- Case study
- Dover New
- United Kingdom
Ninety Marine Pile Seals, Filled From the Surface, Dover Western Docks
Ninety precast corbels had to be landed over 1830 mm steel tubular piles, then stitched to them with a concrete pour. Each corbel needed a stopend seal at its base, underwater, to retain that pour. Doing it with divers would have been slow and weather dependent.
Location
Dover Western Docks Revival, Kent
Client
VolkerStevin Ltd
Designer
VolkerStevin / Boskalis Westminster
System
Tony Gee and Partners LLP
Period
2017
Results at a glance
Seals installed
One stopend seal to each precast corbel, with 93 fabricated in total including the test unit and two spares.
mm gap, ±40 mm
The annulus between a 1830 mm pile and a 2180 mm cast-in recess, at a tolerance no rigid component can be cut to.
Minutes to fill
Achieved at the full scale test, in one continuous pour, with grout arriving halfway up the vent sleeve as designed.
Contents
- Ninety precast corbels had to be landed over 1830 mm steel piles and stitched to them with a concrete pour.
- Each corbel needed an underwater stopend to retain that pour, across a 75 mm annulus at a tolerance of ±40 mm.
- A condensed grout bag seal was pre-fixed into the corbel recess on land, then filled insitu from the surface.
- No divers are needed for filling. The vent sleeve gives the completion signal, and a diver only removes the hoses afterwards.
- A full scale test on a real corbel filled in 11 minutes and established that a five minute pump stop mid-fill is permissible.
The constraints
The Dover Western Docks Revival involved extensive marine construction, including new quay structures, piled foundations and precast elements. This job was one repeated detail within it: ninety precast concrete corbels, each to be landed over a 1830 mm steel tubular pile and then stitched to it with a concrete pour above.
Every one of those corbels needed a stopend at its base to retain the pour, and every one of those stopends was underwater. Forming ninety of them by diver would have been slower and weather dependent.
The geometry is what decides the method. A 1830 mm pile inside a 2180 mm recess leaves a 75 mm annulus, and the tolerance on that annulus is ±40 mm, more than half the nominal gap. Nothing cut to size fits a gap that might be 35 mm or might be 115 mm.
The solution
Proserve supplied a condensed grout bag seal, a remote grout seal pre-fixed into the cast-in recess before the corbel enters the water, and filled insitu from the surface with neat cement grout through pre-fixed filler and vent hoses.
The system works on the deformability and tensile integrity of woven grout-tight fabrics. The component arrives compact and only takes its final shape once it is where it needs to be, which is the same principle as all fabric formed concrete, applied to a joint rather than a slab. The ±40 mm is absorbed by the seal, not by the programme.
The seal is a fabricated unit:
- A circular seal fabricated in two-layer fabric, with one entry sleeve and one exit sleeve.
- An internal mesh diaphragm for filling control.
- A condensing break cover that opens as the seal fills, so the unit is delivered and fitted in its compact state.
- A vent sleeve, which is how the fill is monitored from the surface.
Each unit is strengthened for a combined grout head of 6 m submerged plus 1 m in air, and was designed as temporary works to a seam safety factor of 10.2 against a specified limit of 4. That margin is what makes a fabric component acceptable on a live marine job, on something that is filled once and then has a concrete pour landing on top of it.
Constructability and installation
The reason this suits a live dock is that almost none of the work happens at the interface. The seal is part of the grout bag and sock seal family, and the sequence is five steps that repeat ninety times.
Pre-fix the seal into the recess
The condensed seal is fixed into the 2180 mm cast-in recess before the corbel goes anywhere near the water, with the filler and vent hoses already attached.
Land the corbel over the pile
The corbel is lowered over the 1830 mm pile. The 100 mm recess depth protects the seal from rub on the way in.
Fill from the surface
Neat cement grout is pumped down the filler hose. The seal expands out of its condensed state, opens the break cover and closes the 75 mm annulus, taking up whatever the actual gap turns out to be within the ±40 mm.
Watch the vent, not the clock
Filling stops when grout has risen 0.5 m up the vent sleeve. The vent is the instrument: it confirms the seal is full and gives a visible signal from the surface, with no diver in the water.
Pour the stitching concrete
With the stopend in place the stitching concrete goes in above it. The hoses are cut off afterwards by a diver, or at low tide.
Grout control is specified rather than left to the mixer operator, and if the pump fails the fallback is a gravity tremie fill down the same filler hose.
The full scale test
On 15 September 2017, before any production seals were fitted, one seal was fixed to a real project corbel and inflated against an 1830 mm internal diameter pile offcut, replicating the underwater condition. The vent was elevated 4.0 m above the entry hole on scaffold bars and watched by eye from a viewing platform.
It filled in one continuous pour in eleven minutes, and grout arrived halfway up the vent sleeve exactly as designed.
The more useful part of the test was the failure. A five minute pump breakdown was tested. Grout in the vent fell back to a quarter of its height, then recovered to halfway when the pump restarted, which established that a five minute delay mid-fill is permissible rather than a scrapped seal.
The test also changed the product before any of it was made. Break tags were added to the top fabric afterwards, and the installation guide was reissued and the test report issued the following week.
The corbel was then cut free, recovered intact, cleaned and re-used on the project.
Outcome
Ninety seals were installed, from ninety-three fabricated: the ninety production units, the test seal, and two spares. Delivery ran in batches through the autumn of 2017, with the installation guide setting a maximum of four seals per day.
The approach reduced installation risk, absorbed the tolerance in the component rather than in the programme, and gave a clean interface between the prefabricated and insitu works.
Dover is a useful reference because the conditions were the hard ones: difficult access, time constraints and water management, which together made traditional sealing and bearing systems impractical. The same approach is set out generally on marine pile seals, and the wider principle, that flexible formwork can produce complex shapes economically using simple pumping techniques, is set out in Fabric Formwork Systems Used in Marine Construction (Hawkswood).
Who did what on this project
Contracting party for the seals package.
Delivered the marine works at the Western Docks, and hosted the full scale test.
Consultant to the works.
Port authority for the Port of Dover, and named as client on the Proserve case study for this project.
Condensed grout bag seals, temporary works design, installation guide and the full scale proof test.
Continue
MOSE flood barrier, 719 grout bag foundations
The same condensed-then-filled principle at the other end of the scale: bags pre-fixed beneath 20,000 tonne caissons and grouted from the surface with no divers.
Read the case study →Sealing between precast elements? Send us the gap and the tolerance.
Send us the annulus, the tolerance you have to absorb and the access window, and we will come back with an outline approach: seal configuration, the grout head it has to carry, and how it would be pre-fitted and filled. If a rigid stopend suits your joint better, we will say so.
- The two elements and the annulus between them
- Tolerance range, and whether faces are parallel
- Grout head the seal has to retain, in air and submerged
- Access, tidal window and when the element lands