Question

Explain why hydraulic fill is a poor way to place fill.

Answer 1

The Hydraulic Fill is a poor way to place fill owing to the high susceptibility of the uncompacted, cohesion-less soils in them to liquefaction, Change in volume of the deposits, restrictions in Stability. The feasibility of the design, especially regarding soil conditions and material available in the borrow area, and insight into the volumes and quality of sand in these borrow areas and the dredgeability of these materials. For instance, suitable sand may be overlain with clay or peat or unsuitable fine sands, which have to be removed. The borrow area may be sheltered from waves or positioned offshore exposed to wave action, which is of consequence for the dredging method. In some cases the borrow material may need specific construction and/or treatment methods in order to become suitable fill material.

The quantity of fill available is the in-situ volume of extractable material that is suitable within the boundaries of the borrow area. The volume of the material extracted may change as a result of losses of fine material or bulking, which is a change in density.
Also, the difference between the top and bottom of the suitable deposits and the allowable slope angles of the borrow pit may be restricted because of stability issues or because of other limitations in the surrounding areas like the safety of buildings in the vicinity.

Working with these physical circumstances will require an understanding of the capabilities of various dredging equipment. The subsoil conditions in the fill area itself determine: the extra height of the fill to compensate for consolidation and creep settlements in case of soft underlying clay layers; the method of placement which may be in relatively thin layers; and/or the placement of sand layers in intermittent periods to avoid instability. Soft underlying clay layers may also have to be removed before sand fill is placed or vertical drains may be installed through the soft layers to accelerate consolidation and to strengthen the bearing capacity of the subsoil. In case of removal, such material may be stored in depots on site or removed from the site and applied, if possible, for a beneficial use.

Financial and economic risks include unexpected cost increases caused by adverse soil conditions, which may require different types of or additional equipment and/or a longer execution period, which will involve considerable extra costs. Also costs of arbitration, additional costs caused by inflation (fuel, wages, materials, …) or an increase in royalties for the material from borrow areas, bankruptcy of either party, unstable currency, adverse profit margins or overly optimistic budgeting and a change in economic expectations