CHASM is an integrated slope hydrology/slope stability software package. The dynamics of slope hydrology are computed using a finite difference formulation that accommodates unsaturated and saturated soil water conditions. Slope stability analysis (Bishop method) is undertaken using a grid search procedure, which is implemented continuously during the simulation period. The structure of the code enables a wide range of applications to be undertaken. CHASM developments and applications have been published in both books and research papers, and training courses delivered in the Caribbean, Europe, and the United States.
As well as being featured in the press [including Far East newspapers such as Today, Malaysia] CHASM software has been
The inclusion of dynamic pore pressures is a critical element in enabling a more precise estimate of the slope factor of safety to be made. In certain climatic zones such as the Tropics and sub-tropics this inclusion is of critical importance. Here intense rainfall rates coupled with vegetation commonly found on slopes in these regions, demands an integrated dynamic model capable of capturing the highly dynamic precipitation inputs and consequential pore pressure responses. The hydrological representation within CHASM™ seeks to accommodate these demands by:
The stability assessment techniques used in CHASM is Bishop’s simplified circular method (). At each time step of the simulation, pore pressures, both negative and positive, are incorporated directly into the effective stress determination of the Mohr–Coulomb equation for soil shear strength. The Fredlund et al. (1978) criterion is used for the unsaturated portion. This provides input into the limit equilibrium technique for derivation of the minimum factor of safety (the ratio of restoring to destabilising forces), with temporal variations arising from hydrodynamic responses and changes in the position of the critical slip surface (). This analysis involves a numerical search for the slip surface that minimises the factor of safety.CHASM uses the Bishop method of slices to perform slope stability analysis.
The automatic grid search specification is included which can be specified in terms of grid location, size of grid and search radius. The slip circle is then itself specified in terms of the initial radius and radius increment. The search procedure for the minimum factor of safety is then carried out by successive steps in the slip grid and changes to the radius of the slip circle. This search is carried out every hour during the simulation run time.
Slopes that are not safe enough can be reinforced using a wide range of engineering techniques when they cannot be directly cut to a safer slope angle
CHASM can accommodate slope reinforcement using both geo-textiles/geo-grids and earth nails. Their positive contribution to the shear strength of the slope is incorporated directly within the Bishop equation. In CHASM, earth nails add reinforcement by friction with the surrounding rock/soil and geo-textiles/geo-grids mobilise reinforcement due to the weight of ground above them. For both techniques, a factor of safety can be applied to all parameters supplied by the manufacturer to account for time degradation and parameter uncertainty.
The natural role of vegetation in controlling slope stability is evident in many different areas of the world. For example tree roots can have significant positive effects in terms of adding to the effective strength of the soils. However, what complicates this relationship is the changed hydrological parameterisation of the soil properties that vegetation provides (for example, by increasing the soil hydraulic conductivity).
In major projects as well as more routine maintenance considerations bioengineering is seen as a specific option to enhance slope stability. This can take the form of standard hydroseeding, as well as other more intricate species mixes on slopes:
To accommodate bioengineering CHASM includes a vegetation module which allows changes to be made to the effective soil hydraulic conductivity, the effective rainfall reaching the surface of the slope and the soil shear strength via root reinforcement.
Reference:
Collison, A.J.C., Anderson, M.G. and Lloyd, D.M. (1995) Impact of vegetation on slope stability in a humid tropical environment: a modelling approach. Proc. Inst. Civil Eng. Water Maritime and Energy, 112, 168-175, (paper awarded the Trevithick Premium Triennial award for a research paper published by the Institution of Civil Engineers in 1996).
Landslide travel distance is of importance in a number of applications relating to slope design and maintenance. This is a complex area of research requiring detailed rheology and material parameterisation that is rarely if ever available. Nonetheless it is highly relevant to include some estimation methodology within CHASM to provide some indication of likely landslide impact a s far as runout distance and depth is concerned.
Finlay et al. (1999) used data from the Geotechnical Engineering Office in Hong Kong to carry out an empirical analysis of the landslides recorded. 1100 landslides were included, out of a possible 3000, which had occurred between 1984 and 1993 in man-modified slopes of weathered granite and other volcanics. Most of these landslides had volumes of less than 1000m3 and debris ran out onto a near horizontal surface below the slope.
Multiple regression models based on slope geometry were developed for the prediction of landslide travel across horizontal surfaces at the base of slopes using the geometry of a failure in a cut slope:
and it is this empirical model that is used in CHASM. The required independent variables are tan A (slope angle), D (depth to slip surface) and H (height of landslide). The dependent variables are log L and H4 (depth of debris at base of slope) for cut slopes. Three equations are given for each dependent variable – lower confidence interval (95%), mean predicted value, and upper confidence limit (95%).
CHASM returns the landslide runout distance, R, for each hour of the simulation runtime
Reference:
Finley, P.J., Mostyn, G.R. and Fell, R. 1999. Landslide Risk Assessment: prediction of travel distance. Canadian Geotechnical Journal, 36: 556-562.
Appropriate levels of seismicity can trigger landslides. Significant regions of the world are prone to such triggering events. For example, in the Caribbean it is considered that an earthquake on 18 September 1999 measuring 4.7 on the Richter scale, triggered a significant landslide, whilst in parts of Europe and the Far East similar events have promoted links to be made between the required gravitational acceleration required to initiate landslides and local building codes.
Parameterising seismicity is both difficult and complex. CHASM uses an empirical guide-line method reported by Charalambus 2003 to estimate the horizontal seismic acceleration to induce instability. The software manual provides an illustration of the link between this resultant acceleration and the required Richter scale trigger for selected examples derived from different countries. A degree of local calibration of this form is required in the use of the resultant acceleration.
Reference:and complex. CHASM uses an empirical guide-line method reported by Charalambus 2003 to estimate the horizontal seismic acceleration to induce instability. The software manual provides an illustration of the link between this resultant acceleration and the required Richter scale trigger for selected examples derived from different countries. A degree of local calibration of this form is required in the use of the resultant acceleration.
Charalambus, S 2003 Methodology development for the stability evaluation of natural and man-made slopes against static and seismic loads in a GIS environment. Technical University of Athens, Greece
