Engineering benefits of replacing natural sand with manufactured … – Nature.com

Assessment of shear strength under dry and hydrated conditions

The interface shear tests were designed to understand the effects of overburden stress levels represented through normal stress variations, grain shape effects through natural and Msand having significant difference in their particle shape and hydration effects through dry and wet tests. The shear response of GCL-River sand and GCL- Msand interfaces in dry condition at different normal stresses is presented in Fig.6. Variation of shear stress with displacement is shown in Fig.6a and MohrCoulomb failure envelopes are shown in Fig.6b. Most of these initial tests were repeated to confirm the reproducibility of results. For both GCL-River sand and GCL-Msand interfaces, peak shear stress increased with the increase in normal stress, which is indicative of increased interlocking mechanism between the fiber of geotextile and sand particles under enhanced confinement effect of overburden. Further, the plots show that higher peak shear stress is attained for GCL-Msand compared to GCL-River sand interfaces. Since the test conditions and gradation are maintained identical, the difference in the shear behaviour can only be related to the shape of the sand particles. The internal reinforcing fiber of GCL resist the applied shear force, contributing to the overall shear strength. They transmit the shear force from upper layer to the lower layer of GCL. The post-peak reduction in shear stress can be linked to the extension of reinforcing fiber at large shear strains, which causes loss of tensile strength, leading to the reduction in the interface shear strength11,12,13. The values of interface friction angle () and interface adhesion (ap) for dry GCL-sand interfaces computed from the best-fit lines of the failure envelopes considering peak interface shear stresses at different normal stresses shown in Fig.6b are listed in Table 2. All GCL-sand interfaces showed lesser friction angles compared to sand-sand shear tests. Friction efficiency of GCL-sand interfaces, which is defined as /, is always less than 1.0, as shown in Table 2.

Shear response of GCL- sand interfaces under dry condition (a) stressdisplacement response, (b) failure envelopes.

Bentonite improves the hydraulic performance of GCL, giving it the ability to self-heal. Upon hydration, bentonite increases its volume by 600%, which creates significant impact on the shear strength of GCL. Figure7 shows the crystalline swelling mechanism of bentonite as explained by Ruedrich et al.27. In field, suction of moisture from the interfacing subgrade can lead to bentonite hydration in GCL. The fluctuating groundwater level and infiltrating rainwater can result in abrupt rise of water content of subgrade, thereby negatively impacting the shear strength of GCL-sand interfaces. The worst-case scenario for this reduction of interface shear strength would be the completely saturated condition of the sand subgrade. In this study, interface tests under saturated conditions were conducted to examine the impact of bentonite hydration on the computed interface shear strength parameters. The water content in sand was maintained at 18% in these tests to achieve complete saturation and the GCL specimens were hydrated by suction of moisture from the sand. The normal stresses used for this set of tests were 7kPa, 30kPa and 100kPa. The low normal stress of 7kPa was used to permit high swelling of bentonite and high normal stress of 100kPa was used to facilitate the extrusion of bentonite on to the surface of GCL. Both these phenomena influence the interface shear behaviour of GCL-sand interfaces. The stress- displacement response of GCL-River sand and GCL-Msand under saturated subgrade condition are shown in Fig.8. A significant difference in peak shear stress of river sand and Msand interfaces was observed at higher normal stress of 100kPa, as seen from Fig.8a. When Fig.6a for dry tests and Fig.8a for saturated tests are compared, significant reduction in shear stress at all normal stresses was observed under saturated conditions. The hydration of GCL resulted in the swelling of bentonite, exerting tensile forces on the reinforcing fibers, and thereby impacting the interface shear strength. At higher normal stress, swelling of bentonite is opposed by the overburden stress, leading to the bentonite extrusion onto the interface through the voids of the nonwoven geotextile surface. Figure8c shows the swelling of GCL specimens with the hydration time for normal stresses of 7kPa and 100kPa. As stated earlier, higher swelling is observed in interfaces tested under 7kPa, which signifies the higher volumetric expansion of GCL upon hydration as compared to interfaces tested at a normal stress of 100kPa. At higher normal stress, the swelling is restricted and found to be 57% lower. The volumetric change is observed to be more for GCLs interfaced with river sand. Bentonite is extruded through the voids of the nonwoven surface of the GCL and forms a slimy layer at interface. The extruded bentonite along with the lubricating layer of water at interface, reduces the frictional resistance. Computation of interface shear strength for saturated conditions through Mohr- Coulomb failure envelopes is shown in Fig.8b and the values of interface friction angle () and interface adhesion (ap) are listed in Table 2. As observed, the adhesion and friction angles of the saturated interfaces are significantly lower than those of the dry interfaces. The interface friction angle was reduced by about 10 from dry to saturated condition, both for river sand and Msand interfaces and the interface adhesion was reduced by 710kPa from dry to saturated condition, for the reasons explained above. The reason for the reduction in frictional resistance with saturation is the slimy layer of extruded bentonite under saturated conditions and the interaction of sand particles with this layer, which restricts the efficient sand-fiber interlocking. The lubricating layer of water also reduces the frictional resistance at the GCL-sand interfaces under saturated conditions. Even though identical gradation was maintained for river sand and Msand in this study, Msand interfaces showed significantly higher friction angle and adhesion compared to river sand interfaces in all conditions due to particle morphology effects, which are explained in subsequent sections.

