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Research on the retaining structures for high-steep slopes is extremely significant because of its real-world applications and far-reaching implications. A flexible geocell-reinforced ecological retaining wall as a high-steep slope protection scheme was developed and applied to the slope protection project of the Ji-Lai Expressway by analyzing the reinforcement mechanism of the geocell used. The lateral displacement and Earth pressure distribution on the flexible ecological retaining wall applied to the high-steep slope were studied using finite element numerical simulations and verified using field experiments. Results reveal that the wall maximum horizontal displacement is 2/3 H away from the wall toe because of the replacement of the upper part of soil. There is an obvious bucking effect on the active Earth pressure around the stiffened site, and the flexible deformation of the retaining wall helped effectively release some of the Earth pressure. Consequently, the measured value is lower than the theoretical value. Through this case study, it is demonstrated that the flexible ecological retaining wall as a slope protection technology can be successfully applied to steep slopes with a height of more than 15 m. Moreover, it brings significant advantages for protecting the ecological environment and improving the highway landscape.

High-grade highway slopes are built in the process of filling embankments or during excavations to improve highway networks. To stabilize such slopes, surface protection measures are implemented, such as grout coating, concrete coating, and mortar rubble coating, and common measures, such as flexible retaining walls and rigid retaining walls, are applied to retain a reinforced structure [

Many types of rigid retaining walls have been employed to retain stabilization of slope, including cantilever retaining walls and gravity retaining walls. To promote the application of dry-stone walling to current civil engineering practices, Colas et al. [

Methods for slope protection are usually detrimental to the original vegetation and restoration efforts, resulting in a significant amount of secondary bare land, and the subsequent rainfall and runoff energy can lead to soil erosion and, in some cases, slope collapse [

From the perspective of protecting the ecological environment, enhancing the highway landscape, and maintaining an ecological balance, the application of soil bioengineering technologies should be encouraged. Cao et al. [

Flexible geocell-reinforced retaining walls have drawn considerable research attention in recent years. Currently, the stability of such walls can be studied using methods such as field monitoring, centrifugal model testing [

Clearly, many studies have been conducted on the stability of geocell-reinforced retaining walls under various simulated loads. However, there are some gaps between practical application and theoretical analysis with respect to geocell-reinforced retaining walls under different geological conditions; the study of the effects of lateral Earth pressure and displacement distribution might as well be regarded as an important research topic. In this study, based on a structural analysis and finite element numerical simulation of a geocell-reinforced retaining wall, a high-steep slope protection scheme is applied to a highway slope of the Ji-Lai Expressway. The simulation results were verified using field test monitoring data, and the lateral Earth pressure and displacement distribution were studied. These results can serve as a basis for further research on geocell-reinforced retaining walls as a highway slope protection scheme.

The section of the Ji-Lai Expressway that was evaluated is located on the second-level terrace of a river valley. The slope of the terrace is nearly vertical, and the height of the terrace is in the range of 15.47–16.94 m (Figure

Actual geological site. (a) Terrain. (b) Longitudinal crack at the top of roadbed.

To eliminate hidden dangers and ensure highway engineering quality and driving safety, the necessary engineering measures for slope protection need to be evaluated. The commonly used rigid gravity retaining wall scheme was voted down for the following reasons at the early stage of this project: (1) the JTG D30-2004 Specifications for Design of Highway Subgrades [

An ecological geocell-reinforced retaining wall scheme was proposed owing to the following reasons: (1) a geocell-reinforced retaining wall is lighter than a masonry structure, with only minor foundation treatment required, and the structural damage to the retaining wall due to the uneven settlement of the footing can be avoided; (2) the vegetation cover around the geocell on the retaining wall surface can help resist soil erosion and slope instability, which plays a role in afforestation; (3) the construction cost is relatively low, with the estimated construction cost based on earthwork processing, material cost, and foundation treatment being approximately 542,000 yuan.

Given the many advantages of flexible geocell-reinforced retaining walls, the construction scheme is determined as follows. The main structure is divided into two parts: a 3 m high lower part resting on a 4.8 m wide mortar rubble footing and a 12 m high upper part resting on a 4 m wide geocell-reinforced retaining wall with a slope gradient of 1 : 0.5. Moreover, above the retaining wall is a 3 m high filling slope with a gradient of 1 : 1.5. Figure

Design of a slope protection scheme and layout of test elements.

Studying the reinforcement mechanism of geocells is conducive for practical engineering applications. Under the action of the upper load, the geocell wall provides a good lateral restraint, which is controlled by the tensile strength of the geocell material. The volume of the geocell is assumed to remain unchanged, and according to the rubber membrane theory proposed by Henkel and Gilbert [_{0} is initial diameter of an individual geocell pocket, [m], and _{0} is geocell allowable axial strain.

Under this pressure, anchorage (or interfacial friction, or both) is imposed by the geocell walls (Figure _{b} beneath the concave surface, thus generating a net effect. The equivalent modulus _{s} is upward stress provided by the geocell wall (kPa).

Configuration of a geocell. Reinforcement mechanism of a geocell. (i) Upper load, (ii) geocell wall, (iii) infilled soil, (iv) lateral restraint, and (v) anchorage or interfacial friction or both.

