Method of protecting underground structures in ground liable to be liquefied

A conglomerate layer is formed horizontally on a base of ground which has a high water permeability and easily liquefies. A bottom plate is formed on the conglomerate layer to construct the underground structure thereon. One or more drainpipes are extended upwardly from the conglomerate layer through the bottom plate for draining excessive pore water in the ground.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of protection of an underground 
structure in ground which has a high water permeability and easily 
liquefies, such as sandy ground, and particularly to a protection method 
which is suitable to prevent floating of the underground structure and 
outflow or wash out of ground from under the structure. 
2. Description of the Prior Art 
Generally, ground which has a high water permeability and contains 
excessive pore water easily liquefies when the pressure of the water is 
increased by an earthquake. Further, an underground structure whose 
interior is hollow, like an underground tunnel, generally has a small 
apparent specific gravity. Accordingly, such underground structure 
constructed in such ground in the past frequently has been floated when 
liquefaction of the ground was caused by earthquakes. 
As a countermeasure to solve such a problem, there are known a method of 
improving the ground, a method of forming a gravel drain and a method of 
lowering of the ground-water level. However, to employ these methods, 
various conditions of the ground and restrictions on long term maintenance 
and management of the structures would be required. Thus, these methods 
are not frequently adopted. 
It is possible to apply to such type of underground structure additional 
structures for preventing the ground from being in a liquefied state, e.g. 
as disclosed in Japanese Provisional Patent Publication No. 127823/1983. 
However, such additional structures inevitably are accompanied by the 
following problems. Namely, in such structure, a predetermined number of 
drain pillars are provided in the base ground to support an underground 
structure. The drain pillars each comprise a concrete pillar having water 
permeability. On the outer circumference of the concrete pillar is formed 
a wall having water permeability with materials such as gravel, fine stone 
and slag. By this arrangement, it is attempted to employ the drain pillars 
to prevent an increase of the pressure of excessive pore water in the 
ground. However, it is difficult to construct such drain pillars under the 
underground structure, and even if possible, such construction requires a 
substantially high cost. Moreover, in the above-mentioned structure where 
the drain pillars are formed under the underground structure, the 
underground structure is liable to be floated by the pressure of water 
which elevates in the drain pillars when the ground becomes liquefied. 
3. Object of the Invention 
The present invention has been accomplished in an attempt to solve the 
above problems of the prior art. Therefore, it is an object of the 
invention to provide a method of protecting an underground structure in 
ground which is liable to be liquefied. Moreover, the method can prevent 
floating of the underground structure and can be achieved by a 
constructional operation that can be done easily and at low cost. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method of protecting underground 
structures, such as an underground tunnel, in ground which easily 
liquefies, comprising steps of forming a conglomerate layer horizontally 
on base ground which easily liquefies, forming a bottom plate on the 
conglomerate layer to construct an underground structure thereon, and 
arranging one or more drainpipes extending upwardly from the conglomerate 
layer through the bottom plate, thereby collecting excessive pore water in 
the ground in the conglomerate layer, and then draining the water to the 
surface of the ground or into the underground structure through the 
drainpipes. 
According to the method of the present invention, when the pressure of 
excessive pore water in the ground which is liable to be liquefied, such 
as like sandy ground, is increased by an earthquake, the excessive pore 
water first is collected in the conglomerate layer, and then it is rapidly 
drained to the surface of the ground or into the underground structure by 
the drainpipes extending upwardly through the bottom plate, so that 
floating of the underground structure by the increase of the pressure of 
the water effectively can be prevented. 
These and other objects, features and effects of the present invention will 
be more apparent from the following detailed description with reference to 
the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
In the drawing, an underground structure 10 is located at a predetermined 
depth in sandy ground 12 which has a high water permeability and easily 
liquefies. The structure 10 is formed of concrete and is hollow, such as 
an underground tunnel. 
In the protection method of the invention, a conglomerate layer 16 having a 
predetermined thickness is formed under a bottom plate 14 of the structure 
10 before the structure 10 is constructed. THe conglomerate layer 16 
comprises conglomerate of a predetermined size spread over an area 
corresponding to the area of bottom plate 14 to be constructed. It is a 
matter of course that the conglomerate layer 16 can be formed by spreading 
and compacting conglomerate by usual construction methods. 
After the conglomerate layer 16 is formed in the above manner, the bottom 
plate 14 is constructed on the layer 16, and then side walls 18 and an 
upper plate 20 are constructed so as to form the underground structure 10. 
Moreover, during or after the construction of the structure 10, one or more 
hollow drainpipes 22a-22c are provided to extend upwardly from the 
conglomerate layer 16. In the illustrated embodiment, three drainpipes 
22a-22c of hollow pipe shape are formed. One end of the left drainpipe 22a 
in the drawing is located in the conglomerate layer 16 and the other end 
thereof extends to the ground surface. One end of the central drainpipe 
22b is located in the conglomerate layer 16 and the other end thereof 
opens to a manhole 24 extending from the upper plate 20 of the underground 
structure 10 to the ground surface. The height of the opening of the 
drainpipe 22b is set to be higher than a normal level of ground-water, and 
thus underground water never flows thereinto. One end of the right 
drainpipe 22c is located in the conglomerate layer 16 and the other end 
thereof opens to a catch-basin 26 formed in the ground. 
In the arrangement constructed as mentioned above, when the pore water 
pressure in the sandy ground 12 is increased by an earthquake, excessive 
pore water is collected in the conglomerate layer 16, and such water is 
rapidly drained onto the ground surface or into the hollow underground 
structure 10 through the drainpipes 22a-22c. Thus, both the increase of 
the pore water pressure around the structure 10 and the liquefaction of 
the ground are suitably controlled, so that floating of the structure 10 
and wash out of the base ground thereunder effectively can be prevented. 
Moreover, since some of the excessive water is drained into the underground 
structure 10, the apparent specific gravity of the structure 10 thereby is 
increased, thus farther increasing the ability to prevent floating. 
Additionally, since the protection method of the invention comprises only 
the steps of forming the conglomerate layer under the bottom plate of the 
underground structure and arranging one or more drainpipes extending 
upwardly from the conglomerate layer, the method can be realized with easy 
and low-cost construction of the underground structure.