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Vinyl sheet piles have emerged as a compelling alternative to traditional steel sheet piles, offering significant advantages in terms of corrosion resistance, durability, and environmental friendliness. However, one lingering concern among engineers is their suitability for installation in demanding ground conditions. This article aims to dispel such myths and shed some light on the topic of vinyl sheet pile installation in challenging environments.

From our perspective as one of the worlds’ largest manufacturer of vinyl sheet piles, when confronted with installation concerns addressed by our clients, we noticed that most often, they usually fall into two categories:

Myth: Vinyl Sheet Piles Are Confined to Shallow Depths and Soft Soils

A prevalent misconception surrounding vinyl sheet piles is their perceived limitation to shallow depths and soft soils. This misconception is rooted in older installation techniques that did not utilize steel guides (mandrels), leaving vinyl sheet piles vulnerable to damage in hard-to-penetrate soils. Without a guide, vinyl sheet piles are indeed susceptible to cracking and bending, especially in soils that offer a lot of resistance. The introduction of mandrels helped reduce the risk of damage to the vinyl sheet pile during installation, even at depths of over 10m.

Limited Knowledge: Unveiling the Arsenal of Installation Tools

Another prevalent hindrance to utilizing vinyl sheet piles in difficult ground conditions is a lack of awareness regarding the vast array of specialized equipment and accessories available. From hydraulic hammers to jetting systems, these tools, when employed judiciously, can transform vinyl sheet pile installation into a smooth and efficient process, even in the face of stubborn soil formations.

This article aims to dispel such myths and shed some light on the topic of vinyl sheet pile installation in challenging environments.

Vinyl sheet piles installation: Essential Equipment and Accessories

Vinyl sheet piles are installed using a combination of machinery and specialized accessories. The main machines involved are excavators, cranes, and pile drivers. Each of these machines can be fitted with a vibratory hammer, used for the precise driving of sheet piles into the soil. Each such hammer is compatible with the steel guide facilitating the installation.

The use of a guide is highly recommended by the Pietrucha Group for most vinyl sheet pile projects. This guide acts as a cutting edge, penetrating the soil and protecting the sheet pile from damage. Custom-made for Pietrucha's profiles, each guide must be matched to the corresponding profile to ensure proper alignment. Each profile will require a different steel guide.

How about when profiles with varying lengths are to be used within one project? Do we need several mandrels for such project? We have come up with a practical solution to enable using the same steel guide for installing profiles with different lengths, i.e. the "Slider" accessory which is temporarily mounted on the guide at the appropriate height to ensure that the upper section of the sheet pile fits against it, preventing upward movement during hammering. This allows for the use of a single steel guide for sheet piles with lengths ranging from 12 meters and under.

The upper part of the guide, the head, is the most massive element of the mandrel. It consists of plates with a thickness of at least 30mm, which are welded together in the appropriate places so that the force from the driving machine is transferred as effectively as possible to the knife of the mandrel, which in turn installs the sheet piles. It is important to note that the head of the guide (its dimensions and shape) is also made to fit to the equipment used by the contractor, depending on the type of vibratory hammer.

The vast majority of projects for which we provide 8m long vinyl sheet piles require the use of basic equipment, such as a CAT 320 excavator and a vibratory hammer weighing from 0.5T to 1.5T. But what about when the soils are cohesive soils of low plasticity or sands of high compaction, gravel content, and the sheet piles to be installed are longer than 8m? In this first part of our article, we will focus on projects where vinyl sheet piles are driven in cohesive soils and attempt to explain how we overcome the challenges that may arise in such installation environment.

Cohesive soils

Cohesive soils of low plasticity pose a major challenge for the installation of both steel sheet piles and vinyl sheet piles. This is mainly due to their very high unit weight and the presence of fine particles, which makes them less susceptible to deformation, also due to the cohesion force. When introducing a "foreign body" such as a sheet pile into a very stiff or hard soil, we will not experience the effect of the particles of the soil moving to make room for it. The easiest way to solve this problem is definitely the technique of pre-drilling, which consists in using a auger drill in the spot where the sheet pile would later be driven. The pre-drilling technology is widely recognized when it comes to steel sheet piles, however not everyone knows that it may also be equally successfully applied to install vinyl sheet piles. One must however remember that the above method may change the geotechnical conditions locally, which may affect the behavior of the retaining or a cut-off wall or any structure nearby.

