Effective Scour Protection for Piled Jetties Using Concrete Mattresses

In-situ concrete mattress provides robust revetment protection against vessel and wave-induced forces in ports. The construction technique involves in situ filling of formwork to create a continuous concrete apron over the revetment surface. This article explores the key design principles, technical benefits of the construction process, cost analysis compared to alternative methods, installation procedures, and provides case histories.

Revetment Scour

The geotechnical stability of slopes under piled jetties in ports is a critical factor in ensuring the structural integrity and longevity of these maritime infrastructures. Piled jetties are often exposed to significant environmental forces, including wave action and vessel-induced flow, which can lead to scour – the removal of the slope material around the piles and at the top and toe of the slope. Without adequate protection, this scour increases slope instability and reduces embedment of the piles, potentially leading to structural failure.

To mitigate these risks, effective scour protection measures such as mattress or rock armour is essential. These measures help maintain the stability of the slopes by absorbing and dissipating the energy of waves and currents, thus preventing the erosion of slope and ensuring the continued safe operation of the jetties.

The extent of protection should cover any areas where scouring would decrease structural stability. 4 to 5 metres of scour below bed level are common in sands under container vessels, ferries and cruise ships among others.

Mattress Vs Rock Armour Comparison

In-situ concrete mattress offers significant advantages over traditional rock armour for scour protection, particularly regarding installation and logistical efficiency.

• Prior to filling in situ the formwork is lightweight and easily transported in pallets via air or sea freight
• Concrete mattress can be installed beneath the jetty deck, allowing scour protection work to proceed independently of other construction activities from it from the critical path of construction
• No heavy plant or floating barges are required for installation
• Significant cost and carbon saving achieved from the reduction in overall material required

The concrete apron formed by the mattress formwork is a slab that effectively distributes hydrodynamic forces across the entire surface, providing a stable and robust barrier against scour. In contrast, rock armour relies on the individual deadweight of each rock to prevent displacement, requiring rocks to be significantly thicker than an equivalent concrete mattress (as shown below). Due to the specific size and durability of the required rock, suitable materials may not be available locally, making sourcing challenging. Transporting rock from distant locations can be prohibitively expensive, further complicating the logistics and increasing project costs.

The above visual displays the material saving from a project in Iraq, Al Faw Grand Port. The initial rock armour protection was swapped out for concrete mattress, 430,000m3 of rock armour was replaced by just 26,500m3 of concrete (case studies linked below).

The above graph is a comparison of the relative total installed costs of mattress and rock when comparing the bed velocities from propeller scour. For the average design case, between 4 and 6 m/s, mattress can be 1/3 of the price of rock armour. These costs can also be applied to the level berth pocket for vertical quay walls and block walls. The higher the velocities the more price competitive mattress is due to the size of the rock armour required and the small increase in thickness required for the concrete mattress.

Installation Underneath Deck

By eliminating the need for large, hard-to-source rock armour and providing a streamlined installation process, concrete mattresses offer a more practical and cost-effective solution for maintaining geotechnical stability. This approach ensures efficient project completion and reduces delays, making concrete mattresses the preferred choice for scour protection at piled jetties.

Jetty Refurbishment

When refurbishing or deepening an existing jetty, concrete mattress enables scour protection works under the deck. Localised dredging can be used to prepare the slope ready for mattress installation.

For more information on dredging and concrete mattress please see the article:

Installation

The formwork, like traditional land-based systems, is designed for casting concrete. This lightweight formwork is easily handled and positioned underwater, allowing the casting of interlocking concrete aprons over uneven surfaces, ensuring full thickness on slopes.

Installation Process

1. The diver rolls out the formwork and secures it around the piles and at the top of the slope.
2. The 2nd mattress is lowered and connected in the same manner.
3. The 1st and 2nd mattresses are zipped together.
4. Mattress 1 is pump-filled from the quayside, using a tremie technique, starting at the bottom of the slope. The diver moves the filling hose upwards as filling progresses.
5. This process is repeated: Mattress 3 is zipped on, and then Mattress 2 is filled.

This method ensures the mattress fills to its full thickness, forming interlocking shear joints between the mattress panels.

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Key Installation Preparation:

Proserve ensures the formwork is customised to the project’s needs to minimise diver time and guarantee successful first-time grouting. With specialist expertise, Proserve's engineers collaborate with contractors on construction preparation and mix development using local materials to enable rapid and controlled installation once construction begins.

Project Specific Design & Tailoring

The concrete thickness required for the apron is on average 15-20cm below the wave zone and around 35cm in the wave zone due to additional deadweight required. The formwork is tailored to make installation as fast as possible for the Divers ensuring that the formwork can be quickly attached around the piles and secured for filling. Additional fabric allowances are include to ensure the formwork can fill to full thickness over undulating surfaces without pulling away from the piles.

Porosity holes are specifically designed according to wave height requirements to ensure that wave rundown pressures are dissipated, preventing the apron from lifting or cracking. These holes are fabricated at the necessary intervals and formed during the pump-filling of the mattress. Beneath the mattress, within the wave zone, a layer of stone is placed to provide an additional filtration layer. The stones are slightly larger than the porosity holes in the mattress, ensuring that none of the revetment material can escape through the holes, thus preventing internal erosion of the slope.

Key Formwork Details for Sealed Protection

The concrete apron is much thinner than other pre formed mattress technologies and traditional rock armour due to creating a sealed concrete apron. The key details to ensure sealed protection are:-

• Pile Seals: ensuring a sand-tight seal is achieved around the pile. Proserve have an internal bolster that simplifies diver positioning and fills with the mattress producing a sand tight seal.
• Wall Seal: another bolster is deployed which fills at the mattress edge and against the wall ensuring a sand-tight seal is achieved against the wall structure.
• Filling to Full Thickness: the mattress must fill to full thickness over undulating surfaces, Proserve's Incomat format has a strong internal anchored fix tie position ensuring it can withstand the pressure of undulating slopes while filling to full thickness.
• Shear Joints: form between the mattress panels and down the line of the slope interlocking one panel against its neighbours.
• Perimeter Edge Protection: Rock falling edge aprons are traditionally used to protect the edge of the mattress from underscour. Extensive modelling has been undertaken to develop an articulating or "hinged" mattress edge. Preventing underscour of the of mattresses is critical for long term protection.

For more information on edge protection, please see the article:

More Information

Interested in a team lunchtime webinar or discussing the technical merits of an upcoming project? George Hawkswood , Engineering Director at Proserve, welcomes any enquiries or technical questions you may have. Please feel free to get in touch.

Case Histories

In-situ mattresses have been used on revetment slopes for over 40 years in container and cruise vessel ports. Please refer to the case studies below: