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Dieses Kapitel befasst sich mit der Qualitätskontrolle der Verdichtung von Boden-Gestein-Gemisch (S-RM), einem kritischen Aspekt des Schnellstraßenbaus. Die Studie verwendet eine zweidimensionale (2D) Diskrete-Elemente-Methode (DEM), um die Evolutionsgesetze der Porosität und der Partikelzerkleinerung während des Walzenverdichtungsprozesses zu untersuchen. Schlüsselthemen sind die Auswirkungen von Schichtdicke, Walzenmasse und Walzendurchläufen auf die Verdichtungsqualität des Untergrundes. Die Untersuchung kommt zu dem Schluss, dass eine Schichtdicke von 0,6-0,7 Metern, eine Walzenmasse von 26 Tonnen und 5-6 Walzendurchläufe optimal sind, um die gewünschte Verdichtungsqualität zu erreichen. Die Studie schlägt außerdem eine neuartige Methode zur Qualitätskontrolle der Verdichtung vor, die auf der Differenz zwischen dem Durchschnittswert des kontinuierlichen Index nach den letzten beiden Verdichtungsvorgängen beruht. Darüber hinaus wird in diesem Kapitel der mechanische Zustand des S-RM-Untergrundes beim Walzen beschrieben, der in vier verschiedene Stufen unterteilt ist: schwere Zerkleinerung von Partikeln, ernsthafte Zerkleinerung von Partikeln, schwache Zerkleinerung und Verdichtung. Die Ergebnisse liefern wertvolle Einblicke in den Verdichtungsprozess und geben praktische Empfehlungen zur Verbesserung der Bauweise.
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Abstract
Over-compaction or under-compaction of soil-rock mixture subgrade could lead to diseases after the expressway is put into service. Selecting the appropriate compaction process and compaction quality testing method is the key to controlling the compaction quality of soil-rock mixture subgrade. Based on continuous compaction control technology, the subgrade porosity under rolling was simulated by two-dimensional discrete element method, considering lay thickness, roller mass, and roller passes. The suitable filler type and recommended compaction process for the soil-rock mixture subgrade in Xingtai Section of Taihangshan Expressway were given: the lay thickness of soil-rock mixture subgrade is 0.6–0.7 m; the mass of the roller is 26 t, and the number of roller passes is rolled 5–6 passes. The evolution law of the subgrade porosity considering the particle crushing was analyzed. A method to control the compaction quality of the soil-rock mixture subgrade was proposed: whether the average difference between the in-site measurements from the last two roller passes exceeded 1% of the average value of the last pass.
1 Introduction
Continuous compaction control technology has great advantages in subgrade compaction measurement. The subgrade compaction state is the physical and mechanical properties of the subgrade under the roller rolling. During the roller rolling, the roller passes affect the mechanical properties of the subgrade [1, 2]. The decrease in the number of roller passes could cause under-compaction of the subgrade, while the increase in the number of roller passes may lead to over-compaction of the subgrade [3]. Under-compaction or over-compaction leads to deterioration of the subgrade after the expressway is put into service [4‐6]. Accordingly, reasonable compaction process and subgrade compaction quality testing methods are important.
The particle size of the filler excavated near the Xingtai Section of the Taihangshan Expressway varies greatly, the maximum particle sizes of some blocks can reach 80 mm, and the particle sizes of some soil particles do not exceed 0.25 mm. Moreover, during the rolling process of soil-rock mixture (S-RM) subgrade, the spectral composition of the vibration wheel response signal is complex. It is difficult to establish a general regression relationship between continuous measurements and point measurements in the S-RM subgrade, and using linear regression relationships to determine the compaction quality of the subgrade is not reliable [7‐11].
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And only in-situ monitoring tests cannot effectively provide the meso-behavior of the internal system of materials and the compaction mechanism of S-RM subgrade. The discrete element method (DEM) can reflect the composition characteristics of heterogeneous materials and can simulate the meso-behavior of granular materials [12‐15].
In this study, through two-dimensional (2D) DEM simulation experiments, the recommended compaction process of Xingtai Section of Taihangshan Expressway is given, considering lay thickness, roller mass, and roller passes. We investigated the evolution laws of porosity (n) and particle crushing in subgrade compaction, to provide theoretical support for compaction quality control of S-RM subgrade.
