Geoscience Reference
In-Depth Information
performance-based design are necessary to develop remediation methods to strengthen
existing structures.
To overcome the restrictions mentioned above, several remedial measures which can be
applied to existing structures have recently been developed and applied. Examples of
treated existing structures arebriefly explained below.
4. Remediation methods for existing raft foundations
4.1. PRINCIPLE OFREMEDIATION
The best remediation method for existing structures is to improve the ground under the
structures to prevent liquefaction. Some techniques to improve the ground are available,
if bore holes can be drilled through the floor of a structure. Soil improvement under a
structureisalso possible fromoutside the structure by using special techniques.
Thesecond-bestchoiceistodecreasethesettlementofastructurebysomemethod,such
as installing underground walls around the structure. Liquefaction of the ground under
structures occurs for two reasons: (i) densification of the ground due to the eruption of
undergroundwater,and(ii)movementofthegroundhorizontally,asschematicallyshown
in Figure16.1. As the settlement due to the first reason occurs not only under a structure
butalsoinsurroundingground,relativesettlementbetweenthestructureandsurrounding
ground may be negligible. In contrast, the horizontal movement of the ground under a
structure induces large relative settlement. Therefore, it is important to reduce the hori-
zontal movement to decrease the relative settlement of a structure. By installing under-
ground walls, horizontal movement can be reduced.
4.2. EXAMPLE OF TREATED STRUCTURES
4.2.1. A tank yard in Kawasaki (partially quoted from Ohmori, 1979; JGS, 1998)
As schematically shown in Figure16.2(1), a tank yard was treated by the dewatering
method. Cut-off walls with low permeability were installed around the yard. Then 26
wells were installed and the water level was lowered. The water level is still about 3m
lower than it had been.
Fig. 16.1. Mechanism of liquefaction-induced settlement of building
due tothemovement of the ground under it
