Optical investigation through a flowing saturated granular material

Abstract

It is becoming increasingly necessary to construct on land that was previously considered inappropriate for construction, such as soft clay. The properties of soft clay make it highly compressible and low in shear strength, meaning that bearing capacity failure and excessive settlement are of concern.Piled embankments are a ground improvement technique that can provide a solution for this problem. Piled embankments have the ability to transfer the greater part of the embankment load and any surcharge to more competent material at greater depth due to the 'arching' concept. Consequently, the soft foundation soil has little direct impact on the performance of the embankment. The concept of 'arching' of granular soil over an area where there is partial loss of support from underlying strata has long been recognised in the study of soil mechanics (e. g. Terzaghi, 1943). However, a number of competing theories exist to quantify this behaviour in piled embankments.In addition, the use of geotextile reinforcement in piled embankments placed above the pile caps in principle provides a number of technical as well as economical benefits. As the embankment fill is placed, tension is created in the reinforcement and it is the vertical component of this tension that transfers the embankment load onto the piles and reduces the load carried by the soft clay hence transferring the load of the embankment on to the piles.Differential settlement can be a problem for piled embankments of low height.Significant differential settlement can cause undesirable effects on any structures constructed on the embankment. 'Arching' limits the amount of differential settlement in embankments and the use of geotextile geogrid can also potentially have additional benefits.This thesis presents a series of centrifuge tests examining the performance of unreinforced and reinforced piled embankments constructed over soft subsoil in terms of stress acting on the subsoil, and differential movement at the surface of the embankment. A large range of embankment heights are considered, and the results for stress on the subsoil are compared with existing predictive methods, allowing generic conclusions to be drawn regarding the predictions of various methods.The effect of a 'working platform' below pile cap level and thus directly loading the subsoil is also considered, and used to examine the concept of a 'Ground Reaction Curve' (Iglesia et al, 1999) for arching in the embankment. In principle this can be used to consider compatibility of displacements at the base of the embankment, and thus improve design simultaneously considering the effect of arching in the embankment and underlying support from the subsoil and layers of reinforcement acting in tension.

Centrifuge modelling of the ground reaction curve of fibre reinforced soil

“…The curve shows a characteristic increase in relative settlement with trapdoor width for a given soil height. The curve shows agreement with other observed settlement behaviour in that at high H/B ratios, the settlement tends towards zero (Mckelvey (1994), Aslam, (2008 and Yan (2009)). …”

“…As (s − ) increases to 2.0 the stress on the subsoil does not increase significantly (and thus there is significant evidence of arching), and differential settlement at the embankment surface tends to zero (Aslam, 2008). These McKelvey's plane of equal settlement theory and observations by Iglesia et al (1999) that no additional vertical settlement is present when the embankment height, is increased beyond a critical level.…”

“…Figure 2.13 Relationship between differential settlement and embankment height in piled embankment tests, (Aslam, 2008) The data in Figure 2.13 was obtained at two different prototype scales (30g and 60g). This in effect means that the stress on the model subsoil is increased by a factor of two causing an unequal amount of subsoil deformation, , between the two data sets.…”

“…Nottingham have used the material (Aslam (2008), Liu (2010)). The grain size and natural texture of the material ensures that sufficient detail can be observed in test images to allow the PIV process to accurately track soil displacements.…”

“…The peak and residual friction angles of the soil, and , respectively, were determined by shear box tests conducted by Aslam (2008). Maximum and minimum densities were calculated using Equation 3.6.…”