Simplified Estimation and Graphs for Pre-Design of Geosynthetic-Encased Sand or Gravel Columns as Embankment Foundation
Embankments on soft subsoil supported by piles or stone columns have important advantages compared to "conventional" embankment foundation: no consolidation time is required, there is no import/export of additional embankment soil to accelerate consolidation or to compensate the settlement, practically no additional settlement occurs under traffic etc. The use of this solution is growing recently in Germany. Starting in 1994, a system for foundation of embankments in soft soil areas was developed by the German contractor Möbius and HUESKER Synthetic. The general idea was to create a less expensive alternative to the conventional piles of any kind and to eliminate in the same time the impossibility of constructing e.g. stone columns in very soft soils due to insufficient lateral support. The problem can be solved encasing a compacted sand or gravel column in a high-modular geosynthetic encasement. First projects started successfully in Germany in 1995. Meantime the solution proved to be very efficient for more than 15 huge projects. After intensive modifications over the years, solutions and corresponding high-modular low-creep geosynthetic encasements (Ringtrac®) are in the stage of maturity. Thus, the necessity of quick simplified pre-design procedures as a first guess of column pattern, Ringtrac®-parameters etc. is increasing. Based on series of analytical solutions (although under simplified assumptions) design graphs are developed and presented to meet the needs cited above. They are believed to be a simple pre-design tool for geotechnical engineers.
A series of design calculations were performed for dimensioning of Geotextile Encased Columns (GEC) beneath an embankment on soft soil. Two recent analytical procedures were used, which are believed to be precise enough. A "standard" case was analyzed, varying some important parameters in a typical practice-related range. The results are presented as graphs which can be used in a simple way for rough pre-design calculation of settlements and/or required "percentage" of columns and/or for determining the required tensile module in ring direction of the geotextile encasements (Ringtrac®).
The aims of the work are:
To show the influence mainly of the typical factors ring-tensile module, "percentage" of columns in the embankment base and soft soil oedometric module. (Note, that the first two of them can be varied in a wide range by the geotechnical engineer to find out the optimal solution). To show in a quick way the "lower" and "upper" limits of the system for a given case. To perform "best" and "worst" case quick pre-design calculations for cases with only insufficient information available. To find out which data and parameters are critical for the design in a given case for finding out e.g. is a more precise geotechnical survey necessary for the final design etc.
Some issues should be pointed out:
In the range of parameters on the graphs presented single and double interpolations are allowed with an acceptable loss of precision. The soft soil is assumed to be homogeneous with depth and the diameter of encasement to be equal to the diameter of the installation steel pipe for the purpose of simplicity and due to the lack of place. (More wide-range analyses including also FEM will be published separately). The range of ring tensile modules used corresponds to the short- and long-term values of real geosynthetics (Ringtrac®'s of different types) and is not only a model abstraction. The work presented does not pretend to be complete. The idea is to show a possible way for rough estimation and orientation.