The Case of the New Tagus River Leziria Bridge

Abstract

A brief description of the New Tagus River Leziria Bridge composed by 1695 m North Viaduct, by 970 m Main Bridge and by South Viaduct with a length of 9200 m is presented. The observed thickness of the foundation alluvia material varies between 35 m and 55 m with a maximum value of 62 m. Hundred eighteen boreholes were performed with a depth between 21 m and 71 m and eight boreholes were performed from a maritime platform. Standard penetration tests (SPT) were carried out in all boreholes 1.5 m apart. In addition CPTu tests, seismic cone tests, crosshole and downhole tests were performed. In three boreholes continuous undisturbed sampling with a triple sampler Geogor S was performed. Related with static laboratory tests namely identification tests, triaxial tests, direct shear tests and oedometer tests were performed. In addition for the dynamic characterization reasonant columns tests and torsional cyclic tests were performed. One of the most important considerations for the designers is the risk of earthquakes since Lisbon was wiped out by an 8.5 Ritcher magnitude earthquake in 1755. The seismic studies related to the design spectra were performed. The liquefaction potential evaluation was performed only by field tests taking into account the disturbance that occurs during sampling of sandy materials. In this analysis attention was drawn for SPT and CPT tests as seismic tests have only been used when soil contains gravel particles. The shear stress values were computed from a total stresses model, that gave results on the conservative side using the code “SHAKE 2000”. For the North and South Viaducts 1.5 m diameter piles were used and for the Main Bridge 2.2 m diameter piles were used. For the construction of the piles metallic casings were driven by a vibrofonceur or a hydraulic hammer and the piles length varies between 20 m to 56 m. Static pile load tests (both vertical and horizontal tests) were carried out on trial piles. In addition pile dynamic tests were performed. The construction aspects related with piles and bridge construction are addressed. To assess the integrity of the piles reception tests by sonic diagraphies (crosshole tests) were performed. Some problems that have occurred during piles construction in the Main Bridge, due to the gravel and cobbles dimensions, are described. The bridge was monitored with the purposes of: (i) Validation of design criteria and calibration of mental model; (ii) Analysis of bridge behavior during his life; and (iii) Corrective measures for the rehabilitation of the structure.