Pre-drilling is also very effective in the case of mixed geology, where alternating hard and cohesive soils and compacted sandy soils are present. This allows us to solve the problem of mixed soils, for which we need different mounting approaches. Pre-drilling will be a single and reliable method in such cases. Pre-drilling is also very efficient in the case of presence of coarse-grained fractions in the cohesive soil, such as gravel or even stones.

When it comes to our experience with cohesive soils, we may provide several examples which proved that with the right equipment and approach, it is possible to install sheet piles in the most demanding soil conditions. In the following part of this article, we will move on to discuss two case studies carried out recently.

Case study: Tailings dam in Kazakhstan

In the case of the project carried out in Kazakhstan, vinyl sheet piles GW-610/7.2 and GW-610/9 (wall thickness of 7.2 and 9 mm, respectively) were fully driven into the ground to form a 9 kilometers long cut-off wall around a Tailings Dam. The length of the sheet piles ranged from 6 to 11 m. The structure made of vinyl sheet piles was to form a tight barrier in the ground protecting the surrounding area against the seepage of toxic waste stored in the reservoir, which could have occurred in the layer of permeable soil formed with a layer of sandy silt and very dense silty sands. The solution to the problem was to drive vinyl sheet piles with gaskets to the depth of 1 m into the impremeable layer of fat clay. The task was completed without major problems, thanks to the following factors:

1. Installation equipment: The sheet piles were installed using a heavy piling rig equipped with a 4 ton vibratory hammer.
2. Specially designed, double, reinforced steel guide: Engineers from the Pietrucha Group, after careful analysis of the soil conditions, came to the conclusion that a regular mandrel may not be sufficient in this case. That is why, in Kazakhstan a special type of guide was used and this solution has later become our standard recommendation for the most demanding projects. The special mandrel used in Kazakhstan had two knives, with the first one cutting a preliminary path and loosening the soil for the sheet pile that will be driven in the next hammering operation. The vinyl profiles are attached to second knife which is used for driving vinyl sheet piles, just like a regular mandrel.

Another improvement to the guide is the fact that depending on the conditions, it could be made of  steel with much greater strength, making it stiffer and less prone to deformation during installation, for which enormous forces from the piling rig and vibratory hammer are generated.

This later aspect was particularly instrumental when it comes to the efficiency of installation in such difficult soil conditions. What is interesting, another company attempted to drive steel sheet piles in this place. Steel sheet piles with a wall thickness of 7 mm proved to be impossible to be driven in these soil conditions, as the sandy silt and very dense silty sands and fat clay posed too much resistance to successfully install these relatively thin steel profiles. It would have probably been possible to drive steel sheet piles with a thickness of 12-13 mm, however, the cost of such a solution would have exceeded the investor's budget by several times. To us, the most important was that the reinforced guide made of high-strength steel made it possible to install vinyl sheet piles without any problems.

To verify that the vinyl sheet piles were not damaged during installation, which could result in seepage of dangerous substance through the barrier, the investor performed cyclical tightness tests of the wall made of vinyl sheet piles. Designated sections of the wall were unearthed on both sides and one side was filled with dyed water. No leakage was detected on the other side of the wall, which was the proof that the vinyl sheet piles were not damaged in any way during installation.

Case study: a flood embankment in the Netherlands

In the Netherlands, just as in the case of the Kazakhstan project, vinyl sheet piles with a maximum length of 11.5 m were also fully driven in the ground. The tight wall made of vinyl sheet piles was to prevent against the hydraulic failure of the embankment as a result of rapid rise of the water level at the Waal River. In the case of this project, the greatest installation challenge were extremely variable geological conditions.

The flood embankments in this part of the Netherlands were built several decades ago, using any type of soil available at that time. As a result, the embankments were formed with a mixture of cohesive clays, sands, rocks and often anthropogenic material. The variability of geology was proven by CPT, according to which, at some depths, the Qc cone resistance exceeded 40 MPa, only to immediately fall down to a value below 5 MPa. That is why, despite its massive construction, the flood embankment was susceptible to damage triggered by water internal erosion and the formation of filtration paths.

