Active faults crossing lifeline routes: uncertainties of surface rupture parameters assessment and possible engineering solutions

Abstract

Surface faulting along active faults poses a threat to the facilities that might be affected. It is especially important for lifelines, trunk pipelines in particular, that, unlike other structures that can be just replaced, often cannot be retraced to avoid crossing with active faults able to produce surface faulting associated with large earthquakes. Pipelines' damage may produce significant economic, social and ecological losses. Pipelines, however, can sustain significant offsets that was demonstrated successfully by the Trans-Alaska oil pipeline during the 2002 Denali Fault earthquake. The critical point is that, estimating the design displacement that might occur during anticipated earthquake, researchers and engineers have to deal with significant uncertainties produced by combination of several factors: (1) significant and hardly predictable slip variability along fault strike; (2) very large, up to 1-2 orders of magnitude, scatter of maximal displacement of surface ruptures with the specified rupture length and during earthquakes with certain magnitude; (3) lack of data on the displacement values consistency at a particular fault section during successive rupturing events. Amount of fieldwork required to get such information is, usually, too large to be performed during site investigations for a particular lifeline project. Moreover, traces of older events are poorly recognizable and can be hardly characterized quantitively in the same way as those of modern surface ruptures. The pipeline design solutions, however, are "discrete" – i.e. they are appropriate for some range of the design displacement values. Stress-strain state of the pipeline strongly depends not only on fault displacement value, but also on fault kinematics, on relative orientation of a pipeline and fault at their crossing point, on the shape of a trench, backfill properties, pipe diameter and material. The preliminary analysis of all these factors and the numerical modeling of the pipeline-fault crossing can provide threshold displacement values which excess requires change of the crossing design. Thus, the main task of site investigation is to find if the anticipated displacement will exceed such threshold or not. If not – there is no need to waste time and efforts trying to specify the exact offset value and work can be concentrated on specifying fault kinematics, exact position of the anticipated fault plane, etc. Close cooperation of geologists and engineers at the earliest stages of pipeline projects implementation will help selection of optimal design solutions and optimization of the site investigations program.