Case Studies in Construction Materials 16 (2022) e01004
Available online 10 March 2022
2214-5095/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Experimental study of the role of interface element in earth dams with asphalt concrete core - Case study: Mijran dam
Ashkan Gholipoor Noroozi
a, Rassoul Ajalloeian
a,b,*, Meysam Bayat
aaDepartment of Civil Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran
bProfessor of Engineering Geology, The University of Isfahan, Isfahan, Iran
A R T I C L E I N F O Keywords:
Earth dam with asphalt concrete core Large-scale direct shear test Interface element Interaction ratio
A B S T R A C T
In recent years, the tendency to use asphalt concrete cores in earth dams is very common and is increasing. In such dams, the interaction between asphalt concrete core and soil materials has an important role in dam stability. The present study, as a case study of Mijran Dam located in Ramsar area in Iran, has evaluated experimentally the mechanical behavior of the interface be- tween the filter soil material and the asphalt concrete core in the laboratory. In this regard, the samples were tested using large-scale direct shear machine in different normal stresses and in dry and saturation conditions. Also, tests have been performed on samples with different percentages of bitumen and relative density of filter materials at different shear rates and under different freeze-thaw cycles. In all the above conditions, the shear surface has been considered as the interface and the shear rate used in different tests varied in the amounts of 0.5, 1, 1.5 and 3 mm/
min. The results of this study show that different percentages of bitumen used do not have such an effect on the shear strength parameters of interface. As the relative density increases, the values of the shear parameters increase. If the sample is dry or saturated, parameters such as dilation angle and shear stiffness increase with increasing density, but the interaction ratio increases in the dry state and decreases in the saturated state. As the shearing rate increases, the dilation angle and interaction ratio in the interface materials increase by about 2–3%. On the contrary, increasing the freeze-thaw cycles reduces the mentioned strength parameters.
1. Introduction
The history of using bitumen (asphalt binder) and asphalt concrete in sealing applications in hydraulic structures dates back to thousands of years ago and since the 1920s, there has been a systematic development and improvement of hydraulic asphalt used for such purposes. In the early years, asphalt concrete was used primarily as an impermeable surface for earth dams, flood walls, and reservoirs floor. Asphalt concrete and asphalt mastic membranes have been used to seal concrete dams, as well as canal cover, river bank stabilization, and naval defense operations. According to the experiences gained from the application of asphalt concrete covers, asphalt concrete has also been used in impermeable core walls in earth dams [1]. Asphalt concrete has been used for more than 50 years as a flexible and impermeable layer in dams. Germany has been a pioneer in this industry [2]. Since the construction of the Kleine Dhuenn dam as the first earth dam with dense asphalt concrete core with a height of 35 m in Germany in 1962, the construction of earth dams with asphalt concrete core has received much attention around the world [3].
* Corresponding author at: Professor of Engineering Geology, The University of Isfahan, Isfahan, Iran.
E-mail address: [email protected] (R. Ajalloeian).
Contents lists available at ScienceDirect
Case Studies in Construction Materials
journal homepage: www.elsevier.com/locate/cscm
https://doi.org/10.1016/j.cscm.2022.e01004
Received 21 December 2021; Received in revised form 25 February 2022; Accepted 25 February 2022
Compared to an earth core, the location and compaction of asphalt concrete are less sensitive to adverse weather conditions.
Asphalt concrete is practically impermeable, flexible, erosion-resistant, efficient and compressible, and allows the construction of a seamless core [4]. The interface between the asphalt core and the transition areas in the earth dam plays a key role in transferring stresses and deformations during dam construction and reservoir dewatering [5]. When two different types of materials are in close proximity to each other, the behavior of the materials at the interface between them changes due to a change in mechanical nature.
Now, if one of these two materials includes flexible materials such as soil, due to effects such as changes in soil stress, soil moisture, type of gradation, size and appearance of grains, this issue becomes complexity and the behavior at the interface of these materials needs to be studied more carefully. For example, when piles are used in paving, due to differences in the type of pile materials relative to the soil, the issue of soil-structure interaction is raised; or when t-beams are used below earth dams and ports, the problem of water-soil-structure interaction is considered. The above examples have caused a lot of research on the interaction of various materials such as steel, concrete and wood with different soil materials and its achievements have been used in design and implementation issues [6]. Research and studies have been conducted on static and dynamic analysis of the dam with asphalt concrete core in a laboratory, as well as numerical modeling of this type of dam.
Baziar et al. [7] have analyzed the seismic behavior of Mijran Dam with asphalt concrete core using numerical models and material parameters determined by laboratory tests. The results of this study have shown that according to the elastoplastic characteristics of asphalt concrete, gravelly dams with asphalt concrete core have a satisfactory behavior under seismic loading. In the study of Akh- tarpour and Khodaei [8], nonlinear numerical analyzes for a dam with an asphalt concrete core in Iran (Shur River Dam) have been performed under seismic forces. The results show that the vibration caused by the earthquake can cause the development of small cracks and increase the permeability of asphalt in the upper part of the core. In another study conducted by Tajdini et al. [9], the mechanical behavior of the interface between filter sand and asphalt concrete was investigated by small-scale direct shear tests and numerical analysis performed using FLAC3D software. Akhtarpour and Khodaei [10] in another study have studied the seismic behavior of asphalt concrete used as an impermeable core in earth dams. The results of this study show that asphalt concrete, if used as a sealing barrier in an earth dam, retains its stability even after moderate earthquakes.
