Numerical investigation on the impact resistance of road barriers of Micropile-MSE Wall for subgrade

Computers and Geotechnics 82 (2017) 249–265

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Research Paper

Numerical investigation on the impact resistance of road barriers of Micropile-MSE Wall for subgrade Zhi-chao Zhang a,⇑, Yu-min Chen b, Han-long Liu b,c a

Key Laboratory of Geohazard Prevention of Hilly Mountains, Ministry of Land and Resources of China, Fuzhou, Fujian 350002, China College of Civil & Transportation Engineering, Hohai University, Nanjing 210098, China c Key Laboratory of New Technology for Construction of Cities in Mountain Area, Chongqing University, Chongqing 400045, China b

a r t i c l e

i n f o

Article history: Received 15 June 2016 Received in revised form 25 August 2016 Accepted 7 October 2016

Keywords: MSE wall Micropile Steep slope Model test Numerical simulation Impact resistance

a b s t r a c t The Micropile-MSE Wall, specially designed for mountain roadways, is used to simultaneously increase the MSE wall’s local stability, global stability and impact resistance of road barriers. Model tests were conducted first to validate the viability of the Micropile-MSE Wall. The impact resistance of the road barrier is then studied numerically. The test results indicate that the surcharge-induced earth pressure, base pressure and lateral displacement of Micropile-MSE Wall panels are effectively reduced. The impact loading on the barriers of the Micropile-MSE Wall is actually supported by the whole retaining structure, which increases the impact resistance of the road barrier significantly. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The MSE (Mechanically Stabilized Earth) wall has the merits of high economic efficiency, simple construction and flexible earth work [1], and it can be built vertically, which saves precious land resources in steep terrains. Hence, it may be one promising solution for road construction in mountainous areas with undulate terrain, complex geological structure, insufficient sloping area and limited construction space. However, for MSE walls constructed in steep terrain, the backfill region is inclined to slide along the backslope even if bench excavation is applied. Hence, the interface between the backfill and backslope may be the weak plane with large deformation, which may lead to the global stability failure of the MSE wall. High-grade highways are usually strict in the deformation control of the subgrade. The MSE wall for subgrade, however, is a type of flexible earth retaining structure that may deform noticeably itself, and uneven settlement of the road surface may occur easily under a long-term traffic load, which may affect the speed of vehicles. Hence, it is necessary to take measures to control the defor-

⇑ Corresponding author. E-mail addresses: [email protected] (Z.-c. Zhang), ymchenhhu@163. com (Y.-m. Chen), [email protected] (H.-l. Liu). http://dx.doi.org/10.1016/j.compgeo.2016.10.004 0266-352X/Ó 2016 Elsevier Ltd. All rights reserved.

mation of the backfill region and increase the local stability of the MSE wall. Furthermore, the collision damage of roadside barriers is inevitable in a highway MSE wall system. The complicated landform of western China usually gives rise to complex road conditions with many curves, which may easily lead to traffic accidents. Usually, the outside of the roadside barriers of a mountain highway is cliff over which cars may fall, leading to severe casualties. Hence, it is vital to improve the safety of roadside barriers of highway MSE wall systems. However, the current impact resistance design of roadside barriers mainly focuses on the local part of barriers such as the foundation slab [2,3]; the protection for the barrier is very limited and needs further improvement. Previous reinforcement measures of slope and earth-retaining structures include cables, soil nails, anchors [4,5] and pile foundations [6,7]. Due to the limitations in the construction space in steep terrains, pile foundations with large construction equipment are not applicable. Simply using anchors, cables and soil nails, which are designed for tension resistance, may not serve the purpose of reinforcing the retaining structure on slopes with a complex loading condition and sophisticated structural style. Hence, in practical engineering, anchors, cables and soil nails are usually used in combination with other reinforcement components designed for compression and bending resistance such as a pile foundation, which forms the anchor anti-slide pile, anchored sheet pile [8], etc. The

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anchor anti-slide pile resists the landslide thrust via the synergistic effect of the pile body and the tension of the anchor. As a result, it avoids the mechanism of balancing the landslide thrust simply through the resistance of soil as in the case of the individual cantilever pile, which exhibits a passive load-carrying mode. Therefore, the anchor anti-slide pile performs like a simply supported beam with upper and lower ends hinged rather than a cantilever beam. A micropile is a small-diameter (typically less than 300 mm), drilled and grouted non-displacement pile that is typically reinforced [9]. In contrast to conventional large-diameter bored piles, it has the merits of convenience in construction, small disturbance to existing structures, higher capacity with the same volume and high diversity in the layout form [10]. The use of micropiles has grown significantly since their conception in the 1950s and particularly since the mid-1980s. The working mechanism, calculation method, reinforcement effect and economic efficiency of micropiles in landslide treatment have been studied and proven extensively. Moreover, the combined use of MSE and pile foundations has been researched and deployed to improve the horizontal bearing capacity of MSE walls [11,12]. The micropile is smaller in size and more convenient in construction and will thus be less destructive to the geogrid during construction in MSE compared with conventional pile foundations. Hence, the viability of micropiles constructed in MSE should be higher. Hence, considering the inconvenience of construction, the complexity of the loading condition and structural style of MSE walls on slope areas and the limitations on the earthwork and construction space in steep terrain and also referring to the working mechanism of anchor anti-slide piles, the idea of installing a pair of vertical and inclined micropiles in the form of an A-frame into the MSE wall as a combined system called the Micropile-MSE Wall is put forward in this study, which aims to simultaneously increase

the MSE wall’s local stability, global stability and impact resistance of road barriers. The model tests of MSE walls before and after reinforcement with micropiles and the corresponding numerical simulation were conducted first to preliminarily validate the viability of the Micropile-MSE Wall. The measured results can also verify the numerical simulation by LS-DYNA. Comparative studies were then conducted by LS-DYNA on the impact resistance of the barrier before and after reinforcement with micropiles. It is expected that this study will provide a brand new idea for road construction in mountainous areas and give some guidance for the application of the Micropile-MSE Wall in practical engineering. 2. Introduction of the proposed Micropile-MSE Wall The schematic diagrams of the Micropile-MSE Wall system are shown in Figs. 1–3. The technical features of the system are as follows: (1) A pair of vertical and inclined micropiles in the form of an Aframe penetrates the MSE region (see Fig. 1(a)) to restrain the deformation of the reinforced soil and increase the local stability of the MSE wall. (2) The micropiles penetrate into the foundation and backslope (see Fig. 1(a)) to increase the global stability of the foundation. (3) The road barrier is connected to the grade beam and the underlying micropiles by connecting pieces to increase the impact resistance of the road barrier (see Fig. 1(b)). (4) The pile cap and grade beam are placed on the pile tops to connect the vertical and inclined micropiles (see Figs. 1(b) and 2) and form a longitudinal frame mode along the length of the highway (see Fig. 3) to enhance the soundness of the system.

