Soil Stabilisation - STARES

Authors:
Nigel Balaam

STARES (STability Analysis of REinforced Soil) for Windows is a computer program which enables the user to compute the stability of an earth mass stabilised by reinforcement using an analysis based on Bishop's (1955) simplified method for unreinforced slopes. In these analyses the stabilisation provided by the reinforcement is assumed to be entirely due to the axial force generated along the reinforcement. Thus, the contribution that the shear forces in the reinforcement make towards improving the factor of safety against failure are assumed to be negligible and therefore not included in the analysis. This assumption means that STARES is suitable for assessing the internal stability of the following types of problems:

* wall systems retaining soil stabilised by the inclusion of metallic (e.g. Reinforced Earth) or non-metallic (e.g. Freyssisol by Austress Freyssinet) strips,
* slopes and walls stabilised with soil nails,
* slopes and walls stabilised with geotextiles or geomembranes,

In these analyses a circular surface of rupture is assumed and then the equilibrium of the sliding mass of reinforced soil is considered. This process is repeated for a large number of circles and the minimum factor of safety determined.
The slope (or soil retained by the wall) may be divided into a number of different soil layers with different properties. The reinforcement strips can be conveniently grouped into zones of reinforcement with all the strips in a zone being assigned the same properties.

Pore pressures within each soil layer can be calculated by a number of different methods, viz. from the depth below a piezometric surface, by using a pore pressure coefficient ru, from a user specified grid of pore pressures, or from a grid of pore pressures generated by program FESEEP. This flexibility provides a mechanism for modelling all possible pore pressure variations.

External normal and shear tractions can be applied to segments along the surface of the slope.

STARES for Windows replaces the DOS version that was made commercially available in 1993. It is a fully interactive program that provides the user with options to assemble and edit the data, perform analysis, inspect the output file and graphically check the data and plot the results of the analysis. An interactive analysis option is also available that allows the user to interactively analyse circles as the mouse is moved.

Several features of the program are worth highlighting:

• The circles are sorted into ascending order of factor of safety before being written to the solution file. The sorting routine used is very efficient and was published by Houlsby and Sloan (1984).
• The centres and radii for a user-specified number of circles can be generated automatically. The algorithm provides a good initial estimate of the location of the critical centre. This initial estimate can subsequently be used to determine the location and size of a user-specified grid of circles. Using this approach the critical circle can be determined very efficiently.
• A comprehensive summary table may be generated for a circle, which lists for each slice the values used in computing the factor of safety. This provides the last essential step in design, in which the values of weight, normal force, pore pressure, restraining forces due to the reinforcement etc. for each slice in the critical circle are checked before reporting this circle's factor of safety.
• The methods that are available for defining the pore pressures provide sufficient flexibility so that any pore pressure variation throughout the slope can be adequately modelled. One of these methods involves the use of the pore pressures computed in a finite element steady state seepage analysis using program FESEEP for Windows. This method enables a consistent analysis of the stability of reinforced slopes in which steady state seepage is occurring.
• Interactive analysis can be performed. After the entry and exit points of the circles on the surface of the slope are defined circles are automatically analysed by moving the mouse if the Analysis-Interactive (Mouse move) option is selected. If the Analysis-Interactive (Mouse click) option is selected then a circle is analysed only when the left button on the mouse is pressed. During the interactive analysis a record is kept of the circle analysed with the minimum factor of safety. This circle is plotted in red and its factor of safety value is tabulated in red.