Tajdini et al. [11] have evaluated the mechanical properties of sand materials on the interface with asphalt concrete, using a small-scale direct shear machine whose shear surface has been considered as the interface, in both dry and saturated states. In this evaluation, various parameters affecting the shear strength between these two materials, including the loading level, density per- centage, type and content of bitumen and soil type (angular and round sandy materials) have been considered. In general, it has been concluded that to increase the strength of the sand-asphalt interface, increasing the density of sand or use angular sand, instead of changing the content and type of bitumen is more effective and efficient. Keymanesh et al. [12], in their study, investigated the effect of bitumen content on moisture sensitivity of asphalt mix under freeze-thaw cycles and changes in moisture sensitivity have been compared with changes in a specimen made with optimal bitumen content. The results show that the content of bitumen has a sig- nificant effect on the performance of the asphalt mixture. As the number of cycles increased, the strength of the specimen decreased and with increasing the bitumen content, the amount of this decrease increased. Islam and Tarfder [13] in a study investigated the effects of large freeze-thaw cycles on the stiffness and tensile strength of asphalt concrete. The results showed that the bending stiffness of the specimens exposed to the freeze-thaw cycles has been reduced. Indirect tensile strength did not change significantly under freeze-thaw conditions.
Wang and Hogg [14] conducted a study with the aim of developing a simplified model of materials for different types of loading conditions that an asphalt core undergoes in an earth dam by conducting long-term triaxial creep experiments. According to test results, a simplified material model has been proposed and formulated based on the deviation stress. It is also found that the unloading-reloading behavior and the dynamic behavior are in the linear elastic phase. Gheibi et al. [15] investigated the effect of temperature on the dynamic properties of the asphalt core of an earth dam at different confinement pressures by performing resonant column tests. Based on the results, the effect of confining pressure change on the dynamic properties of asphalt concrete samples was negligible. However, the effect of temperature on the dynamic properties of asphalt concrete is significant.
In a study conducted by Seo et al. [16], the development of an asphalt concrete mix for a rockfill dam with an asphalt core has been investigated. According to the results, among all mixtures, the mixture with the filler content of 10% showed the highest resistance to moisture. Wang et al. [17] investigated the behavior of the interface between the asphalt core and the sand transfer zone in earth dams.
The results of laboratory tests have shown that the interface has undergone a gradual deformation until a shear displacement of 60 mm is obtained, but no change has been made in the properties of the asphalt surface layer. Merga Bayisa [18], in his study, evaluated the performance of earth dam with asphalt concrete core and earth dam with clay core. The results of the analysis showed a good reliability coefficient in the dam with asphalt concrete core. Hernandez Lopez [19] evaluated the effect of water on the triaxial response of asphalt concrete used in the core of earth dams under uniform loading. The results showed that asphalt concrete has a reduced strength due to the saturation process and the loss of adhesion between asphalt and stone materials and a 31% increase in deformability compared to dry conditions. On the other hand, it has been found that the shear strength is affected by the temperature.
In a investigation by Wang et al. [20], a case study was conducted on the rate of asphalt core construction for the Zhaobishan earth dam. According to the study of high rates of asphalt core construction in the test section and in order to determine the parameters of asphalt mixing and construction techniques before the construction of the asphalt core, an extensive laboratory program has been carried out and 4 test sections have been established near Zhaobishan dam site. According to the results, by increasing the construction rate, in many situations, the possibility of overtopping during the construction can be significantly reduced. Cong et al. [21] conducted an experimental study to investigate the effect of freeze-thaw cycles on the performance of asphalt mixtures. The results show that with increasing freeze-thaw cycles, indirect tensile strength and tensile strength ratio decrease. Gemeda [22] conducted a study to replace a clay core with an asphalt core at the Gidabo rockfill dam in southern Ethiopia. In general, based on the results of this study, the core
design of asphalt concrete meets all design requirements and can be used in subsequent designs of rockfill dams.
As observed, in previous researches, the mechanical behavior of the interface of granular materials and core asphalt concrete has been studied statically and dynamically. The effect of factors such as bitumen percentage, moisture content, relative density and angularity of aggregates, as well as the effect of loading level and soil type on shear strength parameters at different levels of normal stresses have been evaluated. Despite the fact that considering the gradation of the aggregates used in the asphalt concrete of the dam core, we must use a direct shear machine with large box dimensions, in previous studies done with the direct shear machine, a small shear box with a size of 100 ×100 ×25 mm has been used. Therefore, there are limitations in the results due to the small size of the shear box. On the other hand, there is no research that has examined the effect of important factors such as shear rate and freeze-thaw cycle number on the behavior of the interface element. The aim of the present study has been to determine the parameters of the interface between asphalt concrete and soil materials in the laboratory, by changing the percentage of bitumen used in asphalt concrete, relative density of soil materials, the shear rate and the number of freeze- thaw cycles, under different normal stresses. Hence, the mechanical behavior of the interface element in the embankment dam with the asphalt concrete core (Case study: Mijran Dam in Mazandaran province, Iran) has been evaluated by determining the shear strength parameters (peak shear stress, cohesion and internal friction angle) and interlocking strength parameter (dilation angle) using a large-scale direct shear machine.
2. Materials and methods
Mijran Dam is an earth dam with asphalt concrete core in 20 km of Ramsar, which is located on Nesarood River. The height of the dam from the foundation is 59 m and its executive operation was completed in 2003 [23]. The cross section of Mijran Dam has been shown in Fig. 1.
2.1. Materials 2.1.1. Soil materials
The soil materials used in this research have been prepared from Akam Shen Company near Tehran. These materials were angular gravel. Experiments performed on soil materials, including particle size analysis test (ASTM-D422) [25], Atterberg limits determi- nation test (ASTM-D4318) [26], maximum specific gravity test (γd max) (ASTM-D1557) [27] and the minimum specific gravity test (γdmin) (ASTM-D4254) [28]. The results of experiments performed on soil materials have been presented in Table 1 and their gradation curve in Fig. 2.
2.1.2. Asphalt concrete
In this research, the modified Marshall method according to ASTM-D5581 [29] has been used for the mix design of core asphalt concrete. Consumption bitumen is bitumen 60/70 which was prepared from Tehran refinery and was used in the amounts of 3.5%, 4.0% and 4.5% by the total weight of asphalt concrete. The particle size distribution of the aggregates was in accordance with the Fuller gradation curve used in the construction of the Mijran Dam and in the range of 0–19 mm and the proposed ICOLD grading range.
The various phases of the Marshall test are shown in Fig. 3 and the weight-volume characteristics of the asphalt mixture, which are determined according to AASHTO-T166 [30] and AASHTO-T245 [31] standards, have been presented in Table 2.
