Comparison Of Field Behavior With Results From Numerical Analysis Of A Geosynthetic Reinforced Soil-integrated Bridge System Subject To Thermal Effects

Abstract

ABSTRACT Geosynthetic Reinforced Soil (GRS) consists of alternating layers of geosynthetic reinforcement and compacted soil. Geosynthetic Reinforced Soil – integrated bridge system (GRS-IBS) has been promoted by Federal Highway Administration (FHWA). It consists of an integral bridge superstructure and sub-structure supported on a GRS abutment. The abutment consists of a reinforced soil foundation (RSF) underlying a GRS abutment. At the ends of the superstructure are approach fills that are also geosynthetically reinforced. The main advantage of GRS abutments over traditional concrete abutment is the savings in time and cost of construction. Unlike traditional concrete abutments, GRS abutments do not require formwork and waiting time for the concrete to set. Moreover, the superstructure can be prefabricated beforehand and quickly placed on the GRS abutments leading to savings in both time and money. Being integral bridges whereby the superstructure is structurally connected to the sub-structure GRS-IBS will undergo volume changes causing movements in the sub-structure and foundation soil during ambient temperature changes. In a previous monitoring study of a 109.5-ft-long GRS-IBS in Lahaina, Maui, a GRS-IBS was observed to undergo cyclic straining. The upper and lower reaches of the superstructure experienced the highest and lowest strain fluctuation, respectively. These non-uniform strains impose not only axial loading of the superstructure but also bending, which in turn cause the vertical pressures beneath the footing and lateral pressures behind the end walls and facing to fluctuate cyclically. Measured vertical footing pressure closest to the stream experienced the greatest daily pressure fluctuation (≈ 2,500 – 3,000 psf), while the one nearest the end wall experienced the least. The toe pressure fluctuations seem rather large. The larger these pressure fluctuations are, the greater will be the cyclic-induced deformations of the GRS abutment. In this study, a finite element analysis of the same GRS-IBS was performed by applying an equivalent temperature and gradient to the superstructure over the coldest and hottest periods of a day to see if the field measured values of pressures are reasonable and verifiable, which indeed they were. This methodology is novel in the sense that the effects of axial load and bending of the superstructure are simulated using measured strains rather than measured temperatures. This simple methodology can be useful to engineers who are interested in estimating thermally-induced cyclic bearing pressures of GRS-IBS and the associated cyclic-induced settlement in the GRS abutment, which can purportedly be more prominent in longer span bridges. FHWA currently suggests limiting the span length of GRS-IBS to 140 ft. This is because it is generally believed that the longer the span length, the more severe the thermal effects will be on the sub-structure behavior and GRS abutment settlement. With modifications to the material and geometric properties of the same GRS-IBS model, the behavior of a bridge that is twice as long was studied. After being subject to temperature loading the average bearing pressures of the 220-ft-long bridge remained within its shakedown limit. The results of this study showed that a GRS abutment is capable of supporting superstructures with a longer span than previously envisioned.