In this paper, three different rock-soil mixtures were reconstituted in laboratory, which were designed to mimic the proportions of coarse and fine particles in the high fill used at the airport construction sites. The shear strength of the reconstituted mixtures was determined by both large-scale direct shear tests (DSTs) with different plate opening sizes and triaxial compression tests. By comparing the test results, the most appropriate plate opening size for DSTs on coarse gap-graded rock-soil mixtures is discussed. The test results indicate that the opening size has a significant effect on the measured shear strength of gap-graded rock-soil mixtures. For DSTs under the same normal stress, the peak strength decreases with increasing plate opening size. For the gap-graded mixture with a small proportion of coarse particles, a plate opening size of one-third to one-quarter of the maximum particle size (
With the rapid economic development in China, many airports are being built in southwestern China (Figure
Distribution of several high fill airport projects and several other high fill projects in China.
In order to save urban land and comply with environmental restrictions, most of the airports are built in mountainous terrain with complex landforms. Enormous amounts of fill are required for airport construction. As a result, most of these airports are high fill airports. The fill materials are usually rock-soil mixtures obtained locally and in most cases by mountain blasting.
The airport fill volume is generally between 1.5 × 107 and 3.0 × 107 m3. In some airports, such as Jiuzhai, Panzhihua, Liupanshui, Changshui, and Chongqing Airports, the fill volume exceeds 5.0 × 107 m3, and that for Changshui Airport is even more than 3.6 × 108 m3. The maximum fill heights are in many cases 20–60 m. However, for some airports, including Jiuzhai, Panzhihua, Liupanshui, Huairen, Tengchong, Lvliang, Wenzhou, and Chongqing Airports, the fill heights are over 60 m. The maximum fill height at Chengde Airport even exceeds 114 m. The problems related to high fill stability in these projects are prominent.
The rock-soil fill most widely used in high fill airport construction in China is composed of soil and blasted rock fragments. The rock-soil mixture contains a complex mix of various particle sizes, resulting in heterogeneous and irregular fill. The maximum particle size is 800 mm, and, in some extreme cases, rock blocks are more than 1000 mm. Rock-soil mixtures without intermediate particle sizes (gap-graded materials) are commonly used for high fill airport construction.
Under normal conditions, the factor of safety for slopes at high fill airports must not be less than 1.30 [
Compared to costly and time-consuming triaxial compression tests, the DST is one of the most effective methods to determine the shear strength index of coarse-grained materials [
The shear test on rock-soil mixture is performed in rigid shear boxes under the constraint of shearing frames and fixed shear surfaces. The orientation and position of coarse gravels are constantly adjusted during testing. However, gravels can hardly deflect or roll in the shear zone when a large normal stress is imposed. Therefore, in order to avoid particle breakage and allow the specimen to shear along the weakest plane, it is necessary to open a gap between the upper and lower shear boxes before shearing. Few studies have been carried out on appropriate plate opening size for DSTs and the opening size is not even mentioned in most studies on DSTs.
Some studies [
In this paper, a series of DSTs were conducted on the typical rock-soil mixtures reconstituted from the high fill of Chengde Airport. After field investigation, three gap-graded rock-soil mixtures were reconstituted to mimic the proportion of coarse and fine particles at the airport construction site. The shear strength of the gap-graded rock-soil mixtures was measured by large-scale DSTs with different opening sizes. By comparing the results obtained by DSTs and triaxial compression tests, the most appropriate DST opening size for the rock-soil mixtures was determined. This research can provide some reference for the design and construction of similar high fill projects.
Gap-graded soil is one of the two types of poorly graded soil [
A small space between the shear boxes may restrict the development of the shear band, but a large opening causes stress reduction and material loss at the specimen edges or small portions of the specimen escaping into the gap [
Based on the DST test results reported by 21 geological exploration institutes in China, Guo [
The plate opening is usually set at 0.5 mm for conventional materials. This value is used widely in the UK [
With a direct shear apparatus, Lings and Dietz [
From the previous studies, it is clear that although the opening size has an important influence on the DST results, the research on the appropriate opening size for rock-soil mixtures is not complete, especially for DSTs on gap-graded rock-soil fill materials. No systematic studies and reasonable suggestions for the appropriate opening size with respect to grain size have been made for large-scale DSTs. It is also questionable whether the opening size of one-third to one-quarter
A series of large-scale DSTs were conducted in laboratory using a direct shear apparatus, ShearTrac-III (GeoComp, Acton, MA, USA) (Figure
Large-scale direct shear apparatus ShearTrac-III.
Opening size adjusting device on ShearTrac-III.
Triaxial compression tests were also conducted on the gap-graded rock-soil mixtures, using a triaxial testing system, LoadTrac-II/FlowTrac-II (Figure
Triaxial compression apparatus LoadTrac-II/FlowTrac-II.
The grain size distributions were determined by field investigations for different high fill materials at a number of airports; test samples were taken from the rock-soil high fill of Chengde Airport (Figure
(a) Rock-soil mixture at high fill construction site for Chengde Airport. (b) The typical particle size distribution of gap-graded high fill for Kunming New Airport.
