A valuable tool

NOT all dams in Japan are designed for power generation or water supply purposes. Since the late 1970s the government has built a series of concrete soil erosion containment dams in mountainous areas throughout the country, designed to prevent soil erosion from blocking rivers and creating artificial lakes. These lakes can become unstable and burst following heavy rainfall, consequently endangering nearby communities.

While the dams are effective at preventing soil erosion from blocking streams and rivers, Japanese scientists have recently become concerned that some of the dams are blocking the migratory routes of various fish species that return up river for spawning each year. By preventing the fish from returning to their spawning grounds, the presence of the dams is threatening the survival of these species and the ecological balance of the river life in surrounding areas.

Following consultations with scientists and ecologists, local governments in a number of prefectures have decided to make a simple modification to concrete soil erosion containment dams to allow fish to swim upstream for spawning. The modification normally involves cutting a narrow slit in the crest of each dam to allow migratory fish to swim through the newly created water outflow channel, while the dam is still able to prevent soil erosion from being washed downstream.

One soil erosion prevention dam, where work has recently been completed cutting a water outflow channel, is Bo dam in Yamagata Prefecture. To ensure the project was carried out correctly, Yamagata prefectural government appointed diamond wire sawing contractor Dai-Ichi Diamond Koji Co of Tokyo to cut the water outflow channel in the centre of the dam's crest.

Built on an 18.1m wide base, Bo dam stands 24m high and measures over 100m in length. The concrete dam has a 2m wide crest.

Following consultations with the local project hydrologists, Yamagata prefectural government asked the company to cut a 17m deep, vertical tapering slit into the crest. The tapering slit measures 2.99m wide and 2m deep at the dam crest, and gradually tapers to the base of the water outlet channel which measures 1.97m wide and 18.15m deep.

According to a Dai-Ichi Diamond Koji spokesman, the project involved sawing and removing a 405.3m3 V-shaped concrete section from Bo dam. The V-shaped slit was cut using one vertical and two vertical slanting cuts so that concrete blocks of a manageable size could be removed.

'We cut concrete blocks standing an average of 1500mm, 1700mm and 1800mm in height and measuring from 1.2-4.5m in length, depending on the crane size available,' the spokesman explained. 'This project was in a mountainous area and it was not possible to bring in a large crane. The cranes used a chemical anchor to remove the blocks. The client did not want the concrete blocks broken too small so that they could be re-used to build a breakwater in a nearby harbour project.'

Owing to the original construction method used to build Bo dam - the concrete had been poured in a series of layers with each pour being allowed to set before the next layer of concrete was poured on top - three downward saw cuts were sufficient to cut the water outflow channel.

To make the vertical diamond wire saw cuts, the Dai-Ichi Diamond Koji wire sawing team first had to drill horizontal cores in the dam wall through which the diamond wire saws could be inserted. The position of each hole was also used to control the wire saw's downward cutting angle. The wire saws used to cut blocks at the lowest level of the water outlet channel each measured 100m in length due to the thickness of the dam wall at the base, and because the wire sawing machines had to be placed some distance away from the cutting site due to difficulty in obtaining convenient access when sawing the dam wall.

The Dai-Ichi Diamond Koji wire sawing team used a Shibuya TS-3501 core drill machine to drill horizontal guide holes for the diamond wire saws. Three core drilling speeds are used when drilling long horizontal holes into concrete. For core horizontal holes up to 9m in length, the core drill drills at 800rpm. The drilling speed is then reduced to 400rpm for the next 5m because more resistance is met as the length of coring increases. To drill cores measuring 18m in length the final 4m were drilled at 150 rpm. Core drilling an 18m long core takes eight hours.

To saw the three vertical cuts Dai-Ichi used Kline wire sawing machines. The cutting speeds varied according to the length of diamond wire being used. According to the spokesman, the wire saw cutting speed was 30m/sec when the longest diamond wire saws measuring 100m in length were used to cut blocks at the base of the water outflow channel, where the dam was 18m thick. At this speed the diamond wire saw was able to cut one square metre of concrete per hour.

As expected, the sawing speed increased as sawing proceeded. As the blocks furthest away from the wire sawing machine were cut and removed, the team were able to shorten the length of the diamond wire saw being used as the cutting surface gradually came closer to the wire sawing machine. Once the wire saw had cut more than half way along the longest 18m cutting path, the wire sawing team shortened the length of the diamond wire saw to allow the cutting speed to be doubled to two square metres of concrete per hour.

Meanwhile, after wire sawing was completed for each layer of blocks, the Dai-Ichi Diamond Koji team used a core drill and a hydraulic block blaster to break the blocks into smaller removable block sizes. A core measuring 20cm diameter was drilled almost to the full depth of each block allowing the block blaster to be inserted.

'Where new concrete was poured on top of dry concrete when constructing the dam wall, the join creates a natural break between the two concrete layers slabs when we use a block blaster,' the spokesman said. 'The only new cuts that needed to be made for this project were the three vertical cuts for the V-shape water outflow channel.'