|Year : 2017 | Volume
| Issue : 2 | Page : 37-44
Negative aging of granular materials
Department of Civil Engineering, College of Engineering, Al Imam Mohammad Ibn Saud Islamic University, Riyadh, Saudi Arabia
|Date of Submission||26-May-2018|
|Date of Acceptance||05-Jul-2018|
|Date of Web Publication||30-Aug-2018|
Dr. Mubashir Aziz
Department of Civil Engineering, College of Engineering, Al Imam Mohammad Ibn Saud Islamic University, Riyadh 11432
Source of Support: None, Conflict of Interest: None
Background: Use of abundantly available soft rocks for embankment construction is continuously increasing due to economic reasons. A detailed knowledge on time-dependent deterioration (negative aging) of such weak geomaterial becomes essential for better serviceability of geotechnical structures.
Methodology: To explore possible effects of deterioration of soil grains on mechanical properties, a series of consolidated drained torsional shear tests were performed under saturated and dry conditions on crushed soft rocks from Japan and Pakistan.
Results: Test results indicated that soils under dry conditions with intact grains exhibited stiffer stress-strain behavior, higher peak stress ratios, and strain softening response after peak shear stress. On the contrary, soils undergoing water-induced deterioration of grain showed enormous loss of strength upon submergence. Mohr-Coulomb failure envelopes elucidated large effects of saturation-induced disintegration of grains on effective angle of internal friction. In addition, peak strength parameters seemed to be relatively more vulnerable to saturated conditions as compared to residual strengths. Conclusion: It is concluded that the observed soil behavior can be crucial to embankments and structural foundations constructed on granular materials obtained from crushed soft rocks.
Keywords: Particle breakage, saturation, shear strength, soil structure, time effects, torsional shear
|How to cite this article:|
Aziz M. Negative aging of granular materials. Imam J Appl Sci 2017;2:37-44
| Introduction|| |
In many regions of the world, population growth, economic needs, and environmental constraints necessitate construction of foundations on widespread residual deposits of soft rock origin along with the use of such cheap but abundantly available granular materials for construction of embankments. With this enormous increase in infrastructure developments on such complex geomaterial, a large number of communities are being exposed to a variety of geotechnical failures that can be triggered either by earthquake and/or heavy rainfall. In this scenario, the detailed knowledge on the time-dependent deterioration of such granular soils becomes essential in geotechnical analysis and design for better serviceability over the lifetime of geotechnical structures and risk analysis. The geologic changes in soil composition and its engineering properties may require hundreds of thousands of years and over this time, the process of soils turning into rocks as well as rock weathering to form soils takes place. In last few decades, it has been realized that there are also changes in the soil properties over a shorter period, like few months or years, which are more relevant to engineers. These changes are called aging effects as schematically described by Aziz et al. in [Figure 1].
Conventionally, the aging effects have been believed to be positive, that is, the increase in strength and stiffness of soils over time under constant effective stresses after deposition. In practice of geotechnical laboratory testing on conventional granular soils, the soil grains are usually considered as durable under working stress-strain conditions. However, naturally occurring sedimentary and residual deposits which are generally treated as granular soils in geotechnical engineering, experience time-dependent disintegration of grains due to different environmental conditions associated with loss of strength and stiffness parameters, hereafter referred to as negative aging. Such characteristics are common in natural geological materials and have a strong influence on the engineering behavior of granular soils and the description of such effects needs to be included in the conventional concepts of soil mechanics.
Granular decomposition of soils
Extensive research work has been published on effects of grain crushing on conventional granular soils. In those studies, the main focus has been made more or less on the effects of extremely high overburden stresses and extensive shear strains. Only few studies,,, are known to exist concerning the water-induced particle disintegration of such granular materials under saturated conditions which cause significant changes in physical properties and overall mechanical behavior of the aggregates. This study explores the physical phenomena controlling the time-dependent geotechnical properties of granular soils because in conventional risk assessment approach for slope failures and other geotechnical risks as described in [Figure 2], the geotechnical investigations and aging effects of soils are often ignored. Keeping this in view, the investigation of water-induced granular decomposition with respect to its influence on strength and deformation characteristics of residual soils has been carried out. Deterioration of soil grains on submergence has been investigated because, irrespective of the bonding nature of particles, the presence of water is always the key source of initiation of weakening processes.