Specification of Soil Test SL 237-1999 [
In the actual project, the particle size of the fill materials generally varies in a large range, and the rock-soil mixture with large difference in particle sizes and the gap-graded fill material are very common. The grain size distribution of typical gap-graded high fill for Kunming New Airport is shown in Figure
Particle size distributions of the gap-graded rock-soil mixtures G1, G2, and G3 for the direct shear and triaxial tests.
According to Standard for Engineering Classification of Soil GB/T 50145-2007 [
The dry densities of the mixtures G1, G2, and G3 are 1.47 g/cm3, 1.78 g/cm3, and 1.77 g/cm3, respectively. The physical parameters of the samples are listed in Table
Physical properties of gap-graded rock-soil mixtures G1, G2, and G3.
Specimen | |||||||
---|---|---|---|---|---|---|---|
G1 | 0.60 | 1.59 | 1.32 | 20.0 | 0.075 | 2.81 | 0.88 |
G2 | 0.60 | 1.92 | 1.60 | 20.0 | 0.075 | 68.67 | 0.02 |
G3 | 0.60 | 1.94 | 1.57 | 20.0 | 0.075 | 11.41 | 9.94 |
Figures
Photograph of rock-soil mixture G1.
Photograph of rock-soil mixture G2.
Photograph of rock-soil mixture G3.
Relative density can reflect grain size, particle shapes, and structure of the specimens and the shape of the particles has an important influence on the physical properties of a soil [
At present, few references and specifications for DSTs on similar gap-graded rock-soil mixtures are available. In consideration of the large amount of laboratory tests, the mixture G1 with more fine particles and the mixture G3 with more coarse particles were tested with priority. The test on G2 is based on the test experience of G3.
The testing procedures suggested by ASTM D3080 (standard method for DST), ASTM D4767 (standard method for the consolidated undrained triaxial test), and SL237-1999 were followed to determine the shear strength of the gap-graded rock-soil specimens. The rock fragments and soil were thoroughly mixed and then loaded into the shear box in four layers. During loading, each layer is tamped until a specified height is reached.
The DSTs and triaxial compression tests on the specimens were carried out sequentially. As the gap-graded rock-soil specimens were air-dried without consolidation or drainage, the normal pressure was 100, 200, 300, or 400 kPa, respectively, and the shearing rate was set to 0.8 mm/min as recommended by ASTM D3080. The total shear displacement was 60.0 mm. For the triaxial compression tests, the confining pressure was 100, 200, 300, and 400 kPa, respectively, the shearing rate was 0.06%, and the axial strain was 18%.
The axially symmetric stress is considered in triaxial compression tests whereas a direct shear test is a plane-strain problem. Hence, the internal friction angle
Large-scale DSTs and medium-scale triaxial compression tests were both carried out on the gap-graded rock-soil mixtures G1, G2, and G3. The failure envelopes are plotted for the triaxial compression tests, which are compared with the peak shear strength determined by the DSTs with reasonable opening sizes. The results are described below.
From Figure
(a) Variation of shear stress with horizontal displacement obtained by direct shear tests on G1 with a plate opening of 5.0 mm. (b) Failure envelope for G1 (plate opening 5.0 mm).
The relative abundances of coarse and fine particles affect the shear strength of rock-soil mixtures directly [
Medium-scale triaxial compression tests were also carried out on the mixture G1. The variations of deviatoric stress with axial strain under different normal stresses are shown in Figure
(a) Variation of deviatoric stress with axial strain for G1. (b) Comparison of shear strength determined by direct shear test (opening size 5.0 mm) and triaxial tests for G1.
The variations of shear stress with horizontal displacement for the mixture G2 are shown in Figure
(a) Variation of shear stress with horizontal displacement obtained by direct shear tests on G2 with a plate opening of 10.0 mm. (b) Failure envelope for G2 (plate opening 10.0 mm).
The variations of deviatoric stress with the axial strain obtained by the triaxial tests are plotted in Figure
(a) Variation of deviatoric stress with axial strain for G2. (b) Comparison of shear strengths determined by direct shear test (plate opening 10.0 mm) and triaxial tests for G2.
When the percentage of the gravel was increased to 75.09% in G3, the gravel formed a skeleton for the rock-soil mixture and the gravel also became dominant in the shear plane (the shear band). For the mixture G3, the shear strength mainly depends on the interactions between the coarse-grained particles.
During shearing, the gravels of larger size move and deflect in the shear zone. The particles slide against each other and roll, and the smaller particles are more likely to roll. Because the main materials for high fill at the airports are produced by hard rock blasting, the surface of rock fragments is rough and the coefficient of friction is high. The interlocking and embedding forces between particles are two of the most important factors causing dilatant deformation of dense rock-soil mixtures. The dilatancy is mainly caused by rotation and rearrangement of the particles on the shear surface or in the shear zone. The rough contact surfaces between the coarse particles will also produce friction and lead to higher shear resistance.
