Enhancing Performance Of Clays And Of Pavements In South Republic Of Benin(West Africa)

To preserve and enhance sustainable development,geotechnical studies of soil are required.This research is based on the performance of clay s and pavements in unstable tropical areas.This research was carried out with the help of pertinent performance data from tropical pavements.The research’s primary objectives were to determine the followings: the respective percentage of hydrated lime and cement,which can be used to improve the bearing capacity of clay,sandy soils,intended to be used in the pavemen t layers.Apart from that,these geotechnical test results are use d to define suitable pavement construction techniques,such as the structural model pavement fit on clay.Then,to evaluate the pavement performance using biaxial geogrid in different pavement layers with recycled asphalt pavement(RAP)materials as the subgrade.And finally,to propose new techniques to preclude the degradation of infrastructures in a hazardous area.The results of these objectives are codified in the form of recomme ndations.The original work is presented in detail as pursuit.(1)In chapter 2,the content is focused on characterization and improvement soil method and its assessment method or techniques.The methods used to examine these objectives are experimental t ests according to French standards such as particle size test,Proctor and Atterberg limits test which are physical tests.Then,dynamic penetration test,direct shear test and oedometer test which are mechanical tests for characterization soil.Finally,Proctor test,California bearing ratio test,compression and tensile strength for the improvement of soil.The results show that the soil is not very deformable because of the particles’ dimensions or aggregates.The soil resistance increases with the depth,whereas the soil is moderately compressible because of the low compression index.Simultaneously,the soil is deformable at other kilometers due to the plastic silt and very much plastic organic soils.The results reveal the unstable and stable areas in tropical areas with clay and give an equation for determining the unstable and stable area.Then,the proportion of percentage lime leading to best performances varies between 2% and 3%,when the proportion of percentage cement leading to best perf ormances requires a minimum of 3%.The CBR index experienced a very appreciable improvement for the subbase layer with hydrated lime and cement addition.Additionally,hydrated lime decreases water content without affecting plasticity and the liquid limit of soil samples.In conclusion,for a successful operation(sustainability),the combination of lime and cement is very relevant in the subgrade.Although the combination of lime and cement is very relevant in the subgrade,it cannot ensure the subgrade and embankment stabilities in a certain area(river area)for the reason of humidity.Thus,it is important to find other techniques to ensure the subgrade and embankment stabilities.(2)In chapter 3,the content is focused on subgrade and embankment stabilities.Based on experimental test such as,geological,environmental,social,and safety studies of river bank slides,the causes of landslide has been studied.Then,from analytical analysis based on Fellinus and Bishop methods,with numerical analysis using Plaxis software,the performance of stabilization by reprofiling and drainage was evaluated.For this,two stabilization systems were considered.Firstly,stabilization systems consisted of a drainage trench upstream of the slope with a safety coeffic ient without an earthquake,varying from 1.50 to 1.67.Then,the case with earthquakes varies from 1.33 to 1.52.Secondly,the installation of a pile curtain downstream of the embankment with a safety factor that varies from 1.34 to 1.54 without an earthqu ake and from1.23 to 1.37 in the accidental case.The results show that a bank’s sliding could result from the sudden receding water recorded in a valley.Therefore,it is the cause of the degradation of infrastructures in a hazardous area.Then,removing sand from the bottom of a riverbed would be an aggravating factor.Furthermore,the slope stabilization technique by reprofiling(using the lateritic gravel sand)and drainage can ensure soil stability in unstable areas by improving safety factor.In conclusion,Bishop’s method is the most realistic compared to the other methods.Then,the slope stabilization technique by reprofiling and drainage can make it possible to permanently solve the problem of landslides in areas near the river.This technique is mainly applicable to soils that are sometimes soft or sensitive.(3)The content of chapter 4 is focused on structural pavement performance and reinforcement technology.The methods used to evaluate the performance pavement using improved lateritic gravel in base layers are numerical methods using Plaxis and ALIZE-LCPC software’s.Then,based on the combination of TRL Road Note 31 method and the Win Julea method,and on the experimental research center and studies in building and public works(CEBTP)method,the structural pavement are evaluated.Furthermore,the pavements design analysis follows three(3)alternatives: the unreinforced is the first alternative that served as a control.The second alternative is where the biaxial geogrid reinforces the base layer,increasing the pavement’s service life.The third alternative is reinforced by the biaxial geogrid,reducing the base layer’s thickness while maintaining its service life.The results show that,the 3.5%cement treatment is the optimal dosage to be us ed in pavement base course for T4 traffic.The modeling of the pavement structure with the Plaxis software,allowed to determine the vertical displacements at the level of the base layer and to observe the failure mechanism as well as the main directions o f the effective stresses of the pavement structure.The structural model pavement from the CEBTP method was slightly more efficient than the structural model pavement from the combination of TRL Road Note 31 method and Win Julea method.Then the two methods designed are complementary according to their technical and economic performance.Geogrid reinforcement reduced the thickness of the base layer by 0.1m and increased the capacity of the cumulated allowable traffic by approximately 10%.As conclusion,the parametric study is of particular interest to geotechnical engineers because it allows to evaluate the influence of parameters on the reliability of modeling results.It can be used to obtain coefficients of variation and to define confidence intervals on modeling results.Furthermore,the structural pavement model from the combination of TRL Road Note 31 method and Win Julea method is more economical because it is due to laterite’s low cost than crushed stones.But.lateritic gravels do not always have the required performances in the base layer.However,this material is suitable for cement improvement.(4)Finally,in chapter 5,the content is focused on the reinforced pavement effects using biaxial geogrid in different layer,according to the choices of t he structural model pavement.The deformation was evaluated based on experimental tests such as the Benkelman beam test and Vizir method.And,based on the Finite Element Method(FEM)computer analysis under static load using Abaqus software,flexible pavements(with and without geogrids)were built and subjected to 13 T load applications as a part of the experimental study.This,to evaluate the vertical stress and the longitudinal strain.The results show that,the recycled asphalt pavement(RAP)materials with moderate plasticity on which the geogrid lay didn’t have any resistance to the embedding of the knots.At the same time,it ensured the stability of the geogrid.Thus,the knots of the geogrid no longer reacted in the structure pavement under the app lication of the loads.In addition,due to the effect of cement stabilization of sub-base layer,the biaxial geogrid knots placed at the interface subbase and base layers,couldn’t be properly embedded in this layer bending movements under the repeated application of the loads or influence traffic.The base layer during its bending movement was subjected to its sub-base layer.Furthermore,the performance of pavement(from CEBTP method)without the biaxial geogrid in subgrade is technically better than the following: firstly,the type of pavement structure with biaxial geogrid placed at the interface of the base layer and subbase layer.And secondly,the same pavement structure with biaxial geogrid in the base layer.Since the mean deflections of these pavements with biaxial geogrid in crushed stones base layer and geogrid in interface crushed stones base-improved lateritic gravel subbase layer are approximately double the mean deflection of the pavement without biaxial geogrid.In conclusion,the pavement made up with geogrid in subgrade is the best.