Numerical simulation of compaction parameters for sand-filled embankment using large thickness sand filling technique in Jianghan Plain district

Abstract

The study uses the finite element method to simulate a new technique of highway sand embankment filling in Jianghan Plain district, which can raise the thickness of sand-filled layer from 30 cm to 70 cm and can significantly shorten the construction period based on the guarantee of sand embankment construction quality. After simulating the three compacting proposals carried out on the field test, the study uses COMSOL software to research on the compacting effects of sand-filled layers in larger thicknesses by 22 ton vibratory roller alone, and then to investigate the steady compacting effect of 12 ton vibratory roller. The simulation results indicate that the sand-filled layer thickness of 70 cm is suitable for the new sand filling technique, and the sand-filled embankment project with tight construction period is suggested to choose the 12 ton vibration roller for steady compaction.

Keywords

sand embankment / compaction in large thickness / numerical simulation / small size vibratory roller / steady compaction

Introduction

As the wide distribution of soft soil foundation in Jianghan Plain district, it’s hard to construct highway in this district because of large consumption of farmland soil for higher embankment construction caused by high ground water level, which is not friendly to local environment. In Jianghan Plain district, Yangtze River contains huge sand resources, which can be used as filling materials to construct highway embankment and solve the problem of large soil consumption. Different from dredger-reclaimed sand embankment that can be filled to a large height in one time [¹,²], Yangtze River’s sand with little mud must be compacted layer by layer in sand embankment construction. However, there is no special specification to instruct sand embankment construction and the thickness of sand-filled layer still refers to the thickness of soil-filled layer from the specifications that should not exceed 30 cm [³,⁴], which will slow the construction progress and do not match the demand of sand-filled embankment project with tight construction period.

In previous researches, a new technique of highway sand embankment filling was proposed to accelerate construction progress, which could raise the thickness of sand-filled layer from 30 cm to 70 cm and solved the problem of tight construction period caused by soft foundation treatment. After solving the safety problem that vibratory roller was often stuck in wet sand [⁵], three compacting proposals were carried out on the field test in the sand-filled embankment of highway project from Qianjiang to Shishou and then optimized the compaction parameters [⁶]. However, it did not further research on larger thickness of sand-filled layer and other compaction pattern combinations as the limitation of construction site condition. In this case, numerical simulation method can be used to solve these issues, and to find out appropriate thickness of sand-filled layer with corresponding compaction parameters, which can significantly shorten the construction period based on the guarantee of sand embankment construction quality and be of great significance to highway sand-filled embankment project.

Currently, researches about sand-filled embankment are focused on experimental investigations of different sand soil properties and key points of both design and compacting plan of filling sand embankment and its construction controlling [⁷‒¹⁰]. In the aspect of numerical simulations, the analysis of slope stability of sand embankment was conducted with shear strength reduction method [¹¹], and mechanical behavior of fine sand filling embankment was analyzed to optimize its structural system [¹²], and the settlement characteristics of the sand drain subgrade under embankment load at soft soil area was investigated [¹³,¹⁴]. Additives are used to reinforce embankment and its performance have been evaluated, such as geotechnical performance and field demonstration of highway embankment constructed using waste foundry sand [¹⁵], interaction mechanism and behavior of hexagonal wire mesh reinforced embankment with silty sand backfill on soft clay [¹⁶], and construction of a test embankment using a sand-tire shred mixture as fill material [¹⁷]. Focused on the feature of soft foundation, a lightweight fill for embankments using cement-treated Yangzi River sand and expanded polystyrene beads has been investigated and the parameters from in-situ measurements of an embankment on soft clays with sand drains was numerical characterized [¹⁸,¹⁹]. In consideration of limit fill height of embankment caused by high groundwater level, effect of reinforced sand cushion on soft clay foundation was investigated and a direct shear strength model of river sand was established to evaluate its application in this district [²⁰,²¹]. Effect of dynamic compaction on red sand soil filling embankment was researched [²²], but few on compaction process of filling materials layer by layer using vibratory roller [²³‒²⁷], and no research on sand compaction process of sand-filled embankment in Jianghan Plain [²⁸].