Through the net effect and owing to the slab configuration, the upper load is uniformly redistributed to a wider area, with a lower intensity to improve the load-bearing capacity of the soil, and to a certain extent, the differential settlement under the action of the upper load is effectively resisted [

The apparent cohesion of sand, even of dry sand, is close to zero. A geocell induces a significant apparent cohesion on the infilled soil [

Development of slip surfaces in unreinforced and geocell-reinforced foundations. (a) Unreinforced foundation: (i) upper load, (ii) footing, (iii) failure surface, (iv) sand, and (v) clay. (b) Geocell-reinforced foundation: (vi) geocell layer.

The above qualitative analysis demonstrates that the geocell helps improve the lateral constraint on the reinforced soil, and the deformation of the soil is decreased. In addition, owing to the lifting force of the geocell, the deformation of the lower part of the soil is reduced. In fact, the overall role of the geocell in the subgrade is much more complex. To apply and verify the above reinforcement theory in engineering practice, several indoor and outdoor tests should be conducted to accumulate the measurement data.

The main physical and mechanical properties of the loess in the section of the Ji-Lai Expressway taken as the test case should be determined, which is conducive to the theoretical calculation, numerical simulation, and practical engineering application (Figure

Construction of an ecological retaining wall. (a) Mortar rubble masonry footing. (b) Geocell-reinforced retaining wall.

Subsoil main properties.

Sampling depth (m) | Water content (%) | Dry density (g·cm^{−3}) | Void ratio | Peak strength of consolidated quick shear | Strength of vane shear test (kPa) | Heavy compaction test | ||
---|---|---|---|---|---|---|---|---|

Friction angle (°) | Cohesion (kPa) | Optimum moisture content (%) | Maximum dry bulk density (kg·m^{−3}) | |||||

5 | 2.9 | 1.41 | 0.93 | 17.6 | 34 | 34.6 | 14.7 | 19.1 |

10 | 15 | 1.47 | 0.81 | 22.4 | 34 | 36.1 | 14.7 | 19.3 |

15 | 4.3 | 1.50 | 0.74 | 25.4 | 38 | 39.2 | 14.6 | 19.3 |

Geocell specifications.

Thickness of geocell wall (mm) | Soldering distance (mm) | Height of geocell wall (mm) | Low-temperature brittleness (°C) | Tensile yield strength (MPa) | Vicat softening point (°C) | Tensile strength at joints (N/cm) |
---|---|---|---|---|---|---|

1.2 ± 0.1 | 680 | 150 | −50 | 24.3 | 114.7 | 161.8 |

The following assumptions are made before conducting the theoretical calculation and analysis:

There are _{m} (m) of each layer is equal.

The anchorage zone containing the geocell stiffener is considered a uniform equivalent zone, and

The tension design value of each layer is determined by the pullout resistance _{i} (kN) of the geocell stiffener, where

where _{i} is ultimate pullout capacity of the ^{−3}); and [_{f}] is dimensionless safety coefficient for pullout stability.

The filling slope gravity and vehicle load acting on the retaining wall are converted to the equivalent soil layer thickness _{0} are load arrangement length and width, respectively (m) (the width of subgrade); and

The Earth pressure

The stability of the retaining wall should be analyzed considering the resistance provided by the retaining wall itself and the resistance of the geocell stiffener. Figure

Analysis of antislip stability:

where _{y}, _{x} are earth pressure vertical and horizontal components, respectively (kN); and

Analysis of antioverturning stability:

where

Analysis of footing eccentricity and stress:

_{f}and

_{f}are footing width and height, respectively (m);

_{f}is footing gravity (kN);

_{max}and

_{min}are maximum and minimum compressive stresses of the footing bottom, respectively (kPa);

Theoretical calculation of the flexible retaining wall.

The overall sliding stability of the retaining wall should be analyzed with the assumption that the geocell stiffener length should not exceed the displacement length of the sliding surface, where it may occur. The Fellenius method of slices can be used for the calculation:_{i}_{i} are cohesion and arc length on the _{i} is dead weight and its applied load on the _{i} is _{i} is angle between the normal and the vertical line of a sliding arc of the

After the calculations are made using the above formula, the flexible geocell-reinforced retaining wall scheme meets all the theoretical requirements.

Backfills and retaining walls are widely regarded as ideal elastoplastic bodies and ideal linear elastomers subjected to the Mohr–Coulomb failure criterion; therefore, a constitutive model based on the Mohr–Coulomb criterion is adopted for the analysis, which can be expressed as follows:_{2} is the second invariant of the deviatoric stress tensor; _{α} is lode angle (−30° ≤ _{α} ≤ 30°) (°), which is the angle between the first principal stress and the deviatoric stress component; and _{1} is the first invariant of the stress tensor. The geocell-reinforced soil triaxial compression test conducted by Chen et al. [_{p} is dimensionless coefficient of passive Rankin pressure;

Furthermore, Madhavi Latha et al. [

The calculation area is divided into several sections: flexible retaining wall, geocell stiffener, rubble footing, backfill, and foundation. Based on the physical parameters measured through laboratory testing and the feasible range of parameter values in the JTG D30-2004 Specifications of Design of Highway Subgrades [

Relevant numerical calculation parameters.

