Improvement of the mechanical properties of compacted clay from the Lambeth group using fibre reinforcement

Lead Research Organisation: University College London
Department Name: Civil Environmental and Geomatic Eng

Abstract

SummarySlope failures related to pore-water dissipation, stress relaxation and desiccation cracks are major problems occurring in our ageing road network. Consequently, the remediation works necessary to correct these problems are known to cause congestion and delays that, in turn, cause financial loss. In order to decrease the recurrent time of maintenance work, Mouchel is running a pilot test using fibres mixed and compacted with natural soil to remediate a small failure occurred in an embankment south of the M25. Research in micro-reinforced soils is still in its infancy and, although laboratory research has shown that the addition of micro-reinforcement improves the strength properties of the composite material significantly, very little is known about their behaviour in situ, or of the effects of the field techniques currently in use to mix and compact the fibres, on their performance. This project, suported by Mouchel and the Highways Agency, is to study the effects of the field techniques in the performance of the composite material, originated from the mixture of clays with polymer tape fibres.The research will focus on the effects of compacting heavily overconsolidated peds (lumps) of clay on the fibre orientation and distribution within the embankment. A few samples of the in-situ compacted material, porvided by Mouchel, and samples prepared in the laboratory, will be dissected, and the results used as a basis to understand the orientation and distribution of fibres. Swelling and triaxial tests will be carried out on large diameter samples; the results will be used to understand and provide good quality data of the mechanical properties of the compacted reinforced and non-reinforced soil. The test results, together with the pilot study run by Mouchel, will provide the data to analyse the performance of the new material and their use in the maintenance of existing slopes along the highway network in UK. The outcome is expected to provide a better understanding of the effects of discrete fibre reinforcement on heavily overconsolidated clays and the effects of in-situ mixing and compaction techniques in the response of the composite soil. This will allow effective guidance in the construction and/or remediation of slope failures and widespread the use of this type of reinforcement as an effective way to reduce maintenance works on embankments. Improvement of soil characteristics using micro-reinforcement can also lead to a more sustainable way of using otherwise unsuitable soils instead of disposing of them
 
Description The research project has concentrated on the study of the mechanical properties of a compacted overconsolidated clay, reinforced with polypropylene fibres. All samples, reinforced and unreinforced, were prepared following a new methodology in order to try and replicate the in-situ procedure. This is well explained in the articles published. All samples were tested using three triaxial apparatuses, under different stress conditions. Also, in order to understand the fibre orientation generated by the sample preparation procedure, samples were desiccated and the angle between the fibre and both the horizontal and the vertical planes were measured. The principal findings of the project are listed below:

1. Although it was expected that the fibre alignment would be random, the study revealed that this is not the case. Nearly 80% of the fibres form an angle with the horizontal plane that is lower than 20 degrees, indicating that they are not randomly distributed.

2. It is well recognised in the literature that adding fibres to reinforce a soil, will increase its shear strength. However, the results obtained in this project, show that the shear strength of the reinforced and unreinforced samples are very similar and no improvement in the shear strength have occurred. The expected benefit did not occur because the percentage of fibres, 0.2%, calculated by the industrial partner and used in this research, was low compared to the percentages quoted in the literature, i.e. higher than 0.5%.

3. Although no increase on shear strength was noticed, the fibres changed the mechanical behaviour of the composite material. This was verified in two stages of the triaxial tests. Firstly, the reinforced samples reach a higher void ratio than the unreinforced samples for the same isotropic stress. Hence the unreinforced samples are denser than the reinforced samples, indicating that the presence of fibres dissipates the energy of the sample preparation procedure in a much larger volume of soil. This, in turn, affects the behaviour of the samples during shearing. Therefore the reinforced samples have a higher tendency to compress, and in undrained tests, generate higher pore-water pressures than the unreinforced samples. The consequences of this effect should be investigated further.

4. The fibres also increased the initial stiffness of the reinforced material. The stiffness of the unreinforced soil showed a common trend, i.e. proportional to the confining stresses applied, while the reinforced soil showed a higher and similar initial stiffness for stresses up to 300kPa. For stresses above 300kPa, the stiffness increases with the increase in confining stress. Such an understanding would benefit predictive modelling.

5. Another important finding of this research is that the sample preparation procedure has demonstrated to be very important to the characterization of the behaviour of this type of soils. As expected, the behaviour of compacted samples of heavily overconsolidated clays will maintain certain features of the original material, therefore any procedure related to the compaction energy applied to these materials must take into account the sample preparation procedure used in this research.
Exploitation Route The objectives were fulfilled in the sense that we were able to characterise the mechanical behaviour of the reinforced material and we are also able to show that in wet conditions, the performance, in comparison to the unreinforced soil is superior.
In order to continue this research more is required to understand the behaviour of the same samples in an unsaturated way. A very difficult task, given the size of the samples and the unsaturated testing requirements, in terms of time and to guarantee that the suction in the sample is homogeneous. In terms of testing, to meet these requirements will require an enormous amount of time. It may not be a feasible option.
Sectors Construction,Environment
 
Description The findings of this research have been used to support the installation of fibres to reinstate a second slope on the M25 on a similar soil. I have maintained few conversations with the contractors to pursue a continuation of this project.
First Year Of Impact 2013
Sector Environment
Impact Types Economic