The gas treatment facilities of the onshore plant at the landfall area of the Zohr Development Project, located west of Port Said, North-East Egypt, include two causeways designed by EniProgetti S.p.A., extending from the Zohr plant shoreline off to about 3 m water depth. The two causeways allow for the installation of the plant tower flares seaside.
Soils in the area consist of alternated layers of firm to stiff clay and medium dense sand/silty sand down to 40 m depth. The presence of sandy soils and the demanding design seismic input considered for the project, concur to determine a significant risk of seismic induced soil liquefaction at the site.
This paper presents results of a probabilistic liquefaction potential assessment and compares them with those resulting from a deterministic approach. The seismic input used for the deterministic analysis is determined according to the Egyptian Seismic Code, while the probabilistic analysis is based on the project specific PSHA conducted for the Zohr area by Rina Consulting S.p.A.
The main advantage of the probabilistic approach with respect to conventional deterministic methods is that all earthquakes potentially affecting the location are considered, summing up the hazard contributions from different magnitudes. In addition, unlike conventional deterministic methods, cyclic soil resistance is modelled using correlations that consider uncertainty explicitly.
Main inputs and results of the probabilistic liquefaction assessment that has been carried out for the design of the Zohr plant marine facilities, connected to the shoreline, are presented. The use of a probabilistic approach allowed downscaling the liquefaction risk at the site with respect to the deterministic approach. Based on this assessment liquefaction risk was eventually determined to be negligible for the project without need of special precautions in the plant design.
Liquefaction is the strength loss and subsequent fluid like behavior which may be exhibited by saturated soils under cyclic loading. Such loadings can result from ground shaking during an earthquake or from storm-generated waves. Cyclic loading causes loose cohesionless soil to compact and decrease in volume. Despite the relatively high permeability of such soils, under rapid loading drainage cannot occur and the tendency to decrease in volume causes an increase in pore water pressure. If the pore pressure increases to the point at which it equals the overburden pressure, the effective stress in the soil becomes zero, determining a complete loss of strength. This condition is referred to as ‘liquefied’ state.