With the vigorous development of high speed rail transit, viaduct solutions are widely adopted in many countries' new high speed railway projects, instead of building tracks directly on at grade subgrades. Many industry clients may wonder: why is the higher cost viaduct approach preferred? What practical value can the extra construction investment bring? This article analyzes the underlying engineering logic behind 'building viaducts in place of subgrades' from engineering technology, economic cost, land use and operational safety perspectives.

1. Strict Operation Requirements Challenge At Grade Subgrades

High speed trains travel at great speeds and demand extremely high track smoothness. Minor settlement or deformation of the track will compromise riding comfort and even threaten operational safety. Track settlement must be controlled within a millimetre range.

Many construction zones feature soft soil, silt and alluvial layers that are prone to compressive settlement. Even with ground improvement for at grade subgrades, long term differential settlement risks remain, together with lengthy pre loading and settlement stabilization periods. Viaducts adopt deep pile foundations that penetrate soft soil layers and rest on stable load bearing strata. This minimizes post construction settlement and maintains long term track alignment stability.

Moreover, high speed railways require large radius curves and gentle gradients. At grade alignments are frequently interrupted by existing urban buildings, highways, rivers, farmland and pipelines, making straight routing difficult. Viaducts can cross above such surface obstacles and easily satisfy the geometric design criteria for high speed tracks.

From an operational safety perspective, viaducts facilitate full line enclosure and isolation, preventing humans and livestock from intruding onto the track. They also keep tracks away from direct impacts of surface flooding, ponding and shallow geohazards, lowering the risk of service disruptions caused by natural disasters.

2. Full Life Cycle Cost Advantage

It is undeniable that the unit construction cost of viaducts is generally higher than that of conventional at grade subgrades, increasing upfront civil works investment. However, project evaluation should not only compare superstructure costs. Land acquisition, resettlement, ground improvement and long term operation and maintenance (O&M) expenditures must be considered for full life cycle costing.

1. Lower land acquisition & resettlement costs At grade subgrades require expropriation of wide strips of land. For viaducts, only pier foundations occupy land, resulting in far smaller land take. Land beneath viaducts can retain its original functions for agriculture, traffic and local development. This significantly scales down land acquisition and compensation for existing structures, which is especially advantageous in densely populated regions with scarce land resources.

2. Competitive in complex geology Where projects pass through poor ground zones such as soft soil areas, tidal flats and swamps, at grade subgrades require extensive composite ground improvement, which sharply raises ground treatment costs. Under such conditions, the total cost of at grade schemes converges with that of viaducts, and viaduct solutions may become more cost effective in certain scenarios.

3. Reduced long term maintenance expenditure Exposed directly to natural environments, at grade subgrades suffer from erosion by rainfall, freeze thaw cycles and long term train induced vibration, triggering various subgrade defects and requiring continuous funding for repairs and rehabilitation. Viaducts are reinforced concrete permanent structures with long design service lives and high structural stability. They demand less routine inspection and maintenance, effectively cutting overall costs over decades of operation.

In short, viaducts increase investment in bridge structures, yet save substantial costs for land acquisition, complex ground improvement and repeated repairs throughout service life.

3. Minimize Spatial Fragmentation

At grade railways physically divide surface space. A large number of culverts and overpasses must be built to guarantee local traffic access, adding auxiliary works quantities. Viaducts preserve connectivity beneath spans without fragmenting existing patterns of towns and farmland, and exert fewer interferences with urban planning, road traffic and agricultural production.

In ecologically sensitive zones, viaducts can also maintain migration corridors for fauna and flora, mitigating construction impacts on local natural environments and balancing infrastructure development with ecological protection.

While viaducts represent a preferred mainstream solution for high speed railways, they are not mandatory for every section.

At grade subgrade schemes are also adopted when geological conditions are favourable with high bearing capacity, project sites are sparsely populated, land acquisition costs are low and surface obstacles are limited. The final alignment form is determined through multi factor comparison covering geology, land use planning and investment.

Conclusion

The extensive use of viaducts for high speed railways is not an engineering choice with disregard for costs. Rather, it is a rational technical decision balancing operational safety, geological conditions, land resources and full life cycle investment. Behind every viaduct lies support from complete sets of bridge construction equipment. Bridge formwork, beam fabrication mold, beam erection systems and form traveler serve as critical guarantees for efficient, high quality viaduct construction.

Shandong Boyoun Heavy Industry serves the global rail transport infrastructure market. We support efficient delivery of high speed rail bridge projects worldwide and deliver reliable formwork for global infrastructure initiatives.