|Freshwater Resources in Arid Lands (UNU, 1997, 94 p.)|
|5: The development of groundwater resources on the Miyakojima Islands|
The site of the cut-off walls was decided from the following points of view:
· The site needs to be located at the most downstream point to the extent that the necessary water volume can be stored;
· The site needs to be located at the narrowest point of the underground valley to reduce the construction cost;
· The site needs to be located at a place where no obstacles can impede construction;
· The site should not be situated over ground containing caverns;
· The depth of the cut-off walls should be less than 70 metres from the surface, taking into consideration the drilling capacity of machines.
As a construction method for the cut-off walls, the consecutive pillar wall using the in-site churning method is adopted in most parts, taking into consideration the geological conditions, the working efficiency, and cost performance. Many different machines are used in this method to make the best use of each machine's capabilities in constructing the cut-off walls. The procedures are as follows:
1. From the surface, augers with a casing are used to make holes down to the crest of the dam;
2. For places deeper than the crest of the dam, powerful augers with a single shaft are used to predrill into the hard limestone layer, making the remaining work easier;
3. Augers with three shafts are used in constructing the cut-off wall. This wall consists of pillars made of liquid cement and crushed limestone;
4. To form the shape of the crest of the dam, another machine with a rectangular-shaped bucket is used.
Figure 5 The Construction Procedures of the Cut-off Wall
A wall as deep as 65 metres from the surface was constructed as the Sunagawa Underground Dam by using this method (fig. 5). The depth, the continuity, and the quality are the most important aspects for the construction of the wall. A computer-aided system has been introduced to manage the construction. The data on the drilling depth, the verticality, and the amount of liquid cement injected were displayed in the control office where operators could issue very detailed and efficient directions.