According to the results and graphs obtained from the Marshall test, the optimal content of bitumen for the preparation of asphalt concrete specimens is equal to 4%.
Fig. 1. Cross section of Mijran Dam [24].
2.2. Test method
In this research, in order to evaluate the shear strength of aggregates (gravelly materials), as well as the shear strength of the interface of granular materials-asphalt concrete, experiments were performed using a large-scale direct shear machine (304.8×304.8×152 mm), By unconsolidated-undrained (UU) method or fast method, according to ASTM-D3080 [32] in both dry and saturated states, under different normal stresses and under strain control conditions and the effect of different factors has been Table 1
Results of experiments performed on soil materials.
Gravel materials sample LL (٪) PL (٪) PI (٪) Unified system classification γd min (kN/m3) γdmax (kN/m3)
18 10 8 GP 16.65 21.24
Fig. 2. Gradation curve of gravel materials.
Fig. 3.The various phases of the Marshall test: (a) Aggregate materials; (b) Bitumen; (c) Making of asphalt concrete; (d) Molding of asphalt concrete; (e) Cylindrical asphalt concrete sample; (f) Hot water bath; (g). Marshall compactor; (h) Marshall jack.
investigated.
Remolding of the specimens for the direct shear test was performed using the minimum and maximum dry density values and the desired relative density values and taking into account the dimensions of the direct shear box. This means that the weight required to achieve the desired relative density has been calculated. Specimens have been made in 3 layers and each layer has been compacted to one third of the height of the shear box. The layers have been driven with a hammer. The formula used to calculate the dry weight of materials for the specimen making is as follows:
Dr=
[ γd− γdmin γdmax− γdmin
][γdmax γd
]
(1) Where, γd: dry density (grams per cubic centimeter); γdmin: minimum dry density (grams per cubic centimeter); and γdmax: maximum dry density (grams per cubic centimeter).
In general, it is recommended that the minimum width of the specimen in a direct shear machine should not be less than 10 times the maximum particle diameter and the minimum initial specimen thickness should not be less than 6 times the maximum particle diameter. The minimum ratio of width to thickness of the specimen should be 2-1 [32]. After making the specimen related to each group of experiments, by placing the loading plate on the specimen and applying the desired normal stress, the shear force is applied at the speed related to each group of specimens. It should be noted that in the direct shear test with the standard method ASTM -D3080, the maximum strain is limited to 20% and we often expect a rupture at the strain of 10–20%. Also, in the saturated state, the entire specimen box is filled with water and normal stresses of 100, 200 and 300 kPa are applied at specified intervals. Then, the shear strength parameters are extracted. Saturation of the specimens was performed for 4 h. The specifications of the specimens made for large-scale direct shear test have been presented in Table 3.
3. Discussion and analysis of the results
3.1. Investigation of the effect of bitumen percentage on the interaction of asphalt concrete core and filter soil materials
In order to investigate the effect of bitumen percentage on the interaction of asphalt concrete and soil materials, specimens con- taining asphalt concrete with different contents of bitumen (3.5%, 4.0% and 4.5%) and GP materials with a relative density of 65%
Table 2
Weight-volume characteristics of asphalt mixtures.
Percentage of
bitumen used (٪٪) Maximum specific gravity (kN/m3)
Real specific gravity (kN/
m3)
Percentage of asphalt concrete void (Va)
Percentage of void of aggregates (VMA)
Percentage of void filled with bitumen (VFA)
Stability
(kN) Flow
(0.25 mm)
3.5 24.14 22.27 7.8 13.1 40.6 16.25 9.0
4.0 23.97 22.53 6.0 12.5 51.9 14.71 10.3
4.5 23.80 22.83 4.1 11.8 65.4 12.58 13.2
Table 3
Names and specifications of specimens made for direct shear testing.
Parameter Specimen name Specimen specification
Bitumen percentage Asphalt (B: 3.5%, 4%, 4.5%)-GP
(65%)-Dry Specimens containing asphalt concrete made with 3.5%, 4.0% and 4.5% bitumen and GP soil with 65% relative density, under dry conditions and with a shear rate of 1 mm/min
Relative density GP(50%, 65%, 80%)-Dry Specimens made using GP soil with relative densities of 50%, 65% and 80%, under dry conditions and with a shear rate of 1 mm/min
GP(50%, 65%, 80%)-Sat Specimens made using GP soil with relative densities of 50%, 65% and 80%, under saturated conditions and with a shear rate of 1 mm/min
Parameter Specimen name Specimen specification
Relative density Asphalt (B:4%)-GP(50%, 65%,
80%)-Dry Specimens containing asphalt concrete made with 4% bitumen and GP soil with relative densities of 50%, 65% and 80%, under dry conditions and with a shear rate of 1 mm/min
Asphalt (B:4%)-GP(50%, 65%,
80%)-Sat Specimens containing asphalt concrete made with 4% bitumen and GP soil with relative densities of 50%, 65% and 80%, under saturated conditions and with a shear rate of 1 mm/min Shear rate GP(65%)-Dry-V(0.5, 1, 1.5, 3 mm/
min) Specimens made with GP soil, 65% relative density, under dry conditions and with shear rates of 0.5, 1, 1.5 and 3 mm/min
Asphalt (B:4%)-GP(65%)-Dry-V
(0.5, 1, 1.5, 3 mm/min) Specimens containing asphalt concrete made with 4% bitumen and GP soil with a relative density of 65%, under dry conditions and with shear rates of 0.5, 1, 1.5 and 3 mm /min
Number of freeze
-thaw cycles GP(65%)-Cycles(10, 20, 30)-Dry Specimens made with GP soil, 65% relative density, under 10, 20 and 30 cycles, dry conditions and shear rate of 1 mm/min
GP(65%)-Cycles(10, 20, 30)-Sat Specimens made with GP soil, 65% relative density, under 10, 20 and 30 cycles, saturation conditions and shear rate of 1 mm/min
Asphalt (B:4%)-GP(65%)-Cycles
(10, 20, 30)-Dry Specimens containing asphalt concrete made with 4% bitumen and GP soil with relative density of 65%, under 10, 20 and 30 cycles, dry conditions and shear rate of 1 mm/min
Asphalt (B:4%)-GP(65%)-Cycles
(10, 20, 30)-Sat Specimens containing asphalt concrete made with 4% bitumen and GP soil with relative density of 65%, under 10, 20 and 30 cycles, saturation conditions and shear rate of 1 mm/min
were made and were tested in the large-scale direct shear machine with UU method at shear rate of 1 mm/min under normal stresses of 100, 200 and 300 kPa. Asphalt concrete is located in the lower half of the shear box and gravelly materials are located in the upper half of the shear box and the shear surface is considered as the interface between the core asphalt concrete and the filter soil materials.
