Reading on the news about the dramatic landslide-mudflows (picture) that occurred (again) in Petropolis near Rio de Janeiro, with significant fatalities, one thinks could this have been predicted and more so avert it?
Even in an area prone to superficial mudflows and thin landslides, it is very difficult to accurately predict the occurrence of such a landslide. This is for the following principal reasons:
- It is difficult to assess the geotechnical parameters of a superficial material that undergoes continuous drying and wetting cycles.
- The tests are usually performed in saturated samples and in higher effective stresses that are usually present in superficial slides.
- A linear Mohr – Coulomb failure criterion is used when it may be more appropriate to use a power function that defines a curved strength envelope with minimum or even zero effective cohesion in very low to zero confinement (especially for uncemented soils).
- The soil is usually unsaturated with some (or even large) suction which is modified during rainfall infiltration. The pore pressure regime is very difficult to assess without continuous specific monitoring.
- Numerical techniques that can accommodate such complex problems are not widely available and not everybody knows how to use efficiently such models.
- The usual triggering mechanism is intense rainfall which increases the pore pressures in the superficial soil. Since the way water infiltrates in the unsaturated soil, depends mostly on permeability, water content at the time of rainfall, the vegetation cover and the angle of the slope, the problem of evaluating such pore pressures becomes even more difficult.
This problem of predicting mudflows is usually assessed based on prior experience and some statistical evaluation of past mudflows and amount and intensity of rainfall. Can we do better?
Unstable soil, rock or combination of both materials, manifests in a land sliding mass when the driving forces are higher than the resisting forces. In essence a landslide forms when the shear strength providing the resisting force becomes smaller than the driving force which may be a combination of weight, external load etc. The usual conditions that trigger a landslide are the increase in water pressure which lead in a reduction of effective stress and shear strength.
The water or pore pressures increase when heavy or prolonged rainfall occurs or when snowmelt takes place. Unfortunately these conditions cannot be controlled and a long term prediction is not possible (rainfall prediction is of a couple of days at most and the magnitude (mm/day) not easily evaluated beforehand).
Water seeps in the soil/rock mass and can increase the external pressure on the backside of an unstable mass (especially in rock slopes), can increase the pore pressures in the sliding surface, and can increase the weight of the soil mass.
It is very interesting to note that although the best way to stabilize a landslide may be by controlling the water pressures, engineers tend to ignore that and try to use more complex methods such as pile walls, ground anchors etc with questionable results. It would be great to have comments on this issue…
If we look at landslides from the earth’s perspective we can define them as a condition towards stress relaxation and minimization of potential energy. The earth is trying to come to equilibrium. Landslides are formed either from natural conditions such as erosion of slope toe, geological uplifting etc or are manmade when extensive excavation or loading is taking place in the foot or the top of a slope respectively. These are the main reasons why landslides form but the triggering mechanisms for a landslide to occur may be different.
In my opinion the most important triggering factor for a landslide to occur is the modification of groundwater or pore pressure conditions. This issue becomes more important if we consider the climate change which is producing different rainfall conditions with abrupt changes on ground water conditions.
Due to weather pattern changes, ancient, dormant or newly formed landslides will start to occur all and all more frequently. This will affect the economies and life worldwide and is something that needs to be addressed not only from geotechnical engineers and geologists but also by government officials, politicians and decision makers in general.
Landslides will become a hot topic in the near future, here are some very interesting blogs dealing with this issue:
People interested in geotechnical engineering are watching closely the colliery landslide at Hatfield Stainforth, in Northern England that is still in progress. Since February 13nth reports and photos on the landslide’s progress are coming in.
Thanks to Prof. Dave Petley’s (Durham University UK) landslide blog, the info and the photographs are really spectacular. According to his first estimation the size and the geometry of the toe bulge suggest a bearing capacity failure. The very wet weather of last few months, triggered the landslide with a rotational geometry.
Due to the size of landslide mass, stabilization will be highly difficult. The railway line is expected to be closed for some time.
In these cases where the amount of soil mass is prohibiting, it might be better to wait for the landslide system to find progressively its own balance, before intervening with further stabilization measures.