(1) Reinforced barrier

Road barrier Anchor

(2) Reinforced MSE

MSE

Pile cap

(3) Reinforced foundation

Wall panel

Grade beam

Foundation

(a) Global graph

Vertical pile

Inclined pile MSE

(b) Local zoom in

Fig. 1. Side view of the Micropile-MSE Wall system.

(a) Global graph

(b) Local zoom in Fig. 2. Front view of the Micropile-MSE Wall system.

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(a) Global graph

(b) Local zoom in Fig. 3. 3D view of the Micropile-MSE Wall system.

The micropiles can be made fully useable under all types of loading conditions in this system and serve multiple purposes in the reinforcement, thus avoiding the waste of materials. In addition, it would be more convenient in practical construction because both the vertical and inclined reinforcement components are micropiles, thus avoiding the alternation of construction equipment compared with the anchor anti-slide pile. Hence, this combination of micropiles in the form of an A-frame should be superior to the anchor anti-slide pile.

depth of micropiles in the foundation is 25 cm, the pile spacing is 33 cm, and the angle between vertical and inclined micropiles is 30°. The geogrid is tied to the wall panel with pre-drilled holes by iron wires. The vertical and inclined micropiles are connected by iron wires at the top of the piles (Fig. 5(e)). It aims to simulate the grade beam and pile cap on the pile top by this simplified way. It should be noted that such connection in the model tests is certainly weaker than the concrete grade beam and pile cap in practical engineering, which should be considered in this study.

3. Validation by model tests 3.2. Loading and monitoring 3.1. Test models of MSE walls The model tests were conducted in a test tank with a height of 1.5 m, a length of 2 m and a width of 1 m. Due to restrictions in the test site and test material, the MSE wall models before and after reinforcement with micropiles are shown in Fig. 4. Fig. 5 indicates the construction process and the monitoring of the test results. Plastic sheets covered the two inner sides of the test tank to reduce the boundary friction (see Fig. 5(a)). Three sections of MSE wall models were built, but only the middle section was measured (see Fig. 5(d)) and studied to weaken the boundary effect. The height of the MSE region built on a 45° slope foundation is 60 cm, and the spacing of the geogrid is 10 cm. The total height of the MSE walls is 120 cm. For simplicity, the cross section of the micropiles is square, with a side length of 3 cm. The anchorage

0.75 m

Backfill sand

dial indicator

The surcharge loading is applied by layering bricks uniformly on top of the MSE region. A layer of bricks is 1 kPa. The multi-stage surcharge loading is applied by 5 levels. The per level is 2 kPa with two layers of bricks at one time, and the maximum surcharge loading is 10 kPa. A piece of galvanized sheet iron with a thickness of 2 mm is placed on the top of the MSE region as a bearing plate. Six dial indicators are placed along the height of the wall panel to monitor the lateral displacement of the wall panel, six earth pressure cells adhering to the inner side of the wall panel are placed in the middle of each backfill layer to record the earth pressure, and four earth pressure cells are placed on the bottom of the MSE to record the base pressure (see Figs. 4 and 5(d)). Strain gages are attached every 10 cm along the pile bodies to measure the strain, which is then changed to the bending moment.

0.4 m

0.75 m

0.4 m

surcharge: 2kPa 5

Backfill sand

surcharge: 2kPa 5

dial indicator 0.6 m

0.6 m

0.25 m 1.2 m

1.2 m

Inclined pile 0.15 m

0.6 m Vertical pile

0.6 m

0.8 m

Clay foundation

0.8 m

Clay foundation 2m

2m

(a) Ordinary MSE wall

(b) Micropile-MSE Wall

Fig. 4. Side view of the MSE wall models before and after reinforcement with micropiles.

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Surcharge load by bricks Subsidence observation

Lateral displacement observation

(a) Clay foundation

(b) Backfill completion

(c) Reinforced sand

(d) Result monitoring

Vertical and inclined Polymethyl methacrylate piles cotton cloth

(e) Connection of vertical and inclined micropiles at the pile top Fig. 5. Model condition in the tests.

It is worth mentioning that bench excavation is usually necessary when an MSE wall is constructed in slope areas. However, for simplicity, a smooth slope surface is employed in this study to compare the MSE walls before and after reinforcement with micropiles under the worst case scenario.

3.3. Test materials and parameters in the numerical simulation 3.3.1. Backfill and foundation soil The backfill soil is dry sand, whereas the foundation soil is clay. Based on lab tests, the sand properties are as follows: unit weight 16 kN/m3, special gravity 2.66, water content 0.32%, nonuniform coefficient 3.1, maximum void ratio 0.86, minimum void ratio 0.48, and friction angle 30°. The clay properties are as follows: water content 12.5%, unit weight 20 kN/m3, maximum dry density 1.9 g/cm3, optimal water content 13%, cohesion 24 kPa, and friction angle 20°. A key to the numerical simulation of the soil deformation is the constitutive model. In this study, the compact elastoplastic Geological Cap Model [13] in LS-DYNA is adopted. The plastic yield surface in this model consists of three regions (see Fig. 6): a shear failure envelope f1(r), an elliptical cap f2(r, j), and a tension cutoff region f3(r), where r is the stress tensor, j is a hardening parameter, I1 is the first invariant of the stress tensor, J2 is the second invariant of the deviator stress tensor, and T is the tension cutoff value. With its plastic parameters c and b set to zero, the Geological Cap Model can be simplified to the Drucker-Prager model, in which the shear strength can be determined by strength parameters a and h only. a and h can be directly related to the classical Mohr-Coulomb parameters c and u. According to the soil properties and based on experience, the soil parameters of the Geological Cap Model in LS-DYNA are shown in Table 1.

Fig. 6. The yield surface of the Geological Cap Model.

Table 1 Parameters of Geological Cap Model for soil in the model tests. Parameters

Backfill sand

Foundation clay

Density (kg/m3) Initial cap surface X0 (kPa) Shear modulus G (MPa) Bulk modulus K (MPa) Shear strength parameter a (kPa) Shear strength parameter h (rad) Plastic parameter b (MPa
1) Plastic parameter c (MPa) Plastic parameter W Plastic parameter D (MPa
1) Shape factor of cap surface R Tension cutoff (kPa)

1600 0 10 30 0 (c = 0 kPa) 0.1667 (u = 30°) 0 0 1 0.0725 4 0

2000 0 10 30 22.55 (c = 24 kPa) 0.1140 (u = 20°) 0 0 1 0.0725 4 20

3.3.2. Geogrid, wall panel and micropiles In the model tests, cotton cloth with a thickness of 0.2 mm is used as a geogrid. The breaking strength of cotton

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The bilinear plastic-kinematic model (see Fig. 7) in LS-DYNA is adopted for the geogrid, whereas the linear elastic material model is employed for the wall panel and micropiles. The material parameters are shown in Table 2, in which Et is the tangent modulus after yielding. 3.4. Finite element models of the model tests

Fig. 7. Bilinear stress-strain curve of the plastic-kinematic model.

cloth with a width of 50 cm is 312.4 N, and its elongation at breakage strain e is 12.32% [14]. Hence, its failure stress ry = 312.4 N/(0.2 mm  50 mm) = 31.24 MPa, and its Young’s modulus E = ry/e = 254 MPa. Galvanized sheet iron with a thickness of 1 mm is used as a wall panel. Polymethyl methacrylate (PMMA) piles with square cross sections with side lengths of 3 cm are used as micropiles. The yield strength of PMMA at 23 °C is 84.9 MPa [15]; therefore, it is supposed preliminarily that the PMMA piles in the tests were far from reaching the yield state.

