Combined Geometric Hydraulic Criteria Approach for Piping and Internal Erosion in Cohesionless Soils

Abstract

Piping is an erosion mechanism that plays a significant role in river barrages, such as dikes and dams, that are founded on poorly graded cohesionless soils like, for instance, fine or medium sands. Likewise, in gap-graded or widely graded cohesionless soil, finer grains can pass through the pore matrix of coarse soil because of seepage. This phenomenon, which is called internal instability or suffusion, can occur in granular filter in or under dams, dikes, barrages, or any other water-retaining structure. For economic reasons, unstable soils are generally used in practice if the hydraulic force or the hydrodynamic energy is smaller than the critical hydraulic force or the critical hydrodynamic energy. Therefore, an approach of combined geometric-hydraulic criterion is expected to yield better results. Based on this premise, laboratory tests of piping and internal erosion are carried out using five cohesionless soils. An analysis of the results considering the instability index shows that the critical hydraulic gradient required to initiate the piping and internal erosion depends on the curve of the grain size distribution, the initial relative density of the soil, the seepage direction, and the initial stress condition in the soil. Based on these laboratory results and the results reported by several other authors, a combined geometric-hydraulic criterion with respect to piping and internal erosion in cohesionless soils is developed and proposed.

Experimental Analysis of Hydraulic Conductivity for Saturated Granular Soils

“…for gap-graded gravely sand Equation (18) For gap-graded gravely sand and water at 20˚C: Figure 12 presents a comparison between the proposed approach (Equation (15) or Equation ( 16)), the Navfac DM-7 (1974) [41] chart, the approach by Carrier (2003) [21], and the recommendations of the committee for waterfront structures harbours and waterways EAU (2012) [42]. For the calculation of the vertical hydraulic in accordance with Carrier (2003) [21] approach, a typical 1/s 2 value of 6.1*10 −4 cm −2 for sand is considered.…”

“…Vertical permeameter tests have been carried out to analyse the hydraulic conductivity of saturated soil materials. For that a special test device have been designed and used for the vertical saturated conductivity in non-cohesive soils under vertical upward seepage (Figure 2, Figure 3 and Figure 5) (Ahlinhan and Adjovi 2019) [18]. The clear internal size of the cylindrical pot is 28.5 cm for the diameter and 47 cm for the height for the vertical saturated hydraulic conductivity under upward seepage.…”

“…This indicates that a steady flow is reached, and the Darcy law is valid. The hydraulic head is then gradually increased, and the seepage outflow is measured again in each time interval of five minutes until It should be noted that from a certain hydraulic head some sand boils occur on the sample surface, which indicate the initiation of the internal erosion or suffusion (Ahlinhan and Adjovi 2019) [18]. This hydraulic gradient that leads to internal erosion or suffusion was not considered for the determination of the saturated hydraulic conductivity, due to significant increase of the hydraulic conductivity of the soil sample and migration of fine particles in the pore matrix that lead to sand boils on the sample surface.…”

“…This explains why most methods of practice for predicting the hydraulic conductivity of saturated soil use the grain size distribution curve (GSDC) instead of the pore space such as the pore size distribution curve (PSDC). Evidently, the PSDC is linked to the GSDC (Ahlinhan and Adjovi 2019 [18]). Simplified descriptions of the pore space, such as bundles of straight tubes, have been used to predict the hydraulic conductivity of saturated soil k sat .…”