The invention concerns a construction for protecting a dike.

Furthermore, the invention concerns an element for a construction for protecting a dike.

The invention also concerns a method for providing in a dike a construction for protecting a dike, Since some 60% of the land area of the Netherlands is susceptible to flooding, it is very important that our dam and dike structures are of a good quality. They must be high and strong in order to offer protection against high water levels and to resist the load exerted by high water levels. For the purpose of the Dutch Water Act (Waterwet), the main dam and dike structures are assessed against the applicable standards every six years. These assessments must consider various aspects including the different types of failure mechanism that may result in a dam or dike structure failing during high water (such as overflow, wave overtopping, inner slope slipping, erosion of the outer slope and piping).

Protecting dikes is not a problem that is limited to the Netherlands. There are dikes in lots of countries, for example in the American Mississippi delta, in China, and in many other delta areas.

In the past few years, the perception has been formed that one of the possible failure mechanisms, i.e. "piping", has been underestimated. When piping occurs, groundwater seeps out of the ground inside the dike, at times of high water. If sand is carried along with it and an open connection occurs between the outside water and the hinterland, this is called 'piping'.

There are possibly hundreds of kilometres of primary dam and dike structures in the Netherlands and thousands or even tens of thousands of kilometres elsewhere in the world that do not comply with this new perception. These water retaining structures will have to be strengthened in the future. According to the more conventional methods, these retaining structures can be made more resistant against piping by improving the foreland through clay encasing, the construction of wide banks, or by providing seepage screens.

However, in many situations these known constructions and work methods are expensive and/or cannot be applied. There may also be a risk that, when installing a protection construction, the dike is weakened temporarily, for example because part of the dike has to be dug away or because very heavy equipment has to be used in order to install the protection construction. An objective of the invention is to provide a construction for protecting a dike that is relatively simple and effective and that can be installed such that, compared to other methods, the risks are avoided.

In order to achieve this, a construction for protecting a dike according to the invention is characterized in that the construction for protecting the dike is provided on the inside of the dike and is located in the soil and contains a number of frame-shaped vertically installed elements interconnected by means of profiled coupling sections, with the frame-shaped elements containing a frame in which a geotextile has been suspended.

The conventional constructions for protecting dikes focus on extending the seepage path, such as applying piping banks or providing the foreland with clay encasing. The invention differs from this in that it is based on preventing sand from being transported under the dike without extending the seepage path. The geotextile walls prevent sand from being transported, but they let seepage water pass through.

The element for a construction for protecting a dike is characterised in that it contains a frame in which a geotextile has been suspended and with profiled coupling sections extending from opposite sides of the frame.

The element forms a window screen with profiled coupling sections on both vertical sides.

The profiled coupling sections are preferably shaped such that a movement in the longitudinal direction of a section will cause the sections to slide together, but a transverse movement will not.

The construction for protecting a dike is also called a sheet pile wall and the profiled coupling sections are called sheet pile locks. The sheet pile wall according to the invention forms a water permeable vertical sheet pile wall that lets the seepage water pass through, so that the "normal" route of the seepage water under the dike is not disturbed, but does not let any sand pass through.

Some existing dikes were built decades or even centuries ago. A more or less natural water balance has come into existence in and near the dike and the "natural" seepage path is a part thereof. Disturbing the seepage path may have unintended effects, as the water balance is changed. Extending the seepage path may cause parts in or around the dike becoming wetter (by water accumulation) or dryer. Both effects can have negative consequences, such as dikes or adjoining structures, such as houses or pumping stations, on or near the dike subsiding. The water-permeable sheet pile wall according to the invention stops sand from being transported, such that the negative effects will not occur at all, or will occur to a lesser extent or at a much later stage.

The geotextile preferably has a D90 value of between 0.01 and 1 mm.

The method according to the invention is characterised in that an element in introduced into an introduction frame, the introduction frame is then driven into the soil after which the introduction frame is brought up again and the element stays behind in the soil after which a further element is introduced into the introduction frame and an introduction frame with a further element is driven into the soil such that the profiled coupling sections of the element and the further element engage.