In this paper, COMSOL finite element software is used to firstly simulate the experimental data of three compacting proposals with the sand-filled layer thickness of 70 cm carried out on the field test for validation of numerical model, and then to investigate the compacting effects of sand-filled layers in larger thicknesses by 22 ton vibratory roller alone and the steady compacting effect of 12 ton vibratory roller. After comparing different compacting proposals, the study recommends applicable sand filling techniques for sand-filled embankment projects with different construction period, and finally compares the large thickness sand filling technique with traditional technique in terms of economic and efficiency.

Large thickness sand filling technique and finite element model

The proposed large thickness sand filling technique makes full use of rich resources of sand and gravel from Yangtze River to construct sand-filled embankment in Jianghan Plain district, and the optimized structure of sand-filled embankment are shown in Fig. 1. After laying nonwoven geofabric and two-way geogrid on soft foundation treated by sand cushion, the sandy gravel layer with the thickness of 50 cm is filled directly and a cross slope of 2% is set to make it convenient for water to drain out of embankment structure. The sand-filled layers with the thickness of 70 cm are constructed respectively and surrounded by soil added with 4% lime, which can form a solid structure and avoid collapsing caused by water loss of sand. The roadbed is filled by stoned-added soil, which can raise the stiffness of the embankment structure and reduce the project cost.

The key problem in sand-filled embankment construction is lack of bearing capacity for sand caused by water content of being too high or too low, which may get the vibratory roller stuck in sand and both slow the construction speed and cause hidden trouble to safety. According to the sand’s California Bearing Ratio tests conducted in different water content [²⁹], the maximum values of sand’s CBR and dry density occurred in the optimum water content and increased with the growth of compaction level, which indicated that the denser of sand and the higher bearing capacity it had. Further, the test that water content of sand changed with time were conducted to find the waiting time for sand to reach its optimum water content in different weather and condition, which supplied data for sand filling construction site to avoid machine getting stuck in wet sand and ensure the construction safety.

When sand is in optimum water content, apply an appropriate compaction pattern combination to the sand-filled layer and raise its bearing capacity step by step can finally make the compaction degree meet the specification. The construction processes of embankment using large thickness sand filling technique are shown in Fig. 2. Firstly, watered the sand-filled layer sufficiently and controlled sand water content to the optimum state by manual watering. Then raised the bearing capacity of the sand-filled layer by loader machine, which could create a safe construction condition for 22 ton vibratory roller to compact on sand-filled layer without getting stuck in wet and loose sand. Applied appropriate pattern combinations of weak and strong vibratory compaction by 22 ton vibratory roller to raise the bearing capacity of sand-filled layer step by step, and finally checked the compaction degree in upper, middle and lower layer respectively [³⁰]. According to results from field tests [⁶], the large thickness sand filling technique could avoid machine getting stuck in wet sand and raised the thickness of sand-filled layer from 30 cm to 70 cm, which could significantly shorten the construction period based on the guarantee of sand embankment construction quality.

In order to break the limitation of construction site condition and further find out appropriate thickness of sand-filled layer with corresponding compaction parameters for sand-filled embankment in Jianghan Plain district, the study focuses on larger thickness of sand-filled layer and other compaction pattern combinations to numerical simulation. As a powerful finite element software with its friendly interactive interface [³¹‒³³], COMSOL is widely used for simulation analysis in subgrade engineering and pavement engineering [³⁴‒⁴⁰], which is chosen for numerical simulation in this study. The study chooses one of the sand-filled layers from 93% compaction degree demanded district in embankment construction site to simulate the sand compaction process in large thickness of sand-filled layer. As a compacted sand-filled layer does not affect the construction process of next sand-filled layer and the vibratory roller compacts the sand layer from one side to other side, the width dimension of the sand model is chosen as approximately the same width as the Roller’s vibratory drum. Moreover, sand is thought at its optimum water content, which makes it possible for vibratory roller not to get stuck in sand. As shown in Fig. 3, it’s the two dimensional finite element model of sand-filled layer with the thickness of 70 cm and the width of 2.5 m. The model with the fixed constraint in the bottom and the free constraint in two sides is meshed by triangular. A numerical linear elastic model is used to simulate the sand, and the material parameters are shown in Table 1.

Two types of single-drum vibratory rollers are chosen to apply several compaction pattern combinations to the sand-filled layer model, which includes static compaction, weak vibratory compaction and strong vibratory compaction. The parameters of single-drum vibratory rollers are shown in Table 2. In order to simplify the simulation process, the load applied to the sand model is roller’s maximum vibratory load when it is passing the cross section of the sand-filled layer as shown in Fig. 3.