Material parameter | Analysis domain | ||||
---|---|---|---|---|---|

Flexible retaining wall | Geocell stiffener | Backfill | Foundation | Rubble footing | |

Elasticity modulus | 22.3 | 1600 | 22 | 25 | 2500 |

Poisson’s ratio | 0.3 | 0.25 | 0.35 | 0.30 | 0.22 |

Bulk density ^{−}³) | 20 | 17 | 19.3 | 22 | 26 |

Cohesion | 60 | — | 21 | 27 | 70 |

Friction angle | 35 | — | 25 | 25 | 45 |

Figure

Numerical model mesh and size of calculation parameters.

Horizontal displacement constraints are applied on the left and right sides of the finite element model. The vertical displacement constraints on the model are free. Horizontal and vertical displacement constraints are applied on the bottom of the model as well. The model finite elements are meshed using a structured pattern made up of high-precision four-node plane-strain quadrilateral elements.

Geocell was modeled as a 2-node linear truss element [

In the process of nonlinear calculation, the final and most important step before applying the loads is to determine the soil initial stress state, which affects not only the initial loading calculation but also all the subsequent loading steps. This determines the success of the numerical simulation, as the stress in the subsequent loading steps is also accumulated on the basis of the previous initial stress. Considering the initial geostress balance [

Finally, a finite element simulation is conducted after the corresponding loads (the 3 m high filling slope above the retaining wall with a slope gradient of 1 : 1.5 is regarded as the dead load) are applied to each section.

The actual distribution of soil pressure and the change in the retaining wall horizontal displacement are determined to verify the numerical calculation results and obtain the working characteristics of the geocell-reinforced retaining wall in practical engineering. Based on the layout of the test elements shown in Figure

Field monitoring device. (a) Calibration of pressure cells. (b) Setting of displacement observation points.

As shown in Figure

The soil pressure test results might be affected by the physical environment, construction disturbance, and quality of the test elements, resulting in a greater dispersion of the soil pressure. In this study, the value with greater dispersion was removed during the analysis of the soil pressure test results.

Figure

Lateral displacement distribution from numerical simulation results. (a) The geocell-reinforced retaining wall color nephogram. (b) The horizontal component of the retaining wall displacement vector.

Figure

Measured displacement with consolidation time.

Although the flexible retaining wall could conform to larger deformation, the maximum horizontal displacement measured does not exceed 42 mm because the entire soil structure inside the retaining wall is not damaged. The lateral deformation due to gravity is negligible, and only small lateral deformation occurs under the action of the roadbed structure and traffic loads. Moreover, as shown in Figure

Figure

Lateral Earth pressure distribution at different cross profiles.

The measured values at the middle and upper parts are in good agreement with the simulated value (Figure

Variation in the lateral Earth pressure at the back of the wall (4 m away from the retaining wall surface) with completion time.

Figure

Application effect of flexible ecological retaining wall for slope protection. (a) After construction completion. (b) One year after completion.

In this study, a flexible geocell-reinforced ecological retaining wall as a high-steep slope protection scheme was developed and applied to a portion of the Ji-Lai Expressway by analyzing the reinforcement mechanism of the geocell used. The lateral displacement and Earth pressure distribution on the flexible ecological retaining wall applied to the high-steep slope were studied using theoretical calculations and a finite element numerical simulation and verified using field experiments. After a detailed analysis of the results, our main findings are outlined as follows:

The simulated horizontal displacement is maximum approximately 1/3 H away from the wall toe, while the measured displacement is maximum 2/3 H away from the wall toe. The deformation is maximum in the middle and upper parts, and the maximum measured horizontal displacement does not exceed 42 mm.

With the increase in the distance from the retaining wall surface, the lateral Earth pressure along the height of the retaining wall presents more fluctuations. The measured value at the back of the wall is lower than that at the middle of the wall.

The measured value in the middle and upper parts is in good agreement with the simulated value; however, there are big differences between the measured and simulated values at the lower part of the retaining wall. This indicates that the complexity of the actual construction environment makes it difficult to evaluate the numerical parameters.

The lateral Earth pressure is maximum approximately 1/3 H away from the wall toe. The measured lateral Earth pressure is less than the theoretical value of the active Rankine pressure. The geocell-reinforced slope protection technology is an effective method to retain slope stability.

Soil consolidation is stable, and the mechanical properties of each part of the soil tend to be uniform six months after construction. The proposed approach brings significant advantages in protecting the ecological environment and improving highway landscape.

Relatively speaking, the finite element simulation result was not the same as actual value due to the complexity of actual construction progress. Therefore, adequate and effective field survey and intensive structure division should be suggested to further consider in similar future studies.

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that there are no conflicts of interest regarding the publication of this paper.

The authors would like to express their sincere gratitude for the funding support provided by the Science and Technology Plan of Education Department of Shandong Province (J07YA05) and Research Project Grant of Shandong Hi-Speed Company Limited.