The element can be driven in by means of pushing or vibration. The same or another introduction frame can be used. The introduction frame protects the element while it is being introduced into the soil. Carrying out extensive excavation work on the dike is not necessaiy. The modular character of the elements also enables the water-permeable sheet pile wall to be applied using relatively few means.

An element, or a further element, that has been introduced is preferably fitted with a guide element up to at least ground level or very close to it, which guide element is fitted as an extension to a profiled coupling section and will guide a further or following element such that the profiled section of the further or following element is guided into the complementary profiled section of a previously fitted element.

This provides for a smooth and proper coupling between elements in a simple manner.

The major advantage of using a guide element, for example a guide rail, is that this enables the elements to be coupled together at a distance of, for example, some metres below ground level. The permeable sheet pile wall tlxen extends a few metres below the surface. A layer of soil is then present over the sheet pile wall. Laying cables or installing roads afterwards, will then usually be quite unproblematic.

This method also allows for elements to be stacked vertically or to be applied in a staggered manner. Elements of different lengths or widths can also be coupled together. In embodiments of the method, samples are first taken of the dike to be protected and a geotextile is used that matches properties that match the characteristics measured, especially the grain size of the aquifers. Embodiments may feature variations in the properties of the geotextile and/or the elements of the sheet pile wall.

These and further aspects of the invention are described below and illustrated by means of the drawing:

The figures contained in the drawing show the following:

Figures 1A to IE show different threats to dam and dike structures

Figure 2 shows an embodiment of an element for a construction according to the invention Figure 3 shows some examples of profiled coupling sections

Figure 4 shows a construction behind a dike

Figure 5 shows a first step of the method according to the invention

Figures 6A to 6H show the introduction of elements

Figure 7 shows deflection and force in the geotextile when exposed to an asymmetrical load The figures have not been drawn to scale; as a rule, like numerals denote like elements.

The primary objective of the invention is to prevent or at least greatly reduce piping.

At times of high water, groundwater flows in aquifers under the dike may be strong. This groundwater will find its way up behind the dike, via a deep toe ditch or via a surface layer bursting open. If the seepage flow is sufficiently strong, sand will be taken along to tlie surface. The process of retrogressive erosion has thus started. However, this does not necessarily mean that the dike will fail since it is possible that a balance will occur between the sand suspension in the pipe and the water flowing to the pipe.

Figures 1A to IE illustrate different threats to dam and dike structures.

The water 2 can pass over the dike 1, as illustrated in figure 1A. For example, this takes place when water levels are veiy high. The water may also be blown over the dike by the wind or carried over it by strong waves, as shown in figure IB. The inner slope may slide down as illustrated in figure 1C. Piping may also occur, as shown in figure ID. And finally, the inner slope may erode, as sketched in figure IE.

In the past few years, tlie perception has been formed that one of the possible failure mechanisms, i.e. "piping", has been underestimated. When piping occurs, groundwater seeps out of the ground inside tlie dike, at times of high water. If sand is carried along and an open connection occurs between the outside water and the hinterland, this is called 'piping'. The new and old piping rules focus on this consideration of a balance. If no balance can be found, the erosion process will continue under the dike and a continuous pipe will be formed. The dike will eventually subside and collapse. This is considered as a failure due to piping. The characteristics of the invention as a measure against piping are described below and compared with the characteristics of conventional piping measures,

Sand tight

The operation of the invention as a measure against piping is based on preventing sand from being transported under the dike without extending the seepage path. This is different from conventional techniques that aim to extend the seepage path, such as applying piping banks or providing the foreland with clay encasing, or installing a watertight sheet pile wall.

Water-permeable

Another characteristic of the invention is that it only has a small effect on the groundwater flow pattern. The water permeability of the invention ensures that the symptoms of water backing up during high water on the upstream side of the screen are considerably less than would be the case with watertight systems such as a heave screen or a cut-off screen.

