Proceedings - Fifth International Conference & Exhibition on Piling and Deep Foundations, 1994, Bruges, Belgium, (DFI)
Little details lead to costly mistakes: problems in design and construction of foundations
Each of the three case histories presented illustrates the consequences of changes made to the original design of a project. Initially these changes may have appeared to produce equivalent designs. However, each resulted in expensive remedial work. The first involves breakage of 70 m long prestressed concrete piles driven through 30-40 m of soft clay. Due to height restrictions, the piles were driven in three segments with two mechanical splices. A preliminary testing program was successful at installting the piles stressed with twelve 11.1 mm strands. In order to reduce fabrication costs on the first production project, alternate designs were prepared with 9 or 6 strands. These were sized (12.7 mm or 15.2 mm) to provide, the same total stressing force. The contractor selected the six strand design. Not until significant changes were made to the splice connection, did pile breakage reduce to an acceptable level. The second involves the corrosion of buried anchor rods. The plans for the construction of the portal walls of a tunnel anticipated the rock to be cleanly cut, and reinforced with short rock bolts. The geologic conditions varied and the contractor's excavation procedures left a wide distance between the rock face and the proposed wall. Field changes required the lengthening of ties through fill. Corrosion issues were addressed by galvanizing the bars. After only seven years, the 31 mm bars were completely corroded. The third involves undetected scour damage. The normally dry climate of the southwestern USA, is interrupted by intense rainfall which causes flash flooding. Unanticipated accumulation of floating debris caused scouring that structurally damages the pile foundations to a point of near collapse. A review of the project records showed that the original piles had been substituted with piles of lower stiffness.
|article #820; publication #36 (IC-1994)|