Simulation and analysis for compaction process of sand-filled layer using 22 ton vibratory roller

Applying several compaction pattern combinations to sand-filled layer with the thickness of 70 cm by the 22 ton vibratory roller, and then check the compaction degree whether it meets the specification. As the compaction degrees decline with depth in sand-filled layer, the study chooses the compaction degree of the bottom sand layer as the test index.

Simulated compaction degrees of three compacting proposals carried out on the field test are shown in Table 3, which indicates that weak vibratory steady compaction promotes the improvement of sand compaction degree to some extent and strong vibratory compaction has an obvious effect on enhancing the compaction degree of the sand-filled layer. The combinations of weak and strong vibratory compaction in proposal 1 and proposal 2 have better effect than the sustained weak vibratory compaction combination in proposal 3. Both the simulation and field test data of proposal 2 can meet the demand of 93% compaction degree in the specification [³,⁴], which indicates that using the combinations of weak and strong vibratory compaction to improve the bearing capacity of sand-filled layer step by step can economically meet the demand of sand filling construction in large thickness and guarantee the sand embankment construction quality. Although there are some differences between simulations and field test data, the final compacting effects of proposals stay the same tendency at last, which indicates the reliability of the sand model.

Further, compaction parameters of proposal 2 are chosen to explore the compacting effect in larger thicknesses of sand-filled layers, such as 80 cm, 90 cm and 100 cm. The simulation results are shown in Fig. 4, which indicates that the compaction degrees decline with the depth in sand-filled layer at the fourth time of strong vibratory compaction, and the sustained strong vibratory compaction are needed to meet the demand of 93% compaction degree in the specification. In addition, the sand filling progress needs to match the progress of surrounding soil filling which should not exceed the thickness of 30 cm according to the specification [³,⁴], and 70 cm is finally chosen as the thickness of sand-filled layer for embankment construction.

Simulation and analysis for compaction process of sand-filled layer using 12 ton vibratory roller

The key problem in sand-filled embankment construction is lack of bearing capacity for sand caused by water content of being too high or too low, which might get the vibratory roller stuck in sand and cause hidden trouble to safety. Applying weak vibratory steady compaction to sand-filled layer to improve its bearing capacity by 22 ton vibratory roller, and then using strong vibratory compaction can reinforce the compaction degree to meet the specification [⁶]. However, previous research did not explore the steady compacting effect of small size vibratory roller as the limitation of construction site condition, which has been discussed in this study and stated below.

Simulated compaction degrees of different proposals combined with steady compaction by 12 ton vibratory roller and strong vibratory compaction by 22 ton roller are shown in Table 4, which indicates that strong vibratory steady compaction by 12 ton roller is effective to enhance the bearing capacity of sand-filled layer and can match the effect of weak vibratory steady compaction by 22 ton roller. Compared with proposal 2, the compaction degrees of the bottom sand layer in proposal 5 can also meet the specification at the same compaction times, which indicates that the compacting effect of compaction pattern combinations by 12 ton and 22 ton vibratory roller are basically the same as by 22 ton vibratory roller alone from the simulative perspective.

Comparison between different sand compacting proposals and economic analysis

Comparison between different sand compacting proposals

Only steady compact the sand layer for enough bearing capacity by using some compaction pattern combinations can make it possible for 22 ton vibratory roller to start strong vibratory compaction without being stuck in wet sand. When sand-filled layer is at the state of optimum water content, the steady compacting effect of strong vibratory compaction by 12 ton roller is basically the same as weak vibratory compaction by 22 ton roller, which can both reach the demanded compaction degree in relatively less compacting times. As shown in Fig. 5, the 22 ton vibratory roller might get stuck in wet sand as the lack of sand bearing capacity if the water content is a little higher than optimum state, but the 12 ton vibratory roller can be used in this construction condition to improve sand bearing capacity and create construction condition for 22 ton vibratory roller.

In different construction weather and environmental conditions [⁵], there is a waiting period for sand-filled layer to reach optimum water content after being sufficiently watered, such as half a day in breeze and cloudy weather. Therefore, combinations of weak and strong vibratory compaction by 22 ton vibratory roller alone are suggested for the sand-filled embankment project with rich construction period, such as proposal 2 in Table 3. However, combinations of steady compaction by 12 ton vibratory roller coordinated with strong vibratory compaction by 22 ton roller are strongly recommended for the sand-filled embankment project with tight construction period caused by time consumption of soft soil foundation treatment or other reasons, such as proposal 5 in Table 4. Although the recommendations concerned with 12 ton vibratory roller will increase some project cost in early construction, but it can insure the rollers against getting stuck in wet sand later, which can significantly shorten the construction period based on the guarantee of sand embankment construction safety and quality.

