Caisson Seawalls

Quay Construction

Modern quay walls are constructed of large adjoining hollow concrete boxes that are filled with sand or other fill. Each of the large boxes is called a caisson, which is the French word for “large box” (similar to the Italian word cassone that has the same meaning).

Figure 1: Caissons, in 1996 in Kobe Japan, lined up on land before being taken out to sea to form a quay wall. Most of each caisson will be submerged, with only the top portion above the surface. For scale, note size of person at the base of the caisson on the right. [UCDavis]

Each caisson (large box) is very big: the size of a multi-story building. For increased strength, the inside of a caisson is partitioned into multiple vertical boxes. A typical caisson has interior walls (all full height), with only one floor (at the bottom of the caisson).

Figure 2: Construction of the next concrete caisson, Kobe Japan 1996. [UCDavis]

One way of constructing and installing the caissons, as shown here in 1996 in Kobe Japan, is to construct the concrete caissons on land and then lift each caisson into the sea with a large offshore barge crane.

Figure 3: Lifting a caisson with an offshore barge crane that will place the caisson in the water, Kobe Japan 1996. [UCDavis]

The caissons rest on the seabed attached to each other, with their own weight holding them down. They are filled in with sand or other fill, and capped with concrete.

Figure 4: Caissons in position to form a quay wall, Kobe Japan 1996, just offshore from a pre-existing quay wall that was damaged the year before by the 1995 Kobe earthquake. The area between the old and new quay walls will be filled in, to form new retained land. The quay cranes will be moved from the old quay to the new quay. Caissons shown at right will be hoisted into the water next the caissons that are in position. [UCDavis]

Lessons have been learned from past caisson failures to provide guidance on new quay wall construction. It is important to prepare the seabed that caissons will rest on, and do a good job filling the new retained land.

The seabed under each caisson must be of suitable material to hold the weight of the caisson without shifting that would cause the caisson to tilt.

This usually involves removing (trenching) the seabed where caissons will rest, and replacing the removed seabed with material that is better suited for holding up a caisson. Compaction may also be used. Wick drains (called prefabricated vertical drains or PVDs) may be used during compaction of soft soils, to substantially shorten the required compaction time.

“Improvement of soft ground by preloading with prefabricated vertical drains (PVDs) is a common practice in the field of ground engineering. PVDs accelerate the consolidation process of soft soils by providing a shorter drainage path for the pore water and thereby increase the strength and stiffness of soft soils over time.”

—

V.A. Sakleshpur, M. Prezzi, R. Salgado, “Ground Engineering Using Prefabricated Vertical Drains: A Review”, Geotechnical Engineering Journal, March 2018.

New retained fill must not be likely to experience shifting (liquefaction) after construction. Precautions may involve removing and replacing seabed that is not strong enough to hold new fill, and carefully placing new fill gradually instead of in a hurry, to allow for proper distribution and settling.

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Caisson Seawalls

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Introduction
Quays
Quay Construct (this page)
Singapore
Tema, Ghana
Le Portier, Monaco
Colombo, Sri Lanka
Hitachinaka, Japan
Slip Form
Seawater Construct
Breakwater

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