2 An overview of peat landslide mechanisms
2.1 Mechanisms and morphology of peat landslides
Peat landslides represent one end of a spectrum of natural processes of peat degradation. Longer term processes of degradation include incision and upslope extension of gully networks by water action (Evans and Warburton, 2005), development of subsurface piping creating extensive sub-surface voids (Holden, 2004; 2005), desiccation cracking and wind erosion (deflation) of the top surface of peat deposits (Evans and Warburton, in press), and structural damage caused by burning of frost action. All of these processes may result in damage to peatland habitats, potential losses in biodiversity and depletion of the peatland carbon store, which globally represents some 30% of the carbon stored in world soils (Immirzi et al, 1992). Human activity, including burning, farming (grazing), afforestation and construction may also act to damage the peat resource.
Two broad groups of peat landslide are reported ( Plates 2.1 and 2.2). The term 'peat slide' is generally used to describe slab-like shallow translational failures (Hutchinson, 1988) with a shear failure mechanism operating within a discrete shear plane at the peat-substrate interface, below this interface, or more rarely within the peat body (Warburton et al., 2004). The peat surface may break up into large rafts and smaller blocks which are transported down slope mainly by sliding. Rapid remoulding during transport may lead to the generation of an organic slurry in which blocks are transported. Peat slides correspond in appearance and mechanism to translational landslides (DoE, 1996) and tend to occur in shallow peat (up to 2.0m) on steeper slopes (5 to 15°). A great majority of recorded peat landslides in Scotland, England and Wales are of the peat slide type.
The term 'bog burst' has been used to describe particularly fluid failures involving rupture of the peat blanket surface or margin due to subsurface creep or swelling, with liquefied basal material expelled through surface tears followed by settlement of the overlying mass (Hemingway and Sledge, 1941-46; Bowes, 1960). They are characterised by pear shaped areas of disturbed (often sunken) blanket bog, arranged in concentric tears and rafts, with little substrate revealed, and without necessarily a clear scar margin. Downslope of the area of subsidence, there is usually a block and slurry runout zone, similar in appearance to that associated with peat slides. Bog bursts correspond in appearance and mechanism to spreading failures (DoE, 1999) and tend to occur in deeper peat (greater than 1.5m) on shallow slopes (2 to 10°) where deeper peat deposits are more likely to be found (Mills, 2002). Reports of bog burst failures are generally restricted to Ireland and Northern Ireland.
There is considerable natural variability in movement types and complex failures may result where the geotechnical properties of the peat vary. Hence there is some degree of overlap in processes and mechanisms between different landslide types.
Peat soils accumulate over thousands of years under generally wet and cool climatic conditions. Changes in the water table govern rates of organic matter decay. The resultant 'soils' are composed of vegetative matter in various states of decomposition rather than mineral particles. Therefore, conventional geotechnical approaches to mineral soil analysis are poorly tested with respect to peat, and the use of slope stability analyses to predict realistic 'Factors of Safety' requires correspondingly greater understanding of site-specific controls.
A number of hydrological and geomorphological preparatory factors operate in peatlands which act to make peat slopes increasingly susceptible to failure without necessarily initiating failure. Triggering factors change the state of the slope from marginally stable to unstable and can be considered as the 'cause' of failure (DoE, 1999). These preparatory and triggering factors are described below.
2.2 Preparatory factors for peat instability
The following are some of the factors which operate to reduce the stability of peat slopes in the medium to long-term (tens to hundreds of years):
(i) Increase in mass of the peat slope through progressive vertical accumulation (peat formation);
(ii) Increase in mass of the peat slope through increases in water content;
(iii) Reduction in shear strength of peat or substrate from changes in physical structure caused by progressive creep and vertical fracturing (tension cracking), chemical or physical weathering or clay dispersal in the substrate;
(iv) Loss of surface vegetation and associated tensile strength; and
(v) Increase in buoyancy of the peat slope through formation of sub-surface pools or water-filled pipe networks.