Economic analysis of sand-filled technique

Compared with traditional thickness sand filling technique that each sand-filled layer’s thickness should not exceed 30 cm, the large thickness sand filling technique proposed in this paper has more than double thickness of each sand-filled layer, which can largely speed up the process of embankment filling and shorten the construction period. In order to compare the construction cost of the two techniques, 22 ton vibratory roller is chosen for compaction alone and each construction process is taken into account, which includes transportation, paving, watering and compaction. The cost of sand transportation process is calculated by the actual number of sand transport trucks, which means that the large thickness sand filling technique will not have extra cost in this process. It takes about two days to water sand-filled layer and wait to reach its optimum water content, and both two techniques have the same cost. In compaction process, it takes about eight times of sustained weak vibratory compaction for traditional technique to reach the demanded compaction degree of 93%, which takes about six times of weak and strong vibratory compaction for large thickness sand filling technique to reach, but the fuel consumption of vibratory roller for the two techniques are more or less the same. In this case, the difference of construction cost for the two techniques is mainly in the paving process.

As shown in Fig. 1, a sand-filled embankment with height of 5 m and length of 100 m is chosen to calculate the construction cost of the two techniques. The trapezoid embankment section is equivalent to a rectangle section with the same area, which assumes that the width of each sand-filled layer stays the same. Construction periods and cost of the two techniques’ paving process are calculated according to the highway engineering budget quota [⁴¹], and the compaction coefficient of sand-filled layer is 1.2. As shown in Table 5, the construction cost of the traditional thickness sand filling technique is almost the same as the large thickness sand filling technique, but the latter can save the construction period by 59 days which makes significant economic benefits.

The large thickness sand filling technique makes full use of abundant resource of Yangtze River’s sand to construct sand-filled embankment in Jianghan Plain district, which can significantly accelerate the construction speed based on the guarantee of the sand embankment construction quality and reduce negative effects caused by construction to local traffic and environment. The environmental advantage of the proposed technique is that it utilizes river sand which has to be removed to retain the profile of the river bed and also be of great importance to the improvement of channels in Yangtze River. Moreover, the saved construction period can be used for soft foundation preloading treatment, which can reduce both the differential settlement after construction and the possibility of pavement distresses appeared widely after traffic opening.

Summary and conclusions

The main objective of this paper is to find out better sand filling techniques to save construction period and guarantee construction quality by simulating the sand compaction process in large thickness of sand-filled layer for highway sand-filled embankment project in Jianghan Plain district. Based on the results presented in this paper, the following key findings are offered:

(1) The simulation results of weak and strong vibratory compaction combinations and the sustained weak vibratory compaction combination are compared with the field test data, which shows the same tendency in conclusions and indicates that using the combinations of weak and strong vibratory compaction to improve the bearing capacity of sand-filled layer step by step can economically meet the demand of sand filling construction in large thickness and guarantee the sand-filled embankment construction quality.

(2) Sustained strong vibratory compaction by 22 ton roller is needed for larger thickness of sand-filled layers to meet the demanded compaction degree. In order to match the progress of surrounding soil filling, 70 cm is finally chosen as the thickness of sand-filled layer for embankment construction.

(3) Strong vibratory steady compaction by 12 ton roller is effective to enhance the bearing capacity of sand-filled layer and can match the effect of weak vibratory steady compaction by 22 ton roller.

(4) Combinations of weak and strong vibratory compaction by 22 ton vibratory roller alone are suggested for the sand-filled embankment project with rich construction period. However, for the sake of shortening the waiting period after sand-filled layers being sufficiently watered and insuring the construction safety, combinations of steady compaction by 12 ton vibratory roller coordinated with strong vibratory compaction by 22 ton roller are strongly recommended for the sand-filled embankment project with tight construction period.

(5) The construction cost of the traditional thickness sand filling technique is almost the same as the large thickness sand filling technique, but the latter can save a lot of the construction period which makes significant economic benefits.

(6) The simulated conclusions in this study need further validation in practical sand-filled embankment project in future.

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