The dimensions of the finite element models are the same as those of the test models. Because the mesh density and mesh design will affect the finite element results significantly, measures were taken to ensure that the meshes of the ordinary MSE walls and the meshes of the Micropile-MSE Wall are the same. As seen in Figs. 8 and 9, compared with the ordinary MSE wall, the extra component in the Micropile-MSE Wall is micropile. Hence, pile holes are built for both the ordinary MSE wall and the MicropileMSE Wall when building the finite element models. Then the pile holes of the former are filled with soil material (see Fig. 8), which means that there is actually no pile in this model, whereas the pile holes of the latter are filled with pile material (see Fig. 9) to represent the micropile. Then the meshes of the ordinary MSE wall model and the meshes of the Micropile-MSE Wall model will be the same. After then, contact relationship is set up to simulate

Table 2 Parameters of cotton cloth, wall panel and micropile. Materials

Geogrid Wall panel Micropiles

Parameters Density (kg/m3)

E (GPa)

Poisson’s ratio

ry (MPa)

Et (MPa)

900 7500 1200

0.254 210 2.9

0.3 0.25 0.25

31.24 N/A N/A

0 N/A N/A

sharing nodes

thickness of geogrid

backfill

geogrid foundation geogrid wall panel backfill

(a) 3D view

(b) Side view of cross section Fig. 8. FE model of ordinary MSE wall.

(a) 3D view

(b) Side view of cross section Fig. 9. FE model of Micropile-MSE Wall.

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the contact between micropiles and the soil around the micropiles. With the same mesh design, it is expected that the difference in the responses of the two models will not be attributed to the mesh but only to the micropiles. In the finite element model, the soil and micropiles are discretized with 8-node constant stress solid elements. The default 8-node solid element uses one-point integration with viscous hourglass control. The wall panel (galvanized sheet iron) is created with 4-node shell elements with a thickness of 1 mm. The geogrid (cotton cloth) is created with membrane elements with a thickness of 0.2 mm that allow no bending resistance, and this is appropriate for the geogrid. The thicknesses of the shell and membrane elements must be included in the mesh to avoid the introduction of initial penetration in the contact interface, which would result in numerical instability. It should be noted that bench excavation is conventional in the construction of an MSE wall in slope areas. However, a smooth backslope is applied in the tests for simplicity to study the reinforcement effect of micropiles under the worst case scenario. The bottom boundary of the finite element model is applied with a fixed constraint, whereas the other side boundaries are applied with a roller constraint. The friction coefficients between PMMA piles and foundation clay, between PMMA piles and backfill sand, and between galvanized sheet iron and backfill sand are 0.16 [16], 0.55 [17] and 0.47 [18], respectively. The close-up of the MSE region is shown in Fig. 8(b). The geogrid elements (i.e., membrane element) and the wall panel elements (i.e., shell element) in the joint share the same nodes. This is consistent with the practical condition of the connection between the geogrid and wall panel in the model tests. Considering the numerical trial and experience, the friction coefficients between different material pairs are shown in Table 3. The gravity and the surcharge loadings in the numerical model are applied by loading time histories (see Fig. 10). The gravity increases gradually and maintained at the constant gravitational acceleration (i.e., 9.8 m/s2) throughout the duration of analysis. After the stabilization of gravity loading, the 5-level surcharge loading with 2 kPa per level is then applied to the top of the MSE region in a stepwise manner, and the maximum surcharge loading is 10 kPa. The total simulation time is 26 s. It should be noted that the loads in the manner of time histories will perform like dynamic loads which may excite dynamic oscillation. Hence, the loading time (or ramp time) of the gravity and surcharge should be long enough to avoid or minimize the dynamic oscillation. That is to say, the loads are applied gradually and slowly so as to achieve the static-loading effect. Specially, the ramp time for the gravity to reach 9.8 m/s2 is 5 s, while the ramp time for the surcharge loading per level (i.e., 2 kPa) is 2 s, and the current surcharge level is then maintained for 1 s during which the surcharge-induced responsees can be read. Generally, this is long enough for the model to reach static state. While this loading approach still does not simulate fully the actual construction sequence, it is believed to be sufficient for establishing a realistic stress state in the soil regions [19].

Table 3 Contact and connection relationships. Materials in contact

Contact type

Friction coefficient

PMMA piles - foundation clay PMMA piles - backfill sand Galvanized sheet iron - backfill sand Galvanized sheet iron - cotton cloth geogrid Backfill sand - clay foundation Cotton cloth geogrid - backfill sand

Frictional contact Frictional contact Frictional contact Sharing nodes Frictional contact Frictional contact

0.16 0.55 0.47 N/A 0.50 0.50

12 10 10 8 8 6

6

4

4 2

gravity surcharge loading

0 0

2

4

6

2

surcharge loading/kPa

254

0 8 10 12 14 16 18 20 22 24 26

time/sec Fig. 10. Loading time histories: gravity and surcharge loading.

3.5. Analysis of measured and numerical results 3.5.1. Lateral displacement of the wall panel Fig. 11 indicates the surcharge-induced lateral displacement of the wall panel of MSE walls before and after reinforcement with micropiles. As the surcharge loading increases, the lateral displacement increases, and the wall panel of the ordinary MSE wall bulges near the middle wall height gradually (see Fig. 11(a)). However, the lateral displacement of the wall panel of the Micropile-MSE Wall is much smaller (Fig. 11(b)), and the consistency between the simulated and measured results validates the numerical method. Generally, the measured displacement is smaller than the simulated values. This may be because the backslope in the numerical simulation is ideally smooth, whereas the backslope constructed in the model tests is not a strictly smooth surface, and the backfill sand may infiltrate into the clay foundation, which increases the frictional resistance between the backfill and foundation. These phenomena may weaken the sliding of the backfill region along the backslope and lead to the reduction of displacement measured in the model tests. Moreover, the boundary effect inevitably exists in the actual tests, which is also a potential reason why the measured results are smaller than the simulated values. 3.5.2. Earth pressure Figs. 12 and 13 depict the earth pressure induced by surcharge loading. Generally, the simulated results fit the measured values well. The earth pressure increases with increasing surcharge loading, and the incremental earth pressure of the ordinary MSE wall (see Fig. 12) is higher than that of the Micropile-MSE Wall (see Fig. 13), which means that the vertical and inclined micropiles in the form of an A-frame in the MSE region can control the earth pressure effectively. As a result, it is easier for the Micropile-MSE Wall to maintain stability. In addition, it can be found that the incremental earth pressure obviously decreases around the middle wall height, especially in the ordinary MSE wall. This is because the wall panel bulges slightly in the middle (see Fig. 11); hence, the incremental earth pressure at the middle wall height tends to the active state and, as a result, is relatively smaller. 3.5.3. Base pressure Figs. 14 and 15 show the surcharge-induced base pressure of the MSE wall before and after reinforcement with micropiles, respectively. Basically, the base pressure increases with increasing surcharge loading. However, the surcharge-induced base pressure of the ordinary MSE wall (see Fig. 14) is obviously larger than