Moreover, many researchers have reported negligible influence of saturated and dry conditions on engineering properties of conventional sands.,, This is true for most of the conventional laboratory silica sands consisting of durable grains. Conversely, Mizuhashi et al. showed an evidence of significant effects of the presence of water on shear strength of weathered soils from drained triaxial tests on specimens from 19 different rainfall-induced slope failures in Japan. Therefore, stability of natural slopes and design of foundations and embankments in such soils will be strongly influenced by such phenomenon. Time-dependent characterization of such geomaterial with continuous in situ monitoring of the rate of deterioration and loss of strength can be helpful to avoid catastrophic geotechnical failures.
The conditions under which soft rocks are degraded due to physical or chemical weathering are well established and few efforts are made towards constitutive modeling of degradable geomaterial.,, They concluded that the weathering affects mechanical behavior due to progressive degradation of bonding and structure loss. It is important to note that the term weathering implies mechanical, physical, and/or chemical changes whereas degradation is a by-product of weathering progression causing loss of strength and stiffness parameters.
Residual soils of sedimentary origin were obtained from Japan (YK01 and YK02) and Pakistan (Hattian Bala (HB), Gulshan Nalah (Pakistan) (GN), and Dera Syyedan (DS)) as shown in [Figure 3] and their index properties have been compared in [Figure 4]. Conventional Japanese Toyoura sand (TS) has been used as a reference material which exhibits similar strength and deformation behavior under saturated and dry conditions. The progression of long-term deterioration of gravels was simulated by disintegration of soil grains due to loss of particle cementation on water-submergence [Figure 5]. Unlike, the conventional granular soils consisting mostly of durable grains, the use of crushed soft rocks in this study was to accelerate the process of negative aging of soils in the laboratory which may take months or years to complete under in situ conditions.
|Figure 3: Microscopic view of soil grains of Toyoura sand and all crushed rock materials|
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|Figure 5: Accelerated negative aging by water-induced deterioration of grains|
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To investigate the effects of water-submergence-induced negative aging on strength and deformation behavior of granular soils, consolidated drained monotonic torsional shear tests as presented in [Table 1] were performed on hollow cylindrical TS and crushed soft rock specimens under saturated and dry conditions under isotropic (Ko = 1.0) and anisotropic (Ko = 0.5) stress states.
|Table 1: Typical test conditions for Toyoura sand specimens (similar tests on all other materials)|
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Evaluation of granular decomposition
Air-dried crushed material was thoroughly mixed before sample preparation to make sure that the physical properties of each specimen are representative of the initial values. According to the findings of Vilar, the use of air-dried samples can successfully minimize the complex suction problems for preliminary estimates of the shear strength parameters of unsaturated soils. Hollow specimens of 200 mm height, 100 mm outer diameter, and 20 mm wall thickness were prepared at desired initial relative densities (DRi) by dry deposition method. Initial relative densities were kept high to simulate the field conditions of construction of embankments and other geotechnical structures as well as to reduce the large volume changes as expected in case of crushed soft rocks during torsional shear tests in the laboratory. To study the effects of Ko on the deformation and strength response, specimens were consolidated at s'h/s'v = 1.0 and 0.5. After consolidating the specimens at desired effective confining stress, drained monotonic and cyclic torsional shear at constant mean effective principal stress (p') under strain-controlled loading were performed.
Grain size distribution (GSD) and particle shape were determined for each dry and saturated tests before and after the shearing as shown in [Figure 6] and [Figure 7]. Using the area under initial and final GSD curves (degradation index [ID]) is defined to quantify the amount of disintegration due to water submergence after saturated tests. This index (ID) has been perceived from previous studies of various researchers like grading index (IG) by Wood and et al. and relative breakage index (Br) by Hardin who used these indices for quantifying particle crushing at very high confining pressures and large shear strain levels.
|Figure 7: Soil grains (a) before and after dry tests; (b) AFTER saturated tests|
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Drained tests at shear strain rate of 0.018%/min were performed to focus on long-term material response as well as to avoid the errors of partial saturation and membrane penetration. The test results were focused on comparison of strength and deformation characteristics of dry and saturated degradable crushed soft rocks with conventional TS of durable silica grains. Therefore, instead of conducting a vast parametric study, the effects of material type, drainage condition, and confining pressure were investigated keeping DRi same for each set of dry and saturated tests. Instead of using relative density after consolidation (DRc) as a parameter of comparison, DRi is used because saturated specimens show change in particle shape and size due to water-induced disintegration of grains.
