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Within the framework of a FEM of the nonlinear behavior of lateritic pavement of Senegal, the effect of truck overloading is studied to estimate its impact on the deformability of road pavement on compacted gravel lateritic soils. For that purpose, various loading conditions were tested to measure the impact on the critical response parameters of road pavement design. The implementation of the models was realized with Cast3M <sup>©</sup> . This study allowed us to point out that the observed variations are linear and would help to plan in advance the impact of axle overloads for a better evaluation within the framework of the mechanistic (M. E.) design of pavements.

The regulation number 14/2005/CM/UEMOA of WAEMU (West African Economic and Monetary Union) of December 16^{th}, 2005 concerning the harmonization of standards and control procedures of truck volumes, weights and axle loads of heavy goods vehicles in the member states of the Union, set the axle load to 12 tons for a single axle with dual-wheels (Uemoa, 2005 [

The reference axle is varied from 13 to 20 tons to take into account the overloading. The effect of a single wheel is tested with a tire pressure of 0.662 MPa for the load of 13 tons and respectively 0.764 MPa, 0.917 MPa and 1.019 MPa respectively for axle loads of the 15, 18 and 20 tons (Samb, 2014 [

The structure consists of a 80 mm thick bituminous concrete, a 200 mm thick base layer of lateritic gravels treated or not, a 250 mm thick subbase of untreated lateritic gravels and of a sandy subgrade of infinite thickness. The materials of the asphalt and subgrade layer are considered linear elastic. The base and the subbase has a nonlinear elastic behavior. To take into account this non-linearity, the elastic modulus is replaced by a resilient modulus which depends on the stress level. Several formulations were suggested by using various terms of stresses (Lekarp, Isacsson, and Dawson, 2000 [

The Uzan model expresses the resilient modulus according to the bulk stress and the deviatoric stress, which allows us to take into account the effect of the shear behavior:

with,

For the Young modulus, the maximal values of the unconfined compression tests are chosen, by considering that the gravel lateritic soils are perfectly compacted in 95% of the Optimum Modified Proctor (OMP). For the asphalt layer and the subgrade (Young modulus (E) and Poisson’s ratio (ν)), these values are chosen in reference to Fall, Senghor and Lakhoune (2002) [

For nonlinear analysis, an incremental iterative procedure is used, the tangent constitutive matrix is updated after and during each load increment (NCHRP, 2004). For this implementation, a direct incremental method of the resilient modulus with very small time steps is used. A first test showed that, for the linear method, 24 time-steps were sufficient to reach a constant result. Besides, for the nonlinear model, the tests showed that the

results vary very widely according to the compulsory number of steps. From 300 steps, the values stabilize, which explains that the value of the number of steps n was set to these aforesaid values. For every interval of load, the resilient modulus is given with the average of the values found for two successive stages. This algorithm is summarized in

Pavement layers | Thickness | Linear model | Nonlinear model | |||
---|---|---|---|---|---|---|

h_{i} (mm) | ν | E (MPa) | k_{1} (kPa) | k_{2} | k_{3} | |

Asphalt | 80 | 0.35 | 1.300 | - | - | - |

Base (untreated) | 200 | 0.25 | 62 | 837.275 | 0.13 | −0.33 |

Base 2% | 200 | 0.25 | 84 | 279.074 | 0.65 | −0.50 |

Base 3% | 200 | 0.25 | 137 | 170.052 | 0.88 | −0.56 |

Subbase (untreated) | 250 | 0.25 | 62 | 837.276 | 0.13 | −0.33 |

Subgrade | 17.500 | 0.25 | 30 | - | - | - |

The effect of overloading was studied with regard to the normalized axle load of calculation (13 tons).

On the other hand, the impact of the axle overload seems to have more effect on deflection at the top layers (asphalt layer and base layer) than for the lower layers. It is necessary to note that the effect of a single wheel was tested. To know the total deflection and strain, it is necessary to take into account the effect of the dual- wheels. Besides, the variation of deflection and deformation for a variation of the axle overload of 1, 2, 3, 5 and 10 tons is given for the linear model (

Deflections | Deflections according to the axle overload (mm) | ||||
---|---|---|---|---|---|

Per tons (mm) | 2 tons | 3 tons | 5 tons | 10 tons | |

δ_{surface} | −0.04877 | −0.09754 | −0.14631 | −0.24385 | −0.4877 |

δ_{BB} | −0.04877 | −0.09754 | −0.14631 | −0.24385 | −0.4877 |

δ_{CB} | −0.0227465 | −0.045493 | −0.0682395 | −0.1137325 | −0.227465 |

δ_{CF} | −0.008193 | −0.016386 | −0.024579 | −0.040965 | −0.08193 |

Deformations | Deformations according to the axle overload (μe) | ||||

Per tons (μe) | 2 tons | 3 tons | 5 tons | 10 tons | |

e_{BB} | −137.42 | −274.84 | −412.26 | −687.1 | −1374.2 |

e_{r} | 56.0705 | 112.141 | 168.2115 | 280.3525 | 560.705 |

e_{CB} | −81.526 | −163.052 | −244.578 | −407.63 | −815.26 |

e_{CF} | −27.518 | −55.036 | −82.554 | −137.59 | −275.18 |

Notations: BB: asphalt layer; CB: base layer; CF: subbase layer.

Variation per tons of overload | ||
---|---|---|

2D Linear | 2D Nonlinear | |

δ_{surface} (mm) | −0.166 | −0.116 |

δ_{BB} (mm) | −0.165 | −0.115 |

δ_{CB} (mm) | −0.131 | −0.098 |

δ_{CF} (mm) | −0.111 | −0.097 |

e_{vBB} (μe) | −177.370 | −87.685 |

e_{r} (μe) | 68.291 | 28.778 |

e_{vCB} (μe) | −115.200 | −24.440 |

e_{vCF} (μe) | −49.345 | −14.245 |

σ_{r} (MPa) | 0.065 | 0.019 |

σ_{CB} (MPa) | −0.008 | −0.013 |

σ_{CF} (MPa) | −0.002 | −0.001 |

the variation of deflections and strains for the linear and nonlinear models by tons of axle overload. We can thus notice that the linear model gives higher strain and deflection variations and lower stress variations than for the nonlinear model.

The results show a linear variation of deflections and deformations at the road layers according to the variation of the axle overload. This overload seems to have more effects on deflections for the top layers level (asphalt layer and base layer) than the lower layers. It is however necessary to note that the effect of a single wheel is tested. To know total deflections and deformations, it is necessary to take into account the dual tires. These results reveal the possibility of obtaining the response values of roads by analyzing real scenarios of load as well as forecasting the impact of the axle overload on deflections and deformations of road pavements.