255

0.9

0.6

0.3

0.0 3.0

normalized wall height

normalized wall height

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2kPa: simulated 2kPa: measured 4kPa: simulated 4kPa: measured 6kPa: simulated 6kPa: measured 8kPa: simulated 8kPa: measured 10kPa: simulated 10kPa: measured

2.5

2.0

1.5

1.0

0.5

0.9

0.6

0.3

0.0 1.2

0.0

2kPa: simulated 2kPa: measured 4kPa: simulated 4kPa: measured 6kPa: simulated 6kPa: measured 8kPa: simulated 8kPa: measured 10kPa: simulated 10kPa: measured

0.9

0.6

0.3

lateral displacement/cm

lateral displacement/cm

(a) Ordinary MSE wall

(b) Micropile-MSE Wall

0.0

Fig. 11. Incremental wall lateral displacement induced by surcharge.

0.0

0.0

-0.1

-0.3

2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

-0.2

depth/m

-0.2

depth/m

-0.1

2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

-0.3

-0.4

-0.4

-0.5

-0.5

-0.6

-0.6 0

1

2

3

4

5

6

0

1

2

3

4

5

Lateral earth pressure/kPa

Lateral earth pressure/kPa

(a) Measured results

(b) Simulated results

6

Fig. 12. Incremental earth pressure of the ordinary MSE wall induced by surcharge.

0.0

0.0 -0.1

-0.3

2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

-0.2

depth/m

-0.2

depth/m

-0.1

2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

-0.3

-0.4

-0.4

-0.5

-0.5 -0.6

-0.6 0

1

2

3

4

5

6

0

1

2

3

4

Lateral earth pressure/kPa

Lateral earth pressure/kPa

(a) Measured results

(b) Simulated results

5

6

Fig. 13. Incremental earth pressure of the Micropile-MSE Wall induced by surcharge.

the corresponding actual surcharge loading applied (i.e., 2, 4, 6, 8, and 10 kPa, 5 stages in total). This is because the base of the backfill region is narrow (15 cm), and the backfill soil is inclined to slide

along the backslope under surcharge loading, which compresses the narrow base and gives rise to the stress redistribution on the narrow base. This illustrates that the construction details can affect

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0

0

10

10

base pressure/kPa

base pressure/kPa

256

20 2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

30

40 0.0

0.2

0.4

0.6

0.8

20

30

40 0.0

1.0

2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

0.2

0.4

0.6

0.8

normalized width

normalized width

(a) Measured results

(b) Simulated results

1.0

Fig. 14. Incremental base pressure of the MSE Wall induced by surcharge.

0

base pressure/kPa

base pressure/kPa

0

10

20 2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

30

40 0.0

0.2

0.4

0.6

0.8

1.0

normalized width (a) Measured results

10

20 2 kPa 4 kPa 6 kPa 8 kPa 10 kPa

30

40 0.0

0.2

0.4

0.6

0.8

1.0

normalized width (b) Simulated results

Fig. 15. Incremental base pressure of the Micropile-MSE Wall induced by surcharge.

the eventual load distribution in the MSE wall system and should be monitored carefully in engineering practice. By contrast, the surcharge-induced base pressure of the Micropile-MSE Wall (see Fig. 15) is approximately the same as the corresponding surcharge loading in each stage. The base pressure decides the required bearing capacity of foundation soil directly. The steep terrain usually has complex geological conditions with relatively low bearing capacity. The application of the Micropile-MSE Wall efficiently reduces the base pressure due to the sliding of the backfill along the backslope, which provides a solution for the bearing capacity problem for a highway retaining wall constructed in mountainous areas. It should also be noted that because the sliding of the backfill region in the tests is weakened compared with that in the ideal numerical modelling, the measured base pressure is generally smaller than the simulated value, and the difference is more obvious in the ordinary MSE wall case. 3.5.4. Bending moment of piles Based on the measured tension and compression strain (+e and
e), the bending moment M can be calculated by M = EI  De/d, where d is the distance between two measure points (i.e., side length of the pile in this study), and EI is the bending stiffness. Figs. 16 and 17 show the incremental bending moments of the vertical and inclined piles, respectively, of the Micropile-MSE Wall due to surcharge.

Generally, the measured and simulated results are reasonably close. The bending moment increases with increasing surcharge loading, and the locations of the maximum bending moment and the inflection point (i.e., 0 bending moment) remain nearly the same. The inflection point of the vertical pile is located 0.075 m above the interface between the backfill and foundation, whereas the inclined one is located 0.15 m above the interface. The bending moment above the inflection point coincides with the sliding direction of the soil (i.e., positive bending moment), whereas the bending moment below the inflection point is contrary to the sliding direction (i.e., negative bending moment). The maximum negative bending moment of the inclined pile is located on the interface between the backfill and foundation (see Fig. 17), whereas the maximum negative bending moment of the vertical pile is located 0.05 m inside the clay foundation (see Fig. 16). These points with extreme bending moment should be considered during the pile design. By comparing Fig. 16 and 17, it can also be seen that the bending moment of the inclined pile is higher than that of the vertical pile. Hence, the inclined pile plays a more significant role in the deformation control of the MSE wall, and it should especially be reinforced in the pile design. In addition, this result reveals that the inclined reinforcement component in the Micropile-MSE Wall is also a micropile, which not only has the same merits as the anchors of a diverse layout form, easy construction and high tensile strength but also has high

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0.00

0.00 2kPa 4kPa 6kPa 8kPa 10kPa

depth/m

-0.30

2kPa 4kPa 6kPa 8kPa 10kPa

-0.15 -0.30

depth/m

-0.15

-0.45

-0.45

-0.60 backfill foundation

-0.60 backfill foundation

-0.75

-0.75

-0.90 10

5

0

-5

-10

-0.90 10

-15

(a) Measured results

5

0

-5

-10

-15

(b) Simulated results

Fig. 16. Incremental bending moment of the vertical pile.