Asphalt specimens prepared for direct shear testing have been shown in Fig. 4 and the failure envelope curve from direct shear testing has been presented in Fig. 5.
According to the diagrams of shear stress-normal stress (failure envelope curve) presented in Fig. 5, it can be found that at the constant relative density of GP materials, the change in the percentage of bitumen used in the construction of asphalt concrete specimens has a partial effect on the shear strength parameters (peak shear stress, cohesion and internal friction angle) of the interface materials. This result is consistent with the results provided by Tajdini et al. [11] and the ICOLD recommendation. Therefore, given the partial effect of bitumen content used on the parameters of the interface, in this study, in other experiments, the optimal bitumen content (4%) was used to make specimens of asphalt concrete at the interface with soil materials.
3.2. Investigation of the effect of relative density on the shear strength parameters and dilation angle of GP materials and interface materials, under dry and saturated conditions
In general, the shear strength of soils consists of two components: frictional strength and particle interlocking. The friction component is a combination of rolling and sliding friction. According to Taylor (1948), interlocking behavior is the reason of changing the volume of soils. Particle interlocking and initial soil mass pressure causes dilation in dense sands and shrinkage (volume reduction) in loose sands [33]. Dilation is the measure of the change in soil volume when the soil is subjected to torsion by shear. If a soil mass is restricted laterally, the dilation angle can be obtained from the following equation:
ψ=tg−1 (d(δv)
d(δh) )
(2) In fact, dilation is defined as the change in volumetric strain relative to the change in shear strain [34]. The tangent line on the vertical displacement diagram relative to the horizontal displacement of the specimen, in its ascending part and especially in the part where the soil has the highest rate of vertical displacement increase relative to the horizontal displacement, indicates the angle of soil dilation.
In this study, to investigate the effect of relative density change on shear strength parameters and dilation angle of GP materials with relative densities of 50%, 65% and 80% and also to investigate the effect of relative density on strength parameters of asphalt concrete-GP materials interface, the specimens have been prepared with specifications presented in Table 3 and tested in large-scale direct shear machine by UU method under normal stresses of 100, 200 and 300 kPa, shear rate of 1 mm/min and in dry and saturated conditions. All experiments were continued up to an axial strain of 15%.
3.2.1. Investigation of the effect of relative density on stress-strain behavior
The shear stress-horizontal displacement variations for both groups of samples, GP materials and interface materials in both dry and saturated states, at different levels of relative density of soil materials, under a normal stress of 300 kPa, have been shown in Fig. 6.
Examination of shear stress-horizontal displacement diagrams for GP and interface materials under dry and saturated conditions is observed that, in both dry and saturated conditions, subjected to a normal stress of 300 kPa, the increase in relative density in small horizontal displacements did not follow a definite trend, and in both categories of GP materials and interface materials, at large horizontal displacements, increasing the relative density has led to an increase in shear stress. In general, in dry conditions, GP ma- terials have ruptured under greater shear stress than interface materials. In saturated conditions, compared to dry conditions, both groups of materials have ruptured in larger horizontal displacements. As can be seen in the figures above, dense materials are failed at smaller horizontal displacements (or axial strains).
Fig. 4. Asphalt concrete specimens made with large-scale direct shear box dimensions.
3.2.2. Investigation of the effect of relative density on shear strength parameters
The variations trend of peak shear stress, as well as the variation trend of cohesion and internal friction angle versus relative density, have been shown in Figs. 7 and 8, respectively.
Observation of the variation trend of peak shear stress for different values of relative density indicates that in all specimens, under Fig. 5. Failure envelope curve for GP materials at the interface with asphalt concrete, prepared using different contents of bitumen.
Fig. 6.Shear stress-horizontal displacement variations for soil and interface materials, under (a) dry and (b) saturated conditions, at different levels of relative density, under a normal stress of 300 kPa.
Fig. 7. Variation of peak shear stress versus relative density for GP materials and interface materials, under dry and saturated conditions.
dry and saturated conditions, the increase in relative density has led to an increase in peak shear stress or shear strength at the moment of rupture. In general, the relative density has a very good correlation with the strength of coarse-grained soils. Compacted soils have higher strength than loose soils [34]. The reason for the increase in shear strength with increasing relative density is the increase in the resistance of gravel particles on the fracture surface against rotation, slipping and collapse. In saturated conditions, compared to dry conditions, there was a higher correlation between peak shear stress and relative density. In both dry and saturated conditions, the peak shear stress values in GP materials were higher than the values of this parameter in interface materials. This can be attributed to the greater friction and inter-particle locking in GP materials than in interface materials. In general, GP materials in dry conditions had the highest peak shear stress. Also, the highest correlation between peak shear stress and relative density was obtained in interface materials under saturated conditions (R2≅0.99). The increase in peak shear stress is consistent with the increase in relative density, according to Hamidi et al. [35].