2 Materials
Located in the Beijing-Tianjin-Hebei regions, the Xingtai section of the Taihangshan Expressway spans 64.2 km. As shown in Fig. 1a, the micro-geomorphic units mainly consist of low mountain terrain and low mountain‒hilly terrain. The monitoring site is located at K8+105-K8+341 in the Xingtai section. The center of the monitoring site has a height of 19.2 m, and the maximum height of the section is 19.8 m, as shown Fig. 1b. The filler for this section was excavated from the adjacent road graben with a lithology of granitic gneiss: deep-extreme weathering (Filler A) and incipient-intermediate weathering (Filler B), as shown in Fig. 1c.
In the subgrade compaction quality control process, the control of filler is fundamental. And the control mainly includes two aspects: (1) Lay thickness. Smaller lay thickness causes over-pressure of the subgrade and larger lay thickness results in under-pressure of the subgrade. (2) Roller rolling. Roller rolling is also a key part of the formation of the subgrade structure. The relevant parameters such as the mass, passes, and the velocity of the roller are crucial.
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In this study, the driving velocity of the roller is nearly uniform, approximately 3 km/h. Considering the lay thickness, the mass of the roller, and roller passes, PFC2D5.0 was used to realize the DEM simulation of the roller rolling process, and the n of the S-RM subgrade was analyzed.
3.1 2D DEM Modeling
The modeling process is shown in Fig. 2. Firstly, a 2D outline of the block in PFC was generated based on a random polygon, and randomly distributed in the box according to the gradation. Then the clumps were filled in geometric shapes, and each clump is numbered. After recording the size and location of each clump, each pebble in the clump was replaced with a ball until all the pebbles were replaced by the ball. And the unbreakable clump turned into a breakable cluster. The balls of soil particles with size of 2–5 mm were generated, and the balls that coincide with the cluster were deleted. Then the boundary conditions were given: the left, right and lower borders were closed, and the upper border was free, as shown in Fig. 3a. A circular wall was used that applied downward eccentric force and horizontal reciprocating motion, as shown in Fig. 3b. The roller traveled from the left to the right at an average speed of 3 km/h. When the roller arrived at the right boundary, it rolled in the opposite velocity. A 2D DEM model of the subgrade compaction process was established. Four measurement regions were monitored, the n of subgrade was calculated by the average value of the n of the four regions.
According to the results of indoor tests, the parameters required for the parallel bonding model and rolling resistance model were selected by trial-and-error method, as shown in Table 1.
Table 1
Selection of 2D DEM parameters for S-RMs
Material
Parameter
Value
Filler A
Density of particles (kg·m−3)
1800
Elastic modulus of particle contacts (MPa)
40
Poisson’s ratio of particle contacts
0.5
Friction coefficient
0.45
Rolling resistance coefficient
0.05
Filler B
Density of particles (kg·m−3)
2400
Elastic modulus of particle contacts (MPa)
100
Poisson’s ratio of particle contacts
0.5
Friction coefficient
0.65
Bond shear strength (MPa)
1.5
Bond tensile strength (MPa)
1.5
Contacts between Filler A and Filler B
Elastic modulus of contacts (MPa)
100
Poisson’s ratio of particle contacts
0.5
Friction coefficient
0.65
Rolling resistance coefficient
0.1
3.2 Lay Thickness
Figure 4 is the section view model of the S-RM subgrade with different lay thickness. The gradation of the filler in Fig. 4 is A:B = 2:5, which is mostly used on site. The length of the four models is 3 m, and the thickness is 0.5 m, 0.6 m, 0.7 m, and 0.8 m, respectively. The number of balls of the blocks in subgrade models of different thicknesses was 18,453, 22,150, 25,761, and 29,634, respectively. The number of balls of these subgrade models was 28,569, 34,282, 39,925, and 45,856, respectively.
As shown in Fig. 5, during the compaction process, the n decreases rapidly during pass 1, and with the increase of the lay thickness, the n gradually decreases. When the lay thickness is 0.5–0.7 m, the n shows a more obvious step shape with the compaction process. With the progress of the compaction process, the subgrade with the lay thickness of 0.5–0.7 m is gradually compacted, and the n reduction rate is basically stable.