The impacts of factors (i) and (ii) are poorly understood, but the formation of tension cracks and pipe networks have been noted in association with many recorded failures. Long-term reductions in slope stability contribute to slope failure when triggering factors operate on susceptible slopes, as described below.
2.3 Triggering factors
Triggering factors act to initiate slope failures, which may be slow to rapid movements and spatially extensive or relatively limited in extent with associated implications for their impacts. Triggering factors may be natural or anthropogenic and can result in either peat slides or bog bursts dependent upon peat characteristics and topography at a particular site.
Natural triggers are reported as follows:
(i) Intense rainfall causing development of transient high pore-water pressures along pre-existing or potential rupture surfaces ( e.g. at the discontinuity between peat and substrate);
(ii) Snow melt causing development of high pore-water pressures, as above;
(iii) Rapid ground accelerations (earthquakes) causing a decrease in shear strength;
(iv) Unloading of the peat mass by fluvial incision of a peat slope at its toe, reducing support to the upslope material; and
(v) Loading of the peat mass by landslide debris causing an increase in shear stress.
Factors (i) and (ii) are most frequently reported for peat mass movements in the UK. Anthropogenic ( i.e. human induced) triggers include some of the following:
(i) Alteration to drainage pattern focussing drainage and generating high pore-water pressures along pre-existing or potential rupture surfaces ( e.g. at the discontinuity between peat and substrate);
(ii) Rapid ground accelerations (blasting or mechanical vibrations) causing an increase in shear stresses;
(iii) Unloading of the peat mass by cutting of peat at the toe of a slope reducing support to the upslope material;
(iv) Loading of the peat mass by heavy plant, structures or overburden causing an increase in shear stress;
(v) Digging and tipping, which may undermine or load the peat mass respectively, and may occur during building, engineering, farming or mining (including subsidence);
(vi) Afforestation of peat areas, reducing water held in the peat body, and increasing potential for formation of desiccation cracks which are exploited by rainfall on forest harvesting; and
(vii) Changes in vegetation cover caused by burning, heaving grazing or stripping of the surface peat cover, reducing tensile strength in the upper layers of the peat body.
Natural factors are difficult to control, and while some anthropogenic factors can be mitigated, some cannot. For these reasons it is essential to identify and select a location for the development and associated infrastructure that avoids or minimises the impact of the development.
2.4 Pre-failure indicators of instability
The presence of preparatory or preconditioning factors, prior to failure, are often indicated by ground conditions that can be mapped or measured remotely or by a site visit. In many cases, sites that have experienced landslides apparently without warning could often have been identified as susceptible to failure by a suitably trained person or through relatively inexpensive monitoring strategies. The nature and signs of instability often differ depending on the type and scale of failure. The following critical features are indicative of potential failure in peat environments:
- Presence of historical and recent failure scars and debris;
- Presence of features indicative of tension;
- Presence of features indicative of compression;
- Evidence of 'peat creep';
- Presence of subsurface drainage networks or water bodies;
- Presence of seeps and springs;
- Presence of cracking related to drying/drainage;
- Concentration of surface drainage networks;
- Presence of clay with organic staining at the peat and (weathered) bedrock interface.
Each of these indicators is considered below with illustrative plates to guide recognition during site visits.
2.4.1 Presence of historical and recent failure scars and debris
The presence of existing landslide scars in a development area may indicate local site conditions conducive to future peat landslide activity. Plates 2.1 and 2.2 illustrate typical peatland morphology associated with historical failure sites. With increasing time since failure, exposed scars will re-vegetate. However, where a bare substrate has been revealed by sliding, a full vegetation cover may take 30-40 years to develop.
Although reactivation of the debris or peat surrounding landslide scars has rarely been noted in the published literature, spatial clustering of peat landslides, separated in occurrence by many years, has been identified on several occasions. Therefore, the existence of a peat landslide scar in a development area provides a strong indicator of potential future peat landslide hazard.
