Barrage de Petit-Saut
2000 | 2001
Petit Saut dam is a large power plant built by EDF (the French Power Company) in French Guyana, on the Le Sinnamary River. The structure consists of a main concrete dam and six lesser dams, two of which (A1 and A2) are located on the right bank and four (B, C, D and E) on the left bank.
Construction of the barrage began in January 1994 and was completed in 18 months. Dams C and D immediately showed problems of hydraulic seal, with water leakages up to 11÷13 US gal/min (40÷50 l/min). Geognostic investigations made it possible to detect areas with high permeability consisting of sand with cobbles (hydraulic conductivity K estimated between 10^-4 and 10^-5 m/sec) and altered clay (K estimated between 10^-5 and 10^-6 m/sec).
The uniform nucleus of sandy clay of the dam bodys rested on alternating of clay and sand bedding variable in thickness from 3 to 12 meters and, going down, sand with blocks of granite - diameter up to 10÷13ft (3÷4 m) - variable in thicknes up to 23ft (7m), altered and fractured granite with thickness from 2 to 20ft (0.5 to 6m) and than a locally cracked granite bedrock.
The aim of the project was the creation of a waterproof jet grout screen with a minimum thickness of 2ft (0.5m), below the dams C and D down to bedrock, to reduce the permeability of soils to K values between 2x10^-7 and 5x10^-7 m/sec.
The entire work was carried out from the crest of the dams after removing stone coating. The project provided construction of a row of columns with a diameter of 1,80m and spacing of 1m. A preliminary hole crossing the body of the dam to reach the level where the treatment was to start had to be made for each column. Each preliminary hole was done by dry drilling with helical rods and fitted with a PVC casing pipe sealed in the hole with injections of slurry so as to eliminate the risk of interference with the draining and monitoring system of the dam. Moreover a preventer was positioned on the top of the pipes during drilling/injection fases to preserve basin waters to contamination.
The screen, built with the Pacchiosi PS3 System, was finally integrated with injections into the bedrock.
The maximum deviation of the holes with respect to vertical was below the limit value required by EDF (1% of drilling length).
During the works, control holes were drilled on 50% of the overlapping zones of the columns. The performance of Lefranc test indicated the hydraulic conductivity (K) was below 2x10^-7 m/sec in 95% of the cases and below 5x10^-7 m/sec in all cases.
The drilling and injection parameters were automatically recorded with the Pacchiosi PRS3 system.
For the duration of the works at the site, a Pacchiosi laboratory was in operation, fully equipped for tests on the cement mixtures with measurement of the density, viscosity, temperature, pH, setting time, mechanical resistance, etc.
