The main purpose of an unbound layer is to distribute the loads applied by the vehicles coming from the upper layers to the underlying subgrade without causing distress in the layer or in the overlying layers. During construction it must guarantee adequate bearing capacity to support load induced by construction equipments; during pavement service life it must be able to withstand the repeated loads induced by traffic without accumulation of permanent deformations. The materials used to build unbound layers have a wide range of properties. A performance based testing approach can facilitate efficient use of different materials guaranteeing that assumptions made in the design process are being, or are likely to be, achieved. At the same time we need to perform more realistic tests, simulating the magnitude and shape of stresses induced by traffic loads. In this context the use of non-destructive deflectometric devices in QC/QA earthwork activities is increasing worldwide. Light Weight Deflectometers (LWD) are systems designed with the aim to reproduce the stress applied by the heavy vehicles to the deep layers. The LWDs with load cell, can vary the drop height and measure stress applied to the testing surface, enabling to investigate the soil stress sensitivity. In addition, innovative systems like DPS (Dual Plate System) and additional external geophones enable to evaluate the influence on Surface Moduli of the stiffer upper layers by means of back-calculation process. Moreover, these particular features of the LWD device enable the engineer to setup specific, quick procedures to estimate the compaction level: the evaluation of the performance characteristics and the estimation of the compaction level achieved on site can be undertaken at the same time with the same instrument. Compaction level is generally evaluated by means of “degree of compaction”: this parameter is calculated as ratio between the measured field density and the optimum laboratory density. Depending upon the method chosen to evaluate field density, they can be time consuming, questionably accuracy, operator dependant (sand cone method), the nuclear density gauge is not allowed in all countries and non-nuclear impedance gauges are not yet fully validated and accepted (). This paper focus attention on the advanced capabilities offered by LWD equipments in evaluating compaction level and on the possibility to setup performance related specifications to be used in conjunction with pavement performance design procedures. Moreover LWD tests can provide a much more detailed investigation than standard approach allowing to identify weakest areas in terms of bearing capacity and to evaluate immediately the improvement in performance characteristics related to additional roller passes and/or to re-compaction under different moisture contents of the material. In summary, the use of LWD allows the enhancement Qc/Qa activities by means of improvement of performance based approach. LWD based procedure for compaction level evaluation allow a very detailed overview of performance of tested area, provide parameters closest to pavement design assumptions, highlight immediately areas to re-compact providing useful results for both contractors and road supervisors.
Earthworks and Pavement unbound layers: a step to the future towards enhanced QC/QA activities
MARRADI, ALESSANDRO;
2011-01-01
Abstract
The main purpose of an unbound layer is to distribute the loads applied by the vehicles coming from the upper layers to the underlying subgrade without causing distress in the layer or in the overlying layers. During construction it must guarantee adequate bearing capacity to support load induced by construction equipments; during pavement service life it must be able to withstand the repeated loads induced by traffic without accumulation of permanent deformations. The materials used to build unbound layers have a wide range of properties. A performance based testing approach can facilitate efficient use of different materials guaranteeing that assumptions made in the design process are being, or are likely to be, achieved. At the same time we need to perform more realistic tests, simulating the magnitude and shape of stresses induced by traffic loads. In this context the use of non-destructive deflectometric devices in QC/QA earthwork activities is increasing worldwide. Light Weight Deflectometers (LWD) are systems designed with the aim to reproduce the stress applied by the heavy vehicles to the deep layers. The LWDs with load cell, can vary the drop height and measure stress applied to the testing surface, enabling to investigate the soil stress sensitivity. In addition, innovative systems like DPS (Dual Plate System) and additional external geophones enable to evaluate the influence on Surface Moduli of the stiffer upper layers by means of back-calculation process. Moreover, these particular features of the LWD device enable the engineer to setup specific, quick procedures to estimate the compaction level: the evaluation of the performance characteristics and the estimation of the compaction level achieved on site can be undertaken at the same time with the same instrument. Compaction level is generally evaluated by means of “degree of compaction”: this parameter is calculated as ratio between the measured field density and the optimum laboratory density. Depending upon the method chosen to evaluate field density, they can be time consuming, questionably accuracy, operator dependant (sand cone method), the nuclear density gauge is not allowed in all countries and non-nuclear impedance gauges are not yet fully validated and accepted (). This paper focus attention on the advanced capabilities offered by LWD equipments in evaluating compaction level and on the possibility to setup performance related specifications to be used in conjunction with pavement performance design procedures. Moreover LWD tests can provide a much more detailed investigation than standard approach allowing to identify weakest areas in terms of bearing capacity and to evaluate immediately the improvement in performance characteristics related to additional roller passes and/or to re-compaction under different moisture contents of the material. In summary, the use of LWD allows the enhancement Qc/Qa activities by means of improvement of performance based approach. LWD based procedure for compaction level evaluation allow a very detailed overview of performance of tested area, provide parameters closest to pavement design assumptions, highlight immediately areas to re-compact providing useful results for both contractors and road supervisors.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