2.4.2 Presence of features indicative of tension
Surface or deeper tension cracking may indicate an accumulation of stress in peat soils as well as generation of surface-to-subsurface pathways for rapid infiltration of water and generation of excess pore-water pressures at depth. Tension features may include tension cracks, which are narrow and deep fissures, frequently infilled with water, and which may be continuous or discontinuous for several tens of metres ( Plate 2.3). Alternatively, shallow tears, which are wider and shallower 'diamond' shapes may indicate tension at the surface only ( Plate 2.4). Concentric tiers of arcuate tension cracks may indicate local displacement, while multiple intersecting cracks may be a precursor to fragmentation of the peat into rafts or blocks ( Plate 2.5).
2.4.3 Presence of features indicative of compression
Compression features usually indicate displacement upslope which has resulted in the formation of ridges ( Plate 2.6), thrusts ( Plate 2.7) or extrusion features ( Plate 2.8). Often, tension features will be visible at the upslope limit of peat displacement.
2.4.4 Evidence of peat creep
Tension and compression features are often associated with creep of the peat blanket on a slope. Zones of tension are often juxtaposed with compression ridges in response to creep of the peat mass and changes in local slope gradient. At the surface such movements can be detected by displacement of walls and boundaries and tilting of fences and posts.
2.4.5 Presence of subsurface drainage networks or water bodies
Subsurface drainage pathways indicate potential for generation of high transient pore-water pressures under conditions of enhanced water supply, e.g. during an intense rainstorm. Soil piping is widespread in upland blanket peat catchments in the UK, with pipes tending to be more prevalent at hillslope summits and footslopes and in areas of peatland subject to moorland gripping (Holden, 2004; 2005).
Such pipe drainage networks ( Plate 2.9) can often only be identified on-site by the sound of running water beneath the surface. Rarely, a pipe ceiling may collapse, leaving a hole in the peat surface ( Plate 2.10). If pipe networks are identified, their size and extent can be ascertained through non-intrusive ground investigation e.g. Ground Penetrating Radar (Holden, 2004). Larger subsurface water bodies, formed where pipes have become blocked or where spring lines are incident below the surface, can be identified by 'trembling' at the peat surface when the surface is walked over (although this is not advisable). Continued supply of water (without release) to a subsurface water body may cause visible swelling of the peat mass over periods ranging from a few hours to several months. Evidence of drainage outlets should also be noted as these usually indicate a well-developed subsurface pipe network. The presence of sediment discharged at pipe outlets often indicates a deep subsurface drainage net with periodically high water pressures.
2.4.6 Presence of seeps and springs
Groundwater seeps and springs are controlled by seasonal rainfall. Large fluctuations in rainfall may increase the rate of groundwater discharges and if this occurs following a period of drought there is an increased risk that peat landslides may be induced.
2.4.7 Presence of cracking related to drying
Drying of peat caused by periods of drought or by drainage (natural or man-made) may also cause cracking, providing pathways for rapid infiltration of water to horizons at depth within the peat mass.
2.4.8 Concentration of surface drainage networks;
Surface drainage pathways also provide a means of supplying water to a susceptible peat area, and are generally manifest in peat as gullies ( Plate 2.11) or flushes / soakaways ( Plate 2.12). Concentration of runoff by artificial drainage networks should be noted especially where the drainage density is greatly increased or runoff is delivered to steep peat-covered hillslopes.
Any of the indicators described in 2.4.1 to 2.4.8 may, in isolation, indicate future potential for peat landslides to occur. Combinations of these features may be indicative of more imminent failure.
2.4.9 Presence of clay with organic staining at the base of the peat
In parts of the Pennines and Scotland, the base of blanket peat may be underlain by clay with organic staining, sometimes no more than 100-200mm in thickness. These clay layers have been observed to provide a failure surface for detachment and subsequent sliding of the peat along the bedrock interface or underlying superficial deposits.