The movement of many landslides is controlled by the force imbalance associated with a reduction in shear resistance caused by a decrease in normal effective stress as pore water pressures increase. This basic premise might lead to an assumption that the movement rate has a simple relationship with the pore water pressure / normal stress state, but previously studies have shown marked differences in this relationship according to whether pore water pressures are rising or falling. This paper reviews examples from the literature in which high resolution monitoring allows the relationship between the movement rate and the pore water pressure / normal effective stress state to be determined. We show that a variety of relationships exist between these parameters with the key determinant appearing to be the peak movement rate of the landslide during the movement event in question. We propose that the key factor is whether the yield stress is exceeded. If so, rate and state friction may dominate; if not then creep decay may be critical.
Landslides are a pervasive hazard on the surface of the earth, responsible for an average of up to 14,000 fatalities per annum (Petley 2012). Triggered primarily by one or more of the effects of precipitation, seismic shaking or slope alteration by humans, landslides also induce substantial socio-economic impacts on society. In many cases these effects are magnified by a lack of insurance cover, resulting from both their socio-economic setting (the majority of loss-inducing landslides occur in comparatively poor countries across Asia and Latin America) and by an unwillingness by insurance companies to provide cover for mass movement hazards in most territories. The latter results from a perceived poor understanding of the geographic distribution of landslide hazard, and the difficulties of determining potential levels of consequential loss. The effects are to increase the impact of landslide hazards relative to other natural hazards.
Whilst the majority of human casualties are associated high velocity landslides, and in particular debris flows, mudflows and soil/rock avalanches, slow moving landslides can cause high levels of financial loss, and, in some cases, loss of life. Thus, understanding these landslides remains a priority. The simple mechanics of these landslides is well-understood in terms of the role of elevated pore fluid pressure leading to a reduction in normal effective stress, and thus failure, and the development of strain, when the yield strength is exceeded. However, observed patterns of movement are more complex than this simple relationship would imply, and are important in terms of understanding, and forecasting, future behaviour for any slow moving landslide. In this paper, the relationship between pore fluid pressure and the rate of movement of landslides is reviewed, demonstrating complex patterns that have hitherto not been fully understood. Interestingly, glaciers display similar behaviour, and a number of hypotheses have been proposed to explain these mechanics across the two types of mass movement. The viability of these models for landslides is examined, and a new framework is proposed to account for the observed complex behaviour in landslide systems.
2 PATTERNS OF MOVEMENT OF SLOW MOVING LANDSLIDES
2.1 A review of landslide movement patterns
It is well established that the movement rate in a slope has a non-linear relationship with pore water pressure (e.g. Bertini, et al, 1984; Gonzalez et al. 2008). In general, once movement has commenced small increments of additional pore water pressure lead to successively greater increases in movement rate; the relationship between pore water pressure and movement rate is sometimes characterised as being exponential. This has sometimes been characterised with a viscosity modification to the Mohr-Columb failure criterion, with some success in predicting the moment patterns of flow type landslides. These models predict a movement rate for any given value of pore water pressure in the landslide, regardless of the dynamic state of that pore water pressure. Of course in reality, factors such as the geometry of the landslide play a key role. Thus, for example, in a rotational landslide the mass becomes increasingly stable as strain accumulates, such that the relationship between strain rate and pore water pressure will change as movement develops (primarily because the static stress state will change). However, in a large landslide this changing relationship will require large strains to become significant.
Some landslides show a simple relationship between pore water pressure and rate of movement in monitoring data. Thus, for example, monitoring of the Vallcebre landslide in the Eastern Pyrenees of Spain showed a simple, non-linear relationship between velocity and the depth of ground water (i.e. the shear surface pore water pressure) (Corominas et al. 1999; Fig. 1). In such cases the movement rate of the landslide can be predicted for any groundwater level.
Figure 1: Patterns of movement of the Vellcebre landslide. Data from Corominas et al. (1999). Figure from Massey (2010)
Interestingly, however, there are a number of documented cases in which this relationship has proven to be more complex than might be expected, even when geometric factors have been taken into consideration. In particular, many landslides show a different movement response when pore water pressure is increasing in comparison to when pore water pressure is falling. But, surprisingly, there is no consistent relationship. The following sections provide some examples. Unfortunately though, there is a surprising paucity of published examples in which monitoring data is of sufficient quality to allow this relationship to be examined in detail.
2.2 La Valette landslide, France
La Valette landslide is located close to Saint-Pons in the Barcelonnette basin, in the Alpes-de- Haute-Provence region of France. Movement began in March 1982 as a reactivation of a pre-existing landslide (Van Asch et al. 2007). The landslide consists of an upper rotational slide that transitions into a mudflow as the displaced blocks degrade. It is large – the estimated volume is about 3.5 x 106 m3, the length is about 2 km and the shear surface depth is 25 to 35 m in the central part of the mudflow. The landslide moves at variable rates, with a total displacement rate of about 1 to 2 m per annum.
La Valette landslide is extensively monitored due to the threat that it poses to the community at the foot of the slope. Van Asch et al. (2007) presented monitoring data for the landslide during a phase of increased pore water (Fig. 2). As expected they found a non-linear relationship (hysteresis) between movement rate and groundwater level, but perhaps less predictably they also found that the movement rate when the ground water was increasing was substantially different from that when groundwater level was declining. In this case a rising groundwater level was associated with a higher movement rate than was the case with a falling groundwater level. This was found to be consistent across two substantial periods of movement. This behaviour appears to be a complex version of strain hardening, in which resistance to movement increases with deformation. In this paper we refer to this style of relationship between movement rate and pore water pressure as strain hardening behaviour. Note however that this is a more complex style of behaviour than is normally ascribed to strain hardening as the increased resistance appears to develop at the point at which pore water pressures start to fall, but not before.