SYMPOSIA PAPER Published: 01 January 1969

Downdrag Measurements on 270-Ft Composite Piles


The Quebec autoroute on the North Shore of the St. Lawrence River between Montreal and Quebec City traverses deep deposits of compressible marine clay. One of the overpass structures at Berthierville is supported on twelve 39-in. (1 m) diameter reinforced concrete piles, which pass through 270 ft (82 m) of soil and finally rest on shale bedrock. The site was preloaded for one year, during which time about 5 ft (1.5 m) of settlement occurred. Steel casings for the piles were then driven and filled with concrete which was reinforced. Each pile was designed to carry 2000 tons (1814 metric tons) of which 1200 tons (1090 metric tons) were allowed for negative skin friction loads. Deformation gages were installed at various levels in two of the piles to measure the axial compression as the negative skin friction accumulated. Contrary to expectations, hydration caused the piles to lengthen about 0.06 in. (1.5 mm) during the 6-week period following placement of the concrete. Five months later this reduced to about 0.04 in. (1.0 mm) as skin friction loads overcame the expansive forces of hydration. In this same period the subsoils consolidated by more than 0.5 ft (0.15 m). Near the ground surface the pore water pressures and settlements were the same adjacent to a pile and at a distance of 22 ft (6.7 m). In November 1966 a cut was made to permit construction of the foundations for the overpass, causing the piles to lengthen about ƈ in. (6 mm). In December, electroosmosis was applied to relieve the test piles of the accumulated skin friction loads imposed by the consolidating clay subsoil. Subsequent measurements showed that they immediately began to accumulate negative skin friction loads, and that in the following 10-month period they compressed more than Ɖ in. (13 mm). Vertical pile deformations were also measured when the overpass was constructed. From these observations the vertical modulus of deformation of the composite piles was calculated and compared with the values obtained from uniaxial compression tests on test cylinders in the laboratory. The paper presents measurements of settlement and pore water pressures in the soil at various depths under the fill, and of axial deformations in two of the piles. It provides a set of relatively rare field observations on which to base an approximate evaluation of the rate and magnitude of negative skin friction development under the described field conditions.

Author Information

Bozozuk, Michael
National Research Council of Canada, Ottawa, Ont., Canada
Labrecque, André
Labrecque, Vezina, and Associates, Consulting Engineers, Montreal, P. Q., Canada
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Developed by Committee: D18
Pages: 15–40
DOI: 10.1520/STP47279S
ISBN-EB: 978-0-8031-6901-2
ISBN-13: 978-0-8031-6644-8