The variation trend of cohesion versus relative density indicates that under dry conditions, with increasing relative density from 50% to 80%, the cohesion has decreased in both GP and interface materials. The rate of cohesion reduction at 80% relative density for GP and interface materials compared to 50% relative density was 41.2% and 41.7%, respectively. On the other hand, under these conditions, the internal friction angle in both GP and interface materials has had incremental trend. In GP and interface materials, with increasing the relative density to 80%, the internal friction angle, compared to its value in the relative density of 50%, has increased by 4.35% and 5.13%, respectively. Under saturation conditions, in both GP and interface materials, with increasing relative density, cohesion and internal friction angle have had an upward trend. In GP materials, the rate of increase in cohesion at 80% relative density compared to its value at 50% relative density was 10.34% and the rate of increase in internal friction angle was 14.70%. In the case of interface materials, the rate of increase in cohesion and the internal friction angle compared to the relative density of 50% were 23.53% and 12.12%, respectively. An increase in the internal friction angle resulting from the direct shear test, with an increase in relative density in coarse aggregates, has been reported in the results obtained by Sezer et al. [36].
In general, the highest correlation between cohesion and relative density was obtained for GP materials under dry conditions, with a correlation coefficient of 0.96. Internal friction angle and relative density in both GP and interface materials and under both dry and Fig. 8.Variations trend of (a) cohesion and (b) internal friction angle versus relative density, for GP materials and interface materials, under dry and saturated conditions.
Table 4
Relationships between shear strength parameters and relative density for different materials in dry and saturated conditions.
Specimen Parameter: Peak shear stress τmax (kPa) =A.Dr (%) +B
A B
GP-Dry 0.4333 308.50
Asphalt (B:4%)-GP-Dry 0.5667 227.17
GP-Sat 1.5667 148.50
Asphalt (B:4%)-GP-Sat 1.5000 131.50
Specimen Parameter: Cohesion C(kPa) =A.Dr(%) +B
A B
GP-Dry - 0.2333 30.833
Asphalt (B:4%)-GP-Dry - 0.1667 21.500
GP-Sat 0.1000 23.833
Asphalt (B:4%)-GP-Sat 0.1333 11.667
Specimen Parameter: Internal friction angle ϕ (˚) =A.Dr(%) +B
A B
GP-Dry 0.0667 42.333
Asphalt (B:4%)-GP-Dry 0.0667 35.667
GP-Sat 0.1667 25.833
Asphalt (B:4%)-GP-Sat 0.1333 26.333
saturated conditions, had a high correlation in the range of 0.75–1. The highest correlation between these two parameters is obtained for interface materials with a correlation coefficient of 1. By fitting the best line to the peak shear stress data, cohesion and internal friction angle, in terms of relative density values, the general equation of these behavioral parameters is determined as shown in Table 4.
3.2.3. Investigation of the effect of relative density on the dilation angle
Changes in the dilation angle versus relative density for the specimens subjected to the normal stress of 300 kPa have been shown in Fig. 9.
Observing the trend of changes in the dilation angle versus relative density, indicates that the increase in relative density has led to an increase in the dilation angle in both GP and interface materials under dry and saturated conditions. In general, under constant stress, the dilation angles in dry conditions were greater than the corresponding angles in saturated conditions.
3.2.4. Calculation of the interaction ratio at different values of relative density
Many soil-structure interaction issues, including retaining walls, pile foundations, and soil reinforcement, involve estimating interface friction between soil and these structures. Determining the interface friction angle between the soil and the pile material is necessary in order to have a good estimate of the axial capacity of the pile [37]. Significant studies have been performed to investigate the interfacial friction between sand and other construction materials, and various devices such as direct shear machine, simple shear machine, etc. have been used for this purpose. In this research, the interface friction angle between GP materials and asphalt concrete has been determined and in order to compare its values with the values of the internal friction angle of GP materials, the interaction ratio parameter has been defined. The ratio of the internal friction angle of the interface to the internal friction angle of the gravelly material (δ/ϕ) is called the interaction ratio. Interaction ratio values for different relative density values for the specimens tested have been presented in Table 5.
Looking at the values of the interaction ratio versus relative density, it can be seen that in the interface materials, under dry conditions, with increasing relative density from 50% to 80%, the interaction ratio has increased. Under saturated conditions, an increase in relative density leads to a decrease in the interaction ratio.
3.3. Investigation of the effect of shear rate (SR) on the shear strength parameters and dilation angle of GP materials and interface materials, under dry and saturated conditions
Although considerable research has been done on the behavior of non-cohesive soils at different shear rates, in the thematic literature, no specific studies are available on the effects of shear rate on the interaction of gravelly materials and asphalt concrete.
Overall, the results of previous research have shown that the effects of shear rate on the final shear strength of non-cohesive soils in undrained experiments are not significant. However, the results of undrained experiments indicate a significant effect of shear rate on the behavior of non-cohesive soils [37].
In this study, to investigate the effect of shear rate on the shear strength parameters and dilation angle of specimens containing GP materials, specimens including GP materials with a relative density of 65% and GP materials with a relative density of 65% - asphalt concrete made with 4% bitumen has been prepared and in large-scale direct shear machine, have been tested in dry conditions, under different normal stresses (100, 200 and 300 kPa) and four different shear rates (0.5, 1, 1.5 and 3 mm per minute). The results and outputs of the direct shear test are presented in the following sections.
3.3.1. Investigation of the effect of shear rate on stress-strain behavior
Diagrams of shear stress-horizontal displacement resulting from the direct shear test, for samples containing GP materials, in dry
Fig. 9.Variation of dilation angle - relative density, for GP materials and interface materials, subjected to normal stress of 300 kPa.
conditions, under different shear rates and a normal stress of 300 kPa, have been shown in Fig. 10.
By observing the shear stress-horizontal displacement variations for the tested specimens under different shear rates, it can be seen that in all specimens, under a normal stress of 300 kPa, increasing the horizontal displacement has led to an increase in shear stress.
Also, in both categories of GP materials and interface materials, the shear stress has increased with increasing shear rate. Due to the reduction of interlocking and the inter-particle contact of soil materials, caused by the presence of asphalt concrete and its flexible nature, the values of shear stress in the interface materials are less than these values in soil materials. Increased shear stress as a result of increased shear rate is more evident in higher horizontal displacements.
3.3.2. Investigation of the effect of shear rate on shear strength parameters
By plotting the failure envelope curves obtained from the direct shear test, the shear strength parameters have been obtained. The trend of changes in these parameters versus shear rate has been shown in Figs. 11 and 12.
