Sheet piles keep feet dry along the river Waal

The section of dyke in question covers more than 23 kilometres and runs from the Dalemsewal in Gorinchem to the A2 near Waardenburg. This area includes not only the provinces of Gelderland and South Holland, but also the municipalities of Gorinchem and West Betuwe, as well as two regions of Rijkswaterstaat – West-Netherlands South and East-Netherlands. With the majestic Waal as the outside water, this stretch also has a special connection with the Nieuwe Hollandse Waterlinie, a UNESCO World Heritage Site.

Challenges and objectives

The dyke section failed the safety test due to failure mechanisms such as height, inward and outward stability, and piping. These rejected dyke sections are often in the vicinity of urban areas, with buildings close to or at some short distance from the dyke. Of particular importance is the section along the Vuren fortress, where the dyke is directly adjacent to the fortress canal.

Problems with dykes

A : Overflow and wave overtopping (height)
If a dyke is not high enough, water can flow right over it. This flowing water erodes the inner side of the dyke, and prolonged exposure to this over-flow can cause the dyke to collapse. It’s a similar concept for wave overtopping – although the dyke is high enough to hold back the water level, the waves skip over it. This skipping water can lead to erosion of the inner side of the dyke.

B : Seepage
Water can and almost always will slowly seep through and under the dyke. We call this seepage. Behind the dyke, this water must then be drained off by means of a so-called seepage ditch. Seepage becomes a problem for the strength of the dyke if too much sand is carried along with the seepage. The dyke is then slowly eroded from the inside and could collapse.

C : Inward stability
If high water levels hit the dyke for a long time, the water will slowly start to penetrate the dyke. The entire dyke then becomes saturated with water on the inside. As a result, the dyke can start to slip on the inside. The inner embankment of the dyke is then not firm nor stable enough enymore.

D : Outward stability
As with inward stability, high water levels can cause water to slowly start to penetrate the dyke. Eventually, the dyke becomes saturated with water, and becomes extra heavy and less stable. If the water level in the river then suddenly drops too quickly, the water will slowly start to drain out of the dyke, causing the dyke to slide outwards

Possible solutions

A : Adding soil inward & B : Adding soil outward
In both cases, a lot of earthmoving is required, which results in a much wider dyke. The disadvantage of this is that the land on either side of the dyke has a different purpose. On the river side, this land is usually used as floodplains which give space for water at high water levels. These floodplains are commonly areas designated for nature. The space on the landside of the dyke has an agricultural or residential function. Often, the required land will be privately owned, and buy-out arrangements would then need to apply.

C : Construction
In areas where it is not desirable or there is no room for dyke widening, the dyke is reinforced with a steel structure, usually in the form of sheet piling. In this case, the dyke keeps its original dimensions whilst being strengthened.

Location

The dyke of the river Waal between Gorinchem and Waardenburg

Equipment

PVE 35VM (2x) with PVE 1000 Stage V (2), Wolrman 55SPP with PVE 4240Z-700

Works

26 Km of sheet piles for reinforcement of the dyke

Contractor

Gebr. de Koning in subcontracting for Aannemerscombinatie Waalensemble v.o.f.

The approach

The sheet piles were vibrated into the ground using a Variable Moment Vibratory Hammer. A VM Vibratory Hammer was chosen because it causes less vibrations than a normal-frequency vibrating block, which is advantageous in the vicinity of houses. In cases where the soil or foundation was too dense and the sheet piling did not reach the correct depth, a switch was made to additional fluidisation. In places where vibration levels remained a concern, we switched to a pressing machine to press the sheet piles into the ground.