Design and construction of the deepest diaphragm walls in Cairo

Abstract

El-Azhar Road tunnels project in Cairo is the first urban underground road tunnel in Africa. It has four ventilation stations along 2.7 km of the tunnels route. The location of the third station was positioned at Port Said street, west of an existing flyover and CWO sewer tunnel. This station was the deepest one due to the crossing of the road tunnels under the sewer tunnel. The Port Said ventilation shaft chalked up two world records for: deepest 87.3 m retaining wall in urban setting; 1.5 m thick, built in separate panels. While the station was 37.3 m deep, Hydro-phrase machine, has been used for the construction of these diaphragm panels. To reduce the lateral deformations of the wall, RC horizontal slabs were adopted at different levels to be a monolithic structure. The ground deformations associated with the construction of the diaphragm walls of the underground stations can be divided into two components, one due to excavation and installation of the diaphragm walls and the other due to wall movements. These deformations are functions of many factors such as depth and shape of walls, soil properties, depth of excavation around the walls, type and stiffness of supporting system, surrounding structures, surcharge loads and time of construction. The comparison of the calculation and monitoring results of diaphragm walls have shown that the traditional methods are not compatible with the field measurements. The reaction modulus relating the horizontal displacements to the soil pressure in the linear range is inconvenient, as it is not a measurable geotechnical parameter and its determination still divides the specialists. Moreover, it does not describe the arching effect caused by the differential deformations of the surrounding soil. Some numerical codes that depend on this coefficient in the analyses do not have realization results and often marked by divergences with in situ measurements. A realistic numerical modelling of the deepest diaphragm walls is considered for evaluating the effect of construction sequence and the associated changes in the characteristics of the surrounding ground. This finite element modelling avoids the main drawbacks that usually appear in empirical or statical design methods. The main shortcomings that usually appear in these analyses are: the simulation of the wall thickness and wall shape; the strength of ground-structure interface; the construction phases; the hydraulic conditions and the dewatering of the excavation. The proposed procedures to simulate the diaphragm walls and the deep excavation of the ventilation station using FE method are presented in numerical example with full scale. The results of the analysis proved the ability of the proposed model to predict successfully the behaviour of interaction between the diaphragm walls and deep excavation of the shaft with retaining horizontal slabs and struts in soft ground. The results also confirmed the need for realistic simulation of concrete elements, combined action and establishment of construction procedure in the numerical model. It is very important to note that there was a very good agreement between the field measurements and those obtained by the proposed numerical modeling. The values of the internal forces from the analysis were lower than those of the contractor's analysis due to shortcomings in the contractor's analysis. However, the contractor analysis is suitable to cover the construction risk in this project. The back-analysis of this deep excavation using FE Method is required to cover the main drawbacks in the analysis and for choosing the soil parameters in major projects. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.