The variation trend of peak shear stress for different shear rates indicates an increase in peak shear stress or strength of specimens at the moment of rupture, with increasing shear rate in both GP and interface materials. The increase amount of the peak shear stress as a result of increasing the shear rate, compared to its value at the shear rate of 0.5 mm/min, for GP and interface materials, has been obtained in the range of 4.1–1.11% and 4.3–8.91%, respectively. In GP materials, there is a higher correlation (R2 =0.9646) between peak shear stress and shear rate.
Observing the variation trend of cohesion versus shear rate, it can be seen that in both GP and interface materials, increasing the shear rate from 0.5 to 3 mm/min has led to an increase in cohesion values. The amount of increase of cohesion values compared to its value at shear rate of 0.5 mm/min, for GP and interface materials, has been in the range of 17.65–135.3% and 62.5–112.5%, respectively. In GP materials, there was a higher correlation (R2 =0.9949) between cohesion and shear rate. Regarding the variation trend of internal friction angle for different shear rates, it can be said that with increasing shear rate, the internal friction angle in GP materials remains constant. In the interface materials, with increasing shear rate from 0.5 to 3 mm/min, a slight increase of 2.5% in the internal friction angle occurred, so that the angle of internal friction has changed from 40 degrees to 41 degrees. By fitting the best line to the peak shear stress data, cohesion and internal friction angle, in terms of shear rate values, the general equation of these behavioral parameters is determined as shown in Table 6.
3.3.3. Investigation of the effect of shear rate on dilation angle
The changes in the dilation angle versus the shear rate under the normal stress of 300 kPa have been shown in Fig. 13.
Observing the variation trend of dilation angle versus the shear rate under different normal stresses, it can be concluded that the Table 5
Interaction ratio values for GP materials under dry and saturated conditions.
Dry conditions Saturated conditions
Relative density (%) Interaction ratio Relative density (%) Interaction ratio
δ/ϕ δ/ϕ
50 0.847 50 0.970
65 0.869 65 0.946
80 0.854 80 0.949
Fig. 10.Shear stress-horizontal displacement variations for soil materials and interface materials, in dry conditions, under different shear rates and normal stress of 300 kPa.
increase in shear rate has led to an increase in the dilation angle in both GP and interface materials. The amount of increase of the dilation angle in GP and interface materials, in comparison with its value at the shear rate of 0.5 mm/min has been in the range of 3.73–6.9% and 6.25–2.25%, respectively.
3.3.4. Calculation of the interaction ratio at different values of shear rate
Interaction ratio values for different shear rates have been presented in Table 7.
By comparing the values of the interaction ratio at different shear rates, it can be seen that by increasing the shear rate from 0.5 to 1 mm/min, the interaction ratio remains constant at 0.87 and, by increasing the shear rate to 1.5 and 3 mm/min, the interaction ratio increased by 2.3% and reached 0.89.
Fig. 11.Variation of peak shear stress versus shear rate, for GP and interface materials, under dry conditions.
Fig. 12.Variations trend of (a) cohesion and (b) internal friction angle versus shear rate, for GP and interface materials, under dry conditions.
Table 6
Relationships between shear strength parameters and shear rate for different materials in dry conditions.
Specimen Parameter: Peak shear stress τmax (kPa) =A.SR (mm/min) +B
A B
GP-Dr65%-Dry 13.1430 314.04
Asphalt (B:4%)-GP(Dr65%)-Dry 8.2857 259.32
Specimen Parameter: Cohesion C(kPa) =A.SR(mm/min) +B
A B
GP-Dr65%-Dry 9.4286 11.607
Asphalt (B:4%)-GP(Dr65%)-Dry 3.1429 8.7857
Specimen Parameter: Internal friction angle ϕ (˚) =A.SR(mm/min) +B
A B
GP-Dr65%-Dry 0 46
Asphalt (B:4%)-GP(Dr65%)-Dry 0.4286 39.857
3.4. Investigation of the effect of freeze-thaw cycles number on the shear strength parameters and dilation angle of GP materials and interface materials, under dry and saturated conditions
Considering the importance of the effect of freeze-thaw cycles number on the performance of asphalt mixtures, in this study, the effect of this item on the strength parameters of GP and interface materials has been investigated. In order to investigate the effect of freeze-thaw cycles number on the shear strength parameters and dilation angle of GP materials and interface materials, the mentioned specimens include GP materials with a relative density of 65% and also GP materials with a relative density of 65%-asphalt concrete made with bitumen content of 4%, prepared and tested in dry and saturated conditions, with different number of freeze-thaw cycles, in large-scale direct shear machine by UU method, under different normal stresses and shear rate of 1 mm/min and the experiments were continued until the axial strain of 15%. It is to be mentioned that mixtures have been subjected to 10, 20 and 30 freeze-thaw cycles (from − 20 to +20◦C for 24 h). The graph of time (in hours)-temperature (in degrees Celsius), obtained from the weight loss test against freeze and thaw, has been shown in Fig. 14.
The results of the direct shear tests are shown below.
3.4.1. Effect of freeze-thaw cycles number on stress-strain behavior
The curves representing shear stress-horizontal displacement variations resulting from large-scale direct shear tests performed on the specimens containing GP materials subjected to different freeze-thaw cycles and a normal stress of 300 kPa have been shown in Fig. 15.
The diagrams of shear stress-horizontal displacement plotted in the above figures show that in dry conditions, under normal stress of 300 kPa, the number of cycles increases from 10 to 20, the shear stress increases and then, with a further increase in the number of cycles to 30, a decrease in shear stress has been observed. A similar trend has occurred in the interface materials. In saturated con- ditions, under a normal stress of 300 kPa, in GP materials, increasing the number of cycles from 10–20 and then from 20 to 30, respectively, reduces and increases the shear stress. In the interface materials, with increasing the number of freeze-thaw cycles, the shear stress has a completely decreasing trend.
