In this thesis an evaluation of the limit values which are stated in CUR 236 – Micropiles is presented. This evaluation has been done by means of a dataset. The dataset contained information on failure tests performed on four different types of micropiles, and was initially used for research on relations between logged installation aspects and the final capacity of the piles; such relations were however not found. This was mainly caused by the shape in which the data was logged, and the lack of details in the data. During this research, significant differences between raw data and data adapted according the limit values stated in CUR 236, were found. This led to a change in research direction towards the evaluation of the limit values.
Limit values are values that are used to build in additional safety in a design, and thus, to prevent unsafe situations. The values are based on the shape of data in which in general a linear relation between maximum mobilized shear stresses (τ_(mob;max)) and cone resistance (q_c). This relation continues up to a certain cone resistance level, after which τ_(mob;max) does not increase significantly anymore. The point where no significant increase in τ_(mob;max) over q_c was seen, was used as the limit value for the cone resistance. The limit for α_t was chosen to be 2.5% and the limit for τ_(mob;max) 2.5% of the cone resistance limit. Data with values above these limits, has to be reduced to the limit values. (CUR236, 2011)
Differences were thus found between raw data, the same data but then adapted according the limit value method stated in CUR 236, and values stated in CUR 236 Table 6.1. For further investigation on how these differences were caused, the available dataset was combined with data from the appendix ‘Bijlage A. Proefbelastingen’ of CUR 236. This showed that the current limit values did not suit the shape of the data very well. Investigation on the usefulness of the limit values showed that they are useful for the design process by preventing under- and overestimations of the capacity. Limit values are thus useful, but not in their current form. New limit values were therefore proposed based on a statistical analysis of the available data points. Next to limit values, also design values (expectancy and lower bound) are added.
Because no detailed data for all micropile types was available, it is advised to re-elaborate the limit- and design values for the types that were left out of the proposal given in this thesis, in a similar, statistical way. Furthermore is it advised to investigate a different method to estimate the shear stresses that can be mobilized along the anchor body of a micropile, based on a more fundamental approach. Currently are those stresses often based on full-scale tests from which τ_(mob;max) is derived, but bounded by the limit values. A different approach to this might be more time- and cost-effective.