The threat category ‘Outside Force’ in a risk assessment typically includes forces such as ground/earth movement, flooding, cold weather, and lightning. When buried pipelines cross sloping terrain, there is a danger that soil movement (down the slope) will impose an excessive load on the pipe that could lead to failure. Slope or earth movements can be described as movement of earth materials (rock, debris, soil, etc) down-slope as a result of the pull of gravity. Defined as movement of mass of rock, debris, or earth down a slope, slope movement can be classified as falls, topples, slides, and flows.
Potentially damaging movements may occur below, above, or through the pipeline location.
More complex landslides may involve combinations of two or more types of movement starting with material moving that eventually with combination of water (from rain or other water sources) form a heavier flow that may have more rocks, soils and other debris. The rate and severity of slope movement depends on factors such as the steepness of the slope, the presence of water, the stability of the soil and other loading or overburden applied to the slope (perhaps from construction and/or maintenance activities). Velocity within flowing mass decreases with depth and laterally.
Earthquakes may also cause damaging earth movement. Earthquake damages/effect are categorize as tectonic and non-tectonic. Tectonic affects deals with fault / surface ruptures and are typically associated only with large earthquake. This phenomenon can cause severe damage to buildings, bridges, tunnels, canals and underground utilities. Non-tectonic are directly related to earthquake shaking and cause liquefaction, earthquake induced slope failures, landslides, tsunamis, and seiches (standing waves). Liquefaction typically occurs in soil with high groundwater table and it is commonly associated with low areas or near water stream. Earthquake induced slope movement is often divided into falls (free-falling of rocks, debris, etc) and slides (movement along the surfaces).
Meteorological effects can cause damages to the pipeline through heavy rain (flooding), cold weather causing frost heaving, strong wind damaging exposed pipeline segments, and lightning strikes. Flooding as a result of heavy rain and poor drainage will not only add to the effect of slope movement and erosion but it also affects soil permeability. Cold weather may freeze ground moisture, preventing water from draining away. Frost heave occurs when the freezing of the soil results in the formation of layers of segregated ice at shallow depths. Thermal loading (expansion by the freezing of water in pores and fractures in cold regions, adds external load onto the pipeline. Lightning strikes have been shown to cause damage on both buried and above-ground facilities.
Mitigation of geohazards is most commonly done in the design phase of a pipeline project. Routing, burial depth, structural strength provisions, anchoring, and buoyancy control are examples of design-phase mitigation opportunities. When mitigation of geohazards on an existing pipeline must be done, it tends to be very situation specific. Anchoring, stress relieving, improved drainage, and pipe insulation are among the opportunities.