Crystalline swelling mechanism of clay minerals by hydration.

Shear response of GCL-sand interfaces under saturated condition (a) stressdisplacement response, (b) failure envelopes, (c) swelling-hydration time response.

The influence of sand grain shape on the shear response of GCL-sand interfaces is evident from the analysis of the test results. The availability of high-end imaging techniques and robust computational tools have made accurate quantification of particle shape possible. By using imaging techniques, soil-geosynthetic interaction mechanisms can be accurately analysed and the same can be correlated to the measured mechanical response to gain deeper insights. In this study, digital imaging techniques were employed to differentiate and quantify the shape parameters of sand grains and to understand the microlevel changes to the tested GCL surfaces to explain the interaction mechanisms at the interface.

The grain shape comprises of three multiscale componentsform (macro-scale), roundness (meso-scale) and surface texture (micro-scale)28. The form, a macro-scale component, describes the deviations in particle proportions. The meso-scale component, roundness, describes the undulations or corners along the particle outline. The surface texture, micro-scale component, defines the minute roughness characteristics on the particle surface. Several shape parameters were defined in literature to characterize the particle shape using particle images and computational techniques. The most widely accepted shape parameters are sphericity, roundness and roughness given by Wadell29,30, which were widely used by many subsequent researchers13,25. Sphericity representing the closeness of the grain shape to a sphere, roundness representing the smoothness of the grain boundary and roughness representing the micro-scale irregularities on the grain boundary, are collectively used to represent the overall grain shape. In this study, an algorithm is written in MATLAB to quantify Wadells shape parameters of sand grains. For this purpose, microscopic images of sand particles were converted into binary images through image segmentation in MATLAB and shape parameter quantifications were carried out on the binary images. Figure9 shows the microscopic and binary images of typical grains of river sand and Msand, both of 0.6mm size. Figure9b,d show the outline of the grain along with the centroid, for river sand and Msand grains, respectively. These grain outlines are plotted in the spatial domain of grain radius in pixels and angle in radians, to obtain the raw profile of the individual sand particles, as shown in Fig.10. The raw profile consists of the three multiscale features of the grain, which are form, roundness, and roughness, which are identified and marked for both the sand particles in Fig.10. While the macro-scale component encompasses the complete raw profile, the meso-scale component corresponds to the major peaks and troughs of the raw profile and the micro-scale component corresponds to the closely spaces clusters of minute deviations in the profile. The raw profile of the Msand particle shows more meso-scale and micro-scale components of shape, indicating the angularity and rough texture of Msand particle compared to the river sand particle. Further shape quantifications were carried out on the binary images of 200 individual particles in different size fractions for both the sands using MATLAB algorithm29 and the average values of shape parameters were computed. The average sphericity, roundness and roughness were obtained as 0.78, 0.38 and 0.0024, respectively for Msand and 0.84, 0.42 and 0.001, respectively for river sand31,32,33. The natural weathering and erosion processes responsible for the formation of river sand particles gave them higher sphericity and roundness compared to Msand particles which were stone-quarried. The average roughness value of the Msand particles is twice the average roughness of the river sand because of the mechanical process involved in crushing of rocks to manufacture the Msand.