0.0

0.0

-0.1

-0.3 -0.4 -0.5 backfill foundation -0.6 -0.7 -0.8 10

2kPa 4kPa 6kPa 8kPa 10kPa

-0.2

depth/m

-0.2

depth/m

-0.1

2kPa 4kPa 6kPa 8kPa 10kPa

-0.3 -0.4 -0.5 backfill foundation -0.6 -0.7

5

0

-5

-10

-15

(a) Measured results

-0.8 10

5

0

-5

-10

-15

(b) Simulated results

Fig. 17. Incremental bending moment of the inclined pile.

compressive and bending strength. Hence, the inclined micropile not only coordinates the loading on the vertical micropile in the Micropile-MSE Wall but also can resist the deformation of MSE directly and provide direct anti-sliding force for the backfill along the backslope, which may better adapt the retaining structure in steep terrain with a complex loading condition and structural style. The test results above preliminarily verify the viability and serviceability of the Micropile-MSE Wall presented in this study. Moreover, the reliability of the LS-DYNA simulation of the model tests under surcharge loading is also validated by the measured results. Hence, engineering questions such as the performance of a new type of retaining structure (i.e., Micropile-MSE Wall) can be reasonably addressed numerically. It is then necessary and meaningful to examine the impact resistance of the Micropile-MSE Wall, which is equally compelling, through the numerical method. 4. Impact resistance of road barrier 4.1. FE models Compatible meshes are also designed for MSE walls before and after reinforcement with micropiles to eliminate the potential interference due to mesh differences of finite elements (see Fig. 18). The height of the MSE region is 6 m, the spacing of the geo-

grid is 0.5 m, and there are 12 layers of backfill in total. For simplicity, the cross section of the micropiles is square with a side length of 0.27 m. The anchorage depth of micropiles in the foundation is 2.5 m, the pile spacing is 3 m, and the angle between vertical and inclined micropiles is 30°. Ross et al. [20] suggested that for impact testing of traffic barrier systems, the length of a metal corrugated barrier should be larger than 30 m, and the length of a rigid barrier (e.g., concrete barrier) should be larger than 23 m. Concrete barriers with a total length of 30 m are used in this numerical simulation (see Fig. 18). This can better meet the suggested requirement because the wheel path of a vehicle is not considered in this study; only the impact loading time histories are applied on the barrier. For simplicity, the cross section of the barrier is an isosceles trapezoid with an upper base of 0.4 m, a lower base of 0.6 m and a height of 1 m. The common continuous distribution form of road barriers in a mountain highway is used. Two adjacent barriers were connected by connecting pieces, with a 3 cm gap between them, and each of the 10 barriers is 2.97 m in length; hence, the total length of the model is 30 m (see Fig. 18). The benefit of the Micropile-MSE Wall lies in that the road barrier is connected to the grade beam and micropiles by connecting pieces (see Fig. 18(b)) to improve its impact resistance. For comparison, the road barrier in the ordinary MSE wall is connected

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30 m

30 m

(a) Ordinary MSE wall

(b) Micropile-MSE Wall Fig. 18. Side view of MSE wall models.

to the backfill soil by connecting pieces with an anchorage depth of 0.5 m in the soil and barrier (see Fig. 18(a)). It should be noted that bench excavation is conventional in the construction of an MSE wall in slope areas. However, a smooth backslope is applied in this numerical study for simplicity. Therefore, it aims to study the reinforcement effect of micropiles under the worst case scenario.

Table 4 Connection and contact relationship. Materials in contact

Contact or connection type

Friction coefficient l

Backfill-foundation soil Concrete-concrete Concrete-soil Geogrid-soil Micropile-grade beam Wall panel-geogrid

Bonded contact Frictional contact Frictional contact Frictional contact Bonded contact Sharing nodes

1 0.50 0.56 0.56 1 N/A

4.2. Contacts and connections The concrete material includes micropiles, barriers, grade beams and wall panels. The contact types and the corresponding friction coefficients between different material pairs are shown in Table 4. The bonded contact is used between micropiles and the grade beam to denote the cementation of concrete. To simulate the continuity between backfill and foundation soil, bonded contact is also applied between the backfill and foundation soil [19]. For simplicity, the connecting pieces (i.e., anchors and dowels, beam elements) are coupled to the surrounding solid continuum

(i.e., soil, barrier and grade beam) to prevent the creation of lowquality elements. This is achieved by the ⁄Constrained_Lagrange_ In_Solid feature in LS-DYNA. The use of this coupling permits the solid mesh to be constructed without consideration of the location of beam elements. The connecting pieces are treated as a slave material that is coupled with a master material comprising the soil, barrier and grade beam. The slave parts (i.e., connecting pieces) can be placed anywhere inside the master continuum part without any special mesh accommodation.

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4.3. Boundary and loading conditions The bottom boundary of the finite element model is applied with a fixed constraint, whereas the four side boundaries are applied with a roller constraint. The impact loading time histories shown in Fig. 19 are applied to the barrier after the stabilization of gravity loading which is maintained at a 1g level throughout the duration of impact loading. Gutkowski and Winkler [2] studied the impact testing of traffic barrier systems by vehicles with a weight of 1950 kg. According to the high-speed camera results, it is estimated that the impulse time is approximately 0.10 s. Based on the momentum conservation law FDt = mDv, the impact force F can then be calculated. Kim [21] simplified the impact force F into triangular loading time histories. Combining the results of Refs. [2,21], the triangular impact loading induced by vehicles with a weight of 1950 kg is also applied in this study. Taking the potential vehicle speeds on the mountain highway into account, the impact speeds are chosen as 6, 12, 18, 24 and 30 m/s with an impulse time of 0.10 s. Based on the momentum conservation law, the peak impact loading force can be calculated as 120, 240, 360, 480 and 600 kN, respectively, and the loading time histories are shown in Fig. 19. The joint between two barriers is the weak point during vehicle impact. Hence, the action point of impact loading is chosen at the joint between two barriers in the centre of the model, and the loading angles are 90° (front impact) and 20° (oblique impact), as indicated in Fig. 20. Because the loading is applied to nodes, the loading area is determined as 0.34  0.6 m2 due to the discretization of the finite element model. 4.4. Constitutive model and parameters 4.4.1. Backfill and foundation soil 4.4.1.1. Geological Cap Model strength parameters a and h. LS-DYNA and the Geological Cap Model have been employed in a number of MSE wall projects [19,22,23], and their feasibility in the simulation of dynamic impact responses has also been validated by many researchers [3,24,25]. From such past usage, there exist sets of experimentally calibrated material parameters for typical soil types and conditions in MSE problems for the Geological Cap Model in LS-DYNA. Use is made of this constitutive modelling database as a guide in this study in selecting the material parameters to describe the site conditions that are most relevant to practice, in lieu of developing an independent experimental program to calibrate the constitutive model for any specific soil. This is consistent with the intended generic nature of this pilot study: to understand the basic mechanics involved and to evaluate the design concept

v=108km/hr v=86km/hr v=65km/hr v=43km/hr v=21km/hr

impact loading/kN

600 500 400

Fig. 20. Barrier under impact and the loading condition.