The general conclusion obtained from these diagrams is that in both GP and interface materials under different freeze-thaw cycles, the shear stress of failure in dry conditions has been higher than the saturation conditions. In addition, at a constant number of cycles, in both dry and saturated conditions, GP materials have ruptured under greater shear stress than interface materials.
3.4.2. Effect of freeze-thaw cycles number on shear strength parameters
By plotting the shear stress-normal stress diagram (failure envelope curves) for the specimens, the shear strength parameters (peak shear stress, cohesion and internal friction angle) are obtained. The trend of variation in these parameters versus the number of freeze- thaw cycles has been shown in Figs. 16 and 17.
Fig. 13.Trend of variation of dilation angle - shear rate, for GP materials and interface materials, under normal stress of 300 kPa.
Table 7
Interaction ratio values for GP materials under dry conditions.
Shear rate (mm/min) Interaction ratio
δ/ϕ
0.5 0.87
1.0 0.87
1.5 0.89
3.0 0.89
Observation of the variation trend in peak shear stress per number of different freeze-thaw cycles shows that under dry and saturated conditions, in GP and interface materials, with increasing freeze-thaw cycles number, the peak shear stress or resistance of specimens to rupture decreases. The highest correlation between peak shear stress and the number of freeze-thaw cycles was obtained for interface materials under saturated conditions (R2=0.9672) and for GP materials under dry conditions (R2=0.9368), respec- tively.
The variation trend of cohesion and internal friction angle versus the number of different freeze-thaw cycles indicates that the Fig. 14.Time (horizontal axis)-temperature (vertical axis) graph, obtained from the weight loss test of asphalt concrete mix.
Fig. 15.Shear stress-horizontal displacement variations for soil materials and interface materials, in (a) dry and (b) saturated conditions, under different freeze-thaw cycles and normal stress of 300 kPa.
Fig. 16.Variation of peak shear stress versus freeze-thaw cycle number, for GP materials and interface materials, under dry and satu- rated conditions.
cohesion increased from 10 to 30 with increasing number of cycles under dry conditions, in GP materials and in interface materials.
The rate of increase in the cohesion for 10 cycles, in GP materials and in interface materials, was 66.7% and 344.44%, respectively.
Under saturated conditions, in both GP and interface materials, an increase in the number of cycles from 10 to 30 resulted in an in- crease in cohesion. With increasing the number of cycles in GP materials and interface materials, the rate of increase in cohesion relative to the cohesion in 10 cycles, was equal to 42.86% and 181.82%, respectively. The maximum correlation between cohesion and the number of freeze-thaw cycles was obtained in GP materials (R2=0.9643) and interface materials (R2=0.9231) under saturated conditions, respectively. Examining the values of internal friction angle and the trend of their changes against the number of freeze- thaw cycles, it can be seen that under dry and saturated conditions, in GP and interface materials, with increasing the number of cycles, the internal friction angle decreases. The rate of reduction of internal friction angle in GP and interface materials, under dry conditions and with increasing the number of cycles from 10 to 30, respectively, was in the range of 6.4% and 15%. Also, the rate of reduction of the internal friction angle for GP and interface materials, under saturated conditions and with increasing number of cycles, is equal to 9.1% and 28.12%, respectively. The highest correlation (R2 =0.9643) between the internal friction angle and the number of freeze- thaw cycles was obtained for GP materials under dry conditions and for interface materials under saturation conditions. Reduction of internal friction angle and consequently, peak shear stress in GP and interface materials by increasing the number of freeze-thaw cycles can be explained by expanding existing cracks and creating new cracks in the specimen after several freeze-thaw cycles, consequently porosity increases and dry specific gravity decreases. In fact, during the freeze cycle, the volume of water in the pores increases due to the formation of ice, which leads to the expansion of pores and the creation of micro cracks in the materials. As the result of thawing, water flows through the cracks thus, strength properties such as internal friction angle and shear strength are reduced. The reduction of the internal friction angle in coarse aggregates with increasing number of freeze-thaw cycles have also been reported in the study done by Ishikawa and Miura [38], Yu et al. [39] and Hosseini and Khodayari [40].
By fitting the best line to the peak shear stress data, cohesion and internal friction angle, in terms of the number of freeze-thaw cycles, the general equations expressing the relationships between these parameters and the number of freeze-thaw cycles, are Fig. 17.Variations trend of (a) cohesion and (b) internal friction angle versus freeze-thaw cycle number, for GP and interface materials, under dry and saturated conditions.
Table 8
Relationships between shear strength parameters and number of freeze-thaw cycles for different specimens in dry and saturated conditions.
Specimen Parameter: Peak shear stress τmax (kPa) =A. (Cycle No.) +B
A B
GP-Dr65%-Dry - 2.00 355.00
Asphalt (B:4%)-GP(Dr65%)-Dry - 0.900 260.67
GP-Dr65%-Sat - 0.950 228.33
Asphalt (B:4%)-GP(Dr65%)-Sat - 2.350 227.00
Specimen Parameter: Cohesion C(kPa) =A.(Cycle No.) +B
A B
GP-Dr65%-Dry 0.400 6.6667
Asphalt (B:4%)-GP(Dr65%)-Dry 1.550 8.6667
GP-Dr65%-Sat 0.450 16.0000
Asphalt (B:4%)-GP(Dr65%)-Sat 1.000 - 0.6667
Specimen Parameter: Internal friction angle ϕ(˚) =A.(Cycle No.) +B
A B
GP-Dr65%-Dry - 0.15 48.667
Asphalt (B:4%)-GP(Dr65%)-Dry - 0.3 39.333
GP-Dr65%-Sat - 0.15 34.000
Asphalt (B:4%)-GP(Dr65%)-Sat - 0.45 37.000
according to Table 8.
3.4.3. Investigation of the effect of the number of freeze-thaw cycles on the dilation angle
Changes in the dilation angle versus the number of freeze-thaw cycles under a normal stress of 300 kPa have been shown in Fig. 18.
Observing the variation trend of dilation angle versus the number of freeze-thaw cycles, it can be seen that in GP and interface materials in both dry and saturated conditions, the dilation angle has decreased with increasing number of freeze-thaw cycles.
