An Integrated Geophysical and Geotechnical Assessment of Hazards Around the Abu Serga Church

The refraction microtremors technique is based on two fundamental ideas. The first one is that common seismic-refraction recording equipment, set out in a way almost identical to shallow P- wave refraction surveys, can effectively record surface waves at frequencies as low as 2 Hz (even lower if low-frequency phones are used). The second idea is that a simple, two-dimensional slowness-frequency (p-f) transform of a microtremors record can separate Rayleigh waves from other seismic arrivals and allow recognition of true phase velocity against apparent velocities. Two essential factors that allow exploration equipment to record surface-wave velocity dispersion, with a minimum field effort, are the use of a single geophone sensor at each channel, rather than a geophone “group array”, and the use of a linear spread of 12 or more geophone sensor channels. Single geophones are the most commonly available type and are typically used for refraction rather than reflection surveying. The advantages of ReMi from a seismic surveying point of view are several, including the following: It requires only standard refraction equipment already owned by most consultants and universities; it requires no triggered source of wave energy, and it will work best in a seismically noisy urban setting. Traffic and other vehicles, and possibly the wind responses of trees, buildings, and utility standards, provide the surface waves. The present study uses the ReMi method to determine the S-wave seismic velocity with depth for Abu Serga church. This is important to determine the depth of the bedrock (any solid rock underlying the soil with S-wave >765 m/s, USGS, 190) as well as other engineering applications.

The paper presents the application of non-pervasive electrical resistance tomography (ERT) subsurface imaging surveys for the rehabilitation and strengthening of Abu Serga church in Cairo, Egypt. The use of several high-resolution geoelectrical methods derived from the field survey techniques proved to be very effective in the Non-Destructive Testing and survey of architectural heritage. In particular, the application of a tomographical approach allowed us to obtain subsurface images of the cross-sections of the bearing soil with complex layers and structures that clearly show the presence of eventual anomalies. Some experiments with geoelectrical tomographic techniques also gave very interesting results when working on historical buildings that seemed hostile to geoelectrics. This is very also interesting due to the velocity of the measurements and the data processing: this means short times and low costs.The use of micro-geophysical techniques offers many advantages with respect to some “classical” techniques under different angles: velocity of execution, non-pervasiveness and costs. The results of ERT are compared to ground penetrating radar (GPR) – they are just as detailed but are often easier to interpret at a lower cost.

Seismic hazard depicts the levels of chosen ground motions that likely will, or will not, be exceeded in specified exposure times. Hazard maps commonly specify a 10% chance of exceedance (90% chance of non-exceedance) of some ground motion parameter for an exposure time of 50 years, corresponding to a return period of 475 years. Such maps depict Peak Ground Acceleration (PGA) with a 10% chance of exceedance in 50 years for a firm soil condition. PGA, a short-period ground motion parameter proportional to force, is the most commonly mapped ground motion parameter because current building codes that include seismic provisions specify the horizontal force a building should withstand during an earthquake.

The probabilistic approach of earthquake hazard analysis aims at estimating the likelihood (probability) that any specified level of ground motion intensity will be attained or exceeded in an arbitrary future time period due to the trigger of earthquakes from potential seismic sources.

The analytical approach used to carry out probabilistic seismic hazard analysis is described in detail by Cornell, 1968; 1971; Esteva, 1969; Merz and Cornell, 1973; Cornell and Merz, 1975 and McGuire and Arabasz, 1990. The probabilistic seismic hazard assessment requires all the available information on seismicity and geotectonics of the examined region and regional attenuation characteristics of the ground motion as well as the adoption of a stochastic model for the forecasting of future earthquake occurrences.

The following basic elements of modern probabilistic seismic hazard had been assessed to find the seismic hazard in Abu Serga church; the compilation of a uniform database and catalogue of seismicity for the historical (pre-1900), and instrumental periods (1900-today) in and around Abu Serga church. The creation of a master seismic source model to describe the spatial-temporal distribution of earthquakes, integrating the earthquake history with evidence from seismotectonic, paleoseismology, mapping of active faults. The evaluation of ground shaking as a function of earthquake size and distance, taking into account propagation effects in different tectonic and structural environments. The computation of the probability of occurrence of ground shaking in a given time period to produce maps of seismic hazard and related uncertainties at appropriate scales.

Abu Serga church is located in the Coptic section of Old Cairo area. It is considered the oldest Coptic Church found in Egypt that represents the cultural heritage of outstanding universal values. It suffers multiple environmental, geotechnical and earthquake hazards.