Table 5 Parameters of Geological Cap Model for soil in the impact simulation. Parameters

Backfill

Foundation soil

Density (kg/m3) X0 (kPa) G (MPa) K (MPa) a (kPa) h (rad) b (MPa
1) c (MPa) W D (MPa
1) R Tension cutoff (kPa)

1600 30–240 7–22 16–48 1.9 (c = 2.5 kPa) 0.2143 (u = 40°) 0 0 1 0.00725 4 0

1600 200 14.5 32 1.9 (c = 2.5 kPa) 0.2143 (u = 40°) 0 0 1 0.00725 4 0

and the possibility of using the Micropile-MSE Wall to solve the impact resistance problem of the road barrier. According to NCHRP Report 556 [22], the soil strength parameters are shown in Table 5. The backfill and foundation soil are applied with a friction angle of 40°, and a small value of cohesion equal to 2.5 kPa was assumed for all soil to provide numerical stability during finite element calculations. 4.4.1.2. Elastic soil modulus. The elasticity of soil increases with increasing stress level. Thus, the shear modulus G and bulk modulus K increase with increasing depth of backfill in this study, as shown in Table 5.

300 200 100 0 17.90 17.95 18.00 18.05 18.10 18.15 18.20

time/sec Fig. 19. Impact loading curves on the barrier.

4.4.1.3. Cap surface parameter X0. The parameter X0 represents the intersection of the initial cap surface with the I1-axis in the stress space and defines the size of the initial elastic domain of the soil. It can be determined from hydrostatic compression test data, i.e., pressure-volume response, when the pressure-volume response transitions from elastic to elastic-plastic. Because the initial elastic domain of the soil is expected to increase with increasing depth in relation to the in situ stress state, X0/3 is estimated to be close to the mean stress rmean.

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Table 6 Parameters of geogrid, connecting pieces and concrete material. Materials

Geogrid Connecting pieces Concrete material

Parameters Density (kg/m3)

E (GPa)

Poisson’s ratio

ry (MPa)

Et (MPa)

1030 7800 2500

0.433 210 25

0.3 0.3 0.2

4.33 N/A N/A

0 N/A N/A

(a) Ordinary MSE wall

(b) Micropile-MSE Wall

Fig. 21. Accumulated displacement under a front impact loading of 600 kN (exaggerated by 5 times).

After the numerical trial, it is found that the actual rmean generally varies from 10 to 80 kPa from the top to the bottom of the MSE region with a height of 6 m. Hence, starting with X0 = 3 ⁄ rmean, the X0 value increases from 30 to 240 kPa along the height of the MSE region linearly (see Table 5). It should be noted that due to the change of the geometric model (e.g., adding the micropiles) and the reinforcement of the geogrid, the rmean at different locations may not be uniform, which means that X0 = 30–240 kPa may not work for all soil elements precisely, but it is expected to reflect the increase of soil elasticity with increasing stress level, which is more reasonable through the choice of varied X0 value and elastic soil modulus. 4.4.2. Geogrid, connecting pieces and concrete material The bilinear plastic-kinematic model has been employed in a number of MSE wall projects [23] and the bilinear stress-strain relationship is thought to be appropriate for geosynthetic reinforcement [26]. Hence, the bilinear plastic-kinematic model is still adopted to simulate the geogrid. The thickness of the geogrid is 2 mm, and based on Ref. [19], the geogrid parameters are shown in Table 6, where ry is the yield stress, E is Young’s Modulus, and Et is the tangent modulus after yielding. The concrete material (i.e., wall panel, barrier, grade beam and micropiles) and the connecting pieces (i.e., anchors and dowels, U32 rebar) between the barrier and grade beam are assumed to be linearly elastic to avoid adding more complexity to the model for the purpose of this pilot study (see Table 6). This will overestimate the reinforcement effect. Hence, it should be considered when assessing the results. 4.5. Result analysis 4.5.1. Barrier deformation The model is initialized to account for gravitational loading on the soil mass, and then, impact loading time histories (see Fig. 19) are applied to the road barrier. Because of space limita-

tions, only the accumulated displacement contour under front impact loading of 600 kN is shown in Fig. 21. The deformations are exaggerated by a factor of 5 to make the comparison more distinct. The barrier of the ordinary MSE wall deforms severely (see Fig. 21(a)), and the deformation expands from the action point of the central barrier, which is nearly overturned to the adjacent two sides of the barriers. In contrast, after the reinforcements with micropiles and grade beams, the barrier of the Micropile-MSE Wall deforms much less (see Fig. 21(b)), and the displacement contour is uniform. The deformation of the barrier due to a 20° oblique impact loading exhibits a similar trend, but the deformation is asymmetric, which is not studied here due to space limitations.

4.5.2. Impact-induced lateral displacement of barrier Fig. 22 indicates the impact-induced peak and residual lateral displacement of the barrier, in which the ‘‘pile” in the legend represents the ‘‘Micropile-MSE Wall” case, whereas ‘‘no pile” represents the ‘‘ordinary MSE wall” case. The following results can be seen in Fig. 22. Under different front impact loading: The peak lateral displacement of the ordinary MSE wall is 17.6–46.5 cm, and that of the residual one is 12–36 cm after impact loading; the peak lateral displacement of the Micropile-MSE Wall is 0.6–4.7 cm, and that of the residual one is 0.27–2.9 cm. Under 20° oblique impact loading: The peak lateral displacement of the ordinary MSE wall is 5.1–25.1 cm, and that of the residual one is 3.5–14.6 cm after impact loading; the peak lateral displacement of the Micropile-MSE Wall is 0.28–1.75 cm, and that of the residual one is 0.08–0.7 cm. Hence, the barrier displacement after reinforcement with micropiles can be reduced by 90% in general, which verifies the impact resistance of the Micropile-MSE Wall powerfully. In terms of mechanism, the impact loading on the barrier is transferred from the top to the bottom of the retaining structure through

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0.5

0.5 no pile,peak value

no pile,residual value

0.4

barrier displacement/m

barrier displacement/m

no pile,peak value pile,peak value pile,residual value

0.3 0.2 0.1 0.0 -100

0

no pile,residual value

0.4

pile,peak value pile,residual value

0.3 0.2 0.1 0.0 -100

100 200 300 400 500 600 700

0

100 200 300 400 500 600 700

impact loading/kN

impact loading/kN

(a) Front impact

(b) 20° oblique impact

120kN 240kN 360kN 480kN 600kN

0.6

0.3

0.9

normalized wall height

0.9

120kN 240kN 360kN 480kN 600kN

0.6

0.3

0.0

0.0 -20

-16

-12

-8

-4

0

normalized wall height

Fig. 22. Comparison of barrier lateral displacement induced by different impact loadings.

