Impervious steel sheet pile walls

Ensuring Water Tightness: Rational Analysis and Practical Solutions for Impervious Steel Sheet Pile Walls

ArcelorMittal, the world’s leading producer of sheet piles, carried out in collaboration with Deltares (Delft Geotechnics) an exhaustive research project, on impervious steel sheet pile interlocks.

The aim of the project was to determine the rate of seepage through SSP with Larssen type interlocks for various interlock filler materials, as well as for empty and welded interlocks.

Two key areas of research were addressed: 

  • setting up a consistent theory to describe the leakage behaviour through individual interlocks; 
  • in situ tests on SSP walls. 

The research results are deployed to enable the designer to make a rational assessment of the rate of seepage for a specific case. A range of possibilities is discussed: highly permeable unsealed interlocks, sealed interlocks for medium permeability and completely impervious welded interlocks.

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The steel sheet piles themselves are completely impervious and therefore the only possible route for the fluid to pass through the wall is via the interlocks. For porous medium like slurry walls, the seepage problem can be treated with the aid of Darcy’s law with a suitably chosen coefficient of permeability K:

where v is the so-called filtration rate and i represents the hydraulic gradient:

In a horizontal plane, it is defined as the ratio of the difference in pressure height (Δp / γᵥᵥ ) and the length of the filtration path (s). 

The (Fig. 1) shows a horizontal cross section of a SSP interlock. The positive pressure difference between the points A and B: p2 –p1 is associated with a flow from B to A. 

The real kind of flow (pipe, potential,...) is difficult to determine, but most likely it will not be a porous media type of flow and consequently Darcy’s law is not applicable to seepage through a SSP interlock. To accommodate this difficulty, researchers at Deltares introduced the concept of “Interlock Resistance”.

In order to allow the design engineer to use equation (3) Deltares and ArcelorMittal carried out field tests on a large number of filler materials. The results of these tests yield values for ρ.

To expose the filler material to extreme site conditions, the sheet piles have been driven in by vibrodriver. Each filler material has been applied in several interlocks. The discharge through each interlock was measured as a function of the applied pressure drop using a special test apparatus, see (Fig. 5). The time dependent behaviour is monitored by taking readings at specific time intervals. 

Table 1 shows the relevant criteria for selecting a water sealing system for an SSP wall and the range of values obtained from the tests for different types of filler materials. The results of the empty interlocks are also shown. It is most important to note that the r -values obtained for empty interlocks strongly depend on the soil properties, the variations being very large. The test results are plotted in which generally confirms that the hypothesis which leads up to formula (3) is well-founded, at least for a certain pressure range.

(Fig. 6) shows a building pit in which the water table has been lowered about 5 m. The toe of the SSP wall goes right down to the bottom layer; the layer is assumed to be virtually impervious. This assumption allows neglecting the flow around the toe (the value of K required to be able to assume an impervious bottom layer will be dealt with in section C). The resulting hydrostatic pressure diagram is easily drawn (Fig. 6): max (Δp) = γᵥᵥ•H

The total discharge through one interlock is obtained: