Impervious Lining Systems on Waterways
BAW assesses materials and processes
So as not to interrupt shipping traffic, impervious lining systems on waterways are usually installed under water during ongoing operation.
Artificial waterways, like motorways, often follow uneven terrain, leading to shipping traffic's need, as with road traffic, for dams, bridges and occasionally tunnels, too. Furthermore, the water needs to be kept securely in the waterways. They therefore receive impervious lining systems in those sections in which the water level lies above the surrounding terrain. This enables, on the one hand, a limitation of water losses (maximum permissible: 2.5ˇ10-8 m3/s per m2), and on the other, the stability of hydraulic structures, such as dams or canal bridges.
A typical cross-section for today's waterways is trapezoidal or rectangular-trapezoidal. The inclination of their embankments is generally designed with a ratio of 1:3. Likewise characteristic are water depths of 4m and water surface widths of 55m or 48.5m. The protection of banks and bottoms mostly consists of a mineral surface lining, and above that a geo-textile separation layer and an erosion-resistant cover layer consisting of loose or partially grouted armour stones (fig. 1). In addition to purely natural materials such as clay, that have been put to use for centuries on waterways, lining is increasingly accomplished using man-made materials, such as bentonite mats, for example, or specially prepared mixtures of sand, water, clay minerals, cement and other additives. Prior to their first application, new lining materials are subject to basic testing by BAW, so as to determine whether they fulfil the legal requirements of the German Federal Waterways and Shipping Administration (WSV) (e.g. ZTV-W, LB 210).
The security of an impervious lining system is not only dependent on the materials, however, but also on the design of joints and overlaps, for instance. Equally, the connection with constructions such as sheet pile walls must be flawless. The installation method is therefore likewise decisive for the serviceability of the protection system. Since shipping traffic, generally speaking, must not be disrupted during maintenance and enhancement measures, the installation of new linings must most of the time take place under water in the presence of moving traffic. This poses particular challenges; all the working equipment, as well as the installation method itself, must be designed such that high-quality laying can be ensured even under the prevailing visually impaired monitoring conditions that exist under water. BAW assesses this before each initial application of a new installation method, likewise, in the course of basic testing (corresponding to ZTV-W, LB 210).
Clay is frequently the lining material of choice
With its very good sealing characteristics and its high flexibility and erosion resistance, processed natural clay fulfils the demands placed on today's lining materials. In order to limit water loss on waterways, a coefficient of permeability of 1ˇ10-9 m/s is required for a layer thickness of 20cm. In principle, three different methods of underwater installation of clay can be applied to ensure an application of the required quality: the clay slab, the extruded clay and the clay cube method. They vary in the means by which the clay is applied and in the design of joints and overlaps.
In the case of the clay slab method, the natural clay is laid in individual, prefabricated square tiles with sides of around 4m on banks and bottom (fig. 2). The prepared clay is initially positioned on a working pontoon - using a hydraulic shovel in the prescribed thickness of 20 or 30cm in a so-called 'clay bed' - and then compressed. Subsequently, the clay tiles thus fabricated are 'pressed out' using a vacuum bell, which then carries them and lays them with the aid of GPS. In the process, the clay tiles are each left standing out by 10cm and this overlap is subsequently compacted by applying pressure. Checking the imperviousness of the individual clay tiles is carried out in the course of the preceding work step by means of vacuum handling technology, in which the individual tiles are lifted from the clay bed. The WSV has successfully been applying the clay slab method for many years.
With the extruded clay method, the prepared natural clay is pumped through a trapezoidal aperture and deposited on the subgrade (fig. 3). According to this method, the individual 20 or 30cm thick liners of around 1.2m width are laid next to one another without overlapping. Joints are successfully sealed in this process by transverse extension of the liners resulting from longitudinal compression during extrusion. It is therefore vital that the equipment's laying speed is precisely synchronized with the speed of clay transportation, so as to ensure on a continual basis the required bearing pressure, and to avoid mistakes in the lining. The WSV has also been applying this method successfully for many years.
In the case of the clay cube method, prepared natural clay is cut into small cubes with sides of around 8cm (fig. 4) and briefly subjected to a vacuum. This process step is necessary in order to prevent deterioration following the underwater installation. The clay cubes are subsequently placed in the desired layer thickness using a dumping frame on the water surface and compressed by means of a vibrating plate compactor. The advantage of this process is that it results in a largely seamless lining. For subsoil susceptible to vibration or settlement, however, it needs to be verified in advance whether the method is applicable at all, since it may not be possible to properly compact the clay on very soft subsoil. Furthermore, there currently exist no valid basic tests for the clay cube method.
New lining methods earn points
The newer types of impervious lining systems on artificial waterways include geosynthetic clay liners, also known as bentonite mats. They consist of two layers of geosynthetics between which a layer of bentonite (usually sodium bentonite consisting of 75% expandable clay mineral montmorillonite) is encased as a lining. With a layer thickness of only 1cm, bentonite mats are a relatively thin lining element. In order to achieve the same effectiveness as a 20cm thick clay lining, bentonite mats must possess a coefficient of permeability of 5ˇ10-11 m/s at most. At the same time it should be noted that an increase of the coefficient of permeability of one power of ten within two to three years is possible when using sodium bentonite, due to ion exchange. In order to prevent damage to the bentonite mats (for instance due to concentrated loads on the cover layer during installation of the armour stones), the bentonite mats are laid together with a sand mat (fig. 5). The maximum permissible weight of an individual stone is limited to 40kg. The process is not permitted for high canal sections.
In the region of waterway beds, permanently plastic linings, composed of sand, water, clay minerals, cement and further additional materials, likewise developed a few years ago, can also be used instead of natural clay. Prepared in high-speed mixers, this highly flowable lining material is easy to apply using pumps and pipes. The result is a seamless lining in the specifically required layer thickness. Its permeability specifications correspond to those of natural clay. Using the correct mixing ratio of the components (low addition of cement), the material does not harden, but instead forms a permanently plastic lining layer (fig. 6). This innovative technique is not permitted, however, for high canal sections.
Since impervious linings not only restrict water losses but are also of relevance for the stability for canal sections, they are required to be reliable in the long term. This is ensured by several means, in addition to the critical examination and approval of new impervious lining systems and installation techniques by BAW. These include careful planning, procurement and execution of sealing works on artificial waterways, as well as the concomitant construction supervision - including the use of underwater divers - and, in addition, auditable quality assurance.