3.4.4. Calculation of the interaction ratio in the number of different freeze-thaw cycles
The values of the interaction ratio for the number of different freeze-thaw cycles, in different specimens, have been presented in Table 9.
Observation of the values of the interaction ratio with the number of freeze-thaw cycles indicates that in both dry and saturated conditions, with increasing the number of cycles from 10 to 30, the interaction ratio has decreased.
3.5. The application of results in the design of the embankment dam with asphalt concrete core
One of the most important engineering features for the design and performance of structures built into or on soil is soil strength, commonly referred to as "shear strength", because the soil has the tendency to fail under shear. Soil shear strength is an essential parameter for the analysis and design for engineering usage of soil materials. Lateral soil pressures and stability of the inclined or sloping soil structures (slope stability) all rely on soil shear strength. Theoretical values of strength can be defined as a combination of components of cohesion strength and frictional resistance under a set of stress conditions [41].
Due to the heterogeneous structure of interface materials, compared to the homogeneous structure of soil materials, the behavior of soil materials and interface materials is different. In general, interface materials have lower shear strength than soil materials due to the reduction of friction and interlocking of the soil materials. Based on the results of this study and the partial effect of asphalt concrete bitumen on the shear strength of interface materials, this decrease can be partially compensated by increasing the relative density of materials and increasing the shear rate. The reason for the increase in shear strength with increasing relative density is the increase in the resistance of gravel particles on the fracture surface against rotation and overturning.
Regarding the effect of shear rate on shear strength, it can be said that at low shear rates, the specimen has enough time and opportunity to deform and its effect on strength is more limited. At high shear rates, the material particles are in contact with each other for a longer period of time and the soil does not have enough time to deform and its deformation is delayed. In other words, at high shear rates, there is less time for failure and reconfiguration of material particles. Therefore, to form the shear surface, all these inter-particle contacts must be yielded, which leads to an increase in the shear strength of the material.
Moisture sensitivity, on the other hand, is a progressive functional failure of asphalt mixtures caused mainly by the infiltration of water due to the loss of adhesion bond between the bitumen and the aggregate surface or the loss of adhesion resistance between the bitumen adhesives. In addition to moisture, naturally, the occurrence of freeze-thaw cycles also affects the internal structure of asphalt mixtures, and with the expansion of voids, the combination of two separate air pores and the creation of new air pores, causes destruction. Water increases in volume during freezing, so if this water is inside the pores of the aggregate and freezes due to the cooling of the environment, because of the increase in volume, pressure is applied to the inner wall of the aggregate, causing cracking and eventually breaking of the rock. Based on the results of this study, increasing the number of freeze-thaw cycles has reduced the shear strength of soil materials and interface materials.
In another part of the present study, by calculating the interaction ratio as the ratio of the friction angle of the interface materials to the internal friction angle of the soil materials, the interaction ratio changes have plotted in terms of different values of relative density,
Fig. 18.Trend of variation of dilation angle-freeze-thaw cycle number, for GP materials and interface materials, under normal stress of 300 KPa.
shear rate and number of freeze-thaw cycles. Changes in the ratio of interaction versus relative density for different materials have been illustrated in Bowles’ book (Fig. 19) [42]. According to this figure, for the design of structures such as sheet piles and retaining walls, the interaction ratio is considered between 0.5 and 0.66 of the internal friction angle of the soil. According to the results of this study, the interaction ratio for different values of relative density in dry and saturated conditions, in the range of 0.847–0.970; For different values of shear rate in dry conditions, in the range of 0.87–0.89 and for different freeze-thaw cycles, in dry and saturated conditions, in the range of 0.565–0.970 is obtained, which can be used in the design of structures like embankment dams with asphalt concrete core or pavement.
4. Conclusion
In this study, the mechanical behavior of the interface materials in the Mijran dam with asphalt concrete core has been investigated using large.
-scale direct shear device and determined the shear strength parameters and dilation angle. The results of this study are as follows:
– At constant relative density of GP materials, the percentage of bitumen used did not have a significant effect on the shear strength parameters of the interface materials.
– Under constant normal stress, increasing the relative density leads to improved shear strength parameters by 5.13% and 12.12%, under dry and saturated conditions, respectively, increased dilation angle under dry and saturated conditions, increased interaction ratio under dry conditions and reduced interaction ratio under saturated conditions, in the interface materials.
– Increasing the shear rate slightly improves the shear strength parameters by 2.5%, increases the dilation angle and increases the interaction ratio, under dry conditions, in the interface materials.
– Increasing the number of freeze-thaw cycles, has led to reduced shear strength parameters by 0.15% and 28.2%, respectively, under dry and saturated conditions, reduced dilation angle, and decreased interaction ratio, under dry and saturated conditions, in the interface materials.
– Under dry conditions, with increasing relative density, the interface materials have ruptured in a larger vertical displacement compared to GP materials. Under saturation conditions, with increasing relative density from 50% to 65%, rupture has occurred in GP materials in a larger vertical displacement compared to interface materials, and this trend is reversed with a further increase in relative density to 80%.
– By increasing the shear rate, increasing the horizontal displacement has led to a decrease in vertical displacement in both GP materials and interface materials. In the interface materials, the reduction of vertical displacement occurred with a relatively slow and uniform slope, but in GP materials, the reduction of vertical displacement continued with a greater slope.
Table 9
Interaction ratio values for GP materials under dry and saturated conditions.
Dry conditions Saturated conditions
Freeze-Thaw cycle number Interaction ratio Freeze-Thaw cycle number Interaction ratio
δ/ϕ δ/ϕ
10 0.851 10 0.969
20 0.565 20 0.970
30 0.773 30 0.770
Fig. 19.Comparison of interaction ratios in sand and some common construction materials [42].
– Under dry conditions, in GP materials and interface materials, increasing the number of freeze-thaw cycles, has increased and decreased the vertical displacement, respectively. With increasing number of cycles under saturation conditions, in GP and interface materials, vertical displacement has a decreasing trend.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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