In general
Currently, a large number of infrastructure works are in the design and implementation stages in the Netherlands. In many cases, sheet pile structures must be used. The impermeability of sheet piles may have to meet specific requirements depending on the application and ambient factors. Tender documents must include specifications for impermeability requirements to realise this (also see CUR 166).

It is very important to create a clear and unambiguous formulation for the impermeability requirements that a sheet pile structure must meet. The recently published EN 12063 European standard is a good manual for the description of impermeability problems.

It is, of course, also important how you can meet the set impermeability requirements.

General specifications
If no special solutions (such as steel sheet piles) are asked, there are two options with regard to the requirements for the impermeability of a sheet pile structure:

- A maximum value of the permeability coefficient k (Darcy`s law) (m/s) in combination with a minimum thickness of the wall (m).
- A maximum ground water flow rate through the wall (m3/s).

The apparently simple method for setting a limit on the amount of water that may pass the wall appears to be very tempting. However, we must always keep in mind that water will always choose the easiest path. A single defect in the complex system blocking the groundwater vertically and horizontally, as well as the connection between the two, could result in significant leakage problems. This could lead to problems in attributing responsibilities to the various parties involved in the design and execution of the construction.

Leaks through steel sheet pile structures
A leak through a sheet pile construction has a different nature to the flow of water through a porous medium such as soil or concrete. The steel itself is naturally fully impermeable and the only way for water to leak is through the interlocks that connect the sheet pile sections to each other. This is shown in Figure 1.

Figure 1: Comparison of leakage between a porous medium and a sheet pile wall. 

The concept of the so-called inverse joint resistance (m/s) was therefore introduced in Annex E of the EN 12063 European standard. This is a factor of proportionality between the flow through the interlock of a sheet pile and the water pressure (see figure 2).

This provides: 
• The flow rate per unit of length of the interlock at depth z (m3/s/m) 
• The difference in water pressure over the sheet pile at depth z (kPa) 
• The inverse joint resistance (m/s) (indicated as [r] below) 
• The bulk density of water (kN/m3

Figure 2: Cross-section of a sheet pile parallel to the direction of the groundwater flow via the sheet pile. It is assumed that the groundwater level is constant and that there is a hydrostatic gradient. Numeric calculation methods must be used for more complicated cases, for instance, for extremely high flow speeds, groundwater flow around the bottom of the sheet pile, etc. 
Laboratory and in situ tests have proven that, in analogy to the coefficient of permeability of soil, the value of [r] displays significant variations.
The value of the inverse joint resistance [r] can significantly be reduced by using interlock sealants.
By using a bituminous filler (e.g. Wadit), a significant reduction in the leakage flow is already achieved.
If the impermeability is subject to very high requirements, a water-swelling agent or a polyurethane sealant can be used. The great advantage of water-swelling agents is that they expand after coming into contact with the groundwater. This ensures that a high degree of impermeability can be achieved. A major drawback is that it is to be implemented continually because the material swells on contact with water. A second disadvantage is that both systems are quite costly in comparison with a Wadit interlock filling.

Full impermeability can only be achieved by welding the interlocks shut. 
Setting exaggeratedly high requirements on the impermeability of a sheet pile automatically increases the price. On the other hand, if the requirements are not strict enough, greater leakage flows than expected could result. This could lead to significant repair and extra drainage costs which could far exceed any original savings gained by not using an interlock sealant. 
This stresses the importance of specifying the impermeability requirements of a sheet pile construction clearly and unambiguously and to handle the material extremely carefully during the implementation.

Specifications for steel sheet pile structures
If sheet piling is to be used for an earth-retaining structure, the impermeability requirement can be specified as follows:
The maximum allowable value for the inverse joint resistance in m/s in combination with the minimum distance in metres between consecutive pile interlocks. It is assumed that the intermediate interlocks have been made fully impermeable.
Various suppliers are able to provide values for the inverse joint resistance of different interlock sealing methods. The values are based on thorough and extensive research. The r values distilled from this research are characteristic values. The choice of design values are to be based on uncertainties concerning the implementation, ground, etc., and on the risk connected to the failure of the system for the building project in question.

Wadit: the perfect interlock sealant

Leakage in any water-retaining sheet pile wall is highly unwelcome. If you fill the sheet pile interlock with Wadit interlock sealant, you will be selecting a very good and cost-effective solution.
Wadit is a bitumen type sealant that is poured at a high temperature in the sheet pile interlocks. Wadit has a very strong bond and an extremely high flexibility including at low temperatures with its (rubber) elastomer modified with styrene-butadiene. Wadit remains plastic after applying up to approx. -20 °C. Above 0 °C, Wadit can be applied without additives while under 0 °C, the flexibility is improved by adding Wadit Flex.
The flexibility of the interlock sealant is of crucial importance for permanent sealing. Every sheet pile will move within specific margins during the use phase. As long as the interlock sealant can continue to follow these movements, the sheet pile wall will continue to be watertight. That is why flexibility (including at high temperatures) and adhesion are two important requirements to be met by the interlock sealant.

Wadit is suitable for:
- Improving the impermeability of steel sheet piles
- Keeping soil away from the piling interlocks so that they heat-up less quickly;
- Keeping soil out of the pile interlocks so that they burn less quickly.

Environmentally friendly
The base materials of Wadit consist of natural, raw materials. Test reports from the German environmental institute LGA conclude that Wadit can be used in groundwater and surface water areas without negatively effecting the environment.

Processing and storage
The Wadit interlock sealant can be heated in full with the environmentally friendly packaging up to approx. 130 – 170 °C. The packaging will dissolve up to 170 °C completely without leaving any residue. Next, pour Wadit in the interlocks using a zinc pouring receptacle by preference. The 25 kg bags must be stored in a closed room at a maximum temperature of 30 °C.

Download here the specifications of the Wadit interlock sealant.

Hoesch PU sealing system

The Hoesch polyurethane sealant system is a highly effective sealant. This machine-profiled seal is applied mechanically to the unused pile interlock and is injected into the middle interlock of the double sheet pile sections. The use of a primer ensures an excellent bond between the sheet pile section and the seal.

The material of the seal is polyurethane. This sealant is resistant to weather and aging, it remains elastic after application and is resistant to (salt) water, mineral oils and many acids. HSP Hoesch Spundwand und Profil has extensive experience in the applicability of this sealant and various contaminated environments, and is happy to advise you on specific situations. 

Download here the data of the Hoesch PU sealant system