DE LA PRESENTACIÓN DE LA SOLICITUD DE REGISTRO II. REQUISITOS DE FONDO

Stabilization of a potential or existing slide area may be accomplished by structural methods such as the use of retaining walls, earth anchors, vertical piles, or sheeting. Since these methods require that resistance to mass movement be obtained from below the slide area, it must be determined where a potential or existing failure plane is located. This will usually require detailed field investigation in order to determine the ground conditions so that the extent and type of slope stabilization can be determined.

Grading of soil or rock slopes is occasionally undertaken to increase soil strength or to decrease compressibility or permeability. However, it is generally quite costly and only justified when the more convential, simpler procedures described previously are unsuccessful.

Seismic Effects

Pipelines in seismic regions are vulnerable to earthquake loads, which could cause significant damage. Most earthquakes have their origin in tectonics, that is the energy causing the motion is produced from the tearing and grinding of the material associated with a slippage movement within an active fault system. A consideration of the geology along the pipeline route, is therefore pertinent to an assessment of potential earthquake activity. It would be appropriate, for example, to consider that the maximum magnitude of earthquake would occur either on an existing fault system or relatively close to it, where supplementary faulting might develop. However, as one moves away from such a region it is reasonable to assume that somewhat smaller magnitudes of earthquake are likely to be developed than along the main fault system. Associated with the magnitude is the local intensity felt at the pipeline, of an earthquake centered deep in the earth beneath the pipeline, or some distance away horizontally but at the same focal depth. In general, large earthquakes occur at focal depths of 70 to 80 km or greater, while those smaller in magnitude have a depth of focus of approximately 50 km. The ground wave attenuates with distance; Esteva (1970) concluded that the analysis of several strong motion accelerograph records ‘‘indicates that the predominant part of strong earthquake ground motions is represented by surface waves.’’ Hence, in order to take account of earthquakes centered some distance away, either directly below the pipeline or offset from it, an appropriate attenuation relationship for the region should be used. While not mandated, the Canadian Code for Oil and Gas Pipeline Systems Z662-03 (2007), in an Appendix, states that the determination of earthquake loads and pipeline displacements from ground motion data shall be based on the response spectrum method, the time history method, or equivalent methods. ASME B31.8 while addressing the subject of soil liquefaction (where the soil behaves like a fluid having lost its shear strength due to intense dynamic cyclic loading), states that the seismic design conditions

used to predict liquefaction shall have the same recurrence interval as that used to determine the operating design strength. On this basis the available historical data, the recurrence rates of earthquakes and their intensity can be assembled for number of predetermined sections or ‘‘links’’ of the pipeline. The seismic risk along the pipeline route can be described in statistical terms as a Poisson process, and the attenuation law for assessing the ground motion intensity at a given location of the pipeline is written as a function of the earthquake magnitude, source to site distance, and the local soil conditions.

The effect of earthquakes on the pipeline is multifaceted. The shaking from the earthquake induces additional stress into the pipe and additional forces into the anchors in the above-ground structures. In addition, the earthquake waves traveling through the ground impose bending, tension, and compression stresses in buried pipe. Shaking of the ground can cause liquefaction or compaction of some granular materials. The ground accelerations during an earthquake also add to the gravity and ground-water seepage forces, which contribute to slope failures.

The shaking of the ground also causes dynamic movements in the above-ground pipe and supports. Shaking perpendicular to the pipe causes the pipe to sway back and forth, thereby inducing bending stresses in the pipe between anchor supports. In addition, the longitudinal shaking of the pipe induces additional forces on the anchors. The stresses and forces become greater as the slope along the pipeline becomes greater.

The traveling seismic wave will induce stresses within the buried pipeline as it moves with the ground. The pipe though is sufficiently flexible in bending to accommodate the large radius of curvature that the ground movements will produce; thus, the induced bending stresses will be quite small. For example, the curvature of the ground caused by an earthquake is approximately the ratio of the maximum ground acceleration to the square of the wave propagation velocity of the surface waves (Newmark 1970). Suppose during a severe earthquake the ground acceleration has a value of 0.33g while the surface wave velocity is 760 m/s (2,500 ft/s). The ground curvature will be of the order of 0.0000016 ft1 and the induced bending stress in a 48 inches diameter pipe would be approximately 100 psi, which is negligible. However, considerably larger compression and tensile stresses can be produced by direct ground strains during the earthquake since the pipe is very rigid with respect to longitudinal strains.

At the surface of the soil, compressional waves are of negligible amplitude. However, the surface waves can have appreciable amplitudes. These waves will produce strains within the pipeline when their direction of propagation is at an angle to the pipeline. The maximum effect in the longitudinal direction of the pipe occurs when the wave is propagating at an angle of 458 to it, with shearing motion occurring in a direction at right angles to the propagation direction. Under these conditions the longitudinal strain in the pipe is approximately given by

"_L¼ maximum ground velocity 2 wave propagation velocity

For a pipe velocity of say 3 ft/s, and a wave velocity of 2,500 ft/s the longitudinal strain in the pipe will be 0.0006, which corresponds to a longitudinal stress of 18000 psi. In reality the pipe will not move entirely in sympathy with the soil, that is the soil in all likelihood will move faster than the pipe, and hence the longitudinal stress calculated above will be conservative. Nevertheless, it must be included in addition to other effects that cause stresses in the pipe in order to ensure its structural adequacy. A reasonable criteria for permissible deformation to avoid rupture is to limit strain to 2% at any section except at stress concentrations where it could double.

Seismic liquefaction: This occurs most commonly in fine-grained loose granular materials that are saturated. In this situation, the grains of soil are loosely stacked and all of the void spaces are filled with water. Upon occurrence of a seismic event, shaking causes the grains of soil to lose contact in an attempt to densify. During that period of time when the water is attempting to drain out so as to allow the densification of the soil grains, the soil mass takes on the characteristics of a dense viscous liquid. When the soil at the base of a slope liquefies the slope will become unstable, and in places where this has the potential to occur, the pipeline should either be rerouted if possible, or the soil suitably stabilized to reduce the risk of slumping.

With above-ground piping the dynamic motion is applied to the base of the anchor blocks and supports and hence their flexibility and freedom to rotate or tilt must be considered. The ground motion can be expressed either as a time history that enables maximum accelerations and velocities to be determined or as response spectra. This information, in either form, can be used as the forcing function in a dynamic analysis of a finite-element representation of the above-ground pipework. Finally, consideration must also be given, for both buried and above-ground pipelines, to the relative motion at faults crossing the pipeline. It is not uncommon to have vertical or horizontal displacements of several feet where faulting occurs. These of themselves will not cause the pipeline to rupture (Dykes 1996) if it has been designed to accommodate such movements. Relative movement of 4 to 5 feet can be sustained without failure by a properly supported above- ground pipeline, although one or two supports may lose contact with the pipe. With buried pipelines such displacements might cause severe distortions or misalignment but not necessarily rupture or collapse. Very useful guidance for the seismic design of water pipelines and hydrocarbon pipelines may be found respectively in the American Lifelines Alliance (2005) and Honegger (2005).