section_path: Science and technology/Structural and geo-technical engineering
section_name: Structural and geo-technical engineering
paragraph_text: Structural and geo-technical engineering
  
  Timber was the chief building material during the Han; it was used to build palace halls, multi-story residential towers and halls, and single-story houses. Because wood decays rapidly, the only remaining evidence of Han wooden architecture is a collection of scattered ceramic roof tiles. The oldest surviving wooden halls in China date to the Tang dynasty. Architectural historian Robert L. Thorp points out the scarcity of Han-era archaeological remains, and claims that often unreliable Han-era literary and artistic sources are used by historians for clues about lost Han architecture.
  
  Though Han wooden structures decayed, some Han dynasty ruins made of brick, stone, and rammed earth remain intact. This includes stone pillar-gates, brick tomb chambers, rammed-earth city walls, rammed-earth and brick beacon towers, rammed-earth sections of the Great Wall, rammed-earth platforms where elevated halls once stood, and two rammed-earth castles in Gansu. The ruins of rammed-earth walls that once surrounded the capitals Chang'an and Luoyang still stand, along with their drainage systems of brick arches, ditches, and ceramic water pipes. Monumental stone pillar-gates, twenty-nine of which survive from the Han period, formed entrances of walled enclosures at shrine and tomb sites. These pillars feature artistic imitations of wooden and ceramic building components such as roof tiles, eaves, and balustrades.
  
  The courtyard house is the most common type of home portrayed in Han artwork. Ceramic architectural models of buildings, like houses and towers, were found in Han tombs, perhaps to provide lodging for the dead in the afterlife. These provide valuable clues about lost wooden architecture. The artistic designs found on ceramic roof tiles of tower models are in some cases exact matches to Han roof tiles found at archaeological sites.
  
  Over ten Han-era underground tombs have been found, many of them featuring archways, vaulted chambers, and domed roofs. Underground vaults and domes did not require buttress supports since they were held in place by earthen pits. The use of brick vaults and domes in aboveground Han structures is unknown.
  
  From Han literary sources, it is known that wooden-trestle beam bridges, arch bridges, simple suspension bridges, and floating pontoon bridges existed during the Han. However, there are only two known references to arch bridges in Han literature. There is only one relief sculpture dated to the Han period that depicts an arch bridge; it is located in Sichuan province.
  
  Underground mine shafts, some reaching depths over 100 m (330 ft), were created for the extraction of metal ores. Borehole drilling and derricks were used to lift brine to iron pans where it was distilled into salt. The distillation furnaces were heated by natural gas funnelled to the surface through bamboo pipelines. These boreholes perhaps reached a depth of 600 m (2000 ft).