Microscopic and binary images of typical sand particles (a) Microscopic image of river sand particle (b) Binary image of river sand particle (c) Microscopic image of Msand particle (b) Binary image of Msand particle.

Raw profiles of typical river sand and Msand particles with multiscale shape components marked.

The sharper elongated and rougher Msand particles generate higher friction upon interaction with other surfaces such as GCL as compared to river sand particles, which is confirmed from the results of interface shear tests. While the sands are being sheared on GCL, apart from the adhesion and friction between sand particles and GCL, there is another important mechanism that significantly contributes to the shear strength of GCL-sand interfaces, which is the sand-fiber interlocking. Through Fig.11, sand particle interlocking within the fibers of GCL can be clearly visualized for GCL-River sand and GCL-Msand interfaces. Using binary image segmentation and region properties function in MATLAB, fibers and particles were differentiated and the percentage area of sand particle entrapment on GCL surfaces was computed. Under dry conditions, the area of entrapment of sand particles for GCL-River sand and GCL-Msand interfaces was 3.44% and 2.29%, respectively at a normal stress of 100kPa. Without other influences, increase in particle entrapment must result in increase in the interface shear strength. However, GCL-Msand interfaces showed higher shear strength compared to GCL-river sand interfaces despite the relatively lesser entrapment. The reason for this higher shear strength is the shape of the Msand particles, which compensated for all other effects.

Images of tested surfaces of GCL showing particle-fibre interlocking (a) GCL-River sand (b) GCL-Msand.

In saturated tests, swelling and extrusion of bentonite greatly influenced the interface shear strength as well as the particle entrapment. Figure12a shows the tested surface of GCL after a saturated test in which extruded bentonite along with a lubricating film of water can be clearly observed. The extruded bentonite forms a slimy sticky layer at the interface, which reduces the friction at the interface. The slimy bentonite layer sticking to the fibers can be seen in Fig.12b, which is the photograph of the GCL dried after a saturated test. This layer causes higher sand particle entrapment because of its stickiness. The area of entrapped sand particles after saturated tests was higher and computed as 35.55% in GCL- River sand and 20.80% in GCL-Msand interfaces, at a normal stress of 100kPa. These results prove that bentonite hydration effects are more in river sand. Shear strength of GCL-Msand interfaces is higher compared to river sand particles even under hydrated conditions, because of the particle shape effects.

Images of tested GCL surface after saturated shear tests taken at 20magnification (a) bentonite extrusion under hydration, (b) extruded bentonite after drying up the GCL.

The interface shear tests, and image analyses carried out in this study bring out the benefits of replacing river sand in liners and capping systems of landfills with Msand and provide scientific explanations for the same. The practical benefits of this study lie in the reduced usage of natural sand in landfill construction, which has long-term environmental benefits. Cost of manufactured sand is much less compared to the cost of river sand, and hence the replacement has high economic benefits. Feasibility of production of specific gradation of Msand to derive maximum benefits in terms of interface shear strength is an added advantage. Findings from the present study can be used to derive empirical relations between the shape parameters of sand and the interface shear strength with GCLs using multivariable regression analysis. However, such relations will be more meaningful if the data includes tests with different GCLs and different water contents in the sand, which can be investigated in future.

View post:
Engineering benefits of replacing natural sand with manufactured ... - Nature.com