Stability and deformation monitoring of geogrid reinforced embankments 1 J A R Ortigao 2 , A R Fahel 3 , E M. Palmeira 2 and A J Simmonds 4 ABSTRACT This paper presents the design and results of stability monitoring for embankments at the abutments of the in the BR-101 highway in south Brazil. Critical stability problems were due to a very soft clay foundation, several metres of depth and a high embankment. In order to cope with stability and settlements the design was carried out with several layers of geogrid reinforcement and prefabricated vertical drains (PVD) or geodrains. Settlement and stability control was monitored by vibrating wire piezometers, inclinometers and a settlement profiler. A stability design chart was prepared based on total and effective stresses and this enabled the selection of reinforcement tensile strength and choice of single stage or multi-stage construction. The latter reduces the necessary strength leading to geogrid savings. This chart was also instrumental for field stability control. The results showed that the presence of the reinforcement layers reduced the soft soil lateral displacements and the damages to the existing structures. INTRODUCTION In July 1997 several embankments failures on soft ground took place during a highway construction in south Brazil. The project was the widening the BR 101 motorway and has strategic importance, as it links three countries: Brazil, Uruguay and Argentina. Geotechnical consultants were called in and a significant design change took place. Site investigation was enhanced by means of in situ piezocone and vane tests. The engineering solution: geosynthetics and construction control by means of instrumentation. Geosynthetic applications for the design of embankments in Brazil on soft ground dates back from late 70’s when nonwoven and woven geosynthetics dominated the market (Ortigao and Palmeira, 1982). At that time, the maximum tensile strength of a geosynthetic was low and in the order of 40 kN/m. Geogrids available today can reach twenty times that strength and prices have become very competitive. Geodrains (or PVD) to accelerate consolidation are also widely used, as modern fast installation techniques and mass production reduced its cost to one tenth of prices practiced twenty years ago. At bridge abutments, where approach embankments on poor ground reached 3 to 5 m in height, stability was critical. The design consisted of the acceleration of settlements by means of geodrains and a temporary surcharge. The geodrains were installed on a square pattern spaced between 1.2 to 1.4 m. The smaller spacing applied to the region close to the bridges, where the required percentage consolidation was 95% of settlements occurring before paving the road. At 50 m away from the bridge, geodrains spacing increased to 1.4 or 1.5 m depending on the depth and consolidation properties of the clay layer. Fahel et al (2000), on a paper on the same project, described in detail the behaviour of the bridge abutments and the reinforcement. This paper, on the other hand, focuses on stability analyses and control measures by means of instrumentation. SITE CHARACTERISTICS AND FAILURES Large, and sometimes rather deep, soft soil deposits are very frequently found in Brazil, particularly along the coastline. In the state of Santa Catarina, in southern Brazil, tropical organic soft 1 TRB Meeting on Geo-Construction Processes, Paper 01-2392, January 2001, Washington DC 2 Federal University of Rio de Janeiro, Brazil 3 University of Brasília, Brazil 4 Geokon, Inc., Lebanon, New Hampshire, USA