Table 1 Possible permanent Stabilizing Configurations and seismic intervention retrofitting measures for the crown zones, Rotunda in the 2nd level and the main pillar in the 3rd level in Catacombs of Kom El-Shoqafa

From: Experimental and finite element assessment of stabilizing configurations for underground heritage sites

(I) Pre-support scheme-Crown zones

Option1: Pre-stressed anchors (Rock bolts)

Option 2: Concrete friction piles

System is envisaged to be installed from surface and will be connected with a buried grid of reinforced concrete beams

System is envisaged to be installed from surface and will be connected with a buried grid of reinforced concrete beams

Pre-stressed anchors (Rock bolts) are a suitable option as long as the rock mass is not in a disturbed state such that pre-stressing could not be achieved due to internal deformations. Beyond the suspension effect (which could also be achieved by a passive system) the important advantage is the minimization of future delamination as a result of the increased shear resistance along the bedding

Concrete friction piles (typical of 100–250 mm diameter with reinforcement bars) would act against sagging, whilst in the process of construction open bedding planes would be grouted

(II) Pre-support scheme-supporting pillars of Rotunda in the second level

Small diameter friction piles (typical of 50–100 mm diameter with reinforcement bars) could be used to upgrade the strength of the pillars of the Rotunda in the second level, as shown in Fig. 5

(III) Pre-support scheme-supporting the main Pillar in the third level

Option1: Concrete friction piles

Option 2: Buttressing and dowelling in combination with a dedicated beam-column support system

This could be pre-supported by reinforced concrete friction piles of large diameter than the above (typical of 200–250 mm diameter with reinforcement bars). This option is considered more appropriate for this main pillar, where construction difficulties in term of boring are minimal

Another proposal for this pillar is by buttressing and dowelling in combination with a dedicated beam-column support system as follows:

- Reinforced concrete beams embracing horizontally the pillar at haunch level will provide the heading of vertical reinforce concrete columns distributed parametrically according to sectional pillar geometry

- The heading beams, typically 170 × 100 mm in section are envisaged to be embedded in haunch replacing the disturbed haunch material

- The columns of anticipated cross Sect. 70 × 70 mm will be structurally reinforced and embedded in the dental concrete against waisting, for buttressing, etc. because of height; they may have to be interlinked via diagonal struts to be installed in holes through the pillars. The columns will be founded in sockets of sound rock

(IV) Innovative Seismic Intervention Retrofitting Measures

Innovative seismic retrofitting of the examined Catacombs, based on the strengthening of the resistant structure, also firstly hypothesized. The strengthening can be achieved also by a current intervention consisting in grouting concrete plates on the faces of the rock pillars. The plates are reinforced with steel net and connected with transversal bars disposed in holes filled with mortar. In addition, the pillars could be reinforced adequately by the steel jacket method, or one of the other innovative measures such as metal (aluminum and steel) shear panel, base isolation with horizontal diaphragm, shape memory alloy braces, steel buckling restrained braces (BRB) frames, SMA, C-FRP, etc

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