The integrated geotechnical and geophysical investigation techniques were done to assess the seismic and other geo-environmental hazards and risks that Abu Serga church suffers from in the present and future. This knowledge is required to protect the building against seismic events. Abu Serga church suffers high risk in terms of seismic and environmental hazards. The main purposes of the present study is to: 1) Assess the level of seismic and other geo-environmental hazards at Abu Serga church. 2) Understand the nature of damage to this historic building caused by the recent earthquakes. 3) Offer technical support and advice on the restoration and repairs of the damaged structure. 4) Build up knowledge and case studies towards the restoration and repairs and survey the historic structure's damage state.

However, Abu Serga church is the oldest Coptic church that was constructed with little or no seismic considerations that represent the most enormous risk to most communities. These significant historical buildings must be assessed, their level of risk determined, and unacceptable risks reduced. This paper focuses on the issues associated with detailed risk and hazard assessment procedures for retrofitting architectural heritage. Many forms of evaluation and related procedures for the assessment of seismic risk for historical buildings have been discussed.

This study suggest a moderate level of earthquake activity at Abu Serga church, and this is in a good agreement with the fact that “Egypt is a part of the stable African Shield”, but the existence of old structure such as the Abu Serga church may reduce the ability to resist any earthquake shaking. This pilot study is essential with respect to the continuous efforts to preserve and restore cultural heritage worldwide.

The results of an integrated geophysical survey at the archaeological site of Abu Serga church, Cairo, Egypt, are presented and discussed. The aim was to investigate the ground conditions of the Church of Abu Serga (St. Sergius), Cairo's oldest. The Church was built in the 4th century, located at Qasr el-Shama in old Cairo in Egypt. In particular, the objective is to study the subsurface geological structures at the church's location and detect and possibly map any ancient remains concealed under the monument. The survey was conducted using two geophysical methods: the groundpenetrating radar (GPR), a fully non-destructive method, and the electrical resistivity tomographies (ERTs). The usefulness of combining conventional geophysical mapping techniques and high resolution imaging methods in delineating shallow targets of archaeological interest at such complex archaeological sites is studied. Ground penetrating radar time slices and 3D electrical tomography depth slices were used for the verification of specific anthropogenic anomalies, which were detected on the geophysical maps Processing of geophysical maps included filtering with the gradient and first derivative operators in the space domain and the upward continuation and Butterworth filters in the wave number domain. The integration of the geophysical measurements revealed that the present Crypt is not the original holy Crypt. The anomalous reflector is detected at a depth of about 5 m below the sanctuary floor, in the form of buried ceiling of the original Crypt. The present Crypt is just a small low subterranean church belong to the 2nd century. High resistivity anomalies and distinct GPR signals were also observed deeper in the inner parts of the church. They are attributed to possible remains of ancient walls and surrounding tunnels or other manmade structures concealed under the monument's floor. The geophysical survey at Abu Serga church also demonstrates that the general features of the foundation soil are heterogeneous with the abundance of fractures; the water table is very high at 1.5m below the sanctuary floors. The benefits of combined geophysical surveys in the case of archaeological investigations at complex sites are highlighted.

The response spectra for the Faiyum earthquake were selected as the best earthquake for the most effective zone near Abu Serga church to construct the design response spectrum for the church. The original acceleration time history was recorded 10 km away from Abu Serga church over the bedrock of Cairo city at Mokattam area (about 10 Km from Abu Serga church). This is considered to be the first response spectrum done for all earthquakes that ever affected Egypt. Fortunately, this earthquake is regarded as one of the most important earthquakes that affected Egypt in the last 100 years. The maximum acceleration response spectrum maintained for Abu Serga church for the church's fundamental resonance frequency, which is 5.5 HZ, is about 100 cm/sec2 for 5% damping. As for the other resonance peaks for the 2nd wooden floor and the roof, nearly the same spectral acceleration occurs (0.1 g).

This research presents the damage mechanism of a historical masonry architecture induced by differential settlement based on 3D FE analysis. The study aimed to investigate the behavior of fully-saturated soft clays subjected to self-weight loading from an old masonry structure of Abu Serga church, the oldest church in Cairo. The church was built in the 4th century and was probably finished during the 5th century, located in misr alqaddima area in Cairo. The church gains its high prestige to having been constructed upon the Holy crypt of the Holy Family where they stayed during their sojourn in Egypt. The main objective of the present study is to accurately record and analysis the geotechnical problems and induced structural failure mechanisms observed and calculated in the field, experimental and numerical studies. The land area is also susceptible to floods. Numerical analysis for such geotechnical problems is largely expected to contribute to the conservation of cultural heritage.

The present research presents an attempt and pilot study to design the PLAXIS 3D FE model to simulate ground problems and distort and analyze the stress of the complex structure of the Abu Serga church, which is loaded on a plane level. Plastic modeling or Mohr Coulomb model in advanced soil was used during the various stages of numerical analysis. Results are recorded and discussed with respect to the stress and volumetric behavior of soil. Finally, the study represents the design studies and implementation of the inter-organizational retrofitting intervention and strengthening project for the oldest Coptic Church in Egypt.