THE ELEMENT ACCORDING TO THE INVENTION

An exemplary embodiment of an element 3 according to the invention is shown in figure 2. It comprises a - preferably steel - frame 4 with profiled coupling sections 6, also called sheet pile locks, on both vertical sides in which a geotextile 5, preferably woven in three directions, has been suspended. This "diamond weave" ensures that there is tensile strength in all directions and undesired openings due to deformations are ruled out or decreased. The preferred combination of proven steel sheet pile locks, a strong steel frame and a robust geotextile, developed specifically for hydraulic engineering, yields a high-quality end product. For certain applications, other materials, such as plastic or composite materials, can also be used for the frame 4 or the sheet pile locks 6.

Figure 3 shows a cross-section of some profiled coupling sections for the sheet pile locks 6. The upper sections can be slid together by both a vertical and a horizontal movement. The lower two sections, including the dovetail, are slid together in a longitudinal direction. These lower two pairs of profiled sections are interlocking, i.e. a movement in a transverse direction to the profiled section will not cause the profiled sections to become detached from each other. The profiled sections are preferably interlocking. Figure 4 illustrates a construction according to the invention that has been installed.

A construction for protecting a dike according to the invention has been applied to the inside of the dam or dike structure 1. This construction contains elements, each with a geotextile 3, that are interconnected and interlocked by means of the profiled coupling sections 6a and 6b.

The arrow W indicates the direction of the seepage water. The seepage water passes through the water-permeable construction, via the geotextiles 3. However, the sand is stopped.

The construction is applied at a distance D below the soil surface, for example at between 1 to 5 metres below the soil surface. In this example, all the elements have the same length and they have all been installed at the same depths, so that the tops and bottoms of the elements are in a straight line. The elements also form a straight line. Differences in height may be effected in embodiments by driving some elements deeper or less deep into the soil. For example, if it is known that a pipeline will be installed in a certain location, the relevant element in this location may be driven more deeply into the soil. And the profiled coupling sections can also be constructed so that screens can be placed at a certain angle to each other, for example an angle of 5 to 10 degrees. This will enable the construction to accurately follow curves in the dam or dike structure.

Figure 5 illustrates a first step of introducing an element into the soil.

An element 3 provided with profiled coupling sections 6a and 6b is inserted into introduction frame 7. It should be noted that, strictly speaking, the outer elements of a sheet pile wall do not have to be provided with outer profiled coupling sections. A guide element 8 is installed on the profiled section 6b. This guide element extends along the direction of profiled section 6b and has roughly the same profile. It is an extension of the profiled section 6b. The guide element is a lock lengthener, it lengthens the lock 6b. The assembly of introduction frame, element 3 and guide element 8 is driven into the soil.

When introducing the construction, the element is introduced into the - preferably steel - introduction frame 7 to minimise the risk of damage while being introduced. While introducing, there is no contact between the soil and the geotextile whatsoever. Once the steel introduction frame has reached the right depth, it is pulled up again in a controlled manner, with soil flowing in from the surroundings, embedding the window screen on both sides. Since the geotextile has been fitted in the centre of the steel window frame, the contact between the steel introduction frame and the geotextile is minimised.

The introduction frame is preferably constructed of two parallel, stiffened plates that are interconnected along their entire length on one side. The introduction frame is supported by the sturdy steel edge of the element 3 on the other side, the open side of the "U". A permanent bottom plate with a small adhesive breaker 9 that extends two centimetres, has preferably been fitted at the bottom. This provides a simple construction that has, on the one hand, sufficient stiffness and, on the other, results in a minimum of soil being displaced.

A major advantage of this construction is that panels can be replaced. Depending on the introduction depth chosen, measured from the top, damaged panels can be pulled out and replaced.

Figures 6 A to 6H sketch the application of elements 3, hereafter also referred to as window screens.

Method

The introduction process consists of the following steps:

1. Laying a guide frame on the ground level for guiding the introduction frame, see Figure 6A;

2. Placing (inserting by sliding sideways) the window screen 3 (i.e. the element) into the introduction frame 7 in vertical position with lock lengthener 8;

3. Optionally sealing the bottom with a bottom panel 9 (this may already be an integrated part of the window screen);

4. Manoeuvring the introduction frame such that the sheet pile lock engages with the sheet pile lock of the previous window screen;

5. Driving in the introduction frame, e.g. by vibration (figures 6C and 6D);

6. Pulling the steel introduction frame 7 in a controlled manner while monitoring that the window screen 3 that has been installed in this work cycle stays at the right depth (Figure 6E), In some embodiments, the window screens are kept in position by means of the lock lengtheners 8.

