Al-Ani, Wisam
(2020)
Numerical analysis of ground improvement using column-like elements.
PhD thesis, University of Nottingham.
Abstract
The issue of constructing foundations and embankments over highly compressible soft soils is of great importance for geotechnical engineers due to their undesirable characteristics such as excessive settlement and low bearing capacity. A number of techniques including column-like elements, such as settlement-reducing ‘piles’ and vibro stone columns, are available for improving strength of weak soils. These techniques are mainly used to reduce the applied pressure on soft soils without altering the soil structure significantly.
Several piled-embankment design procedures (i.e. Hewlett and Randolph, 1988 and the German method (EBGEO), 2004) currently used by industry appear to yield significant difference in predicting the load transfer mechanism within the embankment material due to lack of modelling various aspects of the embankment geometry, soil properties, and load transfer mechanism with the embankment material.
The use of column-like elements, for improving both the settlement performance and bearing capacity of foundations constructed over soft soils is well rehearsed for large groups of columns support an infinitely wide load area such as embankments and slabs where all columns are equally restrained on all sides; but little is understood for lightly loaded low-rise structures supported by pad foundations constructed on stone columns, particularly when a crust layer is available at the top of the soft soil.
A two-dimensional Finite Element analysis (FEA) in conjunction with a hardening soil model was used in this research to investigate the load transfer mechanism within embankment fill using Zaeske’s model (Zaeske, 2001). A frictional design method which depends to a large extent on the coefficient of lateral earth pressure (K) was used to investigate the load transfer mechanism. It is shown in this research that using the friction method in conjunction with a plane of equal settlement method can be very useful tool to investigate the load transfer mechanism within embankment material. It is also shown in this study that BS 8006 (2010) revealed unrealistic results when analysing Zaeske (2001) model test.
This research includes the first comprehensive three-dimensional FEA modelling of the presence of a stiff crust layer overlying soft soil for lightly loaded structures supported by stone columns. The soft soils at Bothkennar site, Scotland, were modelled in this research. The modelling of the Bothkennar field trials has shown that both drained and undrained analyses agree very well in predicting a long-term settlement. The presence of a stiff crust has a significant influence on the deformational mode of the stone columns which is not normally captured by laboratory tests. It was found that the lateral bulging is constrained by the crust at the upper part of the column. It has been demonstrated in this research that the use of geogrid encasement around the column improves the column bearing capacity and their stress-displacement behaviour beyond an average applied pressure of 70 kPa. The greater the length of encasement used, the more the total and differential settlement are reduced. The encasement of the top metre of the column reduces the lateral bulging considerably. However, the lateral bulging can also reduce significantly by extending the encasement below the maximum lateral bulging zone (around 2 times the diameter of the column). It was found that the most common design method (Priebe, 1995) underestimates the settlement of the stone columns; this is attributed to the fact that Priebe (1995) method does not consider the presence of a stiff crust layer, also it does not take account the disturbance of the sensitive soft clay during the installation of the stone columns which will contribute to the increase of footing settlement during the field trials.
Finally, a simplified design method derived by Satibi (2009) has been modified to estimate the relative settlement reduction ratio (RSR) of small and large groups of stone columns with the knowledge of the foundation applied pressure, column diameter, column spacing, and the column installation effects. It appears that the effectiveness (RSR) of the stone columns lies in the range 0.6-0.95 and (0.35-0.6) times the soil layer thickness for small and large groups, respectively. The RSR ratio predicted by the FEA agrees quite well with the analytical model when the soft layer thickness over critical column length ratio is in the range of 2 to 5.
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