The solution to the demanding geology was to use the pre-drilling method to disturb the soil before the proper installation of vinyl sheet piles using a double guide made of reinforced steel. The installation was carried out using a crane and a 3-ton vibratory hammer.

Non-cohesive soils

Having discussed the installation of vinyl sheet piles in cohesive environment, let us now move to the second part of this article, dedicated to non-cohesive soils.

Sandy and gravelly soils have a high angle of internal friction which makes them prone to compacting when exposed to such external forces as for example vibrations generated by a vibratory hammer.  This phenomenon makes non-cohesive soils equally challenging when it comes to the installation of sheet piles.

Based on our field experience, we learned what methods and techniques may be used to approach such challenges. First of all, we observed that in the case of non-cohesive soils, the equipment used for the installation plays a very important role. That is why, we most often recommend a piling rig and a massive vibratory hammer as the most efficient installation equipment.

Another important aspect of successful installation are the operator's skills. In the case of non-cohesive soil, it is more efficient to use higher amplitudes and lower vibration frequencies of the vibratory hammer.

It is worth noting that in the case of non-cohesive soils, the pre-drilling method does not always work. Due to the tendency of non-cohesive soils to compact under the influence of vibrations, pre-drilling may not bring the expected results. It is also important to remember that in the case of non-cohesive soils, pre-drilling can drastically change the geotechnical conditions and facilitate the creation of  filtration paths around the sheet pile structure for the ground water.

One of techniques to deal with compacted non-cohesive soil, in which, for additional difficulty, gravel fractions, or even larger stones, may be present, seems to be Water Jetting, i.e. the technology usually associated with steel sheet piles. This technique uses water jets when driving sheet piles into the ground. The water flushes out the sandy and gravel fractions from under the sheet pile, clearing the path for the profile.

When using water jetting technique, in the case of steel sheet piles, the water nozzles are welded to each sheet pile. In the case of vinyl sheet piles, the water nozzles are welded to a steel guide. This fact may be perceived as a huge competitive advantage on the part of vinyl sheet piles since it significantly saves time and costs of construction, as a single steel guide is usually sufficient to drive several hundred vinyl sheet piles. In the case of steel sheet piles, a single nozzle must be welded to each profile.

Case Study: the most difficult installation we have faced in the history of our company.

Installation in sand with nSPT parameter of 45!

The described example was a field test carried out as part of a R&D project intended to prove the functional efficiency of hybrid sheet piles as retaining structures. The field test consisted of installing 4-8 meter hybrid sheet piles in sandy soil, which formed a retaining wall after they were gradually unearthed on one side. The behavior of the retaining wall was measured under load which gradually increased using different methods. The installation was carried out in sandy soil, which consisted of very dense medium- and fine-grained sands. According to the dynamic probe test, the probe read 55 blows of the DPSH hammer in the most dense layer, which in the case of this type of soil translates to about 45 blows with the SPT probe.

As mentioned above, we already knew that the most suitable equipment for this installation would be a piling rig and a massive vibratory hammer. The steel guide used in this project was made of reinforced steel. Using this equipment we were able to successfully install the sheet piles to the required level. It must however be admitted that the speed of the installation was significantly lower than initially expected.

Summary

In the light of the case studies and detailed technological analyses presented, our article sheds new light on the vinyl sheet piles potential and versatility, also when it comes to difficult ground conditions. Our strong track record of successful installations breaks the myth that the application of vinyl sheet piles technology is limited only to easy soils. We gathered enough evidence proving that the appropriate configuration of installation equipment, an open-minded approach and specialized accessories, such as guides and sliders, open up new possibilities in civil engineering.

The use of reinforced guides, techniques such as pre-drilling and Water Jetting, and the use of equipment adapted to the specific soil, as shown in projects in Kazakhstan and the Netherlands, proves that vinyl sheet piles can be effectively and safely installed even in the most demanding conditions. Additionally, these cases highlight the importance of experience and technical knowledge in the implementation of complex projects.