TXT-tool 1.064-1.1 Field Guide for the Identification and Assessment of Landslide and Erosion Features and Related Hazards Affecting Pipelines

Abstract

This Field Guide was originally prepared by GNS Science in 2007 as part of the Institute of Geological and Nuclear Science (GNS Science) Short Course on the Identification and Assessment of Landslide and Erosion Hazards aimed at pipeline overseers and technicians. The guide was originally designed as a reference for technicians to use in the field during pipeline inspections, but can also be applied to other engineered structures. It contains background information on landslides and other erosion processes, including: some definitions; a short glossary of relevant landslide terms; discussion on observation and recording of geological and geomorphic information in the field; a landslide and erosion classification scheme developed for pipeline technicians; and an example “Relative Hazard Exposure Matrix” that could be used as an initial tool to assess and rank the potential consequences to a pipeline should landslide and other erosion hazards occur.

Appendix 1. Example Relative Hazard Exposure Matrix

This is an example “Relative Hazard Exposure” matrix, established by GNS Science for pipeline engineers working in New Zealand, and reflects the type of landside and other erosion hazards present along a typical gas pipeline in New Zealand. It should be noted that the types of hazard and the impacts on a pipeline should the hazard occur, would change from site to site. This table is one example of a hazard exposure matrix that could be developed once the different landslide and other erosion hazards along a pipeline alignment have been assessed.

