A Note On Design Methodology For Sheet Pile In A River With Large Tidal Variation

Abstract

The stretch of river Ganges from Kolkata to Haldia port experiences high variation of tide and occasionally experiences remarkable phenomena of bore tides which take place under the right hydraulic and astronomical conditions. A tidal bore, consisting of the head-wave of an advancing tide, hemmed in where the estuary narrows suddenly into the river moves rapidly up the river at speeds reaching 10 m/s. Further, the rise of water depth may exceed 2m at specific locations depending upon local geometry and bathymetry. As a tidal bore surges up an estuary, it tends to lift up any floating matter, including vessels and jetties. If the rise is too high and sudden, the upward pull could be severe, often dislodging moorings of floating jetties. One such severe occurrences of a tidal bore in recent times took place on the 20th of September, 2014 when quite a few damages were reported.

Under this environment with unpredictable natural phenomenon, it becomes difficult to design simple Sheet Pile barrier for completely block entry of water to shore side shipbuilding infrastructure like a Dry Dock for carrying out necessary repair to its gate. One such situation had arisen when a medium size shipyard located in Kolkata, India wanted to put up Sheet Pile barrier to carry out modernisation and install a new Gate in the existing Dry Dock. Two attempts to put up water tight barrier were unsuccessful as the high tide in the river slowly scoured away the underground soil with possible piping effect and the barriers collapsed. In the third attempt the author designed a sheet pile barrier considering all the possible forces posed by the river and the barrier is intact for two years since its installation. The present article describes in short the methodology applied, so that it can be replicated in similar situation.

Keywords: Sheet Pile structure, Wave force, Scouring effect, Uplift Pressure and Exit gradient, Current Force, Wave Force, Hydrostatic Force, Soil Pressure.