Mechanical Consequences of Suffusion on Undrained Behaviour of a Gap-Graded Cohesionless Soil - An Experimental Approach

Abstract

Fine particles may migrate in the preexisting pores of an internally unstable soil matrix caused by water flow. This migration changes the fine particle distribution and content at different zones and can affect the mechanical properties of these soils. Due to the different roles that fine particles can play in the force chains of an internally unstable soil, the available geometrical assessment methods do not predict post-erosion behavior of the soil. The fine particles may sit loose in the voids, provide lateral support for the primary soil matrix, or participate directly in stress transfer. This will depend on the fine content, particle size distribution, constriction size, relative density, stress path, and particle shape. However, to evaluate the post-erosion behavior accurately, computational modelling or experimental investigation needs to be conducted. A modified triaxial apparatus connected to a water supply system and collection tank was developed to investigate the post-erosion behavior of an internally unstable cohesionless soil under different loading patterns in undrained conditions. This system allowed all test phases to be completed, including erosion inside the triaxial chamber to remove any possible impact of specimen disturbance. The results suggest that the undrained shear strength of the eroded specimen increased at small vertical strains (0-4 %) under monotonic and cyclic loadings, whereas the initial modulus of elasticity remained unchanged. Also, the eroded specimen showed much higher resistance against cyclic loadings, whereas the non-eroded specimen was liquefied during less than five cycles of loading. This improvement was due to a better interlock between coarse particles due to erosion of fine particles. The hardening strain behavior of the noneroded specimen changed to limited flow deformation due to a decrease in the fine content. The flow deformation of the eroded specimen at medium strain may be due to the local increase in lubrication effect of fine particles in the eroded specimen.

Post-erosion mechanical response of internally unstable soil of varying size and flow regime

“…The increased initially with a decrease in (a ( ) decrease of the residual fine content down to 15.1 per cent) and then decreased with further reduction in equivalent intergranular contact index (decrease in the residual fine content down to 6.9 per cent). Mehdizadeh et al (2017a) showed that for the soil mixture used in this research coarse particles were more angular than fine particles. Therefore, the initial improvement in the undrained shear strength (initial peak shear stress, in Table 3) could be due to a better ′ interlock between the coarse particles but without the loss of semi-active fine particles which helps to prevent collapse at small strains.…”

“…The particle size distribution and physical properties of the soil mixture are shown in Fig. 1 Mehdizadeh et al (2017a) showed that angularity of particles in coarse fraction is higher than that of fine fraction which may enhance the erosion resistance as stated by Marot et al (2012). On the other hand, erosion of the more rounded fines will lead to an overall increase in the average angularity of particles and therefore an increase in post-erosion interlocking.…”

“…The internal instability of this gradation was examined based on methods developed by Kezdi (1969), Kenney and Lau (1986), Burenkova (1993) and Indraratna et al (2011) showing that the soil mixture is internally unstable. Table 1 about here>> The soil specimens with diameters of 50, 75 and 100 mm were compacted layer by layer using the moist tamping technique (Mehdizadeh et al, 2017a), adjusting the thickness of soil layers to ensure that the soil layers in samples with different height still receive almost the same compaction energy. To achieve a high level of saturation, carbon dioxide was injected at the bottom of the specimen using a flow controller at the low rate of 1 L/min for two hours while the cell pressure was maintained constant.…”

“…Vertical strains during the erosion phase were measured at five-minute intervals using the photogrammetry technique (Mehdizadeh et al, 2017a) and are shown in Fig. 7.…”

“…The air above the water was pressurized to the back-pressure applied to the specimen during the test. Details of the modified apparatus, testing procedure and repeatability of tests result were discussed thoroughly byMehdizadeh et al (2017a). Testing was performed in four stages as shown in…”