Archaeological consolidation of UNESCO masonry structures in Oman: the Sumhuram Citadel of Khor Rori and the Al Balid Fortress

A series of missions, starting from early 2005 up to 2009, have been managed to restore the ancient city walls of Sumhuram (4th B.C. – 6th A.D.) in the site of Khor Rori, together with a group of archaeologists of the University of Pisa. Also, from early 2007 a series of missions have been managed to consolidate the dry masonry Fortress in UNESCO site of Al Balid erected about 1100 A.D. with archaeologists of the University of Missouri. Both sites are located in the area of Salalah in southern part of the Sultanate of Oman. The sequence of dry season and rainy season during the Khareef period (Monsoon phenomena) caused several collapses on the excavated masonry structures, with risks for safety of visitors and archaeologists. The walls of Sumhuram, affected by the loss of connection between the two limestone masonry faces, have been rebuilt with the original texture of bricks using a new mix design for mortar joints with the same colors, consistency and proper strength and durability. The safety level of the consolidation works is evaluated by observing on site the shape of the collapse surfaces and using the classical limit analysis approach. The collapses of Al Balid dry masonry walls have been originated by the loss of permeability of the external wall faces due to the progressive filling of joint by soil; consolidation activities consisted to rebuild with the same blocks down and to substitute the earth behind the external layers with gravel and high permeability filling. An analytical model of dry rigid block masonry with frictional interfaces has been used to represent the out-of-plane behaviour, in order to define the maximum height of excavation and the maximum free height of the restored walls to guarantee safety for workers.