Measures for the Optimization of Subgrade and Pavement Design in the Reconstruction and Expansion of Highways

In comparison to the construction of modern highway engineering,several of China’s early pavement construction concerns,such as pavement collapse,are rather clear.Limited by historical and technical factors,the subgrade and pavement design for highways lacks scientificity,thus inducing potential safety problems in the operation.In order to comprehensively improve the subgrade and pavement design as well as ensure the quality and safety of highway engineering projects,this paper takes the reconstructed and expanded highway projects as research subjects and focuses on proposing optimization measures for the subgrade and pavement design of reconstructed and expanded highways,so as to provide adequate reference.

Structures of subgrade/pavement and construction techniques for the Taklimakan Desert Highway

With lots of indoor and outdoor experiments, several key technical issues in construc-tion of the Desert Highway have been solved satisfactorily, on the basis of great achievements ofthe studies in respects of dry compaction on sand base, design parameters, structure combinationof subgrade and pavement, stabilization analysis of sand base strengthened with geotextile and acomplete set of construction techniques. It is the first time the achievements of the study weresuccessfully applied in the Taklimakan Desert where the natural condition is extremely harsh. Ithas been proved that it is economic and reasonable with reliable techniques and simple construc-tion methods. The Desert Highway constructed with the achievements is the first-grade highwayrunning through huge migratory desert for long distance in the world.

Using portable falling weight deflectometer to determine treatment depth of subgrades in highway reconstruction of Southern China

Based on a highway reconstruction project in southern China,this study aims to put forward a method to determine the proper treatment depth of the existing subgrade.First,some field tests including the Beckman beam deflection test and portable falling weight deflectometer(PFWD)test were carried out.The results showed that there was a good correlation between the Beckman beam deflection(L)and PFWD modulus(Ep).Subsequently,a subgrade section was excavated and backfilled with cement-stabilized soil in layers.Compaction test,dynamic cone penetrometer rate test,plate load test and Beckman beam deflection test were performed to evaluate the treatment effect.To make sure,the subgrade was treated deeply enough,the Beckman beam deflection(L)was used as the controlled indicator among all the measured indexes for it was the hardest metric to meet.According to the design deflection and decreasing law of the measured deflections with the different number of the stabilized layers,the treatment depth was finally determined.As the PFWD test is superior to the deflection test in the detection efficiency,and the deflection value can be calculated from PFWD modulus by correlation formulas,thus the latter index can be used as a more suitable parameter for estimating the treatment depth instead of the former.Consequently,based on the measured PFWD moduli of the existing subgrade,six treatment schemes considering different treatment depths were proposed.It was confirmed that the method developed from this study is feasible and worth being extensively applied.

Evaluation of heavy roller compaction on a large-thickness layer of subgrade with full-scale field experiments

Subgrade construction is frequently interrupted due to precipitation,soil shortage,and environmental protection.Therefore,increasing the thickness layer is required to reduce construction costs and to allow highways to be placed into service earlier.This paper presents a series of full-scale field experiments evaluating the compaction quality of gravel subgrade with large-thickness layers of 65 cm and 80 cm using heavy vibratory rollers.An improved sand cone method was first proposed and calibrated to investigate the distribution of soil compaction degree across the full subgrade depth.Results showed that dynamic soil stresses caused by the heavy vibratory rollers were 2.4–5.9 times larger than those of traditional rollers,especially at deeper depths,which were large enough to densify the soils to the full depth.A unified empirical formula was proposed to determine the vertical distribution of dynamic soil stresses caused by roller excitation.It was demonstrated that soils were effectively compacted in a uniform fashion with respect to the full depth to 96.0%–97.2%and 94.1%–95.4%for the large-thickness layers of 65 cm and 80 cm within 6 or 7 passes,respectively.Empirically,linear formulae were finally established between soil compaction degree and the subgrade reaction modulus,dynamic modulus of deformation,dynamic deflection,and relative difference of settlement to conveniently evaluate the compaction qualities.It is demonstrated that increasing the thickness layer by means of heavy rollers can significantly reduce the cost and time burdens involved in construction while ensuring overall subgrade quality.