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Because granulated blast furnace slag is lighter in weight than natural materials, it is used as a material in some sand mats. Steelmaking slag also forms shapes with many edges and rough surfaces, giving it characteristics that are similar to natural crushed stone or sand.
The particle density is 3.3 - 3.6 g/cm3, which is higher than natural stone materials, and the material is heavy, with a unit mass of 21 - 23 kN/m3 when moist or 14 - 16 kN/m3 in water.
The size of steelmaking slag particles is in the range of 37.5 - 0.075 mm, and the angle of shearing resistance is 40 or more even when only lightly compacted. The cohesion is approximately 50 - 100 kN/m2.
Water that is eluted from steelmaking slag satisfies the criteria of the Law on the Prevention of Marine Pollution and Maritime Disaster. Although the pH of the water from slag is ordinarily high, it has been verified that when applied to marine uses the dilution and buffering effect of seawater components result in almost no pH increase in the surrounding water.
Verification of effects on surrounding marine areas |
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Together with the Coastal Development Institute of Technology, Nippon Slag Association has since 1993 been involved in application technology research for the use of steelmaking slag as a material for port and harbor construction, and in 2008 published the Guide to the Use of Steelmaking Slag in Port and Harbor Construction. This research not only examined the physical characteristics of the materials, it also performed trial construction using land sand compaction piles (SCP) at Kobe Port Island, and marine SCP at Hiroshima Port, and investigated the effects on the marine environs.
SCP is a ground improvement method that involves driving a pile with compacted sand into soft ground in order to increase the ground strength. Steelmaking slag for ground improvement has a higher unit mass and greater angle of shearing resistance than natural sand, and it was found that these geotechnical characteristics can be used to reduce the cost of ground improvement work.
Regarding the environmental effects of the use of steelmaking slag on marine areas, although the pH of the water eluted from steelmaking slag is ordinarily high, when slag is used as an SCP filler material it is sealed inside the casing pipe and has almost no contact with seawater. It was verified that there is almost no pH increase in the surrounding marine area.
Because steelmaking slag was evaluated as a possible replacement for natural sand as SCP material, it has been used in ports and harbors in many locations, including approximately 4 million tons at Hiroshima Port. In particular, there are now many local governments that have banned dredging of marine sand in the Seto Inland Sea and other areas for reasons of environmental protection, and this has contributed to rapid growth in the use of steelmaking slag as SCP filler material for ground improvement.
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| Sand compaction pile |
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Hiroshima Port Renaissance, Dejima District |
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Cross section of marine SCP work locations at Hiroshima Port |
Subsidence countermeasure for ultra-soft ground: Kitakyushu Air Terminal |
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During 2000 - 2002, approximately 1.5 million tons of granulated blast furnace slag were used in soft ground improvement work at Kitakyushu Air Terminal. Kitakyushu Air Terminal was constructed approximately 3 km off the Suo-Nada coast. Because dredged soil removed during channel maintenance at ports such as the Port of Kitakyushu and Port of Kanda was used for land reclamation during this construction, steps were needed to prevent subsidence of the ultra-soft ground. Dredged soil in its natural state requires a considerably long time before it compacts to form firm earth. In order to more rapidly create firm earth from ultra-soft soil, sand mats were used during the airport construction, and the paper drain method was used for ground improvement. Because it is lighter in weight than natural materials, granulated blast furnace slag is known to reduce the amount of subsidence of reclaimed land, and was used as part of the sand mat materials. The construction involved laying 60 cm of granulated blast furnace slag (approximately 550,000 m3) on a 90-cm layer of marine sand in the Shin-Moji No. 1 coast project area, and 90 cm of granulated blast furnace slag (approximately 650,000 m3) on a 90-cm layer of marine sand in the No. 2 project area.
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| Kitakyushu Air Terminal |
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Shin-Kitakyushu Air Terminal
(during ground improvement work) |
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