-20

-16

-12

-8

Lateral displacement/cm

Lateral displacement/cm

(a) Ordinary MSE wall

(b) Micropile-MSE Wall

-4

0

Fig. 23. Residual lateral displacement of the wall panel induced by different front impact loadings.

micropiles installed throughout the MSE wall. This increases the nominal anchorage depth of the road barrier significantly. In contrast, the previous research on barrier strengthening mainly focuses on the barrier itself [2,3], in which the protection for the barrier is very limited. Hence, the application of the MicropileMSE Wall can provide a brand new idea for the impact design of a road barrier based on the results of this pilot study. 4.5.3. Residual lateral displacement of the wall panel The front impact-induced residual lateral displacement of the wall panel is shown in Fig. 23. At the bottom part of the wall panel, the lateral displacements before and after reinforcement with micropiles are small in both cases and do not differ significantly. However, the deformation of the wall panel of the ordinary MSE wall is drastically concentrated at the top part (see Fig. 23(a)). This indicates that the MSE wall is a flexible structure that may undergo large deformation under high-intensity loading such as impact from vehicles. This is quite unfavourable for the highway retaining structure demand in terms of the control of deformation. However, through the reinforcement of micropiles, the deformation of the wall panel of the Micropile-MSE Wall is much smaller and more uniform throughout the height of the wall (see Fig. 23 (b)), which avoids the local failure of wall panels. Specifically, the maximum lateral displacement of the ordinary MSE wall under

120–600 kN impact loading is 4.4, 8.6, 12.3, 14.8 and 17.3 cm, whereas it is 0.25, 0.67, 1.26, 2.1 and 3.1 cm, respectively, after reinforcement with micropiles. The reduction in the lateral displacement of wall panels is 82.1–94.3%. 4.5.4. Working mechanism of micropiles The inner force of micropiles reflects the loading transfer in the Micropile-MSE Wall under impact loading directly, which is conducive to the study of the performance of micropiles and the reinforcement mechanism. Hence, the micropiles beneath the barrier under impact loading are chosen for the incremental inner force analysis. Fig. 24 indicates the 600 kN front impact-induced bending moment of vertical and inclined micropiles in the Micropile-MSE Wall. The peak incremental bending moment of the vertical micropile is 111.8 kN m, and it is 165.3 kN m for the inclined micropile, which is larger. Hence, the inclined micropile should be specially reinforced during the impact design. The bending moments of vertical and inclined micropiles exhibit similar trends. At the pile top, the bending moment is relatively smaller due to the constraint of the grade beam. As the burial depth increases, the bending moment rises to the peak rapidly and then decreases, followed by the point of inflection where the bending moment is 0. After that, the bending moment curve exhi-

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0

0

-1

-1

t=0.10 sec

distance to pile top/m

distance to pile top/m

-2 -3 t=0.10 sec -4

after impact

-5

backfill

-6

foundation -7

after impact

-2 -3 -4

backfill

-5 foundation -6 -7

-8 -9 -20

-10

0

10

-8 -20

20

-10

(a) Vertical pile

0

10

20

100

150

(b) Inclined pile

Fig. 24. Bending moment of micropiles induced by 600 kN impact loading in the Micropile-MSE Wall.

0

0

-1

-1

t=0.10 sec

-3

after impact -4 -5 -6

t=0.10 sec

-2

distance to pile top/m

distance to pile top/m

-2

backfill foundation

-4

backfill foundation

-5 -6

-7

-7

-8 -9 -200

after impact -3

-150

-100

-50

0

axial force/kN

(a) Incremental axial force of the vertical micropile

-8 -50

0

50

axial force/kN

(b) Incremental axial force of the inclined micropile

Fig. 25. Axial force of micropiles induced by 600 kN impact loading in the Micropile-MSE Wall.

bits a parabola form, and the peak value occurs at approximately 3 m from the pile top. The maximum bending moment occurs 3 m from the pile top in the MSE region rather than near the pile top, which is closest to the action point of impact loading. This should be paid special attention to during impact design. Then, the second point of inflection occurs near the interface between the backfill and foundation. Inside the foundation, the incremental bending moment of the vertical micropile is minor, while the incremental bending moment of the inclined micropile still fluctuates and is relatively larger. After the impact loading, the bending moment stabilizes and becomes much smaller. Fig. 25 shows the axial force of micropiles induced by 600 kN impact loading, in which positive indicates tension and negative indicates compression. It can be seen that the vertical pile is mainly under compression, whereas the inclined micropile is under tension. The maximum axial force occurs inside the MSE region 3 m

from the pile top. After the impact loading, the incremental axial force decreases obviously and even goes to 0 for the inclined micropile. Based on the impact-induced inner force of micropiles, it can be concluded that through the reinforcement of micropiles penetrating from the road surface into the foundation of the MSE wall system, the impact loading can be transferred and supported by the whole retaining structure from top to bottom gradually. The relatively rigid micropiles play a role in loading transmission in the relatively flexible MSE wall. Hence, they increase the impact resistance of the road barrier significantly via such a rigid-soft combination and avoid the local failure of the MSE wall. 4.5.5. Working mechanism of grade beam The grade beam in the Micropile-MSE Wall not only connects the vertical and inclined micropiles but also connects the sections

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250 200 150 100 50 0 -50 -100 -15

t=0.10 sec after impact

-12

-9

-6

-3

0

3

6

9

12

15

distance to the action point/m Fig. 26. Bending moment of the grade beam induced by 600 kN front impact loading.

Impact loading Fig. 27. Deformation of grade beam before and after front impact (exaggerated by 50 times).

of the MSE wall and forms a longitudinal frame mode along the length of the highway. Fig. 26 indicates the 600 kN front impactinduced bending moment of the grade beam in the MicropileMSE Wall. The incremental bending moment of the grade beam reaches the peak value of 230.7 kN m at t = 0.10 s during impact loading. As it departs from the action point, the bending moment decreases rapidly, which even leads to the point of inflection, and then decreases to 0. After impact loading, the incremental bending moment exhibits a similar trend but is much smaller. In addition, Fig. 27 shows the deformation of the grade beam before and during impact. It can be seen that approximately four barriers were influenced obviously by the impact loading, and the deformation gradually becomes smaller from the action point in the centre to the two sides, which avoids the concentration of deformation on a single barrier. Hence, the impact loading can not only be transferred from top to bottom by the combination of vertical and inclined micropiles connected by the grade beam but can also be transferred horizontally from the action point of impact to the two sides along the road through the grade beam. As a result, the barrier displacement is effectively reduced. 4.5.6. Tension strain of geogrid Fig. 28 shows the tension strain of each geogrid layer before and after impact loading. To show the results more clearly, the strains of different geogrid layers are shown in different scales. The height of the backfill region is 6 m, and the width of the top of MSE region is 7.5 m. The figures are plotted to scale. Before impact loading, the strains of the geogrid of the MSE wall before and after reinforcement with micropiles are relatively close (see Fig. 28(a)). The maximum tension strain occurs at the bottom layer near the wall panel, and the strain level decreases with increasing layer (i.e., the burial depth becomes smaller). Therefore, it is necessary to use a high-strength geogrid and high-quality backfill at the bottom layers, and it is meaningful to use numerical calculation beforehand to provide guidance for the design of MSE walls to achieve the optimization of laying of the geogrid. The connection of the locations of the largest strain in each geogrid layer approximately indicates the potential failure surface of the MSE region [27,28]. The blue dash line in Fig. 28 stands for the potential failure surface in the numerical simulation. The black solid line is the failure surface given by ‘‘0.3H method” [27]. It can be found that the potential failure surface of both MSE walls under gravity approximately coincide with the ‘‘0.3H method” result (see