With a higher
Variation of shear stress with horizontal displacement obtained by direct shear tests on G3 with a plate opening of (a) 5.0 mm, (b) 10.0 mm, (c) 15.0 mm, (d) 20.0 mm, (e) 25.0 mm, and (f) 30.0 mm. (g) Failure envelopes for G3 under direct shear with different plate openings.
As can be seen from Figure
The variations of shear stress with horizontal displacement for G3 corresponding to the opening size of 10, 15, 20, 25, and 30 mm are shown in Figures
Under the same normal stress, the peak shear stress for the mixture G3 decreases as the opening width increases [
The failure envelopes for G3 under various opening sizes are shown in Figure
Peak and residual internal friction angle (
Opening size/(mm) | Peak values | Residual values | ||
---|---|---|---|---|
5.0 | 53.3 | 160.0 | 51.56 | 38.65 |
10.0 | 39.1 | 66.5 | 27.28 | 16.49 |
15.0 | 37.0 | 59.5 | 26.30 | 22.13 |
20.0 | 38.3 | 51.5 | 29.79 | 17.48 |
25.0 | 34.1 | 43.3 | 25.88 | 0.00 |
30.0 | 27.8 | 51.3 | 20.52 | 5.30 |
The relative concentration and proportion of coarse and fine particles in rock-soil mixture directly affect interlocking and embedding of particles during the shear failure process and subsequently have an important impact on the shear strength of the rock-soil mixture [
Residual strength is the residual shear stress on the shear surface after shear damage occurs. It reflects the strength deterioration after a material is damaged. It is also an important parameter for evaluating slope stability. As shown by the DST results in Table
The failure envelopes obtained by the triaxial tests are compared with the peak shear strengths from DSTs on G3 in Figures
(a) Variation of deviatoric stress with axial strain for G3. (b) Comparison of shear strengths determined by direct shear test (plate opening 10.0 mm) and triaxial tests for G3.
The shear strength and friction angle obtained by DST and triaxial compression tests for the three mixtures G1, G2, and G3 are listed in Table
Summary of testing results for direct shear tests and triaxial compression tests.
Specimen/opening size (mm) | ||||
---|---|---|---|---|
G1/5.0 | 29.8 | 44.8 | 29.1 | 88.0 |
G2/10.0 | 36.1 | 118.9 | 36.9 | 48.0 |
G3/10.0 | 39.1 | 66.5 | 41.4 | 17.3 |
For large-scale DSTs on gap-graded rock-soil mixtures with a fairly small proportion of coarse particles, a plate opening size of one-third to one-quarter
Due to the huge volume of excavation and filling in the airport high fill projects, the particle size of the rock-soil mixture changes significantly and is often difficult to control. If the grading requirements for the fill material are too high, a large amount of waste materials will be produced during construction, which directly affects the progress and cost of the airport project. DST is one of the most effective methods to determine the shear strength of the rock-soil mixture, and it is of great practical significance to study the reasonable opening size of the DST for the gap-graded rock-soil mixture which is often encountered in the projects.
Based on the test results on gap-graded rock-soil mixtures in this paper, the possible applications are suggested as follows.
For DSTs on the gap-graded rock-soil mixture under the same normal stress, the larger the plate opening size is, the more easily the particles are rolled and rearranged on the shear surface. Therefore, the opening size has an important influence on the shear strength. In order to obtain reliable shear properties and shear strength indexes of rock-soil mixtures, the plate opening size used during DSTs should be clearly stated, which can provide reference for the design and construction of the increasing number of high fill airports in mountain areas.
The shear strength of rock-soil mixture depends on the interaction between coarse and fine particles. When there is a large amount of coarse particles in the rock-soil mixture, the plate opening size between the upper and lower shear boxes should be increased accordingly.
In addition, it is necessary to consider the particle size distribution of the rock-soil mixture when determining the reasonable opening size for large-scale DSTs on gap-graded rock-soil mixtures. A plate opening of one-third to one-quarter
In this study, three types of rock-soil mixtures were tested by DSTs and triaxial compression tests. The mixture G1 was obtained by adding some gravels to a fairly large volume of fine particles, and the fine particles completely enclose the coarse particles. The mixture G2 l had a higher proportion of coarse-grained particles. However, the coarse particles cannot form a skeleton for the soil, and the fine particles were not enough to fill the space between coarse particles. For G3, the proportion of coarse grains was much higher, and the fine particles partially filled the space between coarse particles. The shear strength is determined by the extrusion, interlocking, and embedding of the coarse particles.
The plate opening size has an important influence on the shear strength of the gap-graded specimens. Under the same normal stress, the larger the plate opening size is, the more easily the particles are rolled and rearranged on the shear surface. As a result, the measured peak strength and residual strength decrease with increasing plate opening size. Therefore, the plate opening size adopted for DSTs should be clearly stated.
During large-scale DSTs on gap-graded rock-soil mixtures, a plate opening of one-third to one-quarter
The data used to support the findings of this study are included within the article
The authors declared that they have no conflicts of interest regarding this work.
This research was supported by the National Key Technologies Research & Development Program (Grant no. 2017YFC0804607) and Independent Research Topics of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin (Grant no. SKL2018TS10).