Typical consolidation time history of TS and HB specimens under saturated and dry conditions are presented in [Figure 8] and [Figure 9], respectively. The rate of increase in cell pressure during consolidation was kept similar for all the test specimens at 3 kPa/min. It can be observed in [Figure 8] that there is an increase in vertical compression of the specimens with increase in p', while the difference between saturated and dry conditions is insignificant. In addition, the volume change in the specimens comes to an end after completion of primary compression and it is certainly due to the durable nature of sand grains which are unaffected by the presence of water.
|Figure 8: Typical time history of consolidation of Toyoura sand specimens|
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|Figure 9: Typical time history of consolidation of a degradable granular soil|
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As shown in [Figure 9], it is observed that with an increase in p', there is an increase in primary compression with an enormous difference between saturated and dry conditions. The saturated tests have shown much faster increase in vertical strains as compared to the dry tests although the rate of consolidation (i.e., increase in effective confining pressure) was kept constant. This progressive compression under sustained effective confining stress, also observed by Mesri et al., is referred to as secondary compression. It is hypothesized that such a phenomenon is certainly due to water-induced disintegration of soil particles as well as relatively loose intrinsic soil structure. This hypothesis is justified by comparing the saturated tests with dry ones. It is quite clear that primary compression of dry specimens is much smaller than saturated cases as well as there is no/very little secondary compression in dry specimens after reaching the maximum mean effective confining stress.
The maximum observed values of volumetric (εvol) and vertical (εz) strains at the end of consolidation along with the ID values are given in [Table 2] from the test results on consolidation response of all test materials. It is important to note that these strains represent only the vertical compression of specimens during consolidation and they do not include saturation-induced vertical strains. The relationship between ID, vertical strains, and volumetric strains are given in [Figure 10]. Along with the increase in volume change with increase in mean effective confining stress, it can be observed that ID has a clear relationship with the deformation characteristics of degradable granular soils during consolidation. Furthermore, it is found that with increase in ID value beyond some threshold (about 0.5), there is a little increase in volumetric and vertical strains and the relationships are relatively stabilized. This is essentially due to the reason that relatively denser packing is achieved in the specimens after undergoing enormous water-induced deterioration of grains.
|Figure 10: Volumetric strain relationships from consolidation data of saturated and dry tests|
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Shear strength parameters of decomposed soil
Many researchers have reported negligible influence of saturated and dry conditions on strength and stiffness as well as volume change characteristics of conventional sands (similar to TS) consisting of durable grains.,, It can be seen from [Figure 11] that the stress-strain response becomes stiffer under both saturated and dry conditions with increase in mean effective principal stress without showing any considerable difference between water-saturated and dry specimens. Moreover, clear peak and residual stress states or strain-softening undersaturated, as well as dry conditions, is observed at relatively higher p'.
|Figure 11: Torsional shear response of Toyoura sand soil under dry and saturated conditions|
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The strength parameters in the Mohr-Coulomb failure envelope were derived by constructing the Mohr stress circles at peak shear stress and residual state (stress at 15% shear strain) for each test. The failure envelopes at peak conditions for TS specimens are represented in [Figure 12]. By comparing the strength parameters and failure envelopes of TS specimens under saturated and dry conditions, it can be inferred that for given test conditions, angle of internal friction of granular soils with durable grains is essentially unaffected by the presence of water.