7. Repeating the actions, using lock lengthener 8 to guide a profiled section of a next element to be installed into the profiled section of an element that has already been installed.

8. Removing lock lengthener 8 if this has been installed.

9. Possibly covering the top with clay if the sheet pile wall has been sunk below ground level.

Material

The geotextile is, for example, of the type SoilTain, type 175/175 DW A30. This type of geotextile has high strength, shearing stiffness and permeability with a relatively small mesh width. The geotextile preferably complies with one or both of the following requirements.

• O ₉₀ < D90 to retain the grain skeleton (preventing a pipe);

• x sand to guarantee water permeability and prevent clogging;

• 0.01 mm D ₉ o≤l ^|1im

An example of an applicable geotextile of the type Soiltain has the following characteristics:

Strength = 175 kN/m,

K _tert ito = 13.10 ^J m/s

D ₉ o = 0.1 mm

Equipment

To introduce an element, a high-frequency hammer-type vibrator with a variable torque is used, for example. Such a vibrator minimises any nuisance for the surroundings. The hammer-type vibrator is guided vertically by means of a kingpost on a foundation machine

The ground pressure under the foundation machine is restricted to 20 kN/m ² by using dragline planks.

The equipment chosen ensures a high reliability of vertical installation of the window screen, which contributes to minimising the shear resistance in the lock construction.

End result

Installing the construction according to the invention results in a continuous wall that is preferably only applied where it is functional, i.e. the aquifers. The installation and long-term operation of the screen will be monitored both while and after installing the invention.

FURTHER REMARKS

While driving in the window screen using a vibrator, there is no contact between the geotextile and the surrounding soil. The window screen is sunk, as it were, into a mini, water-filled room. The geotextile is thus introduced such that it is virtually free from stresses.

A type AZ or similar guide lock is provided between the elements. Since the "free" sheet pile lock is free from soil while introducing the fiame, there is no or hardly any friction on the lock. As a result, there is little risk of the profiled sections slipping out of the lock. If desired, lock feedback sensors may be used to monitor the connection by means of the lock construction. A strong, steel introduction frame is preferably used to install the construction.

If the right depth cannot be reached, different measures are possible in order to achieve improved embodiments:

· The introduction frame has spray lances. Low-pressure water can be supplied to lower the introduction resistance so that the desired depth is reached.

* The soil at the introduction position can be turned over to loosen it in advance.

While pulling the installation frame, a situation may occur where the window screen does not come loose. The window screen that has just been introduced might then be pulled up a bit as well, something which can be detected visually by the lock lengthener at ground level. If this occurs, a pressure force is applied to the window screen via the lock lengthener to prevent dislocation.

A second effect that might occur is that the "stationary" geotextile touches the introduction frame as it is moving up. While pulling the introduction frame up, sand will flow in from the sides at the bottom, filling the cavity under the pulled introduction frame. There is a risk that this does not occur exactly symmetrically. The difference in pressure between the two sides is expected to be a maximum of 10 kN/m ² . When this load is applied, the expected deflection of the geotextile is two centimetres. In an extreme case, the geotextile will just clear or just touch the introduction frame. The inside of the introduction frame has been finished smoothly and the corners are rounded, thus minimising the risk of damage.

Asymmetric embedding occurs while pulling up the introduction frame, but also when a downstream piping channel (starting at the exit point) has reached the sand stopper. Since little information is known about the load, an opposite approach was taken. The load that the geotextile can absorb was assessed. The risk of this load occurring was then estimated based on engineering judgement.

The graph shown in figure 7 is based on a curved cable comparison and shows a calculation of the deflection and force in the geotextile in the event of an asymmetric, evenly distributed load being applied. It has been assumed in the calculation that the geotextile is initially tensioned at a force of 20 kN/m ¹ .

