- Case study
- Eilat New
- Israel
Scour Protection Under an Existing Piled Deck, Port of Eilat
A 60 year old jetty being refurbished needed scour protection placed underneath a deck that was already there, on an undulating slope, through a dense and irregular forest of piles. Some of those piles were more than 400 mm out of tolerance at bed level. Nothing rigid was going to fit, and there was no way to make the slope fit the protection instead.
Location
Port of Eilat, Israel
Client
Israel Ports Development and Assets Co. Ltd.
Designer
SENet
System
Insitu concrete mattress
Period
2020
Results at a glance
Panel designs
Distinct mattress panel designs, developed because the pile spacing was irregular and one repeat panel would not fit.
M per panel
Each panel fully filled on site, with zip connections along the top and bottom edges sealing it to its neighbour.
MM in the wave zone
Thickened for a 3 m design wave height at 6.5 s peak period, stepping to a 300 mm transition compartment and 220 mm for the rest of the slope.
Contents
- A 60 year old jetty at Eilat was refurbished and needed scour protection under the existing deck.
- The slope undulated, the piles were dense and irregularly spaced, and some were over 400 mm out of tolerance at bed level.
- Five distinct panel designs were developed, each 24 m by 5 m, zip-connected along top and bottom edges into a continuous apron.
- Vertical ties woven into the fabric hold thickness automatically, which cuts both diver intervention and concrete volume.
- Thickness runs 575 mm in the wave zone, 300 mm through a transition compartment and 220 mm down the rest of the slope.
The constraints
A jetty undergoing refurbishment at the Port of Eilat required scour protection under the already existing deck. Given the undulating slope and the high density of piles at irregular spacing, the geometry itself was the problem: there was no repeating module to design to, and no access from above to correct anything that did not fit.
Two tolerance figures set the scale of that. Slope levels could vary to +0.1 / −0.3 m, and in some locations piles had been driven over 400 mm out of tolerance at bed level. Any protection system had to absorb both without a redesign on site.
deck
spacing
variation
tolerance
The solution
A geotextile formed concrete mattress had the capabilities the site demanded. The geotextile casts concrete in underwater conditions, and the fabric itself is lightweight and negatively buoyant, which means a single diver can handle it and manoeuvre it into place underwater. That is the whole basis of working under a live deck: the component gets to position by hand, not by crane.
Insitu concrete mattress is the preferred method for scour protection in piled revetments because of its reliability where maintenance access is limited. Rock protection has real advantages, porosity and ease of repair on level beds among them, but it becomes significantly more costly and difficult to install on sloped, piled structures. Increased flow velocities and turbulence around piles on inclined surfaces can also compromise the stability of rock armour, which raises the risk of failure exactly where you can least get back to it.
Pile tolerance and pile seals
This is the part of the job that decided the design. A mattress only works if bed material cannot escape from under it, and on a piled structure every pile is a hole through the apron that has to be closed. At Eilat the piles were not where a drawing would put them: in places they had been driven more than 400 mm out of position at bed level, and the slope they stand in varied by +0.1 / −0.3 m.
For construction on complex undulating slopes, the mattress formwork and the pile seals have to be engineered with surplus material, so the protection conforms to the arrangement of piles and slopes it actually finds. The fabric form takes the tolerance in three ways.
- Internal pile seals, for piles that fall inside a panel. A sealed fabric collar is built into the panel at the surveyed pile position and pumped tight around the pile.
- Half pile seals, at panel edges. The seal is split between two adjoining panels so a pile can sit on a joint line without either panel being cut.
- Internal pile zips, so a panel can be opened, passed around a pile and closed again underwater, rather than being threaded over the pile head.
- Surplus fabric throughout, so the form conforms to the slope and the as-driven pile positions instead of requiring either to be corrected first.
Pile bolsters achieved a sand-tight seal first time, and because the seals were filled as part of the main pour sequence, sealing was not a separate operation after it.
Constructability and installation
Proserve tailored the fabric formwork to the required dimensions. Because of the irregular pile spacing, five distinct mattress panel designs were developed. Each panel measured 24 m by 5 m and was fully filled on site. Zip connections along the top and bottom edges of each panel sealed it to its neighbour, so the formwork achieved continuous protection with interlocking concrete aprons rather than a set of separate slabs with joints between them.
