Evaluation of Unbound Pavement Layers Moduli Using the Miniaturized Pressuremeter Test
Shaban, Alaa Mohammed
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A small version of pressuremeter known as Miniaturized Pressuremeter (MPMT) was designed and built to provide high quality pavement parameters. The device, which is a modified version of PENCEL Pressuremeter (PPMT) has a 6 inch inflatable probe capable of quickly and reliably measuring the stress-strain response of thin pavement layers. The MPMT stress-strain data can be used as inputs to evaluate existing pavement structures as a part of a pavement management system, and as critical inputs in empirical pavement design methods. To check in-situ operational performance of the new probe, twenty four MPMT and PPMT tests were performed in parallel at Heritage Parkway-Phase II. This roadway consists of a natural subgrade, which was classified as a poorly graded sand soil. These initial validation results showed that there was no significant variance between soil parameters obtained from the MPMT probe and those determined by the PPMT probe. After validating reliability of MPMT test results, the device was used to determine static and dynamic properties of unbound granular pavement layers at four roadway projects in Central and South Brevard County, Florida. For measuring static properties of unbound pavement layers, two testing methods were used along with MPMT test: California bearing ratio test (CBR) to assess strength parameters, and one-dimensional creep test to determine long-term deformations. Two comparative analyses were performed to evaluate static predictive capability of MPMT data. First, statistical analyses were established by comparing MPMT parameters with static soil measurements obtained from the standard tests. The results indicated that initial elastic modulus and limit pressure from MPMT correlate well with ultimate bearing strength from CBR. In addition, MPMT creep data have proved feasible for identifying soil creep behavior. The average ratio of predicted to measured strain rate estimated for 10 years was 1.25 and 1.23 for subgrade and base course, respectively. Secondly, finite element analyses were performed to predict CBR as a function of MPMT strain level model. The results indicated that MPMT strain level moduli determined at the top of base layers provide the best CBR prediction, while those measured at depth 6 inches below subgrade surface yield the best CBR values. The average ratio of finite element CBR to measured CBR was 1.01 and 0.91 for subgrade and base course materials, respectively. For measuring dynamic properties of unbound pavement layers, two testing methods were used along with MPMT test: light weight deflectometer (LWD) to measure dynamic moduli and surface deflections; and cyclic triaxial test (CT) to measure resilient moduli and permanent strains. Two comparative analyses were performed to correlate MPMT parameters with dynamic soil measurements from the standard tests. The results indicated that initial and reload moduli from MPMT test can be used to predict LWD dynamic modulus. The results also showed good correlations between MPMT moduli from stress level model with resilient moduli from CT, in addition the MPMT cyclic data yielded reliable results for modeling accumulated strains using the sigmoidal model. Finite element analyses were performed to determine LWD deflections as a function of MPMT moduli. The results revealed that using base course modulus from the strain level model in finite element analysis provides satisfactory predicted deflections, while using the reload modulus model yields the best prediction deflections for subgrade soils.