4. Railway Facilities
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Data Collection Survey on the North-South High Speed Rail Project in the Socialist Republic of Vietnam Final Report 4. Railway Facilities 4.1 Civil Structures (Railway, Stations) 4.1.1 Railway This includes basic conditions related to the civil structures such as topographical, geological and soil, seismic, intersections such as existing railway lines, roads, rivers, construction, as well as surrounding environmental conditions. In addition, economic efficiency, workability, construction time, operation and maintenance after the commencement of operations shall be considered. In particular, economic efficiency will be well considered when selecting the civil structures for the HSR system in Vietnam. The HSR civil structures include earthworks such as embankments and cuts, viaducts, bridges, box culverts, tunnels and station structures. In order to minimize construction costs, for the most part, earth structures are selected in open sections. Viaducts are applied to the areas where earth structure does not much the surrounding environment with high population density, many buildings, roads, and railways. Viaducts are also applied to the areas where the residual settlement of the embankment is expected to be largely due to the weak ground. The points to note for each structure type are shown below. (1) Earthwork (Embankments and Cuts) Various aspects, such as collapsing caused by settlement and rainfall, mud-pumping and maintenance, etc., have to be considered when designing and constructing earth structures. In recent years in Japan, new materials and new construction methods such as layer thickness adjustment materials, reinforcement materials, drainage blankets, slope protection work, and roadbed reinforcement are developed. In addition, the performance-based design methods have made it possible to construct earthworks with the same durability, safety, usability, and recoverability characteristics as concrete structures. It is preferable to adopt soil structure as much as possible for the following reasons: a) Embankment construction costs are relatively low compared to viaducts. b) Relatively good-quality embankment materials can easily be obtained in Vietnam. It is also possible to recycle material and use good quality tunnel muck and soil from cuts for embankments. Construction of very high embankments tends to cause problems for the surrounding environment as they ruin the landscape and require more land. Considering the above, the maximum embankment height construction for Vietnam should be about 9 m on typical sections, and viaducts should be constructed if the height exceeds 9 m, because 9 m high embankments require approximately four times more land than viaducts. Embankment structures divide urban areas and also increase the costs and time required for land acquisition, which in turn can delay construction. This concept matches to the Railway Structure Design Standard for Earthwork edited by RTRI (Railway Technical Research Institute) in Japan. The formation level of HSR embankments require at least a 1 m clearance on the outside of the wind pressure side for a maintenance walkway, and to prevent the top of the slope from collapsing. About 1 m shall also be provided for a maintenance walkway at the toe of the slope. Depending on the soil quality, the slope gradient of embankments and cuts shall, in general, be about 1:1.5 to 1.8 and 1:1.5, respectively. If necessary, a 1.5 m wide berm shall be provided for high slopes. The standard height in installing berms to high slopes is over 6.0 m, according to the 4-1 Data Collection Survey on the North-South High Speed Rail Project in the Socialist Republic of Vietnam Final Report Railway Structure Design Standard for Earthwork (RTRI, 2013). Berms effectively prevent slope erosion by reducing the flow speed and amount of surface water during rainfall, and they can also be used as walkways for slope maintenance. In case that the land where cut structures are applied is difficult, depending on the soil quality, the slope gradient of the retaining wall shall, in general, be about 1:0.35 with an earth retaining wall structure. The use of reinforcement materials is effective in improving the earthquake and rain resistance of embankments. Embankment reinforcement materials are different from thickness adjustment materials and are not used for auxiliary surface compaction; they aid to increase tensile strength and prevent rotational slip. The establishment of the slope protection works is effective for the prevention of slope erosion of embankments and cuts, rainwater infiltration, surface slip, and protection of the environment. The types and performance of the main slope protection works used for embankments and cuts are described