Open Caissons for Deep Foundations

Open caissons are a very efficient method of deep foundation, provided the ground conditions are favorable. Design and construction of a 60‐m deep shaft, 27.0‐m ID for a pumped storage hydroelectric power station is described. The soil consists of about 8 m of soft fluvial deposits on a 200‐m thick layer of homogeneous over consolidated stiff marly clay. The upper 8 m of soft soil were excavated and replaced by 5 m of compacted soil on which the leading ring of the shaft was concreted. Above the leading ring a 20‐cm wide slit filled with stabilized bentonite slurry was provided in order to eliminate skin friction during sinking. The shaft was concreted using slip form work. The method of excavation and verticality control of the shaft axis during excavation and concreting and the construction procedure of the shaft bottom are described in some detail. The shaft was successfully completed within 120 working days. Two examples of open caisson foundation that had to be changed due to unfavorable soil conditions are discussed. It is emphasized that careful soil exploration is prerequisite to success in open caisson foundations.