experiments

The coarse sand barrier is considered as a promising measure to prevent backward erosion piping from causing failure of embankments. A pipe is allowed to progress backwards until it encounters the coarse sand barrier, which prevents it from progressing unless the head difference over the embankment is significantly increased. A three stage experimental programme supported by groundwater flow modelling is carried out to investigate the feasibility of this method. The hypothesis is that the strength of the barrier is characterised by a local gradient at the interface between the barrier and the pipe. Major questions are: can the horizontal gradient as measured in laboratory tests be used to characterise the strength of the barrier material, over which distance should a horizontal gradient be determined, and is this distance the same for models at different scales? This paper presents the background theory and demonstrates the effects using scale dependent criteria. Preliminary results of small- and medium-scale experiments are used to compare the two approaches