in Table 4.1 and Table 4.2. Table 4.1: Types and Performances of Slope-Protection Works for Embankments Parformance Seepage control Prevention against Prevention against Prevention of soil Environmental Type surface layer surface layer slip erosion caused by protection by erosion spring water greening Concrete-block pitching ◎ ◎ ○ ○ ‐ Anti-weed sheets ◎ ◎ ‐ ‐ ○ Lattice frame work ‐ ◎ ◎ ◎ *1 ◎ *2 Rip rap masonry ‐ ◎ ○ ○ ‐ Planting work ‐ ○ ‐ ‐ ◎ ◎: Excellent performance ○: Good performance -: No effect *1: Inside protected by cobble stones *2: Inside protected by planting works Source: Railway Structures Design Standard for Earthwork (Railway Technical Research Institute, 2013) Table 4.2: Types and Performances of Slope-Protection Works for Cuts Parformance Seepage control Prevention against Prevention against Prevention against Prevention of soil Environmental Type surface layer wethering surface layer erosion caused by protection by erosion collapse and peeling spring water greening Concrete-block pitching ◎ ◎ ◎ ‐ ○ ‐ Pre-cast concrete lattice - ○ ○ ○ ◎ *1 ◎ *2 framework Cast-in-place concrete - ◎ ○ ◎ ◎ *1 ◎ *2 lattice framework Spray framework - ◎ ○ ◎ ◎ *1 ◎ *2 Concrete protection work ◎ ◎ ◎ ◎ ○ - Waterproof anti-weed ◎ ◎ ◎ - - ○ sheets Mortar spraying work ◎ ◎ ◎ ○ ○ - Concrete spraying work ◎ ◎ ◎ ○ ○ - Planting work ‐ ○ - - - ◎ ◎: Excellent performance ○: Good performance -: No effect *1: Inside protected by cobble stones *2: Inside protected by planting works Source: Railway Structures Design Standard for Earthwork (Railway Technical Research Institute, 2013) When the supporting ground is soft, pre-loading and vertical drains can be used as countermeasures against soft ground. If soil improvement is uneconomical due to the depth required, viaducts supported by foundation piles are recommended instead of embankment structures. Since there are many soil improvement methods, it is necessary to pay attention to economic efficiency as well as local soil and environmental conditions in order to select an appropriate method. Typical soil improvement methods are listed below. 4-2 Data Collection Survey on the North-South High Speed Rail Project in the Socialist Republic of Vietnam Final Report i) Surface Treatment Method Shallow soft layers up to a depth of about 2-3 m can be treated. Those are replacement, surface mixing with good soil, and laying sand to drain the groundwater. In Vietnam, the replacement method is recommended due to its economic efficiency and workability. ii) Accelerated Consolidation Method The accelerated consolidation method is used to compact cohesive soil consolidation and drainage with loading and drain material. However, it is necessary to observe any adverse effects on nearby structures. If the construction period is long enough, the pre-loading method is more economical. The sand drain method is recommended if the location of the consolidation layer is thick and rapid consolidation is required. iii) Compaction and Tamping Method The compaction and tamping method includes the sand compaction pile and the vibro- flotation methods. These methods are used to press sand into the ground by vibration and impact loading. However, vibration and noise pollution are problems associated with this method. iv) Soil Solidification Method Soil solidification methods include the deep mixing and the lime-pile methods. Soil solidification involves treating deep soil layers by hardening existing soil with solidifying materials. Vibration and noise generally do not cause a problem with this method. If construction cost is high and the area to be treated is large, the cost of this method should be compared to viaduct construction with pile foundations. v) Pile-net Method If the depth of the soft layer is about 3-10 m, the construction cost of this method is cheaper than that of the soil improvement methods, but some vibration and noise occur during pile driving. For this project, taking into account past construction experience with soil improvements in Vietnam and economic efficiency, the accelerated consolidation method shall be applied as a countermeasure against soft ground areas. Roadbeds for ballast track on embankments and cuttings shall be strengthened by using mechanically stabilized crushed stone for the lower layer and asphalt for the upper layers of the roadbed, as this prevents rainwater penetration and improves abrasion resistance. Thus, the stability and durability of the roadbed are improved, and mud pumping caused by train loading can also be prevented. Proper drainage of the embankment is also important, and placing drainage blankets under the toe of the slope is effective in preventing both an increase in pore water pressure inside the embankment and the collapse of the structure. In Vietnam, there are lots of pastured cattle