Internal vertical ties were woven directly into the fabric and anchored between the top and bottom layers of the formwork. Those ties do three jobs at once:
- Automatic thickness control. The system inherently maintains mattress thickness, which reduces the need for diver intervention to adjust or reposition the formwork.
- Reduced concrete usage. Minimised surface undulations give a more consistent thickness, so less concrete is needed to meet the minimum scour protection requirement.
- Improved hydrodynamic stability. A smoother concrete surface reduces the risk of uplift from water forces acting on the mattress.
The other half of the constructability problem is what happens where the mattress meets each pile. A key principle of insitu concrete mattress is sealing around the structure so that bed material cannot be lost from under the concrete apron. On a complex undulating slope, the formwork and the pile seals must be engineered with surplus material, so the protection can conform to whatever arrangement of piles and slope it actually finds.
Pile bolsters were used to achieve a sand-tight seal first time. Sealing around each pile also prevents scour propagating down and around it, which is a known vulnerability in many traditional scour protection systems. The seals:
- They prevent water jetting down the pile line, which would otherwise initiate focussed scour holes.
- They maintain the integrity of the apron, ensuring continuity even at pile penetrations.
- They were installed with close diver supervision and filled as part of the main pour sequence, giving both visual inspection and structural integrity during placement.
Thickness and porosity
Thickness was not uniform, and deliberately so. An increased thickness of 575 mm was used in the wave zone to manage a design wave height of 3 m with a peak period of 6.5 seconds. In that zone the mattress also carries open holes of 80 mm diameter at 1.4 m centres, which provide the porosity needed to resist uplift during wave rundown. A stone layer of larger diameter than the holes is laid under the mattress there, so the porosity does not become a route for bed material to escape.
The drawing also carries the bed preparation the seals depend on. Bed levels on the slope are held to ±0.1 m to ensure the pile seals fit, using a bedding layer of 19 to 50 mm stone, typically 600 mm thick and never less than 200 mm, to assist grading the slope to level. In the OH575 wave zone the mattress sits on that bedding stone layer, which is what stops bed material escaping through the 80 mm holes.
The transition compartment is the detail worth noting. Rather than carrying 575 mm further than the wave climate justified, or dropping straight to 220 mm, a 300 mm step was designed in to optimise concrete usage across the slope.
Edge protection
Underscour, meaning loss of bed material at the edges, can lead to progressive failure of a mattress. Embedment of the mattress edge is fundamental to preventing it, and particularly so where the edge detail is a failing one such as mattress blocks or rock armour.
A rock falling edge apron was used here, designed on four points:
- The edge trench was deep enough to contain the rock armour and large enough that it would not be displaced by flow.
- Without that deadweight, displaced rock armour may roll onto the mattress protection area and affect draft clearance for vessels.
- When edge scour occurs, the rock armour should settle into the localised scour holes.
- Should scour exceed the designed depth, additional rock armour can be added.
The design intent is that the edge is allowed to move. Rock settles into scour holes as they form and keeps the mattress edge buried, and if scour ever goes beyond the designed depth the remedy is simply more rock, not a return to the mattress itself.
Outcome
The refurbished jetty was given continuous scour protection beneath its existing deck without the slope being levelled, the piles being corrected, or the deck being disturbed. The five panel designs conformed to the pile arrangement as it was actually built, including the locations more than 400 mm out of tolerance, and the zip connections closed the panels into a single interlocking apron rather than a set of independent slabs.
Thickness was matched to the wave climate rather than applied uniformly, with the 300 mm transition compartment holding concrete volume down across the slope, and every pile penetration was sealed as part of the main pour rather than as a follow-up operation.
Who did what on this project
Owner of the Port of Eilat and the jetty refurbishment.
Five tailored mattress panel designs, pile seals and bolsters, thickness zoning and the filling method.
Continue
Al Faw Grand Port, 1.75 km piled jetty
The same technology at the other end of the scale: 430,000 m³ of imported rock armour replaced by 26,500 m³ of locally batched concrete, and the works taken off the critical path.
Read the case study →Protecting under an existing deck? Send us the pile layout and the slope.
Send us the pile layout, the bed survey and your wave climate, and we will come back with an outline: how many panel designs the spacing needs, the thickness zones, and how the pile seals would be detailed. If rock suits your bed better, we will say so.
- Pile layout, including spacing and as-driven positions
- Bed survey and slope levels, with the tolerance range
- Design wave height and peak period
- Access under the deck, and what the divers can reach