Fig. 5
The n of S-RM subgrade with different lay thickness
However, when the lay thickness is 0.5 m, the n of the subgrade at the time step of 14 is already at a relatively low level. And then with the rolling progress, the block particles are further broken, and the n rises. The curve is prone to over-pressure phenomenon. When the lay thickness is 0.8 m, the n change form of the subgrade is parabolic and still slowly decreasing, therefore, when the lay thickness is 0.8 m, the compaction quality of the subgrade is also difficult to control, and the rolling of six passes cannot compaction the subgrade, and the subgrade presents a state of under-pressure. Therefore, lay thickness between 0.6 and 0.7 m is a more suitable thickness.
3.3 Roller Mass
The vertical eccentric force has a relatively large influence on the compaction quality of the S-RM subgrade, considering the tonnage of commonly used vibrating rollers: 14, 22, 26, and 33 t. According to the relevant parameters of the roller, the maximum eccentric forces of these four tonnage rollers are 185 kN, 400 kN, 430 kN, and 725 kN, respectively. The n of the subgrade slowly decreases when the mass of the roller is 14 and 22 t, as shown in Fig. 6.
Fig. 6
Effect of different tonnage rollers on compaction quality
The curve does not appear to be stepped down, but rather a parabolic descent. The subgrade shows the characteristics of “under-pressure”, and the compaction of 6 passes cannot compact the subgrade. However, for the roller with the tonnage 26 t, the particles appear less breakage and the n of the subgrade shows a stepwise reduction, as far as the numerical simulation results show that after the six passes of the 26 t vibrating roller, the n of the subgrade gradually flattens and the subgrade is significantly compacted. When the tonnage is 33 t, the n of the subgrade is reduced to a lower level, and the subgrade eventually flattens. The curve is also oscillating, and there is a clear fragmentation behavior, the subgrade shows the characteristics of “over-pressure”.
3.4 Recommended Compression Process
According to the above analysis, this study proposes that the lay thickness of 0.6–0.7 m is proposed as the single-layer filling thickness of the S-RM subgrade; the roller mass is 26 t; the rolling passes are suggested as 5–6 passes.
3.5 The Evolution Law of Subgrade State During Roller Rolling
As shown in Fig. 7, in the process of vibrating roller rolling, the mechanical state of S-RM subgrade can be divided into four sections: severe particle crushing stage (section OA), serious particle crushing stage (section AB), weak particle crushing stage (section BC), and compaction stage (section CD).
Section OA: The subgrade at this stage is continuously compacted, and the numerical simulation test results show that the curve oscillates more intensely. The blocks with relatively large particle size at this stage produce more serious crushing in the process of compaction.
Section AB: The n of the second section first decreases rapidly and then slow down to show a “step” reduction, the compaction quality of the S-RM subgrade at this stage is relatively close to the final stage, and the particle crushing behavior is weakened.
Section BC: The n of this stage is close to the development law of section AB, and the evolution of n is also similar to the n of section AB. The compaction quality of the subgrade is further improved, and the particle crushing behavior is further weakened.
Section CD: The n in this stage is basically flattened, the amplitude of the n curve oscillation is already at a relatively low level, and the particle fragmentation behavior is significantly lower than that of the section BC.
In this study, Fig. 8 show that the difference between the n of the last two passes is 0.001, which is approximately 1% of the mean of the last pass. Based on this result, this study proposes that the compaction quality control method of S-RM subgrade is: whether the difference between the average value of the continuous index after the last two rolling operations is less than 1% is used to assess whether the compaction quality of the S-RM subgrade meets the requirements.
Fig. 8
Analysis of subgrade structure in the last two passes
The DEM can accurately simulate the subgrade rolling process, and the crushing behavior of block particles can be considered. Based on the DEM simulation of soil-rock mixture subgrade rolling in the Xingtai section of Taihangshan Expressway, the lay thickness is 0.6–0.7 m; the mass of the road roller is 26 t; the number of roller passes is 5–6 passes. After the construction of this process, the subgrade filler can be fully compacted, and a certain skeleton can be formed between the soil and the block, and the crushing behavior of the block particles is relatively slight. The results of 2D DEM simulation show that the porosity of the subgrade structure changes very weakly in the average of the porosity of the last two passes. The difference between the average value of the continuous index after the last two rolling operations is less than 1% of the mean of the last pass. In this study, 2D DEM simulation is considered for the subgrade analysis under roller rolling, some shortcomings exist: the variation of the porosity is a three-dimensional problem. Using the three-dimensional discrete element method in subsequent research to study the compaction of soil-rock mixture subgrade would be more convincing.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grants Nos. 42072313), Sichuan Province Science and Technology Support Program (2021-A-2).
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
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