Newport-based firm Cintec has revealed the technical challenges behind its work on a project to save Egypt’s first pyramid.
The 4,700-year-old Step Pyramid of Djoser has been detiorating ever since being damaged by an earthquake that hit Egypt in 1992. By the early 2000s the structure was at serious risk of a catastrophic collapse.
Specialist contractor Cintec was appointed in 2010 to repair the pyramid’s partially-collapsed burial chamber and its crumbling walls in a project worth £7M. The unique scheme took nine years to complete due to delays caused by political unrest in the region. Work on the complex scheme came to a close earlier this week.
Large cedar beams originally supported the pyramid’s burial chamber ceiling, located over an about 8m2 shaft down to a depth of around 23m. However, following the 1992 earthquake, the ceiling caved in, creating a sagging effect.
“The collapse had created a barrel vault shape to the ceiling which was in danger of collapsing completely,” Cintec project manager Dennis Lee told New Civil Engineer.
Before securing the ceiling, all the rubble was removed through long narrow tunnels located at the base of the pyramid, allowing for a scaffold to be erected to a height of 27m, reaching 2m below the ceiling. An additional 2m tall scaffold was erected to reach the top of an arch shape created by collapsed masonry.
Lee described the job of securing the ceiling safely as Cintec’s “most challenging project to date”.
The original mortar between stones in the ceiling had deteriorated to a “dry mud dust” triggering the decision to remove it and replace it with traditional lime mortar.
“The ceiling needed propping first to prevent further collapse and the possibility of a chain reaction if a single stone fell,” Lee said.
“Traditional steel or wooden props were not an option as this would have put pressure on individual stones with the risk of dislodging them,” he added.
To resolve the issue, Cintec used purpose-built air bags formed into cylinders and inflated to the point that they were just touching the stones without actually placing any pressure on them.
Irregularities between the air bags and stones were accounted for through the cutting and shaping of rigid foam so any pressure would be applied evenly over a greater area and thus lessen the chance of a collapse.
“[The air bags] were positioned all around the ceiling with narrow walkways between them to allow the mortar removal and replacement with NHL5 lime mortar which was matched on site using the local materials available, to keep with tradition,” Lee explained.
“This process of old mortar removal and new pointing with lime mortar meant the project took a lot longer than anticipated.”
Lee added: “After the mortar had cured, 52mm holes were dry diamond drilled tangential to the face stones of varying lengths of up to 5m to transfer the load to a higher point within the Mastaba of the pyramid.
“The hard limestone blocks were very difficult to drill and after several trials of varying core drill bits we finally settled on a thin wall soft compound crown,” he added.
Site workers installed structural reinforcement anchors, surrounded with a special fabric sock, into the body of the structure. The anchors were then injected with a specially formulated grout.
The airbags were then deflated and moved to new locations to allow remaining pointing and anchoring to continue. “This went on for several cycles until the air bags could be removed and the anchoring process continued with smaller 1m-long anchors to further secure individual stones,” Lee explained.
Reflecting on the project, Lee told New Civil Engineer: “Although this project was challenging and took a long period of off and on work and loads of trips back and forth to Cairo, it was a huge milestone for Cintec International which had the honour and privilege of working on the oldest and first pyramid whose unique design led on to the building of pyramids all along the Nile delta.”
He added: “We are now hoping to be awarded the work to secure the Pyramid of Sahure in Abusir which dates from around 2480 BC and has similar problems as Djoser Pyramid but this is in the very early proposal stage.”
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