Repairing sheet piling with facing plates
Sheet steel piling walls erected in bodies of water are especially prone to corrosion in the splash zone, i.e. in the transition area from air to water, and particularly in salty tidal water. This area usually comprises +1 metre to -2 metres above or below sea level.
Ultrasonic measurements of wall thickness have indicated that in cases where the wall has corroded in the splash zone, the remainder of the wall - and particularly the sections that are constantly submerged or are below the mudline - generally displays a rusting rate of between 5 and 20 per cent of the original wall thickness.
Thanks to the development of underwater welding techniques over the past ten years, it is now not only possible to stop corrosion by means of patching or using facing plates with concrete-filled cavities, but the statics of the sheet piling can even be reproduced to a large extent by patching using the appropriate steel profiles and sheet steel panels combined with welding seams where a = 6mm.
Repairing sheet piling walls with facing plates is considerably less expensive than driving in new wall sections. Repair work does not, as a rule, require the layout to be changed, e.g. the access points or any waterfront structures supported by the wall.
Current studies are examining the feasibility of reinforcing sheet pilings which do not yet require overhauling, but are subject to increased corrosion.
Another advantage of repairing with facing plates is the fact that access and use of the docks is not restricted while work is being performed. Targeted planning and co-ordinated job assignment permit the renovation work to be split into several smaller phases. Appropriate armouring of the pier also means that work can be performed even while ships are landed.
Chronological sequence of renovation work
1) Preparatory work and measures
The sheet piling is cleaned and all rust is removed using a high-pressure water jet around the affected area plus approx. 10 cm all round. The water pressure should be at least 240 bar in order to obtain a clean metal surface. On the surface, work can be performed using a recoil lance from a stable raft or similar vehicle and wearing protective goggles and suits. The underwater area is blasted by a diver using a recoilless water jet. If necessary, the welding seam area should be treated using a sand blast nozzle or an air/hydraulic flex.
Parallel to cleaning, a template which takes the 30° angle into consideration is produced for cutting out the bottom plates; the new sheet panels are ordered on the basis of this template. When the sheet steel panels, sectional steel and brackets are delivered, they are prepared for installation as follows:
The spacers (T 80) are fitted as required along the entire length. At the top end of the gangway, a groove is burned out and the lower end of the gangway angled at 30° to allow the bottom plate to be inserted at this angle at a later stage.
Brackets are attached to the inside of the facing plates. To allow them to be positioned and secured evenly and swiftly, a light L-steel template (approx. 30x3 cm) is produced. The brackets are then welded all round to the plates.
The bottom plates are delivered ready for installation. A drop retainer with a prescribed inclination of 30° is also made of L-steel for smooth and swift installation. Corresponding long hooked panels are burned out for the installation of the spacers and panels. Secured to the sheet piling, they act as buttresses for wedging on the components referred to above.
Securing the brackets in the sheet piling trenches is facilitated by a template in the form of a double dagger made of L-steel. This means only the position of the first couple needs to be marked.
2) Sealing points rusted through and minor holes
Such damage usually occurs on the flank, but it is often to be found as far as the back of the planking. 4-mm sheet steel panels are laid and secured so that they cover the points in question. They act as a new casing and prevent the concrete from flowing off in an uncontrolled manner into the cavities behind the sheet piling.
3) Replacing severely damaged planking
Any planking that is severely deformed or destroyed has to be replaced with matching U-profiles positioned to overlap. To date, we have only experienced this degree of damage in Larsen profiles where the locks are positioned sideways in the trenches. Work is started by burning off any projections incurred by deformation; then the U-profiles are positioned. In order to press the flange ends flush onto the locks, the following auxiliary materials are used:
Two M20 threaded rods with flat steel welded onto one end are welded into the trenches to the left and right of the extrusion point each with a drop seam.
One U-80 profile 700 mm in length and equipped with two elongated holes is slid onto the threaded rods. Large plain washers are positioned and using two nuts, the planking substitute can be pressed firmly onto the locks and secured. If necessary, the threaded rods are repositioned accordingly. The necessary 3-layer welding process can then follow.
4) Installing the spacers
The prepared spacers are positioned in the middle of the planking back at a point marked by chalk; they are then secured to the top front side, aligned using a water level, wedged using the L-click secured to one side and adhered. It is necessary to change sides regularly when welding in order to avoid the T-profile warping towards one side.
5) Installing the brackets in the sheet piling trenches
The position of the first pair of brackets at the top is measured and marked. The two brackets are secured. The template is suspended from the previous pair of brackets for securing the other brackets. This simple aid considerably reduces the time required for installation, while ensuring that all distances are precise. Once all of the brackets have been secured, they are welded all around.
6) Installing the panels
The prepared panel is secured to the top pair of brackets using a sling on the hoisting device (crane, stacker with winch attached) and manoeuvred into installation position. Aligned flush at the top and evenly at the side, it is wedged to the spacer flanges using the L-clicks and secured. The wedge is removed and the plate welded in full on alternative sides.
7) Installing the bottom plates
The bottom plate serves as encasing and should seal the sheet piling trench at the lower end of the facing. It is passed down to the diver using the drop retainer and specified angle of inclination. The diver presses the sheet metal into the trench and raises the retainer slightly. In this position, the bottom plate and facing can be secured and welded. Nothing needs to be welded in the sheet piling trench as the bottom plate is pressed downwards and wedged via the inclined position and the concrete added later. In order to seal the opening in front of the sheet piling back, a suitable piece of flat steel is welded in at the same angle of inclination.
8) Installing tubular feedthroughs
At the edges of the sheet piling to be repaired, plates are equipped with the appropriate grooves for feedthroughs prior to installation. Pipes are slid into the projections as extensions and should end approx. 2 cm short of the panels. A square cover plate corresponding to the exterior diameter is burned out and positioned before being welded all around with both the plate and the extension pipe.
9) Pumping in the underwater concrete
Once the steel construction section of repairs has been completed, the cavity between the sheet piling and the facing plates can be filled with underwater concrete. This requires a pontoon from which the pump hose can be directed into the cavities between the brackets. It is important to ensure that the hose is lowered as far as the bottom plate to prevent the concrete from bleeding. The cavities are topped up as far as possible over the top edge of the plates to enable the run-off angle to be formed once the concrete has set. It may be necessary to top up the concrete by hand from a container. To ensure that the end result is visually appealing, any concrete that has dripped down the plates as far as the watermark is wiped away with a brush before it sets.
