Archaeologists have discovered typical techniques applied in building the Dai La Citadel in the 8th century at an excavation site at the junction of Dao Tan-Nguyen Khanh Toan in downtown Hanoi.
At three excavation holes on a total area of 600sq.m, scientists have found various layers of citadel walls built by clay compressed with tiles, clay blocks compressed by wooden hammers and by human feet, and big natural block of clay.
Tong Trung Tin, Rector of Vietnam Archaeology Institute said the crucial skill resulting in the steadiness of the citadel lies on a 50cm-thick layer of clay compressed by nails.
Ceramic tile pieces dating back to the Tran dynasty (1225-1400) and an earlier period were found compressed tightly with clay on a layer 0.2m thick and 1.2m wide.
There was also a V channel running between the tile clay compressed wall and the outer citadel wall, which was guessed to be an exit way around the citadel under the Tran reign.
Two layers of clay compressed with gravel have also been unveiled at a depth of 4.5m, which has also contributed to the solidity of the wall.
Various layers of clay dating back to the 8-9th century, the Ly dynasty (1009-1225) and the Tran dynasty making up the 7m-high wall proved the theory that the Dai La Citadel was carefully renovated under Ly-Tran reigns, scientists said.
They hold that the Dai La Citadel was used as a dyke to prevent the inner citadel area from being flooded by the To Lich River.
Between the clay layers, scientists have found four tombs, including two brick-built ones and a clay-built one from the 9-10th century. The rest of the clay-built tomb was found in a layer dated back to the 17-18th century.
Dai La Citadel, built in 767 and underwent many improvements over time, was the former name of Thang Long Citadel, today’s Hanoi.