Structural health monitoring of the first geosynthetic reinforced soil--integrated bridge system in Hawaii

Abstract

Geosynthetic reinforced soil is defined as closely-spaced (≤ 12 inches; typically 8 inches) alternating layers of geosynthetic reinforcement and compacted soil. A geosynthetic reinforced soil-integrated bridge system (GRS-IBS) consists of three main components; a reinforced soil foundation (RSF), a GRS abutment, and an integrated approach. GRS-IBS is being promoted by the Federal Highway Administration (FHWA) where GRS abutments are used to support single span bridge superstructure in their Everyday Counts Initiative, which is focused on accelerating implementation of proven, market-ready technologies. There are many advantages of GRS abutments over traditional concrete abutment walls but two of the more notable ones are: (1) elimination of the need to form, pour and wait for the concrete to cure resulting in accelerated construction and significant cost savings; and (2) reduced carbon footprint with less concrete and hence cement (production of 1 ton of cement releases 1 ton of CO2 into the atmosphere) utilized in the abutment walls. The first GRS-IBS in Hawaii was recently constructed in Lahaina, Maui. The superstructure was instrumented with strain gages to measure the effects of concrete shrinkage and the GRS abutments were instrumented to measure footing vertical pressures, lateral pressures behind the end wall and the GRS facing, bridge footing settlement and lateral displacement of the GRS facing. All gages were monitored remotely with the aid of a data acquisition system. From the recorded data, it was observed that (1) the bridge superstructure continually undergoes thermal expansion and contraction; (2) overall the superstructure compressive strains tend to increase and the end wall lateral pressures tend to decrease with time, which is an indication of superstructure concrete shrinkage; (3) the total footing settlement did not exceed 0.9 inch at the time of writing; (4) the measured pressures underneath the footing are consistent with the estimated stress from the superstructure; during construction and (5) the footing cyclically undergoes rotation about a transverse axis causing the vertical pressures to fluctuate cyclically while the lateral pressures behind the bridge end wall are also undergoing pressure cycles consistent with the effects of thermal loading. A finite element analysis was performed to compare results with the measured field data. The analysis yielded nearly identical trends for footing settlement and facing lateral pressure. This helped verify that the observed trends and measured values are reasonable.