Fig. 28(a)). Hence, the failure surface that passes through the MSE assumed by the ‘‘0.3H method” is approximately substantiated with the results calculated by LS-DYNA. This in turn validates the reliability of the numerical method to some extent. After impact loading, the geogrid strain of the ordinary MSE wall increases drastically at the top layers (see Fig. 28(b)) and exceeds that of the Micropile-MSE Wall obviously; the maximum strain is approximately 6%, and the potential failure surface on the top of MSE wall shifts to near the wall panel. By comparison, the geogrid strain of the Micropile-MSE Wall still maintains a low level. This indicates that the MSE wall is a type of flexible earth-retaining structure that may deform noticeably under intense transient loading such as impact from a vehicle. However, in the Micropile-MSE Wall, the impact loading on the barrier can be transferred vertically by the micropiles from the top to bottom of the MSE wall and horizontally by the grade beam from the action point of the impact to the two sides along the road. This doubly guarantees the local stability of the MSE wall, and the MSE wall can continue to perform well after traffic accidents. 5. Conclusion A Micropile-MSE Wall suitable for mountain roads is put forward in this study, which aims to simultaneously improve the local stability, global stability and impact resistance of an MSE wall road barrier. Model tests and the corresponding numerical modelling were conducted, and then, comparative studies were performed on the impact response of the barrier before and after reinforcement with micropiles by the numerical method. The conclusions are as follows: (1) The test results indicate that the surcharge-induced earth pressure, base pressure and lateral displacement of the wall panel of the Micropile-MSE Wall are obviously reduced compared to those of the ordinary MSE wall, and the corresponding numerical results fit the measured results well. Hence, the viability of the Micropile-MSE Wall and the accuracy of the numerical method are validated preliminarily. (2) The impact loading on the barrier of the Micropile-MSE Wall can be transferred vertically by the micropiles from the top to bottom of the MSE wall and horizontally by the grade beam from the action point of impact to the two sides along the road, which increases the impact resistance of the road barrier significantly. The impact-induced displacement of the barrier can be reduced by approximately 90%, and the maximum wall panel displacement can be decreased by 82.1–94.3% under different impact loadings. (3) Regarding the cases in this study, the impact-induced maximum inner forces of micropiles in the Micropile-MSE Wall are generally located 3 m from the pile top in the MSE region, and the incremental inner force of the inclined micropile is larger than that of the vertical micropile. These

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0.3H method

6m

wall height

Simulated

0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.015 0.000 0.015 0.000 0.015 0.000 0.02 0.00 0.02 0.00 0.03 0.00

6m

wall height

strain

0.3H

ordinary MSE wall Micropile-MSE Wall

0

1

2

3

4

5

6

7

8

distance to wall panel/m

(a) Before impact loading (under gravity) 0.3H

strain

Simulated

0.3H method

0.06 0.00 0.04 0.00 0.02 0.00 0.015 0.000 0.015 0.000 0.015 0.000 0.015 0.000 0.015 0.000 0.02 0.00 0.02 0.00 0.035 0.000

ordinary MSE wall Micropile-MSE Wall

0

1

2

3

4

5

6

7

8

distance to wall panel/m

(b) After impact loading Fig. 28. Comparison of geogrid strain.

special points should be paid attention to during the impact design. (4) The deformation of the geogrid of the Micropile-MSE Wall due to impact is reduced obviously compared with that of the ordinary MSE wall, and the failure surface that passes through the MSE assumed by the ‘‘0.3H method” is approximately substantiated with the results calculated by LS-DYNA.

(5) The roadside barrier can be strengthened by the micropile reinforcement measures taken on the retaining structure itself, which is an extra benefit based on the current specifications. Hence, it can serve multiple purposes and can provide a brand new idea for the impact design of road barriers. Till now, the impact resistance of road barriers of MicropileMSE Wall presented in this study is verified preliminarily. Based

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on the research results of this paper, more resources can be used justifiably and more efforts can be made through full-scale tests or dynamic impact tests with more realistic soil condition and stress condition so as to understand and make use of this new structure more thoroughly. Parametric studies can also be done on different pile conditions to investigate the effect of pile size, angle, spacing and the optimum condition. Because of space limitations, the local and global stability of the Micropile-MSE Wall will be proven in other papers. Acknowledgements The research described in this paper is supported by the National Natural Science Foundation of China (Grant No. 51609040), the Natural Science Foundation of Fujian Province (Grant No. 2016J05112), the Science and technology project of Bureau of Geology and Mineral Resources of Fujian Province (DK2016014). Reference [1] Kibria G, Hossain MDS, Khan MS. Influence of soil reinforcement on horizontal displacement of MSE wall. Int J Geomech 2013;14(1):130–41. [2] Gutkowski RJ, Winkler DJ. Simplified impact testing of traffic barrier systems. Fort Collins: Mountain-Plains Consortium; 2003. [3] Kim KM, Briaud JL, Bligh R, et al. Design guidelines and full-scale verification for MSE walls with traffic barriers. J Geotech Geoenviron Eng 2011;138 (6):690–9. [4] Zhang RJ, Zheng JJ, Li PY, et al. A method for predicting mechanical behaviour of HPJG–Anchors – Part I: Mechanical characteristics and load transfer models. Comput Geotech 2012;45:62–73. [5] Zhang RJ, Zheng JJ, Li PY, et al. A method for predicting mechanical behaviour of HPJG-Anchors – Part II: Prediction procedure, verifications and parametric studies. Comput Geotech 2012;45:44–52. [6] Wei WB, Cheng YM. Strength reduction analysis for slope reinforced with one row of piles. Comput Geotech 2009;36:1176–85. [7] He Y, Hazarika H, Yasufuku N, et al. Evaluating the effect of slope angle on the distribution of the soil–pile pressure acting on stabilizing piles in sandy slopes. Comput Geotech 2015;69:153–65. [8] Bilgin Ö. Numerical studies of anchored sheet pile wall behavior constructed in cut and fill conditions. Comput Geotech 2010;37(3):399–407.

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