Stress regime in compacted fills during wetting and drying cycles under laterally restrained conditions

Abstract

Climate change in the form of global warming is taking place. The impact of it on society is severe and unpredictable. The most noticeable evidences are: rise in land and sea temperatures (in particular in summer time), rise in sea levels, and wet winter combined with flash flooding. Dry summers and wet winters are the main contributors to many of the natural disasters in the world, witnessed in the last few decades. These disasters are directly or indirectly associated with soils (i.e., ground) which immediately response to any climatic changes.  Soil is also an integral part of the modern infrastructure such as embankments, retaining walls, sub-base for roads and railway tracks, and many more. These structures are often referred to as Geo-Infrastructure. 

 The impacts of climate change on the infrastructure are far-reaching. Policy makers and practicing engineers are fully aware of the consequences of climate change and investing resources to develop adaptation technologies to limit the impact and reduce repair and maintenance costs. A particular case examined in this thesis referred to the performance of compacted clays placed behind retaining structures. Compacted soils are often unsaturated at the time of placement and their responses to climate change for example, wetting can be swelling/collapse and reduction in strength. These issues are also influenced by the compaction energy, compaction water content and soil type. Compacted soils placed in accordance with BS guidelines (i.e., engineered fills) could swell upon wetting. If such swelling is restricted (for example conditions behind retaining wall) the horizontal pressure in the compacted soil would increase, raising concern on the stability of the structure. However, if the wetting is cyclic (wetting after prolonged drying) then progressive build-up of stress can lead to eventual failure. 

The above aspect was investigated in laboratory-based research. Three distinctively different soils were considered: Kaolin Clay, London Clay, and Belfast Clay. Samples of these clays were tested in modified stress path cells. In the first stage of the investigation, evolution of stresses during the placement of fills was established. In the second stage of investigations samples of London Clay and Kaolin Clay were subjected to wetting and drying cycles under horizontally restrained conditions. Wetting was induced by injecting water into the sample at a slow rate and drying of sample was executed by circulating dry-air through a slender sand column place in the middle of the sample along the length. In this case suction within the sample was measured using high capacity tensiometers. In the third stage of the investigation, the samples were taken through wetting and drying, still under horizontally restrained conditions. However, a widely adopted known as axis translation technique was adopted to control suction. 

  It is a common practice to assume that the coefficient of earth pressure 𝐾0∗ is one in compacted fills for design purposes. However, the observations obtained on all three soils have clearly indicated that the values of 𝐾0∗ can be very high, and its value would only fall below one at deep depths. Further repeated wetting and drying of samples subjected to constant overburden pressure resulted in a complex behaviour pattern. London Clay exhibited a gradual increase in 𝐾0∗value with repeated wetting and drying cycles, but Kaolin Clay exhibited a noticeable reduction in 𝐾0∗value upon 1st wetting. However, subsequent wetting followed by drying showed a significant recovery of 𝐾0∗ value. The observations obtained from the investigations involving circulating dry-air agreed favorably with that obtained from the investigations involving axis translation technique, thus promoting the potential use of the former in future research.

Thesis is embargoed until 31 December 2028.

Date of Award	Dec 2023
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Pauline MacKinnon (Supervisor) & Vinayagamoothy Sivakumar (Supervisor)