It is clear that the geotextile chosen can absorb major asymmetric loads of more than 60 kN/m ² . The safety as regards tensile strength is more than a factor 2, which is amply sufficient. This high strength guarantees that, even in the event of major scouring downstream of the screen, the upstream sand will be retained.

Introducing the system such that soil is displaced results in some compaction, causing water overpressure. If this water overpressure is confined to the general dimensions of the panel and is of a short duration, given the time needed to introduce a next panel, this will not affect the stability of the system. If this is not the case, the stability of any panels that have already been installed can be affected by a next panel, resulting in subsidence.

To reduce this risk, water pressure gauges are preferably installed on site to monitor and measure the possible water overpressure while passing the frame. Regulating the installation pace enables the zone along which water overpressure occurs to be restricted to the panel width as a maximum. Perpendicularly to the panel, and given the panel thickness, the zone where water overpressure occurs will be restricted to a maximum of one metre from the screen. Damage may be ruled out by measuring and anticipating it, where necessary by adjusting the progress speed. The number of water pressure gauges to be installed will need to be determined in more detail for every sub-location.

Introducing the introduction frame in a manner that displaces soil will influence the pore volume, something which might temporarily lower the permeability very locally. Since the volume of the window screen is highly limited, relaxation of the grain skeleton will occur, as a result of which the equivalent permeability will slowly be restored to the original basis.

Tests, for example cone dissipation tests, can be performed to assess whether there is any significant effect before and after installation.

Soil subsiding

Piping up to the screen may be accompanied by some deformation of the soil over the pipe. Deformations that are caused by a cover layer subsiding due to piping downstream of the invention must not negatively affect the functioning of the invention.

The use of steel sheet pile locks, a strong steel frame and a high-strength geotextile that has been suspended properly, make the screen insusceptible to vertical soil displacement. Local sliding surfaces

The design of a solution shall always consider the effect of the initial stages of piping on the dike's macro-stability. Therefore, the location of the construction in the dike profile is an important part of the design. Due to the modular character of the construction according to the invention and the great liberty to choose and control the position, shape, and the depth location of the elements of the construction, the invention is ideally suitable for this. The construction according to the invention provides a strong screen with robust connections that can absorb both vertical and horizontal deformations in the covering clay layers without the permeable sheet pi le wall of the invention losing its sand stopper function. The sand stopper can also be made suitable for vibrating or pushing the screen to a greater depth under the ground level. This is a major advantage if it has to be possible to absorb major displacements in the overhead bank.

In addition to blocking and clogging, the functional characteristics of the screen can also be influenced by root growth or chemical compounds. Since the policy in place is often that vegetation other than grass must be banned from dikes, there is no risk of root growth. Chemical compounds, and specifically iron compounds, are known to be able to negatively affect the functioning of horizontal drains to a considerable extent. However, a condition for iron deposits is an aerobic environment. Tn cases where the groundwater level is always higher than the upper side of the screen, the chemical compounds identified are not expected to occur.

The design takes into account a decrease in permeability during the lifetime of the sand stopper. If an inspection reveals that the permeability of the sand stopper has decreased too much, the screens can be replaced.

Muskrats usually dig holes to a depth of some 50 cm below ground level. The depth location of the upper side of the sand stopper is some metres below ground level. The danger of the geotextile being eaten by muskrats or other pests is negligible.

The strength of the geotextile is preferably assessed by having tensile strength tests performed by an accredited laboratory. A "trampoline" test is preferably performed on the connection between the geotextile and the steel frame, where a load of, for example, 20 kN/m ² in the flow direction is applied to the geotextile that has been installed.

One or several measuring wires can be woven into the geotextile to enable the integrity of the geotextile that has been installed to be measured. We consider the risk of window screens slipping out of the locks to be negligible. If desired, 'interlocking' can be monitored using lock feedback sensors.

It will be clear that the invention enables many variations and that the invention is not limited to the examples described above.

In summary, the invention can be described as follows:

A water-permeable, sand-stopping sheet pile wall is applied to the inside of a dike. The sheet pile wall is constructed modularly of elements with a frame containing a suspended geotextile and with profiled coupling sections on two vertical sides. The elements are driven into the soil using an introduction frame with the effect that the profiled coupling sections engage with each other.