Hazard class	Impact scenario class
	1. High High risk to pipeline, where the pipeline has been exposed at the ground surface, or where the hazard is highly active and could lead to failure of the pipe	2. Intermediate No immediate risk to the pipeline, however, ongoing development of the hazard could impact the pipeline in the future (months)—hazard assessed as active	3. Low No risk to the pipeline at present or in the future (years)—hazard assessed as inactive
Land movement
(A) Open-slope flows (B) Channelised Flows (C) Slides	Retrogressive failure of existing landslide scarp or new landslide source area has undercut the pipeline. Pipeline may be exposed (even ruptured), or tension cracks and/or displaced blocks (of ground) can be observed across the pipeline alignment OR Landslide debris (from either open-slope or channelised flows), sourcing from up slope (above the pipeline) has impacted the pipeline; either eroding material (exposing the pipe), or depositing significant material (>3 m in height) over the pipeline alignment	Landslide main scarp is <5 m from pipeline alignment, with evidence of ongoing instability (retrogression) in the form of tension cracks, or fresh debris apparent OR Debris from a landslide (either open-slope or channelised flows), sourcing from up slope (above the pipeline) has impacted the pipeline; either eroding material (not exposing the pipe, but < 0.8 m between the pipe invert and ground surface, from probing), or depositing significant material (1–3 m in height) over the pipeline alignment. Or where potential exists for future landslide debris to impact the pipeline	Landslide main scarp is >5 m from pipeline alignment, no evidence of ongoing instability apparent upslope of landslide main scarp OR Debris from a landslide sourcing up slope (above the pipeline) has impacted the pipeline but no evidence of erosion and/or only minor deposition (<1 m) apparent
(D) Slides continued	For slides and where the pipeline alignment passes through the debris associated with an existing slide, reactivation (movement) of the slide block can cause significant damage to the buried pipeline. This movement may not always be visible on the surface, however evidence of fresh landslide scarps, hummocky/rolled ground, tension cracks, deformed vegetation and ponded water in and around the pipeline alignment are indicators that deep ground movements (associated with slides) may have occurred. If possible, the area of movement and movement rates (from knock out pegs, fence post measurements etc.) should be identified. Due to the nature and difficulties in assessing these types of hazard, they should be initially classified as HIGH, pending further investigation
(E) Rock falls	Large, >10 m3 volume of rock fall debris (either as one large block or as multiple smaller blocks), has fallen onto the pipeline alignment, with obvious signs of ground deformation (settlement) around the pipeline alignment	Intermediate, between 1 and 10 m3 volume of rock fall debris (either as one large block or as multiple smaller blocks), has fallen onto the pipeline alignment, with no obvious signs of ground deformation (settlement) around the pipeline apparent. Evidence of future rock fall potential (dilated rock mass and loose boulders visible on slope) apparent	Small, <1 m3 volume of rock fall debris (either as one large block or as multiple smaller blocks), has fallen onto the pipeline alignment
(F) Sheet, Rills and Gullies	Pipeline exposed due to severe rill and gully development in soil (gullies typically >0.5 m deep and >1 m wide), and where >20% of the area is affected	Areas of bare soil, where multiple rills have developed (typically <0.5 m deep and <1 m wide) in and around the pipeline alignment, with <0.8 m between the pipe invert and base of largest rill (from probing). Typically an area where 5–20% of bare soil is exposed	Areas of soil, either partially or totally devoid of vegetation (one or two rills may be apparent on the surface) either along, or immediately adjacent to the pipeline alignment. Typically an area where <5% of bare soil is exposed
(G) Tunnel gullies (sink holes)	Multiple tunnel gullies in and around the pipeline alignment. Diameters of tunnel gully/gullies (pipes) are >0.5 m, and where >20% of the area is affected. Evidence of tunnel gully roof collapse apparent	Multiple tunnel gullies in and around the pipeline alignment. Diameters of tunnel gullies (pipes) are >0.2 m but <0.5 m, and where 5–20% of the area is affected. No evidence of tunnel gully roof collapse apparent	One or two tunnel gullies identified in an area either along or immediately adjacent to the pipeline alignment. Diameters of tunnel gully/gullies (pipes) are <0.2 m, and where <5% of the area is affected
(H) Wind (removal or deposition of material by wind)	Pipeline exposed at surface. Typically where >20% or area affected by wind erosion	Area of active erosion with typically between 5 and 20% of area affected, and where there is <0.8 m of cover (from probing) between pipe invert and ground surface	Area of erosion with typically <5% of area affected, and where there is >0.8 m of cover (from probing) between pipe invert and ground surface
(I) Coastal erosion (cliff line retreat)	Coastal sea cliff actively retreating (evidence of recent landslides, rock falls from the cliff and related debris at toe of cliff) and where the crest of the sea cliff is < 0 m from the pipeline. Or, where the pipeline traverses across a zone/area of weakness (near an actively retreating crest of a sea cliff) and signs of subsidence (sink holes—disc like depressions) are apparent on the surface. Sea cliff erosion (in particular mapping extents of caves/sink holes) is difficult to assess without detailed investigations and therefore should be initially classified as HIGH, pending further investigation		Coastal sea cliff retreating but at relatively low rates (no evidence of recent landslides, rock falls from the cliff and related debris at toe of cliff) and where the crest of the sea cliff is >10 m from the pipeline, with no signs of subsidence apparent on the surface
River erosion
(J) Bank erosion	Pipeline is exposed or < 1 m from an actively eroding stream bank (lateral bank erosion), in very soft to firm, silt, clay and organics; and/or loose sand and gravels (refer to field guide for material and strength definitions)	Where the pipeline alignment is between 1 and <10 m (horizontal distance) from an actively eroding stream bank, in very soft to firm, silt, clay and organics; and/or loose sand and gravels (refer to field guide for material and strength definitions). Or, where the horizontal distance (from pipeline to stream bank) is <10 m and the height of the bank is >2 m	Where the pipeline alignment is: >10 m (horizontal distance) from a stream bank, in very soft to firm, silt, clay and organics; and/or loose sand and gravels (refer to field guide for material and strength definitions); or where the horizontal distance (from pipeline to stream bank) is <10 m and the height of the bank is <2 m
(K) Bed erosion	Erosion (along the stream/river course) has exposed the pipeline in the river bed	Stream/river course has either changed location (typically following storm events), with erosion now covering either a larger area, or different section of the pipeline crossing. Or where there is a constriction to the flow (new area of deposition, e.g. boulders, vegetation etc.) causing concentrated erosion in the area of the pipeline crossing. Due to the unpredictable nature of these hazards any changes to the stream course, which are likely to cause erosion to the pipeline (based on your assessment) should be classified as INTERMEDIATE
Human activity
(L) Placement of Fill (Settlement)	Where fill material >3 m in height has been placed over or adjacent to the pipeline alignment. Or where fill material has been stockpiled and potential exists for the material to form a landslide and impact the pipeline (refer to Hazard types A–C)	Where fill material >1 m in height has been placed over or adjacent to the pipeline alignment, and where soil types are soft to firm, silt, clay and organics; and/or loose sand and gravels (refer to field guide for material and strength definitions), and where there is ponded water apparent, or the area is known to water log (e.g. on river terraces). Or where there are signs of surface instability	Where fill material <3 m in height has been placed over or adjacent to the pipeline alignment, and where soil types are: stiff, silt and clays; dense, sands and gravels; and/or strong to very strong rock (refer to field guide for material and strength definitions). And where the area is not prone to water logging and where there are no signs of surface instability
(M) Slope modification	Where a steep cut slope (>3 m in height) has been created either immediately below or above the pipeline alignment in: soft to firm, silt, clay and organics; loose sand and gravels; or weak to moderately weak rock (refer to field guide for material and strength definitions), with evidence of severe erosion (gullies etc.), or landslide features (tension cracks, landslide scarps, fresh debris etc.) apparent on or around the slope	Where a steep cut slope (>3 m in height) has been created either immediately below or above the pipeline alignment, in: soft to firm, silt, clay and organics; loose sand and gravels; or weak to moderately weak rock (refer to field guide for material and strength definitions), with evidence of minor erosion (sheet and rills), but with no signs of surface water or instability on the surface	Where a steep cut slope (>3 m in height) has been created either immediately below or above the pipeline alignment, and where soil types are: stiff, silt and clays; dense, sands and gravels; and/or strong to very strong rock (refer to field guide for material and strength definitions) with evidence of only minor erosion (sheet and rills), but with no signs of surface water or instability