26
Exercise A - Seepage Underneath a Sheet Pile Wall
Objective
Seepage underneath a sheet pile wall is one of the seepage problems that are most
common in practice. Sheet pile walls are used to reduce seepage under all types of
dams, sea walls, dividing walls, lock walls, coffer-dams and similar structures. They
are also used to reduce leakage from canals, rivers and the sub soils surrounding an
excavation and the like.
It is also this type of seepage which most clearly illustrates the concept of a flow net
where the flow net has a simple and intuitively clear pattern and fully defined
boundary conditions.
i.
Flow Line Visualisation
Prepare about ½ litre of fluorescein solution by slowly adding the chemical
into water until the solution becomes a semi-transparent opal-like liquid of
orange-greenish colour.
Fill the tank with pure sand to a level of about 300mm above the bottom of the
tank.
Adjust the upstream overflow so that its top is about 100mm below the top of
the tank and the downstream overflow so that its top is about 25mm above
the surface of the sand bed.
Adjust the impermeable screen at the middle of tank. Leave about 125 to
150mm of clearance between screen and bottom of the tank. Seal the
contacts between the screen and tank walls with grease or other easily
removable seal.
Apply the seal on downstream side to prevent leakage in case screen moves
under the final pressure difference.
Pour water slowly into the tank. Start with the downstream pool and transfer
the input into the upstream after the lower pool is full. After overflow occurs
both upstream and downstream reduce water input to the minimum needed to
