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Coastal structures

Pier renewal technology via jackets for deepening, seismic strengthening, and improving floor systems

Overview of the technology

Many pile type piers constructed several dozen years ago have aged, and many facilities are left unprepared for the big earthquakes which have been of great concern recently. Under these circumstances, projects to reinforce piers against earthquake resistance are increasing as well as floor system improvement.
In addition, deepening projects are on the increase as well in response to the upsizing of ships.
In terms of the items listed below, renewal through the jacket type pier system is easier to perform than improvement and reinforcement through the pile type pier system, and the onsite construction work period can be shortened.

Item
Pile type pier system
Jacket type pier system
(1) Floor system improvement
Formwork and timbering assembled on site.
Formwork and timbering unnecessary as precast slabs are used.
(2) Improvement of ultimate pier strength
More piles are necessary.
-> Unnecessary
(3) Reduction of pile stress
More piles are necessary, or foundation work or soil improvement must be performed.
-> Unnecessary
(4) Onsite construction work period
Long
Short

Features of jacket type pier system

<High earthquake resistance>

Horizontal rigidity is enhanced by three-dimensional steel pipe trusses, resulting in distinguished earthquake resistance.

<Reduced onsite construction work period>

Jackets are manufactured at the plant and installed on site as is. As a result, the onsite construction work period can be reduced thanks to the smaller number of piles and rapid installation.

Construction work flowchart

Structure features

Floor system (precast deck slabs)

Lower formwork, timbering, and scaffolding are required when constructing the floor system of an ordinary pile type pier. The jacket type pier system does not use lower formwork or timbering because it uses precast slabs, as shown in the figure below. Also, since the jacket is outfitted with girders, scaffolding is easy to build. The precast slabs can be used as overall scaffolding after they are installed, contributing to safety.

There are two types of floor systems using precast slabs:
(1) The type that forms deck slabs by using precast slabs as lower formwork, arranging reinforcing bars on them, and placing concrete onsite.
(2) The type that connects interconnected precast slabs as deck slabs.

The girders and precast slabs of the jacket are joined via the headed studs welded on the upper flanges of the girders.
Although side formwork is required at the ends of any pier, it does not need the large-scale temporary construction members that a pile type pier does, so onsite construction work can be significantly reduced.

Features

Reduced temporary construction members

Precast slabs function as the lower formwork and can bear the load during placement, so timbering is no longer necessary. Therefore, onsite temporary construction members can be reduced, so can costs can be cut.

Reduced onsite construction work period

Since few temporary construction members are installed onsite, the construction work period can be reduced. It is often necessary to reduce the non-operating period when improving an existing pier, and the jacket system can meet this requirement.

How to connect the piles and the jacket (welding and grouting)

The connections of the jacket and the piles need to be designed to convey the load without fail.
The jacket and the piles are connected by welding them using shim plates or by using grout. The former is used only for aerial areas, and the latter is applied to both aerial and underwater areas.
In the latter case, adhesion between grout and the legs or piles is not taken into account.

(1) Connection by welding

Connection by welding using shim plates must be designed to allow the required weld throat thickness against shear force (axial force of the piles) applied to the connections of the jacket and the piles.

(2) Connection by grouting

To convey the load by means of grout connections, provide a shear connector (shear key) on the internal surface of the jacket legs and on the external surface of the piles.