Similarly, stress-strain behavior and failure envelopes of crushed rock specimens (HB) undersaturated as well as dry conditions are presented in [Figure 13] and [Figure 14], respectively. All these tests were performed under strain-controlled conditions at constant, relatively slow shear strain rate of 0.018%/min to a maximum torsional shear strain (γZΘ) of 15%. Similarly, [Table 3] presents the typical shear strength parameters for all soils as obtained from dry and saturated tests. Here, tpeak represents the peak shear stress during torsional shear, R is the maximum principal stress ratio (σ'1/σ'3) and φ' = (sin-1[(σ'1-σ'3)/( σ'1+σ'3)]) is the drained angle of internal friction. From the test results, it is quite clear that the stress-strain response becomes stiffer under both saturated and dry conditions with increase in mean effective principal stress but at the same time, showing a substantial difference between the response of water-saturated and dry specimens. Moreover, clear peak and residual stress states or post-peak strain-softening is not observed in case of degradable granular soils. Although all the specimens were initially at high relative densities (i.e., more than 70%), still, the saturated tests have shown very weak stress-strain response just like loose soils. This is again attributed to the effects of disintegration of soil grains on water-submergence.
|Figure 13: Typical monotonic torsional shear response of a degradable granular soil|
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|Table 3: Shear strength parameters of durable and degradable granular soils|
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[Figure 15] presents the effects of saturated and dry conditions on shear strength of soils. The soils with durable grains essentially lie on the 45° line, i.e., no/little effects of the presence of water. While enormous loss of strength on submergence with its positive as well as systematic correlations with the ID is quite clear.
|Figure 15: Effects of granular decomposition on peak torsional shear stress|
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| Conclusions|| |
The strength and deformation characteristics of degradable granular soils under saturated and dry conditions are not in agreement with our conventional geotechnical approach towards various standard soils of durable grains. It is observed that along with the collapse of metastable structure, slaking phenomena causes loss of intraparticle cementation of the weakly bonded granular materials upon submergence. Consequently, such an evolution of soil grains produces rounded particles with relatively high sphericity which sequentially decreases the interlocking behavior of granular medium.
A ID is used for quantitative assessment of the degree of disintegration and its effects on engineering properties of granular soils. A higher value of ID represents a greater reduction in the strength parameters and a subsequent increase in deformation response of the soil. Apart from evident effects of saturation and material type, it is found that ID for saturated tests is more or less unaffected by the range of mean effective confining stress of 50-200 kPa and maximum torsional shear strain of 40% employed in this study. ID shows clear relationships with maximum vertical and volumetric strains at the end of consolidation for a given effective stress level, Ko value, and relative density. It is inferred that in situ collapse settlement of embankments can be reasonably assessed from these relationships by knowing the ID value of the material. It is also revealed that there is no/little further volume change with increase in ID value beyond some threshold (about 0.5). It is conceivable due to the fact that relatively denser packing is achieved in the specimens after undergoing enormous water-induced deterioration of grains. The volumetric and vertical strain response of saturated soils during anisotropic consolidation at Ko = 0.5 is quite similar to one-dimensional compression. It infers that under such conditions the vertical strains can be reasonably employed for approximate estimation of the overall volume change of soils.
Analysis of monotonic torsional shear test data indicates that the soils under dry conditions with intact grains exhibit stiffer stress-strain behavior, higher peak stress ratios, and strain softening response after peak shear stress. On the contrary, the soils undergoing water-induced deterioration of grain show enormous loss of strength upon submergence. Mohr-Coulomb failure envelopes elucidate the large effects of saturation-induced disintegration of grains on effective angle of internal friction with insignificant effects of Ko on the strength parameters. In addition, the peak strength parameters seem to be relatively more vulnerable to saturated conditions as compared to the residual strengths. Also, with increase in the ID value beyond some threshold, the reduction in mechanical properties appeared to be decreasing/stabilizing.
Enormous volumetric compression during saturation, consolidation, and loading of rockfill materials obtained from soft rock origin as well as drastic loss of strength parameters on submergence are revealed in this study. It is concluded that the observed soil behavior can be critical for the embankments constructed with similar granular materials and bearing capacity of the foundations placed on such soils. The enhanced ability of existing soil mechanics models to predict time-dependent behavior of degradable materials by incorporating the water-induced deterioration of soil grains can simplify several in situ geotechnical problems.
The University of Tokyo and Japanese Ministry of Education, Culture, Sports, Science, and Technology are gratefully acknowledged for research facilities and financial assistance.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15]
[Table 1], [Table 2], [Table 3]