The Benefits of Using Computational Limit Analysis Software
Figure 1: Computational Limit Analysis of working platform problem from BR470 (load bearing on 0.6m working platform of granular material over soft clay), clearly illustrating punching failure and also showing full extent of the failure mechanism. Predicted bearing pressure = 213 kPa, compared to 205 kPa from BR470.
LimitState:GEO featured in an article that appeared in the September 2011 edition of theGeotechnica magazine, illustrating its use in working platform design. A summary of the key points is reproduced below.
To read the full article, visit theGeotechnica online.
Working platform design
Current industry guidance for working platforms is available, for example in the good practice guide ‘Working platforms for tracked plant’, BR470 (Skinner, 2004, BRE). This provides a straightforward semi-empirical design method for determining platform strength and thickness.
As illustrated in Figure 1, computational limit analysis (CLA) software, such as LimitState:GEO, allows a quick check of any design to be performed. LimitState:GEO shows the extent of the failure mechanism and also frees users from many of the restrictions of semi-empirical methods, for example allowing the following to be considered:
- sloped platforms;
- platform edge stability;
- variable strength or multilayer subgrades;
- track interaction;
as well as providing scope for straightforward sensitivity studies.
Comparison with widely-used examples
Figure 2 compares the results of LimitState:GEO analysis with the examples given in the widely-used guidance document BR470 for clay subgrade. It can be seen that a good match (typically less than 4% difference) is obtained between the two approaches for plane strain (2D) analysis. Obtaining the results using LimitState:GEO requires only a few minutes to set up the model. Analyses typically solve in seconds or minutes.
Figure 2: Comparison of BR470 and CLA plane strain results for a range of subgrade undrained shear strength (cu) values.
However, LimitState:GEO really comes into its own when non-standard design situations are involved, e.g. when considering stability on a sloped platform (Figure 3a), a multi-layered subgrade (3b), or when checking how close plant can be taken to a platform edge (3c). In this latter case, the available bearing capacity was reduced by 26% (the same strength of platform material was assumed throughout. In practice reduced strength may be found at the edges, a situation which is easily modelled in LimitState:GEO).
Figure 3: Different working platform configurations, analysed using LimitState:GEO. a) Sloped platform; b) Multi-layered subgrade and c) Collapse mechanism at platform edge